Distribution, Disturbance Tolerance and Conservation of Stackhousia gunnii in Tasmania

Abstract

Although being possibly widespread on the mainland of Australia, where it is recognised as part of the Stackhousia monogyna Labill. complex, S. gunnii Hook.f. is an endangered species in Tasmania, with only 10 known extant populations, all in the drier parts of the southern Midlands. These populations occur largely on roadsides or lightly grazed paddocks in vegetation that was originally either Themeda triandra grassland or Eucalyptus pauciflora woodland. An ordination of floristic data from sites with and without S. gunnii showed that the species occupies a floristically distinct environment. The floristic differences between stands of S. gunnii relate partly to the presence or absence of tree cover and substantially to precipitation. The groups of species that preferentially occur with S. gunnii are geophytes, non-geophytic herbs and exotic plants, while those that are concentrated elsewhere include shrubs, grasses and non-geophytic graminoids. Permanent transects were monitored over a 4-year period at seven sites with varying grazing, firing and disturbance. The number of shoots varied markedly between years in all sites, but not synchronously, even when transects were in close proximity. Although shoot numbers increased after fires occurred on several ungrazed transect lines, they decreased on one grazed line. Deep mechanical disturbance of one site resulted in a dramatic decrease in shoot numbers in the following year. However, they increased steadily over the following 2 years. Shoot emergence occurred at twice the expected rate on bare ground in all years and at all sites, and a substantial proportion of shoots was confined to this type of surface. However, shoots were also recorded in fewer numbers in grass cover, herb cover and non-vascular plant crusts. Like some other threatened plants of grassy ecosystems, S. gunnii seems to be rare because it cannot survive with heavy grazing disturbance, ploughing and fertilisation, and, at the other extreme, because it is susceptible to elimination in the absence of environmental conditions and disturbances that create bare ground.