* Critical Findings
ASSESSMENT
* Logging in the Sierra Nevada
Management Strategies
* Implementing SNEP Forest Strategies
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In contrast, human activities have altered the structure of many forests in the Sierra
Nevada directly and indirectly. Timber harvest has removed trees, snags, and logs,
especially of larger diameters, simplifying forest structure. Denser and less diverse
stands have been purposely created following harvest to accelerate timber production.
The period of aboriginal occupation likely was one of increased fire frequency, with
consequent lower fire intensities. Modern fire suppression has led to the invasion
of shade-tolerant trees into existing older stands, producing greater vertical and horizontal
continuity in canopies and largely excluding shrubs and herbs. This dense in-growth
lacks the structural and ecological diversity of naturally disturbed forests and is vulnerable to high-intensity, stand-destroying fire.
Most of the timber harvest for the last half-century in the Sierra Nevada (on private
and public lands) has been selective (partial) cutting rather than clear-cutting,
although early logging (1850-1920) was often by clear-cutting of large areas. As a
consequence, harvested forest stands often contain substantially more structural complexity,
and more elements of a natural late successional stand, than would have been the
case following clear-cutting. The potential contribution of these managed stands
toward late successional ecological functions of the Sierra depends greatly upon their size
and on the forest matrix in which they occur, but they are an important legacy in
the Sierra and are considered along with the contribution of unharvested stands.
Approach to Late Successional Analysis
In recent years late successional stands in the Sierra were mapped, largely using
remote sensing imagery from satellite and ground sampling and subsequent computer-assisted
classification. SNEP used a novel approach to identify and map remaining late successional forests on Sierran public lands. New approaches were necessary because of
the size of the range itself and the complex spatial distribution of late successional
elements on the landscape. In middle-elevation conifer forests, late successional
forest structures, especially elements of structural complexity, provide readily observed
surrogates for ecological functions (e.g., nutrient cycling, decomposition) and for
species that depend upon late successional forest but that are difficult to observe
directly.
Major elements of the SNEP analysis were (1) adoption of structural complexity as
the measure and surrogate for level of late successional function; (2) creation of
a six-point ranking scale for structural complexity; and (3) identification, mapping,
and characterization of landscape-level units (polygons of 1,000 acres or larger) to serve
as the basic units of analysis (see volume II, chapter 21, for detail on methods).
SNEP mapped conditions on public lands, including national forests, national park
lands, and national resources lands (BLM) of the Sierra.
An experimental pilot mapping effort was applied to the Eldorado National Forest to
test and refine procedures. This pilot effort led to rules and standards for structural
complexity to ensure consistency in mapping over the range. Subsequently, mapping
and characterization were carried out by a large team of resource specialists assembled
from the federal and state land units of the Sierra, directed by members of SNEP.
These specialists were used because of their familiarity with on-the-ground conditions.
A wide variety of source materials, including aerial photographs, satellite imagery,
and maps showing forest conditions and habitat suitability as well as personal knowledge
of forest conditions, was used by the specialists to delineate landscape polygons
and characterize the patches within them.
The polygons, generally of several thousand acres each (although significantly smaller
in the national parks), were delineated on maps based upon overall forest type and
characteristics of structural complexity. More than 2,800 such polygons were mapped
on the public lands of the Sierra Nevada. For each polygon, mappers described and ranked
several large, relatively homogeneous units called patches using late successional
structural features, including numbers of large trees, numbers of large snags and
logs, degree of canopy closure, and history of human disturbances. The ranks of these
patches were then aggregated to provide an overall rank for the larger polygon in
which they occur. Thus the landscape polygons were usually mixes of forest and nonforest
vegetation of varying composition and structure.
The six-point scale for ranking structural complexity and contribution to late successional
forest function in the Sierra Nevada ranged from 0 (low complexity, no contribution)
to 5 (very high complexity and contribution). Examples of areas that received low ratings were structurally simple forests, such as young plantations, areas recently
burned and salvaged, and landscapes that were largely nonforested, such as rock outcrops.
Ranking of 2 included maturing even-aged forests lacking large-diameter trees, snags, and logs. Ranking of 3 included areas that had been selectively logged or burned
but retained significant numbers of large trees and snags or where second-growth
forests were approaching maturity. Old-growth mixed conifer forests with open, parklike structures often produced by frequent low-intensity fire were typically given a ranking
of 4. Forests with the highest levels of structural complexity, including many large
trees, were typical of areas given a ranking of 5. For example, many national park areas outside zones where aggressive fire suppression has occurred were ranked 5.
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