Environmental filtering is an important community assembly process influencing species distributions. Contrasting species abundance patterns along environmental gradients are commonly used to provide evidence for environmental filtering. However, the same abundance patterns may result from alternative or concurrent assembly processes. Experimental tests are an important means to decipher whether species fitness varies with environment, in the absence of dispersal constraints and biotic interactions, and to draw conclusions about the importance of environmental filtering in community assembly. We performed an experimental test of environmental filtering in 14 closely related willow and poplar species (family Salicaceae) by transplanting cuttings of each species into 40 common gardens established along a natural hydrologic gradient in the field, where competition was minimized and herbivory was controlled. We analyzed species fitness responses to the hydrologic environment based on cumulative growth and survival over two years using aster fitness models. We also examined variation in nine drought and flooding tolerance traits expected to contribute to performance based on a priori understanding of plant function in relation to water availability and stress. We found substantial evidence that environmental filtering along the hydrologic gradient played a critical role in determining species distributions. Fitness variation of each species in the field experiment was used to model their water table depth optima. These optima predicted 68% of the variation in species realized hydrologic niches based on peak abundance in naturally assembled communities in the surrounding region. Multiple traits associated with water transport efficiency and water stress tolerance were correlated with species hydrologic niches, but they did not necessarily covary with each other. As a consequence, species occupying similar hydrologic niches had different combinations of trait values. Moreover, individual traits were less phylogenetically conserved than species hydrologic niches and integrated water stress tolerance as determined by multiple traits. We conclude that differential fitness among species along the hydrologic gradient was the consequence of multiple traits associated with water transport and water stress tolerance, expressed in different combinations by different species. Varying environmental tolerance, in turn, played a critical role in driving niche segregation among close relatives along the hydrologic gradient.
Bibliographical noteFunding Information:
We thank the staff and interns at Cedar Creek Ecosystem Science Reserve for greatly facilitating our fieldwork. The common gardens were set up with the help of N. Adams, M.?Wright, and P. Ewing. Plant material was collected at Cedar Creek Ecosystem Science Reserve and several Scientific and Natural Areas with the permission of the Minnesota Department of Natural Resources and the Nature Conservancy (permit #2009-10R). We thank R. Shaw for assistance with the aster model analysis, A. Eule-Nashoba, P. Wragg, and members of the J. Cavender-Bares lab for their feedback on the manuscript. We also thank Oscar Godoy and an anonymous reviewer, whose comments allowed us to improve the paper. All authors contributed intellectually to the study. J. Cavender-Bares conceived of the study and designed the experiment with J. Savage. X. Wei and J. Savage implemented the experimental design. X.?Wei collected the growth, survival, lenticel density, and water table depth data. C. Riggs collected soil nitrogen availability. X.?Wei conceived of and performed the data analysis with help?from J. Cavender-Bares. X. Wei wrote the manuscript with?help from J. Cavender-Bares. All authors edited and helped?revise the manuscript. This study was funded by NSF LTER DEB-060652 and DEB-1234162 and supplements to J.?Cavender-Bares.
- aster fitness models
- environmental filtering
- field experiment
- functional traits
- hydrologic niche segregation
- phylogenetic signal