Inferring mechanisms of community assembly from phylogenetic and functional diversity

A robust ecosystem requires a functionally heterogeneous community of organisms with a wide range of traits that permits broad resource partitioning. Understanding community diversity patterns can help investigate drivers of community assembly and how different metrics reflect the success of restoration in grassland or weed control in cropland. The objectives of my study were to identify assembly drivers influencing community taxonomic, phylogenetic, and functional diversity. The first study examined the effect of different ecotypes of dominant species on grassland community diversity along a spatial aridity gradient during restoration. This study showed that ecotype significantly affected species richness and shaped taxonomic, phylogenetic, and functional diversity. Accordingly, restorations should consider ecotypic variation as a critical biological filter to community assembly in grassland ecosystems. Given the community response across the tallgrass prairie, restoration practitioners should draw attention to regional sources of dominant species because ideal ecotypic sources can affect species richness and even sustain the phylogenetic and functional patterns. The restoration efficacy of existing ecosystems should be evaluated to large-scale diversity patterns to detect gaps and limitations that will reveal which diversity components to highlight for further restoration investment. The second study investigated temporal variations in diversity metrics among dominant species ecotypes and a drought treatment on the importance of external and internal filters in shaping the assembly of grassland communities. In this study, species richness decreased significantly during the early stages of restoration while phylogenetic and functional diversity was maintained over eight years. I also found no significant effect of experimental drought treatment on community biodiversity. Moreover, ecotypic variation as an internal filter played a key role in grassland assembly but the external filter was less strong because of high trait overlap among species within a community. In general, this study highlighted the consequence of integrating both interspecific and intraspecific trait variabilities and the value of concentrating on functional traits to comprehend better how trait variability is coupled with species coexistence. Future investigations are necessary to examine the distinctive origins of variability in plant traits and how they contribute to grassland community assembly. The third study focused on whether weed management tactics such as a glyphosate-resistant (GR) cropping system were beneficial to control weed diversity in the soil seedbank of the agroecosystem. Both phylogenetic and functional diversity of the soil seedbank was relatively stable with different GR cropping systems across six years. The neutral assembly of the soil seedbank may imply that the belowground weed community could be restrained by stochastic mechanisms, such as dispersal and demographic stochasticity of seeds, during agricultural activities. Therefore, I recommend integrated weed management with a sustainable perspective to fight against the evolutionary feedback due to weed herbicide resistance. While the GR cropping system still seems beneficial, future weed control cannot overlook the extensive impact of GR systems on biodiversity variations, the shifts in weed composition, and the resistance evolution of weed species to herbicides in the agroecosystem