Carbon Cycle in the Western United States

Most mountain regions in the western United States are covered by forests, which are for the most part recovering from historical harvesting and have been experiencing active fire suppression over approximately the past 100 years (Tilman and others 2000). Whereas many western landscapes are currently perceived as pristine natural systems, the Rockies, Sierra Nevada, and Cascades were essentially deforested between 1860 and the end of the 20th century, during the era of mining, railroad building, and settlement. Currently, the fraction of old-growth forest remaining in the West is variously estimated at 5 to 15 percent; however, these numbers must be interpreted with caution. In some regions, high-elevation forests of limited current economic value are excluded from the analysis. In other cases, young, naturally disturbed stands are included in the disturbed category, even in forests with normally short disturbance cycles. Forest harvest has generally occurred preferentially in areas of relatively high productivity and standing biomass, so much of the regrowth is occurring in regions with relatively high carbon accumulation potential. Also, in some areas of active fire suppression adjacent to urban corridors, particularly at lower elevations with relatively productive conditions of soil, moisture and light, forests are becoming denser. Although the proportion of total forest land is probably higher than that quoted (taking into account regional variation, high-elevation forests), it remains that a very considerable fraction of the more productive forest lands have experienced some degree of historical disturbance.