Dissecting the hydrological niche: soil moisture, space and lifespan

Garcia-Baquero, Gonzalo; Silvertown, Jonathan; Gowing, David J. and Valle, Cipriano J. (2016). Dissecting the hydrological niche: soil moisture, space and lifespan. Journal of Vegetation Science, 27(2) pp. 219–226.

DOI: https://doi.org/10.1111/jvs.12353

Abstract

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.

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