 * Critical Findings ASSESSMENT * Logging in the Sierra Nevada * Implementing SNEP Forest Strategies
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ASSESSMENT
The forests of the Sierra Nevada are complex in composition, structure, and function,
reflecting wide variations in environmental conditions on both local and regional
scales, and varied histories of natural and human disturbance. This complexity makes
an assessment of forest conditions challenging. The term old growth
has, in common parlance, suggested ancient forests undisturbed and unaltered through
time. In reality, all forests are dynamic, although the rate and spatial distribution
of change varies widely from region to region. Under ideal conditions, Sierran trees may live from several centuries (common) to several thousand years (uncommon), depending
on species. Changes in climate over the past 10,000 years (after the end of the Pleistocene)
have resulted in a continuously changing mix of species aggregations. Fire, drought, insect attacks, wind, avalanches, and other disturbancesoften in combinationhave
typically modified and not infrequently destroyed entire stands of trees. As seedling
trees are added and other trees in a stand grow, mature, and eventually die, both the appearance and the ecological function of the stand and the forest of which
it is a part evolve until they reach a condition we refer to as late successional.   Table 6.1 Actual View (71K) Of these, our assessment of late successional old-growth (LSOG) forests has been directed principally toward the conifer forest types growing at middle elevations, the commercially important west-side mixed conifer, white fir, red fir, east-side mixed conifer, and east-side pine forests (plate 6.1). These are forest types in which structural complexity continues to increase with stand age for at least several centuries, and for which the ecological differences between late successional and earlier successional stages are distinctive and relatively well understood. Conifer forests within the middle-elevation forested zones of the Sierra Nevada that are not disturbed by logging, clearing, or severe fire tend to develop complex structures over time. That is, most often the trees reflect a variety of sizes and conditions and, especially in the case of mixed conifer types, variety of species as well. There are large standing dead trees and down logs present, not as a by-product of timber harvest but through the natural processes of senescence and decay. Patches dominated by large, mature, and old trees are interspersed with openings and younger stands (or even single trees), forming a fine-scale mosaic resulting in both complexity from ground to tree canopy (vertical complexity) and spatial (horizontal) complexity (figure 6.1).  Figure 6.1 (Actual View 18K) Schematic cross section of typical west-side mixed conifer forest illustrating the structural complexity and spatial patterning of high-quality late successional stands ranked 4 and 5. (From volume II, chapter 21. Drawing by Robert VanPelt.) The forest floor itself becomes more complex through the accumulation of organic matter and associated organisms. These late successional forests provide habitats for animals and plants that are not available in areas of extensive young forests, as well as regulating snowmelt, modifying biochemical processes, and moderating temperatures below their canopies. Forests and woodlands composed of other tree species, such as foothill pine woodlands and oak woodlands and forests, riparian hardwood forests, piñon-juniper woodlands, and the several types of subalpine woodlands (e.g., whitebark pine) and forests (e.g., lodgepole pine) represent 40% of the Sierra Nevadas tree-dominated vegetation. These types also undergo structural succession that results in trees that are often very old and very large. They produce ecologically and aesthetically important structural elements, such as large snags and logs, but they generally do not develop the canopy cover, tree density, structural complexity, or patch dynamics over the substantial areas associated with middle-elevation late successional conifer forests. Our understanding of differences in ecology between early and late successional stages of these types is only partial, and although SNEP classified structural complexity in these forests, interpretations about successional status are not discussed here. The structural complexity of natural stands reflects local environmental conditions, such as microclimate, soil depth and chemistry, water table, and disturbance patterns. Particularly at higher elevations, rock outcrops, thin soils, wetlands, and frost pockets further enrich the forest mosaic while constraining the size of the trees themselves (figure 6.2). 
Figure 6.2 (Actual View ?K) An example of an area of higher elevation forest with low structual complexity. Exposed granitic outcrops dominate the site. (Photo by Jerry Franklin.) Prior to the mid-1800s, the most significant disturbances at lower and middle elevations were apparently frequent, usually light to moderately severe fire, which thinned stands, created (usually) small openings, and generated as well as consumed snags and logs. Drought, insects, and disease killed individual trees or aggregations, providing another source of dead woody debris. Large, severe, forest-destroying fire events resulting from the interaction of drought, insect outbreaks, and extreme weather undoubtedly occurred in the Sierra Nevada, but their importance in constructing its successional landscape is a matter of conjecture. 
 
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