Stream nutrient response to contemporary timber harvest practices in western Oregon

Abstract

Timber harvesting has historically been shown to increase nutrient concentrations in stream waters by decreasing vegetative cover and nutrient uptake, allowing more nutrients to be leached into stream waters. Contemporary timber harvest practices, in which a streamside buffer is left in place, have not been studied. This study quantified the effects of contemporary timber harvesting practices, with a streamside buffer, in a Douglas-fir dominated watershed in the Oregon Coast Range, using a paired-watershed design. In the treatment (Needle Branch) and the control (Flynn Creek) watersheds, water quality samples collected from October 2006 through March 2016 were analyzed for nutrients. A clearcut harvest took place in the upper basin in 2009 (Phase 1), and in the lower basin in 2014 (Phase 2), and water samples were tested for nitrate (NO3-N), total nitrogen (TN), ammonia (NH3), orthophosphate (OP), and total phosphorus (TP). Intra-watershed comparisons of nutrient concentrations were made using a Wilcoxon Rank Sum Test to determine statistical significance between sites and treatments. A Before-After Control-Impact (BACI) design was used to compare the treatment watershed to the control watershed across treatments. Results at Needle Branch showed statistically significant increases (α < 0.05) in NO3-N between pre-treatment (0.59 mg/L) and Phase 1 (0.97 mg/L), and between pre-treatment and Phase 2 (0.90 mg/L) at the outlet. TN also showed statistically significant increases between pre-treatment (0.87 mg/L), and Phase 1 (1.06 mg/L), and between pre-treatment and Phase 2 (0.92 mg/L). NH3 was also shown to be statistically significant between pre-treatment (0.011 mg/L) and Phase 1 (0.013 mg/L). OP showed statistically significant increases between pre-treatment (0.018 mg/L) and Phase 1 (0.024 mg/L), and between pre-treatment and Phase 2 (0.022 mg/L), as did TP (0.018, 0.026, 0.020 mg/L during pre-treatment, Phase 1, and Phase 2, respectively). Results in Flynn Creek showed statistically significant increases in NH3 between pre-treatment (0.010 mg/L) and Phase 1 (0.013 mg/L). OP also showed statistically significant increases between pre-treatment (0.029 mg/L) and Phase 1 (0.034), and between pre-treatment and Phase 2 (0.032). TP also showed significantly significant increases between pre-treatment (0.028 mg/L) and Phase 1 (0.036 mg/L). Because similar results were observed in both the treatment and control watersheds, changes in these three constituents within the treatment watershed cannot be attributed to timber harvest. Neither NO3-N nor TN showed any change between phases within Flynn Creek, therefore, changes in these constituents within Needle Branch can be attributed to timber harvest. Contemporary timber harvest practices appear to have similar results as past harvesting practices, regarding nutrient concentrations in stream waters. With a streamside buffer, NO3-N and TN concentrations were significantly increased following harvest. Contemporary timber harvest practices, however, did not affect NH3, OP, and TP concentrations.