The shrublands and woodlands of the Sky Islands of Arizona are diverse ecosystems, maintained by fire before Euro-American settlement. Fire suppression and hotter, drier conditions have triggered a surge in large, high-severity fires throughout the Southwest—events with major impacts on vegetation. Hypotheses about the impacts of fire on alpha diversity (diversity within one site or vegetation type) and beta diversity (degree of variation in species composition across plots) vary widely. Moreover, relatively little is known about how diversity is affected by megafires, especially where they are outside the natural range of variability, as in most Sky Island ecosystems. Using recent analytical approaches, we investigated the effects of the 2011 Horseshoe Two megafire on alpha, beta, and gamma woody plant diversity in Chiricahua National Monument by resampling a network of nearly 200 inventory plots established prior to the fire. We hypothesized that the Horseshoe Two Megafire reduced alpha diversity because of the severity of the fire, but likely enhanced beta diversity because of heterogeneity in fire behavior across the landscape.
Results/Conclusions
Ordination of all vegetation types combined revealed no clear directional changes in species composition from before to after the megafire, but separate analyses of vegetation types uncovered clear shifts away from pines, Douglas-fir, and Arizona cypress towards oaks and manzanita. These compositional shifts corresponded with declines in stand structural diversity, especially in sites subject to high fire-severity. The megafire induced significant increases in woody plant gamma diversity and alpha diversity, although this latter impact depended on vegetation type. The influence of the Horseshoe Two Megafire on beta diversity was complicated, varying with scale, environmental gradients, and analytical approaches. Our results highlight the importance of fire as a regulator of Sky Island vegetation. While increases in woody plant diversity demonstrate the potential benefits of fire as a key disturbance process in this system, the shift in composition and the decline in forest structural complexity may have broad consequences for wildlife. Continued monitoring of temporal changes in vegetation in response to wildfire in Sky Island systems will facilitate a better long-term understanding of the effects of a changing fire regime on the composition, diversity, and structure of these diverse communities.