Fire, insect, and disease-caused tree mortalities increased in forests of greater structural diversity during drought

Lu Zhai, Natural Resource Ecology and Management, Oklahoma State University, Stillwater, OK, David R. Coyle, Department of Forestry and Environmental Conservation, Clemson University, Clemson, SC and Daijiang Li, Louisiana State University

Background/Question/Methods
Structural diversity is an emerging dimension of biodiversity that accounts for size variations in organs among individuals in a community. Previous studies focused on canopy structure measured by Lidar with limited availability, hindering its application to understanding large-scale forest dynamics under disturbance. More available measures of tree stem were used in the other studies to calculate the diversity by stem diameter and height separately, failing to integrate them together given their different eco-physiological effects. To address the knowledge gap, we introduced another way to quantify stem structural diversity (SSD) based on both tree diameter and height. We also calculated species richness, functional diversity, basal area, stand age, and drought magnitude of >2000 plots of USDA Forest Service’s Forest Inventory and Analysis across forestlands in Oklahoma. We then investigated their relative importance in determining fire, insect, and disease-caused tree mortalities during a recent drought.
Results/Conclusions
Our results showed that: (1) SSD was positively associated with tree mortalities caused by all three disturbances; (2) Species richness was negatively associated with the insect and disease-caused mortalities; (3) Functional diversity was negatively associated with the fire and disease-caused mortalities; and (4) The mortalities caused by insect and disease, other than fire, varied significantly across phylogenetics. Together, the tree mortalities during drought associated with SSD more consistently than the other biodiversity metrics. Given SSD’s significant effects and high availability, it could be widely used in predicting forest dynamics and planning management to sustain forest resources under disturbance.