- The greater sage-grouse (Centrocercus urophasianus; hereafter, sage-grouse) is a ground nesting gallinaceous bird that requires large contiguous patches of sagebrush. Sage-grouse populations have declined, especially in the Great Basin where changes in wildfire regimes and the invasion of annual grasses have contributed to habitat loss and fragmentation. During the last 35 years, wildfire activity has increased in the Great Basin and this shift has been linked to decreased populations in sage-grouse. The effect of fire on sage-grouse populations is strongly linked to the reduction of sagebrush cover within their seasonal ranges that occurs after fire. Even at low intensities, fire kills most sagebrush species. Furthermore, the response of vegetation after fire is highly variable both spatially and temporally, and strongly influenced by environmental characteristics as well as pre- and post-fire land management practices. However, high elevation mountain big sagebrush communities are more resilient and recover more quickly than low elevation Wyoming big sagebrush communities. In cases where invasive annual grasses become dominant, the loss of vegetation structural complexity can increase predation risks and reduce thermal heterogeneity for sage-grouse. Thermal variation allows animals to select areas with temperatures that efficiently regulate physiological process. In addition to contributing to general space use patterns, seeking thermal refuge can increase an animal’s nest success and annual survival. There is limited knowledge regarding the short-term (<6 yrs) effects of fire on sage-grouse demographics and their thermal environments. My research builds on a study that began in March 2013 after the Holloway fire burned ~187,000 ha during the summer of 2012 in the Trout Creek Mountains of southeastern Oregon and northwest Nevada.
In chapter 2, I established baseline values and characterized the spatial and temporal thermal variation in 3 dominant sagebrush (Artemisia spp.) communities using a black bulb technique (Tbb), which integrates ambient temperature (Tair), solar radiation, and convective heat transfer. I collected 56,446 Tbb estimates from 144 random points within the breeding range of sage-grouse from 15 April – 23 July in 2017 and 2018. Then, I used linear mixed-effects models where Tbb is a function of Tair to describe differences between unburned and burned sagebrush communities. My findings indicated 1) unburned and burned sagebrush communities exhibited high thermal heterogeneity in Tbb relative to Tair. For example, Tbb varied by 47° C in both unburned and burned communities when Tair was 20° C. 2) Fire altered the thermal environment and reduced the thermal refuge in Wyoming big sagebrush communities (A. tridentata wyomingensis) during low and high Tair more than low sagebrush (A. arbuscula) and mountain big sagebrush communities (A. t. vaseyana). Notably, when Tair was 0° C, unburned Wyoming big sagebrush communities (-12° C) were 7° C warmer than burned communities (-19° C) and when Tair was 35° C, unburned Wyoming big sagebrush communities (55° C) were 9° C cooler than burned communities (64° C). 3) Shrub cover and height were associated with thermal refuge in unburned and burned sagebrush communities. These results elucidate the thermal environment of the sagebrush and associated changes from fire, and illustrate the importance of shrub structure, which can provide thermal refuge for organisms in unburned and burned communities during extreme low and high Tair.
In Chapter 3, I examined sage-grouse nest site selection and nest success with respect to thermal and vegetation characteristics at 77 unburned and 18 burned nest sites as well as 31 unburned and 33 burned random landscape locations. I found that 1) Nest bowls (x̅ = 15.2°C, SD = 9.6, burned and unburned combined) were cooler and had less thermal variation than both nearby microsites (x̅ = 17.5°C, SD = 13.4) and the broader landscape (x̅ = 22.3°C, SD = 14.4). 2) Nest bowls in burned and unburned sagebrush communities more effectively moderated the thermal environment compared to the microsite and landscape. Specifically, nest bowls (burned and unburned combined) remained warmer than the microsite when Tair was <9°C, and cooler than the microsite when Tair was >9°C. Similarly, nest bowls were warmer than the landscape when Tair was <16°C, and cooler than the landscape when Tair was >16°C. 3) Nest bowls from successful nests (burned and unburned nests combined) buffered Tbb more effectively (β ̂ = 1.28, 95% CI: 1.25 to 1.30) than nest bowls from failed nests (β ̂ = 1.34, 95% CI: 1.32 to 1.36) at higher Tair. For example, Tbb at successful nest bowls was 2°C and 3°C cooler than Tbb at failed nest bowls when Tair was 25°C and 35°C, respectively. 4) Visual obstruction at nest sites has a strong, positive effect on daily nest survival. These findings provide some insight into how the thermal environment influences nest site selection and nest success and identifies the importance of vegetation that remains or reestablishes after fire for sage-grouse during nesting.
In Chapter 4, my objective was to quantify the population response of sage-grouse following the Holloway fire and examine sensitivity in the population growth rate, with respect to vital rates. I developed a stochastic 2-age female population matrix model for yearlings and adults during each year (2013 – 2018) of the study using vital rate estimates developed for each year. I found that 1) Nest survival (1st nests: yearlings: x̅ = 0.54, SD = 0.04; adults: x̅ = 0.31, SD = 0.09; renests: x̅ = 0.39, SD = 0.22) was comparable to nest survival values reported for sage-grouse across their distribution, despite extreme reduction in sagebrush cover at nesting sites. In contrast, chick survival (x̅ = 0.27, SD = 0.07) and female survival (yearlings: x̅ = 0.41, SD = 0.16; adults: 0.48 SD = 0.16) was low compared to other values reported for sage-grouse. 2) Annual estimates of λ indicated that the sage-grouse population on my study area was declining during 5 of 6 years. 3) Variation in λ was driven by female survival and chick survival. Moreover, sensitivity in λ with respect to female survival and chick survival, decreased and increased, respectively, during the 6-years post-fire. These results suggest strong negative effects of fire on important vital rates for sage-grouse population growth and illustrate the potential trade-offs among life histories for the species following fire.
My research examined some of the unknown responses of sagebrush communities and sage-grouse following a large wildfire. These findings illustrate how fire effects the thermal environment of sagebrush communities and sage-grouse nest site selection and nest success. Furthermore, my results provide an initial examination into the demographic, population, and life history response of sage-grouse following a large wildfire.