Causes and consequences of variation in snow incidence on the high mountains of Tasmania, 1983-2013

Alpine plant species are considered to have a precarious near future in a warming world, especially where endemic on mountains without a nival zone. We investigated how and why snow patch vegetation and snow incidence varied over recent decades in Tasmania, Australia. Landsat images between 1983 and 2013 were used to calculate the proportion of clear days with snow visible on Mt Field. We compared average annual snow incidence on 74 Tasmanian alpine mountains for 1983-1996 with that for 1997-2013 using the small subset of Landsat runs in which most of Tasmania was clear of cloud. We related the temporal data from Mt Field to Tasmanian climatic data and climate indices to determine the predictors of change. We recorded plant species and life form cover from quadrats in transects through a snow patch on Mt Field in 1983, 2001 and 2014, and mapped decadal scale changes in boundaries and shrub cover at five other snow patches across the extent of the Tasmanian alpine areas from aerial photographs. The incidence of snow fluctuated between 1983 and 2013 at Mt Field with no overall trend. Snow incidence was less on lower elevation alpine mountains in the period 1997-2013 than in the period 1983-1996, but showed a weak opposite trend on mountains higher than 1350 m. The contrast in trends may be a consequence of the effect on lapse rates of stronger frontal winds associated with a steepening of latitudinal pressure gradients. At Mt Field, bare ground decreased, cover of cushion plants and tall shrubs increased and obligate snow patch species were persistent. The trends we observed in both vegetation and snow incidence differ markedly from those observed on mainland Australia. The increase in shrub cover and decrease in bare ground on Mt Field were unexpected, given the constancy in incidence of snow. These results may relate to ongoing recovery from a fire in the 1960s, as the shrub species that have increased are fire-sensitive, obligate seeders and there has been no indication of warming since 1983 in the climatic record for western Tasmania. There is a possibility that some Tasmanian alpine areas might act as long-term refugia from general warming.