Winter foraging ecology of Greater Sage-Grouse in a post-fire landscape

Highlights

• Three-tip sagebrush has a unique chemical profile compared to Wyoming big sagebrush.

• Three-tip sagebrush has a dynamic phytochemical response to fire.

• Sage-grouse used both species of sagebrush relative to availability, indicating no selection.

• Selection of plants and patches was related to specific monoterpenes and plant height.

Abstract

In the Great Basin, changes in climate and associated fire regimes may alter the density and distribution of shrubs, changing the structure and diet quality of plants in burned areas. We evaluated how the structural and phytochemical characteristics of three-tip sagebrush (Artemisia tripartita) relative to Wyoming big sagebrush (A. tridentata wyomingensis) influence the winter foraging ecology of Greater Sage-Grouse (Centrocercus urophasianus) at a site with a known history of fire in south-central Idaho. Three-tip sagebrush had lower protein content and lower chemical defenses compared to Wyoming big sagebrush. We found that time since last fire over a 30-year history was more strongly correlated with changes in phytochemicals in three-tip sagebrush compared to Wyoming big sagebrush. Despite phytochemical differences, both Wyoming big sagebrush and three-tip sagebrush were browsed relative to their availability. However, within a species, smaller plant height and lower concentrations of phytochemicals, specifically two individual monoterpenes, explained diet selection by sage-grouse. Our results indicate that dietary quality of three-tip sagebrush may provide acceptable forage for sage-grouse in post-fire landscapes where other species of sagebrush have not yet recovered. However, relying on re-establishment of one species of sagebrush without consideration of structural and dietary quality may compromise successful conservation efforts.

Introduction

Wildfires are becoming increasingly common in western North America. Coupled with conversion to invasive annual grasses, fires are threatening large expanses of shrubland landscapes. The natural fire regime in sagebrush (Artemisia sp.) landscapes has been altered by invasion of annual grasses resulting in the loss of approximately 11% of available sagebrush over the last 30 years (Brooks et al., 2015). The impact of fire on sagebrush communities contributes to the current and projected long-term declines of Greater Sage-Grouse (Centrocercus urophasianus; hereafter, sage-grouse; Connelly et al., 2000a; Beck et al., 2009; Rhodes et al., 2010; Lockyer et al., 2015; Coates et al., 2016).

Both mountain big sagebrush (A. tridentata vaseyana) and Wyoming big sagebrush (A. t. wyomingensis) do not meet sage-grouse habitat guidelines (Connelly et al., 2000b) for more than twenty years after being burned with only 2% of post-fire restoration sites meeting winter habitat guidelines within 20 years (Arkle et al., 2014). Specifically, Wyoming big sagebrush may take 100 or more years to recover to pre-fire canopy cover after disturbance (Nelle et al., 2000; Baker, 2006; Beck et al., 2009; Rhodes et al., 2010; Arkle et al., 2014). The long-term losses of sagebrush cover after fires may explain reduced nesting success and survival of sage-grouse using burned areas (Foster et al., 2018), decreased lek attendance (Connelly et al., 2000a), and decreased population sizes (Coates et al., 2016; Smith and Beck, 2018).

Fires may reduce availability and quality of sagebrush directly through mortality (Baker, 2006) or indirectly by altering sagebrush communities due to variable responses of plant species to fire (Passey and Hugie, 1962; Lesica et al., 2007; Beck et al., 2009). For example, three-tip sagebrush (A. tripartita) recovers twice as fast as Wyoming big sagebrush species after fires (Beck et al., 2009) because plants can re-sprout instead of re-establishing from seed (Passey and Hugie, 1962; Lesica et al., 2007). Although three-tip sagebrush currently has a relatively small range (Tirmenstein, 1999) compared to big sagebrush taxa, populations of three-tip sagebrush are expected to expand throughout the West due to both decreased fire return intervals (Baker, 2006) and increased temperature (Dalgleish et al., 2011). Expansion of three-tip sagebrush may influence sage-grouse populations. For example, sage-grouse used three-tip sagebrush for nesting cover less than expected based on availability in south-central Idaho, and had lower nesting success under three-tip sagebrush than birds nesting under sympatric Wyoming big sagebrush (Lowe et al., 2009).

Sage-grouse have strong site fidelity, and therefore may continue to use sub-optimal habitat after fires rather than altering patterns of habitat use. The lack of behavioral plasticity in sage-grouse and high site fidelity (Berry and Eng, 1985), coupled with slow recovery of sagebrush (Baker, 2006; Beck et al., 2009), ineffective restoration of sagebrush communities (Nelle et al., 2000; Arkle et al., 2014), and changes in shrub composition (Beck et al., 2009, 2012) may explain reduced nest success and survival of individual sage-grouse (Foster et al., 2018) and overall reductions in population size (Smith and Beck, 2018) in post-burn habitats.

In addition to changes in cover and composition of shrubs that provide critical hiding cover, wildfires may also alter food availability and food quality. Importantly, higher quality diets are correlated with higher nutritional condition and reproductive success for many species of herbivores (Moss and Watson, 1984; Gregg et al., 1994; DeGabriel et al., 2009; Wing and Messmer, 2016). Diet quality is therefore an important consideration in understanding population changes in sage-grouse following large-scale wildfires in winter habitat. Fires may alter the dietary quality of sagebrush present in post-burn habitats through changes in the availability of species that vary in phytochemistry (i.e., three-tip sagebrush versus Wyoming big sagebrush). In addition, fires can alter protein content (DeWitt and Derby, 1955) and concentrations of plant secondary metabolites (PSMs) within a species as plants re-sprout or grow from seeds (Campbell and Taylor, 2007; Bryant et al., 2009). For example, birch trees (Betula sp.) produce more PSMs in areas with greater fire frequencies and higher percent area burned than adjacent areas (Bryant et al., 2009). Because sage-grouse depend on sagebrush for cover and food, both the structural and dietary quality of sagebrush species that remain or are recruited after fires are important for understanding demographic consequences for sage-grouse in post-burn landscapes.

Our overall objective was to examine factors that influence habitat use and diet selection by sage-grouse inhabiting a post-fire landscape dominated by Wyoming big sagebrush and three-tip sagebrush. First, we predicted that three-tip sagebrush would generally be higher in crude protein and PSMs and that these phytochemicals would be influenced more by fire history than Wyoming big sagebrush because three-tip sagebrush has a greater ability to regrow following fires (Passey and Hugie, 1962). Second, we predicted that sage-grouse would select plants and plots with medium heights (25–35 cm) and moderate cover (10–30%) because of the importance of cover for sage-grouse using winter habitats (Beck, 1977; Connelly et al., 2000b; Carpenter et al., 2010; Smith et al., 2014; Holloran et al., 2015). Finally, we predicted that sage-grouse would select species, plots, and individual plants of sagebrush with the highest crude protein and lowest concentrations of PSMs because diet selection by sage-grouse in winter is driven by phytochemicals at different spatial scales (Frye et al., 2013; Remington and Braun, 1985).