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

Background/Question/Methods

Changes in global climate have resulted in increases of deciduous shrubs in the Arctic, a component of the observed ‘greening’ of the arctic. 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. We examined a chronosequence of long-term fertilization experiments at the Arctic Long Term Ecological Research (LTER) site at Toolik Lake, Alaska, composed of experiments fertilized for 4, 14 and 21 years, which has resulted in increasing shrub density with time since fertilization. We examined changes in abundance and composition of litter production in both fertilized and unfertilized plant communities. We further examined potential effects of changing litter species composition on decomposition rates, focusing specifically on a dominant species in moist acidic tundra which declines dramatically with fertilization (Eriophorum vaginatum, cotton grass) and the two species which show the biggest increase (Betula nana, dwarf birch) and (Rubus chamaemorus, cloudberry). We determined species-specific decomposition rates of leaves of these 3 species over 3-years in both fertilized and unfertilized environments, examining both mass loss and also C and N content of the litter over time.

Results/Conclusions

With fertilization, plant communities shift from relatively equal abundance of sedges and deciduous and evergreen shrubs towards dominance by Betula and Rubus, with changes more pronounced in the longest fertilized communities. Increasing shrub density in fertilized plots also resulted in greater litter deposition; fertilized communities produced more 5 times more litter than control communities, with the majority of the litter coming from Betula and Rubus. Differences in litter abundance were less pronounced in more recently fertilized plots. Litter from species more typical of fertilized plots (Betula and Rubus), decomposed faster than Eriophorum litter throughout the first 3 years of decomposition, with Rubus decomposing fastest by the end of the experiment, regardless of the fertilization environment it was decomposed in. Fertilization environment had less consistent direct effects on decomposition rates; litter decomposed in fertilized communities decomposed slower than that in control communities, but only in the first and second year.  Mass loss rates paralleled those for nitrogen loss from litter: Betula and Rubus had the fastest N loss, while Eriophorum accumulated N during decomposition. Our results suggest that as arctic vegetation shifts towards deciduous shrub-dominated tundra, large increases in the abundance of fast-decomposing leaf litter has the potential for increases in carbon turnover rates in arctic ecosystems.