Some like it hot: Increased germination rate and altitudinal range expansion of Sagebrush (Artemisia tridentata ssp. vaseyana) under climate change

Background/Question/Methods

Climate change is progressing rapidly in high elevation ecosystems, resulting in shifts in vegetation communities and ecosystem functioning.  In the subalpine meadows of the Colorado Rockies, experimental heating resulted in an increase in Mountain Big Sagebrush (Artemisia tridentata ssp. vaseyana) growth and cover, which shifted patterns of carbon uptake to earlier and lower peak rates, and reduced carbon stocks in the soil (Harte & Shaw 1995, Saleska et al. 1999).  These results have led to the suggestion that Sagebrush will increase its altitudinal range as the Rockies become warmer and drier.  Here I assess one of the key requirements for an advance of Sagebrush range with warming:  How does Sagebrush germination respond to changes in temperature?

I collected seeds from three Sagebrush source populations at 2500m, 2900m and 3350m near the Rocky Mountain Biological Laboratory (Gunnison Co., CO) in summer 2010.  Additionally, I measured near surface soil temperatures throughout the growing season along the altitudinal gradient. Seeds were placed in a common garden design and exposed to temperatures simulating a diurnal cycle near snowmelt at each elevation, plus elevations warmer and cooler than those of the source populations.

Results/Conclusions

Source populations differed significantly in seed morphology, with seeds increasing in mass with elevation (r²=0.99, p = 0.0627).  All source populations had faster germination time, as measured by days to first germination, at warmer temperatures (p = 0.015), with the seeds from the lowest elevation consistently germinating faster than those from the higher source populations (effect size -0.86 days/⁰C, p=0.0816).  Furthermore, seeds from the lowest elevation site tended to have higher total % germination than those from higher elevations, although this effect was non-significant. 

Together these results indicate that germination in subalpine Mountain Big Sagebrush populations is more rapid and more complete under warmer soil conditions, for all source populations studied.  As the subalpine Rocky Mountains continue to see warmer temperatures and earlier snowmelts, we may see an increase in germination of Mountain Big Sagebrush, particularly at the highest elevations which are warming the most.  These results support the idea that Sagebrush may increase it’s altitudinal range, and thus alter the functioning of subalpine ecosystems, as climate change progresses.