Modeling FoRTE, the Forest Resilience Threshold Experiment

Forested ecosystems represent a large yet uncertain fraction of the global terrestrial carbon sink. Their future state depends on a number of natural and anthropogenic influences; a particularly large uncertainty is how disturbance affects vegetation structure and ecosystem biogeochemistry.  We used the Ecosystem Demography model to explore the ecological and biogeochemical consequences of disturbance as part of the Forest Resilience Threshold Experiment (FoRTE), a dual modeling and manipulative field experiment investigating the effects of disturbance at different severities on a century-old deciduous forest. The field component was conducted at the University of Michigan Biological Station (UMBS), where stem girdling was applied to achieve four different severity levels of disturbance (0, 45, 65, and 85% gross defoliation) before the 2019 growing season. Since then, we have tracked the subsequent changes in vegetation and biogeochemistry. The modeling component attempted to simulate the FoRTE disturbance treatments within its framework. While we were able to instantiate a forest in ED with a similar climatology, soil characteristics, disturbance history, and vegetation of UMBS,  baseline ED is ultimately unable to reproduce the vegetation dynamics and carbon fluxes observed at the UMBS control plots. This is consistent with previous work where the model is not capable of matching observed carbon and vegetation dynamics. However, ED’s response to the disturbance treatments is consistent with observations from UMBS: in both the model and UMBS experimental results, we observed different resiliences and carbon cycle responses with respect to disturbance severity. These intriguing results point to both weaknesses and new possibilities in the modeling of ecosystems facing rising disturbances and climate change.