Tree mortality increases woodland structural complexity on productive sites in the southwestern US

The extent and severity of tree mortality events are increasing in pinyon-juniper ecosystems of the American Southwest, mirroring trends observed in forested systems globally. Although disturbances may alter habitat suitability for numerous species, and indices of structural complexity may serve as useful, easily measured proxies for biodiversity and ecological resilience, few studies have examined relationships between structural complexity and disturbance severity in pinyon-juniper systems. In this study, we surveyed woodland structure across gradients of tree mortality and environmental conditions at 12 pinyon-juniper sites in northern Arizona, USA, and estimated recent tree mortality using stand reconstructions. Results indicated that 58% of sites showed overall tree density declines between 1996 and 2022. Mortality of pinyon pine ( Pinus edulis ) reached as high as 99.8% at one site. We found significant linear relationships among nine individual indicators of structural complexity, while our aggregate structural complexity index showed a significant (p < 0.001), positive linear relationship with actual evapotranspiration and tree loss (trees ha −1 ). We found limited support for a non-linear model that predicted maximum structural complexity at intermediate values of tree mortality. Our findings indicated that woodland structure is rapidly changing due to drought and insect outbreaks, and structural complexity is increasing under moderate levels of tree mortality, particularly on highly productive sites. Understanding these patterns may help managers in planning climate adaptation strategies and conserving biodiversity.