Olivier Alphand

Karine Altisen

Rodolphe Bertolini

Stéphane Devismes

Ali J Fahs

Franck Rousseau

Éviter les collisions dans les r éseaux 6TiSCH

Keywords: IoT, IEEE802.15.4e, TSCH, 6TiSCH, 6top, Scheduling Functions

Les réseaux multi-sauts 802.15.4e TSCH s'appuient sur des échéanciers de communication efficaces. Notre solution complète les algorithmes de construction de tels échéanciers en limitant la réutilisation de cellules temps-fréquence déjà utilisées par des noeuds voisins. Nos simulations montrent un gain significatif par rapport à l'existant.

Introduction

Among many standards dedicated to the Internet of Things (IoT), the initial version of IEEE802.15.4 aimed at low power short range communications. However its MAC layer presented several limitations (no channel diversity, unbounded delay, and poor multi-hop topology support). To overcome these limitations, IEEE802.15.4e extended the standard by proposing a new mode: TSCH (Time-Slotted Channel Hopping). TSCH defines a periodical slotframe consisting of fixed-size cells, each indexed by a specific timeslot offset and a channel offset. Each cell has the same duration and its length should be large enough to accommodate a maximum frame length and its acknowledgment, while ensuring at the same time synchronization between communicating nodes. TSCH also provides channel diversity, enabling parallel communications on different channels within the same timeslot, as well as channel hopping in the same cell from one slotframe to another. The channel of a cell is derived from the channel offset and the absolute slot number. There are two types of TSCH cells: shared cells where multiple nodes may either receive or transmit, and dedicated cells for contention-free link-local communications between two neighbors. IETF (Internet Engineering Task Force) has standardized an IoT stack based on 802.15.4. The integration of TSCH in this stack is currently being carried out by the 6TiSCH IETF Working Group through the 6TiSCH operation sublayer called 6top. It includes the definition of algorithms to schedule dedicated cells and a 6top protocol (6P) to negotiate cells between nodes. We focus here on improving the distributed scheduling algorithms that are mainly designed to address the problem of convergecast traffic towards a unique sink in multi-hop networks. RPL (Routing Protocol for Low-Power and Lossy Networks) organizes the network as a DODAG (Destination-Oriented Directed Acyclic Graph) along which the data generated by each node are relayed up to its root. Scheduling algorithms thus attempt to create efficient TSCH schedules along RPL routes. Most of existing scheduling functions randomly select cells among those that are not already used by either negotiating nodes with their respective DODAG parents or children. However, the cell selection should also preclude dedicated cells already reserved by other neighbors that may interfere leading to collisions and consequently to packet dropping. Our solution makes distributed scheduling algorithms aware of dedicated cells already assigned in the neighborhood by overhearing past cell negotiations exchanged with 6P in shared cells. To evaluate our solution, we compare the performance of the On-the-Fly (OTF) [START_REF] Palattella | On-the-Fly Bandwidth Reservation for 6TiSCH Wireless Industrial Networks[END_REF] scheduling algorithm with and without our approach. Results show a significant improvement. Roadmap. Section 2 gives an overview of 6TiSCH and related works. Section 3 describes our solution and Section 4 details some results obtained by simulations. We conclude in Section 5.

Background

6TiSCH operation sublayer (6top). To integrate TSCH within the IoT stack, the IETF 6TiSCH Work Group defines the 6TiSCH operation sublayer (6top). 6top includes the definition of policies to manage TSCH schedules and the 6top protocol (6P) to control the communication for cell reservation and deletion between nodes. If we consider a transmitting node (Tx) and a receiving node (Rx) that communicate (i.e., Rx is a parent of Tx in the DODAG), 6P supports three types of operations, encapsulated into a transaction, that can be triggered by Scheduling Functions (SF). If the Scheduling Algorithm determines that Tx needs for more (resp. less) cells to communicate with Rx, it will issue a 6top transaction to reserve more cells (resp. to release some TSCH cells) in the TSCH schedule. If the Relocation Algorithm detects that a dedicated cell is facing collisions, this defected cell is replaced through 6top transactions. A 6top transaction consists of a negotiation between Tx and Rx that updates the TSCH table and takes place in two or three steps. The SF decides whether to use a 2-step or a 3-step transaction, or a mix of them. Moreover, the SF has two options to communicate the list of cells: either black listing where the node sends all cells in use, or white listing where the node sends a number of cells available for communication. We detail in Figure 1 the sequence of a 2-step transaction using white listing to allocate two cells for link G → D:

1. The SF of node G detects that two additional cells are needed and selects three candidate available cells {(1, 3), (2, 3), (4, 2)} † before triggering a 6P ADD request towards node D in the shared slot. 2. The SF of D precludes the cell (2,3), selects 2 cells {(1, 3), (4, 2)}, and issues a 6P response to G. As shown in the example, the cell selection is not done randomly among all the cells of the TSCH slotframe. The 6top negotiation natively precludes all the cells in timeslots in which either negotiating nodes communicate with their DODAG parents or children. The selection of the candidate cell is then randomly done in the set of remaining cells. In the previous negotiation, D will not consider timeslot 2 since it already uses it to communicate with its parent B. G has no dedicated cells so it does not remove any cell a priori. Related work. We consider networks that are dynamic in terms of traffic and/or topology. So, centralized scheduling approaches should be given up, since gathering all information by a single node and redistributing the computed schedule to every node would imply a huge overhead in terms of signaling. In distributed solutions, cell scheduling is addressed autonomously by each node on the basis of local information. Thus, the signaling traffic is reduced, but the probability of collisions increases since nodes only have a partial view of the network. Many distributed TSCH scheduling algorithms only rely on the basic 6top random cell selection. The inherent colliding cells are reduced a posteriori by a reactive relocation algorithm. Onthe-Fly (OTF) [START_REF] Palattella | On-the-Fly Bandwidth Reservation for 6TiSCH Wireless Industrial Networks[END_REF] implements a bandwidth reservation mechanism that compares the traffic sent by a node to each of its neighbors and the corresponding scheduled cells. Above/under a certain threshold, it triggers the negotiation to add/delete cells. The resulting cell schedule is not conflict-free but a mechanism called housekeeping [START_REF] Muraoka | Simple Distributed Scheduling With Collision Detection in TSCH Networks[END_REF] relocates underperforming cells. [START_REF] Theoleyre | Experimental validation of a distributed self-configured 6tisch with traffic isolation in low power lossy networks[END_REF] and [START_REF] Domingo-Prieto | Distributed pid-based scheduling for 6tisch networks[END_REF] provide random policies that reduce the collisions and a relocation algorithm; but relocation takes extra time and implies extra 6top traffic which could be avoided a priori. Several proactive algorithms, such as DeTAS [START_REF] Accettura | Decentralized Traffic Aware Scheduling in 6TiSCH Networks: Design and Experimental Evaluation[END_REF] and Wave [START_REF] Soua | Wave: a distributed scheduling algorithm for convergecast in ieee 802.15. 4e tsch networks[END_REF] build a collision-free schedule, but both of them are not entirely distributed. Compared to fully distributed algorithms, they allow to obtain collision-free schedules but increase latency and signaling overhead. Instead, our approach is to improve fully distributed algorithms by using light extra signaling.

Proposed Mechanism

Our approach is complementary to scheduling algorithms as we aim at preventing the scheduling function of nodes from considering cells already allocated in their neighborhood. In Figure 1, the negotiation results in allocating (4,2) to link G → D, which is already allocated to link H → E. If both links are active in this cell, a collision occurs. This could have been prevented if G was aware of the previous allocation negotiated by H with E (n.b., E is in the 1-hop neighborhood of G). More generally, this kind of collision occurs at some reception node when two or more of its neighbors are transmitting in the same scheduled cell. Overhearing. Our approach first consists in overhearing 6top transactions from neighbors. 6top transactions are sent as unicast in a shared slot and received by all neighbors at the PHY layer. Then, all of them except the destination reject the packet due to the MAC filtering. With a slight modification at the MAC sublayer, we disable the filtering of those transactions and collect cells reserved by the neighbors, without any overhead. This mechanism is highly efficient at the bootstrap since no dedicated cell has been scheduled. To allocate their first cells, nodes send their 6P messages in the shared cells defined by the † The tuple (i, j) identifies the cell at timeslot i with channel offset j. 6TiSCH minimal TSCH schedule. Their location is either preconfigured or learnt by nodes at bootstrap. One example of this schedule can be seen in Figure 1 where one shared cell is located at timeslot 0 of the slotframe with channel offset 0. The access to shared cells follows slotted ALOHA rules. So, the higher the number of 6P transactions, the more collisions and retransmissions in shared cells. Once dedicated cells are allocated between a pair of nodes, further 6P transactions should use dedicated cells. In our approach, we always consider 6P transactions that use shared cells to benefit from overhearing. In case the scheduling function uses 2-step transactions, we can collect the cells reserved by Rx from the ADD response, using white listing. Similarly, this approach can be implemented with 3-step transactions by using black listing. Avoid table. The collected cells are saved in a table called an avoid table. The structure of this table is similar to that of TSCH in which each cell is represented by a timeslot and a channel offset. Each cell in the avoid table can be marked as available or reserved. The cell is available if it is not used by any of the neighbor nodes. If it is reserved, it should be avoided because of possible collisions. The last step of the transaction is to force scheduling functions to avoid cells found in the avoid table. Cell buffer. Due to the unreliable nature of low power wireless links, some transactions might not be received by all neighbors leading to inconsistencies among neighbors. Therefore, those neighbors could reserve already allocated cells leading to collisions. To mitigate the effect of lost transactions, we add a cell buffer that stores the last k cells reserved by a node with its children, where k is the size of the buffer. Thus, whenever a cell is reserved, instead of sending only the recently reserved cells, we send the cell buffer. As a result, all the cells in the received buffer are added to the avoid table. This means that each cell is transmitted k times to the neighborhood. This effectively increases the number of successful receptions of the cell reserved by neighbors, and consequently reduces the number of collisions. The value of k is calculated as follows. We assume that the probability that a neighbor successfully receives the transaction is p and that the number of successfully received transmissions (Y ) follows a binomial distribution B(k, p) (each cell in the buffer will be transmitted k times). The probability P of at least one successful reception is then P = P(Y ≥ 1) = 1 -(1p) k . We have evaluated, by simulation, the average value of the Packet Delivery Ratio (PDR) for the neighborhood of each node. As a pessimistic and conservative approximation of those results, we fixed p = 0.3. The equation then ensures that, with a buffer size of k = 10, we have P = 97 % confidence that the cell reserved will be received by all the neighbors.

Results

In our tests, we have used a 6TiSCH simulator ‡ , which is an implementation of IEEE802.15.4e that uses RPL for routing, 6top for the management of TSCH, and OTF (On-the-Fly) for scheduling. We first had to overcome a limitation of the simulator that bypassed the exchange of 6P messages in the existing implementation. Cell negotiations between nodes were "instantaneous", neither resulting in interference, collisions ‡ https://bitbucket.org/6tisch/simulator/src nor retransmissions. We have fixed this issue so that cell negotiations trigger the exchange of 6P packets in shared cells. Second, we have implemented our proposal. The simulations were done over a wide range of topologies and each tested protocol was run on the same topologies to ensure fairness. Below, we summarize the simulation parameters: Comparisons without Housekeeping. First, we have compared the performance of OTF with and without our approach when housekeeping is disabled. Housekeeping is based on a periodic function that checks the underperforming scheduled cells by tracking their PDR. Under a certain threshold, a cell will be relocated.

In Figure 2, Random stands for the original OTF protocol in which nodes only avoid cells already reserved with parents and children. The ME curve corresponds to OTF with our proposed mechanism but with no cell buffer while the curve MECB represents the behavior of the protocol with a cell buffer of size k = 10. At the beginning of the simulation, the increase of collided cells corresponds to the bootstrap of the network: during the creation of the DODAG by RPL, all nodes start requesting dedicated cells by sending 6top messages in shared cells. Collisions in shared cells will be progressively resolved by CSMA/CA before being able to schedule the first dedicated cells. Figure 2, shows a reduction of 60 % in the number of colliding data packets resulting from our approach. The cell buffer mechanism also improves results over the scenario without cell buffer. Despite the reduction of their number, there still remain collisions since: 1. Nodes still have a slight chance of losing 6P transactions. 2. In a specific configuration in which a node Tx2 belongs to the neighborhood of a node Rx1, but Rx2 (the parent of Tx2) has neither Tx1 or Rx1 in its neighborhood, if Rx2 first reserves a cell, Rx1 will not be aware of that transaction and will still be able to reserve the same cell. The first issue should be reduced by intensively using the mechanism of cell buffer. The second issue can be solved using a reactive mechanism like housekeeping (see below). Comparisons with Housekeeping. Housekeeping is another solution to solve the problem of colliding cells. However, it is a reactive solution, which allows the collision to occur while our solution is proactive and prevents collisions to happen in the first place. In Figure 3, we compare OTF with housekeeping alone to OTF with housekeeping and MECB. The results show in particular a decrease of the peak of at least 60% of colliding packets in the bootstrap phase. Indeed, housekeeping alone takes some extra time to relocate the defected cells while MECB, with less colliding cells, speeds up the convergence of housekeeping.

Conclusion

We have proposed a solution to reduce conflicting allocations in TSCH schedules created with 6TiSCH distributed algorithms. The idea is to make nodes able to overhear 6P transactions exchanged by their neighbors to prevent collisions. This mechanism implies a low overhead, provided the transactions are sent in shared slots. The 6P transactions have also been augmented with a short-term memory of past allocated cells, which makes overhearing more robust for the neighbors that may miss some transactions because of intermittent variations in the quality of their wireless links. The performance of our approach has been evaluated through simulations. The results show a significant reduction of collisions.

Figure 1 :

 1 Figure 1: 6P transactions and TSCH schedule update

Figure 2 :

 2 Figure 2: Number of colliding packets per cycle without housekeeping

Figure 3 :

 3 Figure 3: Number of colliding packets per cycle with housekeeping


Thomas Giesecke
email: thomas.giesecke@biologie.uni-goettingen.de

Simon Brewer

Walter Finsinger

Michelle Leydet

Richard H W Bradshaw

Patterns and dynamics of European vegetation change over the last 15,000 years

Keywords: Europe, global warming, migrational lag, pollen, post-glacial, spread of trees, EPD

published or not. The documents may come

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Introduction

The pace of human-induced global warming is forecast to rival the rates of change associated with natural transitions from glacial to interglacial periods [START_REF] Diffenbaugh | Changes in ecologically critical terrestrial climate conditions[END_REF] and the ability of plants to respond remains one of the greatest uncertainties about the impact of global warming on vegetation (e.g. [START_REF] Svenning | Disequilibrium vegetation dynamics under future climate change[END_REF]. The rate of shifting bioclimatic envelopes may not be met by the spread of plants causing a risk for their extinction [START_REF] Ohlemüller | Towards European climate risk surfaces: the extent and distribution of analogous and non-analogous climates 1931-2100[END_REF].

The spread of plants after the last glaciation provides examples of how plants altered their ranges and abundance in response to climate change, which has been researched and discussed for more than a century. [START_REF] Reid | The origin of the British flora[END_REF] showed that the distance covered by Quercus during post-glacial recolonization of the British Isles was too large to be achieved by passive seed dispersal alone. [START_REF] Skellam | Random dispersal in theoretical populations[END_REF] demonstrated this paradox using a diffusion model connecting population growth with the rate of spread, confirming the importance of long-distance dispersal (LDD; [START_REF] Clark | Why trees migrate so fast: confronting theory with dispersal biology and the paleorecord[END_REF]. LDD events are more frequent over shorter distances, but exceptional chance events over large distances do occur [START_REF] Nathan | Long-distance dispersal of plants[END_REF]. The time required for trees and shrubs to produce seed and the number and size of propagules should also impose limits on the rate of spread. Consequently, there should be intrinsic species-specific maximum rates of spread determining how fast plants can shift their ranges in response to climate change.

Limits on post-glacial spread considered a century ago (see discussions in [START_REF] Firbas | Spät-und nacheiszeitliche Waldgeschichte Mitteleuropas nördlich der Alpen[END_REF] led to the interpretation that the intrinsic rate of spread and distance from Last Glacial Maximum (LGM) distributions could explain the sequential appearance of taxa in Europe [START_REF] Bertsch | Geschichte des deutschen Waldes[END_REF]. The delayed arrival of taxa to a region with presumed suitable climate was later described as 'migrational lag' (e.g. [START_REF] Huntley | Migration: species' response to climatic variations caused by changes in the Earth's orbit[END_REF]. However, the alternative dynamic equilibrium hypothesis explains the appearance of taxa and their population expansion as direct reactions to climate [START_REF] Von Post | Ur de sydsvenska skogarnas regionala historia under postarktisk tid[END_REF][START_REF] Rudolph | Grundzüge der nacheiszeitlichen Waldgeschichte Mitteleuropas[END_REF][START_REF] Godwin | History of the British flora, 2nd edn[END_REF] with lags of < 1.5 ka [START_REF] Prentice | Vegetation and climate change in eastern North-America since the Last Glacial Maximum[END_REF]. Pollen-based climate reconstructions (e.g. [START_REF] Bartlein | Pollen-based continental climate reconstructions at 6 and 21 ka: a global synthesis[END_REF] imply the validity of the dynamic equilibrium hypothesis. Nevertheless, the migrational lag hypothesis is often cited as the major factor explaining, for example the late arrival of Fagus and its apparent continued spread to fill its climatic range [START_REF] Lang | Quart€are Vegetationsgeschichte Europas[END_REF]. [START_REF] Svenning | Limited filling of the potential range in European tree species[END_REF] used species distribution models to show that most European trees have not yet filled their current potential ranges, suggesting that the postglacial northward spread of trees is still active and partly determines current tree distributions and diversity gradients [START_REF] Svenning | Postglacial dispersal limitation of widespread forest plant species in nemoral Europe[END_REF]. This emphasises migrational lag as an important factor in past vegetation change and revives the debate about whether intrinsic, species-specific dispersal properties can explain the sequential appearance and dominance of major trees in Europe.

Pollen analysis is powerful for documenting regional abundance changes in anemophilous trees once they have reached densities higher than possibly one tree per hectare [START_REF] Bennett | The rate of spread and population increase of forest trees during the postglacial[END_REF]. [START_REF] Huntley | An atlas of past and present pollen maps for Europe 0-13000 years ago[END_REF] mapped pollen data across Europe using high abundance thresholds and estimated maximum rates of spread for common European trees between 300 and 2000 m year -1 . These high rates of spread were difficult to reconcile with simulations using observed seed dispersal distances that result in much lower rates (e.g. [START_REF] Meier | Climate, competition and connectivity affect future migration and ranges of European trees[END_REF][START_REF] Feurdean | Tree migration-rates: narrowing the gap between inferred post-glacial rates and projected rates[END_REF][START_REF] Snell | Using dynamic vegetation models to simulate plant range shifts[END_REF].

Lower rates of spread are obtained by assuming the spread originated from populations surviving the LGM further north than previously estimated [START_REF] Stewart | Cryptic northern refugia and the origins of the modern biota[END_REF]. This view is supported by finds indicating the occurrence of temperate trees in central-eastern Europe just before the LGM [START_REF] Willis | Trees or no trees? The environments of central and eastern Europe during the Last Glaciation[END_REF], whereas [START_REF] Tzedakis | Cryptic or mystic? Glacial tree refugia in northern Europe[END_REF] discussed that these may not have survived the LGM, advocating the absence of temperate trees in Europe north of 45° N. The time between the end of the coldest phase of the last glaciation and the onset of the Holocene may have allowed small outposts of temperate trees to establish far to the north of their LGM distributions, as was first suggested by [START_REF] Rudolph | Grundzüge der nacheiszeitlichen Waldgeschichte Mitteleuropas[END_REF]. [START_REF] Feurdean | Tree migration-rates: narrowing the gap between inferred post-glacial rates and projected rates[END_REF] considered more northern LGM distributions, assumed that northward spread started with Lateglacial climate warming and ended with the arrival of trees at their current or maximum Holocene northern limit. These assumptions led to much lower overall rates of post-glacial spread between 60 and 260 m year 1 and suggested that the earlier rates of [START_REF] Huntley | An atlas of past and present pollen maps for Europe 0-13000 years ago[END_REF] could be overestimates.

New insights into the post-glacial spread of plants have come from mapping genetic markers in extant populations [START_REF] Hu | Paleoecology meets genetics: deciphering past vegetational dynamics[END_REF]. Independently dated pollen diagrams have increasingly become available with a large number stored in the European Pollen Database (EPD). Based on these sources of information we revisit the palaeoecological evidence from the European continent, 33 years after the summary by [START_REF] Huntley | An atlas of past and present pollen maps for Europe 0-13000 years ago[END_REF]. The uneven distribution of pollen diagrams hampers spatial analysis and we therefore construct interpolated maps of pollen deposition to infer past distribution and abundance of widespread European plant genera and the grass family. The aim of this study was to analyse changes in spatial patterns of plant distribution and abundance during the last 15 kyr and interpret them in the light of the new insights emerging from genetic research and modelling studies. We compare the dynamics and rate of the postglacial spread and abundance changes in the dominant European tree genera. Our analyses provide a basis for comparative studies with vegetation model experiments.

Materials and methods

Study area

Interpolated maps were based on pollen data from the EPD. Sites beyond the European continent but included in the EPD were ignored. European islands without publicly available data were also excluded. Because the distribution of sites becomes increasingly sparse to the east, we limited the analysis to west of a line from the Black Sea to the White Sea. Pollen abundances were interpolated to a 0.5° grid.

Data

We used publicly available EPD sites with associated chronologies based on [START_REF] Giesecke | Towards mapping the late Quaternary vegetation change of Europe[END_REF], resulting in about 780 locations within or close to the study area contributing to the interpolations. All ages are given as calibrated years before present, where 'present' equals the year 1950 Anno Domini (ad). Sample selection is identical to the set used for mapping abundance changes at individual sites [START_REF] Brewer | Late-glacial and Holocene European pollen data[END_REF] with age uncertainties of less than 500 years and extrapolations of < 2000 years [START_REF] Giesecke | Towards mapping the late Quaternary vegetation change of Europe[END_REF]. Pollen counts were converted to percentages based on all terrestrial pollen taxa, excluding Cyperaceae, wetland and aquatic plants, fern and moss spores. Samples were collated into consecutive 500-year bins centred on full 500year intervals between 0 and 15 ka. The values were averaged over all samples where more than one sample per site fell within one bin. The available information differs between time slices, with fewer sites available prior to the Holocene [START_REF] Giesecke | Towards mapping the late Quaternary vegetation change of Europe[END_REF].

Interpolation and classification

Long-distance transport, erosion and redeposition of pollen grains bias the interpretation of small pollen proportions regarding the regional occurrence of the parent tree. To reduce this bias we designed threshold values (T int , Table 1) and interpolated percentages above the threshold using the tricube distance-weighting function [START_REF] Cleveland | Locally weighted regression -An approach to regressionanalysis by local fitting[END_REF]Huntley et al., 1989) with a horizontal search limit of 300 km radius and a vertical threshold of 500 m. Horizontal and elevation differences between grid cells and pollen diagrams are used as covariates, making the procedure well suited for continental interpolations of pollen data with an uneven spatial coverage (e.g. [START_REF] Williams | Late-quaternary vegetation dynamics in north America: scaling from taxa to biomes[END_REF]. The inclusion of elevation limits the propagation of abundance information from a mountainous area into the neighbouring lowlands. Nevertheless, in the case of the Fagus distribution in the north of the Iberian Peninsula it was necessary to manually restrict the extrapolation towards the south, which was done by introducing pseudoabsences from non-existent sites with 0% Fagus pollen.

Acknowledging the many uncertainties in the resulting interpolation, we restrict the mapping and subsequent analysis to four abundance classes capturing the main differences in the abundance of the parent plants (Table 1). The lowest class is designed to approximate the regional presence of the taxon, whereas the highest abundance class was chosen to represent the area within which a species is regionally dominant or characteristic. We determined the threshold values in comparisons between interpolations of modern samples (1700-2010 ad) and species distributional maps and abundance information. The two remaining abundance classes were set to intermediate values with class two representing a more conservative threshold for presence and class three indicating increased abundance. The threshold for presence on the interpolated surface (T1) was necessary to restrict distributions in data-sparse areas. No presence threshold was set for Poaceae because taxa of this family are presumed to occur everywhere in Europe and only changes in abundance through time and space are informative. Pinus and Betula are prolific pollen producers and their northern limit occurs in a landscape of low total pollen production. It is therefore impossible to find pollen percentage thresholds that would adequately describe their northern limits and thresholds were chosen to describe their southern boundaries [START_REF] Lisitsyna | Exploring pollen percentage threshold values as an indication for the regional presence of major European trees[END_REF]. As a consequence, rates of spread cannot be obtained for these genera based on this analysis. The results of the interpolations are provided as supplementary information (https://doi.pangaea.de/10.1594/PANGAEA.865121).

Table 1 Pollen percentage thresholds. The threshold T int is applied before the interpolation procedure. Thresholds T1 to T4 are the lower limit of the four abundance classes applied to the results of the interpolation. Change in area and rate of spread

We adjusted for the latitudinal reduction in area represented by grid cells. Area calculations of lower abundance classes include higher abundances. The general position of inferred distributions and abundance classes was compared with the distance to the Mediterranean and Atlantic coasts by computing the distance for all grid cells and summing all values from grid cells with the respective abundance class.

We restricted the analysis of the dynamics of spread to the nine common arboreal taxa for which the northern distributional limits may be approximated by the pollen percentage thresholds adopted here. We limited the study area to north of 47° N latitude, which is outside the location of possible LGM distributions for the analysed tree taxa with the exception of Picea [START_REF] Tzedakis | Cryptic or mystic? Glacial tree refugia in northern Europe[END_REF]. For this northern half of Europe, we fitted the logistic function to the increase in the area of presence with time and estimated the inflection point, the point at which the initial exponential increase in area starts to slow. We estimated rates of overall spread (V) and area-doubling times for all abundance classes, using a simple model of reaction diffusion V = 2(kD) 1/2 , where k is the intrinsic rate of population growth and D the diffusion coefficient [START_REF] Birks | Holocene isochrone maps and patterns of tree-spreading in the British Isles[END_REF]. The slope of the square root of area increase over time is an estimate of (kD) 1/2 yielding V for the time from the appearance of a taxon with the respective abundance class, until the time when maximum values were reached. The area-doubling time was estimated as ln(2)/r based on a logistic model, where the growth rate r was determined as the slope of the regression between the decadal logarithm of (K N)/N versus time t, with N as the area at time t and K as the carrying capacity set to the maximum area.

We used a more detailed configuration as a starting point for assessing the dynamics of postglacial spread (Fig. 1). The limits were moved north in eastern Europe to avoid large initial dispersal jumps in data poor areas. Prior to calculations, we manually removed the appearance of isolated small occurrences that subsequently disappeared, whereas considering disjointed dispersal jumps growing into larger areas and/or higher abundance classes. This resulted in connected areas of distribution as the basis for analysis and this requirement is the reason why higher abundance classes, which often have disjoint areas, were not considered. Rates of spread were obtained as the shortest geographical distance between the centres of grid cells marking the distributional limits in two consecutive time slices and dividing the value by 0.5 ka. This yields rates of spread in all directions, however, as the analysis was restricted to the area north of the Alps, changes in the southern limits were not considered. We also computed the distance of the northward shift of the 95 percentile of the distributions, with respect to distance to the Mediterranean Sea, using the initial set of taxa and based on the entire area represented by the maps.

Difference maps and dynamics of change

For each grid-cell, we calculated the differences in interpolated pollen proportions between two consecutive time slices and classified the results into four classes of increase and decrease, with one class of no change. Thresholds were obtained per taxon as the standard deviations (SD) of all values over all time slices. The value of zero was assigned to grid cells with <1 SD difference, whereas maximum and minimum values of 4 and 4 were assigned to differences larger than 4 SD.

The resulting maps are provided as supplementary information (https://doi.pangaea.de/10.1594/PAN GAEA.865119).

We visualized these changes by multiplying the numeric value (4 to 4) of each class with the area represented by grid cells assigned to this class. The resulting values were summed over all grid cells per time slice to obtain an index for the dynamics (area 9 amount of change). This was carried out separately for northern and southern Europe with the division at 47° N latitude.

All interpolations and analyses were carried out in R (R Core Team, 2016) using the packages ncdf [START_REF] Pierce | ncdf: Interface to Unidata netCDF data files[END_REF] and raster [START_REF] Hijmans | raster: geographic data analysis and modeling[END_REF].

Results and interpretations

The Lateglacial vegetation in Europe was dominated by herbaceous vegetation types with abundant Poaceae and Artemisia as well as scrublands with Juniperus and Betula nana and boreal woodlands with Pinus and Betula trees. During the Lateglacial the latitudinal tree line formed in central Europe [START_REF] Theuerkauf | Younger Dryas cold stage vegetation patterns of central Europe -climate, soil and relief controls[END_REF][START_REF] Mortensen | Living on the good soil: relationships between soils, vegetation and human settlement during the late Allerod period in Denmark[END_REF], and reached a more northerly position to the east of the Baltic Ice Lake [START_REF] Amon | Tree taxa immigration to the eastern Baltic region, southeastern sector of Scandinavian glaciation during the Late-glacial period (14,500-11,700 cal. BP)[END_REF]. However, this tree line consisting of Pinus and Betula is not adequately represented in the maps (see methods) resulting in biases in their Lateglacial distribution. This problem does not occur for Picea, which was most likely part of the latitudinal tree line east of the Baltic Ice Lake during the Younger Dryas [START_REF] Heikkila | Rapid Late-glacial tree population dynamics and ecosystem changes in the eastern Baltic region[END_REF] and elsewhere during the early Holocene [START_REF] Giesecke | The Holocene spread of Picea abies (L.) Karst. in Fennoscandia and adjacent areas[END_REF].

The effect of the Bølling-Allerød warming and Younger Dryas cooling can be seen in the area of occurrence and/or abundance of some taxa (e.g. Artemisia, Juniperus, Ulmus), although the 500year time slices can only weakly resolve the impact of these climate fluctuations (Fig. 2, see Appendix S1.1 in Supporting Information). The 12 ka time slice represents the situation just before the onset of Holocene warming at 11.7 ka [START_REF] Rasmussen | A stratigraphic framework for abrupt climatic changes during the Last Glacial period based on three synchronized Greenland ice-core records: refining and extending the INTIMATE event stratigraphy[END_REF], however, due to age uncertainty and the 500-year wide sampling window, some initial expansion may already be included. The onset of the Holocene warming set the clear starting point for the expansion in area and abundance of many temperate woody taxa at the European scale. Ulmus and Corylus followed by Quercus and Tilia filled most of their current distributional area within 2 kyr from the onset of the Holocene (Fig. 2). Conversely, Fagus and Carpinus had hardly any response to the onset of the Holocene. They showed a gradual increase in area that only starts after 9 ka and which continues almost to the present. Picea and Ericaceae had similar trends of steady increase through the Holocene. Alnus and Abies showed intermediate patterns with late starting area expansions that stopped in the mid-Holocene.

Figure 1 Study area of Europe restricted towards the east in a line from the Black to the White Sea. The red line marks the starting point for the evaluation of the rate of spread ignoring the regions to the south as here species may have survived the last ice age so that patterns are more difficult to interpret and site density is lower, leading to additional biases. The broken line marks 47° N latitude, which was used as a limit to contrast the dynamics of change between northern and southern Europe.

The areas of occurrence increased through time (Fig. 2) and followed logistic models of population growth. This becomes particularly clear when limiting the analysis to Europe north of 47° N latitude (Fig. 3). The inflection point that was estimated for these logistic increases provides a description for the timing of spread and range filling. Ulmus and Corylus slowed their exponential increase < 1 ka after the onset of the Holocene at around 11.2 and 11.1 ka respectively. Tilia and Quercus followed at 10.3 and 10.1 ka, whereas Alnus started to spread late but rapidly reached the inflection point at 9.5 ka. Picea survived the LGM north of 47° N latitude and its increase in apparent area of occurrence was gradual without a strong sigmoidal shape. The age of inflection point at 6.7 ka characterizes its mid-Holocene expansion. Abies started spreading north of the Alps around 9 ka and slowed its increase in area at around 5.9 ka. Fagus and Carpinus started to slow their spread at 4.9 and 4.8 ka respectively.

Distributions did not shift evenly, but some edges spread faster than others resulting in a range of values (Fig. 4). The box plots and the northward movement of the 95-percentile of distributions for the different abundance classes (see Appendix S1.7) show that the fastest rates of spread were reached during the early Holocene. For Ulmus the uppermost quantile (Fig. 4 beyond plot margin) ranges between 3000 and 1600 m year 1 for the time between 11.5 and 11 ka and the 95percentile shifted over this time by about 1000 km for the first and second abundance classes, which converts to a velocity of 2000 m year 1 . The northward range shift of Ulmus was concentrated in one 500-year interval, whereas the rapid spread of Corylus, Tilia and Quercus had a longer duration, with average values around 400 m year 1 . Nevertheless, the uppermost quantile of the Corylus boxplot for 12-11.5 ka ranges between 1300 and 2800 m year 1 (Fig. 4 beyond plot margin). The late-spreading taxa Fagus and Carpinus started with an initial rapid spread with apparent rates around 400 m year 1 , whereas the later spread occurred generally at slower speed, and only Carpinus shows a sudden increase in higher abundance classes around 2 ka.

The overall rates of spread presented in Fig. 5(a) assume the rate of spread to be constant through time, which was not the case. The highest rate of spread for the entire distribution is estimated for Alnus with 1000 m year 1 , whereas the estimates for most other taxa fall between 600 m year 1 for Corylus and 250 m year 1 for Abies and are thus comparable to average rates in Fig. 4. Only the gradual increase in the distribution of Picea resulted in a slow rate of 150 m year -1 . The fastest rate of spread recorded is for the highest abundance class of Corylus, caused by the rapid and often synchronous population increase in Corylus across large areas of Europe [START_REF] Giesecke | The pace of Holocene vegetation change -testing for synchronous developments[END_REF].

The logistic growth model used in the calculation of the area-doubling time (Fig. 5b) compensates for the saturation phase that reduces the slope in the overall estimate of spread in Fig. 5(a). This results in differences in the rank order, which are most pronounced for Fagus and Carpinus. The three higher abundance classes in Carpinus and the highest abundance class in Fagus have doubling times similar to early spreading taxa like Tilia and Ulmus. The good agreement between the estimates of overall spread (Fig. 5a) based on different abundance classes is remarkable. Early Holocene rapidly spreading taxa Alnus, Corylus, Ulmus and Tilia showed values above 400 m year 1 for all abundance classes. Quercus takes an intermediate position. Its post-glacial spread north started early, whereas the expansion at the northern limits in Sweden lasted until about 3 ka (e.g. Giesecke, 2005b). Picea clearly is the slowest in its overall dynamics, which is partly due to the fact that this analysis focuses on its Holocene dynamics, whereas its area of occurrence was already large during the Lateglacial. The tree was likely widespread in Europe during the coldest stages of the last glacial [START_REF] Ravazzi | Late Quaternary history of spruce in southern Europe[END_REF] and its Lateglacial history remains a focus of study and debate [START_REF] Birks | Comment on "Glacial survival of boreal trees in northern Scandinavia[END_REF]. The position of Abies differs between the panels in Fig. 5, with the overall rate of spread for all abundance classes being similar to Picea, whereas the doubling time for the first abundance class is close to that for Quercus. The obtained doubling times describe the Holocene dynamics of distributions and their use to estimate the size of the LGM distributions by extrapolation may not be appropriate here [START_REF] Magri | Patterns of post-glacial spread and the extent of glacial refugia of European beech (Fagus sylvatica)[END_REF]. However, they indicate that the taxa must have been widespread in the southern half of Europe during the LGM.

The Holocene vegetation history of Europe also provides examples of declines in distribution and abundance. The overall area of occurrence contracted along the northern limits of Corylus, Ulmus, Tilia and Quercus during the last 4 kyr. However, the decline in the area of occurrence is also due to a reduction in abundance below what can be recognized using the applied percentage thresholds. This is the main reason for the strong decline in Tilia (see Appendix S1.1). The area curves for higher abundance classes document these shifts in the abundance of different species across Europe. Curves for the highest abundance class show distinct peaks, some of which replace each other and thus characterize the different phases of European vegetation history (Fig. 6; see also Appendix S1.2). The 16 taxa considered here reach their largest area of the highest abundance class in the following order: Poaceae (15 ka), Juniperus-type (15 ka), Artemisia (12 ka), Betula (10.5 ka), Corylus (9.5 ka), Pinus (8.5 ka), Ulmus/Zelkova (7.5 ka), Fraxinus (7.5 ka), Tilia (6.5 ka), Quercus (6 ka), Abies (5.5 ka), Alnus (4 ka), Carpinus (3.5 ka), Fagus (3 ka), Picea (1.5 ka), Ericaceae (0.5 ka). These peaks mark in most cases the culmination of the expansion phase and are succeeded by a period of decline. The abundance of Ulmus and Tilia characterized many European forests between 9 and 6 ka. The pattern also clearly depicts the well described decline of Ulmus, which is followed by Tilia, whereas the area of abundance rises for Fagus and Carpinus. The deforestation of Europe over the last 3.5 kyr is also clearly depicted by the increase in the area for the highest abundance class of Poaceae, with an associated decline in the area for abundant cover of Fagus, Carpinus, Tilia and Quercus.

Figure 4 Boxplots for rates of spread as obtained by the shortest distance between two grid cells marking the area of inferred occurrence in consecutive time slices. Note that the width of a box indicates the number of observations, with thin boxes indicating that distributional shifts occurred along a small section of the mapped limit. Negative shifts were ignored. The change in area and abundance for each taxon between two time slices is presented in the difference maps (https://d oi.pangaea.de/10.1594/PANGAEA.865119) and these changes, summarized in one index (difference class multiplied by area) are presented in Fig. 7. This comparison shows large symmetry in population expansion and decline between north and south. Some species show more change in the north (Picea) and others in the south (Abies), which is partly due to the centre of their distribution (Fig. 8). The lack of change for Quercus in northern Europe, except around 10 ka, is due to its increase and decline at low rates resulting in small differences over 500 year intervals.

Assuming temperate tree taxa survived the LGM in southern Europe, one would expect populations to start expanding there and subsequently spread northwards where population expansion would be delayed. This pattern is indeed visible for some taxa, particularly for Fraxinus and Abies where southern European dynamics clearly precede northern European changes. However, populations of Tilia and Ulmus expand almost in parallel in the south and north, with a short lead in the south. For Corylus and Alnus the change in the north appears to lead. Fagus populations have their highest rate of increase at the same time in the north and south around 5 ka. The dynamics of declining populations is mirrored for many taxa between north and south. Most symmetry in decline is visible for Corylus, whereas Ulmus declined earlier in the south compared to the north.

To follow the change in the gravitational centre of the different taxa, we calculated the average distance of grid cells within the different abundance classes to the Mediterranean Sea and Atlantic Ocean (see Appendix S1.3,S1.4 and Fig 8). In both cases the average distances for the different abundance classes are near each other for most taxa at most times, indicating that most distributions were symmetrical with higher abundance classes in the centre of the distribution.

Abundance distributions are skewed to the north for the boreal taxa Pinus and Betula, with higher abundance classes found successively further north, whereas the opposite trend is observed for Quercus with a southern affinity (see Appendix S1.3). The symmetry of distributions often deteriorated when abundances declined. For example, the decline in the abundance of Corylus after its initial peak around 9.5 ka occurred mainly in the south and east without affecting the distributional area and consequently the gravitational centre of the abundant areas shifted north and west. The curves for Picea are striking. The distance to the Mediterranean describes the tree as a southern species during the early Holocene, when populations mainly expanded in the eastern Alps and Carpathians. In the Carpathians the dominance of Picea at mid-elevation was replaced by Fagus in the second half of the Holocene [START_REF] Feurdean | Post-glacial patterns in vegetation dynamics in Romania: homogenization or differentiation[END_REF], whereas the tree expanded in Fennoscandia during that time [START_REF] Giesecke | The Holocene spread of Picea abies (L.) Karst. in Fennoscandia and adjacent areas[END_REF]. As a consequence, the mean distance to the Mediterranean shifted for higher abundance classes from southern dominance, as for Quercus, to northern dominance as for Pinus and Betula. At the same time the average distance to the Atlantic Ocean decreased steadily though the Holocene for all abundance classes. This west-ward shift was observed and reported for the two Picea distributions in central and northern Europe separately and it is interesting to see how both distributions shifted simultaneously [START_REF] Giesecke | The Holocene spread of Picea abies (L.) Karst. in Fennoscandia and adjacent areas[END_REF][START_REF] Van Der Knaap | Migration and population expansion of Abies, Fagus, Picea, and Quercus since 15000 years in and across the Alps, based on pollen-percentage threshold values[END_REF]. The two other latecomers Fagus and Carpinus followed the same trend, shifting the centres of their distributions from east to west with the general spread of the distribution in the same direction. While the gravitational centres of distributions shifted north together with the northern edges of the populations, the southern distributional limits did not move north with the early Holocene climate warming (see Appendix S1.3-S1.6). Betula is the only taxon with a slight northward shift of its southern distributional limit and higher abundance classes showed a strong northward shift of the southern limits (see Appendix S1.6).

Discussion

Spread and population expansion

We found that the rates of taxon spread slowed before northern distributional limits were reached, which is demonstrated by the logistic fits in Fig. 3. This may not just be explained by range filling, but rather be an effect of slowed population growth rates near the northern limits, where growing season warmth often limits seed production. This interpretation is supported by findings of lower population growth at higher altitudes and latitudes [START_REF] Mcleod | Postglacial range expansion and population growth of Picea mariana, Picea glauca and Pinus banksiana in the western interior of Canada[END_REF]Giesecke, 2005a) indicating a climatic control [START_REF] Prentice | Paleoecology and plant-population dynamics[END_REF]. It is also consistent with a close link between population growth and rate of spread.

The observation that different abundance classes yield similar overall rates of spread (Fig. 5a) agrees with a simple diffusion model predicting that the speed of the wave can be obtained regardless of the population threshold applied [START_REF] Lubina | The spread of a reinvading species -range expansion in the California sea otter[END_REF]. This would suggest that the wave is stable in space and even though the wave 'front' cannot be captured by palaeoecological methods [START_REF] Bennett | The rate of spread and population increase of forest trees during the postglacial[END_REF], the rate of spread may be obtained from higher abundance classes, where spread is followed by population expansion. A travelling wave of expanding populations has been described for Fagus in North America [START_REF] Woods | Paleoecology of range limits -beech in the upper peninsula of Michigan[END_REF] and Picea in Scandinavia (Giesecke, 2005a) and appears to be the rule rather than the exception, even where spread and population expansion may have been disconnected, as in the case of Picea [START_REF] Giesecke | Changing plant distributions and abundances[END_REF].

This disconnect is a feature of late expanding taxa, and both Picea and Fagus provide some of the best examples for Europe. The mapping of genetic markers in extant Picea populations in Fennoscandia, Russia and the Baltic states documents strong links between the populations in southern Sweden and in the Baltic states as well as between northern Sweden and northern Russia [START_REF] Tollefsrud | Combined analysis of nuclear and mitochondrial markers provide new insight into the genetic structure of North European Picea abies[END_REF]. Macro-fossil evidence of Picea indicates that the tree occurred on the Scandinavian Peninsula soon after deglaciation [START_REF] Kullman | Early postglacial appearance of tree species in northern Scandinavia: review and perspective[END_REF]. On the other hand, pollen data documenting the time of population expansion show a clear wave of expanding populations travelling north-west from southern Finland, crossing the northern extent of the Baltic Sea and then turning south-west into southern Sweden [START_REF] Giesecke | The Holocene spread of Picea abies (L.) Karst. in Fennoscandia and adjacent areas[END_REF]. Thus it seems that the expansion of populations, documented by rising pollen proportions, occurred independently of and in a different pattern to the initial spread of the species. The existence of a Picea haplotype occurring only on the Scandinavian Peninsula [START_REF] Parducci | Glacial survival of boreal trees in northern Scandinavia[END_REF] supports this interpretation, documenting that the initial spread, which left a genetic imprint in populations, and the later population expansion are decoupled. Genetic markers in extant populations confirm earlier claims that Fagus survived the LGM on the Iberian Peninsula [START_REF] Magri | A new scenario for the Quaternary history of European beech populations: palaeobotanical evidence and genetic consequences[END_REF] and possibly also in south-western France (de [START_REF] De Lafontaine | Cryptic no more: soil macrofossils uncover Pleistocene forest microrefugia within a periglacial desert[END_REF], yet these areas contributed little to the post-glacial colonization of Europe.

Similarly, the populations on the Apennine and Balkan peninsulas did not contribute to the postglacial spread of the tree. The dominance of central and north-western Europe by a single chloroplast haplotype is in sharp contrast with the high haplotype diversity of the southern edge of the distribution in northern Greece [START_REF] Magri | A new scenario for the Quaternary history of European beech populations: palaeobotanical evidence and genetic consequences[END_REF]. The simulations by [START_REF] Saltre | Climate or migration: what limited European beech post-glacial colonization?[END_REF] illustrate how the expansion from a more northerly population may block the northward spread from populations in the south. However, we found that Fagus populations on the Balkan and Iberian Peninsulas remained small until the mid-Holocene and grew in parallel with the populations in central Europe (Fig. 7; [START_REF] Giesecke | Towards an understanding of the Holocene distribution of Fagus sylvatica L[END_REF]. Thus the LGM distribution of Fagus is not related to the time of population expansion in central and north-western Europe, as has often been postulated (e.g. [START_REF] Lang | Quart€are Vegetationsgeschichte Europas[END_REF]. Using the proposed LGM distributions as starting points to simulate the post-glacial spread of Fagus as well as Picea [START_REF] Saltre | Climate or migration: what limited European beech post-glacial colonization?[END_REF][START_REF] Lehsten | Modelling the Holocene migrational dynamics of Fagus sylvatica L. and Picea abies (L.) H. Karst[END_REF] does therefore not help develop understanding of the post-glacial history of either species. In both cases similarities between simulated and reconstructed patterns may be accidental, and although we have not yet understood why these two species spread late, our findings do not indicate that the time of population expansion can be explained by migrational lag. Vegetation models that can describe the spread of plants mechanistically are useful as they can test the importance of particular components like interannual variability [START_REF] Nabel | Interannual climate variability and population density thresholds can have a substantial impact on simulated tree species' migration[END_REF], but the models need further improvement before they can provide convincing estimates of the consequence of global warming for vegetation [START_REF] Snell | Using dynamic vegetation models to simulate plant range shifts[END_REF]. The patterns and rates presented here can be used to test these models, but care is needed in the selection of suitable taxa. Modelling the post-glacial colonization of Europe would certainly be useful for Quercus, but is of less value for Picea and Fagus without a better understanding of their early Holocene controls.

Genetic markers in Quercus document that many populations from around the Mediterranean Basin have spread north with post-glacial warming [START_REF] Petit | Chloroplast DNA variation in European white oaks -phylogeography and patterns of diversity based on data from over 2600 populations[END_REF], yet these immigration routes find no reflection in a differentiated time of initial population expansion [START_REF] Brewer | The spread of deciduous Quercus throughout Europe since the last glacial period[END_REF][START_REF] Giesecke | Did thermophilous trees spread into central Europe during the Lateglacial?[END_REF]. High-resolution mapping of haplotypes in extant white oak populations in western France highlights the importance of frequent LDD events, which could explain the observed patchwork of distinct haplotypes of chloroplast DNA markers [START_REF] Petit | Chloroplast DNA footprints of postglacial recolonization by oaks[END_REF]). Simulations using this example also link the frequency of LDD to population density [START_REF] Bialozyt | Contrasting effects of long distance seed dispersal on genetic diversity during range expansion[END_REF].

Comparisons between Europe and North America

Estimated rates of post-glacial spread of woody taxa in eastern North America are nearly always lower than the rates discussed here, regardless of the method. [START_REF] Davis | Pleistocene biogeography of temperate deciduous forests[END_REF] published the first rates of spread based on independently dated pollen diagrams and used the increase in pollen accumulation rates as indication for the local arrival of a taxon. Based on a limited set of sites, she obtained rates between 350-500m year 1 for Pinus banksiana/resinosa and 100 m year 1 for Castanea [START_REF] Davis | Pleistocene biogeography of temperate deciduous forests[END_REF]. [START_REF] Delcourt | Long-term forest dynamics of the Temperate Zone: a case study of Late-Quaternary Forests in eastern North America[END_REF] used a modern data set to interpret the regional arrival of a taxon to produce isochrone maps and evaluate the spread of woody plants along five different north-south tracks. They obtained average rates of spread between 287 m year 1 for Salix and 70 m year 1 for Nyssa, including individual maximal dispersal jumps of 1044 m year 1 for Tsuga and 753 m year 1 for Tilia. Highest rates of spread were observed between 17 and 13.8 ka and thus much earlier compared to Europe, although also at lower latitudes. Ritchie MacDonald (1986) presented a study with the highest rates of spread in North America, for the spread of Picea glauca from south-central Alberta to the Mackenzie Delta with rates of 2000 m year 1 , whereas they reported rates of 2-300 m year 1 in other regions. The apparently high rates in the western interior may, however, result from an eastward spread of populations that survived the ice age in Alaska [START_REF] Anderson | Ice-age endurance: DNA evidence of a white spruce refugium in Alaska[END_REF]. [START_REF] Ordonez | Climatic and biotic velocities for woody taxa distributions over the last 16000 years in eastern North America[END_REF] used well-dated pollen diagrams to model past species distributions using simulated past climate. Core distributions moved northwards at maximal mean velocities of 188 m year 1 for Abies and 153 m year 1 for Alnus during 12-10 ka.

In America the distributional shifts of the southern boundaries occurred at similar rates [START_REF] Ordonez | Climatic and biotic velocities for woody taxa distributions over the last 16000 years in eastern North America[END_REF]. In Europe, the position of most rear-edge distributions had not changed significantly during the last 15 kyr, except perhaps for Betula which declined in abundance at its southern distributional limits in Italy during the early Holocene [START_REF] Magri | Holocene dynamics of tree taxa populations in Italy[END_REF]. All other boreal and temperate trees and shrubs remain present in southern European mountains. Pinus sylvestris for example dominates the northern boreal forest in Scandinavia and grows at or near the tree line on mountains in Spain, Greece and northern Italy. [START_REF] Bennett | Quaternary refugia of North European trees[END_REF] speculated that the southern edge of the distributions of the European woody flora was maintained in these southern European mountains through glacial-interglacial cycles. It may also be argued for North America that the southern distributional limits for temperate species have not significantly moved north [START_REF] Bennett | Quaternary refugia of North European trees[END_REF], however, here temperate trees may have survived close to the ice margin that extended much further south [START_REF] Mclachlan | Molecular indicators of tree migration capacity under rapid climate change[END_REF].

Consequently, studies estimating rates of postglacial spread in eastern North America have considered a wide latitudinal range where the trees in question may have occurred during the LGM.

We have focused on rates of spread north of 45° N in Europe, which in eastern North America corresponds to north of the Canadian border. The northern limits of several temperate species in Europe reach 60° N, and these high northern limits are due to the Gulf Stream pumping heat to high latitudes and the westerly winds pushing it into the continent. As a consequence, January isotherms in Europe have a strong N-S component while they run predominantly east-west in eastern North America. The prevailing westerlies would also propagate rapid shifts in sea surface temperature in the north Atlantic connected to the mode of the North Atlantic Deep Water formation, such as its 'off' mode during the Younger Dryas [START_REF] Rahmstorf | Ocean circulation and climate during the past 120,000 years[END_REF]. Compared to eastern North America, the rate of climate change with the onset of the Holocene was therefore most likely much higher in Europe with highest rates near the Atlantic coast [START_REF] Heiri | Validation of climate model-inferred regional temperature change for late-glacial Europe[END_REF]. Higher rates of climatic changes in Europe might explain the comparably faster apparent rates of spread and higher rates of vegetation change. The slower meltdown of the larger Laurentide ice sheet may have set a slower pace for North American climate and vegetation change during the Lateglacial and early Holocene [START_REF] Shuman | Parallel climate and vegetation responses to the early Holocene collapse of the Laurentide Ice Sheet[END_REF].

Human influence has rarely been discussed in the spreading of tree taxa in North America, although humans were present in the postglacial landscape and probably influenced fire regimes and cultivated crops by 3.8 ka [START_REF] Smith | Initial formation of an indigenous crop complex in eastern North America at 3800 B[END_REF]. Indians may have affected regional distributional limits of fire-sensitive hardwoods at the prairie-forest transition zone for a long time [START_REF] Grimm | Fire and other factors controlling the big woods vegetation of Minnesota in the mid-nineteenth century[END_REF]. In contrast in Europe, there has been considerable discussion about potential human impact on the distribution and abundance change in several trees, particularly Abies [START_REF] Tinner | The past ecology of Abies alba provides new perspectives on future responses of silver fir forests to global warming[END_REF]), Carpinus (Ralska-Jasiewiczowa et al., 2003), Fagus [START_REF] Küster | The role of farming in the post-glacial expansion of beech and hornbeam in the oak woodlands of central Europe[END_REF] and Tilia [START_REF] Turner | The Tilia decline: an anthropogenic interpretation[END_REF]. The effect of humans on European vegetation was particularly pronounced during the second half of the Holocene [START_REF] Molinari | Exploring potential drivers of European biomass burning over the Holocene: a data-model analysis[END_REF] and the conversion of forest to agricultural land is clearly visible in the increase in area dominated by grasses since 4 ka (Fig. 6). The late Holocene spread of Fagus in northern Europe has almost certainly been catalysed by anthropogenic activities [START_REF] Bradshaw | Past anthropogenic influence on European forests and some possible genetic consequences[END_REF] which cannot be separated from a potential response to climate. It may never be known for certain if the tree would have been able to reach its current northern limits in Sweden, western Norway, Scotland and Ireland without human assistance. Genetic markers have provided supporting evidence for likely human translocation of trees such as Castanea sativa and Pinus pinea, whose Mediterranean distributions lack normal genetic structure [START_REF] Fineschi | Chloroplast DNA polymorphism reveals little geographical structure in Castanea sativa Mill. (Fagaceae) throughout southern European countries[END_REF][START_REF] Vendramin | Genetically depauperate but widespread: the case of an emblematic Mediterranean pine[END_REF] and whose ranges have been significantly increased through millennia of cultivation [START_REF] Conedera | The cultivation of Castanea sativa (Mill.) in Europe, from its origin to its diffusion on a continental scale[END_REF].

Two types of change possibly mediated by human activities emerge from our analyses. First, disturbance-mediated population increases, either short-lived as seen with Carpinus in northeastern Europe [START_REF] Ralska-Jasiewiczowa | Some problems of forest transformation at the transition to the oligocratic/Homo sapiens phase of the Holocene interglacial in northern lowlands of central Europe[END_REF] or more sustained as shown by Fagus in northern Europe [START_REF] Giesecke | Towards an understanding of the Holocene distribution of Fagus sylvatica L[END_REF]. Second, declines in abundance that are not necessarily linked to range contractions, best demonstrated for Tilia population declines in connection with forest clearance for agriculture [START_REF] Turner | The Tilia decline: an anthropogenic interpretation[END_REF], but also described for Abies linked to domestic grazing and use of fire [START_REF] Tinner | The past ecology of Abies alba provides new perspectives on future responses of silver fir forests to global warming[END_REF]. Human activities have therefore had a far greater influence on taxon spread and distributional limits in the Holocene of Europe than in North America, yet the pollen record for forest trees has been dominated by natural population dynamics until recent centuries [START_REF] Bradshaw | Past anthropogenic influence on European forests and some possible genetic consequences[END_REF]. Today very little 'natural' forest remains and the current population size of European trees is largely an outcome of recent land use [START_REF] Bradshaw | Forest continuity and conservation value in Western Europe[END_REF].

Explanations for Holocene shifts in tree abundance

The vegetation cover of Europe has changed profoundly over the course of the Holocene with changing dominance of different woody taxa. Their time of dominance corresponds to their time of spread, but it was probably not the species-specific rate of spread that determined the time of dominance in most cases. The late-arriving species Fagus and Carpinus do not show a constant slow spread northward that would explain their late arrival at their northern limits. The expansion of Fagus populations in the Balkan and Iberian peninsulas, where the species occurred during the LGM, was as late as in central Europe where the tree was previously absent. Picea presumably reached mountain tops in Scandinavia during the Lateglacial and early Holocene, but populations did not expand until at least 5 ka later. Thus the late expansion of particularly Fagus, Carpinus and Picea cannot be explained by 'migrational lag', whereas a number of taxa that have not filled their potential distribution and are restricted to southern Europe [START_REF] Svenning | Postglacial dispersal limitation of widespread forest plant species in nemoral Europe[END_REF] show a 'lack of migration'.

The glacial/interglacial cycle first proposed by [START_REF] Iversen | The bearing of glacial and interglacial epochs on the formation and extinction of plant taxa[END_REF] may have some power explaining the late expansion of some trees [START_REF] Birks | The rise and fall of forests[END_REF][START_REF] Kunes | Soil phosphorus as a control of productivity and openness in temperate interglacial forest ecosystems[END_REF]. Glacials reset soil formation particularly in high latitude areas and the young soils gradually leached phosphorus [START_REF] Boyle | A leaky model of long-term soil phosphorus dynamics[END_REF]. Thus while phosphorus may be readily available in young soils during the initial phase of an inter-glacial, with time this nutrient becomes increasingly limiting to plant growth. [START_REF] Kunes | Soil phosphorus as a control of productivity and openness in temperate interglacial forest ecosystems[END_REF] suggest that plants with phosphorus-mining ectomycorrhiza would therefore be favoured during the later stages of interglacials and lists Fagus, Carpinus and Picea among those. The constant increase in area with various abundance classes of Ericaceae pollen over the last 8 kyr (see Fig. S1.1) also attests to gradual changes in soils, namely paludification increasing the amount of peatland as well as podsolization reducing soil fertility. Corylus also possesses ectomycorrhiza, but decreased in abundance during the second half of the Holocene. Its decline, particularly in more continental areas, was unlikely to be due simply to the species being shaded out by more shade-tolerant taxa. There is a simultaneous west-ward shift of Picea distributions towards the Atlantic coast. Corylus benefits from mild winters while they seem to limit the natural distribution of Picea in Europe. Thus a decline of Corylus in more continental areas and a spread of Picea towards more oceanic regions may be interpreted as caused by decreasing winter temperatures in continental areas due to a reduction in the strength of the westerly winds [START_REF] Giesecke | Exploring Holocene continentality changes in Fennoscandia using present and past tree distributions[END_REF].

Thus while migrational lag has little power to explain the shifts in Holocene plant abundance, the combination of climate and soil changes may explain some patterns. Seed dispersal, population growth rates and other plant traits were surely important in shaping the early Holocene dynamics, but cannot explain many patterns.

Figure 2

 2 Figure 2 Post-glacial changes in area of occurrence for the most common European tree taxa based on interpolation and classification of pollen percentages.

Figure 3

 3 Figure3Fitted logistic functions to the increase in area of occurrence in Europe north of 47° N latitude as estimated from interpolation and interpretation of pollen percentages. The inflection point, where the exponential increase starts slowing down, is marked by the vertical line and its age given below the taxon name. Taxa are grouped according to the timing of the area expansion and differences in overall area.

Figure 5

 5 Figure 5 Assessments of the dynamics of area increase: (a) rate of overall spread estimated from the slope of the increase in square root of the area versus time, (b) area-doubling time obtained from the logistic growth model. Symbols: black square = class 1, green triangle = class 2, red triangle point down = class 3, blue diamond = class 4. Taxa are ordered according to their overall rates of spread and doubling time. Horizontal lines separate rapidly expanding taxa.

Figure 6 Figure 7

 67 Figure 6 Post-glacial changes in area of highest abundance or dominance for the most common European tree taxa, Artemisia and Poaceae based on interpolation and classification of pollen percentages. Vertical dashed lines mark times of change

Figure 8

 8 Figure 8 The average distance of areas assigned to different abundance classes to the modern coast of the Mediterranean and the Atlantic respectively. Lower abundance classes include the area of higher abundance classes. The dotted line represents the average for the analysed area. Colour codes: black = class 1, green = class 2, red = class3, blue = class 4.

Acknowledgements

Pollen data were extracted from the European Pollen Database (EPD; http://www.europeanpollendatabase.net/) and the work of the data contributors and the EPD community is gratefully acknowledged. Giesecke acknowledges funding from the DFG grant GI 732/5-1. Brewer was funded by NSF grant EAR-1003848. We thank John Birks, Jack Williams and two anonymous reviewers for comments on the manuscript.

Supporting Information

Additional Supporting Information may be found in the online version of this article:

Data accessibility

All pollen data used for the maps are available from the European Pollen Database (EPD; http://www.europeanpolle ndatabase.net/). The maps generated for this study are available as PDFs as well as numerical values from https://doi.pan gaea.de/10.1594/PANGAEA.865121.

Thomas Giesecke is a palaeoecologist interested in ecosystem dynamics on scales from decades to glacial-interglacial cycles, trying to tease apart internal vegetation drivers such as plant spread from external forcing like climate change. Together with M.L., S.B., W.F. and R.B. he is working with the EPD aiming to increase its usefulness as a resource for the immediate and wider scientific community. The group also uses this resource to address continental scale questions of past vegetation change.

Author contributions: T.G., S.B., W.F. and R.B. had the original idea and M.L. compiled the data; S.B. constructed the maps; T.G. and S.B. performed analyses; T.G. wrote the paper with substantial contributions from M.L., S.B., W.F. and R.B.


Yolande Maury

Information culture and web 2.0: New practices, new knowledge

From the observation of school librarians' usual practices in three secondary schools in Paris (2006Paris ( -2009)), with an ethnographic approach, the aim of this study is to better define information culture, in particular which kinds of knowledge are necessary to learn, live and evolve in the information society context. Thus, the study points up that the web 2.0, in order to be an educational tool, needs an accompanied digital conduct. New training needs appear, characterized by the emergence of new knowledge (digital identity, traceability, informative survey...) and the reconfiguration of some others (information ethics, indexing, mediation, authority, intellectual property, relevance…).

Which knowledge for information culture in the context of web 2.0? Since the 2000s, the changing media landscape, including the development of internet and web 2.0, generated a mini-revolution in information practices. Young people today seem impregnated by diversified media (video games, new modes of mobile communications, and more recently blogs, wikis, forums...). They learned to navigate in a more or less intuitive way in a flow of information, continuously adapting their approaches, and developing empirical skills throughout their practices. They come to school with a "culture" that school librarians and teachers cannot ignore. However, beyond their inventiveness, these practices are indicative of gaps that express a lack of information culture, highlighting new training needs, to make the Web 2.0 an educational tool. The aim of this paper is to investigate what type of knowledge can meet these training needs: next to stable knowledge, sure values, essential to the point of not leaving them to chance, more moving knowledge, to reconfigure in the context of the web 2.0, and emerging knowledge, which are worth being studied because they provide some answers to current, new issues. These points will be addressed by examining information culture through the idea of culture: after presenting the contexts of our study and the methodology used to collect data, we will give off the key features of knowledge, considered as elements of an information culture to build, between permanence and change, before considering a few perspectives. Our approach is comprehensive, focusing on the phenomena of "emergence of meaning", within the meaning of Pierre Paillé and Alex Mucchielli (2003). Information culture is studied in its cultural and social dimensions, in the perspective of identifying the dynamics that emerge.

Context (s)

If this research, by its purpose, information culture, is in the field of information and communication science, it is discussed in the context of this work in an interdisciplinary way (in the orientation of the ERTé Information culture and information curricula, A. Béguin, dir.), borrowing from various fields (education sciences, anthropology ...), to reflect the different dimensions of information culture: cultural, social, cognitive particularly... Information culture is questioned by the idea of culture, as defined by Guy Rocher, inspired by the work of anthropologists [START_REF] Rocher | Introduction à la sociologie générale[END_REF], as "everything a person needs to know to live in a particular society", that is "a vast symbolic set, including knowledge, ideas, thoughts, rules, common to a plurality of social actors, considered in a given space at a given time". Presented in this way, culture is action, it is a behavior related to appropriation (acquisition, learning), the values and symbols that compose it, correspond to "ways of life" ("manières de vivre") in a given society; human groups produce it to meet the needs they share. At the same time, culture is not static (to freeze practices lead to an essentialist view of culture), it is a dynamic set which has a relative plasticity. Describing a culture, it is therefore drawing the outline, showing how it is being developed daily, showing the elements that define it, those that situate it in relation to other cultures, and those that give it thickness. The mere juxtaposition of elements is not sufficient to define a particular culture, it is necessary to determine which organizes it into a more or less coherent set, which binds its components together, and to understand how it is worked by internal and external social relationships. Culture is an historical production, it is experiencing changes, transformations: it is in the interactions that social actors create the social rules, conventions, relations that give it meaning, as a collective process. More specifically, on information culture, we strive to define the contents, as reflected in the observations, which is to try to understand how and in what, offering theoretical tools, information culture can be an instrument to think, live, evolve, and conduct oneself in a world of information [START_REF] Baltz | Une culture pour la société de l'information ? Position théorique, définition, enjeux[END_REF]; or in other words, to become a "member" of this world (and society) of information (the issue is both anthropological and socio-cognitive). Information culture is considered in a dialogic way, by linking the experiences of pupils, their usual practices (information culture as they live it) and information culture more "cultured" that school librarians and teachers are trying to develop during training sessions. We are particularly interested in the evolution of these contents, in their genealogy, in the digital context.

Research methods

As said in introduction, our approach is comprehensive, and resolutely qualitative: from an objectifying observation of info-documentary situations, we seek to discover the meaning of the dynamics involved. Observations are our primary data collection. Through the description of situations, particularly negotiations teacher-pupils, our aim is to bring out the structural elements of information culture, to translate the visible into readable, and to build a knowledge (anthropology) from "seeing" and "writing" on "seeing" (ethnography) [START_REF] Laplantine | La description ethnographique[END_REF].

To this end, observations are discussed in terms of a constructive pluralism, according to an a posteriori methodology in which the researcher is in an inquiry process. At the same time, we opted for a reading "in positif" of situations, more careful to identify and understand their own logic (narrative observation) than their gaps or deficiencies (attributive observation). This second dimension may however be present, insofar as we identify critical moments, explainable by a conceptual vacuum (missing knowledge and concepts), which leads us, to a certain extent, beyond the "look / see", to make inferences. Finally, to achieve the necessary distance and ensure the validity of data collected, but also to better perceive the complexity of our object and allow the discovery of unexpected dimensions, we wanted to cross the findings with the discourse of the actors, giving their point of view on practices: interviews, informal conversations, collection of documents... At this stage of the research, interviews with school librarians, scheduled for the end of observation, have not yet been made, and we mainly rely on informal conversations and other materials gathered during observations. Four school librarians (two beginners and two more experienced ones), in three secondary schools, were observed during training sessions, over time, in various situations, sometimes working alone, sometimes in collaboration with teachers.

Data analysis and findings

Pupils' experiences and training needs

A first observation is that information practices change very quickly in school context with the introduction of new resources (tools of Web 2.0 in particular). Pupils seized these new tools, they arrive at school with an experience of information-documentation. Their personal practices, their "manières de faire" are the reflection of a generation imbued with the screens, they show great creativity, reflecting their ability to make "small arrangements" with technology and information. These practices, mixture of improvisation and adaptation, are also influenced by the family context: references to the practices of parents and elders are very numerous. Empirically acquired, these skills are sometimes sources of gaps, or at least of misunderstandings that highlight the need of an accompanied conduct to allow the construction of meaning ("sensemaking", Dervin). As written by Michel de Certeau, it is not enough to be author of social practices so that there is real culture, it is important that these practices may have meaning for the person who is author of these practices [START_REF] Certeau | L'invention du quotidien 1[END_REF]. With the web 2.0, culture is dynamic, not stable, collaborative, it introduces uncertainty in school, in a world characterized by stability, certainty, peculiar to academic knowledge. The fast flow of information, and the apparent ease of access to a profusion of data, with a single click, give pupils illusion of power control and reinforces their tendency to impatience, in contradiction with the long time for learning. The increase in resources, the fragmentation and granularity of information create new social attitudes, which can bring problems: disorientation, informational confusion, saturation effect, problems to situate and be oneself. To be operational, pupils have to navigate through several worlds and to be at ease with different formats of information: formal and non formal information systems, traditional information structures and their controlled vocabulary and new emerging resource, multimodal, with more chaotic forms. In practice, they are quickly frustrated when they are limited in their projects, that the management of moments of doubt or the respect for codes of good conduct restrains their inventiveness. The challenge for school librarians and teachers is then to find how to maintain their enthusiasm and creativity, encouraging initiative and risk, while promoting a distanced and wise use of new media, which is the condition of a real entry into "culture"; or in others words, ensure that pupils find the flavor to knowledge, considered not as "closed knowledge", utilitarian, but as a "living knowledge", providing answers to the questions asked by the pupils themselves [START_REF] Frisch | Disciplinarisation et didactisation de l'information-documentation[END_REF].

A cultural approach, between permanence and change

Negotiations between teachers and pupils show information culture being built, from the pupils' questions, while refracting their own experiences (and culture of belonging). For school librarians, a way to give meaning and thickness to the knowledge is to link, on a dialogical mode, heritage approach and anthropological approach (Levine, Develay, 2003). In this construction of meaning, knowledge is used as a "tool-to-think", promoting the transition from "information practices" to the "culture of these practices" [START_REF] Tardif | Intégrer les nouvelles technologies : quel cadre pédagogique ? ESF[END_REF]Maury 2005).

Most of the time, school librarians introduce knowledge, integrating it in a "story of problems" (making it "problematical"), related to the founding questions of informationdocumentation. Thus, knowledge is not only considered in the present, but linking past and present, with its genealogy and stakes, on a larger scale, between permanence and change. "Warm knowledge", in permanent change, it is referring to controversial topics, object of debate in society. So, a lot of knowledge needs to be reconfigured in the context of web 2.0.

The concepts of document, information or media, for example, are difficult to define in a definitive way with pupils. In context of the web, the document is less stable, more plastic, it is a process as well as a product. As we have seen, it is not easy for young pupils to distinguish what is a document (or not) and to identify its attributes. Everything "is" document: a stream, an inscription, a digital exchange... Man himself, with his digital identity, becomes a document when he is "traced", documented on the web [START_REF] Ertzscheid | L'homme, un document comme les autres[END_REF].

As for information, the traditional distinction between data and information (machine data and social data) is in question: when information is manipulated, transformed to fit the patterns of the machine, it is treated as a data, demythologized. What's more, it is addressed not only in terms of content (an end in itself), but in terms of exchange, service, sharing of data. It is difficult for the pupils to determine where to draw the limit between information and non-information? From when, for example, is a trace becoming information? And as "everything happens" on the web (school librarian, secondary school), the issue of its construction is also raised (with wikis, blogs ...). Also for media, the traditional categorization (a type of support for an object) is less relevant. Internet, presented as the "ultimate media", is at the same time technology, support, media, and source of knowledge, and it conveys sound, image and writing [START_REF] Cotte | Tout peut-il être média ? Communication et langages[END_REF]. The social networks (blogs, wikis...) are also media; and by extension, the city or the stores are used as media by the signs, with the new communication strategies. However, may anything be a media? So, school librarians have many opportunities to intervene, leading pupils to less naive representations and global perspectives: showing that media are diverse, and a construction of reality, not reality itself, that they are related to values, interests and power relations.

Other change, the crisis of traditional mediators [and media], whereas the monopoly of knowledge by experts is challenged. Intellectual property issues (authorship, copyright, authority, Information Law) join the questions of deontology and ethics: lively discussions between students and school librarian in a high secondary school (School of Photography) shows that, through the questions of law, students are questioned in their own posture towards information, but also in their relationship to the world. What is authority? Is it sufficient to be a writer (in a blog or a wiki...) or a photographer to be an author? Is the traditional publishing or peer review process necessary to declare information valid? It is more difficult to teach authorship and evaluation in an open authorship environment than in a controlled world. Two modes of authorship and validation coexist: a traditional mode, with filtering and upstream validation by a recognized institution, and an "authoritative" one, with self-publishing on the web, and downstream validation, a posteriori, without reference to a printed institution. So, many issues emerge when learners evaluate information they find: popularity, recommendation are not validity, relevance; points of view and opinions are not facts or information. Charters are often discussed and generated, they serve to build a culture of blog or wiki, as a guideline for a sure and responsible behavior and a fair use.

Indexing is also changing in the context of the web, and pupils needs understanding of both worlds, folksonomies and taxonomies. From tags to keywords, then to descriptors, they can access progressively to the concept of indexing, with its three dimensions, social, free, controlled. By developing tag clouds, they discover information relationships in a playful way (broader terms, narrower terms, related terms). If tag clouds are different from the structured controlled vocabulary, they can help to understand the underlying structure of controlled vocabulary and its hierarchy. So, by showing both Boolean logic and descriptors, and popular folksonomies, school librarians lead pupils to have more search power.

Information culture and emergence of new knowledge

In a world in permanent change, it is not enough to know how to select, evaluate, have a fair use of information and contribute positively to the community, pupils need also to control their information world, in order to work independently. New training needs appear, with learning of new skills and emerging knowledge, such as informative survey or digital identity.

In their practice of the web, pupils usually have a practical and functional relationship to information, they seek to collect immediately useful information ("carottage"), they are less attentive to their information process, in the long-term; most of them have not a prospective approach. As we have seen, they limit informative survey to specific topics ("supervised personal work", school search) or hobby (photography, sport…), they don't think to channel information automatically in their direction. So, school librarians, particularly in high secondary school, take pains to teach them how to control their own information world, to seek the information they want to get, not just to receive (pull versus push): by selecting relevant RSS feeds, mailing lists, newsletters, in order to personalize their own stream of information.

These informative surveys are authentic experiences, they permit to conduct original researches, and give them more control. They learn how to extract a sophisticated report from a flow of information in response to their initial questions. These experiences foster a flexible and hybrid thinking, leading to global perspectives (beyond school context), turned towards information dynamics (beyond information literacy).

Another important question in the digital age is the virtualization of identity. Virtual environments are areas of construction of identity: technology changes the nature of identity which is not the same as the identity in everyday life. Identity is socially constructed from interactions within the social world: partly constructed by individuals, partly imposed upon individuals by other people and social systems or structures. Luke Tredinnick notes three entangled elements: virtualization, meaning that the sedimentary record or trace of identity becomes dissociated from corporeal being; sedimentation, meaning that the digital traces of actions or transactions are recorded and stored; and fragmentation, meaning that the unity of identity declines [START_REF] Tredinnick | Digital information culture: the individual and society in the digital âge[END_REF]. Young people don't seem anxious about their digital identity, unlike the adults; they feel protected in school context, school practices give them the illusion of a closed world, whereas their actions are visible to the world. The use of blogs and wikis, with a thinking on the tracks left on the web, offers an opportunity for school librarians to make the pupils conscious of the issue of identity: risks and rights relating to digital identity, protection from harmful information, management of online identity (pseudonyms, profiles). Generally, the purpose is not to restrict choices but to make pupils conscious of potential and limits of an open environment, and of both freedom and responsibilities relating to the community.

Concluding perspectives

There are many changes in terms of information culture in the context of web 2.0. First, web 2.0, by bringing real life (outside life) in school, is the perfect tool to develop information skills; it generates a mini-revolution in information practices, bridging the gap between social practices and academic knowledge. Then, it presents opportunity, via blogs and wikis, for learning not only information retrieval but also expression and communication. As shown in this communication, this mini-revolution in information practices brings a reconfiguration of knowledge, and the emergence of some others (digital identity is an important one): knowledge considered in its continuity, through its genealogy, between change and permanence. So Information culture is much more than the ability to seek, use, create and communicate information. It is not only a matter of reading but of creating meaning out of cultural objects. It implies social and cultural dimensions which participate to define the significance of information, and allow a critical and reasoned use of it. And it includes thinking on the process of knowledge creation and communication: in digital age, the value of knowledge depends on the understanding of this process, and of the underlying assumptions, values, biases, and belief systems.