Print PageWith growing evidence that segregation on fine-scale soil moisture gradients may be a general phenomenon in plant communities dominated by perennial species and that the trade-off that forces plants to specialize in this way may be fundamental, the question arises as to how important this may be as a coexistence mechanism. Since soil moisture gradients and plant distribution are inherently spatial, ecological models must include realistic assumptions about spatial structure and deal with possible confounding effects among different causes of spatial structure. In this paper we introduce an analytical approach that considers spatial autocorrelation and that decomposes spatial structure into a component that correlates with the soil moisture gradient and a component that does not. Plant lifespan is also relevant to coexistence because the regeneration biologies of annuals and perennials are different, although previous studies of the hydrological niche have looked only at perennials. We used a dehesa meadow plant community in Western Iberia that is rich in both annual and perennial herbaceous species to test: 1. whether the community as a whole segregated on a soil moisture gradient, and 2. if life form (annual vs. perennial) affected whether and how species segregated. We then partitioned the variation in within-site species distribution to test the contributions of: 3. hydrological stress and 4. other causes of spatial structure.
Results/Conclusions
Realized niche overlaps (Pianka index) were smaller than expected by chance for both annuals and perennials and there was a trade-off between species’ resistances to water-deficit stress and oxygen-deficit stress. Ecologically relevant nonparametric regression models showed that hydrological stresses shaped the responses of 49% of the annual and 56% of the perennial species in the sample. By partitioning multivariate variation in species composition along a soil moisture gradient we found that hydrological stress explained 36 % of the variation in community structure. Hydrological stress was of slightly greater importance to the community structure of perennials (37%) than of annuals (33.5%), but inherent spatial structure was of greater importance for annuals (17%) than for perennials (12.2%). We conclude that both annuals and perennials in the test community segregate along fine-scale niche axes of water-deficit stress and oxygen-deficit stress and that the dehesa species, like those of meadow and fynbos communities studied previously, have species-specific hydrological niches.
