Old-growth Forests of the Sierra Nevada
There are a variety of definitions for old-growth forests, but they are generally defined as forests in their later stages of development, usually referred to as late-seral or late successional. Approximately 4 million acres of old-growth are remaining in the Sierra Nevada.
Trees sizes for old-growth have been variously defined as trees over 24" diameter with medium to high canopy cover, to 30" trees and 150-200 years in age. Tree size and age-related structure are a result of growing conditions, species type, elevation, and climatic conditions resulting from the diversity of ecosystems in the Sierra Nevada.
There has been an alarming decline in old-growth forest acres and structure in the Sierra Nevada since the 1850's. Estimates vary on the amount of old-growth that existed in the pre-settlement era, depending on what time period is considered. The best estimate (SNEP 1996) states that formerly, late-successional habitat occupied roughly 67% of the mixed conifer forest landscape. Today, it is estimated that only 12% of mixed conifer old-growth forest remains, a staggering loss of approximately 82% of the historical acreage of old-growth found in mixed conifer forests.
Much of this loss is from the high-grading (a type of selective cutting where some or all of the biggest and best trees are cut) of large trees, railroad logging, and clear cutting logging practices, which continued up to the early 1990's on Federal land and continue today on industrial timber lands in the Sierra Nevada. During the logging "hey-days" of the 1980's the Forest Service was producing over 1 billion board feet of saw-timber annually from the Sierra Nevada, much of it in large old-growth trees.
Old-growth forests can play a significant role in storing carbon and helping to mitigate global warming. Old-growth forests in the Northern Hemisphere sequester ten percent of the world's net ecosystem productivity, or NEP, a measure of carbon sequestered. Their value in maintaining a climate that can sustain life far outweighs the short term economic gain from cutting them down for lumber.
Old-growth trees provide a critical habitat structural component of species like the imperiled California spotted owl. A study of California spotted owl nest locations in the southern Sierra Nevada (North 2000) found the mean age of nest trees to be >225 years. Old trees develop important characteristics such as cavities and broken tops, which provide ideal and integral nest sites. Also, the large canopies often found in old-growth stands provide much needed shade in hot summer weather for the heat sensitive spotted owl.
Other imperiled species in the Sierra Nevada that depend on intact old-growth habitat are the Flammulated owl, Northern flying squirrel, Pacific fisher, and Plieated woodpecker. Continued degradation of the remaining old-growth in the Sierra Nevada will likewise continue to put pressure on these threatened wildlife species.
The original 2001 Sierra Nevada Framework Plan called for 4.2 million acres of old forest emphasis areas with specific goals of protecting and enhancing old-growth forests. The 2004 revisions to the Framework allow intensive harvesting of 20"-30" trees in 75% of the old-growth reserves across the Sierra Nevada.