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Woody Wetlands

Forested wetlands are important for their ability to transform inorganic nutrients into organic form, as well as filter out sediment and particulate matter (Lockaby and others 1997). Forested wetlands were considered unproductive up to the 1950s, when many large pine plantations were established on drained forested wetland sites in the lower Coastal Plain of the South (Xu and others 1999). Forested wetlands are characterized by high seasonal water tables and soil surface waterlogging due to flat topography and poor soil drainage. A brief discussion of the impacts of silvicultural activities on forested wetlands is included below; however, a complete discussion of forested wetland characteristics and potential impacts from various land use activities, including silviculture, is included in chapter 20.

The primary silvicultural activities potentially affecting important wetland functions are site drainage and the operation of heavy equipment on wetland soils, usually during site preparation. Site drainage improves access, provides for soil aeration, and increases seedling survival and growth (Segal and others 1987). Site-preparation practices such as mole-plowing and bedding are among the most prominent silvicultural practices in the South (Xu and others 1999). Mole-plowing uses a deep plow to create a channel in poorly drained soils to improve site drainage. Bedding is a common practice that elevates planted trees on beds above the surface of the water table. Minor drainage is often needed to remove excess surface water to permit heavy equipment to be operated without causing extensive soil compaction and rutting (Shepard 1994).

In contrast to upland forests, surface water flow rates are low in wetlands, which typically have little topographic relief and therefore have less energy available to export sediment. In a review of literature on water quality in forested wetlands, Shepard (1994) found that silvicultural activities generally resulted in water-quality impacts, but the impacts were typically small and short-lived. Impacts were greater in upland wetlands where relief is greater and soils are shallower than in lowland wetland forests. Impacts on all common wetland types have not been investigated. In particular, there is very little published information available on the impacts from bottomland hardwood silviculture on water quality. Shepard (1994) concludes that silvicultural activities “do not constitute a permanent threat to the ability of wetlands to maintain or improve water quality.”

Xu and others (1999) examined the effects of clearcutting in the wet and dry seasons and site-preparation activities (bedding and mole-plowing plus bedding) on ground-water levels. The authors found that water tables rose in response to forest removal, with the greatest increases occurring after wet-weather logging. The larger increase associated with wet-weather harvesting was likely due to deeper rutting and greater soil disturbance. No significant differences in ground-water levels were found during the dormant season, indicating that the removal of transpiring vegetation was primarily responsible for the increase in water table levels (Xu and others 1999).

The same study found that site-preparation techniques ameliorated harvest-related elevated water tables by improving site drainage. Bedding reduced ground-water level by up to 22 cm compared to nonbedded sites. Mole-plowing plus bedding had a similar effect on ground-water levels as bedding alone. The recovery of site hydrology was fastest on sites that had been the least disturbed—harvested during dry weather and bedded only. Site hydrology recovered within 2 years of stand establishment (Xu and others 1999).

Miwa and others (1999) also found that wet-weather harvesting had a significantly larger impact on site hydrology than did dry-weather treatment.

Riekerk (1985) conducted a comparative watershed study in the poorly drained pine flatwoods of northern Florida. One watershed was clearcut with minimum disturbance and site preparation (manual shortwood harvesting, slash chopping, soil bedding, and machine planting). The second watershed was clearcut with maximum disturbance and site preparation (machine tree-length harvesting, slash burning, windrowing, soil bedding, and machine planting). The third watershed was an undisturbed control. Runoff increased 2.5-fold on the minimum-treatment watershed and increased 4.2-fold on the maximum-treatment watershed. There was a statistically significant increase in the level of suspended sediment (14 ppm on average) proportional to disturbance, but the absolute levels were low. Significant increases over the control remained for 4 years after both treatments (Riekerk 1985).

Ensign and Mallin (2001) studied the water-quality impacts of clearcutting 130 acres of riparian and seasonally flooded forest in the Coastal Plain of North Carolina. The authors found short-term increases in stream turbidity reaching 111 nephalometric turbidity units (NTU), well above the North Carolina State standard of 50 NTU, but the average increase was not statistically significant. However, compared with an unlogged control stream, suspended sediment concentrations were significantly increased for several months after the clearcut. In addition, statistically significant postlogging increases were reported for both total nitrogen and total phosphorus compared to a nearby control stream.

In aquatic habitats, Ensign and Mallin (2001) found significant decreases in dissolved oxygen that approached anoxia on several occasions after timber harvest. The decreases were attributed to stream algal blooms that formed periodically for two summers after clearcutting. The blooms occurred from a combination of increased nutrient inputs and possibly increased direct solar radiation on surface water. The formation of algal blooms, followed by death and decomposition, created high biochemical oxygen demands leading to decreased dissolved oxygen levels.

Another biotic parameter of interest in streams with human recreation as a designated use is microbial pathogens. Ensign and Mallin (2001) found greatly increased fecal coliform bacterial concentrations in streams following clearcutting. This increase may have occurred due to runoff of pathogens from nearby large-scale swine production facilities, or from the land disturbance itself (Ensign and Mallin 2001).

Lebo and Herrman (1998) examined outflow characteristics in a low-level pocosin with artificial drainage in a 1,161-acre watershed and found that sediment export from the watershed increased nearly 350 percent (4.1 to 14.3 pounds per acre) during a 3-year period that included harvest and site-preparation activities. Minor increases in nitrogen concentrations in streamwater were detected after harvest. These concentrations were typically less than the average value for the control stand. Increases in phosphorus concentrations were more prolonged than for nitrogen, but they decreased to preharvest levels after 3 years.