Limited validation of forecasted northward range shift in ten European tree species from a common garden experiment

Forest Ecology and Management 410 (2018) 144–156

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Limited validation of forecasted northward range shift in ten European tree species from a common garden experiment

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Morgane Merlina, , Anne Duputiéb, Isabelle Chuinea a b

Centre d’Ecologie Fonctionnelle et Evolutive, UMR5175 CNRS-UM-UPVM3-EPHE-IRD, 1919 route de Mende, 34293 Montpellier cedex 05, France Unité Évolution, Écologie, Paléontologie, UMR CNRS 8198, Université de Lille 1, Sciences et Technologies, 59655 Villeneuve d’Ascq cedex, France

A R T I C L E I N F O

A B S T R A C T

Keywords: Climate change Growth Range shift Recruitment Seedlings Species Distribution Models (SDM) Survival

As climate change leads to global warming and modified precipitation patterns, the distribution of forest biomes and tree species is expected to shift towards higher latitudes and altitudes. Such shifts are currently projected by species distribution models fitted to different climate change scenarios. Field validation of these models for several life stages of different tree species is a necessity to adapt forest management and understand the future of forest ecosystems. The study presented here aims to assess whether signs of the projected near-future range shift, contraction or expansion of ten European forest tree species are already observable in survival and growth of their seedlings at the core, leading and trailing edges of their distribution. The results show limited validation of the projected near-future changes in spatial distribution: seedling survival and growth paralleled modelled near-future habitat suitability for the three Mediterranean/Southern species, whereas cold-adapted species showed limited validation of model projections with lower growth and survival at their trailing edge; and widespread species showed inconsistent performance with model projections. Individuals of contrasting provenances did not show strong differences in survival; however they did show substantial differences in growth. The role of extreme events and biotic interactions might prove to be more important factors into shaping the future realized niche and the distribution of these species and thus should be investigated to complete the current studies on latitudinal and altitudinal shifts in relation to climate change.

1. Introduction The distribution of many organisms is strongly controlled by climate factors (Woodward and Williams, 1987), and ongoing and future climate changes are projected to cause geographic range shift, contraction or expansion of a vast majority of species (IPCC, 2013; Parmesan and Yohe, 2003; Root et al., 2003). A number of studies have already detected such changes for mobile organisms such as insects, fishes, and birds (e.g. Parmesan and Yohe, 2003; Burrows et al., 2011; Sunday et al., 2012). Clear evidence of present-day range changes are harder to find and document in sedentary organisms such as plants, and even more in trees with long generation times (but see Crimmins et al., 2011; Delzon et al., 2013; Rabasa et al., 2013; Mathisen et al., 2014; Matías and Jump, 2015) as pressures such as herbivory, insect outbreaks and past land-use changes confound the direct effects of climate change. Species distribution models (SDMs) offer a way to forecast potential changes in tree species distribution range under different climate change scenarios. SDMs have projected upward and poleward range shifts of different magnitude for many tree species across regional and

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continental scales in Europe and North America ranging from a couple hundred meters for altitude shifts to hundreds of kilometers for latitudinal shifts (e.g. Iverson et al., 2008; Cheaib et al., 2012; García-Valdés et al., 2013). The equatorward/lowland trailing edge of tree species distribution is expected to experience severe reductions of tree growth and survival with the projected increase in temperature and in the frequency of extreme events such as droughts (Allen and Breshears, 1998; Fisichelli et al., 2014; Gworek et al., 2007; Matías et al., 2014; Ogaya and Peñuelas, 2004). The poleward/high altitude leading edge of the distributions is conversely expected to experience enhanced tree growth, survival and recruitment in the short to mid-term, as winter conditions will become milder with increased temperatures (Lenoir et al., 2008; Peñuelas et al., 2007; Reinhardt et al., 2011; Saltré et al., 2015; Vitasse et al., 2012). However, in a large number of studies which have investigated tree range shifts during the last decades, detected changes do not appear to be always consistent with climate change (for example downslope and/ or southward range shifts; Crimmins et al., 2011; Zhu et al., 2012; Monleon et al., 2015) and can point out the effects of past forest

Corresponding author at: Department of Renewable Resources, University of Alberta, Edmonton, Alberta T6G 2E3, Canada. E-mail addresses: [email protected] (M. Merlin), [email protected] (A. Duputié), [email protected] (I. Chuine).

https://doi.org/10.1016/j.foreco.2018.01.001 Received 16 August 2017; Received in revised form 2 January 2018; Accepted 3 January 2018 0378-1127/ © 2018 Elsevier B.V. All rights reserved.

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practices, land-use related habitat modification or species interactions in response to global warming (Bodin et al., 2013). It is thus unclear how climate change alone may be the driver of species range shift, and how to quantify this effect in addition to the other biotic and historic effects. A growing body of studies focus on altitudinal range shifts in tree species, as altitudinal gradients in temperature and other climatic factors provide wide climatic gradients on a small-scale where species range shifts should be detected more easily and rapidly (Herrero and Zamora, 2014; Peñuelas et al., 2007; Reinhardt et al., 2011; Zurbriggen et al., 2013). Latitudinal shifts of tree species distribution edges are much less reported. Most of these studies compared either the presence and distribution of adult mature trees and their seedlings synchronously (Vitasse et al., 2012; Zhu et al., 2014) or the presence of adult mature trees through time (Lenoir et al., 2010, 2008). Current adult mature trees have established 20–50 years ago for most of the managed forest ecosystems, hundreds of years ago for less managed ecosystems, and survived the climatic variations and extremes since then (Zhu et al., 2014). Adult mature trees thus established in climatic conditions up to ∼1 °C colder than their current seedlings in Europe (IPCC, 2013). Moreover, the climatic niche of seedlings is thought to be slightly different from that of adults (Grubb, 1977; Quero et al., 2008), especially regarding drought resistance (Cavender-Bares and Bazzaz, 2000). In particular, seedlings are expected to be more sensitive to climate change effects on temperature and water supply than adult mature trees (see for review Walck et al., 2011). Moreover, SDMs generally use the combined distributions of adults and seedlings to estimate a species’ niche, hence may overestimate its ecological niche (Ashcroft et al., 2017) or fail to identify it. To understand the observed tree dynamics at the range limits and understand how climate mediates these dynamics, we need to delve into the climatic controls of a much less studied stage, regeneration. This step is crucial to understand how future climatic conditions will control seedlings performance at both the leading and trailing edges, but only a few studies have tackled this issue so far (Carón et al., 2015; Matías and Jump, 2014; Putnam and Reich, 2017; Reinhardt et al., 2011). To form a better view on future seedlings performance at the edges of their distribution, we tested in this study the SDM-projected range shift in a very near future of ten of the most economically important and widespread European forest tree species. For each species, seeds from three to four provenances were sown at three common gardens in France showing contrasted climatic conditions, which would become part either of the leading edge, or the trailing edge, or would remain within the range of the species within France. Survival and growth of the seedlings were monitored for three years. More specifically, we aimed to answer the three following questions:

Fig. 1. Climate type and geographic location of the three common gardens in France (adapted from Joly et al., 2010).

locations in France to account for a potential genetic effect linked with local adaptation to climate in the species responses (see Supplementary Material Fig. SA.1 for the location of each provenance within the species distribution). Note that the provenances used for P. halepensis, L. decidua and Q. ilex, species with more limited distribution in France, were from closer geographic locations than for the other species, thus interpretations of the provenance effects in subsequent analyses should be taken with caution. 2.2. Study sites Three common gardens were installed in 2006 to conduct two experiments running from 2007 to 2010. These gardens show very different climatic conditions (Fig. 1): (1) a Northern-Oceanic (NO) site located in Guéméné-Penfao (47°37′51″N, 1°49′53″W, 20 m a.s.l) with oceanic climate (de La Broise, 1987); (2) a Medium Elevation (ME) site located in Peyrat-le-Château (45°48′49′′N, 1°46′25′′E, 570 m a.s.l.) with a continental climate with some oceanic influence (Poly, 1987); (3) a Southern Mediterranean (SM) site located in Les Milles (43°30′13′′N, 5°23′08′′E, 120 m a.s.l.) with Mediterranean climate (see Table 1). The three gardens show slightly different sandy soils typical of forest soils in France: sandstone heavy acidic soils at NO, sandy acidic soils with high humus content at ME and silty sand soils at SM.

(i) Do seedling’s growth and survival correlate with current and/or future SDM-derived habitat suitability, and especially, are survival and growth higher at a species’ leading edge than at its trailing edge? (ii) Does genetic differentiation lead to differential recruitment success when transferred at trailing and leading edges? (iii) How much is seedling performance dependent on climatic conditions during recruitment? 2. Material and methods 2.1. Study species

2.3. Hypotheses building

The study focused on the main forest tree species encountered in Europe: silver fir Abies alba Mill., sweet chestnut Castanea sativa Mill., European beech Fagus sylvatica L., European ash Fraxinus excelsior L., European larch Larix decidua Mill., Norway spruce Picea abies L., Aleppo pine Pinus halepensis Miller, Scots pine Pinus sylvestris L., holm oak Quercus ilex L., and pedunculate oak Quercus robur L. For each species, we used three to four provenances from geographically distinct

Habitat suitability of the study sites was modelled at the European scale as in Duputié et al. (2013). To calibrate the models, we used occurrence data from the Atlas Flora Europaea, EuroVegMap, EUFORGEN, JRC and ICP datasets, downscaled or upscaled to the resolution of 10′ (Duputié et al., 2013). Five climatic variables were used 145

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slightly less than half-way between the 2000 and 2030′s projections, we expect to observe patterns in trees’ survival and growth to be more intermediate to what is expected for the 2031–2050 projections and for the 1981–2000 projections.

Table 1 Observed and modelled climatic characteristics at the three sites NO (Guéméné Penfao), ME (Peyrat le Chateau) and SM (Les Milles). The data comes from four different sources: the observed historical record from the closest meteorological station (less than 15 km away), i.e. Derval, Eymoutiers, and Aix en Provence respectively, “Obs 2007-2010”; observed conditions during the experiments, “Obs 1981-2000”; historical climatic conditions used in the SDMs available from CERFACS (Centre Européen de Recherche et de Formation Avancée en Calcul Scientifique), “CERFACS 1981-2000” & “CERFACS 20312050”; and the projected climatic conditions used in the projected SDMs available from CERFACS. Tmin: average daily minimum temperature over the year, Tmax: average daily maximum temperature, Tmean: average daily mean temperature. Site

Data source

Tmin (°C)

Tmax (°C)

Tmean (°C)

Annual rainfall (mm)

NO

Obs 1981-2000 CERFACS 19812000 Obs 2007-2010 CERFACS 20312050 Obs 1981-2000 CERFACS 19812000 Obs 2007-2010 CERFACS 20312050 Obs 1981-2000 CERFACS 19812000 Obs 2007-2010 CERFACS 20312050

7.2 8.3

16.7 15.8

12.0 12.1

768 780

6.2 9.7

17.5 17.6

11.9 13.6

795 722

7.3 7.3

15.1 14.2

11.2 10.8

1182 1191

6.3 8.9

14.7 16.5

10.5 12.7

1080 1071

8.3 8.2

20.2 18.8

14.3 13.5

586 600

6.9 9.3

20.5 20.4

13.7 14.8

629 521

ME

SM

2.4. Experimental set-up Seeds from each provenance and species were obtained in 2006 and 2007 through the Tree Seed Center of the French National Forest Office and from the Research Unit on Mediterranean Forest, INRA Avignon. In spring 2007, seeds were sown in plastic pots in greenhouses or sheds at ME and SM, and treated with minimum fertilization (12 fertilizations using Hakaphos® Blau 15-10-15 throughout the growing season) and fungicide (one application using Proplant™ - Propamocarb hydrochloride, 7 mL m−2). In November 2007, the 8 months old seedlings were randomly distributed among the three common gardens where they were transplanted to the final density of 100 individuals per square meter. A mulch of bark chips was laid around each individual seedling to prevent competition with herbaceous species and mimic a forest litter layer. This first experiment (called hereafter 2007 seedling experiment) allowed us to evaluate the effect of climatic conditions on the performance of established seedlings in their second and third year. To get closer to the natural regeneration conditions, evaluate the effect of climatic conditions on germination rate and seedlings performance during their first year, as well as replicate the experiment with different climatic conditions during recruitment, a second experiment (called hereafter 2008 seed experiment) was conducted. Another lot of seeds were sown directly into the ground in the three gardens during spring 2008, with the number of seed for each seedling position determined by the horticultural optimum for each provenance and species, linked to germination potential. Extra individuals were later on removed to keep one individual per seedling position in each plot. Sowing density was at 100 individuals per square meter for deciduous species and 400 individuals per square meter for coniferous species. At NO and ME gardens, they were sown under a Scots pine stand, whereas in SM garden they were sown under a shelter mimicking shade produced by Scots pine (reducing incoming light by 30%). Seeds were watered until germination, and thinned to the same final density as in the 2007 seedling experiment. Climate effect on germination rate is thus mainly restricted to temperature effect. The 2007 seedling experiment included all species but silver fir (Abies alba), and a few provenances differed between the two experiments due to seed shortage (see Supplementary Material Table SA.1 for a map of the provenances used). Castanea sativa was removed from further analysis in the 2008 seed experiment as only 4 individuals were obtained. Each common garden received 96 individual seedlings of each provenance for the three repetitions (32 seedlings per repetition). Some species x provenance combinations had fewer seedlings (76–94) due to a shortage of viable seedlings at thinning. A split-plot design was used in order to prevent competition among species with three repetitions. Each repetition was divided into two units, one with deciduous species and the other with coniferous species. Each plot in each unit of each repetition contained seedlings from one provenance of one species; plots were randomly assigned a position within each unit. Each species x provenance plot was composed of (at most) 22 border unmeasured seedlings and 20 measured seedlings. Individuals from both experiments were observed from spring 2008 to the end of autumn 2009 in the three sites.

to build correlative distribution models: minimal mean temperature of the coldest month (°C); mean yearly growing-day-degrees above 5 °C (°C); mean total yearly amount of precipitation (mm); seasonality of precipitation as expressed by the coefficient of variation of precipitation across the four trimesters (dimensionless); and an index of summer drought, the 20-year mean value of the minimum value of the monthly Thornthwaite drought index each year (usually in September). Models were calibrated using these climatic variables averaged over 1951–1970 derived from the CRU TS 1.2 dataset. Models were then projected for 1981–2000 and 2031–2050 (using environmental variables derived from the CRU TS 1.2 and TYN SC 1.0 dataset (Mitchell et al., 2004) with the HadCM3 general circulation model and under SRES A1Fi – equivalent to RCP 8.5 – Rogelj et al., 2012; IPCC, 2013). SDMs were calibrated using the Biomod library in R (Thuiller et al., 2009), using four algorithms: Artificial Neural Networks (ANN), Classification Tree Analysis (CTA), Flexible Discriminant Analysis (FDA) and Generalised Linear Models (GLM). Pseudo-absences were generated where the species was not deemed as “present”. We generated as many pseudo-absences as there were “presence” records. Models were calibrated three times using a random set of 70% of the available presences and pseudo-absences, and evaluated against the remaining 30% of the dataset, using the Area Under the receiver operating Curve criterion (AUC, Swets, 1988) (Supplementary Material Table SB.2). This procedure was repeated three times, to provide threefold internal cross-validation. Binary model projections were then weighted according to the models’ AUC and averaged to yield an “ensemble model” for each species at the European scale, leading to the habitat suitability values shown here. Maps of the projected European distribution of the ten studied species are shown in Supplementary Material Table SB.1. The study focuses on the projected distribution at the scale of a single country, France, as shown in Table 2. Projected suitability for each site, species and period was retrieved from the European models for France. Comparison of this metric among sites for the current and future climatic conditions allowed us to formulate hypotheses on changes in species’ performance and to qualify sites as being within or outside the species’ potential distribution, at its leading edge or at its trailing edge in France. As this study took place

2.5. Measurements Seedlings height and stem collar diameter were measured two (in the 2008 seed experiment) to three times (in the 2007 seedling experiment) during the course of the experiments using a graduated Stanley® tape and a caliper with 1/10 mm precision respectively. 146

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Table 2 Observed and modelled distribution of the ten studied species over France. Observed distributions are based on observations from four databases (EuroVegMap, EUFORGEN, AFE, JRC). On simulated habitat suitability maps, warmer colors indicate a higher probability of occurrence. Hypotheses on species projected performances at each experimental site are based on these projections (∼: difference < 0.05; < or > : difference below 0.3, ≪ or ≫: difference above 0.3 in the suitability measurement). The black triangles on the maps represent each experimental site – see Fig. 1 for more details on the location of the three sites, and Supplementary Material Table SB.1 for distributions at the scale of Europe. Spatial projection: in the Lambert 93. Castanea sativa was classified here in the Mediterranean/Southern species as it originates from southern Europe, although it is now planted throughout France. Species

Observed distribution

Modelled suitability 1981-2000

Modelled suitability A1Fi 2031-2050

Mediterranean/Southern species Castanea sativa

Projected change

All sites remain in the range, with lower performance expected in SM (trailing edge)

ME~NO>SM

ME~NO>>SM NO and ME are at the leading edge and show increased performance. SM remains in the range with high performance

Pinus halepensis

SM>NO>>ME

SM>NO>ME

Quercus ilex

All sites are in the range with higher performance (SM within the range, NO and ME at the leading edge)

SM>NO~ME

SM~NO~ME

Cold-adapted species Abies alba

Decreased probability of presence at ME (trailing edge) but increased at NO (leading edge) and no change at SM (outside range)

ME>>NO>SM

NO>ME>SM (continued on next page)

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Table 2 (continued) Species

Observed distribution

Modelled suitability 1981-2000

Modelled suitability A1Fi 2031-2050

Projected change

NO and ME are at the trailing edge; SM remains outside the range, small changes between 2000 and 2050

Larix decidua

ME>>NO>SM

ME>>NO>SM

Picea abies

ME is at a trailing edge and shows decreased performance, and SM and NO remain outside the range with very low performance

ME~NO~SM

ME~NO~SM No change at NO and SM (outside the range) and decreasing performance at ME (trailing edge)

Pinus sylvestris

ME>NO~SM

ME~NO~SM

Cosmopolitan species Fagus sylvatica

Decreased performance expected at NO and ME (within range), low performance at SM (outside range)

ME~NO>>SM

ME>NO>>SM (continued on next page)

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Table 2 (continued) Species

Observed distribution

Modelled suitability 1981-2000

Modelled suitability A1Fi 2031-2050

Projected change

Marked decreased performance expected at NO and SM (trailing edge); ME remains within the range

Fraxinus excelsior

ME~NO>>SM

ME>NO>>SM

Quercus robur

Marked decreased performance at NO (trailing edge) whereas ME remains inside and SM outside of the range

ME>NO>>SM

ME>NO>>SM effects models from the binomial family with zero-mean normal priors for fixed effects were used for each species with the bglmer function from the blme package to incorporate repetition as a random effect. Site, provenance and the interaction between the two were fixed effects and their significance assessed. We additionally tested whether individuals died preferentially at a particular season, using the 2007 seedling experiment at the NO and SM sites and similar Bayesian generalized linear mixed-effects models. Season, site and their interaction were set as fixed effects and their significance assessed. Repetition was added as a random effect. We also tested the effects of site, provenance and their interaction for each species and sowing year separately on final growth (height and diameter) using linear mixed-effects models. All statistical analyses were performed with the R statistical software (R Development Core Team, 2015). For all analyses, the significance threshold was set at 0.05. Details on the different models used for the analyses can be found in the Supplementary Material (Section C).

Germination rate before the thinning step as well as survival of 1year old seedlings (i.e. survival once germinated to one year old after thinning) were assessed in the 2008 seed experiment, survival of 2-year old seedlings was assessed in both experiments (from first to second winter), and survival of 3-year old seedlings was assessed in the 2007 seedling experiment (from second to third winter). Seasonal survival was additionally assessed in the 2007 seedling experiment at the NO and SM sites. Because of a severe flooding at the SM site in 2008, all measurements at this site for the 2008 seed experiment had to be cancelled. 2.6. Statistical analysis Germination rate differences among sites for each species all provenances confounded were assessed with one-way Anova for the 2008 seed experiment. Seed lot was also tested using Student’s t-test to assess if germination levels observed in the field in the 2008 seed experiment were potentially due to seed lot quality when compared to germination in controlled conditions for the 2007 seedling experiment. For survival analysis, each species and each sowing year were considered separately. In the 2007 seedling experiment, we analyzed the survival of 2-year old seedlings (between spring 2008 and spring 2009) and 3-year old seedlings (between spring 2009 and fall 2009). In the 2008 seed experiment, we analyzed the survival of seedlings in their first year (between spring 2008 and spring 2009) and in their second year (between spring 2009 and fall 2009). Quasi-complete separation arose when analyzing survival between sites and provenances for each species. Quasi-complete separation occurs when the outcome variable separates a projector variable or a combination of projector variables to a certain degree, leading to the absence of a unique estimate of maximum likelihood for that projector variable which prevents the use of classic survival analysis methods. Bayesian generalized linear mixed-

3. Results 3.1. Germination rate Germination rate in the field varied greatly among species in the 2008 seed experiment. Percentages of germinated individuals in spring 2009 relative to the number of seeds sown are: P. halepensis: 95.2%, P. sylvestris: 80.8%, P. abies: 80%, F. excelsior: 71.7%, L. decidua 69.4%, Q. robur: 56.7%, A. alba: 32.4%, F. sylvatica: 22.8%, Q. ilex: 12.9%. Germination was significantly higher at the NO site for F. excelsior, P. abies, P halepensis (one-way Anova: NO > ME = SM p < .001, NO > ME p < .01 and NO > ME = SM p < .01 respectively) and higher at the ME site for Q. robur (one-way Anova: ME > NO > SM, 149

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Fig. 2. Survival of seedlings of each species (rows) and provenances (x-axis) in autumn 2009 in the 2007 seedling experiment (panel (a)) and the 2008 seed experiment (panel (b)) where each column represents a site for each experiment. D1: 1st year mortality; D2: 2nd year mortality, Dp2: mortality post 2nd year. See Figure SA1 for the location of the provenances. Note that (1) there is no 1st year mortality represented for the 2007 seedling experiment as seedlings were installed in their first year on the plots and spent most of their first year in nursery, (2) there is no 3rd year mortality represented for the 2008 seed experiment which stopped at the end of the 2nd year.

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3.4. Seedlings growth

p < .001). The germination rate was not significantly related to the seed lot provenance, as the germination of individuals coming from lots used in both 2007 and 2008 was not significantly different from the germination of individuals coming from different seed lots in 2007 and 2008 (Student’s t-test p = .361).

Final height from the 2007 seedling experiment measured in autumn 2009 showed different species-specific growth patterns across sites (Fig. 3). All species and provenances had the tallest seedlings at the NO site, with the exception of the southern Alps provenance of P. abies (Fig. 3, Supplementary Material Table SC.5 and Fig. SC.1). C. sativa, F. sylvatica and L. decidua showed a significant decrease in height from NO to SM with ME intermediate, whereas F. excelsior, P. halepensis and P. sylvestris showed a significant decrease in height from NO to ME with SM intermediate. The patterns for P. abies and Q. robur are more complex due to site x provenance interactions: the north eastern provenance overall was composed of the tallest individuals for P. abies whereas the southern provenance did best for Q. robur (Fig. 3, Supplementary Material Table SC.5 and Fig. SC.1). Seedlings sown in 2008 showed the same pattern as in the 2007 seedling experiment (taller seedlings at NO than at ME), except for A. alba (no trend) and Q. ilex (opposite pattern; Fig. 3, Supplementary Material Table SC.6). Looking at provenance effect, again the southern Alp provenance for P. abies showed smaller seedlings’ height at the NO site and ME site, as for the southern provenance of F. sylvatica at the ME site but the opposite was true for F. excelsior at the NO site where the southern provenance performed best compared to the Parisian basin provenance. The two southern provenances for P. sylvestris showed lowest performance compared to the other two northern ones at both sites. When comparing seedlings’ height after two growing seasons in the 2007 and 2008 seed experiments (at NO and ME sites), no significant difference was found between the two experiments (p = .39). Diameter showed similar results in both experiments and only Q. robur and P. sylvestris showed a significant interaction between provenance and site in both experiments (Supplementary Material Tables SC.7 and SC.8).

3.2. First year survival First-year survival among the germinated individuals in the 2008 seed experiment, i.e. survival from spring or summer 2008 (depending on the germination date) to spring 2009, was high for all species, with mortality varying from 1.5% (F. excelsior) to 26.8% (A. alba) all provenances and sites confounded (shown in light grey, Fig. 2). Seedlings survival differed among sites in two species (P. halepensis and Q. ilex) with higher mortality at the NO site compared to the ME site. The interaction between provenance and site was significant for A. alba with a higher mortality of the Massif Central provenance (401MC) at the ME site, as well as for P. abies with higher mortality of the Alpine provenance (506PAN) at the ME site (Fig. 2, Supplementary Material Table SC.1). 3.3. Seedlings survival up to their third year Seedlings from the 2007 seedling experiment survived significantly better up till autumn 2009 at ME and NO sites compared to SM site in 4 species out of 9 (C. sativa, F. sylvatica, L. decidua and P. abies), whereas seedlings from the 2008 seed experiment survived significantly better at NO than at ME for 6 species out of 9 (F. excelsior, P. abies, P. halepensis, P. sylvestris, Q. ilex, Q. robur) with no significant site x provenance interaction (shown in black, Fig. 2). Most of the post first-year mortality at the SM site occurred in 2009 whereas it occurred mostly in 2010 at the NO and ME sites in the 2007 experiment. In the 2008 seed experiment, mortality occurred in 2009 at the NO site and 2010 at the ME site (except for Abies alba) (Fig. 2). Provenance had a significant effect only for 4 species in both experiments (F. excelsior, P. abies and P. halepensis, and F. sylvatica in 2007 and P. sylvestris in 2008). In the 2007 seedling experiment, provenances from north (Parisian basin) and eastern locations (Jura-Vosges mountain ranges) showed a higher survival in general than other locations for species other than P. halepensis. In the 2008 seed experiments, the north eastern locations survived better than other locations. As for P. halepensis, the 700RM location, closest to the SM site fared better in the 2008 seed experiment but not in the 2007 seedling experiment (Fig. 2, Supplementary Material Tables SC.2 and SC.3). Mortality did not occur evenly along the year for some of the species at the NO and SM sites (no data on seasonal mortality was available for the ME site). Significantly more individuals died during the winter compared to the summer (C. sativa and P. halepensis at NO and SM sites, L. decidua and P. abies at SM site) (Table 3, Supplementary Material Table SC.4).

3.5. Is seedlings’ performance consistent with ongoing climate change? Over the 28 combinations of species and pair-sites comparison (10 species × 3 comparisons, with 2 combinations unavailable for Abies alba, Supplementary Material Table SC.9), 11 showed no consistency between the hypothesized performance based on the model’ suitability probabilities in the near future and seedlings observed performance (negative result) and they are the most numerous (5 combinations) for the NO-ME comparison. Another 11 combinations showed seedlings performance consistent with the hypothesized performance based on the model’ suitability probabilities (positive result) and they are the most numerous (5 combinations) for the ME-SM comparison. Other combinations showed either consistency of survival only or growth only with hypothesized near-future conditions (3 combinations), or consistency with seedlings performance in historical conditions only (3 combinations). Mediterranean/Southern species (P. halepensis, Q. ilex and C. sativa) are projected to widen their distribution range or shift it northward (Table 2). For these species, seedling’s growth and survival followed the expected patterns with climate change (Table 4): higher or equal survival and growth at the leading edge (the NO site as compared to the SM site located within the range for P. halepensis and Q. ilex), or lower performances at their trailing edge (SM site) than within their range (C. sativa). Cold-adapted species like P. sylvestris, P. abies, A. alba and L. decidua are projected to retreat upslope and northwards (Table 2). For these species, the overall very high survival observed was opposite of what was expected in regards to the modelled current and projected suitability for L. decidua, P. abies and P. sylvestris whereas it reflected the modelled expectations better in the case of Abies alba (Table 4). These results are dominated by a higher performance at the NO site especially in terms of growth, which may be related to a milder oceanic climate at this site during the growing season. Cosmopolitan species like F. sylvatica, Q. robur and F. excelsior are projected to retreat northwards, but

Table 3 Comparison of mortality rates between season during the 2007 seedling experiment at the SM and NO sites. Only significant relationships (p-value < .05) are summarized in this table. “ns”, “ > ”, “ < ”, - indicate respectively not significant, higher, lower and similar mortality rate during a particular season. Species

Season

C. sativa F. sylvatica F. excelsior L. decidua P. abies P. halepensis P. sylvestris Q. ilex Q. robur

Winter > spring – summer Winter > summer > spring ns Winter > spring – summer in SM Spring > summer in NO; winter > spring – summer in SM Winter > summer ns ns ns

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Fig. 3. Height measurements per species at the end of the experiment in autumn 2009. (a) 2007 seedling experiment (2.5 years old seedlings), (b) 2008 seed experiment (1.5 years old seedlings). Mean and standard error bars are represented for each species and site. When standard errors are too small, points may overlap the entirety of the error bar.

the more northern provenances of these species at the northern site NO (Supplementary Material Tables SC.6, SC.7 and SC.9, Fig. SC.1).

their performances are not hypothesized to change drastically at the three experimental sites, except for a decrease at NO for Q. robur and F. excelsior (Table 2). For these species, we observed an overall high survival at all sites without seeing a decrease in the seedlings performance at NO site. The NO site remained the best performing site in terms of growth (Table 4). Looking at the provenance level, southern provenances of L. decidua (2007 seedling experiment), P. abies and P. sylvestris (both experiments) had a lower growth rate than northernmost provenances at all sites. In contrast, the southern provenance of Q. robur (2007 seedling experiment) and F. excelsior (2008 seed experiment) outperformed in growth

4. Discussion Seedlings survival and growth of ten European tree species and 36 provenances showed only limited evidence of the projected northward and upward shift as a result of climate change in our study. More precisely, our results suggest climate change driven range shift is observable at a seedling stage only for Mediterranean species (i.e. P. halepensis, C. sativa and Q. ilex). Quercus ilex is the only species showing a 152

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Table 4 Species modelled suitability (current and projected) and observed performance at each of the three common gardens at the end of both the 2007 seedling and 2008 seed experiments. The first two columns present the suitability in current and projected (2050) conditions based on the results of the SDMs presented in Table 2, in percentage. The columns “Final height” and “Final survival” show the final seedlings’ height in the 2007 seedling experiment (the gardens ranking in the 2008 seed experiment is similar to the one in 2007) as well as the average survival rate of the seedlings of each species in both the 2007 seedling and the 2008 seed experiments. The grey shading represents the quartiles each number belongs to: 75–100%: dark grey, 50–75% medium grey, 25–50%: light grey and 0–25%: white. The “Conclusions” column recapitulates the conclusion drawn from our study compared to the expected change in suitability based on the results of the SDMs.

individual plant (Warren and Bradford, 2011) with often seedlings’ niche narrower than adults’ niche (Jackson et al., 2009; Stohlgren et al., 1998). Yet, SDMs try to capture the climatic niche of the species as a whole. This may lead SDMs to overestimate range widths and range shifts (Black and Bliss, 1980; Cavender-Bares and Bazzaz, 2000), with minor climatic changes impacting seedlings in a potentially severe way (Lloret et al., 2004). Second, these differences could arise from a climate sampling effect during the experiments. This is illustrated in Table 1 showing climatic conditions used for the SDM simulations and climatic conditions during the experiments. Noticeably, at the NO and SM sites, average temperature and rainfall during the experiments were similar to those of the historical period but maximum temperature was similar to that of the future period; and at the ME site, temperatures were similar to the historical period but rainfall was similar to the future period. This is also illustrated by the contrasted results obtained between the 2007 and 2008 seed experiments at the NO and ME sites in 43% of the cases (for survival and growth), and the contrasted results in survival rate between years and sites more generally. Adult trees have potentially resisted to some rare extreme events that occurred after their establishment that may not be sampled in the time course of an experiment or a monitoring. Extreme events are likely to play an important role in species’ range future dynamics, with high mortality induced by extreme events accelerating either dieback at the trailing edges or colonization of competing species at their leading edges.

clear validation of a latitudinal shift in survival and growth consistent with northward expansion of the species climate envelope (2007 seedling experiment). Though seedlings performance of Pinus halepensis does not agree with the detailed projections, we observed high survival at the ME site and increased growth at the NO site in accordance with the expected northward expansion of this species. Our results also show a lower survival of seedlings at the southern site (SM) in half (F. excelsior, P. sylvestris, Q. robur) of the species projected to retreat from southern France (F. sylvatica, F. excelsior, L. decidua, P. sylvestris, P. abies, Q. robur). Overall, growth was the highest for all species in both experiments at the NO site which has the least limiting climate for growth. Effects of genetic differentiation among geographical provenances on growth and survival did not reveal clear patterns. 4.1. Discrepancies between SDM projections and seedling performances Several hypotheses can explain why seedlings performance differs from projected changes in species probability of occurrence, setting aside the well-known sources of uncertainty in SDM projections (Hargreaves and Annan, 2014; Rocchini et al., 2011). First, seedlings and adult trees are likely to have different climatic niches. For example, a shift in nutrients requirement over ontogeny may lead to a shift in the realized niche as an individual grows from seedling to a tree (Bertrand et al., 2011). Abiotic conditions and biotic interactions also play an important role in shaping the niche over the different life stages of an 153

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In terms of growth performance however, provenances differed in our study. Populations from the trailing edges of receding species such as L. decidua, P. abies and P. sylvestris performed poorly in terms of growth compared to other populations, even when transplanted to higher latitude sites. Such differences in growth between provenances might affect survival later on.

4.2. Importance of meteorological conditions during the highest risk transition phases in tree regeneration The comparison between the 2007 and 2008 seed experiments shows that a large proportion of individuals do not germinate in a natural setting whereas they might in optimal greenhouse conditions. This result has to be related to the common bottlenecks to seedling establishment: the fraction of seeds that germinate (Clark et al., 2007; Koller et al., 1962) and the fraction of seedlings that survive (James et al., 2011). Note that seed availability does not play a role in our study as a fixed number of seeds were hand sowed. However, seed sources were different between the two experiments for some of the species due to logistic constraints. We might have expected that a lower seed quality for some the species was responsible for the higher proportion of non-germinated individuals and the higher mortality in the 2008 seed experiment. However, our results show this is not the case. The low survival rate during the 2008 seed experiment could be explained by differences in species germination requirement. Environmental and microsite conditions during recruitment are critical to establishment success (estimated here with survival in autumn 2009 and final growth). It is often observed for example that germination rate can be very low as a result of quick topsoil drying during brief periods of drought, or of fungus contamination, herbivory and other environmental stresses when sowing seeds directly on the ground (Ammer et al., 2002; Baskin and Baskin, 1988). However, the climatic conditions in 2008 were considered as normal in France in terms of precipitation and temperature with no extreme events. Microsite effects were minimized by the randomization of provenance x species plots in each repetition. A non-measured variable or stochastic factor inherent to the germination process and maybe linked to dormancy mechanisms (Dalling et al., 2011; Walck et al., 2011) may thus have been the cause of the low germination in the 2008 seed experiment. Germination is the highest-risk transition stage during recruitment and probably the least projectable. It is therefore crucial to take germination success into account when attempting to project the future distribution of plant species in response to climate change. Although process-based models of germination and seedling emergence do exist since the 1990 s (see for review Forcella et al., 2000), most of them were developed for weeds and still have not been integrated into SDMs (but see Forcella et al., 2000; Manso et al., 2013; Midmore et al., 2015). Incorporating such models into process-based SDMs would be a first step toward taking into account the impact of climate change on the early life stages (Mouillot et al., 2001).

4.4. Limits of this study and recommendations This study aimed to assess experimentally whether trees seedlings’ potential distribution is already expanding in response to climate change using three common gardens representing an array of trailing and leading edges for ten European tree species. The underlying assumptions of our study were that the climatic conditions experienced during the experiments would be intermediate between historical and near future climatic conditions. However, during the course of the experiment at our three sites, climatic conditions were closer to historical than to the projected near future conditions for some variables and not always the same ones depending on the site (Table 1). This may explain why experimental result confirmed only partly SDMs projections (Table 4). Additionally, future studies should consider extracting climate predictions from an average of accepted global circulations models instead of using a unique model as done in our study, thus accounting for variability between predictions and reducing bias linked to each model. The absence of extreme climatic events during the span of our experiments (apart from the flooding at the SM site) may also limit the conclusions of this study. The high planting density applied to the seedlings in this experiment (100 individuals per square meter for deciduous species and 400 individuals per square meter for coniferous species) is commonly used in common gardens for seedlings (Birot, 1972), however such high density is likely to have influenced our results, especially in the more stressful sites such as SM (Maestre et al., 2005), increasing the effects of competition on survival and performance at this site. Ideally, several additional repetitions of this experiment while controlling for competition between seedlings would be required to confirm our results. Finding evidence for climate driven changes in tree species distribution is difficult as it involves different life stages (seedlings, saplings and mature trees) that interact with each other (Collins and Carson, 2004) but that may be limited by different climate drivers and biotic factors (Sittaro et al., 2017; Wason and Dovciak, 2017). Planning an efficient forest management for the near future climatic conditions can thus be a challenge when model projections are still not transferable to field observations at the species level. Refining the projections and field experiments at the provenance level might be a solution to efficiently work towards a sustainable forest management for the near future. Our study provides partial validation of expected distribution change for the upcoming decades for ten European tree species, where Mediterranean species displayed a high performance at their northern leading edge and species expected to retreat from southern locations showed a lower survival (but this result may be due to a milder climate in the oceanic site at their leading edge). This means Mediterranean species perform well in locations where they are expected to migrate under warmer climates. At the seedling stage, cold-adapted and cosmopolitan species tend to perform well in locations at or beyond their trailing edge: hence this experiment needs to be complemented by competition studies to determine whether they will persist under warmer climates (which seem to be in their ecological niche), or be displaced by more warm-adapted competitors.

4.3. Genetic effects on seedlings’ performance Local adaptation of populations from the same species spurred the idea of assisted migration, where the effects of climate change could be mitigated on economically and culturally important plant species by implanting southern populations from the species trailing edge to more northern locations where the climate is expected to reach similar conditions as their original southern location (Vitt et al., 2010). In terms of survival, genetic differentiation seemed low in our experiment, with very few species displaying a provenance effect on their final survival and almost none a site x provenance interaction (with the exception of P. abies). We therefore cannot provide guidelines to forest managers towards the use of particular provenances for future reforestation. Using provenances from geographically more distant locations, i.e. at a continental scale might have yielded significant differentiation among provenances (Eilmann et al., 2013; Lu et al., 2014) though a few studies have found significant differentiation at smaller spatial scales (Taïbi et al., 2014). Some studies showed strong local adaptation along elevation gradients in physiological traits and growth in trees growing at the alpine tree line ecotone (Reinhardt et al., 2011; Vitasse et al., 2009) but not for survival, for which trends are present but not significant (Castanha et al., 2013).

Acknowledgments The authors are grateful to Patrice Brahic, Marie De Castro and Viviane Cogo for conducting the experiments at Les Milles, Hervé Le Bouler, Jean Pierre Huvelin, Michel Rondouin for conducting the 154

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experiments at Géméné Penfao; Sébastien Guérinet and François Montagnon for conducting the experiments at Peyrat Le Château, and Stéphanie Brachet for her help in the supervision of the experiments. They are also grateful to Bruno Fady (INRA Avignon) and Joel Conche (ONF) for seed supply; and to Christian Pagé and Laurent Terray from CERFACS Toulouse (URA 1875) for providing the CERFACS climatic data. This study was supported by the Biodiversity call of the French Agence Nationale de la Recherche 2005 within the QDiv project. We thank three anonymous referees for their useful comments that greatly improved the first version of this manuscript.

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