Variable-retention riparian harvesting effects on riparian air and water temperature of sub-boreal headwater streams in British Columbia

A 5-year (2002–2006) before–after control impact study was initiated in three watersheds of the British Columbia central interior to assess the ability of a variable retention riparian treatment to maintain fish habitat conditions in small sub-boreal streams (<2 m width). This paper presents findings for the stream shade and air and stream temperature component of the study. Eight streams were studied to assess stream shade, riparian air, and stream water temperature response during summer months to a policy retention level of at least 10 stems of merchantable timber per 100 m of channel length. After harvesting there was a significant decrease in shade as well as an increase in air and stream temperature at all treatment sites. Riparian harvesting reduced stream shade by 30–50% from pre-harvest levels but shade was recovering to pre-harvest levels 2–3 years after harvesting. Mean weekly average and maximum air temperatures at treatment sites increased more than 3 °C compared to control locations. Mean weekly average and maximum stream temperatures at treatment sites increased by as much as 5 and 6 °C, respectively. Despite the recovery of shade measured at the water surface, mean and maximum water temperatures remained significantly higher at treatment sites than control sites. The discrepancy between shade recovery and temperature response indicates that vegetative surface height receiving radiation must be considered along with shade. Shade from overstory may be more effective at maintaining riparian air and stream temperatures than lower understory because it can limit energy transfer to lower layers of the forest canopy and ground surface.

► We measure riparian retention policy influence on shade, water and air temperature.
► Three sub-boreal watersheds were studied for 5 years using a BACI approach.
► Riparian harvesting decreased shade and led to increased air and stream temperature.
► Although shade increased after harvesting, air and water temperature remained high.
► Riparian retention requires variable vertical structure as well as water surface shade.