The Impact of disturbance on tree size distributions in the United States

Authors: EICHENWALD, ADAM - University Of Maine; GRADY, JOHN - University Of Maine; KNOTT, JONATHAN - Forest Service (FS); Read, Quentin; RODRIGUEZ, J. MARCOS - University Of Maine; RECORD, SYDNE - University Of Maine

Submitted to: Global Ecology and Biogeography
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/22/2025
Publication Date: 8/7/2025
Citation: Eichenwald, A., Grady, J.M., Knott, J., Read, Q.D., Rodriguez, J., Record, S. 2025. The Impact of disturbance on tree size distributions in the United States. Global Ecology and Biogeography. https://doi.org/10.1111/geb.70102.
DOI: https://doi.org/10.1111/geb.70102

Interpretive Summary: Forest trees in the continental USA range in size from tiny seedlings to giant redwoods. Ecological theory tells us that the distribution of tree sizes should follow certain mathematical laws. If the forest is disturbed by fire, wind, or some other event, the distribution will change. We explored how disturbance changes tree size distribution by fitting statistical distribution models to over 180,000 forest plots surveyed by the U.S. Forest Service’s Forest Inventory and Analysis program between 2001-2023. We asked whether disturbance, forest age, and maximum tree height affected the shape of the tree size distributions. We found that disturbances that affect the smallest trees, such as grazing and ground fires, change the shape of the distributions in predictable ways. On the other hand, disturbances that affect all sizes of trees, like crown fires, do not consistently change the shape of the distributions. These findings are important for understanding how disturbance affects forest structure, which can help us predict how forest biomass will change over space and time.

Technical Abstract: Aim: Tree size distributions are an emergent feature of forests that reflect community dynamics and patterns of carbon storage. Theory suggests that undisturbed forests have characteristic size-abundance distributions, but disturbance should also lead to predictable deviations. We aim to quantify how disturbances affect forest size-abundance distributions. Methods: We analyzed size-abundance distributions in over 180,000 Forest Inventory and Analysis (FIA) plots in the contiguous United States from 2001-2023 for both disturbed and undisturbed forests, examining changes in slope associated with disturbances, stand age, and maximum tree height with Bayesian mixed models. A structural equation model (SEM) was also used to evaluate whether disturbance effects on size-abundance slopes were indirectly mediated by changes in stand age or maximum height. Results: We found that disturbances specifically targeting smaller trees (e.g., animal grazing, ground fire) result in predictable, shallower shifts in size-abundance slopes, whereas disturbances less likely to target specific tree sizes (simultaneous ground and crown fires) do not shift slopes in predictable patterns. Maximum tree height and stand age were also reliable predictors of size-abundance slopes, but a structural equation model revealed that the effects of disturbance on forest structure did not indirectly influence slope via stand age or maximum tree height. Main Conclusions: Disturbances result in predictable changes to forest size-abundance slopes. As climate change and increased disturbance frequencies produce younger and shorter forests, future size-abundance slopes are expected to be steeper than those in undisturbed forests. Immediately after a disturbance, however, we predict a temporary flattening in the size-abundance relationship. These findings underscore the importance of understanding disturbance impacts on forest size structure to improve predictions of forest biomass and other ecological variables across broad spatial and temporalscales.