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Management Intensity

This section describes the gradient of potential water-quality impacts across a variety of silvicultural management techniques. The activities discussed include: (1) the harvesting method (single-tree selection, group selection, and clearcutting); (2) the degree of mechanization used in felling and collecting logs (hand felling, feller-bunchers, and cable yarders); and (3) the site-preparation method (windrowing, shearing, disking, prescribed burning, and use of fertilizers and herbicides). Other aspects of timber management associated with management intensity but not related to site disturbance and sedimentation, such as the conversion of hardwood and natural pine stands to pine plantations, are covered in the final section of this chapter. A more thorough discussion of forest operation technologies, including various site-preparation techniques and their impacts on the environment, is included in chapter 15.

In general, as management intensity increases, so does the level of site disturbance. Similarly, the greater the site disturbance, the greater the nonpoint-source impacts, particularly increased erosion and potential for sediment delivery into streams (Riekerk 1985). For example, in the poorly drained pine flatwoods of northern Florida, Riekerk (1985) found increases in total runoff, pH, suspended sediment, and potassium and calcium concentrations proportional to site disturbance in the year after harvest.

Effects of harvest method—It is widely acknowledged that the majority of effects from silvicultural activities can be attributed to operation of heavy machinery on roads and skid trails near water bodies. Rice and Wallis (1962) found no detectable change in stream channel conditions following harvest other than impacts directly resulting from logging equipment and logging debris. Physical alterations included stream channel scouring or filling by bulldozers, slash and debris in channel crossings, and diversion of water down logging roads at stream crossings and road cuts. The diversions caused severe gullying.

McMinn (1984) compared a skidder logging system and a cable yarder for their relative effects on soil disturbance. With the cable yarder, 99 percent of the soil remained undisturbed (the original litter still covered the mineral soil), while the amount of soil remaining undisturbed after logging by skidder was only 63 percent. Currently, cable yarding is primarily limited to the steepest slopes in the Appalachian Mountains and is otherwise rarely used in the South.

Other studies have demonstrated that the intensity of harvest, depending on the silvicultural prescription, may increase concentrations and loadings of sediment during storms. In watershed research studies in Arkansas and Oklahoma, Scoles and others (1996) found that soil loss increased with harvest intensity (clearcutting versus selection harvesting). Site-preparation activities consisted of crushing and burning residual vegetation. No special erosion control measures were applied. In both studies, statistically significant increases in annual soil loss were found in the first year after clearcutting compared to selectively harvested and control sites. Annual soil losses averaged 211 and 251 pounds per acre on clearcut watersheds in Arkansas and Oklahoma, respectively.

Research conducted by Beasley and Granillo (1985) demonstrated that selective cutting generated lower water yields and sediment yields than did clearcutting. Selective cutting resulted in sediment yields 2.5 to 20 times less and water yields 1.3 to 2.6 times less than those resulting from clearcutting.

Eschner and Larmoyeux (1963) completed a study that compared the water-quality impacts from four harvesting methods: (1) commercial clearcut, (2) intensive selection (trees over 5 inches diameter breast height (d.b.h.) were cut), (3) extensive selection (trees over 11 inches d.b.h. were cut), and (4) diameter limit (trees over 17 inches d.b.h. were cut). However, each of these harvest methods was combined with varying road designs, to determine their overall effectiveness in protecting water quality. It was concluded that the amount of trees removed, or harvesting method, was not the primary factor affecting water quality, as measured by turbidity. Water-quality impacts were shown to be related to the care taken in logging and planning skid roads. The extensive selection method, combined with some nonpoint-source controls (20-percent road grade limits, no skidding in streams, water bars on skid roads), produced higher maximum levels of turbidity than did intensive selection (210 and 25 turbidity units, respectively) with additional control practices (10-percent road grade limits, skid trails located away from streams). Harvesting by diameter limit without any restrictions on road grades or stream restrictions increased maximum turbidity by 200 times over intensive selection (5,200 and 25 turbidity units, respectively). Commercial clearcutting with no controls increased maximum turbidity by over three orders of magnitude compared to harvesting by diameter limit (56,000 and 25 turbidity units, respectively).

Effects of site preparation—Shearing, disking, drum-chopping, or root-raking a site with large tractors may heavily disturb the soil over large areas and has a high potential to deteriorate water quality (Beasley 1979). Site-preparation techniques that remove vegetation and litter cover, compact the soil, expose or disturb the mineral soil, and increase stormflows due to decreased infiltration and percolation all can contribute to increases in sediment loads (Golden and others 1984). However, erosion rates typically decrease as vegetative cover grows back. Prescribed burning and application of herbicides and fertilizers also have potential negative effects on water quality. These activities are discussed separately in sections that follow.

Shearing, which exposes large amounts of bare soil while removing logging debris, and windrowing resulted in higher levels of soil loss in the Texas Coastal Plain and Athens Plateau (Scoles and others 1996). Shearing also reduced the soil’s ability to absorb water in the Texas study. Douglass (1977) found that total soil loss from sites that had been cleared was approximately 580 pounds of soil per inch of runoff. However, runoff from sites that were both cleared and disked was twice that from sites that had been cleared only.

Blackburn and Wood (1990) reported that harvesting and shearing a watershed in east Texas increased phosphate and total phosphorus concentrations in the year after harvest, while harvesting and chopping had no effect on phosphate and total phosphorus concentrations.

As described previously, Xu and others (1999) determined that site-preparation activities (bedding and mole-plowing plus bedding) reduced water table levels significantly in forested wetlands.

Effects of prescribed fire—Prescribed fire can impact water quality by heating the soil and killing soil organisms, thereby altering nutrient transformation rates and bioavailability. These impacts depend on the severity and intensity of the fire. Prescribed burning of slash can increase erosion and sediment delivery to streams by eliminating protective cover and altering soil properties (Megahan 1980). The degree of erosion after a prescribed burn depends on soil erodibility; slope; precipitation timing, volume, and intensity; fire severity; cover remaining on the soil; and speed of revegetation. Swift and others (1993) found erosion after burning to be spotty and did not leave the treated site or reach stream channels. The prescription for this burn, however, was to maintain a low-fire intensity and avoid consuming the compacted litter or organic layers. Burning may also increase stormflow in areas where all vegetation is killed. Such increases are partially attributable to decreased evapotranspiration rates and reduced canopy interception of precipitation. Erosion resulting from prescribed burning is generally less than that resulting from roads and skid trails and from site preparation that causes intense soil disturbance (Golden and others 1984).

Knoepp and Swank (1993) found that clearcutting and burning increased streamwater nitrate concentrations from less than 0.01 mg per L to a maximum of 0.075 mg per L. This small increase was associated with a slight increase in nitrogen transformations and little movement of inorganic nitrogen off the site (Knoepp and Swank 1993). Concentrations returned to pretreatment levels within 9 months after burning.

In a paired watershed study, Van Lear and others (1985) examined soil and nutrient export in ephemeral streamflow after three low-intensity prescribed fires prior to harvest on the Clemson Experimental Forest in the upper Piedmont of South Carolina. Minor increases in stormflow and nutrient and sediment concentrations in the water were identified after low-intensity prescribed fires. It was suggested that erosion and sedimentation from plowed fire lines accounted for the majority of sediment from all watersheds. Following the prescribed fires, the overstory in the burned watersheds was harvested, and runoff, sediment, and nutrient export were monitored for 3 years after harvest. Sediment levels were elevated after harvest, but the magnitude and duration of these effects were considerably less than from other studies (Douglass and Goodwin 1980, Fox and others 1983, Hewlett 1979) that utilized mechanical site-preparation techniques instead of prescribed burning (Van Lear and others 1985).

Landsberg and Tiedemann (2000) thoroughly reviewed the effects of wildfires and fire management on water quality. The following specific management measures were identified as ways to reduce the magnitude of the effects of fire on water quality: (1) limit fire severity, (2) avoid burning on steep slopes, and (3) limit burning on sandy or water-repellent soils.

Effects of Fertilizers, pesticides, and herbicides—Although fertilizer application is uncommon in hardwood forests in the East, forest fertilization is routine—and possibly increasing (Dubois and others 1999)—on many intensively managed pine plantations in the South (Shepard 1994). A brief discussion of the use of fertilizers and pesticides (herbicides and insecticides) in forest operations is included in chapter 15. In a periodic survey of the cost of forest practices, Dubois and others (1999) report that the number of fertilized acres increased between 1996 and 1998. Few studies have looked at the impacts of this practice on water quality (Shepard 1994). Studies typically show that forest fertilization is not a problem; most studies have shown that nutrient increases are too small to degrade water quality (Binkley and Brown 1993, Fisher and Binkley 2000). Many forest streams are nutrient limited, so the application of fertilizers has a greater potential for impacts in nutrient-poor aquatic ecosystems.

Fertilizers, pesticides, and herbicides reach streams either directly through aerial or hand application, or indirectly by surface runoff and subsurface flow. BMPs typically restrict application to nonriparian zones. However, in practice, riparian zones are difficult to avoid in aerial applications. The effects of fertilizer application on aquatic ecosystems are the same as described for nutrients in the section “Aquatic Habitat and Biota.”

Pesticides can have both direct and indirect effects on ecological processes. Aquatic organisms can be affected through direct exposure to pesticides in the streamwater or through ingestion. There have been too few studies on the impacts of insecticides to make generalizations about the impacts on fish populations. Some 1- to 2-year studies (Reed 1966) have concluded that short-term reductions in insect populations—an important food source for fish—may occur. Insect communities should recover within a few years due to their short life cycles (National Council for Air and Stream Improvement 1994).

Herbicides can impact aquatic communities directly through increased organic matter inputs and indirectly through other effects on riparian vegetation. These secondary impacts can include changes in physical properties of streams, such as increases in water temperature and sedimentation, due to loss of riparian vegetation. Other secondary impacts to stream properties can result from changes in riparian vegetation, including increased nitrate inputs, decreased slope stability, and altered food-web structure in streams. No critical indirect effects have been documented for normal forest use of herbicides (National Council for Air and Stream Improvement 1994).

In a literature review on forest fertilization with nitrogen and phosphorus and water quality, Binkley and others (1999) found that without the use of BMPs, short-lived elevated nitrate and phosphorus concentrations were often found in receiving waters, but that national drinking water-quality standards (for nitrogen) and/or suggested criteria (for phosphorus) were rarely exceeded. No studies were identified that reported adverse affects on aquatic biota.

The effects of fertilizer application on water quality were studied in three North Carolina plantations (Campbell 1989). Fertilization temporarily elevated levels of ammonium, total nitrogen, total phosphate, orthophosphate, and urea in streams draining plantations. Concentrations returned to pretreatment levels within 3 weeks. Net exports were small compared to the total amount of fertilizer applied: net export of total Kjeldahl nitrogen was 0.3 percent of total nitrogen applied, net export of ammonium was 0.02 percent of total nitrogen applied, and net export of urea was 0.03 percent of total applied urea. Several other studies reported similar results (Fromm 1992, Herrmann and White 1983).

Segal and others (1987) studied the effects on water quality of applying fertilizer and herbicide in a pine flatwood in eastern South Carolina. They identified a strong pulse of nutrient concentrations in July and attributed this to higher mineralization rates of forest floor litter and higher soil temperatures after clearcutting. Nutrient concentrations in ground water did not appear to be outside the range of natural seasonal nutrient dynamics. Furthermore, ground-water quality did not appear to be negatively affected. All nutrient levels returned to pretreatment levels within 200 days after fertilizer application.