Damla Cinoğlu and Caroline Farrior, University of Texas at Austin, discuss their article: Small disturbances and subsequent competition for light can maintain a diversity of demographic strategies in a neotropical forest: Results from model-data integration
Tropical forests are one of the most diverse ecosystems on Earth. They are fundamental for managing the global carbon budget and supporting biodiversity. If we were walking in a tropical forest, we could really appreciate the many different kinds of trees, with various leaves, branches, and trunks, making it challenging for us to find trees that look exactly alike. These forests are usually closed-canopy multi-tree-layer environments, with occasional gaps in the canopy, due to large-tree mortality that allows trees to compete for the light that reaches the forest floor. But this remarkable scene has puzzled ecologists for over a century. How can hundreds of tree species survive side by side, all depending on the same set of limited resources? Why has a “perfect” tree not yet evolved to dominate these tropical forests?
Grouping trees by how they live
On Barro Colorado Island (BCI) in Panama, an influential patch of forest, scientists have recorded 282 tree species living together. Despite this overwhelming diversity, they have found that tree species can be grouped based on how they use the limited resources available to grow, survive, and reproduce. Specifically, some tree species prioritize growth instead of lifespan (a fast versus slow growth strategy), and some others prioritize growth and lifespan over contributions to the next generation (a stature versus recruitment trade-off). These trade-offs seem to be fundamental to how trees share space and resources in these crowded tropical forests.
The role of gaps: How falling giants shape the forest
In our study, we were excited to test how changing competition for light after gaps are formed by large trees falling may help explain the coexistence of not just one perfect species, but the four types of species grouped as Slow (low growth, long lifespan), Fast (high growth, short lifespan), LLP (high growth, long lifespan, low recruitment), and SLB (low growth, short lifespan, high recruitment). Importantly, we find that simulation models that include this demographic diversity maintain long-term coexistence of species for longer and more stably than scenarios in which all the species are of one type (neutral models). But this diversity is only maintained when those gaps and the competition for light after the gaps are present. It is important to note that our findings are the result of an exploration of the empirical recruitment space. We do not initially find long-term coexistence when we use recruitment parameters summarized at the scale of the 50-ha BCI plot, but there is a suite of empirically reasonable parameter combinations for which the competition for light and treefall gaps supports the long-term coexistence of four differing strategies.
The bigger picture: Why this matters for ecology
We also explore how diversity plays out over time in our models. Specifically, we find species can bounce back from low numbers, a sign of stable coexistence, and that when a species becomes too abundant, it suffers – its own success limits its future performance and growth. This pattern, conspecific negative density dependence, is a well-known pattern helping to maintain diversity in real forests and has emerged from the mechanisms within this simple model.
Our work shows that even a simple mechanism like competition for light following large tree fall gaps can support some of the most basic and important forms of diversity of tropical forests. It also suggests that we are just scratching the surface and can likely explain further diversity with the inclusion of additional mechanisms, like competition for additional resources and interactions with natural enemies like herbivores and pathogens.
