The contribution of the spatial hydrological niche to species diversity in rare plant communities of English floodplain meadows

The hydrological niche is one of the few below ground spatial environmental niches, which has been shown to structure English floodplain meadows and other European and African herbaceous ecosystems. However, both the relative contribution of hydrological heterogeneity to the structure of English floodplain meadows across spatial scales and the forms of the individual species’ responses to hydrological heterogeneity remain largely unknown. Here, we use a survey database of 2440 evaluation units sampled in 15 English floodplain meadows to dissect the spatial architecture of this metacommunity and describe the relationship between the abundance of individual species and hydrological heterogeneity. Of the tested species, 65% responded to spatial hydrological heterogeneity, with both monotonic and hump-shaped responses. We found that between-site beta-diversity is much stronger than within-site beta-diversity, with between-site scale hydrological variation explaining twice as much variation in community structure as within-site scale. This leads to the conclusion that a conservation strategy of rare plant communities should include not only the preservation of the diversity of local hydrological regimes, but also, specially, the inclusion in the conservation system of as many and environmentally varied local plant communities as possible.


Introduction
components at each spatial scale by means of permutation-based variation partitioning 122 (Borcard et al. 1992). Prior to analysis, abundance data were Hellinger-transformed  (Table S2 in Online Resource 1) were recorded, although only 50 of them 154 (26.4%) had relative frequency > 10%. Among these, 49% were 'graminoids' (Poaceae,155 Cyperaceae and Juncaceae) and 51% were 'forbs' (all other species). The forbs Ranunculus 156 acris, Cardamine pratensis and Rumex acetosa, and the grasses Anthoxantum odoratum, Beta-diversity component, the between-site -β2, is the largest observed component 171 (Fig. 2); moreover, it is significantly greater than expected by chance (Fig. 3). Within-site 172 diversity -β1 appeared to be smaller than β2. That shows the greatest source of plant diversity 173 variation occurs between, not within field sites. Besides, β1, the beta-diversity component at 174 the within-site scale (within field sites) is significantly smaller than expected by chance.

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The contribution of hydrological heterogeneity to the explanation of beta-176 diversity, in general terms, is c. 6% of multivariate variation in community structure ( hydrological descriptor for additive (GAMM) and linear (RDA) explanatory modeling 208 purposes has been based on judgment that considered statistical and pragmatic criteria. Thus, 209 and given co-linearity, preference has been given (i) to direct measurements on a continuous 210 scale (e.g. AWTD or mWTD) over derived measurements (e.g. EA or EB) or measurements 211 on a discrete scale (e.g. DF or DB), (ii) to comparatively symmetric descriptors (mWTD) 212 over comparatively skewed descriptors (e.g. DF or DA), and (iii) to descriptors lacking, also 213 comparatively, extreme values (Online recourse 1, Fig. S5). These criteria led us to judge that 214 the use of mWTD was the best descriptor among the available ones for explanatory purposes.

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Between-site beta-diversity component (β2) is spatially-structured. It is the largest diversity 216 component and, besides, it is (much) greater than expected by chance alone (Fig. 2-3). This 217 means that the greatest observed source of variation in meadow structure (61% of species) 218 occurs at this scale, i.e. among field sites (Crist et al. 2003;Summerville et al. 2003). On the 219 other hand, the largest contribution of hydrological heterogeneity to the explanation of beta-220 diversity also occurs at this scale, with hydrological heterogeneity at the between-site scale 221 contributing to the explanation of community structure twice that at the within-site scale 222 (Table 1). This confirms the insights from Figure 1. However, the amount of variability in 223 meadow structure explained by spatial hydrological heterogeneity at this scale is relatively 224 small.

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Most variability in between-site meadow structure was unexplained by spatial  Within-site beta-diversity component, β1 ( Fig. 2-3), is smaller than the between-site 239 beta-diversity component and, besides, it is significantly less (28% of species) than expected 240 just by chance (50% of species). The contribution of spatial hydrological heterogeneity to the 241 explanation of beta-diversity at the within-site scale is about half that at the between-site 242 scale, but still significant (Table 1).

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At this local spatial scale, besides the contribution of hydrological heterogeneity to   (Table 2) show that 65% of meadow species that were sufficiently frequent to be 253 tested, respond to hydrological heterogeneity in a way that is consistent both with Figure 1 254 and the environmental/spatial models summarised in Table 2   The modelling process here implemented via GAMMs (a flexible approach for 293 univariate regression) and RDA (a technique based on multivariate linear regression) has 294 tested, broadly speaking, the null hypothesis that the distribution of vascular plant species in 295 floodplain meadows is not related with hydrological heterogeneity that occurs in space. For 296 so doing, both species composition and hydrological heterogeneity have been expressed at 297 the level of evaluation units (quadrats), using, for the latter variable, a location measure (the 298 median) of a hydrological descriptor (Water Table Depth) which was in fact measured also in 299 time. This means that in order to gain the ability of testing for the spatial hydrological niche,

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We conclude that median Water Table Depth (mWTD) is the best of the ten available 309 hydrological descriptors, at least for its use as an explanatory variable of meadow community 310 structure in additive and linear spatial models. While this choice allows for modeling the 311 spatial hydrological niche of meadow plants, it does not facilitate observation of temporal 312 variation.

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Most tested species (65%), forbs as well as graminoids, responded significantly to 314 hydrological heterogeneity, with the normalised abundance (cover) of species being related to 315 hydrological heterogeneity either with monotonic (increasing or decreasing) or with hump-316 shaped responses. Species hydrological optima (maximum normalised abundance) are 317 observed throughout the whole measured hydrological gradient.

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Hydrological heterogeneity drives, after our estimation, c.6% of meadow community 319 structure (i.e. species abundance and composition), and does so at two increasing spatial scales (within-and between-site). Partitioning of plant beta-diversity, and the subsequent 321 explanation of this partitioning in terms of latent and hydrological processes, have led us to 322 suggest that a conservation strategy of English meadows based on the present results should 323 promote not only the preservation of local hydrological regimes, but, specially, the inclusion 324 in the conservation system of as many and environmentally varying field sites as feasible.

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Further research about the effect of restoring connectivity between field sites on meadow 326 community structure is likely to be relevant for conservation purposes.     decreasing. See Fig. 4 and Online Resource 1, S3-S5.