Post-fire ponderosa pine planting in the southwestern U.S.A.

摘要

Burned areas are increasing in size and severity across the West, and natural conifer regeneration in dry southwestern forests is sporadic, episodic, and difficult to predict. In some burned areas, conifers do not regenerate for decades, raising concerns over lost carbon stocks, altered wildlife habitat, and long-term deforestation. Where timing and funding allow, forest managers are planting trees, typically on federal lands, to accelerate regeneration or reduce the risk of deforestation. However, little is known about success of plantings and natural regeneration after intense burning. We compared post-fire planting sites to unplanted sites at eight intensely burned wildfires in Arizona and New Mexico where planting occurred 5 - 9 years ago on national forest land. We measured seedling density and size, plant competition, presence of seed source and nurse objects, and abiotic site variables (e.g. slope, aspect, elevation, and burn severity). We found that planted sites have on average 14% greater seedling density than unplanted sites, and that effects of planting varied by fire and by elevation. At planted sites, 34 plots (74%) had seedlings present, and 12 plots (26%) had no seedlings present. At unplanted sites, 15 plots (32%) had seedlings present, and 31 plots (68%) had no seedlings present. Seedling densities were greater in planted than unplanted plots at five of six fires that had seedlings present. Survival of planted seedlings was 70% at the Borrego Fire, 42% at the Rodeo-Chediski Fire, 33% at the Lakes Fire, 32% at the Montoya Fire, 20% at the Ponil Complex, 12% at the Viveash Fire, and 0% at the Indian and Pine Canyon Fires. Pooled over planted and unplanted sites, seedling presence was more likely with increasing years since fire and increasing forb cover. The effect of planting also varied with shrub canopy cover and ponderosa pine basal area. Planting treatment effects were stronger with increasing shrub canopy cover, and decreased with increasing ponderosa pine basal area. On planted sites, seedling presence was positively associated with forb cover, whereas seedling density was positively associated with elevation and precipitation since fire. On unplanted sites, seedling presence was positively associated with litter cover. Seedling density on unplanted sites was positively associated with elevation and forb cover, and negatively associated with coarse woody debris and lower eastness values. At five of eight fires, natural regeneration was sufficient to regenerate low to medium density stands of ponderosa pine in future decades. Where managers decide to plant, waiting more than three seasons and selecting sites with no residual basal area of ponderosa pine, slight/low percent cover of shrubs and forbs, high elevation and precipitation should improve planting survival.