Effects of Timber Harvest on Water Quantity and Quality in Small Watersheds in the Piedmont of North Carolina

Abstract

This paired watershed study tested the effects of timber harvest on water quantity and quality in the North Carolina Piedmont physiographic region. Four headwater watersheds at Hill Demonstration Forest (HF1, HF2, HFW1, and HFW2) and two at Umstead Research Farm (UF1 and UF2) were continuously monitored for discharge and water quality from 2007 to 2013. The HF1 and UF1 watersheds were clearcut (treatment), leaving a 15.2-m vegetated riparian buffer around the streams to protect water quality as described in the North Carolina Neuse River Basin Riparian Buffer Rule. HF2 and UF2 were uncut and used as reference watersheds. Merchantable timber was selectively removed from the riparian buffer, reducing tree basal area by 27% in HF1 and 48% in UF1. HF1 and HF2 were nested within HFW1; thus, HFW1 was considered a partial cut where 33% of the watershed area was harvested, and HFW2 was the reference. We found that discharge in treatment watersheds increased dramatically, averaging 240% in HF1 and 200% in UF1 and 40% in HFW1 during the postharvest period, 2011–2013. Total suspended sediment export in the treatment watersheds also increased significantly in HF1 after harvest, probably due to the increase of discharge and movement of in-channel legacy sediment. Stormflow peak nitrate reached its maximum concentration during the first 2 years after harvest in the treatment watersheds and then declined, corresponding to the rapid regrowth of woody and herbaceous plants in the riparian buffer and uplands. We found that 36% of the UF1 streambank trees were blown down but did not cause a measurable increase in mean daily stormflow total suspended sediment concentration. Most buffer tree blowdown occurred during the first few years after a harvest. Bioclassification of benthic macroinvertebrates indicated that stream water quality remained good/fair to excellent in the treatment watersheds after the harvest. We conclude that the temporary increases in discharge were relatively large for the Piedmont region compared with those for other regions in the southeastern United States. However, the increases in channel sediment transport and nutrient exports associated with the hydrologic change did not have a measurable impact on the indicators of aquatic invertebrate community health or bioclassification rankings. Management and Policy Implications: There are three distinct land provinces across North Carolina: the mountains, Piedmont, and coastal plain. Understanding how region-specific watershed hydrology responds to land management and natural disturbances can provide useful information to land managers as they set flow targets needed to maintain ecological integrity in surface waters or to design riparian buffers for water quality protection. We found that after a clearcut timber harvest with a riparian buffer zone around the streams, the percent increases in annual discharge and mean nitrate concentrations tended to be higher in the Piedmont region than in the other two regions. Our study also found that tree blowdown in the riparian buffer was more likely to occur in the Piedmont Triassic region than in the Piedmont Carolina Slate Belt region. In the Triassic region, additional management activities should be part of the preharvest planning process as they may help mitigate windthrow and uprooting of streambanks and improve the overall riparian buffer functions and flow dynamics. For example, land planners or loggers should assess whether soils have Triassic or Slate Belt characteristics to determine if they need to refrain from creating gaps and retain more windfirm species within the riparian buffer. Although there was variation in blowdown occurrences among regions, Triassic soils comprise about 3.5% of North Carolina's total land area, so any statewide implications would be low and recommendations would be localized.