The shadeshed: A lidar-based variable width shade buffer and riparian core for headwater streams

The high density of headwater streams within working forests poses challenges to forest managers seeking to maintain riparian ecosystem functions while balancing forest production. Buffer guidelines vary among jurisdictions, stream size, and land ownership, but most incorporate a fixed-width buffer structure that is assumed protective of ecosystem functions and administratively simple to implement. However, prescriptive one-size-fits-all buffer rules often fail to achieve the intended goals when uniformly applied across spatially diverse and temporally dynamic watersheds and stream systems. Therefore, new approaches are needed that address spatial variability and focus on riparian ecological functions not only to improve effectiveness of riparian management but increase certainty of outcomes. To address these knowledge gaps, we experimentally tested the effectiveness of a variable-width-shade buffer (VWSB) to provide effective shade (ES), maintain stream temperature, and optimize riparian tree retention across nineteen headwater streams in Washington, USA. We employed GIS to delineate the riparian forest area that obstructs midday direct beam solar radiation (i.e., shadeshed zone) and used the shadeshed zone to guide the location and size of VWSBs. We used Lidar and the Light Penetration Index (LPI) to estimate effective shade retained within the shadeshed zone and validated the accuracy of LPI with hemispherical photography to produce continuous and accurate estimates of effective shade over streams. We estimated pre- post-harvest changes in effective shade and water temperature to evaluate the relative effectiveness of the VWSB as compared to the fixed-width discontinuous forest practice buffer (FPB) specified under current Washington State (USA) Forest Practice rules. Our results demonstrated that shade losses at VWSB units were generally similar to or less than those observed at FPB units and that post-harvest shade is maximized when the percentage of shadeshed zone area buffered is maximized. Also, the VWSB proved to be more efficient than the FPB for minimizing riparian timber retention to maintain ES and improve buffer performance across a wide range of buffer sizes. Water temperature responses to buffer treatments were highly variable and no difference was discernible in the mean summertime seven-day moving average of daily maximum stream temperature between treatments. Exploratory analyses suggested a temperature-shade relationship that varied depending on the amount of shrub cover and the percentage of study reach length with surface flow, and potentially groundwater inflow in combination with shade loss. The collective findings demonstrated that the VWSB and shadeshed concepts provide an innovative and effective new approach for designing variable width shade buffers in complex topography.