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.