Can shrubs accelerate the uphill movement of subalpine species through the provision of stepping-stones?

Laurel M. Brigham and Katharine N. Suding, Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO, Laurel M. Brigham and Katharine N. Suding, Ecology and Evolutionary Biology, University of Colorado, Boulder, CO

Background/Question/Methods
Microclimates can accelerate plant range shifts by providing suitable habitat in an otherwise unsuitable matrix. Through interactions with snow, wind, and solar radiation, topography and structure-forming species (e.g. shrubs, trees) drive water and energy balances across the landscape. There is a group of structure-forming species that is expanding in alpine areas across the globe—shrubs—and thus increasing the number of microclimates across the landscape. However, it is unclear how important this biotic shift will be in mediating the response of other species to global change. Here, we ask whether shrub microclimates enhance subalpine herb establishment in the alpine. To address this question, we seeded a subalpine forb (Erigeron glacialis), and a common alpine grass (Deschampsia cespitosa) to serve as baseline for germination in the area. Our plots were located at Niwot Ridge in the Front Range of Colorado, placed either on the leeward side of a shrub or in open tundra. Half of the plots across both plot types were randomly selected to have their vegetation removed prior to seeding to investigate neighbor effects. We measured a suite of microclimatic variables—seasonal soil moisture, soil temperature, and photosynthetically active radiation—to contextualize our findings.
Results/Conclusions
Shrubs enhanced soil moisture, while the neighbor removal treatment resulted in increased maximum soil temperatures. We found that 13 and 16 out of our 28 plots demonstrated germination of E. glacialis and D. cespitosa, respectively. The likelihood of E. glacialis germination was not shaped by shrub presence or neighbors, only by soil moisture where higher soil moisture enhanced germination. The likelihood of D. cespitosa germination was also positively related to soil moisture. Around 40% and 50% of E. glacialis and D. cespitosa germinants, respectively, survived the growing season. The proportion of E. glacialis survivors was increased by shrub presence. These results suggest that our subalpine species may be able to germinate in the alpine regardless of shrub presence, but that the survival of our subalpine species could be increased by shrubs. Hence, the conditions promoted by shrubs may accelerate the uphill movement of subalpine species through the provision of stepping-stones. Additionally, our results suggest that the presence of neighbors may not be a limiting factor for establishment of our subalpine species in the alpine.