Historical Changes in Ponderosa Pine Forests Since Euro-American Settlement

ABSTRACT

Most ecosystems require periodic disturbances to maintain their integrity, and human modification of natural disturbance regimes (e.g. fire frequency) often leads to deleterious changes. Ponderosa pine (Pinus ponderosa P. & C. Lawson) forests in the western United States are selected as examples. They were maintained in an open, park-like condition because of frequent low-intensity disturbances: fires and/or pest outbreaks. Fires enhanced grass cover, helped to reduce the buildup of organic matter, eliminated weak trees, and controlled pests. Euro-American settlement changed this balance by allowing heavy livestock grazing, which caused damage to the grass cover and contributed to soil erosion and depletion of the nutrient pool. Later, the policy of fire suppression promoted the establishment of a greater density of ponderosa pine and understory thickets at the expense of the grasses and caused excessive accumulation of coarse organic matter. Productivity of the forest subsequently declined and the thickets stagnated. Today, even unlogged ponderosa pine forests exhibit profound signs of stress: slow production and growth, decreased rate of nutrient cycling, simplified vertical and horizontal structure, and increased extent of disease. Some ecosystem services, such as the provision of soil and water quality, high biodiversity, and aesthetic value, are impaired.