Applying Ecological Principles to Management of the U.S.National Forests
Published by the Ecological Society of America Number 6, Spring 2000 Applying Ecological Principles to Management of the U.S. National Forests Issues in Eco logy
Issues in Ecology Number 6 Spring 2000 Applying Ecological Principles to Management of the U.S.National Forests SUMMARY The U..National Forest System is a diverse and unique resource that must be managed within the cntextf competing and shifting socil expectations.The policies under which the system operates have changed over the century. along with the values wood production. tion.Proposals nagemer orests are once g debated. h among ecologists is that nd on an unde managed as orest management practices erstanding of how natural forest ecosys- ems wo We have identified majorc niderations that should be in sound forest managemen policy nc cks that hold the fture forest sit. 9 t es al d le ore large cutover quality and yield na pre an ides call for greater attention to the ative mpacts logging roads ne value o undis urbe zones along streams and rivers Conservation of forest bic iversity will often require reducing forest fragmentation by clearcuts and roads iding harvest in vulnerable areas such as hardwood or old growth stands and riparian zones,and restoring atural struct ural complexity to cutover sites Planning at the landscape level is needed to address ecological conce ns such as biodiversity.water flows,and ntatio Repeated overcutting of National Forests lands in the past has been linked to lack of planning at the landscape scale Increasing pressures on forests due to human population growth and global change oblige land managers to be alert for climate-related stresses as well as damage from ground-level ozone,acid rain,and acidification of soils and watersheds. This pane as analyzed assumptins.both explicit and implicit,that underlie a number of curent es in National Forest management.Key assumptions i some of these proposals are unsupported or irradicted by current knowledge of forest ec We are confident that Despite natural disturbance and successional change.forest reserves are much more likely to sustain the full biological diversity of forests than lands managed primarily for timber production. No evidence supports the view that natural forests or reserves are more vulnerable to disturbances such as wildfire,windthrow,and pests than intensively managed forests.Indeed,there is evidence natural systems may be more resistant in many cases. Traditional beliefs that timber harvesting can duplicate and fully substitute for the ecological effects of natural disturbance are incorrect.although newer techniques such as retaining trees and large woody debris on harvest sites can more closely mimic natural processes. There is no scientific basis for asserting that silvicultural practices can create forests that are ecologically equivalent to natural old-growth forests.although we can certainly use our understanding of forest ecology to help restore managed forests to more natural conditions. Proposals to ban all timber harvesting on National Forests would leave managers without a valuable tool that can be used selectively to restore early successional habitat,reduce fuel loads.and contain pest and pathogen outbreaks in some forests. Creativity is needed in designing forest management policies for the future.but simple solutions are almost never adequate for sustaining a complex system that must fulfill diverse expectations.Sustainable management policies must make full use of current ecological knowledge.The goal of our policy efforts today should be to design forest manage- ment practices that assure the value of our forest resources for future generations
1 Issues in Ecology Number 6 Spring 2000 Applying Ecological Principles to Management of the U.S. National Forests SUMMARY The U.S. National Forest System is a diverse and unique resource that must be managed within the context of competing and shifting social expectations. The policies under which the system operates have changed over the century, along with the values society places on wood production, wilderness protection, recreation, and biodiversity conservation. Proposals for major changes in the management of the National Forests are once again being debated. The consensus among forest ecologists is that all forests, despite their complexity and variability, should be managed as ecosystems. Sustainable forest management practices must be based on an understanding of how natural forest ecosystems work. We have identified major ecological considerations that should be incorporated in sound forest management policy and their potential impacts on current practice: • Maintenance of soil quality and nutrient stocks that hold the key to current and future forest productivity may necessitate adjusting timber harvest rates and leaving more large woody debris on cutover sites. • Protection of water quality and yield and prevention of flooding and landslides call for greater attention to the negative impacts of logging roads and the value of undisturbed buffer zones along streams and rivers. • Conservation of forest biodiversity will often require reducing forest fragmentation by clearcuts and roads, avoiding harvest in vulnerable areas such as hardwood or old growth stands and riparian zones, and restoring natural structural complexity to cutover sites. • Planning at the landscape level is needed to address ecological concerns such as biodiversity, water flows, and forest fragmentation. Repeated overcutting of National Forests lands in the past has been linked to lack of planning at the landscape scale. • Increasing pressures on forests due to human population growth and global change oblige land managers to be alert for climate-related stresses as well as damage from ground-level ozone, acid rain, and acidification of soils and watersheds. This panel also analyzed the ecological assumptions, both explicit and implicit, that underlie a number of current proposals for changes in National Forest management. Key assumptions in some of these proposals are unsupported or directly contradicted by current knowledge of forest ecology. We are confident that: • Despite natural disturbance and successional change, forest reserves are much more likely to sustain the full biological diversity of forests than lands managed primarily for timber production. • No evidence supports the view that natural forests or reserves are more vulnerable to disturbances such as wildfire, windthrow, and pests than intensively managed forests. Indeed, there is evidence natural systems may be more resistant in many cases. • Traditional beliefs that timber harvesting can duplicate and fully substitute for the ecological effects of natural disturbance are incorrect, although newer techniques such as retaining trees and large woody debris on harvest sites can more closely mimic natural processes. • There is no scientific basis for asserting that silvicultural practices can create forests that are ecologically equivalent to natural old-growth forests, although we can certainly use our understanding of forest ecology to help restore managed forests to more natural conditions. • Proposals to ban all timber harvesting on National Forests would leave managers without a valuable tool that can be used selectively to restore early successional habitat, reduce fuel loads, and contain pest and pathogen outbreaks in some forests. Creativity is needed in designing forest management policies for the future, but simple solutions are almost never adequate for sustaining a complex system that must fulfill diverse expectations. Sustainable management policies must make full use of current ecological knowledge. The goal of our policy efforts today should be to design forest management practices that assure the value of our forest resources for future generations
Issues in Ecology Number 6 Spring 2000 Applying Ecological Principles to Management of the U.S.National Forests by John Aber',Norman Christensen.Ivan Fernandez,Jerry Franklin,Lori Hidinger.Malcolm Hunter. James MacMahon.David Mladenoff,John Pastor.David Perry.Ron Slangen.Helga van Miegroet INTRODUCTION Today we are experiencing another period of shifting values as well as conflicting pr posals for maior changes in The U.S.National Forest System is a diverse and unique the management of the National Forest System.Some seg resource.encompassing approximately 192 million acres and ments of society propose to increase forest harvesting dra representing most of the continent's major forest types.The matically while others want to eliminate harvesting alto system itself is entirely a creation of twentieth century po gether (e.g..Oliver et al.1997.McKinney 1999).Policies litical and social forces.and society's expectations for it have regarding the role of natural disturbances such as fire are changed repeatedly over the century.As the values society also under review.Recently,the U.S.Forest Service began places on timber production.wilderness protection.recre reviewing its mission based on the recommendations of the ation,and cor servation of biological dive ersity have shifted. Committee of Scientists Report(199)commissioned by the so have the policy directives under which the system operates Secretary of Agriculture his committee of 13 academic The various legislative mandates under Mhich the Na and professionals concluded that ecological sustainability and with pubic ownership and ipation are key guiding principles esta the lirst po gemen an Dac drop.the forests them 0estabishnation The act gave chang ing hun on pu selves a dyr onst anging in respon e stress, climate y d spec th.po ange,and extended this b five thi door recreation.range and fodder.and watersheds (Wiersum ina howy the natural systems work and developing manage 1995).The National Forests Manage ent Act of 1976 in ment prescrintions consistent with that knowledge Wher tuspecified that this policy of multipleuse be incorporated political pressures are strong.however.it is all too easy for into a mandated planning process. land managers and decision makers to lose sight of the ex- and providing for well-regulated,high-quality streamflow. Private forest lands,western Washington state
Issues in Ecology Number 6 Spring 2000 2 by John Aber* , Norman Christensen, Ivan Fernandez, Jerry Franklin, Lori Hidinger, Malcolm Hunter, James MacMahon, David Mladenoff, John Pastor, David Perry, Ron Slangen, Helga van Miegroet Applying Ecological Principles to Management of the U.S. National Forests INTRODUCTION The U.S. National Forest System is a diverse and unique resource, encompassing approximately 192 million acres and representing most of the continents major forest types. The system itself is entirely a creation of twentieth century political and social forces, and societys expectations for it have changed repeatedly over the century. As the values society places on timber production, wilderness protection, recreation, and conservation of biological diversity have shifted, so have the policy directives under which the system operates. The various legislative mandates under which the National Forest System has operated began with The Organic Administration Act of 1897 that established the first policy for national forest use and management. The act gave the President authority to establish national forests on public lands in order to improve and protect the forest within boundaries, or for the purpose of security, favorable conditions of waterflows, and to furnish a continuous supply of timber for the use and necessities of citizens of the United States (Fedkiw 1999). The 1960 Multiple Use Sustained Yield Act extended this by specifying five things that were to be sustained on public landstimber, fish and wildlife, outdoor recreation, range and fodder, and watersheds (Wiersum 1995). The National Forests Management Act of 1976 in turn specified that this policy of multiple use be incorporated into a mandated planning process. Today we are experiencing another period of shifting values as well as conflicting proposals for major changes in the management of the National Forest System. Some segments of society propose to increase forest harvesting dramatically while others want to eliminate harvesting altogether (e.g., Oliver et al. 1997, McKinney 1999). Policies regarding the role of natural disturbances such as fire are also under review. Recently, the U.S. Forest Service began reviewing its mission based on the recommendations of the Committee of Scientists Report (1999) commissioned by the Secretary of Agriculture. This committee of 13 academics and professionals concluded that ecological sustainability and pubic ownership and participation are key guiding principles for managing the National Forests. Behind this changing human backdrop, the forests themselves are also dynamic, constantly changing in response to stress, disturbance, and climate, yet always constrained by their underlying physical, chemical, and biological processes. The stresses on forested ecosystems and the plant and animal species they harbor are continually increasing because of human population growth, pollution, climate change, and other threats (Figure 1). The key to responsible forest management is understanding how the natural systems work and developing management prescriptions consistent with that knowledge. When political pressures are strong, however, it is all too easy for land managers and decision makers to lose sight of the exFigure 1 - Continuous clearcutting of forests can create major environmental problems, such as in maintaining biological diversity and providing for well-regulated, high-quality streamflow. Private forest lands, western Washington state. Photo by Jerry Franklin. * Authors in alphabetical order.
Issues in Ecology Number 6 Spring 2000 tent and value of the knowledge base that has been developed forest ecosystems and also how management practices af on forest ecosystem dynamics and response to disturbance. fect soil quality (eg.Cole 1995 and Perry and rose 1998) The purpose of this report is to outline key ecological Although very little research has been published on systems considerations that should underlie sound forest manage for evaluating or monitoring soil quality.defining it and ini ment.The complexity and variability of forest ecosystems tiating programs to evaluate its maintenance and promo throughout the United States make it difficult to formulate tion are central to achieving demonstrable sustainability in ecological principles that apply uniformly to all.Yet there is our National Forests.The ability to define and measure so consensus among forest ecologists about one generalization: quality is important for applications at a number of scales All forests should be regarded and managed as ecosystems from monitoring soil compaction and nutrient supply at spe ecosystems that represent a variety of resources and val- cific .em est this report we management in tive catego Wha cycles blend of minerals. .living .In n pro of the suppo explicitly a te em propo mite h rests (USDA NRCS 1996) tha Healthy soil e critical ec ical functions (An in forested eco ont cy lino.a on the scientific hasis of for de nted by david perry (1998)in the annual reu dead organic matter and release vital nlant nutrients such a and phosphorus for reuse.This activity accounts A single overarching prir nciple sets the context for this for the majority of nutrients taken up by plants in matur report:the national forest system should be viewed as a forests.Second.healthy soil enables a forest to maintain multifaceted resource of continuing value.and current man some productivity (tree srowth)during periods of shortase agement policies and practices should not devalue the re- especially droucht Third.healthy soil is capable of retain source for future generations.Any set of management prac ing fertility and thereby facilitating plant recovery follow tices should therefore be sustainable for the indefinite future ing disturbances such as fire or timber harvesting.The lat ter capability quickly degrades,however.when plant cover ECOLOGICAL CONSIDERATIONS IN is removed and the soil is left bare(Perry 1998). R5MANA Soi Structure and Oranic Matter From the early the.Forest Ser A significant concern in the maintenance of forest soil has had two soil o ing compa wers e expande agency s matter includ sity c m he as catego 180 de signed to the d h ms,many which perform th ersity:(4 scape level iss and (5) As lon of soil car ine lit SOIL AND NUTRIENT CYCLES will be th ofc the st Soil quality is central to sustainable forest tus soil nutrients because it defines the current and future productivity of the More problematic is the replenishment of those co land and promotes the health of its plant and animal com nents of soil carhon that are derived from large wo munities (Doran and Parkin 1994).A great deal is known bris,especially tree stems(Figure 2:Harmon et al.1986) about the importance of soil quality for the functioning of The practice of leaving tree stems on site is not common in
3 Issues in Ecology Number 6 Spring 2000 tent and value of the knowledge base that has been developed on forest ecosystem dynamics and response to disturbance. The purpose of this report is to outline key ecological considerations that should underlie sound forest management. The complexity and variability of forest ecosystems throughout the United States make it difficult to formulate ecological principles that apply uniformly to all. Yet there is consensus among forest ecologists about one generalization: All forests should be regarded and managed as ecosystems ecosystems that represent a variety of resources and values for different forest users. In the first section of this report, we discuss ecological considerations for forest management in five broad categories: 1) soil and nutrient cycles, 2) hydrology, 3) biodiversity, 4) landscape level issues, and 5) global change. In the second section of the report, we examine and critique some of the ecological assumptions that explicitly or implicitly underlie several current forest policy proposals. In particular, we analyze acceptable or desirable levels of direct human manipulation and use of federal forests based on current ecological understanding. Rather than presenting a comprehensive review of the literature, we discuss principles that are generally accepted among ecological scientists. (An excellent review of the literature on the scientific basis of forestry was presented by David Perry (1998) in the Annual Review of Ecology and Systematics.) A single overarching principle sets the context for this report: The National Forest System should be viewed as a multifaceted resource of continuing value, and current management policies and practices should not devalue the resource for future generations. Any set of management practices should therefore be sustainable for the indefinite future. ECOLOGICAL CONSIDERATIONS IN FOREST MANAGEMENT From the early days of its creation, the U.S. Forest Service has had two primary goals: to support local industry and to protect and sustain watersheds. Over time, new laws and policies have expanded the agencys mission to include recreation, biodiversity conservation, and maintenance of soil quality and natural processes. We examine here five broad categories of ecological considerations that should go into management practices designed to fulfill this complex mission and to sustain forest resources into the future: (1) soil and nutrient cycles; (2) hydrology; (3) biodiversity; (4) landscape level issues; and (5) global change. SOIL AND NUTRIENT CYCLES Soil quality is central to sustainable forest management because it defines the current and future productivity of the land and promotes the health of its plant and animal communities (Doran and Parkin 1994). A great deal is known about the importance of soil quality for the functioning of forest ecosystems and also how management practices affect soil quality (eg., Cole 1995 and Perry and Rose 1998). Although very little research has been published on systems for evaluating or monitoring soil quality, defining it and initiating programs to evaluate its maintenance and promotion are central to achieving demonstrable sustainability in our National Forests. The ability to define and measure soil quality is important for applications at a number of scales, from monitoring soil compaction and nutrient supply at specific sites to addressing global concerns about the amount of carbon sequestered in the wood of the worlds forests. What is soil? Soil is a unique and complex blend of minerals, living organisms, and the organic products of organisms. It provides habitat and physical support as well as sustenance for a teeming array of creatures, from bacteria and fungi to mites, earthworms and plants. The soil and its living community store and cycle nutrients, regulate water flows, and also filter, buffer, degrade, immobilize, or detoxify a myriad organic and inorganic materials (USDA NRCS 1996). Healthy soil performs three critical ecological functions in forested ecosystems. One is nutrient cycling, a process carried out by invertebrates and microbes that decompose dead organic matter and release vital plant nutrients such as nitrogen and phosphorus for reuse. This activity accounts for the majority of nutrients taken up by plants in mature forests. Second, healthy soil enables a forest to maintain some productivity (tree growth) during periods of shortage, especially drought. Third, healthy soil is capable of retaining fertility and thereby facilitating plant recovery following disturbances such as fire or timber harvesting. The latter capability quickly degrades, however, when plant cover is removed and the soil is left bare (Perry 1998). Soil Structure and Organic Matter A significant concern in the maintenance of forest soil quality is assuring the replenishment of surface and soil organic matter and avoiding compaction of the soil (Powers et al. 1990). Soil organic matter includes highly decomposed material called humus, less decomposed leaf litter and other detritus, and large woody debris such as branches and stems. This organic material stores nutrients and water and supplies the carbon to nourish the myriad belowground organisms, many of which perform the critical tasks of releasing the mineral nutrients necessary for continued plant growth. As long as plant communities regrow vigorously after timber harvesting, losses of soil carbon derived from fine litter will be replenished. Regrowth, of course, depends on the status of soil nutrients, soil carbon, and soil biology after harvest. More problematic is the replenishment of those components of soil carbon that are derived from large woody debris, especially tree stems (Figure 2; Harmon et al. 1986). The practice of leaving tree stems on site is not common in
Issues in Ecology Number 6 Spring 2000 intensive forestry today.and in fact.doino so has been seer to another flevated nitrate levels in streams followino har as a waste.The question of how many trees to leave to vest or forest disturbance represent a threat to wate r qua nt and will ity hecsuee e nutrient fouline can lead to a wide reguire further research on the ecological functions of large problems from algal blooms,loss of oxygen.and fish kills to dead wood.Yet retaining trees on site as future sources of degradation of drinking water.In general.forest ecosy large woody debris must be a major component of sustain- tems with higher levels of nitrogen mineralization (release o able forest management nitrogen from decomposing soil organic matter)have been shown to exhibit higher rates of nitrate production and loss Nutrient Cycling and these losses are further increased by the removal of trees Another major factor in sus and corresponding elimination o taining soil quality is maintaining nitrogen uptake by the trees pools of essential plant nutrients Hibbert 1969.Likens et al.1970 and assuring these are steadily Hornbeck et al.1996). available in forms that plants can Computer modeling of nutri use.Undisturbed forests seldom ent requirements for forest growth experience significant losses of nu as well as studies on waters trient stocks u an importan and forest ecosystems agree tha b orestry i n principl rvesting whol anagement practice est in the long- those nutrients t Until rec nd oth he tant nutrient limiti in te rate and boreal fo 177 and by far the majority of research al.1986.Johnson and Todd has focu ised on nitrogen losse sociated with timber harvest and nrona site preparation (Johnson 1992) sites for planting signific ntly im Losses from a harvested site take nact soil fertility esne cially the use three forms:removal of the nitro of heavy equipment to push slash gen contained in the harvested and other organic matter into wood.nitrogen leached and oiles.a practice called windrow eroded from disturbed soil.and ni ne(Powers et al.1990).In a sus During the last 30 years it has become appar trogen volatilized and lost to the logs and atmosphere during slash burning oer woody debris fulfill m tainable forest management pro y eco the gram.therefore.rates of tree re The extent and impact of these Note person in red for scale moval and other management ac losses vary depending on numer- by Jerry Franklin tivities should be planned accord ous site-specific factors such as ni ing to nutrient budgeting tech trogen ava limate and also on management prac niques in order to reduceor deter long-term degrada tices (Col 1995).In the n nitrogen-poor tion of soil nutrients. em u.s. for example. dslash burn ing ogen-ricl as have increased of North A meric nts shov of r h gen ecognized that h 05 proc le due to tree ha soils to stre espe able fron ne has bee into the
Issues in Ecology Number 6 Spring 2000 4 intensive forestry today, and in fact, doing so has been seen as a waste. The question of how many trees to leave to sustain soil quality is not easily answered at present and will require further research on the ecological functions of large dead wood. Yet retaining trees on site as future sources of large woody debris must be a major component of sustainable forest management. Nutrient Cycling Another major factor in sustaining soil quality is maintaining pools of essential plant nutrients and assuring these are steadily available in forms that plants can use. Undisturbed forests seldom experience significant losses of nutrient stocks. Thus an important element in sustainable forestry is taking care that management practices do not result in longterm reductions in a forests nutrient capital or in the long-term availability of those nutrients to plants. Until recently, nitrogen has been considered the most important nutrient limiting tree growth in temperate and boreal forests, and by far the majority of research has focused on nitrogen losses associated with timber harvest and site preparation (Johnson 1992). Losses from a harvested site take three forms: removal of the nitrogen contained in the harvested wood, nitrogen leached and eroded from disturbed soil, and nitrogen volatilized and lost to the atmosphere during slash burning. The extent and impact of these losses vary depending on numerous site-specific factors such as nitrogen availability and climate and also on management practices (Cole 1995). In the nitrogen-poor forests of the western U.S., for example, losses in wood removal and slash burning far exceed those in leaching, while in more nitrogen-rich eastern forests, leaching losses can be quite high. Watershed-scale studies and harvesting experiments show that total nitrogen lost from a site after clearcutting varies widely among forest types. Since nitrogen is considered the major nutrient limiting tree growth in most systems, postharvest losses are regarded as a long-term threat to forest productivity. Nitrogen losses in the form of nitrate leached from soils to streams are especially variable from one forest to another. Elevated nitrate levels in streams following harvest or forest disturbance represent a threat to water quality because nutrient fouling can lead to a wide range of problems from algal blooms, loss of oxygen, and fish kills to degradation of drinking water. In general, forest ecosystems with higher levels of nitrogen mineralization (release of nitrogen from decomposing soil organic matter) have been shown to exhibit higher rates of nitrate production and loss, and these losses are further increased by the removal of trees and corresponding elimination of nitrogen uptake by the trees. (Hibbert 1969, Likens et al. 1970, Hornbeck et al. 1996). Computer modeling of nutrient requirements for forest growth as well as studies on watersheds and forest ecosystems agree that, in principle, harvesting whole trees and using short intervals between harvests on a site lead to significant reductions in soil nitrogen stocks, nitrogen availability, and productivity. Large losses of phosphorus, calcium, magnesium, potassium, and other nutrients also occur in association with whole-tree harvest and short rotations (Kimmins 1977, Smith et al. 1986, Johnson and Todd 1987). Some practices used to clear logging slash and prepare sites for planting significantly impact soil fertility, especially the use of heavy equipment to push slash and other organic matter into piles, a practice called windrowing (Powers et al. 1990). In a sustainable forest management program, therefore, rates of tree removal and other management activities should be planned according to nutrient budgeting techniques in order to reduce or deter long-term degradation of soil nutrients. Nitrogen Saturation Concerns have increased across much of North America and Europe about the overabundance of nitrogen entering forests due to the human-driven buildup of airborne nitrogen. It is now recognized that human activities such as burning of fossil fuels and production of nitrogen fertilizers have effectively doubled the supply of biologically available nitrogen. Thus, research on nitrogen shortages due to tree harvest has been augmented by investigations into the effects of Figure 2 - During the last 30 years it has become apparent that logs and other woody debris fulfill many ecological functions and persist for centuries, as in the case of this giant sequoia log. Note person in red for scale. Photo by Jerry Franklin
Issues in Ecology Number 6 Sprine 2000 excess nitrogen availability and consequent nitrate leaching HYDROLOGY due to increased airborne nitrogen entering forest soils The headwaters of the nation's largest rivers.which sup as dry deposition or acid rain (Aber 1992.Fenn et al. ply much of our fresh water.originate on National Forest 19971. land.Cutting of timber in these watersheds raises three con Increasingly.a phenomenon known as"nitrogen satu- cems:changes in the volume of water flowing to streams,tim ration"from atmospheric deposition has been observed in ing of those flows.and water quality,especially sediment loads some forest ecosystems where growth is normally limited by the availability of nitrogen. Nitrogen saturation occurs wher Water Yield and Flooding inputs of nitrogen exceed the rate at which soils.plants,and Accurate generalizations about the impacts of clearcutting on the volume and exces los timing of flows are er eastern U.S nigh var both te high tha rece d some areas of the West.ho ially in er ha s.Also. n saturation is much in conifer forests and chanarra the stands surrounding the Los Ang snow dominates.whether fog les Basin,nitrogen deposition is so drip from canopies is significant high and has been occurrino fo affect the volume and timing of so long that these systems have stream flow. been highly impacted by nitrogen The clearest effects of har saturation. vest on water flows have been ob Although elevated nitroger tained from experimentally paired deposition could potentially offset small watersheds (Reiter and harvesting losses,it is also likely Beschta 1995). watershed to exacerbate the acidification o studies generally show 1989,Federer et a learcutting increase water yield As negatively charged ni Figure 3.Deadwood (logs)and other woody debris An exception may be found in foggy trates seep awa into stre s of provide a maior contribution to the structure of ripar ons v here tree c rake sig grou th arry an zones like in this small headland stream.sequoia or lo charge Kings Canyon National Park.CA.Photo by Jerry In suc vater vield Franklin may de Pea d s its fertility Forest har thi can shnntakyand d tha ut and th ent of the for ng har nd dis With the g valence of ni and floodplain forests have the orea retaining a healthy gr n cover at all times est likelibood of increasing the maonitude and dur ion o either through retention harvests or regrowth of early suc peak flows and the threat of flooding (Reiter and Rescht cessional plants (or both).will become increasingly impor- 1995).Sustainable forest management should limit such tant to conserve soil nutrient capital after logging practices in vulnerable watersheds
5 Issues in Ecology Number 6 Spring 2000 excess nitrogen availability and consequent nitrate leaching due to increased airborne nitrogen entering forest soils as dry deposition or acid rain (Aber 1992, Fenn et al. 1997). Increasingly, a phenomenon known as nitrogen saturation from atmospheric deposition has been observed in some forest ecosystems where growth is normally limited by the availability of nitrogen. Nitrogen saturation occurs when inputs of nitrogen exceed the rate at which soils, plants, and microbes can use or store it, and the excess is lost to streams, groundwater, or the atmosphere. In the eastern U.S., this saturation has been witnessed in forests at intermediate to high elevations that receive large amounts of nitrogen deposition. In the western U.S., the early stages of nitrogen saturation have been observed in high elevation ecosystems of the Colorado Rockies Front Range. In some areas of the West, however, nitrogen saturation is much more advanced. For example, in mixed conifer forests and chaparral stands surrounding the Los Angeles Basin, nitrogen deposition is so high and has been occurring for so long that these systems have been highly impacted by nitrogen saturation. Although elevated nitrogen deposition could potentially offset harvesting losses, it is also likely to exacerbate the acidification of soils (Schulze 1989, Federer et al. 1989). As negatively charged nitrates seep away into streams or groundwater, they carry along positively charged minerals such as calcium, magnesium, and potassium. Loss of these alkaline minerals acidifies the soil and decreases its fertility. Forest harvesting and associated nitrate leaching can intensify this chemical imbalance and lead to potentially severe limitations on forest growth. In ecosystems rich in nitrogen, excessive control of early successional vegetation that resprouts following harvest removes an important biological dam and may greatly increase leaching of nitrate and other nutrient elements. With the growing prevalence of nitrogen saturation in forest ecosystems, retaining a healthy green cover at all times, either through retention harvests or regrowth of early successional plants (or both), will become increasingly important to conserve soil nutrient capital after logging. HYDROLOGY The headwaters of the nations largest rivers, which supply much of our fresh water, originate on National Forest land. Cutting of timber in these watersheds raises three concerns: changes in the volume of water flowing to streams, timing of those flows, and water quality, especially sediment loads. Water Yield and Flooding Accurate generalizations about the impacts of clearcutting on the volume and timing of stream flows are extremely difficult because of the high variability of such flows, both over time and from one forest system to the next. Because of natural variability in flows, only dramatic impacts of tree removal on stream hydrology are statistically detectable in short-term studies. Decades-long records are often necessary to discern trends, especially in larger basins. Also, a variety of factors from harvest practices to bedrock geology, topography, and climate (whether rain or snow dominates, whether fogdrip from canopies is significant) affect the volume and timing of stream flow. The clearest effects of harvest on water flows have been obtained from experimentally paired small watersheds (Reiter and Beschta 1995). These watershed studies generally show that clearcutting increases water yield. An exception may be found in foggy regions where tree crowns rake significant water from clouds or fog. In such fog-drip forests, water yields may decline following harvest. Peak flows are of more concern environmentally and economically because high peak flows can result in damaging floods. Often clearcutting increases peak flows, although that can vary with the extent and rate of logging within a basin, how the logging is carried out, and the extent of the forest road network. Practices such as intensive site preparation, prevention of shrub and grass regrowth on the site, extensive roading, and disruption of streambank and floodplain forests have the greatest likelihood of increasing the magnitude and duration of peak flows and the threat of flooding (Reiter and Beschta 1995). Sustainable forest management should limit such practices in vulnerable watersheds. Figure 3 - Deadwood (logs) and other woody debris provide a major contribution to the structure of riparian zones like in this small headland stream. SequoiaKings Canyon National Park, CA. Photo by Jerry Franklin.
Issues in Ecology Number 6 Spring 2000 Impacts of Logging Roads exnerience splash erosion as rainfall knocks sediment loose splash Studies in western Oregon demonstrate that clearcutting compaction as rain packs down the soil.or gully erosion. and roads rgistically to alter hydrology in a for val of tr and resultin iner in deep (Harr 1976).Removal of trees from a site ter can threaten the stability of slopes and increase the r duces water loss to evapotranspiration (evaporation from sibility of landslides.Unless the reduced evapot nir plant surfaces and transpiration from leaf pores)and also caused by clearcutting is accompanied by increased wate increases snow accumulation and speeds melt since no trees flow to streams the result can be wetter soils and decreased shade the snowpack.As a result.deep-soil water storage soil cohesion.Plant roots also play an important role in slope increases on cutover sites,and this effect persists for de stability.and management practices that decrease root density cades until the leaf canopy of deep-rooted trees and shrubs or vitality can destabilize slopes and contribute to slope failures has fully recovered. In poorly drained areas.water tables although these may not occur until several years after vegeta rise in clearcuts(Burger and Pritchett 1988).sometimes trig- tion removal.The long-term impact of such practices will de gering bog formation (Perry 1998). pend on how quickly the roots of new vegetation expand in Roads.on the other hand.cut into hillslopes and allow relation to the decay of roots from the harvested trees. In addition, water quality and aquatic systems can b 1975).In two watersheds on the Andrews Ex degraded by leaching of nitrate from nirogen-saturated soils perime Forest in the Oregon Cascades The primary result of excess nitrogen in forest ecosystemss same on a watershe nat was groun ater or sur ace wate ut leaching to aquatic sys em roads rant 199 incl estuaries and increased toxicity to year surlace waters threa grea 20 tha he cuts, aqu les in sm tream s after the harvest,peak (Fen et a still hi 5t 40 percen tha create or track gre Sedimentation frosion and landslide ial fo The effects of fo tation hav trin heen vater flo s he nent of fo cause backeround variability is much less est management hecause of their canacity to slow such ove eroded from Once again.the best-docu nded sedim nts to settle out and u mented studies come from tal wat sheds.alt timately reduce siltation of streams.A program of sustain these are supported by evidence from historical observati able forest management should embrace such solutions and and logged and unlog eed watershed comnarisons take care to avoid practices that result in greatly increased Sediments associated with forestry come from four or irreversible loading of sediment to rivers and streams mary sources:surface erosion from roads,surface erosion from clearcuts,mass transport during slash burns,and land- BIODIVERSITY slides associated either with roads or clearcuts.Studies or The term"biodiversity"encompasses the full variety of the H.J.Andrews Experimental Forest found that landslides life on earth,from genes and species to ecosystems and land especially from poorly designed roads during major storms scapes.as well as ecological processes that both sustain and pulsed large amounts of sediment in brief f episodes.while are sustained by living things.Both laws and emerging soci surface erosion from roads and clearcuts was more etal values have made forest managers responsible for pro (Swanson et al.1989).As studies of water flow have shown tecting biodiversity as well as the habitats and processes roads Eleven years atte that maintain it nent lost from a roaded watersh d tha ects c timber harvesting on biodi rsity depend percent clearcut ate on nte of har well as ho 88e di D e mom one area do not ne n in pa the nd b nsive rainsto estry pr e ha d uch
Issues in Ecology Number 6 Spring 2000 6 Impacts of Logging Roads Studies in western Oregon demonstrate that clearcutting and roads act synergistically to alter hydrology in a forest (Harr 1976). Removal of trees from a site necessarily reduces water loss to evapotranspiration (evaporation from plant surfaces and transpiration from leaf pores) and also increases snow accumulation and speeds melt since no trees shade the snowpack. As a result, deep-soil water storage increases on cutover sites, and this effect persists for decades until the leaf canopy of deep-rooted trees and shrubs has fully recovered. In poorly drained areas, water tables rise in clearcuts (Burger and Pritchett 1988), sometimes triggering bog formation (Perry 1998). Roads, on the other hand, cut into hillslopes and allow deep-soil water to surface and run rapidly to streams (Harr et al. 1975). In two watersheds on the H.J. Andrews Experimental Forest in the Oregon Cascades, for instance, peak stream flows were the same on a watershed that was 100 percent clearcut but had no roads and one that was only 25 percent clearcut but had roads (Jones and Grant 1996). For the first five years after harvest, peak flows averaged greater than 50 percent higher than before the cuts, then began to decline. However, 25 years after the harvest, peak flows were still higher by 25 to 40 percent. Sedimentation, Erosion, and Landslides The effects of forest management on sedimentation have been easier to demonstrate than effects on water flows because background variability is much less very little soil is eroded from undisturbed forests. Once again, the best-documented studies come from experimental watersheds, although these are supported by evidence from historical observations and logged and unlogged watershed comparisons. Sediments associated with forestry come from four primary sources: surface erosion from roads, surface erosion from clearcuts, mass transport during slash burns, and landslides associated either with roads or clearcuts. Studies on the H.J. Andrews Experimental Forest found that landslides, especially from poorly designed roads during major storms, pulsed large amounts of sediment in brief episodes, while surface erosion from roads and clearcuts was more chronic (Swanson et al. 1989). As studies of water flow have shown, roads and clearcuts act synergistically. Eleven years after harvest, suspended sediment lost from a roaded watershed that was 25 percent clearcut averaged 57 times greater than sediment losses in an unroaded, unlogged control watershed. In contrast, a similar watershed that was 100 percent clearcut but unroaded experienced sediment losses averaging 23 times greater than in the undisturbed watershed. Absolute amounts of erosion from one area do not necessarily extrapolate to others because erosion varies depending on slope steepness, soil, rock type, and snow and rainfall patterns. Areas with large expanses of bare mineral surface, especially in regions where intensive rainstorms are likely, can experience splash erosion as rainfall knocks sediment loose, splash compaction as rain packs down the soil, or gully erosion. Removal of trees and resulting increases in deep-soil water can threaten the stability of slopes and increase the possibility of landslides. Unless the reduced evapotranspiration caused by clearcutting is accompanied by increased water flow to streams, the result can be wetter soils and decreased soil cohesion. Plant roots also play an important role in slope stability, and management practices that decrease root density or vitality can destabilize slopes and contribute to slope failures, although these may not occur until several years after vegetation removal. The long-term impact of such practices will depend on how quickly the roots of new vegetation expand in relation to the decay of roots from the harvested trees. In addition, water quality and aquatic systems can be degraded by leaching of nitrate from nitrogen-saturated soils. The primary result of excess nitrogen in forest ecosystems is elevated loss of nitrate to groundwater or surface water. The impacts of increased nitrate leaching to aquatic systems include eutrophication of estuaries and increased toxicity to surface waters. These can pose serious threats to sensitive aquatic organisms, especially fish communities in small streams (Fenn et al. 1998). Management practices that create ruts or tracks can greatly speed the flow of water across the landscape and thus increase the potential for gully erosion and sediment transport. Buffer strips of undisturbed vegetation along streams and floodplains can be a critical component of forest management because of their capacity to slow such overland flows, allow suspended sediments to settle out, and ultimately reduce siltation of streams. A program of sustainable forest management should embrace such solutions and take care to avoid practices that result in greatly increased or irreversible loading of sediment to rivers and streams. BIODIVERSITY The term biodiversity encompasses the full variety of life on earth, from genes and species to ecosystems and landscapes, as well as ecological processes that both sustain and are sustained by living things. Both laws and emerging societal values have made forest managers responsible for protecting biodiversity as well as the habitats and processes that maintain it. The effects of timber harvesting on biodiversity depend on scale, intensity, and method of harvest, as well as how individual animal and plant species respond to harvesting. In general , however, forestry practices affect biodiversity principally by changing the age of a forest, its horizontal and vertical structure, and its species composition. As commonly practiced, forestry structurally simplifies natural landscapes and also adds new elements. Some species increase in numbers while others are jeopardized. While some species may adapt to the changes imposed on the land by intensive forestry practices, none have evolved in such settings.
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 vulnerable 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 species); 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 different 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 create gaps of mixed sizes depending on cause. These can range from a single tree-fall gap to large blowdowns caused by hurricanes 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 meantime they serve as important structural elements to prevent erosion and provide habitat for many organisms. Most woodpecker species, for example, nest in cavities they excavate from standing dead trees, and fallen dead trees provide habitat for numerous species, both on land and in streams (Figure 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 microclimate at the forest edge may also affect seed dispersal, movement 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 imperiled species and also keystones species that play a disproportionately 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 maintaining forest stability and diversity. Warm Springs Indian Reservation (previously Mount Hood National Forest), Oregon. Photo by Jerry Franklin.
Issues in Ecology Number 6 Spring 2000 forest management should consider the mosaic of fores Another maior research task is to determine what sain patches on the landscape and the connectedness of habitat in biodiversity actually accrue from retaining mature g for forest species in plannine future cuts in uity.results to date show that Forests managed for timber harvest can be fragmented with remnant old trees support a greater abundance of some by roads as well as clearcuts.Roads may affect biodiversity old-growth-associated species than do uniformly youn in a number of ways princinally hy creating barriers to move stands and also sustain some species not found at all i ment.access routes for predators (including humans).and young stands (Hansen et al.1995,Peck and McCune 1997 corridors for invasions by noxious weeds and pathogens(Perry Schowalter 1995). 1988.Small and Hunter 1988).Nu Ecosystems are sensitive to change merous studies have shown that popu in the number and kinds of species lation sizes of bears,wolves,moose found in their communities.Because and mountain lions decline as roac species can vary dramatically in thei density increases(e.g..Brocke et al contribution to ecosystem function 1989) ing.the identity of the species present The spread of both native and in a com unity is important.Declir exotic pests and pathogens in many ngs到 ecies richnes can lead to dete forest systems can be linked to sim on in overa ecosys plific ng6emanom the em ores on arcuts and upy partic web (gra id have est 99 tre P dN with the fre ity of long-term success in ntly viewed de ing forests against pests and pathd ods are mportant because the eens is to encou naintenance act as bio a diverse set of controls such as oc logical dams following disturbance. curs in nature vide unique habitat and food for ure 5.Interior of old orowth Doualas.fir stand animals.and modulate fire severity. Structural Complexitu showine the structural complexity of natural for research has demonstrated tha LANDSCAPE-LEVEL ISSUES IN structural complexity is an important ests provided by multiple canopy layers.snags.and fallen logs.Mt.Baker-Snoqualmie National Forest. foREST MANAGEMENT PLAN feature of natural forest ecosystems Photo by Jerry Franklin. NING (e.g..Perry 1994).Vegetation of While most forestry research ha different heights provides a variety of habitats for the rich focused on the management of individual stands.most im diversity of species associated with healthy torests.Iree har portant forest policy issues require consideration of land vesting usually reduces elements of this structural scape-l mena and concerns (se also the Commit including mu Itiple canopy layers, dead snags.an rge faller tee Report 99).Sustainab stry necess logs(Figure 5). Ecologists gating how tates tha much structural complexity is larger spatial s Ices su es on edg the ests in rder to tat fr nd fects em functic d,the ailur
Issues in Ecology Number 6 Spring 2000 8 forest management should consider the mosaic of forest patches on the landscape and the connectedness of habitat for forest species in planning future cuts. Forests managed for timber harvest can be fragmented by roads as well as clearcuts. Roads may affect biodiversity in a number of ways, principally by creating barriers to movement, access routes for predators (including humans), and corridors for invasions by noxious weeds and pathogens (Perry 1988, Small and Hunter 1988). Numerous studies have shown that population sizes of bears, wolves, moose, and mountain lions decline as road density increases (e.g., Brocke et al. 1989). The spread of both native and exotic pests and pathogens in many forest systems can be linked to simplification and fragmentation of the forest during harvesting, certain replanting practices on clearcuts, and the ready travel corridors provided by extensive road networks. Problems with pathogens and tree-eating insects in forestry have often been associated with widespread planting of a single tree species (Perry 1998). From an ecological standpoint, the strategy with the greatest probability of long-term success in protecting forests against pests and pathogens is to encourage maintenance of a diverse set of controls such as occurs in nature. Structural Complexity Research has demonstrated that structural complexity is an important feature of natural forest ecosystems (e.g., Perry 1994). Vegetation of different heights provides a variety of habitats for the rich diversity of species associated with healthy forests. Tree harvesting usually reduces elements of this structural diversity, including multiple canopy layers, dead snags, and large fallen logs (Figure 5). Ecologists are currently investigating how much structural complexity is necessary and whether forest management practices such as long cutting rotations and variable-retention harvesting (which leaves more live trees, snags, and downed logs on a cutover site) can maintain ecologically important structural features while still allowing timber harvest (Franklin et al. 1997). Despite gaps in our understanding, sustainable forestry strategies should seek to return managed forests to the structure more typical of natural forests in order to assure protection of biodiversity and ecosystem functioning. Another major research task is to determine what gains in biodiversity actually accrue from retaining mature green trees in perpetuity. Results to date show that young stands with remnant old trees support a greater abundance of some old-growth-associated species than do uniformly young stands, and also sustain some species not found at all in young stands (Hansen et al. 1995, Peck and McCune 1997, Schowalter 1995). Ecosystems are sensitive to changes in the number and kinds of species found in their communities. Because species can vary dramatically in their contribution to ecosystem functioning, the identity of the species present in a community is important. Declining species richness can lead to deterioration in overall levels of ecosystem functioning. Loss of functional groups or reductions in the number of species that occupy a particular level in the food web (grazers, browsers, predators, decomposers) can also cause declines in ecosystem functioning (Tilman et al. 1997). Two important groups of species in conifer-dominated forests are hardwood trees and shrubs, which are present at one or more stages of succession and are frequently viewed as weeds. Hardwoods are important because they enhance nutrient cycling, act as biological dams following disturbance, provide unique habitat and food for animals, and modulate fire severity. LANDSCAPE-LEVEL ISSUES IN FOREST MANAGEMENT PLANNING While most forestry research has focused on the management of individual stands, most important forest policy issues require consideration of landscape-level phenomena and concerns (see also the Committee of Scientist Report 1999). Sustainable forestry necessitates that individual stands be managed in the context of larger spatial scales. Issues such as regulation of water yields, conservation of biological diversity, and maintenance of aquatic ecosystems require the application of principles from landscape ecology. These principles include concern for size and shape of forest patches, edge effects, and connectivity that is, the movement of organisms and materials through the landscape. Lack of attention to these landscape-scale concerns in stand-by-stand planning can lead to further habitat fragmentation and cumulative negative effects on forest communities. Indeed, the failure to consider larger landscape Figure 5 - Interior of old-growth Douglas-fir stand showing the structural complexity of natural forests provided by multiple canopy layers, snags, and fallen logs. Mt. Baker-Snoqualmie National Forest. Photo by Jerry Franklin.
Issues in Ecology Number 6 Spring 2000 issues underlies policies and practices that have been blamed mosphere.a phenomenon expected to drive warming of the for repeated over-cutting of national forest lands.Consid global climate.However,one way the earth maintains a erations of spatial pattern are essential in management plans balanced atompshere is by storing or "sequestering"car that strive to assure normal ecological functioning within bon (Perry 1994).Forests help mitigate the accumualtion the landscape(Harris 1984.Franklin and Forman 1987). of CO,in the atmosphere by absorbing this trace gas from As an example,consideration of forest patch size and the air to fuel photosynthesis.Half of the carbon absorbed shape and the extent of edge influences is critical in assess- is released back to the atmoshpere during respiration.while ing whether the interior forest conditions required by many the other half is sequestered in soils.sediments.and wood species are actually present in a landscape.The influenceof This makes forests a significant global resevoir for carbon recently cut areas on adjacent forest patches (edge effects) but it is unknown how much this sequestration will mitigate For instance,micro tic increasing emi sions of( rcut patch pernaps greater sunlig ner tem orest rworld with incr ed at y extend or 10to 30 meters mospheri are ha to pre KE earch on individua meters) etting es y El gro en e 1993 ant g et a dscape in whi such fin ng 981 empera dictable t in the landscape are not would require availability to duce th nes associated extra wood.roots.and lea eindications that lands lakes and n ds are nd nutrient eveling example.In many for some re earch show and gr rowth by producing leaves and other tissues with upon the root strength of the standing forest:clearcut this lower nitrogen concentrations (Schlesinger 1997)Organic forest and the likelihood of landslides increases dramatically matter low in nitrooen decomposes more slowly.raisino the Other sensitive areas in a landscape may be source areas for specter of reduced nitrogen availability and constraints on woody debris and sediments for streams and rivers,rock potential increases in plant growth. outcrops and scree slopes.and calving areas for deer.elk. A larger scale consequence of altered global climate pat and other ungulates.All of these types of sensitive areas terns could be changes in the distribution of species,includ should be recognized and protected as part of landscape ing the geographic regions suitable for important forest spe level forest management planning cies.Recent predictions for the eastern U.S.suggest,for Some have proposed that spatial issues can be adequately example,that changes in climate could lead to the complet addressed simply by providing that a certain percentagef loss of species such as sugar maple,with its new range lying each watershed be maintained in a series of four or five struc entirely in Canada.In the central states,a north vara shit 1997.How of loblolly pine populatio from Ok and ever,p and I s pr adequately com with the of loblo y pines mo Gu ccurs an orgla 94 of ho espec ally 03 est ed.populated. -altere nd goa dition d de ides alter ing the and climate.nollutio pend upor regional c ext and il fl h manageme direct i acts whic GLOBAL CHANGE:IMPLICATIONS FOR ear the Farth's su e is a very rea FOREST MANAGEMENT tive.short-lived gas that accumulates mainly on hot Combustion of fossil fuels.deforestation.and other hu- nant summer days ozone damac s plants by penetratin man activities are contributing to the buildup of carbon di- the leaf pores (stomata)and oxidizing cell membranes and oxide(CO)and other so-called greenhouse gases in the at- other structures.The result is a reduction in net photosyn
9 Issues in Ecology Number 6 Spring 2000 issues underlies policies and practices that have been blamed for repeated over-cutting of National Forest lands. Considerations of spatial pattern are essential in management plans that strive to assure normal ecological functioning within the landscape (Harris 1984, Franklin and Forman 1987). As an example, consideration of forest patch size and shape and the extent of edge influences is critical in assessing whether the interior forest conditions required by many species are actually present in a landscape. The influence of recently cut areas on adjacent forest patches (edge effects) can be very extensive. For instance, microclimatic influences of a clearcut patch perhaps greater sunlight, higher temperatures, drying winds may extend for 10 to 30 meters (with extremes of 200 to 400 meters) into an adjacent oldgrowth forest patch (e.g., Chen et al. 1993). Edge influences can be so pervasive that a landscape in which 10- hectare patches of cutover and forest are interspersed will entirely lack interior forest conditions after half of the landscape has been harvested (Franklin and Forman 1987). Recognizing the sensitivity of portions of a landscape is another essential element in landscape planning. All parts of the landscape are not created equal. Floodplains, banks, and shallow-water zones associated with streams, rivers, wetlands, lakes and ponds are examples of sensitive areas. Areas of unstable soil provide another example. In many forested mountainous landscapes, soil stability depends substantially upon the root strength of the standing forest; clearcut this forest and the likelihood of landslides increases dramatically. Other sensitive areas in a landscape may be source areas for woody debris and sediments for streams and rivers, rock outcrops and scree slopes, and calving areas for deer, elk, and other ungulates. All of these types of sensitive areas should be recognized and protected as part of landscapelevel forest management planning. Some have proposed that spatial issues can be adequately addressed simply by providing that a certain percentage of each watershed be maintained in a series of four or five structural stages of development (e.g., Oliver et al. 1997). However, proponents of this approach use structural stages that do not adequately represent complex structural development that occurs in natural forest stands, and they often fail to incorporate principles of landscape ecology just discussed. Such simplifications have value in dealing with plantations or intensively managed stands, but not in designing sustainable policies for National Forest lands managed to meet diverse values and goals. The proportion of various forest conditions or stages present in a given landscape should depend upon regional context and management objectives. GLOBAL CHANGE: IMPLICATIONS FOR FOREST MANAGEMENT Combustion of fossil fuels, deforestation, and other human activities are contributing to the buildup of carbon dioxide (CO2 ) and other so-called greenhouse gases in the atmosphere, a phenomenon expected to drive warming of the global climate. However, one way the earth maintains a balanced atompshere is by storing or sequestering carbon (Perry 1994). Forests help mitigate the accumualtion of CO2 in the atmosphere by absorbing this trace gas from the air to fuel photosynthesis. Half of the carbon absorbed is released back to the atmoshpere during respiration, while the other half is sequestered in soils, sediments, and wood. This makes forests a significant global resevoir for carbon, but it is unknown how much this sequestration will mitigate increasing emissions of CO2 . Forest responses to a warmer world with increased atmospheric CO2 are hard to predict. Research on individual plants in controlled settings indicates that a primary effect of rising CO2 will be enhanced plant growth (DeLucia et al. 1999). However, it is unclear how such findings will translate to ecosystems in the field over time. A changing climate will likely bring not just a shift in temperatures but unpredictable changes in precipitation, cloudiness, disturbance patterns, and perhaps timing of growing seasons (Perry 1994). Increased tree growth in enriched CO2 environments would require increased nutrient availability to produce the extra wood, roots, and leaves. Yet there are indications that decomposition and nutrient cycling could be hampered. For example, some research shows plants respond to increased CO2 and growth by producing leaves and other tissues with lower nitrogen concentrations (Schlesinger 1997). Organic matter low in nitrogen decomposes more slowly, raising the specter of reduced nitrogen availability and constraints on potential increases in plant growth. A larger scale consequence of altered global climate patterns could be changes in the distribution of species, including the geographic regions suitable for important forest species. Recent predictions for the eastern U.S. suggest, for example, that changes in climate could lead to the complete loss of species such as sugar maple, with its new range lying entirely in Canada. In the central states, a northward shift of loblolly pine populations from Oklahoma, Tennessee and North Carolina to central Illinois and Indiana is predicted, with the southern limit of loblolly pines moving from the Gulf Coast into central Alabama and Georgia (Perry 1994). Such projections raise the question of how quickly plant and animal species will be able to migrate as their suitable climatic range shifts, especially when they must migrate across fragmented, populated, and otherwise human-altered landscapes. Besides altering the atmosphere and climate, pollution from fossil fuel burning could have direct impacts on forest distribution by raising levels of tropospheric ozone, which accumulates near the Earths surface. Ozone is a very reactive, short-lived gas that accumulates mainly on hot, stagnant summer days. Ozone damages plants by penetrating the leaf pores (stomata) and oxidizing cell membranes and other structures. The result is a reduction in net photosyn-
