Armin Bischoff
email: armin.bischoff@univ-avignon.fr

Sandy Hoboy
email: sandy.hoboy@lpr-landschaftsplanung.com

Nadja Winter
email: nadja.winter@landkreis-wittenberg.de

Guido Warthemann
email: guido.warthemann@lpr-landschaftsplanung.com

Hay and seed transfer to re-establish rare grassland species and communities: How important are date and soil preparation?

Keywords:

European floodplain grasslands are threatened by land use intensification or abandonment. Hay transfer using plant material from species-rich reference communities may be a valuable tool to restore such grasslands. However, large differences in seed production periods and strong competition are still obstacles that limit the efficiency of hay transfer. Using continental Cnidion meadows (FFH habitat type 6440) as a model system, we tested the effect of cutting date and of soil disturbance intensity on community and target species for eight years in a full-factorial hay transfer experiment.

The cutting date treatments were early (June), late (October), combined and no hay. Soil disturbance included shallow, deep and no tillage prior to hay transfer. We estimated the plant cover on donor and restoration sites before and after hay transfer. We additionally counted the individual number of seven specialist species of Cnidion meadows considered as target species. In a sowing experiment, seedling establishment was recorded for a subset of target species and compared to establishment in the hay transfer experiment.

Hay transfer was successful in transferring target species but community structure was still quite different from the reference grassland. Target species were only transferred with late hay but early hay added several nonspecialist species of Cnidion meadows. Strong competition by pre-existing vegetation prevented target species from establishing without soil disturbance but differences were small between shallow and deep tillage. In conclusion, a combination of early and late hay and moderate soil disturbance were the most appropriate treatments to restore Cnidion meadows.

Introduction

European floodplain grasslands are potentially diverse plant communities comprising many specialist plant species that do not or rarely occur in other grasslands [START_REF] Joyce | European Wet Grasslands: Biodiversity, Management and Restoration[END_REF][START_REF] Bischoff | Dispersal and establishment of floodplain grassland species as limiting factors in restoration[END_REF][START_REF] Krause | Habitat loss of floodplain meadows in north Germany since the 1950s[END_REF][START_REF] Wesche | Fifty years of change in central European grassland vegetation: large losses in species richness and animal-pollinated plants[END_REF]. They are man-made but result from long-term traditional land use allowing an adaptation to this specific environment [START_REF] Härdtle | Vegetation responses to environmental conditions in floodplain grasslands: prerequisites for preserving plant species diversity[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]. However, recent changes in land use such as drainage, fertilization and the conversion to arable fields have degraded or destroyed these floodplain grassland communities [START_REF] Hundt | Zur Veränderung der Wiesenvegetation Mitteldeutschlands unter dem Ein-fluß einer starken Bewirtschaftungsintensität[END_REF][START_REF] Joyce | European Wet Grasslands: Biodiversity, Management and Restoration[END_REF][START_REF] Krause | Habitat loss of floodplain meadows in north Germany since the 1950s[END_REF]. Specialists or indicator species have become rare and plant communities are usually protected now via European habitat directive such as the continental Cnidion dubii meadows (habitat type 6440, FFH, 92/43/EEC; European Commission, 2007). Cnidion meadows can be found in Central and Eastern European river valleys from the French-German border to South-Western Siberia including the Rhine, Elbe, Danube and Oder floodplains and its tributaries [START_REF] Šeffer | Management models for habitats in natura 2000 sites[END_REF][START_REF] Ludewig | Vegetation patterns of floodplain meadows along the climatic gradient at the middle Elbe River[END_REF].

In the Eastern German Elbe valley, intensification was particularly strong from the 1960s to 1980s since the former German Democratic Republic (GDR) aimed at self-sustainable food production resulting in a high conversion to arable land and increased livestock densities [START_REF] Hundt | Zur Veränderung der Wiesenvegetation Mitteldeutschlands unter dem Ein-fluß einer starken Bewirtschaftungsintensität[END_REF][START_REF] Warthemann | Die Pflanzengesellschaften des Auengrünlandes im Biospärenreservat Mittlere Elbe (Sachsen-Anhalt) im historischen, räumlichen und syntaxo-nomischen Vergleich[END_REF]. After German reunification in 1990, agri-environment schemes were introduced to restore former species richness. The major objective of these schemes was to reduce fertilization and related grazing or cutting frequency to the traditional extensive level through land use contracts with farmers. However, restoration success was poor in grasslands that did not include remnant populations of specialist floodplain grassland species [START_REF] Bischoff | Dispersal and establishment of floodplain grassland species as limiting factors in restoration[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]. Grassland restoration is often dispersal limited and may be unsuccessful if source populations are too far away [START_REF] Bischoff | Dispersal and re-establishment of Silaum silaus in floodplain grasslands[END_REF][START_REF] Donath | The impact of site conditions and seed dispersal on restoration success in alluvial meadows[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].

To overcome dispersal limitation, an active introduction of grassland species propagules has been suggested by several authors [START_REF] Kiehl | Species introduction in restoration projects -evaluation of different techniques for the establishment of seminatural grasslands in central and North-Western Europe[END_REF][START_REF] Scotton | Practical Handbook for Seed Harvest and Ecological Restoration of Species-Rich Grasslands[END_REF]. The use of seed mixtures involves propagation in stock by commercial producers. Seeds of less common species such as specialists of floodplain grasslands are usually not available. The transfer of hay collected in close-by reference communities may be a cost-efficient alternative to seed production [START_REF] Kiehl | Species introduction in restoration projects -evaluation of different techniques for the establishment of seminatural grasslands in central and North-Western Europe[END_REF]. Although different techniques have been used to concentrate seeds in the cuttings, the transfer of green fresh hay is still the most widespread method and has also been successfully tested in floodplain grasslands [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] Edwards | Hay strewing, brush harvesting of seed and soil disturbance as tools for the enhancement of botanical diversity in grasslands[END_REF][START_REF] Klimkowska | Wet meadow restoration in Western Europe: a quantitative assessment of the effectiveness of several techniques[END_REF][START_REF] Engst | Functional community ecology meets restoration ecology: assessing the restoration success of alluvial floodplain meadows with functional traits[END_REF].

Two problems may, however, limit the success of floodplain grassland restoration by hay transfer. First, seedling recruitment may be hampered by pre-existing grassland swards of already established 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] Pywell | Enhancing diversity of species-poor grasslands: an experimental assessment of multiple con-straints[END_REF][START_REF] Schmiede | Enhancing plant biodiversity in species-poor grassland through plant material transfer -the impact of sward disturbance[END_REF]. Due to the higher nutrient and water availability, productivity and interspecific competition are much higher than in dry or calcareous grasslands [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] Kiehl | Species introduction in restoration projects -evaluation of different techniques for the establishment of seminatural grasslands in central and North-Western Europe[END_REF]. In the Rhine valley, restoration by hay transfer was therefore more efficient starting from bare soil (arable fields) than from established grasslands [START_REF] Donath | Large-scale application of diaspore transfer with plant material in restoration practice -impact of seed and microsite limitation[END_REF]. In the latter case, soil or sward disturbance may be required to improve restoration success [START_REF] Schmiede | Enhancing plant biodiversity in species-poor grassland through plant material transfer -the impact of sward disturbance[END_REF].

Second, the phenology of floodplain grassland species is quite different resulting in different ripening periods. Early species may have already shed seed when late species start to produce seeds resulting in incomplete species assemblages if hay is only cut once [START_REF] Kiehl | Evaluation of initial restoration measures during the restoration of calcareous grasslands on former arable fields[END_REF][START_REF] Edwards | Hay strewing, brush harvesting of seed and soil disturbance as tools for the enhancement of botanical diversity in grasslands[END_REF]. Multiple transfers with plant material harvested at different dates are required to include early and late fruiting species but the thicker litter layer of multiple transfers may hamper germination [START_REF] Kiehl | Evaluation of initial restoration measures during the restoration of calcareous grasslands on former arable fields[END_REF][START_REF] Scotton | Practical Handbook for Seed Harvest and Ecological Restoration of Species-Rich Grasslands[END_REF].

In this study, we used a full factorial experiment to test the influence of soil preparation and of harvest date on the efficiency of green hay transfer. While [START_REF] Schmiede | Enhancing plant biodiversity in species-poor grassland through plant material transfer -the impact of sward disturbance[END_REF] obtained similar seedling establishment for deep tillage by ploughing than for medium disturbance by rotovation, [START_REF] Schnoor | Soil disturbance as a grassland restoration measure -effects on plant species composition and plant functional traits[END_REF] recommend rotovating rather than ploughing in calcareous grassland restoration. We used shallow and deep soil tillage to evaluate the need for sward opening or destruction prior to hay transfer. We combined these soil tillage treatments with hay transfer at different dates. Most specialist species of Cnidion meadows only produce seeds in late summer following regrowth after the first cut [START_REF] Šeffer | Management models for habitats in natura 2000 sites[END_REF][START_REF] Warthemann | Renaturierung von Brenndolden-Auenwiesen durch Mahdgutübertragung in der Elbeaue bei Dessau[END_REF]. Other typical but more widespread species flower much earlier and shed seeds before summer. We tested an early transfer corresponding to a traditional first cut, an early autumn transfer corresponding to a late second cut and a multiple transfer combining both. We hypothesised that specialist Cnidion species best establish from a late hay transfer whereas a full species set can only be obtained by transferring early and late hay. We additionally sowed hand-collected seeds of four specialist species to analyse seedling recruitment more in detail and to compare establishment from sown seeds and from hay.

We present classical community-based comparisons of reference and restoration grasslands but we focus on the re-establishment of target species populations because the protection of these species is a major aim of Cnidion meadow conservation. We specifically analyse the following research questions: i) Is hay transfer a suitable method to restore subcontinental floodplain grasslands and is it as efficient as hand sowing? ii) Can specialist (=target) species of Cnidion meadows be established and do their populations increase in the long run? iii) What is the best harvest time for transfer and are multiple transfers required? iv) Does previous soil disturbance improve re-establishment?

Material and methods

Study site

The experimental site is part of the "Neue Wiesen" grassland area 5 km west of Dessau (51°51′09.9″N, 12°08′45.5″E). In the 1970s, large parts of this area were ploughed and re-sown using a species-poor grass mixture, followed by a period of intensive grassland use until the breakdown of the former GDR in 1989 [START_REF] Warthemann | Renaturierung von Brenndolden-Auenwiesen durch Mahdgutübertragung in der Elbeaue bei Dessau[END_REF]. Since 1993, most of these grasslands have been under agri-environment schemes excluding fertiliser use, soil tillage and re-sowing. At the beginning of our experiment, after fourteen years of restoration, on average 21 species were found in 50 m 2 plots but re-colonisation by target species specialised on Cnidion meadows was very low. Only some Silaum silaus individuals occurred before hay transfer (5% of the plots, mean cover: 0.025%). The area is frequently inundated in spring after snowmelt in the surrounding mountain ranges, but flooding may also occur during summer following heavy rainfall in large parts of the river catchment. The site is only 1.25 m above the level of the next channel connecting the area with the Elbe River [START_REF] Bischoff | Succession of floodplain grasslands following reduction in land use intensity: the importance of environmental conditions, management and dispersal[END_REF]. Ground water levels may still decrease to 1.5 m below the surface during summer. pH-values are about 6.4, plant available N (ammonium, nitrate), P and K contents of the soils are 0.5 mg100 ml -1 , 4.4 mg 100 ml -1 and 10.4 mg 100 ml -1 , respectively. The grassland is usually mown twice a year, mid-June and early September.

A close-by (1 km) Cnidion meadow of the same altitude was used as a reference and donor site for hay transfer. Seven target species described as regional specialist species of this plant community [START_REF] Schubert | Bestimmungsbuch der Pflanzengesellschaften Mittel-und Nordostdeutschlands[END_REF] were identified on the reference site: Allium angulosum, Cnidium dubium, Galium boreale, Pseudolysimachium longifolium, Sanguisorba officinalis, Selinum carvifolia and Silaum silaus. Compared with a large-scale analysis of 33 grassland6s in the same region [START_REF] Bischoff | Succession of floodplain grasslands following reduction in land use intensity: the importance of environmental conditions, management and dispersal[END_REF], only one target species (Serratula tinctoria) was missing.

Experimental design

Green hay transfer

The experiment was set up in June 2007 using a randomised block design. Each of the five replicate blocks comprised twelve treatment combinations and two replicates of each treatment combination resulting in 24 plots per block and 120 plots in total. Plot size was 5 m × 10 m. Prior to hay transfer the three soil disturbance treatments were established. Shallow tillage was realised using a rotary harrow that cuts rhizomes at the soil surface (10 cm) but not in deeper soil layers. A deep cultivator was used to destroy below-ground plant organs to a depth of 20-25 cm by deep tillage. One third of the plots were left undisturbed (control). At the following day (18 June, early transfer), hay was cut in a close-by reference community (1 km) and directly (without drying) transferred to the restoration plots (green hay). For this purpose, a bar-mower suction combination was used throwing the hay automatically on a loader wagon. The green hay was manually spread to the early hay and the combined hay plots representing half of the block area. In two of the five blocks, wild boar grubbing in deep and shallow tillage plots resulted in a second unintended soil disturbance. Hay transfer was repeated at 4 October 2007 by spreading the harvested material (210 g m -2 ) to the late hay and the combined hay plots resulting in four different hay transfer treatments: no hay (control), early hay, late hay and combined hay. The position of the twelve treatment combinations was randomised within blocks. Hay was collected from a reference site that was four times larger than the sum of the recipient plots (donor-recipient area ratio 4:1) resulting in a transferred layer of 596 g hay per m 2 (air-dried before weighing).

Transfer of hand-collected seeds

One week after the late hay transfer, hand-collected seeds of four target species were sown to three additional plots in each block. To avoid cross contamination a buffer strip of 2 m separated sowing and hay transfer experiment. Similar soil disturbance treatments (no, shallow, deep tillage) were established prior to sowing using a rototiller and adjusting it for different depths. For the deep tillage treatment, the soil was tilled twice. Compared to the hay transfer experiment plot size was reduced to 5 m × 5 m in order to obtain a sufficient seed density. The four selected target species were A. angulosum, C. dubium, S. officinalis and S. tinctoria. Seeds were collected within a distance of 60 km from source populations of different reference communities. They were manually separated from fruits and sown by hand in a density of 2400 seeds per plot (96 seeds m -2 ). For Sanguisorba officinalis, seed density was only 600 per plot (24 m -2 ) since less seeds were available.

Measurements and observations

The vegetation of the reference community was analysed in 2007 prior to hay transfer. The cover of all vascular plants was estimated in four randomly chosen quadrats of 100 m 2 . In order to evaluate seed transfer rates, the density of seeds was measured for three species: C. dubium, S. officinalis and Leucanthemum vulgare. L. vulgare is a typical species of floodplain grasslands but not a specialist Cnidion species since it occurs in different grassland communities. It was included in the analysis as a species that is predominantly transferred with early hay -contrary to the target species. The number of umbels (C. dubium) and flower heads (L. vulgare, S. officinalis) was counted in five representative 1 m 2 quadrats. Fifty umbels and flower heads were sampled to count the number of seeds allowing an estimation of seed number per m 2 . This seed density of the reference community was compared to the seed density arriving on the restoration sites with the transferred hay. The seed density of the hay spread to the restoration sites was analysed using 1 × 1 m 2 cardboard seed traps. Five traps per block were placed on the ground prior to transfer. The hay samples were weighed after drying at ambient temperature. In five hay subsamples per block of together 100 g, seeds were counted allowing the calculation of seed density per m 2 .

The vegetation of all plots of the experimental site was recorded in 2007 prior to transfer and soil disturbance, in 2008 and in 2011 using the same releve technique as in the reference community (estimation of cover % of all vascular plant species). Additionally the seven target species occurring at the reference site were counted. Due to their small size, seedlings were difficult to find in the beginning (2008,2009) and we limited observations to three 1m 2 quadrats. In 2011 and 2015, the higher visibility of plants allowed an analysis of the whole plot area. The small seedling size also compromised the identification of different Apiaceae species in the first year (three target species and Daucus carota) Thus, 2008 seedling records were not taken into account in the present analyses. Seedling density of the sowing experiment was analysed in the same way and at the same date as in the hay transfer experiment.

Data analyses

Seed transfer rates for C. dubium, L. vulgare and S. officinalis were calculated for each block using seed density per m 2 at source (reference community) and recipient sites (hay spread) divided by the area ratio: = seed transfer rate

x 100 seed density (hay) x area (recipient site) seed density (source) x area (source site)

. In order to calculate the average number of transferred species in each plot, species composition prior to transfer was compared with that after transfer and in the reference community. Species occurring after the transfer (2008 and 2011) were only considered as transferred if they occurred in the reference community but not in 2007 releves on the restoration sites prior to transfer. So, transferred species that occurred already before transfer as well as newly occurring species not found in the reference community were not taken into account. The latter case was quite common in the soil disturbance treatments since arable weeds absent from aboveground vegetation in previous releves emerged from the soil seed bank.

Univariate response variables such as species richness, number of transferred species and abundance of target species were analysed using generalised linear models (GLM). The hay transfer experiment model included the main effects of soil disturbance (three levels: no, shallow and deep tillage) and hay transfer treatment (four levels: no, early hay, late hay, combined hay), their interaction and the effect of block. Normally distributed response variables (with normally distributed residuals) were tested using Gaussian error with identity link function. The left-skewed data on number of transferred species and target species abundance were analysed using Poisson error with log-link function. In the case of overdispersion, the analyses were run with negative binomial errors and loglink function. A reduced model was applied to the sowing experiment only including soil disturbance and block effects. As in the hay transfer experiment a Poisson (not overdispersed) or negative binomial error (overdispersed) with log link function was used to analyse target species abundance. The glht function (multcomp package) in R (version 03.3.1, R Development Core Team ( 2013)) was used to run multiple comparisons within significant main effects or interactions.

Multivariate models were applied to analyse treatment effects on plant species composition. A Detrended Correspondence Analysis (DCA) was run to test whether a linear or a unimodal approach is better adapted to the data set. The gradient length was lower than three standard deviations, advocating for a linear approach [START_REF] Ter Braak | CANOCO Reference Manual and User's Guide version 5.0. Biometrics[END_REF]. A principal component analysis (PCA) was run to compare the species composition of all twelve treatments with that of the reference community. The effects of soil disturbance and hay transfer on plant species composition were tested using MANOVA. The model is the same as in the univariate analysis (two main effects, their interaction, block) but Pseudo-F statistics of partial Redundancy Analysis (RDA) modelled in Canoco 5 was used to evaluate significance. Different partial RDA were calculated to obtain Pseudo-F values for main effects, interaction and block as recommended by Ter Braak and Smilauer (2012). The different partial RDA models included one factor as explanatory variable whereas all other factors and interactions were fitted as co-variables. A full model including block as a covariate was calculated to obtain the residual variation. Except for multiple comparisons in GLM, Statistica 12 and Canoco 5 were used to run univariate and multivariate models, respectively.

Results

Except for Allium angulosum, all target species of the reference community were found on the restoration sites with hay transfer. With a mean cover of < 1%, A. angulosum was quite rare in the reference community explaining the absence of this species. The quantitative analysis of transfer rates showed that losses are very different among species. The seed density of S. officinalis was low compared to the other analysed species (41 m -2 ) but the transfer rate was high (85% of the reference community seeds, Fig. A1). C. dubium seed density on the recipient sites was 612 m -2 but transfer rate was only 15%. And the non-target species L. vulgare occurred with highest seed densities (930 m -2 ) but showed an extremely low transfer rate (4%) suggesting high losses during transfer.

In the first year, the hay transfer treatments had no significant influence on species richness but the influence of soil preparation was highly significant (Fig. 1A). The response to soil preparation was similar in all hay transfer treatments explained by the occurrence of annual species emerging from the soil seed bank such as Barbarea stricta, Capsella bursa-pastoris, Chenopodium polyspermum, Mentha arvensis, Myosotis arvensis, Polygonum aviculare and Rorippa palustris. These species disappeared until 2011 resulting in a non-significant soil disturbance effect on species richness in the fourth year after hay transfer (Fig. 1B). In this year, a significant hay transfer effect appeared with highest species richness in the combined hay treatment and lowest in the control without hay. The number of transferred species was in the first and in the fourth year positively influenced by previous soil disturbance and hay transfer. In the first year, transferred species number was highest in the early hay and the combined hay treatment (Fig. 1C). In the fourth year, this number largely increased in the late and the combined hay treatment whereas numbers remained unchanged in the control (close to 0) and in the early hay treatment (Fig. 1D). In the long run, more species were transferred with the late than with the early hay and combined hay provided highest species numbers (significant to early hay: Tukey P = 0.003). The soil disturbance effect was explained by differences between the undisturbed control and the two tillage treatments (marginally significant to shallow tillage: Tukey P = 0.060) whereas no consistent difference was observed between deep and shallow tillage (Tukey P = 0.446).

Plant species composition in the fourth year was significantly affected by soil preparation and hay transfer (Table 1). The PCA plots showed that plant species composition of all restoration plots was still quite different from the reference community (Fig. 2). Without soil disturbance, no differences between hay transfer treatments can be identified (Fig. 2A). However, after deep tillage, the combined hay treatment and the late hay treatment were closer to the reference community than the early hay treatment and the control (Fig. 2B) indicating that in particular the transfer of late hay species is required to drive plant succession towards the reference system. The abundance of target species clearly increased by late and combined hay transfer but not by early hay (Fig. 3, Tukey P < 0.001 for early vs. late hay/combined hay). Target species abundance was close to zero and not higher than in the control indicating that only late hay contained target species. In the first survey (second year), combined hay showed a lower target species abundance than late hay indicating a negative effect of the thicker litter layer (Fig. 3A). However, this difference disappeared in year four and eight (Fig. 3B,C). Soil tillage prior to hay transfer had a significantly positive influence on seedling recruitment of target species. However, the influence was not significant in the first survey (second year). Wild boar grubbing in two of the five blocks may have increased inter-block variation and seedling mortality in the beginning. In the fourth year, target species abundance was significantly higher in the shallow (Tukey P = 0.019) and deep tillage treatment (Tukey P = 0.040) compared with the undisturbed control whereas differences between the two tillage treatments were not significant. The positive effect of soil disturbance was still marginally significant in the eighths year (Fig. 3C). Target species abundance largely decreased from the second to the fourth year indicating high seedling mortality. However, abundance re-increased by the eighth year suggesting a secondary spread from established individuals. In Cnidium dubium and Sanguisorba officinalis, the seedling density was only 0.14 and 0.03 m -2 resulting in final establishment rates of 0.02% and 0.07%, respectively, compared with seed densities in the transferred hay.

All four species of the sowing experiment got established but Allium angulosum was only found eight years after sowing and individual number (only four in total) was too low to analyse soil treatment effects. Seedling establishment rates of Cnidium dubium (0.07% of sown seeds) and Sanguisorba officinalis (0.38% of sown seeds) were much higher in the sowing than in the hay transfer experiment. The best establishment was found for Serratula tinctoria (0.57%) absent from the source community of the hay transfer. In these three species, soil tillage had a clearly positive effect on plant abundance confirming the result of the hay transfer experiment (Fig. 4). No C. dubium and very few S. officinalis individuals were found in controls without soil disturbance. While S. officinalis benefitted from a deep soil tillage the other species did not show consistent differences between shallow and deep tillage. As in the hay transfer experiment C. dubium abundance increased with time whereas S. officinalis abundance remained constant. S. tinctoria showed a decline in individual but was still the most successful species in year eight of the experiment.

Discussion

Hay transfer to establish species rich Cnidion meadows

The transfer of freshly cut, green hay was successful in transferring specialist species of Cnidion meadows considered as target species of nature conservation and restoration in the Middle Elbe region. All target species occurring with at least medium densities in the donor community established on the restoration grassland. Additionally, other less specialised species lacking on the restoration site were successfully transferred and contributed to the increase in species richness. Compared to other studies, the number of transferred species seems to be low (reviewed by [START_REF] Kiehl | Species introduction in restoration projects -evaluation of different techniques for the establishment of seminatural grasslands in central and North-Western Europe[END_REF]. However, most of the cited studies only indicated the total number of transferred species per site or do not include control plots without transfer. A similar increase in average species numbers was observed by [START_REF] Edwards | Hay strewing, brush harvesting of seed and soil disturbance as tools for the enhancement of botanical diversity in grasslands[END_REF]. [START_REF] Engst | Functional community ecology meets restoration ecology: assessing the restoration success of alluvial floodplain meadows with functional traits[END_REF] found a higher increase but started from lower species richness in the beginning.

The test on seed transfer rates demonstrated that high numbers of target species seeds arrived on restoration grassland. Seed density of Cnidium dubium and Leucanthemum vulgare in the hay was higher than average seed density per species in hay transfer studies on other European floodplain grasslands [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] Schmiede | Enhancing plant biodiversity in species-poor grassland through plant material transfer -the impact of sward disturbance[END_REF]. However, transfer ratios that, to our knowledge, have not yet been tested in other studies were largely species-specific. Depending on ripeness and morphology, seeds may drop to the soil when touched by the mowing machines but there may also be losses during the transport. Seedling recruitment rates were low indicating strong competition by the already established vegetation. Final establishment rates relative to the number of sown seeds were also below 0.1% in eight floodplain grassland species tested in a similar plant community [START_REF] Schmiede | Establishment of rare flood meadow species by plant material transfer: experimental tests of threshold amounts and the effect of sowing position[END_REF]. Our study confirmed that in mesophilic systems such as floodplains, competition represents a particularly strong environmental filter that often limits the re-establishment of target species [START_REF] Pywell | Enhancing diversity of species-poor grasslands: an experimental assessment of multiple con-straints[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] Schmiede | Enhancing plant biodiversity in species-poor grassland through plant material transfer -the impact of sward disturbance[END_REF].

Our study also demonstrated that long-term monitoring is required to evaluate restoration success. Several species were not (A. angulosum in the sowing experiment) or poorly (Cnidium dubium) visible in the beginning but occurred after eight yearsin the case of C. dubium with a relatively high abundance. In general, a decline was observed in the beginning due to seedling mortality followed by a subsequent increase eight years after transfer. The increase may be explained by a low visibility of seedlings in the beginning (risk of overlooking) but also by a secondary spread through sexual reproduction or vegetative propagation. In particular, C. dubium shows a high small-scale turn-over with clonal growth from lateral roots forming short-lived monocarpic ramets [START_REF] Geißler | Ramet demography and ecological attributes of the perennial river corridor plant Cnidium dubium (Schkuhr) Thell. (Apiaceae)[END_REF]. Once established, the species continuously produce such ramets that were counted as individuals in our study.

The establishment of target species tested in the sowing experiment and also occurring in the donor community of the hay transfer experiment (C. dubium, S. officinalis) was three to five times lower in the latter one. The non-reproductive biomass transferred with the hay may reduce germination and seedling recruitment [START_REF] Eckstein | Interactions between litter and water availability affect seedling emergence in four familial pairs of floodplain species[END_REF][START_REF] Hovstad | Physical and chemical effects of litter on plant establishment in semi-natural grasslands[END_REF][START_REF] Schmiede | Establishment of rare flood meadow species by plant material transfer: experimental tests of threshold amounts and the effect of sowing position[END_REF]. The most important factor is the reduction in light availability but also physical barrier effects and chemical litter compounds may hamper the establishment of plant species. [START_REF] Hovstad | Physical and chemical effects of litter on plant establishment in semi-natural grasslands[END_REF] found inhibitory effects on germination and seedling growth already for litter amounts of 400 g m -2 whereas facilitative effects prevailed in the study of [START_REF] Schmiede | Establishment of rare flood meadow species by plant material transfer: experimental tests of threshold amounts and the effect of sowing position[END_REF] up to 800 g m -2 under controlled conditions. In our study, potentially negative effects of litter were confirmed by a lower target species density in the combined hay treatment that received much more vegetative biomass (with the first hay) than the late hay treatment but the same amount of seeds.

Date of cutting and transfer

The date of hay transfer was crucial for the re-establishment of target species. Target species were almost absent in the first hay treatment. Due to their late flowering period, target species are usually unable to produce seeds until the first cutting date determined by the phenology of major grass species [START_REF] Bischoff | Succession of floodplain grasslands following reduction in land use intensity: the importance of environmental conditions, management and dispersal[END_REF]. Target species of Cnidion meadows typically flower and produce seeds in the aftermath when competition by high growing grasses is lower [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] Baasch | Enhancing success in grassland restoration by adding regionally propagated target species[END_REF]. Aftermath cutting is therefore the better strategy to transfer target species and to improve the conservation value of such grasslands. It is also in agreement with agricultural use of the grasslands since quality and quantity of the first cutting are higher. However, aftermath grazing as the most common form of grassland use in the study region [START_REF] Bischoff | Succession of floodplain grasslands following reduction in land use intensity: the importance of environmental conditions, management and dispersal[END_REF] needs to be limited.

Although the best strategy to re-establish Cnidion target species, the late hay transfer did not add all the species of the donor community that were lacking on the restoration sites. More widespread species not limited to Cnidion meadows such as Euphorbia esula, Leucanthemum vulgare and Ranunculus acris were predominantly transferred with the early hay. Accordingly, the average number of transferred species was still higher in the combined hay treatment than in the late hay treatment. Multiple transfers are useful if the phenology of plant species is very different [START_REF] Kiehl | Evaluation of initial restoration measures during the restoration of calcareous grasslands on former arable fields[END_REF][START_REF] Scotton | Practical Handbook for Seed Harvest and Ecological Restoration of Species-Rich Grasslands[END_REF]. In our study system, however, we observed a trade-off between an increase in species richness and a reduction in target species establishment due to the litter layer of the early hay (see above). Additionally, early hay may contain a high number of undesirable seed of highly competitive grasses that shed seed quite early in the season [START_REF] Hölzel | Restoration of a species-rich flood-meadow by topsoil removal and diaspore transfer with plant material[END_REF][START_REF] Donath | Influence of competition by sown grass, disturbance and litter on recruitment of rare flood-meadow species[END_REF]. Nevertheless, a multiple transfer of early and late hay is useful if restoration grassland is particularly species poor and/or if typical grass species are lacking.

Previous soil tillage

Soil tillage clearly increased the success of hay transfer treatments. The average number of transferred species as well as the abundance of target species was higher in plots that had been tilled prior to hay transfer. The sowing experiment confirmed this positive effect of soil tillage on seedling recruitment and establishment. The effect was stronger for the relatively small-seeded C. dubium than for S. officinalis and S. tinctoria with large seeds confirming the theory that small seeded seeds suffer more from competition and litter during seedling recruitment [START_REF] Hölzel | Seedling recruitment in flood-meadow species: the effects of gaps, litter and vegetation matrix[END_REF]. However, even the large-seeded species benefitted significantly from soil disturbance.

At community level, hay transfer was only successful in changing plant species composition towards the reference community when soils were tilled before. Floodplains of lowland rivers receive high amounts of nutrients with inundation events resulting in a high productivity of floodplain grasslands [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] Ludewig | Effects of reduced summer precipitation on productivity and forage quality of floodplain meadows at the Elbe and the Rhine River[END_REF]. Microsite limitation increases with productivity and may hamper seedling recruitment [START_REF] Pywell | Enhancing diversity of species-poor grasslands: an experimental assessment of multiple con-straints[END_REF][START_REF] Schmiede | Enhancing plant biodiversity in species-poor grassland through plant material transfer -the impact of sward disturbance[END_REF]. Accordingly, restoration was more successful in Cnidion meadows of the upper Rhine valley when starting from bare ground on former arable fields than from existing species-poor grasslands [START_REF] Donath | Large-scale application of diaspore transfer with plant material in restoration practice -impact of seed and microsite limitation[END_REF]. Similar positive effects of previous soil tillage on the re-establishment of grassland species and communities were shown in studies on nutrient poor calcareous grassland and in mesic lowland meadows [START_REF] Edwards | Hay strewing, brush harvesting of seed and soil disturbance as tools for the enhancement of botanical diversity in grasslands[END_REF][START_REF] Schmiede | Enhancing plant biodiversity in species-poor grassland through plant material transfer -the impact of sward disturbance[END_REF][START_REF] Schnoor | Soil disturbance as a grassland restoration measure -effects on plant species composition and plant functional traits[END_REF]. In particular on lowland sites of high productivity, intensive soil disturbance by ploughing or turf stripping had a more positive effect on restoration success than shallow sward disturbance by harrowing or rotovation [START_REF] Edwards | Hay strewing, brush harvesting of seed and soil disturbance as tools for the enhancement of botanical diversity in grasslands[END_REF][START_REF] Schmiede | Enhancing plant biodiversity in species-poor grassland through plant material transfer -the impact of sward disturbance[END_REF] whereas on sandy, calcareous sites shallow rotovation provided the best results [START_REF] Schnoor | Soil disturbance as a grassland restoration measure -effects on plant species composition and plant functional traits[END_REF]. In our study, deep tillage destroying rhizomes down to 25 cm favoured seedling recruitment and establishment but differences to shallow tillage by harrowing were small or absent in tested target species. Deep tillage, in particular ploughing has negative effects on soil erosion, nitrogen mineralisation (increase), soil structure, biological activity and diversity of soil organisms [START_REF] Holland | The environmental consequences of adopting conservation tillage in Europe: reviewing the evidence[END_REF]. A small advantage of deep tillage in terms of restoration success may not compensate for such negative effects.

Conclusions

Our study demonstrated that green hay transfer may be an appropriate technique to re-establish grassland communities of high conservation value including rare target species that are specialists of particular grassland types. However, success depended on date of hay making and on soil disturbance prior to hay transfer. In Cnidion meadows, target species produced seeds after the traditional first cutting date. Restoration of such meadows was thus only possible if the transferred plant material contained aftermath hay regrown after the first cut. Such late hay transfer is, however, not sufficient in transferring all species of the reference community. Species-poor restoration sites often need an additional early hay transfer to restore the typical community structure. In order to avoid negative effects of early hay litter layer on species emerging from late hay, late hay may be transferred to different areas resulting in a spatial separation of both treatments. Alternatively, a combination of hay transfer and sowing of seeds produced by local seed companies may be a more efficient solution if the phenology of few species does not correspond to the most appropriate cutting date [START_REF] Baasch | Enhancing success in grassland restoration by adding regionally propagated target species[END_REF][START_REF] Engst | Functional community ecology meets restoration ecology: assessing the restoration success of alluvial floodplain meadows with functional traits[END_REF].

Hay transfer in productive lowland grassland needs soil disturbance to reduce competition. In particular, target species showed very poor seedling recruitment in undisturbed control plots. Such a destruction of pre-existing grassland sward needs to be tolerated in nature reserves and other protected areas. We only found a small advantage of deep tillage compared with shallow tillage not justifying the higher negative impact on soil structure and biota. Our study also confirmed that longterm monitoring is required to evaluate restoration success in productive 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] Baasch | Enhancing success in grassland restoration by adding regionally propagated target species[END_REF]. Within usual study periods of three to four years, results may be misleading since self-thinning due to seedling mortality reduced target plant number in the first years but later on secondary spread increased abundance.

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biocon.2018.02.033.

Fig. 1 .

 1 Fig. 1. Effect of hay transfer and soil preparation on plant species richness (A: 2008, B: 2011) and on the number of transferred species established in restoration grassland (C: 2008, D: 2011). Mean values ± SE. Note that scale of y-axis is different in C and D. Results of GLM with Chi-Square (χ 2 ) and significance are indicated: + P < 0.1 P < 0.05 * P < 0.01 ** P < 0.001 ***, ns: not significant.

Fig. 3 .

 3 Fig. 3. Effect of hay transfer and soil preparation on the density of target species. A: 2009, B: 2011, C: 2015. Mean values ± SE. Note that scales of y-axis are different. Results of GLM with Chi-Square (χ 2 ) and significance are indicated: + P < 0.1 P < 0.05 * P < 0.01 ** P < 0.001 ***, ns: not significant.

Fig. 4 .

 4 Fig. 4. Effect of soil preparation on the density of sown target species. A: Cnidium dubium, B: Serratula tinctoria, C: Sanguisorba officinalis. Mean values ± SE. Note that scales of yaxis are different. Significance of soil disturbance effect (GLM) is indicated above columns: P < 0.05 * P < 0.01 ** P < 0.001 ***, ns: not significant.

Table 1

 1 Multivariate analysis of variance comparing the effect of hay transfer and soil preparation on plant species composition of grassland restoration sites in the fourth year (2011).

	Factor	df	Total SS	Pseudo-F	P-value
	Block	4	0.5681	46	0.002
	Transfer	3	0.0459	5	0.002
	Soil	2	0.0466	7.6	0.002
	Transfer × Soil	6	0.0167	0.9	0.666
	Residual	44	0.3227

Fig.

2

. PCA on species composition in different hay transfer treatments compared with the reference community in the fourth year (2011). A: no tillage, B: deep tillage. Enclosing polygons are used to illustrate treatment positions in ordination space.

Acknowledgements

We thank Astrid Eichhorn (Word Wide Fund for Nature, WWF, Mittlere Elbe) and Hendrik Pannach (Biosphere Reserve Mittlere Elbe) for administrative support and information on field sites. Julia Knopf, Jeanne Poulet and Mathias Henning assisted in field surveys. The study was funded by the German Federal Agency for Nature Conservation (BfN), the WWF and the Federal State of Sachsen-Anhalt in the framework of the programme "large-scale conservation project (Naturschutzgrossprojekt) Mittlere Elbe"


Martin F Breed

Peter A Harrison

Armin Bischoff

Paula Durruty

Nick J C Gellie

Emilie Gonzales

Kayri Havens

Francis F Kilkenny

Siegy Krauss

Andrew J Lowe

Emily K Gonzales

Marion Karmann

Siegfried L Krauss

Pedro Marques

Paul G Nevill

AND Pati L Vitt

Anna Bucharova

Priority actions to improve provenance decision making

Keywords: assisted migration, ecological restoration, local adaptation, restoration genetics

come

T he restoration sector's demand for seed is enormous and is rapidly increasing with the growth in the global restoration effort [START_REF] Verdone | Time, space, place, and the Bonn Challenge global forest restoration target[END_REF]. Choosing the geographic origin-the provenance (see the glossary in box 1 for definitions)-of seed used in restoration is an early and fundamental decision faced by restoration practitioners [START_REF] Miller | A framework for the practical science necessary to restore sustainable, resilient, and biodiverse ecosystems[END_REF]). The traditional practice has been to source seed local to the restoration site-local provenancing-because it is expected to maintain the evolutionary history of plant populations (e.g., local adaptation), minimize maladaptation, and limit deleterious genetic effects (e.g., outbreeding depression).

Global change has sparked a broad and robust debate on whether nonlocal seed should be used in restoration and, if so, under what circumstances [START_REF] Hamilton | Is local provenance important in habitat creation? A reply[END_REF][START_REF] Wilkinson | Is local provenance important in habitat creation[END_REF][START_REF] Broadhurst | Seed supply for broadscale restoration: Maximizing evolutionary potential[END_REF][START_REF] Mijnsbrugge | A question of origin: Where and how to collect seed for ecological restoration[END_REF][START_REF] Byrne | Assessing genetic risk in revegetation[END_REF][START_REF] Breed | Which provenance and where? Seed sourcing strategies for revegetation in a changing environment[END_REF][START_REF] Williams | Next generation restoration genetics: Applications and opportunities[END_REF][START_REF] Havens | Seed sourcing for restoration in an era of climate change[END_REF][START_REF] Prober | Climate-adjusted provenancing: A strategy for climate-resilient ecological restoration[END_REF], Breed et al. 2016b, Christmas et al. 2016b[START_REF] Bucharova | Assisted migration within species range ignores biotic interactions and lacks evidence[END_REF]. There are two central tenets to this argument: habitat fragmentation and climate change. Habitat fragmentation generally increases inbreeding and reduces effective population sizes of plant populations, which can have an impact on local seed quality by reducing fitness and adaptive potential [START_REF] Vranckx | Meta-analysis of susceptibility of woody plants to loss of genetic diversity through habitat fragmentation[END_REF][START_REF] Breed | Mating patterns and pollinator mobility are critical traits in forest fragmentation genetics[END_REF]. Because climate is a strong and common agent of selection in plants (Davis andShaw 2001, Petit et al. 2008), it has been argued that provenance selection should match values of climatic variables predicted into the future [START_REF] Breed | Which provenance and where? Seed sourcing strategies for revegetation in a changing environment[END_REF][START_REF] Williams | Next generation restoration genetics: Applications and opportunities[END_REF][START_REF] Havens | Seed sourcing for restoration in an era of climate change[END_REF][START_REF] Prober | Climate-adjusted provenancing: A strategy for climate-resilient ecological restoration[END_REF], Breed et al. 2016b, Christmas et al. 2016b).

Consequently, several alternative provenancing strategies have been proposed to supplement local provenances with nonlocal provenances. These strategies generally fall into two categories: those that attempt to increase the adaptive potential of plants at a restoration site by increasing the genetic diversity of the seed mix (e.g., relaxed local, composite, and admixture provenancing) and those that recommend matching provenances with anticipated future environmental conditions of a restoration site (e.g., predictive and climate-adjusted provenancing). Although there are theoretical underpinnings to these alternative strategies, experimental field-testing in restoration contexts is limited.

In this article, we identify several knowledge gaps and practical barriers that need priority action by researchers, policymakers, and practitioners to improve provenance decision-making from an international perspective. We discuss these priority actions in light of the global restoration challenge of restoring degraded terrestrial landscapes from grassland to forest ecosystems, but we acknowledge that the success of a provenancing strategy will be context dependent (e.g., landscape heterogeneity and age; plant species biology, demography, and evolutionary history; and management priorities, scale, and resources). By highlighting these priority actions, we hope to improve support for restoration, offer guidance for policy development, bridge knowledge gaps from science into practice, and ultimately improve restoration outcomes now and in the future.

Strategic goals and practitioner priorities

We identify four gaps in provenance decision-making that are commonly faced by both policymakers and practitioners. These include (1) the lack of guidance when departing from a strict local provenancing strategy, (2) the need for greater evidence-based restoration practices, (3) the potential risks associated with using nonlocal provenances, focusing on outbreeding depression, and (4) the risks and benefits of using cultivars in restoration and when they may be required. Each of these are discussed below.

Improving provenance strategy choice. Local provenancing has been the preferred approach for many years. However, a strict interpretation of local provenancing may halt restoration projects if local seed supply is insufficient [START_REF] Wilkinson | Is local provenance important in habitat creation[END_REF][START_REF] Broadhurst | Seed supply for broadscale restoration: Maximizing evolutionary potential[END_REF]. Local provenancing should not be so restrictive that it stops projects, even if this means that nonlocal provenances are introduced. In cases in which broader provenances are used or a change in practice is implemented, documentation and ongoing monitoring will be important in order to evaluate the impacts of such changes on restoration success. Indeed, determining whether plant failure at a restoration site is due to provenance selection can be inferred through personal Box 1. Glossary of terms.

Admixture provenancing: Supplementing local provenances with seed collections across a species' natural distribution.

Climate-adjusted provenancing: Supplementing local provenances with targeted nonlocal provenances collected along a climate gradient in line with climate-change projections.

Composite provenancing: Supplementing local provenances with seed from multiple nonlocal provenances within gene-flow distances to mimic natural gene flow among populations.

Cultivar:

The selective breeding of a genotype for favorable attributes such as fast growth or high-salinity (or other extreme environment) tolerance.

Habitat suitability model:

A model of the statistical relationship between species occurrences and environment that conceptually represents the predicted spatial abiotic and biotic limits of a species suitable habitat. These models are also referred to as species distribution models or ecological niche models and draw on ecological niche theory.

Heterosis:

The increased fitness of the hybrid product between two divergent provenances or species compared with parental genotypes.

Local adaptation: Superior fitness of a local population over other provenances when grown in its home environment.

Local provenance: Seed from either within the restoration site (strict local provenance) or within close geographic proximity to the restoration site (relaxed local provenance). Defining a "local" provenance is a difficult task and is rarely articulated well in papers that discuss this issue, which hinders progress in the provenancing debate. Adopting a genetically informed definition of a local provenance is probably the most useful approach, and examples include spatial delineation of provenances based on significant genetic differentiation in neutral and/or adaptive genetic markers or functional traits. However, such an approach is perhaps the least attainable for practitioners because of the costs, time, and skills required.

Local provenancing: Only using a local provenance.

Outbreeding depression:

The decreased fitness of the hybrid product between two divergent provenances due to disruption of locally adapted gene complexes.

Provenance:

The geographic location of a seed source that is used to describe the genetic material from that location.

Provenancing:

In an ecological restoration context, refers to a seed-sourcing strategy, with a focus on the geographic location of seed source(s).

Phenotypic plasticity:

The nongenetic capability of one genotype to produce a variety of different phenotypes.

Predictive provenancing: Deriving a matched seed source with predicted future home-site conditions; optimally should be based on provenance trials.

Restoration:

The process of reducing the damage and/or degradation of an ecosystem by resetting the trajectory of the ecological community toward that of a reference state. This definition should potentially be broadened to capture restoration of sites that do not have a reference (e.g., pollinator habitat in agricultural matrices or biodiverse urban green spaces) and to also include sites that have been degraded to such an extent that restoration to a reference state is likely an unachievable goal (e.g., some postmining sites and novel ecosystems).

Seed zones: Discrete geographic regions that have similar environments. Transfer within zones should result in few detrimental effects on mean population fitness. Optimally should be based on progeny testing of multiple provenances.

observations, but without experiments and monitoring, such evidence remains anecdotal.

Departing from a strict local provenancing strategy relies on the development of robust seed transfer guidelines, such as seed zones. Seed zones generally refer to regions that have similar environments, and transfer within zones should result in few detrimental effects on mean population fitness [START_REF] Hufford | Plant ecotypes: Genetic differentiation in the age of ecological restoration[END_REF]. Such zones are well developed in many forestry sectors because of large commercial interest in the final crop, but they are less well developed for restoration. Ideally, seed zones should reflect the likelihood of adaptive differentiation of the focal species [START_REF] Kilkenny | Genecological approaches to predicting the effects of climate change on plant populations[END_REF][START_REF] Jørgensen | What's the meaning of local? Using molecular markers to define seed transfer zones for ecological restoration in Norway[END_REF], Bucharova et al. 2017b). Geographic distance, for example, is often used as the basis of seed transfer zones [START_REF] Mortlock | Local seed for revegetation: Where will all that seed come from?[END_REF]. However, geographic distance is a theoretically poor predictor of adaptive differentiation unless it correlates with environmental distance [START_REF] Leimu | A meta-analysis of local adaptation in plants[END_REF]. Indeed, genetic differentiation among populations can occur over short distances because of microsite differences, as well as over broad scales reflecting clinal variation or more major geographic barriers to gene flow [START_REF] Richardson | Microgeographic adaptation and the spatial scale of evolution[END_REF]). As such, seed zones should reflect this environmental heterogeneity, because this is likely a better predictor of adaptive differentiation than geographic distance [START_REF] Hereford | A quantitative survey of local adaptation and fitness tradeoffs[END_REF]. Seed zones for restoration have been defined in some countries, such as the provisional seed zones in the United States [START_REF] Bower | Generalized provisional seed zones for native plants[END_REF] and Herkunftregionen in Germany [START_REF] Durka | Genetic differentiation within multiple common grassland plants supports seed transfer zones for ecological restoration[END_REF]). However, there is a need to develop seed-zone boundaries more generally to better reflect ecological and/or genetic differences rather than political boundaries.

Sourcing seed across political boundaries is a prerequisite when nonlocal provenances are used, requiring the exchange of seed from outside and potentially across established supply networks (i.e., among seed collectors and/or producers). Creating a mechanism to share seed across political boundaries will further help to achieve better restoration results through greater provenance options. Indeed, stakeholders in the United States have developed a strategic framework to help alleviate provenance supply issues, with the goal of improving the availability of appropriate seed for restoration. To increase the cost-effectiveness and availability of appropriate seed, the Plant Conservation Alliance (2015) developed the National Seed Strategy for Rehabilitation and Restoration, which fosters coordination among parties involved along the seed supply chain, from wild seed collection to the application in restoration projects. Similar strategic frameworks exist elsewhere (e.g., in São Paulo state, Brazil; [START_REF] Chaves | On the need of legal frameworks for assessing restoration projects success: New perspectives from São Paulo state (Brazil)[END_REF]. We encourage formulation of such frameworks in other counties, because they will likely help (a) establish best-practice provenance decision-making, (b) manage and potentially mitigate the risks of poor decision-making, and (c) ultimately improve accountability of provenance-choice decisions.

Growing the evidence base for restoration. There is an increased need for evidence-based restoration to achieve optimal outcomes. Embedding well-designed common garden experiments into restoration projects is a demonstrated way to obtain empirical evidence on the effect of provenance choice (figure 1). Despite the clear benefits of such researcher-practitioner collaborations, differences in training, terminology, objectives, and reward systems can create a lag in research uptake and a barrier to collaboration [START_REF] Guerrero | Using a social-ecological framework to inform the implementation of conservation plans[END_REF]. For example, most restoration funders demand on-ground actions over research. There need to be better cross-sector communication of the benefits of and increased funding for embedding experiments into restoration projects. Ultimately, these experiments will help inform future restoration and should become a standard part of as many restoration projects as possible.

Monitoring embedded experiments is a key part of building the evidence-based feedback loop into restoration practice (figure 1) and should be prioritized higher by funders to improve provenance decision-making. Monitoring can efficiently be done by aligning and collaborating with researchers when, for example, student projects or contracted work can be undertaken to collect, analyze, and interpret data from such experiments (e.g., [START_REF] Bailey | Plantings for carbon, biodiversity and restoration in dry rural landscapes[END_REF][START_REF] Breed | Which provenance and where? Seed sourcing strategies for revegetation in a changing environment[END_REF], 2016a, Gellie et al. 2016, 2018). In fact, it is often the case that researchers seek more opportunities to work with practitioners to broaden the impact of their research [START_REF] Evans | Academics do want to engage with business, but need more support[END_REF]. Nevertheless, it is important that researchers are brought in early into the restoration process, because well-designed studies (e.g., with controls and randomized treatment replicates) are more powerful than post hoc analyses (Bucharova et al. 2017a). Furthermore, the maintenance of well-documented databases on the provenances used in the experiments (e.g., geographic location, degree of fragmentation, or number of sampled mothers in a bulked seedlot) will assist in interpreting the data generated from such experiments and help to determine the factors influencing long-term success in restoration projects. However, there are few examples of good provenance-recording frameworks that allow such monitoring and analysis to take place in the absence of research collaborations. Implementing a tracking system, such as the yellow-tag system employed in the United States [START_REF] Young | Verification of germplasm origin and genetic status by seed certification agencies[END_REF], could improve the quality of provenance information across the provenance supply chain.

Mitigate the risk of outbreeding depression. The risks associated with mixing provenances for restoration have been thoroughly discussed (e.g., maladaptation and outbreeding depression; see [START_REF] Byrne | Assessing genetic risk in revegetation[END_REF][START_REF] Weeks | Assessing the benefits and risks of translocations in changing environments: A genetic perspective[END_REF]. Unlike maladaptation, which is difficult to predict, there are practical recommendations about how to predict outbreeding depression-the reduced fitness of progeny resulting from mating between two genetically dissimilar individuals [START_REF] Templeton | Coadaptation and outbreeding depression[END_REF][START_REF] Frankham | Predicting the probability of outbreeding depression[END_REF][START_REF] Weeks | Assessing the benefits and risks of translocations in changing environments: A genetic perspective[END_REF]. The risks of outbreeding depression can largely be predicted by identifying whether there are ploidy or major chromosomal polymorphisms between provenances with karyotyping or flow cytometry [START_REF] Frankham | Predicting the probability of outbreeding depression[END_REF].

The frequency and phylogenetic patterns of these intraspecies ploidy polymorphisms are an active area of research. The publicly available Chromosome Counts Database (CCDB; [START_REF] Rice | The Chromosome Counts Database (CCDB)-A community resource of plant chromosome numbers[END_REF] indicates that approximately 16% of plant species show ploidy polymorphisms. However, only 69 polymorphic species have been well sampled (i.e., range-wide and n > 100 sampling; [START_REF] Kolář | Mixedploidy species: Progress and opportunities in polyploid research[END_REF]. Of these 69 species, 21 were Asteraceae and 7 were Poaceae, supporting taxonomic trends identified by Kramer and colleagues (2018). Interestingly, 16% of 39 species in Kolář and colleagues (2017) showed intrapopulation polymorphisms, indicating that ploidy variation between and within populations may occur at similar rates.

Inexpensive checks for ploidy polymorphisms should be more broadly employed, especially for plant groups expected to exhibit intraspecies ploidy differences, to improve long-term restoration success by helping to mitigate the risk of outbreeding depression. For example, ploidy of species used for restoration should be initially screened in publically available databases (e.g., the CCDB). For some species, there are phenotypic correlates with ploidy that can be the basis of rapid screening (e.g., seed mass in big sagebrush, Asteraceae; [START_REF] Richardson | Genetic and environmental effects on seed weight in subspecies of big sagebrush: Applications for restoration[END_REF]. Taxa in Asteraceae and Poaceae should be priority candidates for further ploidy screening when used for restoration, because of their apparent high frequency of intraspecies polyploidy.

When to use cultivars. Restoration often takes place at sites that have deviated far away from their reference state, resulting in novel environments (e.g., high salinity or nutrient levels postagriculture and novel soil postmining). It may be difficult to establish native seed, regardless of provenance, at these sites. In such cases, selectively bred cultivars may be appropriate. Indeed, the use of cultivars in restoration has gained momentum, particularly in the United States. The genetic background of cultivars can vary greatly, with some being clonal or highly inbred, whereas others are derived from polycrosses consisting of multiple pollen parents from the same or different provenances.

The use of cultivars may be problematic if they lack genetic variation or if they have traits that counteract the

Figure 1. A schematic representation of the evidence-based feedback loop that embedded provenance trials within restoration plantings can have on improving provenance decision-making.

restoration process [START_REF] Chivers | The merits of artificial selection for the development of restoration-ready plant materials of native perennial grasses[END_REF][START_REF] Nevill | Seed production areas for the global restoration challenge[END_REF]). Furthermore, when cultivars have a narrow genetic base, they risk generating genetically uniform populations that may be less resilient to responding to environmental changes compared with those produced by wild provenances. The habitat provided by these cultivars can also have a negative impact on dependent organisms in terms of both supporting a limited set of species and disrupting ecosystem functions [START_REF] Barbour | Genetic specificity of a plant-insect food web: Implications for linking genetic variation to network complexity[END_REF][START_REF] Mody | Crop cultivar affects performance of herbivore enemies and may trigger enhanced pest control by coaction of different parasitoid species[END_REF]). Nevertheless, the use of polycrosses may counteract some of these concerns [START_REF] Chivers | The merits of artificial selection for the development of restoration-ready plant materials of native perennial grasses[END_REF]. We emphasize that cultivars should be used in a risk-assessment framework to minimize their impact on natural populations [START_REF] Byrne | Assessing genetic risk in revegetation[END_REF][START_REF] Weeks | Assessing the benefits and risks of translocations in changing environments: A genetic perspective[END_REF].

Research priorities

The push for greater evidence-based provenance decisionmaking has highlighted several key areas for future research. These include (a) understanding the role of phenotypic plasticity in climate resilience of provenances, (b) developing improved species habitat suitability models to guide provenance choice, (c) understanding provenance adaptation to both biotic and abiotic factors, and (d) understanding the provenance effects on dependent organisms in restoration plantings. Each of these are discussed below.

Phenotypic plasticity. The capability of one genotype to produce a variety of different phenotypes-phenotypic plasticity-is poorly understood for most species used in restoration. Despite this poor understanding, plasticity will likely be important in a restoration context because it can allow shortterm acclimation to novel environments and thereby stabilize fitness during environmental change [START_REF] Nicotra | Plant phenotypic plasticity in a changing climate[END_REF]. Phenotypic plasticity can be quantified by growing genotypes in a variety of environments (e.g., with and without herbivores or across an environmental gradient) and determining the variation in traits produced by each genotype across environments. A broader understanding of plasticity in functional and fitness traits (e.g., water-use efficiency, specific leaf area, and relative growth rate) from theoretical, empirical, and applied-research perspectives would help provenance decision-making by understanding how trait plasticity within a particular provenance influences its potential to survive and persist under changing conditions (e.g., [START_REF] Mclean | Plasticity of functional traits varies clinally along a rainfall gradient in Eucalyptus tricarpa[END_REF]. Although there is clear theoretical understanding of the potential benefits of plasticity, such as facilitating rapid evolution [START_REF] Rice | Managing microevolution: Restoration in the face of global change[END_REF] that increases climate resilience of restoration plantings, there is a greater need to evaluate its importance for provenance decision-making.

Habitat suitability models. A principal concern in the seedsourcing debate is climate change, and much of our current predictive knowledge relies on habitat suitability models that incorporate climate projections [START_REF] Elith | Species distribution models: Ecological explanation and prediction across space and time[END_REF]. Although habitat suitability models provide a way to select provenances for restoration plantings [START_REF] Harrison | Integrating climate change and habitat fragmentation to identify candidate seed sources for ecological restoration[END_REF], they are often based on environmental layers that assume equal weighting of these factors on fitness. Extensions of these models to include both genetic information [START_REF] Kilkenny | Genecological approaches to predicting the effects of climate change on plant populations[END_REF][START_REF] Ikeda | Genetically informed ecological niche models improve climate change predictions[END_REF]) and ecophysiological limits (e.g., mechanistic models; [START_REF] Kearney | Integrating biophysical models and evolutionary theory to predict climatic impacts on species' ranges: The dengue mosquito Aedes aegypti in Australia[END_REF][START_REF] Caddy-Retalic | Bioclimatic transect networks: Powerful observatories of ecological change[END_REF]) provide an increased realism to their predictions. However, further research is required, with an initial focus on key species used in restoration. Predictions of plant performance based on habitat suitability models need to be empirically validated through reciprocal transplant experiments or at least an understanding of the role climate and nonclimate factors have on the study system.

Components of local adaptation. Populations are often locally adapted [START_REF] Leimu | A meta-analysis of local adaptation in plants[END_REF][START_REF] Hereford | A quantitative survey of local adaptation and fitness tradeoffs[END_REF][START_REF] Oduor | Invasive plant species are locally adapted just as frequently and at least as strongly as native plant species[END_REF]. Local adaptation can have both climate and many nonclimate components, including other abiotic factors such as soil and photoperiod [START_REF] Savolainen | Ecological genomics of local adaptation[END_REF], Christmas et al. 2016a), but biotic factors such as antagonists and mutualists are also important [START_REF] Crémieux | Potential contribution of natural enemies to patterns of local adaptation in plants[END_REF], Gellie et al. 2016, Potts et al. 2016[START_REF] Gehring | Tree genetics defines fungal partner communities that may confer drought tolerance[END_REF][START_REF] Urbina | Specificity in Arabidopsis thaliana recruitment of root fungal communities from soil and rhizosphere[END_REF]). Some nonlocal provenances may have no history of exposure to local pests and pathogens [START_REF] Potts | Evolutionary history shapes the susceptibility of an island tree flora to an exotic pathogen[END_REF], and pests and pathogens may themselves be shifting their ranges [START_REF] Burke | Consequences of distributional asymmetry in a warming environment: Invasion of novel forests by the mountain pine beetle[END_REF], potentially increasing the risk of maladaptation [START_REF] Gellie | Local maladaptation in a foundation tree species: Implications for restoration[END_REF]. Little is known about the relative importance of climate compared with these other factors for most plant species, but reciprocal transplant and experimental treatments (e.g., herbivore and pathogen exclusion or exposure trials) can help tease these effects apart.

Ecological networks. The debate on seed-sourcing strategies has generally focused on mean provenance fitness. However, there are largely unappreciated extended effects of provenance choice on organisms that colonize and inhabit restoration plantings [START_REF] Whitham | A framework for community and ecosystem genetics: From genes to ecosystems[END_REF][START_REF] Crémieux | Potential contribution of natural enemies to patterns of local adaptation in plants[END_REF][START_REF] Bucharova | Assisted migration within species range ignores biotic interactions and lacks evidence[END_REF]). Provenance effects have been shown to affect individual organism responses, as well as biotic communities both above- [START_REF] Bucharova | Plant ecotype affects interacting organisms across multiple trophic levels[END_REF][START_REF] Gosney | The extended community-level effects of genetic variation in foliar wax chemistry in the forest tree Eucalyptus globulus[END_REF] and belowground [START_REF] Senior | Phylogeny explains variation in the root chemistry of Eucalyptus species[END_REF][START_REF] Gehring | Tree genetics defines fungal partner communities that may confer drought tolerance[END_REF][START_REF] Urbina | Specificity in Arabidopsis thaliana recruitment of root fungal communities from soil and rhizosphere[END_REF]. The coevolutionary relationships between local provenances and their dependent communities (e.g., [START_REF] Toju | Imbalance of predator and prey armament: Geographic clines in phenotypic interface and natural selection[END_REF]Sota 2005, Gehring et al. 2017) further emphasize the importance of considering the extended effects of using local or nonlocal provenances on ecological networks.

Conclusions

The restoration sector aims to return biodiverse, functional ecosystems at unprecedented scales [START_REF] Verdone | Time, space, place, and the Bonn Challenge global forest restoration target[END_REF], as well as to restore valuable ecosystem services (see the Call to Action from the 2017 Society for Ecological Restoration conference at www.ser.org/general/ custom.asp?page=7thWorldConference). However, the ability to meet these aspirational goals relies on supplying vast quantities of appropriately provenanced seed. Although the suitability of different provenancing strategies will no doubt be context dependent, the debate continues. To progress the debate from largely theoretical to empirical underpinnings, we recommend here a set of priority actions that need to be adopted to improve restoration outcomes from provenance decision-making: (a) Increase testing of provenance decisions by embedding provenance experiments into restoration projects. (b) Develop policies and standards on provenance choice that are dynamic, are evidence based, are influenced by context, and incorporate risk-mitigation strategies. (c) Establish stronger research-practitioner collaborations to promote research on provenance choice and encourage the adoption of research outcomes for future restoration projects.

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Acknowledgments This article was the output of a workshop at the Society for Ecological Restoration conference in Foz do Iguaçu, Brazil, held on 28 August 2017, titled "Using Non-Local Plants in Restoration: Fallacy or Fundamental?" This work was supported by the Australian Research Council (no. DE150100542 awarded to MFB, no. DP150103414 awarded to MFB and AJL; no. LP100100620 awarded to SLK; no. ICI150100041 awarded to SLK and supported PGN; no. LP120200380 supported PAH), the National Climate Change Adaptation Research Facility funding to MFB and AJL, and the Great Basin Native Plant Project to FFK. The authors declare no conflict of interest. Supplemental material Supplementary data are available at BIOSCI online.