Fifty years of storm runoff response to forest succession at the H.J. Andrews Forest

Forest harvesting frequently increases storm induced discharge, but it is not known how long such responses persist. Few studies have examined storm event response over multiple decades after harvest. We examined storm runoff coefficients (quickflow/storm precipitation) in five paired-basin experiments in the H.J. Andrews Experimental Forest, Western Oregon, USA to test how storm response varied by season over 50 years after forest harvesting and how these responses might reflect changes in canopy interception including epiphytes, changes in evapotranspiration by different forest structures and ages, and changes in snowpack and water storage. The largest percent increases in runoff coefficients occurred in the early part of the water year (fall) in the first decade after harvest (+8 to +94%), but these percent changes declined over time, and early wet season storm event discharges became smaller than pre-harvest by the 4^th decade (-1 to -15%) apparently as a result of forest succession, which produces multi-decade declines in summer streamflow that lead to greater recharge and less stormflow in the early wet season. In contrast, initial increases in storm runoff coefficient were smaller during the wet (winter) and late wet (spring) seasons than in the early wet season (fall), but these wet and late wet season storm runoff coefficients remained above the pre-harvest levels (higher relative quick flow) throughout the 5^th decade. This is likely because 50-yr-old planted forests are only 10% of the age of the old-growth forests they replaced, and hence have less capacity to intercept and store water in the forest canopy and to access water from deep in the soil profile. The relative increases in runoff coefficients also remained higher in the late wet season (spring) than in the wet season (winter) throughout the 5^th decade, perhaps because of different snow accumulation and melt in 50-yr-old forests compared to the old-growth forest they replaced.