CHAPTER 6 Late Successional Old-Growth Forest Conditions ❆ CRITICAL FINDINGS ality, all forests are dynamic, although the rate and spatial distribution of change varies widely from region to region. Status of Current Late Successional Forests Late successional Under ideal conditions, Sierran trees may live from several old-growth forests of middle elevations (west-side mixed conifer, red centuries (common) to several thousand years (uncommon), fir, white fir, east-side mixed conifer, and east-side pine types) at depending on species. Changes in climate over the past 10,000 present constitute 7%–30% of the forest cover, depending on forest years (after the end of the Pleistocene) have resulted in a con- type. On average, national forests have about 25% the amount of tinuously changing mix of species aggregations. Fire, drought, the national parks, which is an approximate benchmark for pre-con- insect attacks, wind, avalanches, and other disturbances— tact forest conditions. East-side pine forests have been especially often in combination—have typically modified and not in- altered. frequently destroyed entire stands of trees. As seedling trees are added and other trees in a stand grow, mature, and even- Forest Simplification The primary impact of 150 years of forestry tually die, both the appearance and the ecological function of on middle-elevation conifer forests has been to simplify structure (in- the stand and the forest of which it is a part evolve until they cluding large trees, snags, woody debris of large diameter, canopies reach a condition we refer to as late successional. of multiple heights and closures, and complex spatial mosaics of veg- Old growth is incorporated within the broader category of etation), and presumably function, of these forests. late successional forest conditions in the following analysis. Contribution to late successional forest function refers to the Distribution of Late Successional Forests Four Sierran national ability of a stand or landscape to provide habitat for species parks, Lassen Volcanic, Yosemite, Sequoia, and Kings Canyon, pro- that prefer or require late successional forest conditions and vide most of the remaining large contiguous areas of late successional to carry out ecological functions of the types and at levels forests in middle-elevation conifer types. characteristic of late successional forest ecosystems, such as regulation of hydrologic regimes. Thus old growth and late Historic Conditions of Federal Lands Much of the best of the ac- successional are used as interchangeable terms here. Some of cessible pine forest was cut before the national forests were created. the ecological functions peculiar to the late successional stage Many national forest lands were created from the leavings: cutover can operate at the scale of an individual stand; others require lands, steep canyon walls, high montane forests, and relatively inac- much larger landscapes of intact forest. cessible timberlands. SNEP used ten principal forest types for late successional analysis in the Sierra Nevada (table 6.1). Of these, our assess- Continuous Forest Cover Despite 150 years of Euro-American tim- ment of late successional old-growth (LSOG) forests has been ber harvest activity in the Sierra Nevada, clear-cut blocks larger than directed principally toward the conifer forest types growing 5–10 acres are at present uncommon in the conifer forests of the at middle elevations, the commercially important west-side Sierra Nevada, and tree cover is relatively continuous. mixed conifer, white fir, red fir, east-side mixed conifer, and east-side pine forests (plate 6.1). These are forest types in Forest Mortality Over the past decade, as they have many times in which structural complexity continues to increase with stand the past, Sierra Nevada conifer forests have experienced widespread, age for at least several centuries, and for which the ecological locally severe mortality caused principally by bark beetles infesting differences between late successional and earlier successional trees stressed by drought, overdense stands, and pathogens. 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, clear- ing, 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, ASSESSMENT variety of species as well. There are large standing dead trees and down logs present, not as a by-product of timber harvest The forests of the Sierra Nevada are complex in composition, but through the natural processes of senescence and decay. structure, and function, reflecting wide variations in envi- Patches dominated by large, mature, and old trees are inter- ronmental conditions on both local and regional scales, and spersed with openings and younger stands (or even single varied histories of natural and human disturbance. This com- trees), forming a fine-scale mosaic resulting in both complex- plexity makes an assessment of forest conditions challeng- ity from ground to tree canopy (vertical complexity) and spa- ing. The term old growth has, in common parlance, suggested tial (horizontal) complexity (figure 6.1). The forest floor itself ancient forests undisturbed and unaltered through time. In re- becomes more complex through the accumulation of organic Modoc NF Alturas Distribution of Five Forest Types Lassen NF Eastside mixed conifer White fir Lassen Volcanic NP Montane mixed conifer Upper montane red fir Plumas NF Eastside pine, some eastside mixed conifer Administrative boundary SNEP study area boundary Reno Lake Tahoe Basin Tahoe NF Management Unit South Lake Tahoe Sacramento Eldorado NF Toiyabe NF Stanislaus NF Yosemite NP Inyo NF Sierra NF Fresno Sequoia and Kings Canyon NPs 0 10 20 30 40 50 60 70 80 90 100 Kilometers Sequoia NF 0 10 20 30 40 50 60 70 80 Miles Tehachapi SNEP GIS Center PLATE 6.1 Distribution of five forest types in the Sierra Nevada that were assessed for successional status. (From volume II, chapter 21.) TABLE 6.1 Characteristics of the major forest type groups of the Sierra Nevada. (From volume II, chapter 21.) Dominant Trees Presettlement Fire Regime Forest Type Northern Sierra Southern Sierra Landscape Patterns Primary Disturbances Northern Sierra Southern Sierra Foothill pine and oak Foothill pine, ponderosa Foothill pine, ponderosa Mostly open structure, Fire, insects, pathogens, Low-severity regime: Same pine, blue oak, live oak, pine, blue oak, live oak limited patches of dense drought frequent, low-intensity Douglas fir forest, frequent natural fires openings (chaparral and outcrops) West-side mixed conifer Douglas fir, ponderosa Ponderosa pine, sugar Primarily continuous forest Fire, insects, pathogens, Low- to moderate-severity Low-severity regime: pine, sugar pine, white fir, pine, incense cedar, with few extensive natural drought regimes: areas > 50 inches dominantly low-intensity incense cedar, black oak, black oak, giant sequoia, openings (e.g., outcrops) annual precipitation likely fires tan oak Jeffrey pine mixture of low- and moderate-intensity fires in complex mosaic with sufficient variability in interval to perpetuate Douglas fir; areas < 50 inches annual precipitation likely more dominantly low- Sierra Nevada Ecosystems Sierra Nevada intensity fires; infrequent large-scale high-severity fires White fir White fir Same Same as west-side mixed Insects, pathogens, fire, Moderate-severity regime: Same? conifer drought frequent but variable 93 extent or frequency, variable intensity with small patches of moderate to high intensity Red fir Red fir, lodgepole pine, Same Fine- to moderate-scale Insects, pathogens, fire, Moderate-severity regime Moderate-severity regime western white pine high patch diversity of drought, wind, avalanche (same as white fir) (same as white fir) natural openings (meadows, outcrops) and open or closed forest; large, extensive patches limited Jeffrey pine Jeffrey pine Same Generally extensive Insects, pathogens, fire, Low-severity regime: low Low-severity regime (upper montane) uniform patches of very drought intensity and/or small open forest or woodland extent of fires due to interspersed with small discontinuous fuels pockets of denser forest Subalpine Lodgepole pine, mountain Lodgepole pine, mountain Highly variable patterns Avalanche, wind Low-severity regime: low hemlock, western white hemlock, western white but generally diverse intensity and/or small pine, whitebark pine pine, whitebark pine, patch mosaic with large extent of fires due to foxtail pine, limber pine, meadows, small patches discontinuous fuels and western juniper of dense forest embedded infrequent ignitions (due in a large matrix of open to precipitation forest or scattered trees associated with lightning) and rock outcrop continued TABLE 6.1 (continued) Dominant Trees Presettlement Fire Regime Forest Type Northern Sierra Southern Sierra Landscape Patterns Primary Disturbances Northern Sierra Southern Sierra East-side mixed conifer and White fir, ponderosa pine, White fir, Jeffrey pine Variable patterns, most Fire, insects, pathogens, Low- to moderate-severity Same white fir Jeffrey pine (some often occur in a coarse- drought regime: dominantly frequent Douglas fir, sugar pine, scale mosaic with low-intensity fires but with incense cedar) east-side pine related to variable intervals, aspect enabling recruitment of Douglas fir and white fir to large