Ponderosa pine forest

Ponderosa pine forest is a plant association and plant community dominated by ponderosa pine and found in western North America. It is found from the British Columbia to Durango, Mexico. In the south and east, ponderosa pine forest is the climax forest, while in the more northern part of its range, it can transition to Douglas-fir or grand fir, or white fir forests. Understory species depends on location. Fire suppression has led to insect outbreaks in ponderosa pine forests.

Ponderosa pine forest is the largest western forest type in the United States. Ponderosa pine is the principal species on over 110,000 km² (27,000,000 acres) and is present on an additional 55,000 km² (14,000,000 acres). Within the western United States, California alone contains the greatest concentrations of ponderosa pine (20,700 km² or 5,100,000 acres) closely followed by Oregon with 19,000 km² (4,700,000 acres) and, when combined, Arizona and New Mexico contain an additional 25,000 km² (6,200,000 acres) of ponderosa pine.

Since ponderosa pine has a rather wide range of adaptability and can dominate some of the less mesic true forest sites, it occupies low mountains and foothills in many places; yet in mixtures with other species, it is found at moderate elevations.

Local topography can be gentle, as on plateau tops and low mountains, or it can be steep, as on canyon walls and faces. Semiarid, low mountain sites are common, but it is difficult to characterize the physiography of the ecosystem because it varies greatly.

Ponderosa pine forest occurs when the average mean temperature is between 42 and 50 °F (6 and 10 °C) and the July/August mean temperature is 62 to 70 °F (17 to 21 °C).

The length of the frost-free season at any location in the ecosystem depends largely upon the latitude and elevation. In general, in much of the northern area the frost-free period is 120 days, whereas in many foothill areas and in parts of the Sierra Nevada and the Gila Wilderness in Arizona, it can be 240 days. Annual precipitation is about 15 to 20 inches (380 to 510 mm) in the more pure pine areas, but where there are combinations of pine and other conifers at moderate elevations, the precipitation can total 30 inches (760 mm). In the far western sector, only 25 percent of this moisture falls in the warmest months. In the Northern Rocky Mountains, 40-50 percent of the precipitation occurs in the warmest months. In the middle to southern Rockies, 66 to 75 percent occurs in the warmest months. In the southern areas and those at low elevations, precipitation during the frost-free season is only 20 percent of the evaporation potential. In northern areas precipitation during the growing season may be as high as 40 percent of the evaporation potential.

Ponderosa pine forest can occur in different types of ecological site. Each of these sites can be characterized by its potential vegetation type (PVT) after succession reaches equilibrium or climax community.

In the southern and extreme eastern portion of the range, ponderosa pine grows primarily on ponderosa pine PVTs. On these settings, quaking aspen is the most frequent early-seral tree species. Ground-level vegetation includes oaks (Quercus spp.), grasses (Festuca and Andropogon spp.), and low shrubs (e.g., snowberry (Symphoricarpos spp.) and spirea (Spirea spp.)). Russet buffaloberry (Shepherdia canadensis), a frequent shrub in these forests, stands out for its nitrogen fixing properties which is the process of making elemental nitrogen in the atmosphere available to plants.