Abstract:Nine small (2Ð5 ha) and four large (70-135 ha) watersheds were instrumented in 1999 to evaluate the effects of intensive silvicultural practices with best management practices (BMPs) on runoff and stream water quality in the Western Gulf Coastal Plain of East Texas, USA. Two treatments were implemented in 2002: a conventional treatment with clearcutting and herbicide site preparation, and an intensive treatment that added subsoiling, fertilization and a release herbicide application. Watershed effects were compared with results from a previously conducted study on the same watersheds in 1981, in which two combinations of harvesting and mechanical site preparation without BMPs were evaluated. Due to the reduction in evapotranspirational demand, total storm runoff increased on all six treated small watersheds following harvest by 0Ð94 to 13Ð73 cm in 2003. Runoff increases were not statistically significant on the treated large watersheds. Total first-year sediment loss was significantly greater on two of the conventional and one of the intensive small watersheds. The greatest first-year increase was 540Ð1 kg ha 1 , only one-fifth of that observed on these watersheds from shearing and windrowing without BMPs in 1981. First-year sediment loss was significantly greater on the intensive large watershed following harvest, but not on the conventional large watershed. These data suggest that BMPs are very effective in reducing potential water quality impacts from intensive silvicultural practices.
Nine small (2.5 ha) and four large (70-135 ha) watersheds were instrumented in 1999 to evaluate the effects of silvicultural practices with application of best management practices (BMPs) on stream water quality in East Texas, USA. Two management regimes were implemented in 2002: (i) conventional, with clearcutting, herbicide site preparation, and BMPs and (ii) intensive, which added subsoiling, aerial broadcast fertilization, and an additional herbicide application. Watershed effects were compared with results from a study on the same small watersheds in 1981, in which two combinations of harvesting and mechanical site preparation without BMPs or fertilization were evaluated. Clearcutting with conventional site preparation resulted in increased nitrogen losses on the small watersheds by about 1 additional kg ha(-1) each of total Kjeldahl nitrogen (TKN) and nitrate-nitrogen (NO(3)-N) in 2003. First-year losses were not significantly increased on the large watershed with a conventional site preparation with BMPs. Fertilization resulted in increased runoff losses in 2003 on the intensive small watersheds by an additional 0.77, 2.33, and 0.36 kg ha(-1) for NO(3)-N, TKN, and total phosphorus, respectively. Total loss rates of ammonia nitrogen (NH(4)-N) and NO(3)-N were low overall and accounted for only approximately 7% of the applied N. Mean loss rates from treated watersheds were much lower than rainfall inputs of about 5 kg ha(-1) TKN and NO(3)-N in 2003. Aerial fertilization of the 5-yr-old stand on another large watershed did not increase nutrient losses. Intensive silvicultural practices with BMPs did not significantly impair surface water quality with N and P.
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