Alpine shrub leaf litter decomposition across mountain summits in south-eastern Australia

Climate warming has been linked to shrub expansion in alpine regions and the decomposition of shrub leaf litter and subsequent release of nutrients has been proposed as a mechanism to facilitate shrub growth. We quantified the rate of alpine shrub leaf litter decomposition (measured as mass loss) over the course of a year in four locally occurring alpine shrub species that grow across four alpine summits. We measured a range of environmental attributes at the study sites, and via a standard litter bag approach, we evaluated the effects of site elevation, the depth of litter bag deployment, the removal time, the species-specific leaf area (SLA) and the accumulated growing degree days at each site on the total per cent and rate of litter decomposition (as mass loss). The higher elevation sites were cooler with more snow days than the lower sites. Soil moisture was higher early in the snow-free season at the higher elevation sites. Linear mixed effect models indicated no significant effects of elevation on total and rate of litter decomposition, but there were significant positive effects of deployment depth and removal time and a significant negative effect of species SLA. There were significant negative relationships between the rate of decomposition and growing degree days, as decomposition slows through time. The modelled mean rates of shrub litter decomposition for each species indicated that there would be more and faster decomposition if winter and early spring conditions were to persist for a whole year, compared with the modelled rates of average annual conditions persisting for a whole year. Our results indicate that Australian alpine shrub litter decomposes readily, with the highest rates of decomposition occurring soon after deployment, which in this study was after a snowy winter at the start of the growing season in spring.