Julien Duforest
email: julien.duforest@junia.com

Benoît Larras

Deepu John

Olev Märtens

Antoine Frappé

Slope-Based Event-Driven Feature Extraction For Cardiac Arrhythmia Classification

Keywords: Artificial neural network (ANN), electrocardiogram (ECG), cardiac arrhythmia classification

To detect cardiovascular diseases (CVD), electrocardiogram (ECG) of a patient must be recorded and analyzed for a long period. For an effective diagnosis, the ECG recording system must automatically adapt to new patients. This paper presents a low-complexity artificial neural network that exclusively uses the consecutive slopes of ECG signal as inputs. These features are extracted using a level-crossing ADC and a simple TDC-based event-driven processing chain. The proposed clockless system can detect arrhythmias in ECG with 98.4% accuracy and reduce the ANN hardware complexity by more than half compared to recent literature. It is perfectly adapted to integrated wearable monitoring systems and shows good adaptability to new patients.

I. INTRODUCTION

diseases (CVD) are one of the major causes of deaths in the world [1]. Electrocardiogram (ECG) is used for early CVD diagnosis; and wearable at-home monitoring systems can avoid patient examinations at the hospital. To enable wearable health monitoring systems with a long battery life, their power consumption must be greatly reduced. The near-sensor computing paradigm suggests that processing data in the sensor reduces the total amount of data to send to a data center for further analysis. It enables the early detection of abnormality using low-power feature extraction and classification subsystems.

Besides, ECG signals do not hold uniformly distributed information in time. Event-driven sampling, which relies on generating a sample, when a certain amplitude level is crossed, is best suited for signals such as ECG signals that have large changes in short periods and small changes over longer periods, compared to Nyquist sampling. It is demonstrated in [START_REF] Van Assche | Power Efficiency Comparison of Event-Driven and Fixed-Rate Signal Conversion and Compression for Biomedical Applications[END_REF] that level-crossing ADCs (LC-ADCs) offer two main advantages, compared to traditional uniform sampling, for low-to-medium resolution applications (< 8 bits). Fewer samples are generated, which leads to less energy-hungry processing, and the majority of these samples are located at timings of interest, i.e. around the R-peaks, which are more relevant to process.

Solutions exist in the literature to classify ECG signals using non-uniform sampling. In [START_REF] Rovere | A 2.2-μ W Cognitive Always-On Wake-Up Circuit for Event-Driven Duty-Cycling of IoT Sensor Nodes[END_REF], an event-driven wake-up circuit detects abnormal signals that do not fit a learned pattern, but the classification is not robust and results in an accuracy below 75%. Moreover, no information is given on the data used for training and validation. In [START_REF] Zhao | A 13.34μW Event-driven Patientspecific ANN Cardiac Arrhythmia Classifier for Wearable ECG Sensors[END_REF], low-complexity 96-bit features are extracted and the classification is performed with a patient-specific Artificial Neural Network (ANN). Although the reported accuracy is higher than 99%, the architecture requires a global timer to calculate the precise time between two R-peaks of the ECG signal and digitally encode it as the main feature. This increases the power consumption of the system and counteracts the clockless nature of the event-driven system.

In this work, we want to combine the higher accuracy and adaptability of ANNs for classification with the simplicity and adaptability in the event-driven feature extraction stage. To that end, the slopes between two consecutive non-uniform samples are used as input features for the ANN, since the slopes hold information of both time and amplitude between samples. We demonstrate in this paper that the event-driven extraction of slope is extremely simple and that the simulated classification results in the case of premature ventricular contraction (PVC) promise accuracies up to 98.3%.

This article is organized as follows. In Section II, the global architecture of the slope-based event-driven system is presented and the slope extraction, quantization, and heartbeat classification blocks are detailed. The results of arrhythmia classification is presented in Section III. Conclusions are finally drawn on the efficiency of the method.

II. SLOPE-BASED EVENT-DRIVEN FEATURE EXTRACTION

The input ECG signal used for testing and evaluation is extracted from the MIT-BIH arrhythmia database. The records from this database is sampled at 360 Hz. The database is oversampled to 10 kHz to increase the time resolution between two non-uniform consecutive samples. Figure 1 depicts an overview of the different functions implemented to perform the classification. First, it is composed of a 7-bit LC-ADC to perform the non-uniform sampling of the ECG signal. Then, a 7-state Time-to-Digital Converter (TDC) extracts the quantized values of the slopes. The framing and R-peak detection blocks select a sequence of N consecutive slopes centered around the R-peak of the heartbeat, before feeding them in a feed-forward fully connected ANN. It outputs either a "normal beat" or "abnormal beat" class.

A. Non-uniform sampling

The input data is non-uniformly sampled with an LC-ADC acting as an asynchronous delta encoder [START_REF] Marisa | Pseudo Asynchronous Level Crossing adc for ecg Signal Acquisition[END_REF]. The LC-ADC outputs two signals REQ and DIR. REQ triggers a pulse each time a level is crossed, DIR indicates the direction of the crossed level. Using a 7-bit LC-ADC, the resulting number of samples is lower than in the uniform sampling case, with an average compression ratio of 3.2 for the database records with PVCs. The LC-ADC uses a floating window, avoiding the generation of samples created by the noise around an amplitude level and guaranteeing a minimal slope between two consecutive samples.

B. Time-to-digital conversion

The non-uniform samples are converted to slopes that are quantized using a TDC. For each record, slopes between consecutive event-driven samples are calculated. These slopes are approximated from the oversampled data as we cannot have the exact duration between two samples of the MIT-BIH database. As an example, the distribution of all the calculated slopes for record 119 is plotted as a histogram in Fig. 2. The quantization thresholds are set according to the vertical lines shown in this Figure . They divide interesting and meaningful portions of the histogram (for positive slopes) and are symmetrical around 0 to be able to use the same hardware for positive and negative slopes. For record 119, thresholds are set to -100 mV/s, -30 mV/s, -4 mV/S, 4 mV/s, 30 mV/S, and 100 mV/s for a total of 7 quantized slope values. Each distribution of slopes is patient-specific, but the histogram can always be divided as illustrated with record 119, providing 7 quantized slopes (Q.S.). In the architecture, the slope thresholds will be defined with respective delays used in the 7-state TDC. The 7 states can then be digitally coded with 3 bits, 1 for the sign of the slope and 2 for the magnitude (MAGN.), as shown in Fig. 1.

The architecture of the TDC is presented in Fig. 3. The block converts the REQ and DIR signals into a 3-bit nonuniformly sampled data stream at the output. The time interval between two samples, following the REQ signal, is quantized using 3 delays, corresponding to the thresholds defined previously, giving four possible quantized time ranges: t < T1, T1 < t < T2, T2 < t < T3, and T3 < t.

The integrated digital delays are usually realized with thyristor-like architectures, controlled either by discharge currents [START_REF] González | A Wide Tuning Range Delay Element for Event-Driven Processing of Low-Frequency Signals in 28-nm FD-SOI CMOS[END_REF] or by digitally-controlled capacitor arrays [7]. The delays should be configurable to adapt the slope quantization thresholds for each patient. The time interval between two rising edges of the REQ signal is thermometrically coded on 3 bits (D1, D2, and D3 in Fig. 3) and converted to a 2-bit binary word to constitute the magnitude (MAGN.) bits of Q.S., '11' for the highest magnitude and '00' for the lowest. The SIGN bit is directly derived from the DIR signal of the LC-ADC, 1 for a positive slope and 0 for a negative one. A second flip-flop is inserted in the SIGN path for synchronization.

It is to note that for slowly-varying signals, when T3 < t, the direction of the change signal is not relevant, thus not considered and the resulting states ('000' and '100') are aggregated into a unique state ('000').

C. Data framing

The data fed to the ANN is a vector of N consecutive 3bit slopes. The framing block is thus implemented by a 3-bit shift register of length N. The center of the frame is positioned around the R-peak position, as indicated in Fig. 1, and is provided by an R-peak detection block. As a reference, R-peak detection can be performed by simple QRS detection blocks as presented in [START_REF] Ravanshad | A Level-Crossing Based QRS-Detection Algorithm for Wearable ECG Sensors[END_REF] or [START_REF] Zhang | A 300-mV 220-nW Event-Driven ADC With Real-Time QRS Detection for Wearable ECG Sensors[END_REF] with more than 97% detection rate, or by detecting the maximum of the signal reconstituted by the extracted slopes. It is to note that this latter method would be ultra-low-power but would give a reduced accuracy. In the scope of this paper, ideally placed R-peaks are considered for further classification.

D. Heartbeat classification

The ANN is a feed-forward fully connected neural network with an input layer with N_input = 8 neurons, a hidden layer with N_hidden = 4 neurons, and two output classes (normal or arrhythmic), as shown in Fig. 1. These dimensions have been found optimal considering the tradeoff between the accuracy of the classification and the complexity of the network. The activation functions for each layer are symmetric linear functions with saturation. The output layer uses a soft maximum. Since the normal and arrhythmic classes from the ECG records are imbalanced, a conditional grouping scheme like [START_REF] Zhao | A 13.34μW Event-driven Patientspecific ANN Cardiac Arrhythmia Classifier for Wearable ECG Sensors[END_REF] is used. Once balanced, 70% of the data is used for training, 15% for validating, and 15% for testing. The ANN is trained for each specific record independently.

III. SIMULATION RESULTS

A. Arrhythmia classification

The system is modeled using Matlab software and includes all the described blocks for feature extraction and classification. The records chosen for the tests are 105, 106, 109, 114, 119, 203, 205, 210, 215, 219, 221, 228, and 233 from MIT-BIH since their arrhythmias mostly consist of PVCs. The accuracy portrays how well the beats are globally classified, while the sensitivity and specificity depicts how well the normal beats and arrhythmic beats are classified, respectively. Finally, the false positive rate represents the percentage of arrhythmic beats classified as normal beats.

The classification results are given in Fig. 4 for two configurations. In the case indicated in blue, the slope thresholds are set at the values defined in Section II-B and shown in Fig. 2 for all the records. In the case indicated in orange, the slope thresholds are adapted to each record. For the different values of input frame size N, the accuracy, sensitivity, specificity, and false positive rate are shown. In all cases, using adapted slopes leads to better classification results, but it is interesting to highlight that, even with nonadapted thresholds, the obtained accuracies are good enough to provide a low-cost embedded solution. The maximum obtained accuracy for PVC classification is 98.4%. The measured accuracies increase with the size of the frame N. For frames larger than 32, the accuracy does not improve because of the possible overlap between consecutive heartbeats. This proves that the information contained in the quantized slopes is sufficient for the correct determination of PVC and there is no necessity to use other information from the neighboring beats. This is an advantage for the system as the slope thresholds represent values that can be memorized on-chip, so no information is needed on top of the samples created from the LC-ADC. The threshold adaptation can increase the classification results, provided that enough samples are recorded to plot a patient-specific histogram like the one in Fig. 2. The accuracy could be increased further with a more complex selection method for the threshold or by adapting them actively during the process, but it would increase the system complexity. The ANN used in this work implements a total of 296 MAC operations for the maximum value of the input frame size, i.e. 32 slopes.

B. Comparison with state-of-the-art event-driven PVC classifiers

As shown in Table I, the PVC classification in [START_REF] Rovere | A 2.2-μ W Cognitive Always-On Wake-Up Circuit for Event-Driven Duty-Cycling of IoT Sensor Nodes[END_REF] is performed using a small 16-state finite state machine using the DIR signal from the LC-ADC along with counters and comparators. Globally, the implementation is simpler in computation compared to an ANN. However, the accuracy is 25 percentage points lower compared to other state-of-the-art classifiers and has not been tested on a cited database.

Although the work in [START_REF] Zhao | A 13.34μW Event-driven Patientspecific ANN Cardiac Arrhythmia Classifier for Wearable ECG Sensors[END_REF] displays the best performance in terms of classification accuracy, the feature extraction operation uses an RR-interval parameter which needs to refer to the neighboring heartbeats. It can be problematic when the neighboring heartbeats are not normal beats and the classification results may be biased. Besides, to extract this parameter, a global timer and an 11-bit TDC are implemented, which is in contradiction with the event-driven character of the system.

The work presented here can reach an accuracy similar to the system in [START_REF] Zhao | A 13.34μW Event-driven Patientspecific ANN Cardiac Arrhythmia Classifier for Wearable ECG Sensors[END_REF] (only 1.5% lower accuracy) for the classification of PVC arrhythmias. It is done without the use of information from neighboring beats, nor the use of a global timer. The ANN computation requirements are reduced by 50% in terms of MAC operations compared to [START_REF] Zhao | A 13.34μW Event-driven Patientspecific ANN Cardiac Arrhythmia Classifier for Wearable ECG Sensors[END_REF], which also implements 3072 additional accumulation operations.

IV. CONCLUSION

In this paper, a classification method for arrhythmia detection is proposed specifically for wearable health monitoring. The low-complexity artificial neural network takes as input a vector of consecutive slopes extracted from the ECG signal. An efficient event-driven feature extraction architecture is derived and shows promising classification results, with up to 98.4% accuracy when the quantization thresholds are adapted to the patient. This accuracy result is very close to the state of the art, but the system offers a lower complexity, involving a classifier with less than 300 MAC operations and not requiring any information from the RR interval or neighboring beats. This work provides a promising architecture for ultra-low-power event-driven integrated health monitoring systems.

Fig. 1 .Fig. 2 .

 12 Fig. 1. Proposed event-driven architecture for the feature extraction and classification of ECG signals

Fig. 4 .

 4 Fig. 4. Average Accuracy, Sensitivity, Specificity and False Positive Rate for all the records tested, in function of the size of the ANN input frame. In dashed blue, the slope thresholds are the same for all records. In doted orange, the slope thresholds are adapted for each record.

ACKNOWLEDGMENT

This work was supported by the EU Regional Development Fund (Estonian Centre of Excellence in ICT Research EXCITE TAR16013), the CHIST-ERA grant JEDAI, Mobilitas+ project Mobera20, Irish Research Council, and French Research Agency project ANR-19-CHR3-0005-01.


IMBE Aure Durbecq
email: aure.durbecq@gmail.com

Margaux D'ambly

Elise Buisson

Renaud Jaunatre

Alexandre Cluchier

Armin Bischoff

Seedling recruitment in mountain grassland restoration: effects of soil preparation and grazing

Keywords: Bromopsis erecta, brush harvesting, community ecology, ecological engineering, grassland restoration, grazing, harrowing, montane grasslands, Rhinanthus alectorolophus, seedbed preparation, seedling establishment, survival

Questions: Seedling recruitment is a key step

INTRODUCTION

Seedling recruitment is an important process driving the composition and diversity of plant communities [START_REF] Grubb | The Maintenance of Species-Richness in Plant Communities: The Importance of the Regeneration Niche[END_REF]Zeiter et al., 2006). Understanding the factors constraining seedling recruitment is thus crucial to predict and improve restoration success. Dispersal [START_REF] Münzbergová | Seed, dispersal, microsite, habitat and recruitment limitation: identification of terms and concepts in studies of limitations[END_REF][START_REF] Frances | Importance of Seed and Microsite Limitation: Native Wildflower Establishment in Non-native Pasture[END_REF] and microsite limitation [START_REF] Myers | Seed arrival, ecological filters, and plant species richness: a meta-analysis[END_REF][START_REF] Long | Seed and microsite limitations mediate stochastic recruitment in a low-diversity prairie restoration[END_REF] are known to be such constraining factors in re-colonisation processes. In grassland restoration, the small soil seed bank of typical late-successional species may result in insufficient seedling recruitment [START_REF] Grman | Dispersal and establishment filters influence the assembly of restored prairie plant communities[END_REF][START_REF] Buisson | Species transfer via topsoil translocation: lessons from two large Mediterranean restoration projects[END_REF]. Poor seed dispersal, as well as the lack of connectivity between source populations due to landscape fragmentation, often limit the seed rain [START_REF] Münzbergová | Seed, dispersal, microsite, habitat and recruitment limitation: identification of terms and concepts in studies of limitations[END_REF][START_REF] Bischoff | Succession of floodplain grasslands following reduction in land use intensity: the importance of environmental conditions, management and dispersal[END_REF][START_REF] Muller | Effect of topsoil removal and plant material transfer on vegetation development in created Mediterranean meso-xeric grasslands[END_REF] and successful restoration may require seed addition to overcome seed limitation [START_REF] Öster | Dispersal and establishment limitation reduces the potential for successful restoration of semi-natural grassland communities on former arable fields[END_REF][START_REF] Lengyel | Grassland restoration to conserve landscape-level biodiversity: a synthesis of early results from a large-scale project[END_REF]Valkó et al., 2016). Thus, seed addition methods such as sowing local seed mixtures, hay transfer or soil transfer, are increasingly used in grassland restoration [START_REF] Kiehl | Species introduction in restoration projects -Evaluation of different techniques for the establishment of seminatural grasslands in Central and Northwestern Europe[END_REF][START_REF] Scotton | Practical handbook for seed harvest and ecological restoration of species-rich grasslands[END_REF][START_REF] Kiss | Establishment Gaps in Species-Poor Grasslands: Artificial Biodiversity Hotspots to Support the Colonization of Target Species[END_REF].

Seed-addition experiments demonstrated that limited recruitment is the next major constraint influencing plant establishment [START_REF] Tilman | Community Invasibility, Recruitment Limitation, and Grassland Biodiversity[END_REF]Turnbull et al., 2000;[START_REF] Pywell | Restoration of species-rich grassland on arable land: assessing the limiting processes using a multi-site experiment[END_REF]. Unfavourable microsite conditions may hamper germination and reduce seedling survival often resulting in microsite limitation [START_REF] Bissels | Effects of different mowing regimes on seedling recruitment in alluvial grasslands[END_REF]. Soil disturbance, such as topsoil removal or soil tillage, increases recruitment opportunities [START_REF] Hölzel | Restoration of a species-rich flood meadow by topsoil removal and diaspore transfer with plant material[END_REF][START_REF] Myers | Seed arrival, ecological filters, and plant species richness: a meta-analysis[END_REF][START_REF] Long | Seed and microsite limitations mediate stochastic recruitment in a low-diversity prairie restoration[END_REF][START_REF] Klaus | Enriching Plant Diversity in Grasslands by Large-Scale Experimental Sward Disturbance and Seed Addition along Gradients of Land-Use Intensity[END_REF]. Soil preparation (also called "seedbed preparation"; [START_REF] Shaw | Seed use in the field: Delivering seeds for restoration success[END_REF] may promote the recruitment of plant species by (i) limiting the competition of an already established vegetation, and by (ii) improving abiotic conditions of seedling recruitment [START_REF] Kiehl | Species introduction in restoration projects -Evaluation of different techniques for the establishment of seminatural grasslands in Central and Northwestern Europe[END_REF]. A low pre-existing vegetation cover may have a facilitating effect on recruitment by protecting seedlings from high solar radiation and drought stress [START_REF] Callaway | Competition and Facilitation: A Synthetic Approach to Interactions in Plant Communities[END_REF]. However, if pre-existing vegetation is dense, the competitive effect on introduced species may prevail because already established species limit available space and resources [START_REF] Kupferschmid | Dispersal and microsite limitation in an abandoned calcareous grassland of the southern prealps[END_REF][START_REF] Poschlod | Diaspore and gap availability are limiting species richness in wet meadows[END_REF].

Preparing the soil may favour seedling recruitment of new species by opening the vegetation and destroying the root systems of competitive perennial species [START_REF] Bischoff | Hay and seed transfer to reestablish rare grassland species and communities: How important are date and soil preparation?[END_REF]. Soil preparation further creates micro-reliefs improving seed and water retention as well as seed adhesion [START_REF] Chambers | Seed movements and seedling fates in disturbed sagebrush steppe ecosystems: implications for restoration[END_REF]. However, soil disturbance increases nitrogen mineralisation increasing plant available nitrogen in the soil (eutrophication) and may negatively affect the soil fauna and soil structure potentially favouring erosion [START_REF] Di | Nitrate Leaching in Temperate Agroecosystems: Sources, Factors and Mitigating Strategies[END_REF][START_REF] Holland | The environmental consequences of adopting conservation tillage in Europe: reviewing the evidence[END_REF]).

Human land management by grazing and mowing is a key factor regulating the functioning and structure of semi-natural grasslands [START_REF] Hejcman | Origin and history of grasslands in Central Europe -a review[END_REF]. Several studies confirmed the essential role of grazing in maintaining plant species richness of grassland ecosystems [START_REF] Dupré | Differences in species richness and life-history traits between grazed and abandoned grasslands in southern Sweden[END_REF][START_REF] Pykälä | Effects of restoration with cattle grazing on plant species composition and richness of semi-natural grasslands[END_REF][START_REF] Saatkamp | Vegetation and soil seed bank in a 23-year grazing exclusion chronosequence in a Mediterranean dry grassland[END_REF]. However, in the initial stages of grassland restoration, grazing may hamper seedling recruitment since seedlings are particularly vulnerable to trampling and biomass removal by herbivory [START_REF] Bakker | Herbivores as mediators of their environment: the impact of large and small species on vegetation dynamics[END_REF][START_REF] Buisson | Limiting processes for perennial plant reintroduction to restore dry grasslands[END_REF]. Trampling may prevent species from germinating by crushing and damaging seeds or coleoptiles [START_REF] Rother | Demographic bottlenecks in tropical plant regeneration: A comparative analysis of causal influences[END_REF]. Moreover, trampling on slopes may lead to soil destabilization [START_REF] Tasser | Effects of land use in alpine grasslands on the probability of landslides[END_REF], increases soil erosion [START_REF] Farrell | Seeding alters plant community trajectory: Impacts of seeding, grazing and trampling on semi-arid re-vegetation[END_REF] and soil compaction [START_REF] Allington | Long-Term Livestock Exclusion in an Arid Grassland Alters Vegetation and Soil[END_REF]. However, in mountain ecosystems, in which the particular pedo-climatic conditions (soil erosion, shallow and stony soils) reduce seedling recruitment, micro-reliefs created by hoof prints may increase seed trapping [START_REF] Isselin-Nondedeu | Contributions of vegetation cover and cattle hoof prints towards seed runoff control on ski pistes[END_REF] and improve seed adhesion to the soil [START_REF] Chambers | Seed movements and seedling fates in disturbed sagebrush steppe ecosystems: implications for restoration[END_REF]. Accordingly, [START_REF] Eichberg | Sheep trampling on surface-lying seeds improves seedling recruitment in open sand ecosystems[END_REF] found an increased seedling recruitment in a trampling simulation experiment on sandy soils also suffering from low stability and high run-off. Biomass removal by herbivory damages seedlings and often reduces initial seedling survival [START_REF] Buisson | Limiting processes for perennial plant reintroduction to restore dry grasslands[END_REF]Vidaller et al., 2019b). However, [START_REF] Farrell | Seeding alters plant community trajectory: Impacts of seeding, grazing and trampling on semi-arid re-vegetation[END_REF] only found a combined negative effect of trampling and herbivory whereas herbivory alone did not affect seedling establishment. Furthermore, [START_REF] Kladivová | Interacting effects of grazing and habitat conditions on seedling recruitment and establishment[END_REF] even revealed a positive effect of grazing on both seedling recruitment and establishment but the grazing effect depended on habitat conditions at the specific microsite.

In our study, we aim to test the effect of soil preparation and grazing on seedling recruitment (combining germination and early survival) in a seed-addition experiment conducted in semi-natural mountain grasslands. Using brush material transferred from a species-rich donor grassland to restore degraded sites, we addressed the following questions:

(1) Does soil preparation by harrowing before sowing improve seedling recruitment and survival of the transferred species? (2) What is the effect of early grazing on seedling recruitment and survival of the transferred species?

MATERIALS AND METHODS

Study area

The study area is the upper Durance valley ("Haute-Durance") in the Southern French Alps. The construction of a high-voltage transmission line led to a local destruction of species-rich grassland communities within a 1 km wide and 100 km long strip from L'Argentière-la-Bessée to La-Batie-Neuve (44°78'78''N, 6°59''41'E; 44°57'93''N, 6°20''77'E). The experimental sites representing a subsample of these grasslands are located between 1100 and 1300 meters above the sea level (upper montane altitudinal belt). The climate is temperate to sub-Mediterranean depending on slope, exposition and elevation. At the closest meteorological station (Embrun), the annual precipitation is about 740 mm and the mean annual temperature 10.7°C. Considering the lower elevation of the station (880 m) the temperature is roughly 2°C lower resulting in an annual mean of 8.7°C at our experimental sites. The bedrock is predominantly calcareous with some quaternary glacial deposits. Non-degraded grasslands of the study area belong to Natura 2000 priority habitats: "Semi-natural dry grasslands of Festuco-Brometalia and scrubland facies on calcareous substrates" (N6210*, EU directive habitat 92/43/EEC; [START_REF] Calaciura | -Brometalia) (*important orchid sites)[END_REF] that are traditionally managed either by mowing, cattle and sheep grazing. On transitory access tracks and construction platforms, the vegetation was removed, and the soil compacted to facilitate the transport of heavy equipment. To recreate former soil conditions after electricity line construction the soil was de-compacted by deep tillage between May and July 2018.

Donor site and seed harvest

The donor site to collect brush material is located within the study area at Freissinières at an elevation of 1100 m (44°73'61''N; 6°56'72''E). The soil and climatic conditions of the donor site corresponded to the average of the five experimental sites. The seed material was harvested in July 2018 using a brush harvester mounted on a quad and equipped with an integrated vacuum system. On a surface of 4000 m², we obtained 2.6 kg of brush material. The brush material comprised seeds (60% of total mass) and vegetative parts (40%). Seed counts in ten subsamples of 0.5 g revealed an average density of 887 seeds/g of brush material. A single harvest was considered as sufficient since the short summer season in the study area has resulted in a concentration of the seed production period for most species.

Experimental set-up and design

The experiment was set up in October 2018 using a full factorial split-plot design replicated on five sites (blocks) to test the effect of previous soil preparation, grazing and its interaction on seedling recruitment (see [START_REF] Durbecq | Identifying reference communities in ecological restoration: the use of environmental conditions driving vegetation composition[END_REF] for the position of experimental sites). Each of the five sites comprised eight plots of 16 m² each (4 m × 4 m) resulting in a total of 40 plots.

Four plots representing a half-block were exposed to extensive grazing and the remaining four were fenced preventing grazing (whole-plot factor). The sowing and soil preparation treatments (split-plot factors) were randomly assigned within half-blocks. The distance between plots was 1 m except for the Embrun site where this distance was reduced to 50 cm due to limited space.

The full design also comprised unsown control plots randomly assigned to half-blocks.

The unsown plots were used to evaluate which plant species of the donor grassland already occurred at the restoration sites before the brush material was transferred (soil seed bank, seed rain). Average vegetation cover of these unsown controls was 28% compared with 90% at the donor site. Species occurring in the control plots of a given site were considered as "spontaneously emerging" for this site. Only plant species of the donor site not occurring in the control plots were considered as "transferred" (Appendix S1). The method may underestimate the density of transferred species since species occurring in unsown control plots may still have been transferred to the sown plots.

Sites already covered with spontaneous vegetation were mown prior to experimental set-up. Harrowing was selected as a soil preparation method using a rotary harrow adjusted to a working depth of 8 cm. In October 2018, just after harrowing, the brushed material was spread at a density of 104 g/plot corresponding to a seed density of 3.75 g/m² and approximatively 5765 seeds/m². The chosen seed density was based on recommendations of a previous research project in the French Alps [START_REF] Koch | Les semences locales dans la restauration écologique en montagne. Production et utilisation de mélanges pour la préservation[END_REF]. In agreement with the traditional local grazing regime, the unfenced experimental plots were grazed twice a year: in late October just after experimental set up, and in early June. Two sites were grazed by cattle, three sites by sheep, corresponding to the average proportion of both grazing types in the study region. The sites were neither fertilised nor irrigated.

Data collection

Vegetation relevés at the donor site and germination tests in the greenhouse

Prior to seed harvest, the cover of all occurring vascular plant species was estimated in five representative 3 m × 3 m plots as the vertical projection cover of above-ground vegetation in late June 2018. We used [START_REF] Tison | Flore de la France méditerranéenne continentale[END_REF] for plant species identification and as reference for plant species names. At the harvest in July 2018, the average phenological stage of all species was recorded in order to evaluate seed availability. Eighty percent of the species recorded in June were found to have seeds in July.

In order to obtain information on the germination potential of seeds contained in the brush material and to evaluate germination in the field compared with germination potential, we set up a germination test in November 2018. Subsamples of the brush material were transferred to five trays (0.03 m 2 ) filled with standard peat substrate and placed in an unheated greenhouse. Each tray received 0.54 g of brush material corresponding to 18 g/m² and a density of approximately 18 000 seeds per m 2 . The trays were regularly watered, and germination was recorded until April 2019. Greenhouse germination of the harvested brush material was recorded once a week for four months and seedlings were removed after identification.

Seedling survey and vegetation relevés at restoration sites

To quantify seedling recruitment of both transferred species and spontaneously emerging species, plant numbers were counted in three quadrats of 40 cm × 40 cm placed along the diagonal of each plot. Since we were not able to distinguish seedlings and plants re-sprouting from below-ground organs, the seedling counts may include young ramets of clonally growing plants. In order to identify spontaneously emerging species, we conducted at the same time vegetation relevés on the unsown control plots (9 m²).

Seedling survival of focal species

Seedlings of two typical species of the donor site, Bromopsis erecta (Huds.) Fourr. (former name: Bromus erectus) and Rhinanthus alectorolophus (Scop.) Pollich were monitored over 11 weeks in order to assess seedling survival. B. erecta is the characteristic species of semi-dry grasslands (Mesobromion type sensu [START_REF] Ellenberg | Vegetation Mitteleuropas mit den Alpen in ökologischer Sicht[END_REF] occupying a large range in Europe. It is further one of the typical species of the habitat type (N6210*, EU directive habitat 92/43/EEC; [START_REF] Calaciura | -Brometalia) (*important orchid sites)[END_REF]. B. erecta is a perennial grass growing on calcareous and often nutrient-poor soils. R. alectorolophus is a summer annual, hemi-parasitic species growing in semi-dry to mesophilic calcareous grasslands. As B. erecta, it is among the characteristic species of the N6210*-habitat type. R. alectorolophus has a wide range of hosts that includes grasses and legumes [START_REF] Sandner | Multiple choice: hemiparasite performance in multispecies mixtures[END_REF]. The species is used as a tool in restoration of species-rich grasslands as it reduces competition by grasses and may thus have a positive effect on plant diversity [START_REF] Bullock | Rhinanthus: a tool for restoring diverse grassland?[END_REF][START_REF] Heer | Hemiparasite-density effects on grassland plant diversity, composition and biomass[END_REF]. In all sown plots (four plots per site), ten randomly chosen seedlings of both species were tagged in May 2019 using coloured rings and poles (Appendix S2). A total of 400 seedlings were tagged over the five sites. The position of each individual was mapped, and each seedling was numbered in order to facilitate localization and recognition. Seedling survival and reproduction of these two focal species were recorded in late July 2019.

Data analysis

We analysed (i) seedling numbers and species richness of the total plant community ("all species" = spontaneously emerging species + transferred species), and of transferred species only, as well as (ii) the survival of the two focal species, Bromopsis erecta and Rhinanthus alectorolophus.

Generalized linear mixed models were applied to assess the effect of soil preparation (harrowing: yes/no) and of grazing (yes/no) on seedling recruitment and the survival of B. erecta and R. alectorolophus. The full model included the two factors and the grazing-by-soil preparation interaction as fixed effects and site as a random effect. In order to include the splitplot design of the experiment, a grazing-by-site interaction (random) was fitted to the model to test the whole-plot factor (grazing). Harrowing (split-plot factor) and the site-by-harrowing interaction were tested against the model residuals. Abundance data were right-skewed but models with Poisson error distribution and log-link function showed overdispersion. Thus negative binomial error distribution and log-link function were finally used to test total abundance and abundance of transferred species. Species richness was tested using Gaussian error distribution and identity link. Both models additionally included quadrat within plots as a random factor. Survival was analysed at plot level and did not include quadrat. A binomial error distribution was fitted using a logit-link function.

Non-Metric Multidimensional Scaling (NMDS) based on Bray-Curtis similarity was applied to compare plant species composition in different treatments (Borcard et al., 2011). A permutation multivariate analysis of variance (PERMANOVA, Anderson 2001) with 9999 permutations was used to analyse whether the community composition was significantly different between the grazing and soil preparation treatments (R package "vegan", [START_REF] Oksanen | Vegan: Community Ecology Package. R package version 2[END_REF]. All analyses were performed using R 3.5.3 (R Core Team, 2019).

RESULTS

Seedling emergence

A total of 21 plant species germinated in the greenhouse corresponding to 37.5% of the total species number observed at the donor site. At the restoration sites, 29 donor site species (51.8%) were found. Taking into account the difference in the amount of seed material used in the greenhouse test and at the restoration sites, the final germination was 40 times higher in the greenhouse (7.31 seeds/g) than in the field (0.17 seeds/g; Appendices S3). In particular, Plantago media L., Festuca marginata (Hack.) K.Richt. and Schedonorus pratensis (Huds.) P.Beauv. showed a much higher greenhouse germination whereas the difference between greenhouse and field was much smaller in B. erecta (4 times higher in the greenhouse). R. alectorolophus did not germinate in the greenhouse.

Seedling recruitment

Soil preparation by harrowing reduced overall seedling recruitment including that of spontaneously emerging species. Grazing had a marginally significant negative effect on seedling recruitment (Table 1, Fig. 1a). The opposite effect of harrowing was observed for transferred plant species. Twice as many seedlings of transferred species were found in the harrowed than in the non-harrowed plots (Fig. 1b). Grazing resulted in a 50% reduction of the seedling density of transferred species, but due to the high between-site variation the grazing effect was only marginally significant . The grazing-by-soil preparation interaction was not significant for both response variables. Table 1. Effect of harrowing and grazing on number of seedlings per m², species richness and the survival of two focal species. Results on seedling number and richness are presented separately for the whole plant community and the subsample of transferred species. H x G = grazing-by-harrowing interaction. Results of GLMM with Chi-Square (χ2) and significance levels:

. P < 0.

Plant species richness

The treatment effects on overall species richness were not (grazing) or marginally (increase when harrowed) significant (Table 1, Fig. 2a). The species richness of transferred species was 30% higher in the harrowed compared to the non-harrowed plots (Fig. 2b). Similarly to the abundance of transferred species, the richness of transferred species was reduced by grazing but the effect was only marginally significant. No significant grazing-by-soil preparation interaction was observed. marginally significant) and "=" (not significantly different).

Plant species composition

The species composition was not clearly separated between treatments resulting in a large overlap of NMDS polygons (Fig. 3). Neither harrowing (P = 0.198), nor grazing (P = 0.392) nor the grazing-by-soil preparation interaction (P = 0.461) were significant. The plots without soil preparation nor grazing showed higher variation in species composition resulting in a larger

Seedling survival of two focal species

Seedling survival of B. erecta was high in all treatments ( 75%, Fig. 4a). The main effects of grazing and soil preparation were not significant, but a significant grazing-by-soil preparation interaction indicated that the grazing effect depended on soil preparation (Table 1). Grazing reduced seedling survival only in non-harrowed plots. With few exceptions, B. erecta did not reach the reproductive stage in the first year. The survival rate of R. alectorolophus was low (<45%) compared to that of B. erecta (Fig. 4b). Similarly to B. erecta, no significant main effect was found for R. alectorolophus but a significant grazing-by-soil preparation interaction explained by a negative grazing effect in non-harrowed plots (Fig. 4b). Most surviving R.

alectorolophus individuals of the un-grazed, non-harrowed plots were flowering in July, with a significant negative effect of grazing (χ 2 = 7.348, df = 1, P = 0.007; Appendix S3). Few individuals reached the reproductive stage in the three other treatments types. Similarly to survival, the significant grazing-by-soil preparation interaction (χ 2 = 4.100, df = 1, P = 0.043) was explained by a negative grazing effect in non-harrowed plots. Significant grazing-by-soil preparation interactions are presented using "*" (P<0.05).

DISCUSSION

The low field emergence compared with greenhouse germination showed that seedling recruitment may be a bottleneck in ecological restoration of grasslands (Öster et al., 2009). In our study on mountain grasslands, soil preparation largely increased the recruitment of transferred species, whereas competition by spontaneously emerging ruderal species was reduced. Grazing had only a weak negative effect on seedling density. While neither harrowing nor grazing had an effect on focal species survival when applied alone, a significant interaction effect indicated that the grazing effect depended on soil preparation or vice versa. The negative grazing effect was generally stronger in non-harrowed plots. Despite significant grazing and harrowing effects on seedling recruitment and early survival, none of the treatments influenced the first-year plant community composition.

Several studies have demonstrated that soil preparation prior to seed addition increases germination success and emergence of added seeds because it limits competition of preexisting vegetation (Edwards et al., 2007;[START_REF] Bischoff | Hay and seed transfer to reestablish rare grassland species and communities: How important are date and soil preparation?[END_REF][START_REF] Harvolk-Schöning | Floodplain meadow restoration revisited: Long-term success of large scale application of diaspore transfer with plant material in restoration practice[END_REF].

However, pre-existing vegetation or a mulch layer may also facilitate seedling recruitment [START_REF] Donath | Large scale application of diaspore transfer with plant material in restoration practice -Impact of seed and microsite limitation[END_REF][START_REF] Scotton | Practical handbook for seed harvest and ecological restoration of species-rich grasslands[END_REF]. In particular in dry and open habitats, the absence of vegetation cover increases drought stress reducing germination and seedling survival [START_REF] Callaway | Competition and Facilitation: A Synthetic Approach to Interactions in Plant Communities[END_REF][START_REF] Eckstein | Interactions between litter and water availability affect seedling emergence in four familial pairs of floodplain species[END_REF]. Our restoration sites in the Upper Durance valley clearly benefitted from previous harrowing indicating that the reduction of competition and improved conditions for germination [START_REF] Chambers | Seed movements and seedling fates in disturbed sagebrush steppe ecosystems: implications for restoration[END_REF] prevailed over the reduction of facilitation effects. Similarly, [START_REF] Schmiede | Enhancing plant biodiversity in species-poor grassland through plant material transfer -the impact of sward disturbance[END_REF] and Harvolk-Schöning et al.

(2020) found that the cover of target species was higher following soil disturbance and seed introduction in a floodplain grassland. To our knowledge, our study is the first showing a simultaneous negative effect of soil preparation on spontaneously emerging species and a positive effect on transferred species. This effect was observed shortly after construction work leading to vegetation degradation and may be even stronger at restoration sites with higher spontaneous vegetation cover (here only 28%). The reduced competition by already established species created favourable microsite conditions for the recruitment of transferred species. Soil tillage particularly reduced the abundance of ruderal, competitive species (sensu Grime, 1988), such as Anisantha sterilis (L.) Nevski, Capsella bursa-pastoris (L.) Medik., Atriplex patula L., and Polygonum aviculare L. Without soil disturbance, such competitive early successional species hamper the establishment of the transferred late-successional species [START_REF] Donath | Large scale application of diaspore transfer with plant material in restoration practice -Impact of seed and microsite limitation[END_REF][START_REF] Jaunatre | Topsoil removal improves various restoration treatments of a Mediterranean steppe (La Crau, southeast France)[END_REF]. Regardless of competition, harrowing has also been shown to create favourable micro-reliefs improving the adhesion of the transferred seeds to the soil and thus favouring recruitment and establishment [START_REF] Isselin-Nondedeu | Contributions of vegetation cover and cattle hoof prints towards seed runoff control on ski pistes[END_REF]. [START_REF] Chambers | Seed movements and seedling fates in disturbed sagebrush steppe ecosystems: implications for restoration[END_REF] showed that seed movements and seedling establishment clearly depended on soil surface structure with holes limiting seed removal and increasing germination. The quantity of removed seeds also depends on the number of transferred seeds and their morphology (size, weight, shape). A better understanding of interactions between soil preparation, seed adhesion to the soil and seed morphology may help to increase the recruitment of transferred species and thus improve the efficiency of restoration measures [START_REF] Jakobsson | A comparative study of seed number, seed size, seedling size and recruitment in grassland plants[END_REF].

Contrary to the soil treatment, the effect of grazing was not selective between transferred species representing the target species of restoration, and non-target species.

Grazing had a small but generally negative effect on seedling recruitment. The negative grazing effect was smaller than in other studies of the same geographical region but under Mediterranean climate [START_REF] Buisson | Limiting processes for perennial plant reintroduction to restore dry grasslands[END_REF]Vidaller et al., 2019b) in which the transferred species clearly established better when grazing was excluded. Grazing animals negatively affect seedling recruitment by trampling and later on by biomass removal. Trampling by livestock does not only damage seedlings but also strongly affects the soil by erosion [START_REF] Farrell | Seeding alters plant community trajectory: Impacts of seeding, grazing and trampling on semi-arid re-vegetation[END_REF] or destabilization [START_REF] Tasser | Effects of land use in alpine grasslands on the probability of landslides[END_REF], particularly in mountain grassland slopes. On the other hand, trampling may have a positive effect on seedling recruitment of transferred species, since pre-existing vegetation is damaged by bruising, crushing, plant displacement or burial in mud [START_REF] Bilotta | The Impacts of Grazing Animals on the Quality of Soils, Vegetation, and Surface Waters in Intensively Managed Grasslands[END_REF], thus limiting competition with non-target species. Similar to soil preparation, trampling may also create holes reducing seed removal of transferred species [START_REF] Isselin-Nondedeu | Contributions of vegetation cover and cattle hoof prints towards seed runoff control on ski pistes[END_REF]. Soil preparation and grazing may thus have a common positive effect: the creation of microsites favourable to recruitment by limiting competition, increasing seed adhesion and water retention [START_REF] Isselin-Nondedeu | Contributions of vegetation cover and cattle hoof prints towards seed runoff control on ski pistes[END_REF][START_REF] Chambers | Seed movements and seedling fates in disturbed sagebrush steppe ecosystems: implications for restoration[END_REF]. The outcome of the trade-off between microsite creation and seedling damage depends on the productivity of the sites and the timing of grazing. Biomass removal by grazing or mowing is particularly required on highly productive sites where competitive grasses often hamper seedling recruitment and establishment [START_REF] Schmiede | Enhancing plant biodiversity in species-poor grassland through plant material transfer -the impact of sward disturbance[END_REF]Kladivova and Münzbergova, 2016). Nutrient-poor sites rather need protection of seedlings against grazing than biomass removal since competition is much lower [START_REF] Kirmer | Sowing of low and high diversity seed mixtures in ecological restoration of surface mined-land[END_REF][START_REF] Scotton | Practical handbook for seed harvest and ecological restoration of species-rich grasslands[END_REF]Vidaller et al., 2019aVidaller et al., , 2019b)). The marginally significant negative grazing effect in our study may thus be explained by the intermediate productivity level of our restoration sites. Post-pasture sowing has been cited as a method to optimise timing, because it allows seedlings to develop before the next grazing, reducing trampling and herbivory at the most vulnerable stage [START_REF] Eichberg | Sheep trampling on surface-lying seeds improves seedling recruitment in open sand ecosystems[END_REF]. Our study sites were first grazed before seedling emergence, 2-3 weeks after brush material transfer, and the following grazing period occurred seven months later. Thus, the first grazing probably had a positive effect in reducing competition by pre-existing vegetation, but the second grazing period may have negatively affected young seedlings that predominantly emerged in spring, finally resulting in a slightly negative effect of grazing. The vegetation period usually starts after snow melt in March resulting in a low probability to attain maturity before the June grazing period. The timing of grazing needs to be taken into account to limit the damage on seedlings, particularly if grazing exclusion is not possible. Livestock type may influence grazing effects (Tóth et al., 2016). In our study, we focused on an overall grazing effect including sheep and cattle, and the grazer species effect was pooled with experimental site. It may be interesting for future studies to distinguish effects of cattle and sheep grazing.

The analysis of seedling survival of two focal species confirmed these conclusions. The grazing effect was small and limited to plots without soil preparation. The vegetation of these non-harrowed plots was denser and thus more attractive for grazers explaining the higher grazing effect. Grazing livestock prefers dense vegetation with higher food resource over sparse vegetation resulting in a higher biomass removal (Török et al., 2014;[START_REF] Meuret | When Art and Science Meet: Integrating Knowledge of French Herders with Science of Foraging Behavior[END_REF]. B. erecta showed a generally high survival rate after three months suggesting that the species, characteristic of semi-dry soils and semi-natural sites, is very well adapted to the pedoclimatic conditions of our study zone and resistant to grazing [START_REF] Calaciura | -Brometalia) (*important orchid sites)[END_REF].

Rhinanthus alectorolophus showed, however, a much lower survival since the abundance of potential host plants was low during the first year of restoration. The species is a hemi-parasite and its performance depends on the availability and performance of host plants [START_REF] Matthies | Response of a root hemiparasite to elevated CO2 depends on host type and soil nutrients[END_REF][START_REF] Bullock | Habitat-specific dispersal: environmental effects on the mechanisms and patterns of seed movement in a grassland herb Rhinanthus minor[END_REF]. It cannot establish under low vegetation cover and its population may go locally extinct as it is an annual species whose year-to-year survival relies on the production of seeds and seedling recruitment [START_REF] Coulson | Colonization of grassland by sown species: dispersal versus microsite limitation in responses to management[END_REF][START_REF] Bullock | Habitat-specific dispersal: environmental effects on the mechanisms and patterns of seed movement in a grassland herb Rhinanthus minor[END_REF].

Whereas the grazing effect on the survival of R. alectorolphus was not significant, the effect on flower production was negative. The result suggests that the species may survive grazing, but grazing damage prevent species from flowering.

The differential effects of soil preparation and grazing on spontaneous and transferred species did not result in a change in first-year plant species composition. The abundance of spontaneously emerging species was between six and ten times higher than that of transferred species, explaining that the observed significant soil treatment effect on transferred species

was not yet visible at community level. The soil of disturbed sites generally contains high numbers of seeds of annuals and/or ruderal species [START_REF] Bischoff | Dispersal and establishment of floodplain grassland species as limiting factors in restoration[END_REF][START_REF] Donath | Large scale application of diaspore transfer with plant material in restoration practice -Impact of seed and microsite limitation[END_REF]. These species often dominate the vegetation of early successional stages before being replaced by typical grassland species that are usually perennials developing later in the succession [START_REF] Kiehl | Species introduction in restoration projects -Evaluation of different techniques for the establishment of seminatural grasslands in Central and Northwestern Europe[END_REF]Valkó et al., 2016;[START_REF] Kiss | Establishment Gaps in Species-Poor Grasslands: Artificial Biodiversity Hotspots to Support the Colonization of Target Species[END_REF]. The first year after the restoration of Eastern European grasslands, [START_REF] Lengyel | Grassland restoration to conserve landscape-level biodiversity: a synthesis of early results from a large-scale project[END_REF] observed that annual weeds were dominant, but the cover of those weeds decreased dramatically after the third year due to the growth of perennial grasses. The authors observed an increase in the cover of target species from the first to the fourth year of restoration. A similar low initial restoration treatment effect due to initial low establishment of transferred target species and a subsequent high long-term restoration success (eight years) is known for riparian mesophilous grasslands [START_REF] Auestad | Near-natural methods promote restoration of species-rich grassland vegetation -revisiting a road verge trial after 9 years[END_REF][START_REF] Bischoff | Hay and seed transfer to reestablish rare grassland species and communities: How important are date and soil preparation?[END_REF]. Monitoring the plant community over several years will be necessary in order to evaluate whether the observed initial differences in seedling recruitment and early survival significantly influence plant species composition and restoration success in the long run.

CONCLUSION

Most species of the donor site were found at the restoration sites indicating that the applied brush harvesting technique was appropriate and allowed testing soil preparation and grazing effects. Similarly to studies in mesophilous grasslands (Edwards et al., 2007[START_REF] Bischoff | Hay and seed transfer to reestablish rare grassland species and communities: How important are date and soil preparation?[END_REF], soil preparation had a clearly positive effect on the seedling recruitment. The reduction in overall seedling density dominated by spontaneously emerging species demonstrated that the reduction in competition was the principal mechanism explaining the positive effect of previous soil tillage by harrowing on the recruitment of transferred species (Edwards et al., 2007;[START_REF] Schmiede | Enhancing plant biodiversity in species-poor grassland through plant material transfer -the impact of sward disturbance[END_REF]. We thus recommend soil preparation before seed-based ecological restoration even in semi-dry grasslands to improve seedling recruitment of transferred target species. Our study did not confirm a strong negative effect of grazing on seedling recruitment found by several other authors [START_REF] Scotton | Practical handbook for seed harvest and ecological restoration of species-rich grasslands[END_REF]Vidaller et al., 2019b). The marginally significant negative effect of extensive grazing may be tolerated if farmers are reluctant in putting up fences as in our study area. Further research is needed on the timing of sowing relative to grazing periods, in order to benefit from positive grazing effects by reduction of competition while avoiding seedling damage.

Fig. 1 .

 1 Fig. 1. Number of seedlings per m 2 of (a) all species, and of (b) transferred species only, in the

Fig. 2 .

 2 Fig. 2. (a) Species richness of seedlings of all species and of (b) transferred species in the four

  polygon. Independent of treatment, semi-dry grassland species such as B. erecta, Saponaria ocymoides L. and Eryngium campestre L. occurred in the upper part of the ordination plot, and ruderal species such as Capsella bursa-pastoris (L.) Medik., Atriplex patula L., Anisantha sterilis (L.) Nevski (former name: Bromus sterilis), Veronica polita Fr. and Polygonum aviculare L. in the lower right part.

Fig. 3 .

 3 Fig. 3. Plant species composition in the four restoration treatments using NMDS. Polygons

Fig. 4 .

 4 Fig. 4. Final seedling survival of (a) Bromopsis erecta and (b) Rhinanthus alectorolophus. The

  1, * P < 0.05, ** P < 0.01, *** P < 0.001, NS not significant.

		df	Seedling number	Species richness	Survival
			All	Transferred	All	Transferred	Bromopsis	Rhinanthus
			species	species	species	species	erecta	alectorolophus
	Harrowing 1 7.097 ** 6.435 *	3.476 .	6.308 *	2.484 NS 0.741 NS
	Grazing	1 3.726 .	3.440 .	2.661 NS 2.804 .	1.799 NS 2.585 NS
	H x G	1 0.560 NS 0.011 NS	0.014 NS 1.095 NS	5.089 *	4.851 *

ACKNOWLEDGMENTS

We would like to thank Daniel Pavon for his valuable help in plant species identification. We thank Alice Dupré-la-Tour and Sophie Duhautois for assistance in logistics and project coordination. We are grateful to Jean-François Alignan, Antoine Beltrame, Sarah Kalkas, Jeanne Pénaud, Tiago Toma and Christel Vidaller for their help in the field and for seed counting.

Zeiter, M., Stampfli, A., & Newbery, D. M. (2006). Recruitment Limitation Constrains Local Species Richness and Productivity in Dry Grassland. Ecology, 87(4), 942-951. https://doi.org/10.1890/0012-9658(2006)87[942:RLCLSR]2.0.CO;2

Funding information

Financial support was provided by RTE (Electricity Transmission Network), the ecology consultancy ECO-MED and ANRT (National Agency of Research and Technology) via a grant CIFRE N° 2017 / 0478. in any seed-based ecological restoration project. preparation is required to reduce competition facilitate seedling recruitment, and ii)

Török, P., Miglécz, T., Valkó, O., Kelemen, A., Tóth, K., Lengyel, S., & Tóthmérész, B. (2012)


Csaba Tölgyesi

Elise Buisson

Aveliina Helm

Vicky M Temperton

Péter Török

Urgent need for updating the slogan of global climate actions from "tree planting" to "restore native vegetation"

Keywords: afforestation, biodiversity loss, Bonn Challenge, climate mitigation, EU Biodiversity Strategy, forest restoration, grassland restoration, tree planting, Trillion

The prevailing nature-based solution to tackle climate change is tree planting. However, there is growing evidence that it has serious contraindications in many regions. The main shortcoming of global tree planting is its awareness disparity to alternative ecosystem types, mainly grasslands. Grasslands, where they constitute the natural vegetation, may support higher biodiversity and a safer, soil-locked carbon stock than plantations and other forests.

We suggest replacing "tree planting" by "restore native vegetation." This improved action terminology reduces the risks of inappropriate afforestation and, by diversifying target ecosystem types, does not reduce but increases potential land area for nature-based climate mitigation.

Introduction

Nature-based climate strategies, such as the Bonn Challenge or the Trillion Tree Campaign, have become exceedingly popular to counteract anthropogenic carbon emissions. The first and foremost method of these strategies is planting trees worldwide [START_REF] Bastin | The global tree restoration potential[END_REF][START_REF] Cook-Patton | Mapping carbon accumulation potential from global natural forest regrowth[END_REF][START_REF] Fleischman | Pitfalls of tree planting show why we need people-centered natural climate solutions[END_REF][START_REF] Holl | Tree planting is not a simple solution[END_REF], which is often limited to planting monocultures of exotic tree species [START_REF] Lewis | Restoring natural forests is the best way to remove atmospheric carbon[END_REF][START_REF] Heilmayr | Impacts of Chilean forest subsidies on forest cover, carbon and biodiversity[END_REF].

However, there is growing concern that global afforestation, that is, the establishment of woody vegetation in historically or naturally non-forested areas, may have critical side effects in many regions. Such inappropriately chosen target areas include temperate and tropical grasslands and savannas, despite strong evidence that these habitats can store large quantities of carbon belowground, are less exposed to devastating fires, and absorb less solar radiation than closed-canopy forests [START_REF] Carvalhais | Global covariation of carbon turnover times with climate in terrestrial ecosystems[END_REF][START_REF] Dass | Grasslands may be more reliable carbon sinks than forests in California[END_REF][START_REF] Temperton | Step back from the forest and step up to the Bonn Challenge: how a broad ecological perspective can promote successful landscape restoration[END_REF]).

Many of these ecosystems are also very biodiverse [START_REF] Murphy | The underestimated biodiversity of tropical grassy biomes[END_REF][START_REF] Zhao | Grassland ecosystem services: a systematic review of research advances and future directions[END_REF], and their afforestation is not only a missed opportunity to truly mitigate climate change but also increases biodiversity loss [START_REF] Veldman | Comment on "The global tree restoration potential[END_REF].

Global afforestation programs may thus generate conflict with local to regional restoration, conservation, and climate targets (see also [START_REF] Suding | Committing to ecological restoration[END_REF]. They might have limited effect not only on climate mitigation but also on livelihood goals [START_REF] Coleman | Limited effects of tree planting on forest canopy cover and rural livelihoods in Northern India[END_REF]. There seems little progress toward a prompt solution for this issue as even its recognition is largely lacking among pro-afforestation groups. As a reinforcement of this global deficiency of climate strategies, the recently declared EU Biodiversity Strategy for 2030 also ignores the climate benefit of grasslands by setting the plantation of 3 billion trees as a major action, which is not counterbalanced by any other proposed form of major habitat restoration (Gomez-Gonzalez et al. 2020;[START_REF] Selva | Misguided forest action in EU Biodiversity Strategy[END_REF]. Why is this?

Grassland awareness disparity

We believe that the mismatch between the ultimate goal of global climate strategies and their tree planting action is mainly due to grassland awareness disparity, i.e. a general undervaluation of the diverse flora, fauna and ecosystem services of grasslands (and other non-forested vegetation types) compared to those of forests, leading to an ignorance of grasslands as possible alternatives for forest-based solutions. For centuries, the western world has considered forests the climax of ecological organization; hence, nature tends to equal forests for many people [START_REF] Pausas | Humboldt and the reinvention of nature[END_REF]. In colonial times, this notion was spread all across developing countries, where it continues to persist until present days (Davies and Robbins 2018). Tropical grasslands and savannas have traditionally been mistaken for degraded forests with little biodiversity conservation value [START_REF] Veldman | Clarifying the confusion: old-growth savannahs and tropical ecosystem degradation[END_REF]; therefore, it is no surprise that climate actions have not embraced them yet. In contrast, the biodiversity of temperate grasslands in the northern hemisphere has long been acknowledged but, to date, no global climate strategy considers them as targets. Grasslands lack large and obvious aboveground carbon stores such as massive tree trunks and their microclimate is generally more hostile (i.e., warmer and drier) compared to that of forests [START_REF] Tölgyesi | Underground deserts below fertility islands? Woody species desiccate lower soil layers in sandy drylands[END_REF], hindering their appreciation as efficient climate solutions by the general public and policy makers.

However, grasslands may sometimes represent larger and more stable carbon sinks belowground than forests below-and aboveground combined [START_REF] Dass | Grasslands may be more reliable carbon sinks than forests in California[END_REF]. The attention of the scientific community is also biased for forests, as a recent synthesis highlighted that nearly three times more papers are available in the Web of Science portal on the restoration of forests than grasslands [START_REF] Török | The present and future of grassland restoration[END_REF]). This bias is particularly expressed in the Global South, where grassland restoration is still in its infancy [START_REF] Buisson | A research agenda for the restoration of tropical and subtropical grasslands and savannas[END_REF]). Nevertheless, the number of studies emphasizing the importance of grassland restoration (e.g. [START_REF] Török | Grassland restoration on former croplands in Europe: an assessment of applicability of techniques and costs[END_REF][START_REF] Buisson | Resilience and restoration of tropical and subtropical grasslands, savannas, and grassy woodlands[END_REF]) and the inclusion of this action in large-scale biodiversity conservation programs, such as the UN Decade on Ecosystem Restoration [START_REF] Dudley | Grasslands and savannahs in the UN Decade on Ecosystem Restoration[END_REF]) is increasing, especially in the Global North. As long as climate actions do not embrace grasslands and handle them equally to forests, though, the conflict between afforestation and the conservation or restoration of non-forested ecosystems will persist and hinder activities on both sides (see also [START_REF] Silveira | Biome Awareness Disparity is BAD for tropical ecosystem conservation and restoration[END_REF].

Where to plant what?

Ecological theory predicting whether forest or grassland (or other non-forested vegetation types) should occur under natural conditions in a specific location is well-founded. Where climate, soil and/or topography are restrictive for closed-canopy natural forests, the restoration of alternative ecosystems, such as grasslands and other open habitats, should be encouraged. The success rate of afforestation in areas that are too arid for trees would be low and hence the carbon targets are unlikely to be met. Intensive forestry techniques may decrease the mortality of trees but entail heavy soil disturbance [START_REF] Yildiz | Experimental arid land afforestation in Central Anatolia, Turkey[END_REF], compromising soil carbon stocks and questioning the net effect on carbon storage. Moreover, afforestation in arid areas can have detrimental regional side-effects, including soil salinization and negative water balance [START_REF] Jackson | Trading water for carbon with biological carbon sequestration[END_REF][START_REF] Wang | Assessing the water footprint of afforestation in Inner Mongolia, China[END_REF][START_REF] Li | Time and space catch up with restoration programs that ignore ecosystem service trade-offs[END_REF].

Afforested arid lands are also more prone to fire, and when burnt, exhibit a heavier overall land degradation, including soil erosion, than non-forested sites [START_REF] Nunes | Afforestation, subsequent forest fires and provision of hydrological services: a model-based analysis for Mediterranean mountainous catchment[END_REF]. In contrast, grasslands in arid areas preserve native biodiversity and safely store carbon in the soil, even when fires sweep through [START_REF] Dass | Grasslands may be more reliable carbon sinks than forests in California[END_REF]. The amount of carbon potentially stored in the soil of arid grasslands, such as the prairie and steppe grasslands, is well indicated by their typical, black chernozemic soil, which is among the most carbon-rich soils on Earth [START_REF] Mikhailova | Organic carbon stocks in the Russian Chernozem[END_REF]. Similarly, open habitats with water-logged conditions, such as peatlands, are also advised against for afforestation, as it changes their carbon-rich soil from sink to source, preventing a net gain in ecosystem carbon stocks [START_REF] Friggens | Tree planting in organic soils does not result in net carbon sequestration on decadal timescales[END_REF]).

On the other hand, historically forested regions, located mostly in humid but not water-logged tropical, temperate or boreal areas, should be reforested if currently cleared.

However, monocultures should be avoided and the natural diversity of the forests should be restored, since plantations harbour little biodiversity and store little more carbon, on average, than the land cleared to plant them [START_REF] Lewis | Restoring natural forests is the best way to remove atmospheric carbon[END_REF][START_REF] Martin | People plant trees for utility more often than for biodiversity or carbon[END_REF]. Furthermore, care should be taken in that indiscriminate reforestation of agriculturally utilized land may lead to deforestation or the destruction of grasslands elsewhere to meet the demand for food and feed products [START_REF] Fleischman | Pitfalls of tree planting show why we need people-centered natural climate solutions[END_REF][START_REF] Holl | Tree planting is not a simple solution[END_REF].

In a broad zone between these two extremes, i.e. historically treeless grasslands or closed canopy forests, savannas and forest-steppes can be found, featured by a mosaic of woody vegetation and grassland [START_REF] Staver | The global extent and determinants of savanna and forest as alternative biome states[END_REF]). Here, environmental conditions are within the tolerance limit of closed-canopy forests; therefore these ecosystems are among the major targets of afforestation plans (e.g. [START_REF] Bastin | The global tree restoration potential[END_REF]. Historically, wildfires, herbivores and/or millennia-long traditional land-use have partially or temporarily suppressed woody species in this zone, thereby making room for grassy vegetation [START_REF] Sankaran | Woody cover in African savannas: the role of resources, fire and herbivory[END_REF][START_REF] Erdős | The edge of two worlds: A new review and synthesis on Eurasian forest-steppes[END_REF]. The mosaic structure of these ecosystems allows for exceptionally high species diversity and large capacity for ecosystem service provisioning [START_REF] Archer | Woody Plant Encroachment: Causes and Consequences[END_REF].

Grasslands within these regions are among the globally most threatened habitats due to conversion to cropland or land abandonment and subsequent woody species encroachment, and will be further decimated if they fall victim to a tree planting frenzy [START_REF] Bond | Open Ecosystems[END_REF]. A key concern about the afforestation of savannas and forest-steppes lies in the fact that ongoing climate change has already shown an increase in the frequency of droughts and other extreme weather conditions, compromising the sustainability of forests [START_REF] Mátyás | Forests in a water limited world under climate change[END_REF][START_REF] Anderegg | Climate-driven risks to the climate mitigation potential of forests[END_REF] and their capacity to efficiently capture atmospheric carbon dioxide [START_REF] Anderegg | Pervasive drought legacies in forest ecosystems and their implications for carbon cycle models[END_REF]. Another important aspect to consider is that increasing atmospheric carbon dioxide tends to increase soil organic carbon in grasslands but not in forests [START_REF] Terrer | A trade-off between plant and soil carbon storage under elevated CO2[END_REF]. Restoration in these transitional areas, whether for carbon storage or biodiversity conservation, should thus focus on the re-establishment of both woody and grassy vegetation in appropriate proportions and spatial configuration. Implementation of new concepts for multifunctional landscapes could be a promising way forward [START_REF] Manning | Redefining ecosystem multifunctionality[END_REF][START_REF] Svenning | Rewilding should be central to global restoration efforts[END_REF].

Conclusions

Considering the high number of situations where tree planting is disadvantageous, its use as an action or a general slogan to address policy makers and the general public is highly problematic. As catchy as it may sound, the fact that well-established science does not back it up in many cases may confuse target groups (i.e. benevolent policy makers, nongovernmental organizations [NGOs], and the general public), thus, threatening the credibility of climate mitigation strategies and generating conflicts between climate mitigation and biodiversity conservation. We recommend that global climate strategies should include grasslands and replace the slogan "tree planting" by "restoring native vegetation" as a naturebased solution, which can be forest, grassland, or combinations thereof, depending on present or projected future conditions of an area. This general approach fully includes reforestation if the trees are planted in the right places, but also grants equal consideration to alternative vegetation types, mostly grasslands or grassy ecosystems. Thus, the first step in all localities would be, using comprehensive ecological knowledge, to assess what the target ecosystems should be. This approach is more advanced than a simple call to plant trees and requires the involvement of more expert knowledge to aid policy makers and NGOs, but would (1) remove the current dangerous mismatch between actions and goals imposed by the risks of inappropriate afforestation [START_REF] Anderegg | Climate-driven risks to the climate mitigation potential of forests[END_REF]) and the loss of benefits from non-forested vegetation types,

(2) provide reconciliation between actions of climate mitigation and biodiversity conservation, and, most importantly,

(3) expand the potential land area that can be considered for nature-based climate solutions.

'Restoring native vegetation' in climate strategies would be an action with a self-sustaining outcome that has no critical adverse effects, so the reliability of promoting organizations will not be undermined. The use of 'restoring native vegetation' applied at whole landscape and watershed scales would pave the way for a more efficient global consensus on the activities to mitigate climate change and the biodiversity crisis.

"Restoring native vegetation" in climate strategies would be an action with a self-sustaining outcome that has no critical adverse effects, so the reliability of promoting organizations will not be undermined. The use of "restoring native vegetation" applied at whole landscape and watershed scales would pave the way for a more efficient global consensus on the activities to mitigate climate change and the biodiversity crisis.

Overbeck, Fernando A. O. Silveira and four anonymous reviewers improving the former manuscript drafts are greatly acknowledged.

Acknowledgements

The authors were supported by the National Research, Development and Innovation Office of Hungary (PD132131 -CT, K119225 -PT) and the Hungarian Academy of Sciences (Momentum Program) during manuscript writing. CT was also supported by the Bolyai János Research Scholarship of the Hungarian Academy of Sciences. The help of Gerhardt


Use our forests to create more value, diffuse and stable labour all over the country Tell the link between emotions, anchorage in geography and history and modernity

_1From

  http://reports.weforum.org/toward-the-circular-economy-accelerating-thescale-up-across-global-supply-chains/mounting-pressure-on-resources/ dominique.luzeaux@polytechnique.org 5/11/2014 N°2 From linear (cradle to grave) to circular economy (waste is food) ry political leaders and responsible business leaders alike work together… to pursue economic development on the basis urce conservation and build a circular economy. Hu Jintao, President, People's Republic of China http://www.mosim2014.org/sites/mosim2014.org/files/pdf/ Pleniere_D.Luzeaux.pdfLes grandes éoliennes (3-5 MW) peuvent être regardées comme une « fuite en avant technologique » Le bois peut être regardé comme de l'énergie solaire stockée et flexible Wood may be viewed as stored, Source : Augusto et al. (2014) -Les intrants sont-ils nécessaires pour développer durablement la fonction de production ? Carte de France du pH des sols forestiers, échelle 1km, ©IFN, LERFoB Recycle exported nutrients back to forests, possible strategies : soils/limestone → acid soils

Increasing license to operate in forestry and overcoming conflicts of representation with new narrative options Jean-François Dhôte, Alice Roux (INRA) Brigitte Musch (ONF) / 29 th June, 2017 Why investigating narrative options to speak about the climate challenges of forestry ?

	❖ Background :
	❖ forests : you cannot handle separately adaptation to climate change,
	mitigating it through supply to the bioeconomy and securing the provision of
	ecosystem services
	❖ Examine ways to overcome/solve :
	❖ Growing conflicts on solutions and representations
	❖ Low license to operate (harvesting, planting, changing things in forests)
	❖

❖ rational point of view : efficiency, flexibility, diversity enhancement, resilience of ecosystems and management systems… ❖ popular representations of forest disturbances by management as harmful to landscape persistence, Nature, authenticity… ❖ good image of wood VS bad perception of forest management (« vous ne pouvez pas être pour le saucisson et contre tuer le cochon », proverbe breton)

Sustainability standards (eg FSC, PEFC) increasingly inadequate

	Rationalizations of bioeconomy
	(forbidding practices that would be the most appropriate for CC-adaptation
	and supply to the bioeconomy)

Private land 100 ha, 50% grassland, 50% forest, elevation 1000 m (280 hab. community near Pontarlier, east France) Optimal site for Norway Spruce production

Same overall impact across time, until 2050 very ≠ arbitrages between ecosystem storage (labile) and avoided emissions(permanent)