COS 250-3 - The impacts of annual and seasonal weather on big sagebrush (Artemisia tridentata) growth - a growth ring analysis

Abstract: Covering 19 million hectares, the big sagebrush ecosystem is the largest dryland in the US. These ecosystems are dominated by big sagebrush (Artemisia tridentata) and have an understory of grasses and forbs. The composition of shrub and herbaceous species is correlated with the depth of available soil water; shallow soil water from rain events tend to favor herbaceous plants whereas deep soil water recharged by snowmelt favors shrubs. Big sagebrush has annual growth rings that are easily identifiable due to a cork layer between each year's growth. Previous studies have shown that climate can influence the annual ring width. The aim of our study was to identify which annual and seasonal climate variables best explain the annual ring growth of big sagebrush. We sampled 100 plants of different size classes across 21 sites from 3 elevation classes (low: 1700-2000 m, mid: 2100-2200 m, and high: 2300-2500 m) in the upper Green River Basin in western Wyoming. We used ImageJ to measure ring widths and correlated the widths with climate data from PRISM. We explored the relationship between annual ring width and annual and monthly temperature (maximum, minimum, and mean; ^oC), precipitation (mm), and vapor pressure deficit (maximum and minimum; hPa). We also examined the relationships for two seasons, winter (October - March) and summer (May - September). The big sagebrush individuals ranged from 3 to 105 years and the age of an individual was correlated with size class (r=0.5). At low elevations, we found precipitation (annual, winter, and summer) had the highest correlations with ring width (r=0.3-0.5). Precipitation also had the highest correlations with annual ring width at mid elevations (r=0.2-0.3). At high elevations, temperature variables had the highest correlation with ring width - mean temperature: annual (r=0.4) and winter (r=0.3); and minimum temperature: annual, winter, and summer (r=0.3). Our results demonstrate positive relationships between big sagebrush annual ring width and climate variables; precipitation at low and mid-elevations and temperature at high elevations. Climate change models have projected an increase in temperature and variable shifts in precipitation timing across the big sagebrush region. Our results suggest that these projected shifts in climate variables could impact ring growth of big sagebrush and, thus, their ability to survive in their extant habitat.