Effects of Multiple Wildland Fires on Ponderosa Pine Stand Structure in Two Southwestern Wilderness Areas, USA

Abstract

The effects of 30 years (1972-2003) of Wildland Fire Use for Resource Benefit (WFU) fires on ponderosa pine forest stand structure were evaluated in the Gila Wilderness, New Mexico, and the Saguaro Wilderness, Arizona. Tree density, diameter-class distributions, basal area, and stand density index were compared among areas that burned with different frequencies since 1972 and areas that burned mid-century (1940-1950) and again during the WFU era (1972-2003). In both the Saguaro Wilderness and the Gila Wilderness, significantly fewer small-diameter (5 em to 22.5 em) trees occurred in areas that burned multiple times since 1972 compared to areas that were unburned (p < 0.05) during this time. The density of large-diameter (>45 em diameter breast height) trees in the Gila Wilderness was highly variable and did not differ significantly among fire treatments (p > 0.32). In the Saguaro Wilderness, significantly more large-diameter trees (>45 em dbh) occurred in areas that burned mid-century and again during WFU than in all other fire treatments. Mean 10-year basal area increment growth rates (1840 to the present) of trees in the Gila Wilderness that experienced mid-century fires suggest that those fires may have had a thinning effect. Ponderosa pine forests in the Gila Wilderness and Saguaro Wilderness are structurally diverse and resistant to fires burning during the natural fire season, suggesting that repeated WFU fires have restored forest resilience to fire.