Effects of Forest Harvest on Stream-Water Quality and Nitrogen Cycling in the Caspar Creek Watershed1

The effects of forest harvest on stream-water quality and nitrogen cycling were examined for a redwood/Douglas-fir ecosystem in the North Fork, Caspar Creek experimental watershed in northern California. Stream-water samples were collected from treated (e.g., clearcut) and reference (e.g., noncut) watersheds, and from various locations downstream from the treated watersheds to determine how far the impacts of these practices extended. Additionally, a detailed nutrient cycling study was performed in a clearcut and reference watershed to gain insights into changes in nitrogen cycling after harvesting activities. Stream-water nitrate concentrations were higher in clearcut watersheds, especially during high stream discharge associated with storm events. Elevated concentrations of nitrate were due to increased leaching from the soil as mineralization (i.e., release of nutrients from organic matter) was enhanced and nutrient uptake by vegetation was greatly reduced after harvest. The elevated nitrate concentration in stream water from clearcut watersheds decreased in the higher-order downstream segments. This decrease is believed to be primarily due to dilution, although in-stream immobilization may also be important. Although elevated nitrate concentrations in stream water from the clearcut watershed might suggest a large nitrogen loss after clearcutting, conversion to a flux indicates a maximum loss of only 1.8 kg N ha-1 yr-1; fluxes decreased to <0.4 kg N ha-1 yr-1 3 years after the harvest. Nitrogen fluxes from the reference watershed over the same period were <0.1 kg N ha-1 yr-1. The increased nitrogen flux was due to both higher nitrate concentrations and an increased water flux from the clearcut watershed. In contrast to many forest ecosystems that show large nutrient losses in stream water after harvest, this redwood/Douglas-fir ecosystem shows relatively small losses. The rapid regrowth of redwood stump sprouts, which use the vast rooting system from the previous tree, is capable of immobilizing nutrients in its biomass, thereby attenuating nutrient losses by leaching. Rapid regeneration also provides soil cover that appreciably reduces the erosion potential after harvest. Removal of nitrogen, primarily in the harvested biomass, results in an appreciable loss of nitrogen from the ecosystem. These data suggest that nitrogen fixation by Ceanothus may be an important nitrogen input that is necessary to maintain the long-term productivity and sustainability of these ecosystems. The effects of forest harvest and postharvest practices on nutrient cycling were examined for a redwood/Douglas-fir ecosystem in the North Fork, Caspar Creek experimental watershed in northern California. This ecotype is intensively used for commercial timber production, and streams draining these ecosystems are an important salmon-spawning habitat. Although the effects of forest harvest practices on stream flow and sediment generation have been intensively studied (e.g., Keppeler and Ziemer 1990, Rice and others 1979, Thomas 1990, Wright and others 1990, Ziemer 1981, and papers contained in these proceedings), the impacts of harvest practices on nutrient cycling processes have not been rigorously examined for the coastal region of northern California. Water quality and long-term nutrient sustainability are major components addressed within the ecosystem approach to forest management (Swanson and Franklin 1992). Forest harvest practices are often considered to have adverse impacts on water quality and sensitive aquatic communities owing to enhanced sediment and nutrient losses due to erosion and leaching (Hornbeck and others 1987, Likens and others, 1970, Martin and others 1986). The loss of plant nutrients in drainage waters, suspended sediments, and biomass removed by harvesting may further affect nutrient sustainability of forest ecosystems (Hornbeck and others 1987, Johnson and others 1982, 1988). Sustainable forestry is based on the premise of removing essential nutrients at a rate less than or equal to that which can be replenished by natural processes. As forest ecosystems become more intensively managed, it is imperative that best-management practices be developed and used to minimize environmental impacts and assure long-term ecosystem sustainability. This paper specifically examines the effects of forest harvest and postharvest management practices on the nitrogen cycle because nitrogen is the mineral nutrient required in the largest amount by the vegetation and it is believed to be the most limiting nutrient in this ecosystem. In addition, results for 10 additional nutrients are available in the final project report (Dahlgren 1998). This research uses a biogeochemistry approach to nutrient cycling that examines processes and interactions occurring within and between the atmosphere, hydrosphere, biosphere, and geosphere (fig. 1). The process-level information obtained can be used to decipher the complex interactions that occur in nutrient cycling processes at the ecosystem scale. Results from this research can be applied to the development of best-management practices to maintain long-term forest productivity while minimizing adverse environmental impacts from forest management. Methods Study Site Characteristics Headwater catchments in the North Fork of Caspar Creek were selected for this study (fig. 2). The watersheds are located in the Jackson Demonstration State Forest, 11 km southeast of Fort Bragg, California, and approximately 7 km from the Pacific Ocean. The North Fork of Caspar Creek has a drainage area of 473 ha and 1 An abbreviated version of this paper was presented at the Conference on Coastal Watersheds: The Caspar Creek Story, May 6, 1998, Ukiah, California. 2 Professor of Soil Science and Biogeochemistry, University of California, Department of Land, Air and Water Resources, One Shields Avenue, Davis, CA 95616. ([email protected]) Coastal Watersheds: The Caspar Creek Story Effects of Forest Harvest on Caspar Creek Watershed Dahlgren USDA Forest Service Gen. Tech. Rep. PSW-GTR-168. 1998. 46 ranges in elevation from 37 to 320 m. The topography of the North Fork watersheds ranges from broad, rounded ridgetops to steep inner gorges. Slopes within the watershed are <30 percent (35 percent of the area), 30-70 percent (58 percent of area), and >70 percent (7 percent of the area) (Wright and others 1990). The climate is Mediterranean, having dry summers with coastal fog and mild temperatures ranging from 10 to 25 °C. Winters are mild and wet, with temperatures ranging between 5 and 14 °C. The average annual rainfall is about 1,200 mm with no appreciable snowfall (Ziemer 1981). Soils are dominated by welldrained Ultic Hapludalfs and Typic Haplohumults formed in residuum derived predominately from sandstone and weathered coarse-grained shale of Cretaceous Age. The North Fork of Caspar Creek was originally clearcut logged and burned in approximately 1910 (Tilley and Rice 1977). Current vegetation is dominated by second-growth redwood (Sequoia sempervirens (D. Don) Endl.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) with minor associated western hemlock (Tsuga heterophylla (Raf.) Sarg.) and grand fir (Abies grandis (Dougl.) Lindl.). The mean stand density based on cruise data from three watersheds was 207 redwood stems ha (mean DBH = 56 cm) and 86 Douglas-fir stems ha (mean DBH = 66 cm). Timber volume at the onset of this study before logging was estimated at about 700 m ha (Krammes and Burns 1973). Solid-Phase Soil Analyses Sites for six soil pits were randomly selected within a clearcut (KJE) and reference (MUN) watershed (fig. 2). Soil pits (1.5 by 0.5 by 1-1.2 m; L × W × D) were excavated to a depth corresponding to the limit of the major rooting zone (BC horizon; 100-120 cm). Each pedon was described, and soil samples for chemical analyses and clods for bulk density measurements were collected from across the entire pit face for each morphological horizon. All soil samples were collected during September 1992, the month during which the soil is driest. Soil samples were air-dried, gently crushed, and passed through a 2-mm sieve; roots passing through the sieve were removed with a forceps. Bulk density was determined by the paraffin-coated clod method using three replicate clods per horizon (Soil Survey Staff 1984). Total carbon and nitrogen were determined on ground samples (<250 μm) by dry combustion with a C/N analyzer. Soil carbon and nitrogen pools (kg ha) were calculated for each soil profile (n = 6) by summing the nutrient content of all horizons within the major rooting zone. Nutrient pools for each horizon were determined from the nutrient concentration in the <2 mm fraction, mean horizon thickness, and bulk density of each horizon with a correction for the coarse fragment (>2 mm) volume. Exchangeable Ions Vegetation Immbilization Microbes Sorbed Primary minerals