Five decades of exclusion of vertebrate animal grazing from alpine vegetation in Lutruwita/Tasmania does not result in a succession from shrubs to tall herbs and grasses

Context The multidecadal response of treeless subalpine vegetation on fertile sites in Lutruwita/Tasmania to the exclusion of vertebrate grazing animals might be expected to mirror the transition from grassy shrubland to complete vegetation cover dominated by tall herbs and grasses, as documented for alpine vegetation in the Australian Alps. Aims To determine whether long-term protection from vertebrate herbivore grazing results in a transition from treeless subalpine grassy shrubland to tall alpine herbs and grassland. Methods We conducted repeat sampling in treeless subalpine vegetation at a long-term monitoring site on Liawenee Moor, Lutruwita/Tasmania. Vegetation surveys were conducted in 20, 5 m × 5 m quadrats, and along 20, 10 m line transects in a vertebrate grazing exclosure in 1997, 2011, and 2018. Data on species presence and cover and the cover of lifeforms were used to track changes in vegetation over time. On-ground vegetation surveys were supplemented by drone imagery in 2022 to measure changes in the cover of the dominant shrub, Grevillea australis, across the whole experimental site. Key results The large shifts in species composition and cover from 1997 to 2018 were not in accordance with the expected trajectory, from shrub-dominated to herbaceous vegetation. Cover of both tall shrubs and tall herbs declined over time, whereas grass cover remained constant. There was no corresponding increase in cover for any lifeform groups that could account for the decline in litter. Overlapping vegetation cover decreased significantly over time, with most of the change in this variable occurring between the first two sampling periods (1997 and 2011). Changes in frequency and cover of individual species was variable within lifeform groups; however, the frequency of several short herbs decreased dramatically between 1997 and 2011. Warmer temperatures and soil moisture deficits are likely to be partly responsible for changes in vegetation over time. Conclusions After 50 years of vertebrate herbivore exclusion there is little evidence to suggest that the ungrazed full exclosure plot at Liawenee Moor is transitioning from a shrub-dominated community to one that is dominated by tall herbs and grasses. Overlapping vegetation cover was significantly lower in 2018 than in 1997, most likely related to the reduction in cover of the dominant shrub, G. australis. Climate signals such as warmer minimum temperatures in combination with soil moisture deficits during the growing season, may be partly responsible for changes in vegetation. The decline in cover of G. australis across the experimental site, regardless of grazing treatment, maybe a response to drought and has been observed in shrubs elsewhere on the Central Plateau and in the Australian Alps. Observed declines in some smaller-statured plants may also relate to soil moisture deficits and increasing air temperatures. Further monitoring of the whole experimental site, including the plots subjected to grazing, will help disentangle the impacts of a warming climate from responses to grazing pressure. Implications This is one of the oldest long-term monitoring sites in Lutruwita/Tasmania and is important for investigating changes in vegetation dynamics under a warming and drying climate.