| Summary: | Living organisms face a changing physical environment. A major challenge in ecology is understanding the ecological and evolutionary role that this changing physical environment has in shaping a community. One fundamental question is how environmental variation affects species coexistence. Modern understanding of environmental variation emphasized the hypothesis that possible adaptations to a fluctuating environment allow species to use different environments in different ways. Species can partition temporally their use of resources. Persistent stages in the life cycle such as prolonged longevity can buffer species through unfavorable environments. Differences in longevity will also lead to different nonlinear responses of population growth rate to fluctuating in resources. Questions arise: how do these possible adaptations to environmental fluctuations affect coexistence. Do they act through multiple coexistence mechanisms, how strong are the mechanisms, and do the mechanisms interact? A framework has been developed for quantifying coexistence mechanisms in models. Being able to quantify coexistence mechanisms in the field is critical to understand different processes contributing to species coexistence in a community: whether a process prevents species dropping out of the community (stable coexistence), or slows down species losses (unstable coexistence), or both. In many respects, applications of those techniques for quantifying coexistence mechanisms have the potential for substantial improvements. In particular, very few studies directly quantify coexistence mechanisms for perennial plants. Coexistence of plant is often puzzling because they share similar resources. Environmental variation has been suggested as an important factor for niche partitioning but challenges for studying it in perennial plants are unclear. The long generation time poses challenges to controlled experiments. Moreover, perennial plants have complex life histories. Vital rates change with size. In addition, tremendous temporal variation is observed in various life history processes. Seedling recruitment and individual growth can both be highly sensitive to fluctuation in the physical environment. Furthermore, different processes in different stages of the life history can interact with environment and competition in different ways. Using perennial plants as a specific system, our study reveals a crucial role in theory development to summarize understanding of such a complex system. I start with the simplest model for perennial plants, the lottery model, to study the relative importance of two coexistence mechanisms: the storage effect and the relative nonlinearity. Then I extend the model by showing that variation in individual growth can also lead to stable coexistence similar to the effect of variation in seedling recruitment. Species can benefit most from variable environments when the processes contributing most to capturing resources on average are also very sensitive to environmental fluctuations. New mechanisms arise through shifts in size structure, which depend on how vital rates change through ontogeny.
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