Soil and vegetation predictors of shrub expansion in arctic Alaska

Rapid climate change is already occurring in the Arctic, where temperature has increased at twice the rate of the rest of the world in the last three decades. In arctic Alaska, a widespread increase in the area occupied by several species of deciduous shrubs has occurred concomitantly with increasing temperature. Increases in shrub cover and biomass have been observed directly from repeat photography, in experimentally warmed tundra, and have been inferred from increased pollen signatures during previous warm periods in paleoecological records. A transition from tundra to shrubland will alter the structure and function of arctic ecosystems, driving changes to productivity, surface energy balance, C and N dynamics, hydrology, and wildlife habitat. In this study, we sought to identify areas susceptible to shrub expansion and assess the effects of expansion on ecosystem carbon storage by sampling soil and vegetation characteristics along transects at two known expanding and non-expanding shrub sites on Alaska’s North Slope. Transects were then compared to 9 other arctic sites with established shrub populations, but unknown expansion history. Using ordination, we found that understory vegetation composition of unknown sites was more similar to that of expanding versus non-expanding sites. Although there was no overall relationship between canopy shrub cover and soil carbon content, active layer characteristics including pH and bulk density explained some floristic variation at expanding and non-expanding sites. These results will help identify areas to monitor for future shrub expansion, and assess possible impacts of shrub expansion on carbon balance in Arctic ecosystems dominated by shrubs.