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Vertical Variation in Tree Canopy Soil Communities

Cary Scientist(s): Dr. Evan Gora, Dr. Jane M. Lucas

Much like the communities of bacteria in our guts influence our physical and mental health, the number and types of bacteria in the environment can have important impacts on soil fertility, carbon sequestration, and more. This project aims to uncover how climatic gradients drive microbial community assembly, with the goal of improving predictions of microbial responses to climate change and other environmental stressors.

Specifically, we are studying microbial communities in the soils that accumulate on the branches of tropical trees. Although tree canopies can hold vast amounts of soil — reaching more than five feet deep on some branches — their inhabitants have rarely been studied. Our project will be the first to characterize how canopy soil communities vary across different heights, from the ground level to the canopy. Environmental stress increases from the floor to the canopy, where conditions are drier, more variable, and temperatures are around 3 degrees C higher than at ground level — comparable to 100 years of unmitigated climate change. This gradient provides an excellent model system for understanding how microbial communities respond to climate change.

The new model system will also allow us to test the theory that microbes that are good at tolerating stressful abiotic environments aren’t as good at competing against other microbes, and vice versa. Given the reduced abiotic stress near the ground, we hypothesize that communities in these lower environments will favor competitive traits and exhibit lower abiotic stress tolerance, while those in the upper canopy will be more specialized for coping with extreme environmental conditions.

Our field work takes place in the Santa Fé National Park in Veraguas, Panama, at a site that includes mapped trees and infrastructure for safely accessing the canopy. In 24 trees, we will monitor microclimates and sample canopy soil microbes at five different heights: the ground level, understory, subcanopy, canopy, and the emergent layer (the highest part of the tree).

forest vertical gradient

Metagenomic analyses, metabarcoding, and quantitative approaches will help us identify which fungal, bacterial, and archaeal communities are present in each sample, whether they have genes for tolerance versus competition (see examples in the chart below), and how the communities change along the vertical gradient.

We will also transplant soil samples between different canopy heights to test the abiotic and biotic controls on community assembly at each location. Lab-based experiments will allow us to specifically evaluate the roles of heat and water stress in shaping community assembly under more controlled conditions.

These experiments are timely because they will provide key insights into which microbial taxa and genes are most sensitive and resilient to future global change, helping to predict which groups will be “winners” or “losers” in a hotter future.