COS 173-3 - Relative Importance of Multiple Stand Attributes and Site Conditions in Forest Temporal Productivity Stability Under Drought

Abstract: Ecosystem stability is the ability of an ecosystem to maintain functioning and growth over a period of time. Forest stability is important for sustaining ecosystem health and services including biodiversity, timber production, wildlife habitat, and long-term carbon sequestration. However, forest productivity stability under the impending drought stress driven by climate change has scarcely been investigated leading to a lack of mitigation strategies, consequentially threatening long-term stability. Using USDA Forest Service Forest Inventory Analysis (FIA) data we calculated forest biomass productivity stability across the southcentral U.S. that experienced drought from 2010-2015. Using the Random Forest algorithm, we quantified the interactive effects of multiple stand attributes and site conditions with drought on forest productivity stability to determine the relative importance of these variables in forest stability under drought.
From the stand attribute perspective, the productivity stability was most sensitive to basal area, stand age, and functional identity by specific leaf area. The most important site conditions of stability were soil conditions, specifically percent organic matter (POM) and percent sand, and seasonal precipitation. However, when interacting with drought, the most important variables interchanged from the stand attributes to the site conditions, the greatest being topographic aspect, temperature, soil moisture, and POM. For the stand attributes interacting with drought, functional identity described by rooting depth, and leaf carbon:nitrogen ratio were respectively the most important. Notably, functional identity being more important than other diversity metrics (e.g., functional diversity, species richness, and structural diversity) to the stability, supporting the mass-ratio (i.e., selection) effect over complementarity effect hypothesis when interacting with drought. Moreover, the projection of drought impacts on forest stability could focus more attention on functional identity metrics due to their critical stability roles. Our results could inform drought mitigation strategies for long-term forest stability, e.g., site selection considering aspect and soil with higher levels of POM, and increasing OM effects with the application of nutrients (e.g., nitrogen and phosphorus).