A Reanalysis of Forest Disturbance Impacts on Streamflow

Abstract:

This study demonstrates that stream flow changes are detectable several decades after forest disturbance. Evidence is based on empirical data available from US long-term paired watershed studies. Changes in the long-term streamflow records are continuous and occur over a range of flow conditions reflecting forest disturbance severity, regrowth dynamics, vegetation type, and watershed characteristics. Whereas many published studies have focused on annual or seasonal flows, we show that high and low flow percentiles are more sensitive to forest disturbance over decades. We developed a new method to quantify relative and absolute changes in streamflow for ten groups of flow percentiles including the statistical significance between reference and treatment watersheds. Increases in lower flows occur at all sites immediately after forest removal or removal with vegetation suppression mostly ranging between 50-100% relative flow increase. In the regrowth phase, however, most sites show decreases in low flows due to increases in transpiration. Perhaps surprisingly, while low flows may still be reduced decades after disturbance, some watersheds had increased high flows (e.g., at 80^th and 90^th flow percentiles) 20 to 30 years after disturbance. The long-term change in seasonal runoff yield showed variable responses among the nine long-term paired watershed studies. For several of the disturbed watersheds, the cumulative yield increased up to 14 times the summer runoff within 30-40 years. In light of continuing loss of forest land or changes to forest cover globally and in North America, we recommend maintaining and establishing highly valuable long-term hydrological records of paired watershed studies that empirically allow for the examination of long-term change and variability of streamflow response to forest management and disturbance. These records are also important for testing models in their ability to reproduce the long-term forest disturbance effects as shown in this study. The ability to detect legacies of forest disturbance emphasizes the need to consider not only the sensitivity of the watershed to climate, but to coupled climate and vegetation change for addressing hydrological responses to global change.