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5 FOREST SUCCESSION, FIRE, AND LANDSCAPE DYNAMICS /NTRODUCT/ON Disturbances such as fire, wind, and insect and pathogen outbreaks occur naturally in all Pacific Northwest forest types, although the frequency, intensity, and spatial extent of such events vary considerably. Natural disturbancesespecially fireshape the distribution and abundance of many forest species and influence key ecosystem processes across Pacific Northwest forest landscapes. Much of the debate and discussion regarding the status and management of Pacific Northwest forest ecosystems focuses on assertions regarding the status and dynamics of landscapes before European settlement and the manner and extent to which current management practices simulate them. Among those assertions are · Suppression of fire and, to a lesser extent, other disturbance factors have resulted in ecosystem charges (e.g., the accumulation of fuels) that put landscapes at risk for catastrophic disturbance. · In some cases, fire, insects, and pathogens disturbed forests with return times that roughly approximated timber rotations. · Logging activities represent a reasonable surrogate for natural disturbances. This chapter presents a conceptual framework for understanding the factors responsible for varying patterns of natural disturbance and the successional processes that derive from them. The specific role of fire 108
Forest Succession, Fire, and Landscape Dynamics 709 with regard to the dynamics of Pacific Northwest forested landscapes is discussed, with particular attention to the effects human activities have on fire regimes across the region. Finally, the effects of disturbance associated with forest management and timber-related activities are compared with those of natural disturbance processes. CONCEPTS OF SUCCESS/ON AND LANDSCAPE DYNAMICS Many of the land- and natural-resource management policies and protocols developed in the first half of this century were strongly influenced by a comprehensive theory of succession described first by Cowles (1910) and subsequently refined by Clements (1916, 1928), both of whom worked primarily in forested regions. Central to that theory was the view that any disturbance initiated a linear and directional successional process cuhninating in a stable climax community with a structure determined largely by the constraints of climate. Such climax communities were thought to be composed of species able to reproduce successfully in the context of small-scale disturbances, such as tree-fall gaps. Large-scale disturbances, such as fire, were viewed largely as the result of human interventions, and presettlement North America was thought to have been dominated by vast, unbroken expanses of climax forests. Climax communities were thought to be the most stable species assemblages, and it seemed to follow that successional change pro- ceecled on an inexorable path toward increasing stability (Odum 1969~. Over the past 3 decades, virtually every aspect of this classical theory of succession has come under question (e.g., Drury and Nisbet 1973; Connell and Slatyer 1977; Egler 1977; Sousa 1984), and the change in understanding of succession has major implications for the management of Pacific Northwest forests. We now recognize that fire and other disturbance processes operate over a wide range of spatial scales and were indeed an integral part of the primeval landscapes. Through evolutionary time, recurrent disturbances of varying intensity and frequency have selected for adaptations in the biota, making some species more resistant to and other species more resilient from the effects of disturbances. Life histories of many tree and other species have evolved to be dependent on such disturbances.
1 7 0 Pacific Northwest Forests Successional change does not necessarily lead to increased resistance to disturbance. Rather, successional change might facilitate future disturbance. For example, as forests develop, changes in structure, species composition, and the accumulation of organic matter alter fire susceptibility. Those changes may increase the likelihood of fire or other disturbances, as happens with the ingrowth of shade-tolerant firs and accumulation of woody fuels In Eastside ponderosa pine forests. Conversely, old-growth Douglas-fir and ponderosa pine forests tend to be less susceptible to crown fires than younger forests (Perry 198Sa; Franklin et al. 1989~. They are well buffered against wind; hence, they remain relatively moist. Tree crowns are high in the air, so the probabil- ity of fire moving from the ground into the crowns is low, at least in the absence of understory fire "ladders" (smaller understory trees). The thick bark of outgrowth Douglas-fir and ponderosa pine also increases their likelihood of surviving a ground fire, and ground fires (which historically were frequent in many forest types) reduce fuel loads while leaving the overstory of older trees frequently scarred but still intact. Disturbances vary in intensity, spatial extent, frequency, seasonality, and predictability. That variation results in different patterns of response often involving different suites of species within a given ecosystem (Agee 1993~. A landscape can be viewed as a collection of disturbance-reduced patches of varying size and undergoing change that is conditioned not only by the history of disturbance of a patch, but also by the character and position of the patches that surround it, against a background of yearly and decadal variation (Delcourt and Delcourt 1991~. The importance of the overall forest matrix on the patterns of change within individual stands or patches makes a landscape approach to understanding and managing ecosystems compelling. The temporal analog to spatial relationships"legacies" (Franklin 1 993b) provides additional complexity. Disturbances do not wipe the slate clean, but rather leave legacies such as residual debris, seed banks, advanced regeneration, and surviving plants and animals that influence future trajectories of change. The nature of such legacies depends on the status of the ecosystem before the disturbance (e.g., evenly aged thinning forest versus mixed-age old-growth forest), as well as the behavior (intensity and variability) of the disturbance itself. The legacies vary among fire regimes and between natural disturbances
Forest Succession, Fire, and Landscape Dynamics 1 1 7 (such as fire and wind storms) and management interventions (such as logging). Current theories of succession, although less simplistic than the classical ones, do postulate the existence of predictable patterns of change following catastrophic disturbances. One description of the pattern is a four-stage mode} of forest succession (see Feet and Chris- tensen 1987~: · The first stage-estabZishmentis characterized by extensive seedling establishment and rapid growth, during which competitive interactions among individual plants are somewhat limited. · Closure of the tree canopy initiates the second stagethinningduring which competition among more or less evenly aged, individual trees is intense. Overall stand biomass continues to increase rapidly cluring this stage, but the number of trees decreases as shaded and less-vigorous individuals die. Shade-tolerant trees may gradually invade the subcanopy during this stage. · Eventually, tree density (number of trees per unit area of lanct) decreases to the point where individual tree deaths create gaps that cannot be filled by the lateral growth of adjacent trees. That initiates the third stagetransitionduring which new individuals begin to fill gaps, creating an unevenly aged forest. During this stage, living biomass might remain constant or decrease while total soil and above-ground biomass might increase, owing to the accumulation of woody debris. · In the final stagesteady statethe first three stages occur as a mosaic of patches within the forest (Watt 1947; Bormann and Likens 1979~. How frequently this final stage occurs in most forested land- scapes is uncertain (Peet and Christensen 1987~. Variations on this four-stage process are common. For example, in some areas of the Pacific Northwest, the earliest forest canopy may be formed as an even-aged stand of red alder that is subsequently replaced by a more or less even-aged stand of Douglas-fir. Where the red alder cover is particularly dense, seedling establishment of Douglas-fir may be inhibited. Where seedling establishment is sparse and initial tree density or stocking generally low (e.g., many ponderosa pine forests), competitive thinning (stage 2) might not occur.
1 72 Pacific Northwest Forests Small-scale disturbances in forests (e.g., gaps created by the loss of one to a few trees) tend to facilitate the replacement of Douglas-fir by shade-tolerant species. An old-canopy, Douglas-fir tree that dies as part of natural tree mortality is likely to be replaced by hemlock or cedar seedlings expanding into the canopy opening (Spies et al. 1990~. Similarly, winter storms that blow downisolated trees or groups of trees usually do not open sufficient areas to permit Douglas-fir regeneration. If sufficient area is opened to permit regeneration, root rots can prevent seedling survival. Consequently, small-scale disturbances typically promote regeneration of shade-tolerant hemlock and red cedar on the Westside. However, if windthrow provided sufficient fuel accumula- tion, a subsequent fire could denude the site and reinitiate succession. In the absence of fire that eliminates hemlock and cedar, a Douglas-fir forest eventually will be replaced by a more diverse forest of shade- tolerant species. In over situations, patterns of fuel accumulation and frequency of ignition are such that the process is interrupted before reaching the later stages. Shade-tolerant trees may not become successfully established in forests with frequent ground fires (as in ponderosa pine and Douglas-fir on drier sites). FIRE AND LANDSCAPE DYNAMICS Among the various sources of natural disturbance, fire has been most important in affecting the forested landscapes of the Pacific Northwest. The frequency and intensity of fire within the region varies considerably depending on temperature and moisture conditions of the site, ignition patterns, and the characteristics of individual tree species and their susceptibility to fire (Agee 1981~. The most obvious differences occur between the west and east sides of the Cascade Mountains. Fires are less frequent in the moist climate of the Westside, and fire-return intervals are relatively long and highly variable. More frequent drought on the Eastside, coupled with different patterns of fuel accumulation, favor shorter fire-return intervals with relatively low-intensity surface fires (Agee 1993~. The relationship between moisture availability and fire frequency is complex (Martin 1982~. Biomass production and the rate at which flammable fuels accumulate particularly are correlated with available .
Forest Succession, Fire, and Landscape Dynamics 1 73 water. In seasonally dry areas, such as the Eastside, fuel production is sufficiently high to carry fires at more frequent intervals, and dry conditions that are favorable for ignition and fire spread are likely. Increased moisture and humidity favor higher rates of fuel production; however, drought periods during which fuels dry sufficiently to burn decrease in frequency. In general, fire Among the various sources of narura/ intensity (expressed disturbance, fire has been most as fuel consumed or important in affecting the forestecl energy expended /andscapes of the Pacific Northwest. per unit area) Is ~n- versely related to frequency. In forests typified by short fire-return intervals, the flammable fuels of the forest floor are spatially separated from the flammable needles and branches of the canopy. Fires In those forests tenet to burn only litter and small woody debris. Where fires burn less frequently, fuels are distributed more continuously from forest floor to canopy. Fires in these forests are more likely to spread to tree crowns. Presert/ement Fire Regimes and Successiona/ Change Before IS50, infrequent, severe fires ("stand-replacement fires") with a highly variable return interval of more than 100 years were common In the western hemlock/Douglas-fir forests, Pacific silver fir forests, and subalpine forests (Agee 1993~. Stand-replacement fires are followed by major changes in plant-species composition (Agee 1993~. Moderate- severity fireswith a 25- to 100-year interval between fireswere common in dry Douglas-fir, mixed evergreen, red fir, and lodgepole pine forests (Agee 1993~. Moist Douglas-fir forests in the central Cascades had a moderate fire regime, at least during the 1SOOs (Morri- son and Swanson 1990; Teensma et al 1991~. Fires of moderate severity often resulted in stands with two or more age classes of trees (Agee 1993~. Low-severity fireswith a 1- to 25-year interval between fireswere typical in oak woodlands, Ponderosa pine, and mixed- conifer forests and tendecl to maintain those ecosystems' which are tolerant of fire (Agee 1981).
7 74 Pacific Northwest Forests Huff (1984) noted that variations in fire regimes are highly correlated with the relative prevalence of Douglas-fir and hemlock. Douglas-fir, although relatively tolerant of fire, is intolerant of shading, whereas hemlock and cedar are less tolerant of fire but tolerant of shade. When fire-return time is 100-300 years, Douglas-fir is dominant; longer return times result in a mixture of Douglas-fir and ponderosa pine. Fire-return times exceeding 600 years favor hemlock dominance. The long intervals between fires on the Westside favor Douglas-fir domination early in the successional sequence, but late-successional forests increasingly become dominatect by western hemlock; hemlock begins to replace Douglas-fir at about 250 years. It can take several hundred more years without fire for western hemlock to attain stand dominance (Agee 1993~. A consider- able portion of the old growth of Westside forests was characterized by long-return fires (interval of more than 500 years) (Huff 1984~. In areas dominated by spruce and hemlock, very moist conditions and rapid decomposition of fine fuels result in fire return times of 500- 1,100 years (Fahnestock and Agee 1983; Huff 1984~. In northern California and southern Oregon, where the coastal redwood is impor- tant, fire regimes are much more variable. Return times of 400-500 years can be the norm in moist areas, and more frequent surface fires (every 50 years) typify drier locations. Severe winds can disturb these forests more frequently (Ruth and Harris 1979; Harcombe 1986), and large- scale (many hectares in extent) Slowdowns occur in these forests with a return time of 300-400 years (Harcombe 1986~. On the Eastside and over much of the Idaho-Montana region, frequent low- and moderate-intensity fires maintain an open, parklike forest dominated by fire-tolerant species, such as ponderosa pine and larch. Those forests often have open canopies with a heterogeneous understory of grasses and shrubs. Low-intensity fires have been frequent, with return intervals of 4-7 years common in some places (Arno 1980; Bork 1985~. Frequent fires have kept the stands open, providing sites for advanced regeneration of pines as canopy trees die from windthrow, disease, or occasional fire "hot spots" (Gruell ~ 985; Merrill et al.1980~. Succession after low-intensity fire events is rapid. Large trees are generally unaffected, and most herbs and shrubs resprout rapidly from belowground rootstocks, bulbs, and burls. Intense disturbances, such
Forest Succession, Fire, and Landscape Dynamics 7~5 as crown-kiHing fires, favor invasion by shrubs such as greenIeaf manzanita ancl deer brush. The mineral seedbecE and open light conditions after low-intensity fires favor heterogeneous patterns of pine establishment and thinning in localized patches. With subsequent thinning, pine canopies are kept open by repeated low-intensity surface fires. Long fire-return intervals in these forests favor growth of the shrubby understory species and invasion of shade-tolerant firs and Douglas-fir. On the Eastsicle and in Idaho and Montana, lodgepole pine forests are Apical of higher and moister elevations. Surface fires occur as fre- quently as every 50-80 years and stand-replacement fires occur with a 150-300 year return time. The intervals are considerably longer and less preclictable on the moist west slope of the Cascades. In those areas, lodgepole stands often are unevenly aged, and disturbance factors other than fire, such as bark beetles, can be far more important in determining stand structure. Large forest fires in 1910, 1919, and 1934 burned much of northern Idaho and western Montana, creating a variety of successional stages from shrub field to young conifer stancis, which characterize much of the area at present (Wellner 1970; Arno ~ 980; GrueD 1985~. Interspersed among the regrowth are old western hemlock, western red cedar, and Douglas-fir. Many of those communities are mixed-species forests, with western larch and western white pine also prominent. In the more southerly portions of the region, a history of frequent fires has also contributed to the development of a mosaic pattern of forests. Extensive stands of lodgepole pine characterize higher-elevation forests, while a complex of Douglas-fir and ponderosa pine forests dominate at lower elevations. Montana and Idaho forests dominated by Douglas-fir generally have longer fire intervals than ponderosa pine forests. Grand fir, western red cedar, and western hemlock forests have still longer intervals, but when burned (or logged and burned), a tall shrub complex develops that persists for as long as 50 years and that is eventually replaced as the conifer forest develops (Mueggler 1965; WitUnger et al. 1977; Crane et al. 1983; Stickney 1986; Morgan and Neuenschwander 1988~. Higher- elevation spruce-fir forests might be replaced by lodgepole pine in the overstory when burned, but understories characteristic of the spruce-fir
7 7 6 Pacific Northwest Forests communist resprout and are not replaced by a tall shrub complex (Lyon 1984). Human A/teratior' of Fire Regimes Over the past century, human activities in the Pacific Northwest have altered fire regimes enormously. On the Westside, development of urban areas and transportation networks, increased recreational use of forests, and timber activities have increased the frequency of fires in many areas (Pyne 1995~. Residual fuels and patterns of post-fire succession have facilitated successive and sometimes devastating reburning of many areas (e.g., the Tilamook fires) (Pyne 1982~. But reg~onwide generalizations are difficult because human land use and transportation corridors have broken the landscape and altered the movement of fires in complex ways that are not totally understood. Foresters have long recognized Over the past century, human the differences in activities in the Pacific Northwest have susceptibility to altered fire regimes enormously. fire of different age classes of forests. Andrews and Cowlin (1940), in their analysis of wildfires in western Oregon and Washington between 1924 and 1933, noted that, "A much higher proportion of the Douglas-fir seedling and sapling areas, of recently cut-over land, of old deforested burns, and of the noncommercial types is burned over annually than of saw timber areas." Because flammable fuels are more continuously distributed vertically and available for burning in early successional forests than they are in old-growth or late-successional forests, early successional forests are more susceptible to intense fires than their more mature counterparts. The extent and landscape-level continuity of those forests has increased owing to fires, timber activities, and other human actions, resulting in more flammable conditions in many areas of the Westside. Fire hazard has increased throughout much of western Oregon and Washington because of the increase in area of young forest plantations (Perry 198Sa; Franklin et al. ~ 989~. It has been known for many years that plantations are more vulnerable to crown fires than are healthy old- growth forests (e.g., Andrews and Cowlin 1940; Cowlin et al. 1942~.
Forest Succession, Fire, and Landscape Dynamics 777 Once a landscape reaches some critical proportion of susceptible types of stands, disturbances propagate across boundaries and affect even relatively unsusceptible types (Perry 1 98Sa; Turner et al. 1994~. Human activities have undoubtedly increased the frequency of forest ignition on the Eastside as wed; however, active suppression of fire has played a much greater role in determining the current status and fire regimes of forests in this region. Fire suppression and selective logging of old-growth trees on the Eastside has led to encroachment of fire- intolerant and pest-susceptible Douglas-fir and true firs in those forests (Anderson et al. 1987; Wickman et al. 1992; Mutch et al. 1993; Covington et al. 1994~. That in turn has led to more devastating fires and pest outbreaks in recent years than likely occurred in the past when fuels and host abundance were limiting factors (Anderson et al. 1987; Agee 1993; Hessburg et al. 1993; Wickman et al. 1992; USFS/BEM 1994~. Wilcifire-contro! programs over the past 75 years have been enor- mously successful in reducing the area burned each year, nationally and regionally (MacCleery 1995 ). At the same time, those programs have created "a different fire regime, one characterized by uncontrollable fires burning in heavy fuels" (Mutch et al. 1993~. Increasecl vulnerability to crown fires in Eastside forests can be traced to the same factors that have exacerbated insect end drought problems (see Chapter 3~. The combina- tion of successional ingrowth associated with fire exclusion ancE the development of more densely stocked stands after logging has increased the likelihood of extensive surface fires in the region. That condition probably extends to higher-elevation forests as wed (Perry 1 988a). in northern Idaho, where severe fires are more frequent than in most of Oregon and Washington, tree mortality after fire was approximately 70% in mature stands, 80% in thinning stands, and more than 90% in early-successional stands (Hutchison and Winters 1942~. Some of the high mortality In young stands on cutover sites in the early part of the century might have been due to burning residual logging slash. However, even with more thorough slash removal, young stancis are more vulnerable than mature forests and old-growth forests. For example, in the 1987 Silver Firethe most severe in southwest Oregon in at least 50 years 45% of the old-growth stands had less than 10% mortality, but this was true of only 18% of the small sawtimber stands (USES 1988a).
7 7 8 NATURAL DISTURBANCE AND HUMAN MANAGEMENT: AN ECOLOG/CAL COMPARISON Pacific Northwest Forests Much current debate regarding forest-management practices centers on the relationship and similarities between the patterns of natural disturbance and change discussed above and silvicultural practices (Hansen et al. 1991, Swanson and Franklin 1 992; Perry 1995a; Kohm and Franklin 1997~. Patterns of ecosystem response (e.g., nutrient transfor- mations, changes in species composition, and accumulation of flamma- ble fuels) vary considerably among fire and silvicultural regimes as a function of predisturbance history, site conditions, disturbance characteristics, end postdisturbance environment. Furthermore, itis not necessarily the case that simply because a disturbance is natural (having happened without human intervention), it is more benign or more conducive to sustaining forest health and ecosystem processes than forest-management activities (Christensen 1988; Christensen et al. 1989~. Landscape Considerations Patterns of natural disturbance, especially fire, were highly variable on landscapes before European settlement. Landscape pattern influences where fires burn and their intensities. When weather conditions are extreme (as they were during the Yeliowstone fires of 1988 or the great fires of 1910 in Idaho) fires may cross major patch boundaries, and a new mosaic is created (Turner et al. 1994~. Without intervention, fires will burn up to natural boundaries, such as rivers; ridges; and low-fuel ecosystems, such as rocky scree or feldfields. Except in the most extreme situations, variable fire behavior results in environmental variability, even within burn boundaries. The spatial scales over which timber and other resource management activities typically take place rarely are those for fire, and the boundary conditions usually are quite different. Boundaries of fogging units often are related to ownership boundaries typically do not coincide with ecosystem boundaries, such as watersheds and ridges. The checker- board patterns of cubing on railroad lands represents one extreme example of such arbitrary patterns. Harvest activities, especially clearcutting, generally leave a much less variable postdisturbance
Forest Succession, Fire, and Landscape Dynamics 7 79 environment than do fires. It is possible to devise silvicultural systems that mimic patterns typical of natural disturbances (Hansen et al. 1991; Swanson and Franklin 1992; Perry 1995b). However, patterns of ownership and economic considerations can make such systems impractical, and more research is needed to understand the potential benefits of such systems with regard to management goals (see Chapter in. Human development and building on fire-prone landscapes has increased the financial liability and risk to human life associated with catastrophic fires that can occur as a consequence of extensive fuel accumulations. Furthermore, development severely constrains the variety of management interventions that can be used to remedy the situation. For example, prescribed fire in heavy fuels close to homes or other structures is risky and expensive and can be applied on only limited scales. The use of prescribed fire as a management strategy is limited in some places owing to the effects of smoke on air quality near aIreacly polluted urban centers. Fuels Logging is often cited as a means of reducing fuels and, thereby reducing fire danger, particularly in areas where fire suppression has resulted in considerable fuel accumulation and where public liability from fire is high. But the direct effect of fire on fuels is highly variable. Repeated, low-intensity fires tend to produce fuel loads and structures (e.g., vertical stratification) that are not conducive to high-intensity, crown-killing fires. However, crown-killing fires often result in an increase in the load of flammable fuels in He form of charred trunks, and some of the largest burn complexes in the Pacific Northwest (e.g., the Tillamook Burn) occurred as reburns. Logging has been proposed as a possible surrogate for fire in reducing fuel accumulations with the added benefit of economic return (Agee 1993), but logging or clearcutting do not necessarily reduce flammable fuels. Residual slash can carry intense fires, and rapid regeneration of early-successional shrubs and trees can create highly flammable fuel conditions within a few years of cutting. Without adequate treatment of small woody residues, logging may exacerbate fire risk rather than
720 Pacific Northwest Forests lower it (Agee 1993~. if cutting such as that allowed under recent salvage logging legislation is to reduce fuel accumulation, considerably greater attention must be paid to logging and postharvest management practices that will accomplish that goal. Nurrient Fluxes Fire results in immediate oxidation of large amounts of carbon and rapid fluxes of nearly all nutrients. The loss of nutrient capital (especially nitrogen and phosphorus) can be substantial and is largely dependent on fire intensity and the quantity of fuel consumed. In fine fuels (small- diameter fuels) burning at high temperatures, as much as 70% of the nitrogen in the consumed fuel can be volatilized or gasified (phosphorus loss is usually considerably lower); however, in heavy fuels, that loss is usually more in the range of 20-40%. Nutrients not volatilized are added to the soil as char and ash, and a considerable portion of the above-ground nutrient capital can remain in burnt snags and woody debris that decompose slowly. Fire often results in an immediate flush of nutrient availability as a consequence of ash and a postfire environment conducive to m~neraliza- tion (e.g., Crier 1975; Christensen 1985~. That nutrient flush is important to the establishment of many forest plant species, and the removal of the litter mat and exposure of mineral soil by fire is important to successful germination and establishment of many conifer species. Increased nutrient mobility also increases nutrient loss to leaching, adding to the overall loss of nutrient capital, although those losses for nitrogen amount to less than ~ % of the total prefire capital. The flush of nutrient availability is short-lived (a year or two), and in some cases, subsequent soil-nutrient availability can be lower than prefire conditions. It is conventional wisdom that the loss of nutrient capital owing to forest cubing is considerably greater than that due to fire, and it seems obvious that clearcu~ng coupled with slash burning results in large nutrient losses. However, carefully controlled studies comparing nutrient budgets among different fire regimes or cutting treatments have not been conducted. Whether nutrient losses associated with natural or human-caused disturbance are detrimental in the long term depends on the patterns and rate of recovery of nutrients. in the case of phosphorus or cations,
Forest Succession, Fire, and Landscape Dynamics 127 replenishment of nutrient capital is primarily by atmospheric inputs and can be quite slow. Nitrogen fixation accelerates inputs of nitrogen. Although claims have been made that fire increases activities of nonsymbiotic, nitrogen-fixing microbes, solid evidence is lacking. But the importance of symbiotic, nitrogen-fixing plants, such as buckbrush and red alder, in postfire successional ecosystems is well known. Bio/ogica/ Diversity Given the close tie between the life history of some species and fire (e.g., serotinous cones in lodgepole pine and heat-stimulated germination in some shrubs) the responses of many species to fire differ from their responses to cutting. The greatest differences are found in the years immediately after disturbance. The high degree of spatial variability generated by burning probably facilitates higher levels of species diversity than less-variable postharvest environments, such as those typical of clearcuts (Christensen 1988~. Where management is inten- sive, many silvicultural activities are explicitly designed to diminish diversity and increase production and yield of target species.