The impacts of an exceptional drought on forest demographics and biomass in east Texas, USA

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Tilak Chaudhary, Department of Biological and Health Sciences, Texas A & M University-Kingsville, Kingsville, TX, Weimin Xi, Biological and Health Sciences, Texas A&M University-Kingsville, Kingsville, TX, Sandra Rideout-Hanzak, Department of Rangeland and Wildlife Sciences, Caesar Kleberg Wildlife Research Institute, Texas A & M University-Kingsville, Kingsville, TX, Haibin Su, Department of Physics and Geosciences, Texas A & M University-Kingsville, Kingsville, TX and Stephen Clarke, Forest Health Protection, USDA Forest Service, TX

Abstract
Background/Question/Methods
Forest mortality due to drought coupled with climate change has been reported in all continents. Droughts as a major natural disturbance, have increased tree mortality and negatively affected forest structure, tree composition, and ecosystem services. Texas experienced an exceptional drought in 2011, which killed an estimated 301 million trees, including 60.5 million in forested east Texas. Previous studies showed that drought-induced tree mortality and biomass loss in east Texas has increased significantly. However, factors affecting variations in tree mortality, growth, recruitment, and biomass losses and its long-term effects are still less known. The objective of the study was to understand effects of drought and other factors influencing forest demographics and biomass loss rates in east Texas. In total 1549 Forest Inventory and Analysis (FIA) plots that were measured 3-4 times across 20 years (2001 - 2019) were used. Data were regrouped into three periods: base period (FIA Cycle 7 (2001-2003), pre-drought (between 2003 to 2010-07-30 after base period), drought (2010-08-01 to 2016-12-31), and post-drought periods (2017 and after). General linear mixed models (GLMM) were used to examine correlation and variation among forest demographic rates at the stand level and climatic variables at the regional level. Forest stability index was used to estimate forest stability after drought events.
Results/Conclusions
Weather-related agents (drought & hurricane) are the leading cause of tree mortality and biomass loss followed by wildfires and insects/diseases. Tree mortality and biomass loss to mortality increased significantly during drought period (14.9% and 6.44 % Year-1 respectively). In contrast, annual basal area growth rates declined during drought period, but recruitment rate increased in the post-drought period (1.56% Year-1). Mortality, recruitment, basal area growth, and biomass loss varied across species, diameter, tree height, stand age, stand size, stand origin, site productivity, and ecological divisions. Small trees (as measured by diameter and height) are more vulnerable to drought-induced tree mortality. Stem density, basal area density, total basal area, and species richness were endogenous factors explaining forest mortality, recruitment, basal area growth, and biomass loss. Drought severity was negatively correlated to recruitment, biomass growth, basal area growth, and drought length to mortality. Temperature, precipitation, and drought length were negatively related to basal growth. As result of increased mortality and variable response in different species and diameter classes, east Texas forests have been undergoing structural and compositional changes as indicated by the forest stability index. The forest stability index indicates that of most abundant species, southern red oak is experiencing the greatest decline within its range across east Texas, whereas Chinese tallow is expanding. And continued mortality and delayed basal area growth suggest forests have compromised resilience and might be approaching tipping point in the future under climate change. However, during this study period the forests were still stable, but were experiencing a re-organization stage.