Long-Term Impacts of Woody Crop Conversion on Hydrology and Biogeochemistry in Forest Watersheds

High-yield silvicultural practices (e.g short-rotation woody crops (SRWCs)) increase the pressure on the hydrological and biogeochemical cycles of forest watersheds. These forest systems experience more intensive mechanical and chemical practices compared to traditional forestry. Despite a plethora of studies on the effects of forest management on catchment function, some of the long-term effects of SRWC cultivation on water quality and quantity remain poorly understood.

Following harvest of existing mature loblolly pine stands, we implemented intensive silvicultural management, using high fertilization rates, high tree planting density, and competition control with herbicides on pine plantations covering approximately 50% of two first-order watersheds (B and C) in the southeastern U.S. Coastal Plain. Over a nine year period, we monitored streamflow, stream chemistry, and groundwater chemistry in these watersheds and an adjacent reference watershed (R) prior to and after harvest and planting. The objective of this watershed manipulation experiment was to evaluate the changes on catchment hydrology and biogeochemistry and relate that to the efficiency of the applied traditional forestry best management practices (BMPs).

Our results suggest a significant initial shift in hydrological processes and the catchment water balance, with increased streamflow following clear-cutting. Over time, we observed a return to baseline conditions. The water quality response was variable between chemical compounds and different across watershed compartments. For example, nitrate levels in groundwater increased post-fertilization with no drop in the 6-year post-harvest observation period. Contrary, we detected no significant water quality changes in the riparian groundwater nor stream water, likely due to effective denitrification and nutrient uptake in the riparian zones of these groundwater-dominated watersheds.

The experiment suggest that watershed-scale conversion to SRWC loblolly pine systems may cause short-term alterations in catchment processes of Coastal Plain watersheds. Our findings highlight the critical role of riparian zones in mitigating impacts on stream water quality. However, elevated nitrate concentrations after the last application of fertilizer stresses the critical need for observations to fully characterize long-term water quality impacts of SRWCs across entire watersheds.