Shifting from Local to Landscape Controls of Disturbance Size and Severity: A Tree-Ring Reconstruction of Fire, Spruce Beetle Outbreaks, and Species Dynamics of the Pinaleño Mountains

Forest disturbances exert a strong control over species composition and structure. In the Sky Islands of the American Southwest, steep elevation gradients give rise to moisture and temperature gradients that support a diverse array of forest types and disturbance regimes in a relatively small geographic area. In the Pinaleño Mountains of Southeast Arizona, pre-settlement lower elevation pine and dry mixed-conifer forest structure was maintained by frequent, low severity fire, whereas upper elevation mesic mixed conifer and subalpine forest structure experienced less frequent, mixed-severity or stand-replacing fire. Forest structure and age distributions in these mesic forest types were controlled by bark beetle outbreaks during long fire-free periods. Widespread logging beginning in the 1870s followed by a century of aggressive fire suppression led to significant structural changes of lower elevation forests and coincident increase in synchrony of seedling recruitment in all forest types. Over the past two decades, a series of large fires spreading across several forest types and the most severe series of insect outbreaks in the past century of monitoring suggest a series of fundamental changes to the disturbance regimes of the mountain range. Using a grid of 78 demography sampling plots, 121 fire history samples, and tree-ring measurements from 21 spruce-containing plots, we reconstruct 325 years of fire, bark beetle outbreaks, and changes to species distributions through time and space. Comparison of pre and post settlement fire regimes reveal significant changes to the fire-climate relationship likely driven by a shift from fuel-limited to drought-limited landscapes. Reconstructed spruce beetle outbreaks track of the expansion of spruce from a few refuge populations following the 1685 fire to the complete area over 2,835 msl (9,300 ft.) elevation, with outbreak size and severity increasing with the size and distribution of available hosts.