Consequences of a changing fire regime in the Southwest: How do ponderosa pine (Pinus ponderosa) forests recover?

Background/Question/Methods

Recent large fires in the southwestern U.S. are expected to hold dramatic consequences for forest ecosystems over the long term. Concern is particularly high for species that rely on legacy seed sources, such as ponderosa pine (Pinus ponderosa), because seed production, germination success, and dispersal capabilities may limit re-establishment in large openings created by severe fire. To gain a better understanding of how ponderosa pine forests recover in relation to spatial patterns of burning as well as local factors, we quantified stem density and age structure of recovering forests in Spring 2005 at two study sites that burned in severe fire: La Mesa (New Mexico, 1977) and Saddle Mountain (Arizona, 1960). Based on pre- and post-fire aerial photography, sample plots were located within areas where all trees were killed by the fire event across a range of distances to legacy forest. Using generalized linear models, we determined the relationship of ponderosa pine stem density to three spatial burn pattern metrics: 1) distance to nearest edge of lower severity; 2) neighborhood severity, measured at varying spatial scales, and 3) scaled seed dispersal kernel surfaces.

Results/Conclusions

Pine stem density corresponded most closely with particular scales of measurement in both seed dispersal kernel and neighborhood severity. Elevation, heat load, soil characteristics, and ground cover also influenced tree density, but resprouter cover (e.g., Quercus spp., Populus tremuloides) was not a significant predictor in the models. Spatial patterns of burning remained important even after inclusion of subsequent disturbance and other variables in the models. Age structure analysis revealed that cumulative number of trees steadily declined across the range of distance to edge of legacy forest, but some regeneration was recorded at distances greater than 200 m. Trees gradually filled in areas at close distance, with slow progress in early post-fire years; greatest increases occurred from ca 1985-1995 at La Mesa and ca 1975-1985 at Saddle Mountain. In more recent years, less growth was initiated at locations closer to edge. Recovering populations apparently spread into openings in a moving front, as well as by remotely dispersed individuals. Based on our results, recent large fires cannot be summarily dismissed as catastrophic. As large fires continue to influence southwestern forests, natural recovery of post-fire landscapes will include areas with conservation value. In some places, patterns of recovering forests will differ from that expected under historic conditions. Nevertheless, diverse forest structures are potentially valuable components of sustainable ecosystems.