Stream temperature response to 50% strip-thinning in a temperate forested headwater catchment

Winkler

Hope

2022

This study documents the natural variability of true colour and the effects of forest harvesting in three snow‐dominated headwater catchments at the Upper Penticton Creek Experimental Watersheds, located in south‐central British Columbia. Although colour does not pose direct health risks, high values of colour can reduce the effectiveness of ultraviolet treatment systems, and can also indicate high concentrations of dissolved organic compounds, which can produce trihalomethanes following chlorination. The study employed a paired‐catchment design with an unlogged control (240 Creek) and pre‐ and post‐harvesting observations at two treatment catchments (241 and 242 Creeks). True colour varied seasonally, with high values during freshet followed by a general decline through summer, with some marked increases in association with autumn rain events. In the absence of harvesting effects, colour at all streams frequently exceeded the drinking water standard of 15 true colour units, especially for two catchments that contained organic soils, and there were significant positive relations between colour and the logarithm of stream discharge for all three catchments. The paired‐catchment analysis revealed significant increases in colour values once harvest reached 28% of the 241 Creek watershed and 21% of the 242 Creek watershed. For 241 Creek, the harvesting effect appeared to be independent of the colour value at 240 Creek. In contrast, the harvesting effect appeared to manifest in an increase in the slope of the relation for 242 Creek. The post‐harvest increase in colour did not appear to depend on streamflow for 241 Creek, whereas the colour response for 242 Creek had a strong positive relation with streamflow. These patterns of responses are hypothesized to reflect differences in the spatial patterns of imperfectly and poorly drained, wet soils in relation to locations of harvesting units, as well as differences in the morphology of the stream channel and riparian zone. Novelty Statement This is the first paired‐catchment experiment to document variations in true colour and its response to forestry in snow‐dominated headwater catchments. A unique aspect of the study is the availability of a soil map showing the locations of imperfectly and poorly drained soils and organic soils. The combination of the experimental design, which included both pre‐ and post‐harvest monitoring and a control catchment, and the soils map, supported the formation of process‐based hypotheses to explain the natural seasonal and among‐catchment variability of colour and the responses to forest harvesting.

Section: Discussionmentioning

confidence: 99%

Streamwater colour in snow‐dominated headwater catchments: Natural variability and the effects of forest harvesting

Winkler

Hope

2022

“…Hillslope runoff typically increases following forest harvesting (Brown et al, 2005; Kirchner et al, 2020) which may also influence the stream thermal regime. Increased hillslope contributions to the stream, without a change in hillslope runoff temperature, should offset some of the stream warming due to greater flow volumes (Oanh et al, 2021). However, soil temperatures can also be altered due to harvesting.…”

Section: Discussionmentioning

confidence: 99%

“…The lake moderates peakflows and augments baseflows, which combine to alter both the volume and velocity of water flowing through the stream reach. A greater volume and depth of water will reduce the thermal response to energy inputs, and higher water velocities will result in shorter travel times within the stream reach and reduce potential for heating (Brown, 1972; Oanh et al, 2021). During rainfall events, the lake scenario has lower stream discharge response due to the storage capacity of the lake and it is during these periods that the lake thermal response to harvesting is greater than for the no‐lake response (Figure 7).…”

Section: Discussionmentioning

confidence: 99%

“…Hillslope runoff and soil temperature can respond to forest harvesting (Brown et al, 2005; Kreutzweiser et al, 2008) with the potential to impact stream temperature via changes in the advective heat flux associated with lateral inflows (Oanh et al, 2021). We do not account for these potential post‐harvest hydrologic changes in our virtual experiment because the direction, magnitude and timing of streamflow response to harvesting documented in previous studies is highly variable (Brown et al, 2005; Zhang et al, 2017).…”

Section: Methodsmentioning

confidence: 99%

“…Most empirical studies typically focus on summer stream temperature metrics (e.g., daily maximum or weekly mean) and the period immediately and up to 5 years following harvesting. Documented post‐harvest stream temperature responses have ranged from no detectable change up to 13°C increase (Bladon et al, 2018; Moore, Spittlehouse, et al, 2005a; Oanh et al, 2021). This variability is due to a number of factors including type of harvesting treatment, extent of harvesting, hydrogeomorphic characteristics of the streams and their catchments, as well as experimental design.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Do headwater lakes moderate downstream temperature response to forest harvesting? Illustrating opportunities and obstacles associated with virtual experiments

Leach

Laudon

et al. 2022

There are concerns that environmental changes, such as climate variability and forest harvesting, are altering stream thermal regimes and impacting aquatic ecosystems. Previous studies have suggested that the abundant headwater lakes found in northern landscapes may moderate downstream temperature response to forest harvesting. We investigated this hypothesis using a virtual experimental approach based on detailed field measurements made at boreal catchments in northern Sweden coupled with a process‐based stream temperature model. We simulated streamside harvesting for stream reaches with and without a headwater lake. Mean daily summer stream temperature response to harvesting was generally between 0.5 and 1.5°C higher for the stream without a lake than for the stream with a lake. However, during rain events the stream with the lake showed a greater stream temperature response than the stream without a lake. Headwater lakes typically store and delay runoff from rain events, augment baseflow, and have elevated outflow temperatures. These differences in upstream boundary conditions, in terms of flow and water temperature, were the key drivers for the contrasting harvest responses between streams with and without headwater lakes. These findings were generally consistent across different harvesting scenarios; however, uncertainty in the hyporheic term and post‐harvest microclimate conditions influenced the simulated magnitude of post‐harvest stream temperature response. Our study highlights the utility of virtual experiments for gaining insight on systems understanding but caution is needed when using models for predictions outside the conditions for which models are calibrated.

Assessing stream temperature response and recovery for different harvesting systems in northern hardwood forests using 40 years of spot measurements

Leach

Hudson

2022

Stream temperature is a critical control on aquatic habitat and a key forest management concern in many jurisdictions. Most research on stream temperature response to forest harvesting is from coniferous forests in rain‐dominated watersheds and focused on the first few years following harvesting. In contrast, we know less about the harvesting impacts on stream temperature for silviculture approaches typically used in northern hardwood forests that are influenced by snow. We addressed this knowledge gap by using four decades (1980 to 2020) of spot water temperature measurements recorded at three treatment and two reference catchments (areas 4.5 to 69 ha) as part of a long‐term water quality monitoring programme at the Turkey Lakes Watershed study near the eastern shores of Lake Superior. We were able to control for diel and seasonal biases in the spot temperature measurements and found that clearcut harvesting showed a summer temperature increase that persisted for 5 to 7 years after harvesting. Shelterwood and selection harvest did not exhibit a detectable change in stream temperature. These responses are consistent with observed changes in forest canopy through time and between harvesting approaches. In addition, the stream temperature responses were likely muted due to the streams being short and characterized by intermittent flow conditions, as well as the potential moderating influence of increased subsurface runoff following harvesting. Our results highlight how insights can be extracted from routine water quality programmes that were hitherto unrecognized.