Habitat differentiation among closely-related willow species along a water table gradient

Background/Question/Methods

Niche-based processes are considered important mechanisms in community assembly and in promoting species co-existence. However, empirical evidence for niche partitioning can be challenging to find in plant communities, given that plants require the same general type of resources to survive. Stochastic, neutral processes have also been proposed to play an important role in plant community assembly. Here we present a study testing for evidence of habitat differentiation among a group of co-occurring, closely-related willow species along a hydrological gradient in temperate wetland ecosystems. Our goal is to better understand the relative importance of these contrasting mechanisms in driving the assembly of willow communities.

We propagated cuttings from natural populations in and near the study site and planted them in a series of common gardens established along a water table gradient that represents the typical range of conditions in the willows’ natural habitats. To reduce competition between species to a negligible degree, we provided sufficient spacing between individual cuttings. We monitored individual survival and growth for two consecutive growing seasons

Results/Conclusions

Results support the hypothesis that willow species co-exist, in part, by partitioning a hydrologic gradient. A quadratic model was fitted to the survival of individuals across water table depth for each species. Based on the predicted water table depth at which they achieved maximal survival, the thirteen species were sorted into a “wetland specialist” group and an “upland specialist” group. Furthermore, the sorting of species into these two groups agree with the distributions of natural populations along the water table gradient observed in the study site. We hypothesize that the contrasting survival and distribution patterns among the two groups of species are caused by differential tolerances to both drought and flood stress, which vary in intensity along the water table gradient. Our study provides empirical evidence that habitat differentiation along an abiotic gradient promotes coexistence among closely-related plant species in local communities.