Sarah Smith-Tripp, Alden Griffith, Valerie Pasquarella, Jaclyn Hatala Matthes

Repeated moderate severity forest disturbances can cause short- and long-term shifts in ecosystem processes. Prior work has found that stand-replacing disturbances (e.g., clear-cutting) increases streamflow in temperate forests, but streamflow responses to repeated moderate severity disturbances are more equivocal. This study examined a moderate disturbance caused by an unexpected population irruption of the invasive insect Lymantria dispar (common name: gypsy moth) in 2015-2017. This irruption resulted in major but spatially heterogeneous defoliation due to leaf consumption by larvae during the summer growing season, which reduced leaf index area and associated evapotranspiration. Our regional approach in southern New England, USA used data from 88 U.S. Geological Survey (USGS) stream gages to assess whether changes in watershed runoff ratios and instantaneous streamflow characteristics were associated with satellite-derived metrics of forest condition (i.e., defoliation). Using stream gages with at least 15 years of baseline flow data, we calculated subwatershed runoff ratio values for a baseline period and quantified anomalous departures from baseline runoff ratios for the 2015-2017 period of L. dispar defoliation. We found a small linear increase in seasonal runoff ratio anomalies that was associated with defoliation intensity. We also found larger volumes of high and medium instantaneous streamflows compared to baseline flow conditions in watersheds with more intense defoliation. This study provided important insights into the impacts of moderate disturbance on ecohydrology in mesic temperate forests with a methodological approach that assessed the impacts of spatially heterogeneous and repeated moderate disturbance at a regional scale.