Consequences of drought, flooding, and insect herbivory on the distributions of closely related willow and poplar (Salicaceae) species across hydrologic gradients in Minnesota wetlands

Abstract

Understanding the mechanisms underlying species diversity patterns is a central and long-standing issue in ecology. Beta diversity, the variances in species composition among sites, is an important aspect of species diversity that links local diversity patterns to regional diversity patterns. One of the community assembly processes that known to influence beta diversity is environmental filtering. Besides environmental filtering, biotic interactions can also affect beta diversity, if biotic factors exert differential effects on species performances across environmental gradients. Contributions of different community assembly processes to beta diversity can be tested with multiple approaches. First, experimental approaches allow direct tests of effects of abiotic and biotic factors on species performances across environmental gradients. Second, functional traits can be used to infer the community assembly processes underlying species diversity patterns, as differences in performance responses between species are caused by differences in relevant traits. Last, closely related species tend to (but not always) share similar ecological attributes; therefore, phylogenetic information may be used to predict functional traits and infer community assembly processes. This dissertation examined the effect of environmental filtering and insect herbivory on distributions of 14 willow and poplar species across hydrologic gradients in Central Minnesota, combining field and greenhouse experiments, functional traits data, and phylogenetic analyses. At our study site, Cedar Creek Ecosystem Science Reserve, the species showed differential distributions across a water table depth gradient, suggesting environmental filtering mediated by water stresses likely caused habitat segregation among species. Furthermore, species in the Salicaceae family are known to host a variety of insect herbivores, which makes insect herbivory another possible process influencing beta diversity among the Salicaceae communities. Chapter 1 tested environmental filtering in the Salicaceae species using a field experimental test. Cuttings of the 14 species were transplanted into 40 common gardens established along water table depth gradients in the field, where competition was minimized and herbivory was controlled. Species fitness response to the hydrologic environment was estimated based on cumulative growth and survival over two years using aster fitness models. Variation in nine drought and flooding tolerance traits were examined; these traits were expected to contribute to performance based on a priori understanding of plant function in relation to water availability and stress. Fitness variation of each species in the field experiment was used to model their water table depth optima. These optima predicted 75% of the variation in species observed hydrologic niches, based on peak abundances in naturally assembled communities in the surrounding region. Multiple traits associated with water transport efficiency and with 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 or integrated water stress tolerance as determined by multiple traits. In conclusion, differential fitness among species along hydrologic gradients is the consequence of multiple traits associated with water transport and water stress tolerance, expressed in different combinations by different species. Varying environmental tolerances, in turn, play a critical role in driving niche segregation among close relatives along hydrologic gradients. In chapter 2, the effect of insect herbivory on the growth of the Salicaceae species across hydrologic gradients was examined using the same common garden experiment mentioned above. An insect exclusion treatment was performed nested within the gardens, by installing real and sham cages to individual experimental plants and comparing species growth in the different cage treatments. Concentrations of nitrogen, carbon, and two groups of defense compounds, phenolic glycosides and condensed tannins in leaves were measured, and phylogenetic signals in these foliar traits were analyzed. The results showed that insect herbivory reduced plant growth, was different between species, and varied across the water table depth gradient in a hump-shaped manner. However, herbivory did not promote habitat segregation among the species because there was no interaction effect between species and water table depth on either herbivory damages or the cage treatment effect on growth. Furthermore, variations in leaf traits could partially explain the variation in herbivory between species but not variation across hydrologic gradients. Last, closely related species did not share similar defense traits: although secondary metabolite richness was phylogenetically conserved, the concentrations of the defense compounds and nitrogen were not. In conclusion, although insect herbivory did not promote beta diversity among the Salicaceae communities across hydrologic gradients, the dissimilarity in defense chemistry might promote the coexistence of close relatives within local communities through density-dependent effects. In the last chapter, a greenhouse experiment was performed to examine responses of growth and physiological traits in seven willow (genus Salix) species to a six-week long flooding treatment followed by a six-week long recovery period. These seven species were selected to represent the full ranges of mean water table depth and season water table depth fluctuation in the natural habitats of the 14 species. The flooding treatment increased plant growth and carbon assimilation by improving plant water status: plants received the flooding treatment showed higher stomatal conductance and predawn leaf water potential than plants received the control treatment. Furthermore, species distributed in wetter habitats had higher stem growth rate in the flooding treatment; and the species distributed in habitats with greater water table depth fluctuations showed greater variations in growth between the flooding and the recovery period. The results of this experiment suggest differential tolerances to flooding and water table depth fluctuation may contribute to habitat segregation among the species.