Data from: Dissecting the hydrological niche: soil moisture, space and lifespan

Questions: Are communities structured on a hydrological (soil moisture)
gradient? Is there spatial segregation into hydrological niches? What is
the shape of the hydrological niches of individual species? Controlling
for spatial autocorrelation, how much of the spatial structure in the
community is due to variation in hydrology? Do annuals and perennials
behave alike with respect to the above questions? Locations: La Mina in
Moscosa Farm, Salamanca, western Spain (dehesa community) and Laguna Larga
in the Urbión Peaks, Soria, central-northern Spain (alpine grassland).
Methods: The presence of plant species was sampled in two contrasting
field sites, for which we also built hydrological models. First, we
reduced the dimensionality of the plant distribution data (non-metric
multidimensional scaling) and measured the correlation between the
resulting ordination and the hydrological gradient. Then we defined
hydrological niches and tested niche segregation of plant species against
null models (Pianka metrics). Finally, we characterized the hydrological
niche of each species using generalised additive mixed models and
partitioned the species distribution variance into (1) a hydrological
component, (2) a linear trend component and (3) and a spatial component,
defined through sets of spatial variables (Moran's eigenvector maps).
Results: Both plant communities were primarily structured along
hydrological gradients, and spatial segregation into hydrological niches
occurred among perennial species, although not among annuals in the dehesa
community. Dehesa annuals were spatially aggregated in the driest niches.
Hydrological variation shaped the responses of 60% of the annual and about
70% of the perennial species in both the dehesa meadow and the alpine
community. Most responses were either monotonic or hump-shaped. Finally,
spatially structured hydrological variation proved to be the main driver
of spatially structured species composition in all cases. Conclusions:
Linearly (gradient of slope) and topographically (at a fine scale)
structured variation in hydrology is the main driver of spatially
structured species composition in both communities. Our results support
the ecological hypothesis that spatial niche segregation on soil moisture
gradients is an important mechanism of co-existence for perennials in both
test communities, although not for the species-rich sub-community of
annuals in the dehesa meadow.