Spatial and temporal dynamics of disturbance interactions along an ecological gradient of the Pinaleño Mountains, Arizona, USA

The onset of fire exclusion in the western United States began one of the largest and most effective landscape ecology experiments in human history. Structural and species changes to forests of western North America have transformed how these ecosystems now respond to natural and anthropogenic disturbances and may contribute to recent trends of more frequent high-severity wildfires and insect outbreaks. In this study I examine interactions between forest successional dynamics, disturbance processes, and climate variability in high elevation forests of the Pinaleño Mountains of southeastern Arizona. I use a gridded sampling design to reconstruct the spatial and temporal components of landscape-scale changes from 1640 to present. I found: (1) stability of forest species assemblages following fire exclusion is a function of site productivity and historical disturbance frequency, (2) patterns of recent fire size, severity, and climate associations deviate from historical precedent, (3) spruce beetle outbreak size, frequency, and severity have increased over the past 300 years, and (4) that 20th century spruce beetle outbreaks are associated with antecedent warm summer temperatures followed by persistent spring drought. I use the results from reconstructed spatial patterns of fire frequency to validate a model of fire and vegetation dynamics for high elevation forests of the Pinaleño Mountains prior to fire exclusion. Cascading interactions among disturbance events can destabilize entire ecosystems as a result of anthropogenic pressures on individual forest types. Current forests are unstable and prone to more high-severity fire and insect outbreaks, raising concerns about the long-term viability of the once distinct forest assemblages of the Pinaleño Mountains. By focusing intensive restoration efforts on one or a few keystone forest types, we may be able to return resilience to the entire system.