Issues in Ecology Number 6 Spring 2000 Under intensive forestry management.the most vulner: Snaos or standing dead trees.alone with other wood able communities are the unique and biologically rich ones debris.provide important functions in forests(Harmon et al associated with forests older than harvest age (over 20 to 1986).Over the long term.of course.they contribute to 100 years depending on forest type and product:Amaranthus soil fertility through their decomposition.but in the mean et al.1994,Franklin et al.1981,Marcot 1997);hardwoods time they serve as important structural elements to prevent because repeated cutting of conifers on short rotation cycles erosion and provide habitat for many organisms.Most wood discourages the establishment of these late-successional spe- pecker species.for example.nest in cavities they excavate cies):and from standing dead trees.and fallen dead trees provide habi tat for numerous species,both on land and in streams (Fig ure 3:McArthur 1989.Sedell et al.1988). ere ar natura age,size Extensive c of many fore ms into sma open,cutover h I).Eac actors plays ucture of lorest ecosy gants the n while specie and e ferer ag For spe in mi iable mate at the forest edoe may also affect seed di leave trees of various ac some partially function osition rates and size of n others dead.which con bute to the reger and animal nonulations forest and provide microhabitat for man Forest managers must examine effects of fragmentation Timber harvesting.especially clearcutting.leaves large on a species-by-species hasis with emphasis placed on imner swaths of onen area.in contrast.natural disturbances cre iled species and also "kevstones"-species that play a dis ate gaps of mixed sizes depending on cause.These can range proportionately vital role in an ecosystem relative to thei from a single tree-fall gap to large blowdowns caused by hurri abundance and whose removal has large ripple effects on canes and tomadoes.Tornadoes in boreal forests,for example other plants and animals as well as on ecological processes may create clearings measuring over 100,000 hectares. To reduce the impact of timber harvesting on biodiversity sure 4-Timber harvest on federal lands has favored a dispersed patch clearcutting technique in many ns.including th Pacific Northy sed resulted in the fraementation of ma mall fores patches.which do not provide intact forest conditions.and immense amounts of edge.which create many problems in main. taining forest stability and diversity.Warm Springs Indian Reservation(previously Mount Hood National Forest).Oregon. 7 Issues in Ecology Number 6 Spring 2000 Under intensive forestry management, the most vulner￾able communities are the unique and biologically rich ones associated with forests older than harvest age (over 20 to 100 years depending on forest type and product; Amaranthus et al. 1994, Franklin et al. 1981, Marcot 1997); hardwoods (because repeated cutting of conifers on short rotation cycles discourages the establishment of these late-successional spe￾cies); and riparian zones, wetlands, and streams (Gregory et al. 1987, Kuenzler 1989,Thomas 1979). Changes in Forest Structure At the stand level, there are three important differences between natural and harvested forest stands: age, size of gap openings, and abundance and distribution of large dead woody debris (Morrison and Swanson 1990, Sharitz et al. 1992, Spies and Franklin 1991). Each of these factors plays a key role in functioning and structure of forest ecosystems. Clearcutting results in even-aged regeneration of trees, while natural disturbances such as fire and wind can result in uneven-aged regeneration. For example, fire creates dif￾ferent effects on individual trees in a stand depending on temperature, time of day, and position in the burn, and it also influences establishment of seedlings. These variables leave trees of various ages, some partially functioning and others dead, which contribute to the regeneration of the forest and provide microhabitat for many species. Timber harvesting, especially clearcutting, leaves large swaths of open area. In contrast, natural disturbances cre￾ate gaps of mixed sizes depending on cause. These can range from a single tree-fall gap to large blowdowns caused by hurri￾canes and tornadoes. Tornadoes in boreal forests, for example, may create clearings measuring over 100,000 hectares. Snags or standing dead trees, along with other woody debris, provide important functions in forests (Harmon et al. 1986). Over the long term, of course, they contribute to soil fertility through their decomposition, but in the mean￾time they serve as important structural elements to prevent erosion and provide habitat for many organisms. Most wood￾pecker species, for example, nest in cavities they excavate from standing dead trees, and fallen dead trees provide habi￾tat for numerous species, both on land and in streams (Fig￾ure 3; McArthur 1989, Sedell et al. 1988). Forest Fragmentation Extensive clearcutting has resulted in the fragmentation of many forested ecosystems into smaller patches that have more forest edge exposed to open, cutover habitats (Figure 4; Harris 1984). The effects of such fragmentation on forest remnants include changes in the microclimate (Chen et al. 1995), in species composition, and in species behavior. Changes in species composition may include loss of some species as a result of unsuitable forest micorenvironment, competitive interactions with species at the forest edge, or insufficient total foraging habitat. The change in microcli￾mate at the forest edge may also affect seed dispersal, move￾ment of flying insects, decomposition rates, and size of plant and animal populations. Forest managers must examine effects of fragmentation on a species-by-species basis with emphasis placed on imper￾iled species and also keystones species that play a dis￾proportionately vital role in an ecosystem relative to their abundance and whose removal has large ripple effects on other plants and animals as well as on ecological processes. To reduce the impact of timber harvesting on biodiversity, Figure 4 - Timber harvest on federal lands has favored a dispersed patch clearcutting technique in many regions, including the Pacific Northwest. Unfortunately, the technique used resulted in the fragmentation of many landscapes, creating small forest patches, which do not provide intact forest conditions, and immense amounts of edge, which create many problems in main￾taining forest stability and diversity. Warm Springs Indian Reservation (previously Mount Hood National Forest), Oregon. Photo by Jerry Franklin.