Fifty years of runoff response to conversion of old‐growth forest to planted forest in the H. J. Andrews Forest, Oregon, USA

Abstract

Long-term watershed experiments provide the opportunity to understand forest hydrology responses to past logging, road construction, forest regrowth, and their interactions with climate and geomorphic processes such as road-related landslides. We examined a 50-year record from paired-watershed experiments in the H. J. Andrews Experimental Forest, Oregon, USA in which 125 to 450-year-old conifer forests were harvested in the 1960s and 1970s and converted to planted conifer forests. We evaluated how quickflow and delayed flow for 1222 events in treated and reference watersheds changed by season after clearcutting and road construction, including 50 years of growth of planted forest, major floods, and multi-decade reductions in snowpack. Quickflow runoff early in the water year (fall) increased by up to +99% in the first decade, declining to below preharvest levels (À1% to À15%) by the third to fifth decade after clearcutting. Fall delayed flow responded more dramatically than quickflow and fell below pre-treatment levels in all watersheds by the fifth decade, consistent with increased transpiration in the planted forests. Quickflow increased less (+12% to 70%) during the winter and spring but remained higher than pre-treatment levels throughout the fourth or fifth decade, potentially impacted by post-harvest burning, roads, and landslides. Quickflow remained high throughout the 50-year period of study, and much higher than delayed flow in the last two decades in a watershed in which road-related changes in flow routing and debris flows after the flood of record increased network connectivity. A long-term decline in regional snowpack was not clearly associated with responses of treated vs. reference watersheds. Hydrologic processes altered by harvest of old-growth conifer forest more than 50 years ago (transpiration, interception, snowmelt, and flow routing) continued to modify streamflow, with no clear evidence of hydrologic recovery. These findings underscore the importance of continued long-term watershed experiments.

Seventy years of watershed response to floods and changing forestry practices in western Oregon, USA

“…Mean annual precipitation ranges from 2200 to 2700 mm depending on elevation and slope orientation. More than 80% of precipitation occurs between October and April (Bierlmaier & McKee, 1989; Crampe et al, 2021; Jones & Perkins, 2010). Average (1998–2007) snow water equivalent peaks in February at 100 mm at low elevation and in April at 750 mm at high elevation (Jones & Perkins, 2010).…”

Amount and reactivity of dissolved organic matter export are affected by land cover change from old‐growth to second‐growth forests in headwater ecosystems

“…Like other trenched hillslope studies, some of which appear in this special issue (Coweeta Hydrologic Laboratory [Hewlett & Hibbert, 1963], H.J. Andrews Experimental Forest [Crampe, Segura, & Jones, 2021], Maimai New Zealand [McDonnell et al, 2021], Panola Mountain Research Watershed [Aulenbach et al, 2021]), findings from this experimental scale can be validated and strengthened by long‐term catchment scale hydrology and climate data.…”

Streamflow response to native forest restoration in former Eucalyptus plantations in south central Chile

“…These findings are also relevant globally, such as in North America, where evidence is accumulating that intensively managed plantations of native species can reduce streamflow (Gronsdahl et al, 2019; Perry & Jones, 2017; Segura et al, 2020). Drought and climate change can exacerbate these reductions (Crampe et al, 2021; Iroumé et al, 2021). The findings presented here indicate that native forest restoration may offset expected streamflow reductions from climate change and help meet Sustainable Development Goals.…”