The structural impacts of wildfire and defoliation on Mexican spotted owl (Strix occidentalis lucida) nesting habitat

Disturbance plays a major role in shaping forest structure and composition, but disturbance regimes across the globe are changing because of human activities. Structure-altering disturbances, including defoliating insect outbreaks and fire, historically occurred in southwestern forest systems, but anthropogenically driven disturbance regime change is rapidly reshaping spatial and temporal patterns of these historical disturbances. These forests are trending towards increased defoliator outbreaks and stand-replacing wildfire due to fuel accumulation and/or climate change. Despite the increasingly prominent presence of defoliation and fire in Mexican spotted owl (Strix occidentalis lucida) forest habitat, there is little understanding of how these disturbances effect the threatened subspecies through altered forest structure. No studies to date have examined how defoliation affects the owls, and in comparison to the two other spotted owl subspecies, the relationship between fire and the Mexican spotted owl is poorly studied, despite its range being projected to experience the largest increase in percent burned area of the three subspecies by the 2080s. To help fill these knowledge gaps, I addressed the impacts of both fire (at varying severity and time since fire—up to 99 years) and defoliation (via Janet’s looper; Nepytia janetae) on forest structure in mixed-conifer forest of the Sacramento Mountains, South-Central New Mexico. I quantified the structural response in burned, defoliated, and control areas, comparing the forest structure in these areas to structural thresholds defined as desired for Mexican spotted owl nesting habitat, and I analyzed spatial patterns of successful nest locations in response to a 2007 Janet’s looper outbreak. In line with the evolutionary history of this landscape, forest that burned at low to moderate severity provides and maintains adequate Mexican spotted owl nesting habitat structure soon after fire, but high severity fire would take greater than 100 years to recover to desired levels for nesting. Structure in a recently (13 years) defoliated area was suitable for nesting, but the spatial patterning of nest locations 1-4 years after the defoliation event demonstrated an immediate negative response to the disturbance—a possible result of temporal discrepancy between nest site data and forest structure data, a sampling bias against more severely defoliated areas, or a response to other disturbance. Ultimately, forest structure that is understood to support Mexican spotted owl nesting will be promoted by the restoration of frequent, low-to-moderate severity fire and minimal activities that incur large structural changes, such as stand-replacing fire and logging.

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