Intense wildfire in southeastern Arizona: transformation of a Madrean oak–pine forest to oak woodland

Abstract

Fire exclusion in southwestern USA and northern Mexico has decreased fire frequency and increased fire intensity in many forest types. I examined the effects of intense crown fire in a Madrean oak–pine community in southeastern Arizona, an ecosystem characterized by frequent, moderate-intensity fire during presettlement times. I sampled seedling establishment and sprouting response of crown-killed pines and oaks after the 10,330-ha Rattlesnake wildfire of 1994 and a smaller intense fire. About 90% of the Quercus hypoleucoides sprouted from top-killed stems. Sprouting occurred in none of the Pinus engelmannii and 13–23% of the P. leiophylla, a low level that nevertheless may help maintain this species after intense fire. Seedlings were very rare for all species, especially P. engelmannii. Height of pine sprouts and seedlings lagged far behind that for Q. hypoleucoides. Results of this case study suggest that anomalous crown fires radically changed community structure and composition for the long-term, from a lower canopy of oaks with large, emergent pines to a more homogeneous oak woodland. Such transformation, if repeated across the landscape, could seriously threaten the maintenance of biological diversity in these ecosystems.