Effects of increasing shrub abundance on leaf and root litter decomposition in arctic tundra

Background/Question/Methods

Changes in global climate have resulted in a ‘greening’ of the Arctic as the abundance of deciduous shrub species increases. Increasing shrub cover not only changes the distribution and abundance of species in the living plant community, but also in the litter community, which can affect carbon turnover patterns. Much of the plant biomass in arctic tundra occurs belowground, and differences in species effects between leaf and the less commonly examined root litter are likely to have large effects on overall decomposition patterns. We examined potential effects of changing litter species composition (both root and leaf litter), focusing specifically on how litter of the deciduous shrub, Betula nana, affects the decomposition of litter from the dominant members of two other growth forms common in moist acidic tundra: evergreen shrubs (Ledum palustre and Vaccinium vitis-idaea) and graminoids (Eriophorum vaginatum). The species-specific decomposition rates of these four species singly, and mixed with Betula, for both leaf and root litter respectively, were measured to determine the mixing effects of Betula litter on the decomposition rates. Measurements were carried out seasonally over two years, focusing separately on decomposition patterns that occur during the winter and growing seasons.  

Results/Conclusions

Species identity strongly affected decomposition rate in both leaf and root litter. Betula leaf litter decomposed faster than the other three species, with Eriophorum leaves decomposing the slowest. We also found significant, non-additive effects of mixing on decomposition of leaf litter; combining Betula with any other species resulted in slower decomposition than would be predicted based on monoculture decomposition rates. This reduction was caused by a decrease in the rate of Betula leaf litter decomposition in mixture rather than changes in the decomposition rates of the other species present. In contrast, there were fewer effects of species identity on root decomposition rates; Ledum roots decomposed slower than the other three species, none of which were significantly different from each other. Further, only a single combination of species, Betula-Vaccinium, showed negative non-additive effects on root decomposition. Finally, all species-specific litter decomposition effects occurred during the winter months (September – May), with differences in decomposition during the growing season being legacy effects of winter processes. Our results suggest that as arctic vegetation shifts towards deciduous shrub-dominated tundra, species composition both above and below ground, as well as winter decomposition processes, are essential components in predictions of future carbon turnover patterns in arctic ecosystems.