Pine vs. oaks revisited: Conversion of Madrean pine-oak forest to oak shrubland after high-severity wildfire in the Sky Islands of Arizona

Fire regimes have changed dramatically in most dry woodlands and forests of the American Southwest from frequent surface fires prior to Euro-American settlement, to fire suppression in the 20th century, to a current era characterized by large, higher-severity wildfires. The recent increase in fire activity and consequent changes in forest communities are key management concerns across the region. We use sampling from before and after the 1994 Rattlesnake and 2011 Horseshoe Two Wildfires in the Chiricahua Mountains to address the extent to which higher-severity fire is converting Madrean pine-oak forest to oak shrubland in the Sky Islands of Arizona, Plant communities changed from mixed pine-oak forest before the wildfires to oak shrublands and grasslands by 2016 where fires burned at high-severity. In those sites, nearly all stems were killed above ground, oaks regenerated vigorously, mainly by resprouting, and pines recruited at very low levels. These patterns were consistent after each fire and for up to 14 years after the Rattlesnake Fire. Across the Horseshoe Two Fire, seedling recruitment of pines and oaks declined with increasing fire-severity, but oak resprouting increased. Differential recruitment success of oaks over pines was amplified by their far greater juvenile height and more ramets per resprouting genet. Resprouting in Pious leiophylla (Chihuahua pine) after top-kill was low, but sufficient to suggest that this behavior may maintain this species at low density after high-severity fire. The impact of fire depended on topography, as less exposed plots (e.g. in drainages) experienced lower-severity fire and less conversion to oak shrublands, suggesting that these sites provided refugia for species, such as pines, sensitive to high-severity fire. Low pine recruitment occurred in all plots, not only in sites subject to high-severity fire, suggesting that vegetation conversion may have been exacerbated by the extreme drought of the past two decades. Given the episodic nature of their regeneration, pine recovery is possible in the future, but projections call for intensification of aridity and fire activity in the Southwest, which could lead to continued transition of Madrean pine-oak forests to more fire-and drought-resilient communities dominated by oaks. The results of this study point to the need for protective vegetation management and restoration experiments targeting pines.