Floristic composition and variability of subalpine grasslands in the Mt Hotham region, north-eastern Victoria

Abstract

Subalpine grasslands in the Mt Hotham area, Victoria, are a common feature of cold-air drainage valleys at elevations of 1260–1660 m. Here, the pooling of cold air prevents trees from establishing and results in a distinct grassland community, composed of tussock grasses and a wide variety of intertussock species. Despite their common occurrence in the region, such grasslands have yet to be fully described. This study focused on identifying the floristic composition of subalpine grasslands across 51 sites in the vicinity of Mt Cope, Dinner Plain and Mt Hotham. The vegetation was sampled from 172, 20-m² quadrats which were analysed by multivariate ordination techniques. Environmental variables were quantified (i.e. soil depth, pH, aspect, slope, biomass, grazing intensity, altitude). Analysis revealed that the grassland sites varied greatly in their composition and richness. There was a gradual rather than abrupt change in species composition across grassland sites, thought to be related to both the geographic proximity of the sites and environmental factors such as geology. The following five grassland types were identified from the entire data set and defined primarily by the dominant species: Poa hiemata, Poa costiniana, Poa sieberiana, Poa labillardierei and Themeda triandra. Vector-fitting revealed significant correlations between the location of the quadrats in ordination space and altitude, biomass, pH and soil depth. Both increasing altitude and biomass were associated with the P. costiniana grasslands and some of the P. hiemata grasslands. The P. hiemata grasslands were widely distibuted across altitudes and geology (i.e. basalt and metamorphic). The lower-altitude grasslands (P. labillardierei, P. sieberiana, T. triandra) were associated with increasing pH and increasing soil depth. These grasslands were of limited extent and usually occupied small areas within larger grasslands dominated by P. hiemata or P. costiniana. The floristic composition of the Hotham grasslands (1260–1630 m a.s.l.), when compared with previously published data from the higher-elevation subalpine grasslands of both the Dargo High Plains (1450–1680 m a.s.l.) and Bogong High Plains (>1700 m a.s.l.), showed that there was no distinct differentiation between grasslands of these areas. However, there was a suggestion of gradual floristic change across this geographic range. This study highlights the need for ongoing conservation of grasslands in the Hotham area, particularly those at lower altitudes (1260–1450 m a.s.l.), which represent the upper limits of many temperate grassland species.