Title: The Role of Arbuscular Mycorrhizal Fungi, and Natural Enemy Communities on Seedling Dynamics
Description: Identifying the mechanisms that prevent competitive exclusion in tropical forests is a key goal of tropical ecology. Because trees are long-lived organisms, it is complicated to test theory related to coexistence. However, the seedling stage, during which tree mortality is the highest, offers an ideal proxy to evaluate mechanisms that promote or hinder tree species coexistence. This dissertation utilizes both theory and empirical approaches to investigate two mechanisms thought to influence seedling dynamics and tree species coexistence: negative feedbacks from tree natural enemies and positive feedbacks from arbuscular mycorrhizal (AM) fungi. Tree natural enemies might promote tropical tree species coexistence by acting as agents of negative density-dependent mortality. Simultaneously, tropical seedlings associate with arbuscular mycorrhizal (AM) fungi, which can increase seedling growth and survival through enhanced nutrient and water uptake. However, little is known about the effects of AM fungi on tropical tree community dynamics.

In chapter 1, I developed a theoretical model that combines competition between trees, predation by tree natural enemies, and mutualism with AM fungi, and showed that a subtle balance between negative and positive feedbacks is required to reach tree species coexistence. In chapters 2 and 3, I used empirical data collected from El Yunque (Puerto Rico) to gain a better understanding of the distribution of soil fungi and tree natural enemies in a secondary tropical forest and to test some of the assumptions of my theoretical model. In chapter 2, I found evidence that soil characteristics and the tree community were important to structure soil fungal communities, and I demonstrated long-lasting effects of past human land use. If AM fungi are important to promote tropical tree species coexistence as suggested by my theoretical model, past land use could influence tree species coexistence by altering AM fungal communities, emphasizing the need for additional studies about land use legacy effects on AM fungal communities.

In chapter 3, I showed that seedlings at intermediate conspecific density and from intermediate abundance tree species, hosted the richest community of natural enemies, suggesting that negative density-dependent processes might be non-linear, and partially supporting my theoretical model. Finally in chapter 4, I investigated seedling mortality and showed that natural enemies increase seedling mortality, whereas AM fungal diversity decreases seedling mortality, counteracting the local effects of natural enemies. I also found evidence that AM fungal diversity rescues rare tree species, and natural enemies reduce survivorship of more abundant species, thereby preventing competitive exclusion. Therefore, at the community scale, AM fungal diversity and natural enemies act in the same direction, promoting tropical tree species coexistence, which is consistent with the findings of my theoretical model. In conclusion, this dissertation jointly investigated the effects of negative and positive feedbacks on tropical tree species coexistence, and demonstrated the importance of combining demographic processes that are known to occur simultaneously.