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 satu￾ration 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 satura￾tion has been witnessed in forests at intermediate to high elevations that receive large amounts of ni￾trogen 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 Ange￾les 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 ni￾trates seep away into streams or groundwater, they carry along positively charged minerals such as calcium, magnesium, and po￾tassium. Loss of these alkaline min￾erals acidifies the soil and decreases its fertility. Forest har￾vesting 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 follow￾ing 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 for￾est ecosystems, retaining a healthy green cover at all times, either through retention harvests or regrowth of early suc￾cessional plants (or both), will become increasingly impor￾tant to conserve soil nutrient capital after logging. HYDROLOGY The headwaters of the nation’s largest rivers, which sup￾ply much of our fresh water, originate on National Forest land. Cutting of timber in these watersheds raises three con￾cerns: changes in the volume of water flowing to streams, tim￾ing 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 ex￾tremely difficult because of the high variability of such flows, both over time and from one forest sys￾tem to the next. Because of natu￾ral variability in flows, only dra￾matic impacts of tree removal on stream hydrology are statistically detectable in short-term studies. Decades-long records are often necessary to discern trends, espe￾cially in larger basins. Also, a va￾riety of factors from harvest prac￾tices to bedrock geology, topog￾raphy, and climate (whether rain or snow dominates, whether fog￾drip from canopies is significant) affect the volume and timing of stream flow. The clearest effects of har￾vest on water flows have been ob￾tained 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 sig￾nificant 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 in￾creases peak flows, although that can vary with the extent and rate of logging within a basin, how the logging is car￾ried out, and the extent of the forest road network. Prac￾tices such as intensive site preparation, prevention of shrub and grass regrowth on the site, extensive roading, and dis￾ruption of streambank and floodplain forests have the great￾est 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 ripar￾ian zones like in this small headland stream. Sequoia￾Kings Canyon National Park, CA. Photo by Jerry Franklin.