A multicatchment analysis of headwater and downstream temperature effects from contemporary forest harvesting

Abstract

Stream temperature is a key physical water-quality parameter, controlling many biological, chemical, and physical processes in aquatic ecosystems. Maintenance of cool stream temperatures during summer is critical for high-quality aquatic habitat. As such, transmission of warm water from small, nonfish-bearing headwater streams after forest harvesting could cause warming in downstream fish-bearing stream reaches with negative consequences. In this study, we evaluate (a) the effects of contemporary forest management practices on stream temperature in small, headwater streams, (b) the transmission of thermal signals from headwater reaches after harvesting to downstream fish-bearing reaches, and (c) the relative role of lithology and forest management practices in influencing differential thermal responses in both the headwater and downstream reaches. We measured summer stream temperatures both preharvest and postharvest at 29 sites-12 upstream sites (4 reference, 8 harvested) and 17 downstream sites (5 reference, 12 harvested)-across 3 paired watershed studies in western Oregon. The 7-day moving average of daily maximum stream temperature (T 7DAYMAX ) was greater during the postharvest period relative to the preharvest period at 7 of the 8 harvested upstream sites. Although the T 7DAYMAX was generally warmer in the downstream direction at most of the stream reaches during both the preharvest and postharvest period, there was no evidence for additional downstream warming related to the harvesting activity. Rather, the T 7DAYMAX cooled rapidly as stream water flowed into forested reaches~370-1,420 m downstream of harvested areas. Finally, the magnitude of effects of contemporary forest management practices on stream temperature increased with the proportion of catchment underlain by more resistant lithology at both the headwater and downstream sites, reducing the potential for the cooling influence of groundwater.

Summer stream temperature changes following forest harvest in the headwaters of the Trask River watershed, Oregon Coast Range

“…In agreement with our predictions, summer temperature regimes in headwater streams exhibited variability related to annual weather conditions and local site conditions prior to management. The observed treatment effects were of similar magnitude to other studies of small, nonfish streams in the Pacific Northwest studies of small, nonfish streams (e.g., Bladon et al, ; Bladon et al, ; Janisch et al, ; Johnson & Jones, ; Kibler et al, ) where streams without riparian buffers experienced increases in maximum and mean temperatures after forest harvest. For the TRWS, the temperature responses to a range of silvicultural harvest prescriptions added variability in headwater stream temperatures at all percentiles of the distribution.…”

“…Prior studies that addressed management-related temperature responses in small, fishless streams of the Pacific Northwest (e.g., Bladon, Segura, Cook, Bywater-Reyes, & Reiter, 2018;Janisch et al, 2012;Johnson & Jones, 2000;Kibler et al, 2013) did not assess changes in the context of thermal requirements for resident aquatic species. Headwater streams provide important habitats for some temperature-sensitive species, including some amphibians (Homyack, 2010;Huff, Hubler, & Borisenko, 2005).…”

Empirical Stream Thermal Sensitivities May Underestimate Stream Temperature Response to Climate Warming

“…Empirical research on stream temperature response to environmental change has been dominated by three general approaches: (1) landscape-scale studies in which statistical models are applied to relatively large data sets of stream temperature, air temperature and sometimes discharge, or catchment characteristics (Isaak et al, 2016;Moore et al, 2013;Wehrly et al, 2009); (2) detailed local-scale studies that focus on one or a small number of streams and attempt to quantify the energy and water balances (Brown, 1969;Garner et al, 2014;Story et al, 2003;Webb & Zhang, 1999); and (3) experimental studies focused on quantifying the effect of a land-cover change such as forest harvesting (Brown & Krygier, 1970;Bladon et al, 2018;Gomi et al, 2006). An advantage of the landscape-scale approach is that more real-world variability is sampled, leading to a stronger inferential basis for generalization.…”

Effects of forestry on summertime low flows and physical fish habitat in snowmelt‐dominant headwater catchments of the Pacific Northwest

“…Although many previous studies have documented how forestry affects streamflow and water quality (e.g., Bladon, Segura, Cook, Bywater‐Reyes, & Reiter, ; Bywater‐Reyes, Bladon, & Segura, ; Gomi, Moore, & Hassan, ; Moore, Spittlehouse, & Story, ; Moore & Wondzell, ; Perry & Jones, ), EFN assessments have historically been concerned with regulation and consumptive use of water resources, such as power generation and agriculture, and have neglected to consider the impacts of forestry on the natural flow regime (e.g., Lewis, Hatfield, Chilibeck, & Roberts, ). In the context of EFN, the impacts of forest harvesting on summertime low flows are of particular interest.…”