Regional Differences in Stream Water Nitrogen, Phosphorus, and Sediment Responses to Forest Harvesting in the Conterminous USA.

Forest harvesting and management techniques were hypothesized to result in significant differences in stream water N (NO-N), P (total P [TP]), and total suspended sediment (TSS) responses among regions of United States. The objectives were (i) to determine the mean response periods after harvesting for each water quality variable, (ii) to compare the regional response yields, and (iii) to determine relationships among water quality, rainfall, and flow. Watershed-scale studies where best management practices were implemented provided a basis for water quality analyses. A mixed model was used to estimate the time from harvest to time when the harvested site yielded similar export as the reference site (response period). Normalized water quality yields were calculated as response yields (kg ha yr) times estimated response periods. Significant differences among yields were identified using ANOVA and Tukey test (α = 0.05), and relationships between water quality and hydrologic variables were identified using multivariate analysis (α = 0.05). The ratio of estimated mean response period for TSS to NO-N and TP, each individually, was approximately two. The mean normalized NO-N response yield was greater for the northern than the southern and/or western regions. Normalized NO-N and TSS response yields were greater for plantations than for other harvest types. The TSS export significantly increased with discharge from plantations. The literature-based response periods used in this study were not fully monitored, and soil surface manipulations after harvesting pose a significant influence on sediment export in the United States.