Spatial and temporal variability in the effects of wildfire and drought on thermal habitat for a desert trout

Highlights

• Wildfire had immediate and mid-term effects on thermal responses of stream habitats in a Great Basin ecosystem.

• Post-fire drought extended warming of streams with ecological consequences for a threatened trout.

Abstract

We studied how drought and an associated stressor, wildfire, influenced stream flow permanence and thermal regimes in a Great Basin stream network. We quantified these responses by collecting information with a spatially extensive network of data loggers. To understand the effects of wildfire specifically, we used data from 4 additional sites that were installed prior to a 2012 fire that burned nearly the entire watershed. Within the sampled network 73 reaches were classified as perennial, yet only 51 contained surface water during logger installation in 2014. Among the sites with pre-fire temperature data, we observed 2–4 °C increases in maximum daily stream temperature relative to an unburned control in the month following the fire; effects (elevated up to 6.6 °C) appeared to persist for at least one year. When observed August mean temperatures in 2015 (the peak of regionally severe drought) were compared to those predicted by a regional stream temperature model, we observed deviations of −2.1°-3.5°. The model under-predicted and over-predicted August mean by > 1 °C in 54% and 10% of sites, respectively, and deviance from predicted was negatively associated with elevation. Combined drought and post-fire conditions appeared to greatly restrict thermally-suitable habitat for Lahontan cutthroat trout (Oncorhynchus clarkii henshawi).

Introduction

The influences of climate on Earth are often portrayed as systematic trends through time (Loarie et al., 2009), but change itself is not stationary (Milly et al., 2008). For example, as climate changes, it is broadly anticipated that the probability of episodic disturbances will also change (Running, 2008). Ecologically, this means that species will have to contend with a host of acute and chronic exposures (Foden et al., 2013) to changing climates. Such uncertainties regarding climate itself provide many challenges for anticipating ecological responses to climate change (Wenger et al., 2013). Here we focus on two common episodic, climate-driven ecological disturbances: drought (Lake, 2011) and wildfire (Westerling, 2016). Both of these disturbances are driven by the intersection of precipitation deficits and warm temperatures, and are thus likely to manifest at the same time within a given area (Diffenbaugh et al., 2015, Ganguli and Ganguly, 2016).

In this study we consider the influences of drought and wildfire on stream temperatures. Stream temperature is well-known to be sensitive to both meteorological variability (i.e. drought-related; Caissie, 2006, Diabat et al., 2013, Luce et al., 2014) and wildfires (Hitt, 2003, Dunham et al., 2007, Mahlum et al., 2011). In the northern Great Basin desert, the onset of meteorological and hydrological drought (Wilhite and Glantz, 1985) has increasingly coincided with large wildfires (Denison et al., 2014, Westerling, 2016). Wildfires often burn riparian vegetation, leading to loss of shade and warming of stream temperatures (Dunham et al., 2007, Mahlum et al., 2011), which may further warm if streams experience reduced flows during drought. Because a host of factors can influence stream temperatures and sensitivity to drought and wildfire (e.g., riparian and topographic shading, surface-subsurface heat fluxes) specific responses can be variable and difficult to predict. We used a combination of opportunistically available records of stream temperature (from temperature data loggers in operation before and after a large wildfire), a regional model of stream temperatures from before a wildfire and drought (Isaak et al., 2016a), and a network of temperature loggers deployed after a fire and during the height of an ongoing drought (i.e., 2015; Fig. 1) to evaluate how both wildfire and drought influenced temperatures across a large, desert stream network.

Specifically, our objectives were to: 1) quantify the magnitude and duration of influences of wildfire on stream temperature at sites with pre- and post-wildfire temperature data, 2) evaluate spatial variation in responses of summer stream temperature throughout the entire stream network during the latter stages of the drought to a baseline of predictions from a stream temperature model assembled with data from 1993 to 2011 (Isaak et al., 2016a), and 3) assess the ecological consequences of conditions observed during the 2015 drought year for a sensitive species (Lahontan cutthroat trout, [Oncorhynchus clarkii henshawi]; Jones et al., 1998). Collectively, results of this work provide novel insights into how aquatic ecosystems respond to the combined influences of drought and wildfire, two interactive disturbances that are more likely to occur as climate change proceeds (Lake, 2011, Diffenbaugh et al., 2015, Westerling, 2016).