John Guittar
email: guittarj@msu.edu

Deborah Goldberg

Kari Klanderud

Astrid Berge

Marta Ramírez Boixaderas

Eric Meineri

Joachim Töpper

Vigdis Vandvik

Marta Ramírez Boixaderas

Quantifying the roles of seed dispersal, filtering, and climate on regional patterns of grassland biodiversity

Keywords: Alpine grasslands, climate change, community assembly, dispersal dynamics, range expansion, seedling survival, metacommunities, species sorting

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INTRODUCTION

Plant community assembly in a landscape or metacommunity context includes important roles for seed dispersal among communities and local filtering [START_REF] Leibold | The metacommunity concept: a framework for multi-scale community ecology[END_REF][START_REF] Alexander | Metapopulations and metacommunities: Combining spatial and temporal perspectives in plant ecology[END_REF]. Despite empirical evidence illustrating how dispersal and filtering can shape community membership on their own [START_REF] Choler | Facilitation and competition on gradients in alpine plant communities[END_REF][START_REF] Ehrlén | Long-term assessment of seed limitation in plants: Results from an 11-year experiment[END_REF][START_REF] Armas | A field test of the stress-gradient hypothesis along an aridity gradient[END_REF][START_REF] Laliberte | Environmental filtering explains variation in plant diversity along resource gradients[END_REF], there is a lack of data documenting how dispersal and filtering interact in natural plant communities. This lack of data represents a missing link between our theoretical understanding of how plant diversity patterns can form and how they actually form. Moreover, it hampers our ability to predict community responses to climate change, because different species likely have different dispersal abilities and/or different constraints to expansion, leading to variable responses [START_REF] Graae | Stay or go -how topographic complexity influences alpine plant population and community responses to climate change[END_REF].

The absence of a clear empirical picture of how dispersal and filtering interactively shape plant community membership and regional diversity patterns is due in large part to the logistical challenges of connecting dispersed seeds to individual plant performance. Plant propagules are often tiny, numerous, difficult to identify and track, capable of traveling great distances, and can remain dormant in the soil for years prior to germination [START_REF] Baskin | Seeds: ecology, biogeography, and, evolution of dormancy and germination[END_REF]Baskin 1998, Vandvik et al. 2016). Some researchers have tried to infer dispersal and filtering dynamics using indirect methods [START_REF] Alexander | Metapopulations and metacommunities: Combining spatial and temporal perspectives in plant ecology[END_REF]. The "nearest-neighbor" approach, for example, assumes connectivity in a metacommunity to be proportional to inter-patch distance [START_REF] Calabrese | A comparison-shopper's guide to connectivity metrics[END_REF]Fagan 2004, Jacobson and[START_REF] Jacobson | Quantifying and disentangling dispersal in metacommunities: How close have we come? How far is there to go?[END_REF]. Other approaches infer dispersal patterns from historical population range shifts over periods of climate change [START_REF] Kelly | Rapid shifts in plant distribution with recent climate change[END_REF]Goulden 2008, Bertrand et al. 2011) or by analyzing population genetic structure. These indirect methods are useful when modeling species distributions phenomenologically, but fall far short of evaluating whether species fail to arrive or fail to persist after arrival [START_REF] Calabrese | A comparison-shopper's guide to connectivity metrics[END_REF]. The small number of studies that have directly assessed dispersal patterns, either by manually marking seeds (e.g., [START_REF] Xiao | Using seed-tagging methods for assessing postdispersal seed fate in rodent-dispersed trees[END_REF] or connecting seeds to parents using parentage analyses (e.g., [START_REF] Cain | Long-distance seed dispersal in plant populations[END_REF], are often conducted for single species and/or over short distances, and are of limited use when considering responses of dozens of species at regional scales [START_REF] Nathan | Spatial patterns of seed dispersal, their determinants and consequences for recruitment[END_REF].

Here, we compare the seed rain, seed bank, seedlings, and adult plant communities at 12 sites falling along temperature and precipitation gradients in southern Norway to shed light on how seed dispersal and filtering interact in situ to shape community membership and maintain regional climate-associated vegetation patterns. Although our questions align with those of metacommunity theory [START_REF] Leibold | The metacommunity concept: a framework for multi-scale community ecology[END_REF]), our sites exist within a broad and contiguous patchwork of grassland habitat, making our system unsuitable to explicitly empirically evaluate between-patch dispersal dynamics, and therefore motivating us to develop our own approach. Specifically, we divide species at each site into two groups: "locally-transient" species, which occur as seeds but are rare or absent as adults (i.e., they arrive from outside the site but are filtered out), and "locally-persistent" species, which occur consistently as adults in annual vegetation surveys. We use adult vegetation surveys to infer which species dispersed outside of their realized climate niches (i.e., were found outside of the climate ranges where we found them to persist as adults), and how their realized climate niches compared to local climate conditions (i.e., if they have dispersed into warmer/wetter/cooler/drier climates). We then ask how and why locally-transient species were selectively disfavored during community assembly at each site, and how this process varied by climate. Our framework recalls the core-satellite framework of [START_REF] Hanski | The Core and Satellite Species Hypothesis[END_REF], but differs in that we allow species status to vary by site, enabling us to examine how species performance varies by environment.

Our study takes place in a network of 12 alpine and subalpine grassland sites in southern Norway, a region with unusually high spatial climate variability. The sites were selected according to their mean summer temperatures and mean annual rainfalls such that they form an orthogonal climate grid (Fig. 1), facilitating independent assessment of these two important climate drivers in community assembly. Prior work in our system used subsets of the data used in this study to compare diversity patterns in the seed bank and mature vegetation [START_REF] Vandvik | Seed banks are biodiversity reservoirs: Species-area relationships above versus below ground[END_REF][START_REF] Meineri | Functional traits, not productivity, predict alpine plant community openness to seedling recruitment under climatic warming[END_REF], to understand how trait-based community composition varies with climate [START_REF] Guittar | Can trait patterns along gradients predict plant community responses to climate change?[END_REF], and to evaluate the relative balance of competition and facilitation in seedling recruitment [START_REF] Klanderud | Biotic interaction effects on seedling recruitment along bioclimatic gradients: testing the stress-gradient hypothesis[END_REF]. We combine these previously published data with new data on seed rain and seedling survival to ask how dispersal and filtering interactively govern community membership and regional vegetation patterns. After dividing species into locally-transient and locally-persistent groups at each site, we ask the following questions:

1. How many seeds and species arrive to a site but fail to establish locally-persistent adult populations, and how does this vary with climate?

2. At what life stages are locally-transient species disfavored? Are they less likely to emerge as seedlings, establish as seedlings, compete as adults, or a combination of the three?

3. Why do locally-transient species fail to persist? Is it because they dispersed from outside of their realized climate niches and are disfavored due to their climate preferences? Is it because they differ from locally-persistent species in their functional traits, offering mechanistic hypotheses for their arrival and selective removal?

4. What do our results mean for how these grasslands will respond to climate change?

MATERIALS AND METHODS

Study area. The study area comprises 12 semi-natural calcareous grassland sites in southern

Norway that host at least 144 non-woody vascular plant species at the adult life stage, and at least 126 at the seed stage (Appendix S1: Table S1). Sites have similar bedrock, land use histories, slopes of approximately 20°, and southwest aspects, but differ in mean summer temperature, defined as the mean temperature of the four warmest months per year, and/or mean annual precipitation, such that they form a grid with approximately orthogonal climate axes (Fig. 1). Interannual variation in mean summer temperature and annual precipitation at each site over the sampling period (2008 -2013) was modest, with standard deviations of 0.79 °C and 238 mm, respectively, on average across sites. Each site has five blocks, placed in representative areas of grassland vegetation within a 75 -200 m 2 area, with individual positions that vary according to local topography and accessibility. Each block has a variable number of 25 x 25 cm plots used for the data sets described below, as well as other experiments and surveys. Blocks were protected from grazers with electric fences and manually mowed once a year to evenly simulate biomass loss due to grazing. Taxonomic identifications follow [START_REF] Lid | Norsk flora[END_REF]. Woody species and bryophytes were not recorded in all datasets and therefore were excluded from analyses.

Seed rain data. We collected seed rain over two periods to target winter (September 2009 to June 2010) and summer (June 2010 to September 2010) seed deposition. We trapped seeds in 25

x 25 cm artificial turfs placed in gaps about 50 cm from seedling monitoring gaps (see below) in four blocks at each site, for a total of 48 traps across all sites. The small synthetic filaments in artificial turfs effectively catch and retain small particles like seeds. Turfs were gathered and flushed with water to free collected seeds. The rinse water was passed through 500 µm and 125 µm diameter sieves to discriminate seeds by size and remove debris. Seeds were counted and identified taxonomically using a stereomicroscope. We included fruits, bulbils, and viviparous seeds in our working definition of "seeds." Rates of seed predation on artificial turfs are likely similar to those on soil, thus seed predation should not bias our results.

Seed bank data. To characterize seed bank diversity we haphazardly selected one 64 x 64 cm plot at each site and excavated soil to a depth of 3 cm in October 2008. Because the seed bank survey area at each site (0.41 m 2 ) was larger than the total areas used for seed rain and seedling surveys at each site (four 25 x 25 cm plots, 0.25 m 2 ), we randomly subsampled 61 % (0.25 m 2 / 0.41 m 2 ) of the original seed bank community in silico and discarded the remainder to standardize sampling effort across plant life stages. To exclude the seeds from that year's seed rain from our seed bank surveys, we removed the aboveground vegetation, including moss and litter. Soil samples were stored for three months at 2 -4 °C and ambient moisture. Soil samples were then sowed into a standard mixture of sterile subsoil and placed in 30 x 60 cm trays. The trays were incubated in a greenhouse with a diurnal cycle of 16 hours light (25 °C) and 8 hours darkness (15 °C). The diurnal cycle was continued for four months, followed by six months of cold stratification in darkness (4 °C), followed by another four-month period of diurnal cycling.

Emerging seedlings were counted and removed once identifiable to species. This method of characterizing the seed bank community effectively accounts for seed viability because nonviable seeds would not have emerged as seedlings.

Seedling data. Like most perennial grasslands, seedling recruitment in our system is highly dependent on disturbances and occurs only rarely in intact vegetation due to strong competitive effects from adult plants [START_REF] Silvertown | Mapping the microenvironment for seed germination in the field[END_REF][START_REF] Eriksson | Seedling Dynamics and Life Histories in Clonal Plants[END_REF][START_REF] Bullock | Gap colonization as a source of grassland community change: effects of gap size and grazing on the rate and mode of colonization by different species[END_REF][START_REF] Vandvik | Gap dynamics in perennial subalpine grasslands: Trends and processes change during secondary succession[END_REF][START_REF] Berge | Seedbank, seedrain and seedling recruitment along climate gradients in southern Norway[END_REF], Klanderud et al. 2017). We therefore monitored seedlings in experimental gaps where they were relatively free from competitive effects of adult plants, but still exposed to environmental stress and other biotic interactions, such as herbivory, disease, and potential resource competition among seedlings. One 25 x 25 cm gap was created in each of four blocks at each site in spring 2009, for a total of four gaps per site and 48 gaps overall. The gaps were made by cutting along the inner edges of a metal frame mounted in metal pipes marking the corners of the plot, and peeling away the natural vegetation and its thickly interwoven root mat.

Seeds and topsoil were returned to gaps by vigorously shaking excavated vegetation and passing it through a 4 mm sieve to remove plant remains. Emerged seedlings in the plots were ID-tagged in one of three censuses (late summer 2009, early summer 2010, late summer 2010) using numbered plastic toothpicks and plot coordinates. About 70 % of seedlings were identifiable to species; the remaining 30 % were unidentifiable or died before they could be identified and were lumped into two generic groups for graminoids and forbs. Seedlings were differentiated from emergent clonal ramets by looking for cotyledons or signs of above-or below-ground connections. Seedling survival and potential establishment were recorded twice yearly from spring 2010 to spring 2012. We approximated seedling emergence rates by dividing the density of emerged seedlings by the sum of seed rain and seed bank densities. We marked seedlings established when they had grown to a size greater than what could be derived exclusively from their maternal subsidies, which we estimated to be when stems were longer than 2 cm, and if a forb, also when they had grown their first non-cotyledonous leaves. Although we took pains to census each site at times of peak seedling emergence, some seedlings may have emerged, died, and disappeared before ever being recorded; even if this where the case, however, we contend that it is unlikely to influence our overall conclusions because the factors discouraging seedling survival during the earliest stages of establishment are likely the same as those discouraging seedling emergence (e.g., late spring frosts), and are distinct from the factors influencing seedling survival establishment (e.g., drought, competition for light, predation, early fall frosts, and late spring frosts in the year after seedling emergence).

Mature vegetation data.

We surveyed mature vegetation at peak biomass (July or August) in 2009, 2011, 2012, and 2013. At each site, we used two 25 x 25 cm plots in each of five blocks at each site, for a total of 10 plots per site (except for the third-wettest second-warmest site, which only had nine plots), and 119 plots overall. These plots were controls for a turf transplant experiment and included five undisturbed controls and five transplant controls (i.e., turfs dug up and replaced in the same location). The two types of controls did not differ in species composition or any other aspect of community structure at any of the survey periods [START_REF] Guittar | Can trait patterns along gradients predict plant community responses to climate change?[END_REF]. We visually estimated the percent cover of each species in each plot using a 5 x 5 cm grid overlay, and then pooled the data by site. To ensure that site vegetation was not undergoing successional changes that could bias our conclusions, we used an NMDS ordination to confirm that site composition did not change substantively between years (Appendix S1: Fig. S1). Stagespecific species abundance data are provided as a supplementary file.

Trait data. We used four commonly used plant traits, and four traits related to clonal growth strategy, an important facet of competitive ability in perennial grasslands. Each trait has hypothesized associations to dispersal ability and/or the ability to compete for resources. Seed mass (mg), a reflection of species regeneration strategy (Kraft et al. 2008, Cornwell and[START_REF] Cornwell | Community assembly and shifts in plant trait distributions across an environmental gradient in coastal California[END_REF], was drawn from the Seed Information Database [START_REF] Royal | Seed information database (SID)[END_REF].

Maximum canopy height (m) data, which relates to both dispersal ability and light competition [START_REF] Westoby | A leaf-height-seed (LHS) plant ecology strategy scheme[END_REF][START_REF] Falster | Plant height and evolutionary games[END_REF][START_REF] D'andrea | Counting niches: Abundance-by-trait patterns reveal niche partitioning in a Neotropical forest[END_REF], were mined from [START_REF] Lid | Norsk flora[END_REF]. Leaf area (mm 2 ) and specific leaf area (SLA; m 2 /kg), two traits indicative of where species fall along a continuum of slow-to-fast resource use strategies [START_REF] Reich | From tropics to tundra: Global convergence in plant functioning[END_REF][START_REF] Ackerly | Convergence and correlations among leaf size and function in seed plants: A comparative test using independent contrasts[END_REF][START_REF] Reich | The world-wide "fast-slow" plant economics spectrum: A traits manifesto[END_REF], were estimated using a combination of field data [START_REF] Guittar | Can trait patterns along gradients predict plant community responses to climate change?[END_REF]) and data from the LEDA online trait database [START_REF] Kleyer | The LEDA Traitbase: a database of life-history traits of the Northwest European flora[END_REF]. Leaf area, SLA, maximum height, and seed mass values were log-transformed. Clonal traits included the number of offspring per parent per year ("0" = 1 offspring; "1" = ≥ 2 offspring), persistence of plantoffspring connections ("0" = < 2 years; "1" = ≥ 2 years), rate of lateral spread ("0" = ≤ 1 cm/year; "1" = > 1 cm/year), and number of buds per ramet (an integer score ranging from "1" = few buds either belowground or aboveground, to "8" = many buds both below and aboveground). Clonal attributes are thought to help plants integrate over spatially heterogeneous resources [START_REF] Eilts | Resource heterogeneity, soil fertility, and species diversity: effects of clonal species on plant communities[END_REF], recover from disturbances [START_REF] Klimešová | Bud banks and their role in vegetative regeneration -A literature review and proposal for simple classification and assessment[END_REF], and provide sustained maternal subsidies to new ramets as they grow horizontally and vie for local establishment [START_REF] Herben | Community-level effects of plant traits in a grassland community examined by multispecies model of clonal plant growth[END_REF]. Clonal trait data were drawn from [START_REF] Klimeš | CLO-PLA2a database of clonal plants in central Europe[END_REF] and converted from categorical to quantitative formats to enable calculations of community means. Trait data are provided as a supplementary file.

Assigning local species status. Each species observed at each site at any life stage was labeled as "locally-persistent" if adults were recorded in more than half (i.e., at least three of four) of the site vegetation surveys conducted from 2009 to 2013, or otherwise labeled "locally-transient." In using this cutoff, we distinguish species that are consistent community members from those that are not. Because locally-transient/locally-persistent species status assignments were potentially sensitive to the depth at which we characterized local site community composition, we used rarefaction to ensure that we had sufficiently surveyed the mature vegetation such that the number of locally-persistent species observed at each site had stabilized (Appendix S1: Fig. S2).

We also re-ran our analysis with all possible locally-transient/locally-persistent cutoffs to assess the sensitivity of our conclusions to our methodology.

Assigning putative climate origins. For each locally-transient species at each site, we identified the sites and climates where we knew it had persistent adult populations, and then used these to infer whether it was dispersing outside of its realized climate niche. If a locally-transient species at a given site had persistent adult populations at other sites with similar temperatures and/or precipitations, we assumed it dispersed from a site with approximately the same temperature and/or the same precipitation, such as from a neighboring site with a similar climate but with potentially different topographical, edaphic, or biotic characteristics. If a locally-transient species at a given site had persistent adult populations only at sites with warmer/cooler or wetter/drier conditions, we then assumed it dispersed from a warmer/cooler or wetter/drier site (i.e., it dispersed into a cooler/warmer or drier/wetter site). Again, we are not assuming that a given seed of a locally-transient species literally dispersed from a given climate, but that the seed is occurring outside of the climate range at which it is a common and persistent community member, and is presumably on the cusp of being locally excluded.

Statistical approach. We used linear regressions to test for baseline trends in the total abundance and richness of locally-transient and locally-persistent species in the seed rain, the seed bank, and the two combined along temperature and precipitation gradients. Unlike other data in this study, seed bank data were not collected in replicate across blocks within a site, thus we performed regressions on data aggregated by site (N = 12). Each linear regression model assumes normally distributed errors and takes the form of 𝑦 𝑗 ~𝑀𝐴𝑃 𝑗 or 𝑦 𝑗 ~𝑀𝑆𝑇 𝑗 , where 𝑦 𝑗 is the response variable being examined at site 𝑗, and 𝑀𝐴𝑃 𝑗 and 𝑀𝑆𝑇 𝑗 are the mean annual precipitation (centered to zero) and mean summer temperature (centered to zero) at site 𝑗. We also used linear regressions to test for baseline trends in species richness in the mature vegetation with temperature and precipitation.

We used four sets of generalized linear models (GLMs) to test potential predictors of species performance during seedling emergence and seedling establishment. The dependent variable for seedling emergence GLMs was the number of emerged seedlings of each species at each site (N = 692, the number of unique seed and/or seedling species-by-site combinations), and the dependent variable for seedling establishment GLMs was the number of established seedlings of each species at each site (N = 279, the number of unique seedling species-by-site combinations). The GLMs used negative binomial error distributions and log link functions.

Because each site has a unique combination of temperature and precipitation values (i.e., there is no nestedness), it was not appropriate to include site as a random effect while also testing for the effects of climate. When using seed and seedling numbers as predictors of emergence and establishment, respectively, we normalized their highly skewed abundance distributions with Yeo-Johnson transformations, which are similar to Box-Cox transformations but can be used with zeros. The lambda values used for the Yeo-Johnson transformations were those which maximized normality, as quantified by Shapiro-Wilk normality tests; specifically, the lambda used to transform seed abundance data was -0.204 and the lambda used to transform emerged seedling data was -0.35.

In the first, baseline set of GLMs ("null" models), local species abundances in the combined seed rain and seed bank were used as predictors of local species abundances of emerged seedlings, and local species abundances of emerged seedlings were used as predictors of local species abundances of established seedlings. Formally, we modeled the number of emerged seedlings 𝑔 for species 𝑖 at site 𝑗 as 𝑔 𝑖𝑗 ~𝑠_𝑡𝑟𝑎𝑛𝑠 𝑖𝑗 , where 𝑠_𝑡𝑟𝑎𝑛𝑠 𝑖𝑗 is the Yeo-Johnson transformed abundance of seeds (seed rain + seed bank) of species 𝑖 at site 𝑗. In other words, our null expectation was that all seeds were equally likely to emerge, and all seedlings were equally likely to establish. In the second set of GLMs ("site climate" models), we added model terms 𝑀𝐴𝑃 𝑗 and 𝑀𝑆𝑇 𝑗 for local site mean summer temperature and mean annual precipitation at site 𝑗 to evaluate how these climate variables improved predictions of seedling emergence and seedling establishment rates.

In the third set of GLMs ("site climate + species status" models), we evaluated how well locally-transient/locally-persistent species status predicted performance differences during seedling emergence and seedling establishment. Specifically, we added model terms specifying the status of each species at each site, and interactions between species status and local climate.

Formally, this is

𝑔 𝑖𝑗 ~𝑠_𝑡𝑟𝑎𝑛𝑠 𝑖𝑗 + 𝑀𝐴𝑃 𝑗 + 𝑀𝑆𝑇 𝑗 + 𝑝 𝑖𝑗 + (𝑀𝐴𝑃 𝑗 • 𝑝 𝑖𝑗 ) + (𝑀𝑆𝑇 𝑗 • 𝑝 𝑖𝑗 ),
where 𝑝 𝑖𝑗 is the factor indicating local species status, and the other variables as described above.

Likewise, we modeled the number of established seedlings 𝑒 for species 𝑖 at site 𝑗 as

𝑒 𝑖𝑗 ~𝑔_𝑡𝑟𝑎𝑛𝑠 𝑖𝑗 + 𝑀𝐴𝑃 𝑗 + 𝑀𝑆𝑇 𝑗 + 𝑝 𝑖𝑗 + (𝑀𝐴𝑃 𝑗 • 𝑝 𝑖𝑗 ) + (𝑀𝑆𝑇 𝑗 • 𝑝 𝑖𝑗 ),
where 𝑔_𝑡𝑟𝑎𝑛𝑠 𝑖𝑗 is the Yeo-Johnson transformed abundance of emerged seedlings of species 𝑖 at site 𝑗, and the other variables are as described above.

In the fourth and final set of GLMs ("site climate + species status + species climate origin" models), we dropped 𝑝 𝑖𝑗 , the model term for locally-transient/locally-persistent species status, and replaced it with 𝑜 𝑖𝑗 , a new term denoting the putative temperature/precipitation origins of each locally-transient species 𝑖 at site 𝑗. Specifically, the 𝑜 𝑖𝑗 term tags each species at each site with one of five labels: (1) locally-persistent, (2) locally-transient but likely dispersed from an adjacent site with similar temperature/precipitation, (3) locally-transient and likely dispersed from a cooler/drier site (i.e., into a warmer/wetter site), (4) locally-transient and likely dispersed from a warmer/wetter site (i.e., into a cooler/drier site), (5) locally-transient with no locally-persistent populations at any of our grassland sites (i.e., an 'unknown' climate preference). We dropped the interaction terms from these GLMs to reduce excessive model complexity. Formally, we modelled the number of emerged seedlings g as 𝑔 𝑖𝑗 ~𝑠_𝑡𝑟𝑎𝑛𝑠 𝑖𝑗 + 𝑀𝐴𝑃 𝑗 + 𝑀𝑆𝑇 𝑗 + 𝑜 𝑖𝑗 , and the number of established seedlings e as 𝑒 𝑖𝑗 ~𝑔_𝑡𝑟𝑎𝑛𝑠 𝑖𝑗 + 𝑀𝐴𝑃 𝑗 + 𝑀𝑆𝑇 𝑗 + 𝑜 𝑖𝑗 , Finally, we asked if systematic differences in the traits of locally-transient and locallypersistent species at each site offered mechanistic explanations for performance differences between the two groups. To do this, we averaged the trait values of all species (not weighted by their relative abundances) in the combined seed rain and seed bank at each site, grouped by local-transient/locally-persistent species status. We used paired t-tests, paired by site, to identify which traits, if any, differed consistently between locally-transient and locally-persistent species across all sites. For the traits with significant differences between the two groups, we performed linear regressions to see whether the magnitude of the difference trended with site temperature or precipitation. We did not test for trait-based differences by species status in seedling communities because the number of locally-transient species was too low to provide confidence in calculations of within-site trait means (only 4 ± 3 locally-transient species occurred on average as emerged seedlings at each site; Appendix S1: Table S1). All scripts in this study were written in R.

RESULTS

Mature vegetation rarefactions.

Rarefactions indicated that the numbers of plots of mature vegetation surveyed at each site were more than sufficient to stabilize the compositions of locally-persistent species (Appendix S1: Fig. S2), lending confidence to our locallytransient/locally-persistent species status assignments. Furthermore, no locally-transient species by itself ever represented more than 0.4% of total cover at any site (Appendix S1: Fig. S3), illustrating the minor overall contribution of locally-transient species to local community structure.

Evidence of dispersal and filtering. Seeds of locally-transient species occurred at all 12 of our grassland sites, representing, on average, 4 of 42 species in the combined seed rain and seed bank. In the combined seed rain and seed bank, the number of locally-transient species and their total abundances increased significantly with temperature (Fig. 2; species richness: p = 0.045, R 2 = 0.34; total abundance: p = 0.016, R 2 = 0.46). These trends were driven primarily by increases of local-transients in the seed bank (species richness: p = 0.031, R 2 = 0.39; total abundance p = 0.035, R 2 = 0.37), not the seed rain (Appendix S1: Fig. S4), underscoring the fact that the composition of the seed rain likely differs between years, and that the seed bank likely serves as an important reservoir of local plant diversity. The strength at which locally-transient species were disfavored during seedling emergence did not change with temperature (Table 1; refer to the lack of significance for the transient*local temperature interaction term). There were no significant trends in species richness or total abundance with precipitation in either the seed rain, the seed bank, or the two combined. Species richness in the adult vegetation rose significantly with temperature (p = 0.0063, R 2 = 0.54) but not with precipitation (p = 0.871).

About 10% of all seeds were from locally-transient species, with more transients represented in the seed bank (ca. 14% of total seed bank) than in the seed rain (ca. 4% of total seed rain) (Appendix S1: Table S2). Not all locally-transient populations had locally-persistent adult populations at sites with similar climates (Appendix S1: Table S2), pointing to an exchange of seeds among climate zones. Operating on the assumption that locally-transient species dispersed from the most-climatically similar sites at which they persist as adults, seeds of locally-transient species were about four times more likely to have dispersed outside of their realized climate niches into warmer sites (i.e., from cooler climates) and slightly more likely to have dispersed outside of their realized climate niches into drier sites (i.e., from wetter climates) (Appendix S1: Table S2).

To determine how sensitive our conclusions were to changes in our operational definition of locally-transient/locally-persistent species status, we explored how results changed under each of the four possible cutoff scenarios offered by our data. That is, we looked at how results changed as the definition of locally-persistent species shifted from those species present in at least one, at least two, at least three, or all four annual surveys of mature vegetation at each given site. As the cutoff for locally-persistent became more stringent, and the cutoff for locallytransient (by definition) relaxed, the total number of locally-transient species-by-site combinations in the combined seed rain and seed bank rose from 119 (1989 seeds), to 149 (2549 seeds), to 167 (3665 seeds), to 205 (5007 seeds). However, these differences did not alter our main conclusions that (1) the realized effect of local filtering on community membership increases with temperature, and that (2) grassland sites in southern Norway are connected by dispersal, albeit primarily among sites with similar climates.

The stage-wise removal of locally-transient species. Locally-transient species were

outperformed by locally-persistent species during seedling emergence (Fig. 3, Table 1), but not seedling establishment (Fig. 3, Appendix S1: Table S3). The lower emergence rates of locallytransient species appeared to be driven primarily by the species that had putatively dispersed outside of their realized climate niches into cooler climates, drier climates, or those that had dispersed from unknown climates (see "Origin-based predictors" in Table 1). In addition, after accounting for local seed abundances, warmer sites tended to have higher rates of seedling emergence than cooler sites ("General predictors" in Table 1), linking climate and overall seedling performance. No GLM of seedling establishment outperformed null expectations (Appendix S1: Table S3), suggesting that neither climate, species status, nor putative seed origins were meaningful predictors of seedling establishment rates.

To confirm that these results were not artifacts of how we combined seed rain and seed bank data (e.g., if seeds of locally-transient species emerged at lower rates because most were from the seed bank, and the seed bank had overall lower rates of emergence), we re-ran GLMs with only seed rain data and observed qualitatively similar results (data not shown). To determine how sensitive our conclusions involving seedling performance were to changes in the operational definition of locally-transient/locally-persistent species status, we re-ran the GLMs for emergence and establishment using each of the four possible locally-transient/locallypersistent cutoff scenarios offered by our data. Changes to the locally-transient/locally-persistent cutoff did not alter our overall conclusion that locally-transient species are disfavored during emergence but not establishment (Appendix S1: Fig. S5).

Trait-based mechanisms of local filtering.

Locally-transient and locally-persistent species in the combined seed rain and seed bank differed significantly in three functional traits (Fig. 4). Specifically, when averaged across species at the site level, locally-transient species were significantly shorter, had smaller seeds, and had less persistent vegetative connections among ramets than locally-persistent species. Of the three traits that differed consistently between locally-transient and locally-persistent species, the only instance where those differences varied significantly with site climate was an increase in the magnitude by which locally-transient species had less persistent connections than locally-persistent species with increasing temperature.

DISCUSSION

In this study, we compared the composition of seed rain, seed bank, seedlings, and adult plant communities along temperature and precipitation gradients to shed light on how dispersal and filtering interactively shape community membership, and how this process varies across a landscape. Overall, our results point to four main conclusions: (1) the total numbers of seeds and species that arrived, but failed to establish locally-persistent populations, rose with temperature, indicating an increase in the realized effects of local filtering on community assembly, as well as an increase in the number of species poised to rapidly colonize those warmer sites if local conditions change in their favor, (2) locally-transient species were selectively filtered out during seedling emergence, but not during seedling establishment, (3) selective filtering was partly driven by species climate preferences, exemplified by the poor performance of seeds dispersing outside of their realized climate niches into foreign climates that were more stressful (colder and drier), and (4) locally-transient species had traits that may have made them better dispersers (i.e., smaller seeds) but poorer competitors for light (i.e., shorter statures and less persistent clonal connections) than locally-persistent species, potentially explaining why these species arrived to new sites but failed to establish locally-persistent adult populations. We elaborate on these conclusions below, and end with a discussion of what our results mean for how these alpine grasslands will likely respond to climate change.

Community assembly and the determinants of species' ranges

Community membership and the potential for compositional change over time are influenced by the numbers and identities of seeds dispersing, or not dispersing, into a given site. We found relatively few species to be dispersing outside of their realized climate niches, emphasizing the important limiting role that dispersal plays in shaping population distributions, at least on shorter time scales. At the same time, the seeds of the few species that did disperse outside of their realized climate niches were the ones most strongly disfavored by filtering processes (Table 1), highlighting the role that filtering also plays in shaping community composition in our system. These results empirically demonstrate how dispersal and filtering interactively drive community assembly.

Within the general detection of filtering across all communities and climates (Table 1), we observed the numbers of locally-transient seeds and species to increase from the coldest (highest altitude) to warmest (lowest altitude) sites (Fig. 2), suggesting that the realized effect of local filtering on community membership (i.e., the numbers of seeds and species that arrive but are filtered out) increases with temperature. Such a temperature-based trend could arise for at least three non-exclusive reasons. First, warm-adapted species could have spatially larger seed dispersal shadows than cold-adapted species, resulting in more seeds traveling to unsuitable patches, such as those with undesirable topographic, edaphic, or biotic conditions. Factors affecting seed dispersal distances include wind speed and plant height [START_REF] Thomson | Seed dispersal distance is more strongly correlated with plant height than with seed mass[END_REF], although neither are likely to be important in our experimental system: the colder, alpine sites are consistently windier than the warmer, lowland sites, due to their more exposed topography and less protective tree-cover, and the variation in mean maximum plant height among our sites is minor (< 30 cm) [START_REF] Guittar | Can trait patterns along gradients predict plant community responses to climate change?[END_REF]) compared to the differences in plant height known to affect dispersal distance [START_REF] Thomson | Seed dispersal distance is more strongly correlated with plant height than with seed mass[END_REF]. A second explanation could be that seeds are simply more likely to disperse downhill (i.e., from colder high-altitude sites to warmer low-altitude sites) than the reverse, again leading to more seeds arriving at unsuitably warm patches than unsuitably cold patches. A third explanation could be that competition for resources is fiercer at warmer temperatures, resulting in more local extirpations in the adult plant communities at warmer sites, although, importantly, this explanation presumes that similar numbers of seeds and species are dispersing to all sites, which may not be the case. Nevertheless, other studies have found support for such an explanation: competition for light is hypothesized to be stronger at warmer sites due to higher productivity, more developed soils, and overall less stressful growth conditions [START_REF] Grime | Competitive exclusion in herbaceous vegetation[END_REF][START_REF] Olsen | From facilitation to competition: Temperature-driven shift in dominant plant interactions affects population dynamics in seminatural grasslands[END_REF]). In our study, species richness fell steeply with temperature in the adult vegetation (from an average of 64 species at the coldest sites to 42 species at the warmest sites), but only slightly in the seed rain and not in the seed bank (Fig. S4), consistent with a scenario in which similar numbers of species arrive to all sites, but more species are competitively excluded by adulthood at warmer sites. Future work could track individuals after establishment but before mature adulthood to determine if the degree to which locally-transient taxa are disfavored increases with temperature, substantiating this hypothesis. Surprisingly, locally-transient species were disfavored more strongly during the first lifehistory transition, seedling emergence (Fig. 3, Table 1), than later, during seedling establishment (Fig. 3, Appendix S1: Table S3). This suggests that germination and early seedling survival are more important for success than seedling establishment, at least in our system. This result contradicts conventional expectations of weak filtering during seedling emergence but strong filtering during seedling establishment [START_REF] Moles | Seedling survival and seed size: A synthesis of the literature[END_REF]). Differential germination rates among species could be driven by species preferences for soil [START_REF] Evans | The effect of soil water potential on seed germination of some British plants[END_REF]Etherington 1990, Benvenuti 2003) or climate conditions. It should be noted that while some individuals could have been filtered out after meeting our operational definition of establishment (i.e., had stems at least 2 cm long and, if a forb, also had non-cotyledonous leaves), but before becoming reproductively active (i.e., before establishment sensu stricto), this does not appear to be the case. Across sites, the mean proportion of species that were classified as locally-transient among established seedlings was only slightly higher than that found in the adult vegetation data (~14.6 % vs. ~13.6 %, respectively), indicating that any locally-transient species which successfully emerged were essentially just as likely as locally-persistent species to survive long enough to be captured by our mature vegetation surveys, although these species were not present consistently enough to be classified locally-persistent. In other words, locally-transient species evidently failed to persist not because they failed to establish, but because they failed to arrive in high enough numbers, because their seeds failed to germinate, and/or because their post-establishment juveniles failed to reach sexual maturity. The lower emergence rates among locally-transient species were largely driven by seeds that had putatively dispersed outside their realized climate niches into cooler climates (i.e., from warmer climates), into drier climates (i.e., from wetter climates), and those from unknown climates (Table 1), illustrating how climate-based niche preferences can influence community membership and limit range expansion. Climate is known to play a pivotal role in the release of seed dormancy in plants [START_REF] Probert | The Role of Temperature in the Regulation of Seed Dormancy and Germination[END_REF], and variation in germination timing is known to occur even among populations of the same species at different climates [START_REF] Shimono | Intraspecific variations in seedling emergence and survival of Potentilla matsumurae (Rosaceae) between alpine fellfield and snowbed habitats[END_REF][START_REF] Bischoff | Seed provenance matters-effects on germination of four plant species used in ecological restoration[END_REF][START_REF] Spindelböck | Conditional cold avoidance drives between-population variation in germination behaviour in Calluna vulgaris[END_REF]. If the seeds of locally-transient species dispersing outside of their realized climate niches into colder and/or drier climates were predisposed to emerging before spring is safely underway, this could explain their particularly low emergence rates. A second, complementary explanation could be that seeds dispersing outside of their realized climate niches into colder and/or drier climates simply find these conditions more stressful and are thus less likely (i.e., not adapted) to successfully emerge as seedlings.

Locally-transient species differed consistently from locally-persistent species in their functional traits, offering some mechanistic hypotheses for why they dispersed into new sites but failed to persist as adults (Fig. 4). First, locally-transient species had consistently smaller seeds than locally-persistent species, which is thought to increase dispersal distance [START_REF] Greene | Seed mass and dispersal capacity in wind-dispersed diaspores[END_REF]Johnson 1993, Westoby 1998), and may also decrease performance upon arrival due to their smaller maternal subsidies [START_REF] Moles | Seed size and plant strategy across the whole life cycle[END_REF]. Second, locally-transient species were shorter in stature and had less long-lasting ramet-ramet connections, on average, than locallypersistent species, suggesting that the latter species' ability to grow both vertically and exchange resources horizontally confer important advantages for long-term survival in mature vegetation.

Maximum potential height and the capacity for clonal growth are associated with the ability to compete for light [START_REF] Falster | Plant height and evolutionary games[END_REF] and soil resources in spatially heterogenous systems [START_REF] Oborny | Survival and competition of clonal plant populations in spatially and temporally heterogeneous habitats[END_REF]Kun 2003, Eilts et al. 2011), respectively, and can work synergistically to confer competitive ability in herbaceous communities [START_REF] Gough | Incorporating clonal growth form clarifies the role of plant height in response to nitrogen addition[END_REF]). These two traits predicted competitive ability in a turf transplant experiment in the same system [START_REF] Guittar | Can trait patterns along gradients predict plant community responses to climate change?[END_REF], offering additional support for the importance of architectural traits in driving grassland species performance. Equally noteworthy were the generally weak correlations between locallytransient and locally-persistent species in the remaining functional traits. The weak correlations in these traits does not imply that they have no influence on the selective removal of locallytransient species, but that their influence may not be consistent or predictable across sites.

The trait-based differences between locally-transient species and locally-persistent species align with a general tradeoff between colonization and competitive ability [START_REF] Levins | Regional coexistence of species and competition between rare species[END_REF][START_REF] Tilman | Competition and biodiversity in spatially structured habitats[END_REF][START_REF] Amarasekare | Spatial heterogeneity, source-sink dynamics, and the local coexistence of competing species[END_REF][START_REF] Yu | The competition-colonization trade-off is dead; long live the[END_REF], and thus suggest that such a tradeoff, when combined with disturbance, is an important driver of succession and community assembly dynamics. That is, the smaller seeds of locally-transient species may promote their probability of arriving to fresh openings in the canopy, while their lower statures and decreased capacity for clonal growth may decrease their chances of establishing locally-persistent populations. The same trade-off underlies the core-satellite hypothesis [START_REF] Hanski | The Core and Satellite Species Hypothesis[END_REF], and likely operates alongside climate-based performance differences among species to shape regional diversity patterns. While our trait-based analysis offers no obvious mechanistic explanation for why locally-transient species were disfavored during seedling emergence, the trait-based differences may correlate with other unmeasured traits that do influence species performance during seedling emergence, and thus may serve as important proxies.

Implications for community responses to climate and climate change.

Community response to climate change will depend both on species' abilities to track environmental changes via dispersal, and on niche-based performance differences among species in different environments [START_REF] Graae | Stay or go -how topographic complexity influences alpine plant population and community responses to climate change[END_REF]. Southern Norway is expected to grow warmer and wetter as climate change progresses [START_REF] Hanssen-Bauer | Klima i Norge[END_REF], so species will have to disperse into cooler (upslope) and drier (more inland) sites to maintain their current climate conditions. However, of the 122 species with persistent adult populations at one or more of our sites, only 10 species (representing 0.3 % of total seeds) dispersed outside of their realized climate niches into cooler sites, and 13 species (representing 1.9 % of total seeds) into drier sites (Appendix S1: Fig. S4, Appendix S1: Table S2), suggesting that many if not most species will likely fail to rapidly shift their populations into cooler and drier climates, and are at risk of being extirpated as competitively-superior species arrive to their communities [START_REF] Alexander | Novel competitors shape species' responses to climate change[END_REF]). Yet, our finding that immigrant seeds from other climates were strongly disfavored provides a potential silver lining: if a species can disperse into a site with a newly suitable climate, it should be strongly favored over local residents, and thus be able to quickly establish a locally-persistent population. Overall, our results provide insight into the dynamic way in which communities will respond to climate change, and emphasize the need for further work on how species will vary in their ability to disperse and compete in different community contexts.

Table 1. Standardized coefficients (z-scores) from different GLM models (columns) predicting numbers of emerged seedlings by species and site. In column headers, "Sp. status" refers to whether the species is locally-transient or locally-persistent, and "Sp. pref. temp./precip." refers to the nearest temperatures/precipitations at which we found the species to have a persistent adult population, which we used to infer the climate from which they likely dispersed. The predictor "Seed no. (transformed)" refers to the numbers of seeds of each species at each site, normalized with Yeo-Johnson transformations (refer to Methods). Data consisted of all recorded seeds and seedlings that could be identified to species. N is equal to 692, the number of unique emerged seedling and/or seed speciesby-site combinations. Asterisks denote significance (*: p < 0.05, **: p < 0.001). Dashes denote predictors that were not included in a given model. Shapes are filled with color only when values differ significantly between locally-transient and locally-persistent species across all sites (paired t-test; p < 0.05). SLA: specific leaf area. Bud number is an integer score that ranges from 1 (few buds either belowground or aboveground) to 8 (many buds both below and aboveground); percent lateral spread refers to the proportion of species with rates of lateral spread greater than 1 cm/year; percent offspring refers to the proportion of species that commonly produce two or more vegetative offshoots per parent per year; percent connection persistence refers to the proportion of species with inter-ramet connections that persist for two or more years.

Figure 1 .

 1 Figure 1. Climates (a) and locations (b and c) of the twelve field sites in southern Norway. Mean

Figure 2 .

 2 Figure 2. Seed density (a) and species richness (b) in the combined seed rain and seed bank,

Figure 3 .

 3 Figure 3. Life-stage transition probabilities, grouped by locally-transient/locally-persistent

Figure 4 .

 4 Figure 4. Mean trait values of locally-transient species relative to locally-persistent species at

Acknowledgements. We thank the landowners for allowing us to set up the experiments on their land, and the Norwegian Research Council (KLIMAFORSK project 184912/S30), NSF GRFP (JLG; Grant Number: 1256260), and Norge-Amerika Foreningen for funding. We thank Siri L.

Olsen, Mari Jokerud, Pascale Michel, Hilary H. Birks and Berhe Luel for contributions to the data collection.