Introduction

Streamside buffer strips have been used for years as a management tool in protecting riparian environments from the adverse effects of timber harvesting. Still, questions about what constitutes adequate buffer widths persists. State and Federal agencies have developed different policies regarding timber harvest practices in riparian zones. These differences concern the width of buffer strips along streams and the activity allowed within these zones.

To determine an optimum buffer width, one that protects the integrity of the riparian ecosystem while allowing profitable timber harvest, researchers have examined riparian processes as a function of distance from stream channels. These studies include the role of root strength on slope stability (Burroughs and Thomas 1977; Sidle et al. 1985; Wu 1986), delivery of large wood to streams (McDade et al. 1990; Vansickle and Gregory 1990; Andrus and Lorenzen 1992), shade (Reifsnyder and Lull 1965; Steinblums 1977; Beschta et al. 1987; Takentat 1988; Chen 1991), water quality (Broderson 1973; Darling et al. 1982; Lynch et al. 1985; Castelle et al. 1992), and wildlife, including benthic invertebrates (Erman et al. 1977; Roderick and Milner 1991).

Few, if any, studies have examined the effects of buffer width on air temperature and relative humidity in riparian zones. This relationship is important because a buffer strip of insufficient width may allow an increase in direct and reflected solar radiation into a stream environment, increasing the air temperature and lowering the relative humidity on warm days. These effects are most pronounced during the afternoon hours of the summer months when the highest concentration of solar radiation is present (Chen 1991).

Methods

From June 7 to August 31, 1994, air temperature and relative humidity in a stream riparian zone were measured at two sites in the Mad River Ranger District, Six Rivers National Forest, California. Both sites were on southwest-facing slopes that had recently been clearcut (1993 for Site 1, and 1992 for Site 2), leaving buffer strips of varying widths between the clearcuts and the streams. The study period, aspect, and slope were chosen to represent conditions where the greatest incident solar radiation and highest air temperatures occur within the study area. Measurements were collected over each stream at six collection sites, where the buffer widths were 150 meters, 90 meters, 60 meters, 30 meters, 15 meters, and 0 meters (clearcut). Measurements were taken once a week during the afternoon when the sun was at a right angle to the buffer strips. This occurred during the hours of 1100-1300 for Site 1 and 1000-1200 for Site 2.

Results

Air temperature above the streams increased exponentially with decreasing buffer width (Figure 1). There was a 6.5°C increase in mean air temperature along the riparian zone between the 150 meter and 0 meter buffer width collection sites. A power function best modeled this relationship with the equation y = 27.739 * x^(-0.055033) R2 = 0.98117, where y is air temperature (°C), and x is buffer width in meters. Mean air temperature rose sharply, 1.6°C/10 meters, in the riparian zone where the buffer width was 0 to 30 meters wide. Where the buffer strip was 30 to 150 meters wide, the rise in mean air temperature was more gradual at 0.2°C/10 meters.

Relative humidity was inversely proportional to air temperature (Figure 2). There was a 19% decrease in mean relative humidity along the riparian zone between the 150 meter and 0 meter buffer width collection sites. This relationship was modeled with the equation y = 34.172 * x^(0.086176) R2 = 0.92435 where y is relative humidity (%), and x is buffer width in meters. At the 0 to 30 meter collection sites, mean relative humidity along the riparian zone dropped sharply at 3.8%/10 meters. Between the 30 and 150 meter buffer width collection sites the drop in mean relative humidity was more gradual at 0.6%/10 meters.

Discussion

Earlier research supports the findings that buffer width affects air temperature and relative humidity. In a study examining microclimate gradients from the edge of a clearcut to 240 meters into an upland forest, Chen (1991) found that during the afternoon on a west-facing slope, air temperature decreased exponentially from the edge into the forest at an overall mean rate of 0.4°C/10 meters. The greatest rate of change was found within the first 30 meters where air temperature decreased at a mean rate of 1.0°C/10 meters before decreasing to a rate of 0.4°C/10 meters from 30 to 180 meters into the forest. In the same study, Chen (1991) found that relative humidity increased at a mean rate of 3.7%/10 meters between the edge and 30 meters, and 2.0%/10 meters between 30 to 180 meters before leveling off.

Changes in air temperature and relative humidity were found up to the 150 meter buffer width collection site, which was the control for the study. This indicates that buffer widths greater than 150 meters may affect riparian microclimate. Chen (1991) recorded changes in air temperature, relative humidity, and wind velocity up to 240 meters into an upland forest from the edge of a clearcut, while solar radiation, soil temperature, and soil moisture were influenced up to 90 meters.

Changes in microclimate conditions can alter the ecosystem of the riparian environment. Buffer widths that allow increased direct and indirect solar radiation into the riparian zone will increase air temperature and decrease relative humidity in that area. If these measurements move beyond the tolerance levels of terrestrial riparian flora and fauna, these species may perish or be forced to find other suitable habitat to complete their life cycle. Rudolph and Dickson (1990) reported amphibian and reptile populations were significantly lower in aquatic habitats with narrow buffer widths (<30 meters) than those with wider buffer strips due to greater shading (i.e., less solar radiation and lower air temperatures) and open understory vegetation. Evapotranspiration rates increase with increasing air temperature and may contribute to a lowering of the groundwater table and soil moisture content. This may prematurely dry up intermittent streams, depriving flora and fauna of an important water source during the dry season. Increased solar radiation and air temperature may also raise the water temperature in a stream to sublethal or lethal levels for resident aquatic life. For example, Northwest fall chinook salmon require stream temperatures between 10.6°C to 19.4°C to migrate upstream, and prefer stream temperatures of 5.6°C to 13.9°C for spawning (Bjornn and Reiser 1991). Water temperatures have been shown to exceed this tolerance level of salmonids along stream reaches where vegetative shading has been reduced (Brown et al. 1970; Beschta et al. 1987).

Land managers who wish to avoid significantly altering the microclimate of a riparian zone may want to leave buffer strips over 30 meters wide in regions similar to the study area. Buffer strips wider than 30 meters will still affect the microclimate of a riparian zone, but at a lower rate of change. The effect of small changes in microclimate (e.g., 1°C or 2°C increase of air temperature) on riparian species has not been extensively studied. Further studies are needed to determine the effects of incremental changes in microclimate on the riparian ecosystem.

Buffer Width Considerations in Forest Management

Establishing a buffer width necessary to maintain a functioning riparian ecosystem is dependent on many factors including local climatic conditions, topography, geology, and vegetation. Arbitrarily set buffer widths will not address the specific conditions and processes of each site. However, using available research, general guidelines for minimum widths can be determined. Much of the data on buffer strips, including this study, indicates that a minimum buffer width of 30 meters (~100 feet) is necessary to avoid significantly impacting riparian environments (Erman et al. 1977; Steinblums 1977; Rudolph and Dickson 1990; Chen 1991; Spackman and Hughes 1994). For many processes such as sediment flow and delivery of large woody debris, this minimum width may be increased to 60 to 80 meters or one site potential tree (Broderson 1973; Beschta et al. 1993; Thomas et al. 1993). Under the Northwest Forest Plan for federal forestlands in the range of the northern spotted owl, the "interim" buffer widths listed for fish-bearing streams (300 feet or the average height of two site-potential trees) and permanently flowing streams (150 feet or the average height of one site-potential tree) exceeds the minimum buffer width recommended by most studies and provides a high degree of protection pending more detailed analysis and site-scale design.

Watershed analysis can be used to identify critical hillslope, riparian, and channel processes affecting riparian and aquatic functions on a ecosystem or watershed level (Thomas et al. 1993). Site specific considerations of these processes can be used in determining buffer widths for each project (Thomas et al. 1993). The use of site specific analysis and minimum buffer width guidelines based on research, should allow for riparian zones to be managed for a variety of objectives while maintaining viable riparian ecosystems.-
The Effects of Buffer Strip Width on Air Temperature and Relative Humidity in a Stream Riparian Zone.

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