Ponderosa Pine Forest

Abstract

Ponderosa Pine Forest is the lowest-elevation, most extensive forest in the American Southwest. It occurs in an elevational band below Mixed Conifer Forest and above Pinyon-Juniper vegetation, Gambel Oak Shrubland, and Interior Chaparral Shrubland. Stands are dominated by ponderosa pine, and are divided into moist, mesic, and dry stand types with decreasing elevation and moisture availability. A historical fire regime of frequent, low-severity surface fires is widely documented, but there is growing evidence of historical mixed-severity and high-severity fires, especially for steep slopes in areas of heterogeneous topography. Other important natural disturbances include insect outbreaks and drought. Late nineteenth century livestock grazing initiated fire exclusion, which was continued by fire management through most of the twentieth century. Other anthropogenic drivers are modern climate change, invasive species, recreation, and nearby land use. Vegetation dynamics are dominated by tree regeneration, thinning, and succession. Historical conditions ranged from open-canopied stands with a well-developed, often grass-­dominated understory – more woodland than forest – to denser stands. Stand densities increased during the twentieth century because of the exclusion of surface fires. Vegetation dynamics are illustrated in a nested, three-tiered set of conceptual models. Key conclusions and challenges for researchers and land managers are summarized.