I am a forest ecologist investigating how disturbance and decomposition shape forest ecosystems in the context of global change. My research combines extensive field work, remote sensing, and genomic techniques with modern quantitative tools to investigate the mechanisms regulating forest ecosystems. My approach is to develop a conceptual and mathematical understanding of local processes, and then use big data (e.g., satellite sensors, plot networks, etc) to determine how these processes influence ecosystems at regional to global scales. My work revolves around three fundamental questions.
Why do trees die?
Forests play dominant roles in global biodiversity and carbon cycling. Consequently, the local disturbances that regulate forest dynamics – particularly via tree death – have global implications. Although we know what can kill trees, the relative importance of factors that do kill trees is largely unknown. Much of my current and forthcoming research investigates how lightning, pathogens, and drought interact to cause tree death, including how drivers of mortality vary with tree size.
Most of my past work on this topic has focused on the direct (i.e., non-fire) effects of lightning in tropical forests. In a collaboration with ecologist Steve Yanoviak and atmospheric physicists Phillip Bitzer and Jeff Burchfield, we have been locating lightning strikes in near real-time throughout a lowland forest in Panama. This work revealed that lightning strikes in this forest typically cause cryptic group tree damage and death events, and initiate interactions with other agents of tree death. I am now starting separate research projects investigating lightning in other regions and testing the physiological and electromagnetic processes by which lightning causes tree death.
Pictured are a large group of trees directly killed by lightning in Peru. Locations where electric current jumped between branches in the canopy are noticeable between the branches in adjacent tree crowns.
This image of a lightning strike was captured by our lightning triangulation system. We combine images like this from multiple cameras to triangulate lightning strikes and quantify their ecological effects.
How do disturbances shape forest communities and Ecosystems?
After developing an understanding of local disturbances, I use my quantitative toolkit to scale these findings up to landscape and global processes. In our work with lightning, I calculated the contributions of lightning to total tree mortality by combining descriptions of individual lightning disturbances with decades of forest dynamics data and remotely sensed estimates of lightning frequency. Using this approach, we showed that lightning was the primary cause of large tree mortality in central Panama.
This work suggested that lightning might be more likely to strike and kill the largest trees within a forest stand. Although the idea that lightning strikes larger trees seems intuitive, there was no empirical evidence of this effect. To assess this idea, I worked with my colleagues to develop a mechanistic model of lightning strikes to a forest. We then tested this model using our lightning strike data and a fine-scale map of tree crowns within a forest dynamics plot. The key results of this study were straightforward: taller trees and trees with larger crowns are more likely to be struck by lightning than are shorter trees and trees with smaller crowns. This mathematical work is a key component of a fully mechanistic model of lightning-caused disturbance that is currently under development.
The tendency of lightning to kill large trees suggests that it plays a major role in forest biomass dynamics and carbon cycling. The largest 1% of trees in a forest contain approximately 50% of forest biomass and the distribution of these trees determines differences in forest carbon storage. If lightning is a major cause of death for these large trees, then differences in lighting frequency should influence total forest biomass stocks and turnover rates. We combined multiple datasets of pantropical lightning frequency to test this idea and found surprisingly strong evidence that lightning shapes forest structure and dynamics. Specifically, higher lightning frequency was associated with fewer large trees per hectare, higher rates of woody biomass turnover, and less total aboveground biomass. We are further expanding on this work to directly tie the effects of lightning to forest carbon cycling.
Maps of lightning frequency and ecosystem types throughout tropical America (a), Africa (b), and Asia/Australia (c). Lightning frequency only includes lightning strikes that reached the ground in terrestrial ecosystems.
How does dead wood decompose and contribute to carbon cycling?
Tropical forests are disproportionately important to the global carbon cycle and the vast majority of aboveground carbon is stored in woody tissues. This carbon is released as dead wood decomposes, yet the process of wood decomposition is poorly studied in tropical forests. I am investigating the distribution and decomposition of dead wood in tropical forests. The current stage of this work uses forest plots to understand wood decomposition at large scales and over long time frames.
I use a variety of approaches to study dead wood dynamics in tropical forests. In a collaboration with Emma Sayer, Ed Tanner, and Ben Turner, we used a long-term litter removal and addition experiment to determine how soil nutrient availability affects decomposition over a 15 year period. Separately, I worked with Helene Muller-Landau to quantify the stocks, fluxes, and spatiotemporal variability of dead wood on Barro Colorado Island in Panama. We found that ca. 50% of wood necromass is separated from the soil, yet we know almost nothing about decomposition above the forest floor. To address this knowledge gap, I am investigating how decomposition rates change vertically and how these changes are associated with environmental factors (microclimate and nutrient availability) and microbial communities (bacteria, archaea, and fungi). The next stage of this work is taking an ambitious approach to quantifying long-term and large-scale patterns of decomposition.