John Morgan 

Ongoing changes in global climate are altering ecological conditions for many species and the consequences of such changes are most evident at the edges of the geograph­ical distribution of species. Alpine plant species’ distributions are shifting to higher elevations in response to climate change. Most evidence for upslope migration of alpine plants comes from the northern hemisphere, with relatively limited evidence for elevational shifts in southern hemisphere plants. To address this knowledge gap, Auld et al. (2022) used historic data (herbarium records, field observations), combined with a recent field survey, to determine if alpine plants in Australian high mountains have migrated upslope over the last two to six decades. By comparing historic records with their new survey, they suggested that plant elevational shifts were occurring, with ~30% of species (n=36 species in the study) showing evidence for upslope movements. Five species were suggested to have contracted their range, and two had expanded their range.

While the evidence presented in Auld et al. (2022) suggests that range changes may be occurring over time – with change couched as species’ mean, upper and lower elevation shifts – such data cannot (by themselves) attribute change to any specific mechanism(s). The authors codify their primary question in terms of climate change (more specifically, increasing temperatures) but there are other explanations for the changes observed that do not invoke climate change responses. 

Australian alpine landscapes have been subjected to a century (or more) of ungulate grazing by domestic stock, and associated burning practices (Good 1992), with significant impacts on wetland, grassland and heathland structure and composition. Grazing in the Kosciuszko National Park (KNP) was banned 70 yrs ago. Recovery of alpine species and ecosystems from decades of prior disturbances has been well-studied in Australia (Carr and Turner 1959; Costin et al. 1979, Wahren et al. 1994; Scherrer and Pickering 2005). Wahren et al. (1994), for example, show that recovery of grazing-sensitive alpine grassland species was still continuing more than 45 yrs after cattle grazing was excluded. In KNP, livestock grazing caused the major decline of palatable species (such as grasses and herbs), while grazing and fire led substantial soil erosion in wetlands and herbfields. Indeed, soil erosion of the catchment was so bad that rehabilitation of groundcover condition was a key focus for 30 yrs after the cattle were removed (Good 1992). Hence, the historic elevation range captured by historic records used in Auld et al. (2022) are a range that encapsulates species recovering (some quickly, some slowly) from past disturbances, i.e. it may not represent the species’ climatic distribution but rather, likely captures the impact of land use that made many species rare in the landscape. 

Several plant species are known to have been much reduced in extent due to cattle grazing. For example, Costin et al. (2000) note that Chionochloa frigida, a tall tussock grass, was “common on the eastern slopes of Kosciuszko but with grazing and burning it almost became extinct there by the 1930s. Since 1944, when the Kosciuzko summit area became protected from grazing, C. frigida has been recovering”. Ranunculus anemoneus is shown by Auld et al. (2022) to be moving downslope, but this is just as likely a landscape recovery from refuge areas amongst rock outcrops after cattle grazing (Good 1992) as it is to climate change.  Separating out the relative contribution of these factors is difficult, but evidence hints that several of the changes observed over recent decades should be couched as landscape recovery change via dispersal and regeneration. Costin et al. (2000) suggest that “prior to the cessation of livestock grazing, R. anemoneus was almost grazed out of existence, but is now making a spectacular recovery”. Good (1992) echoes this sentiment: “… species are now slowly but actively recolonising available areas of previous habitat”. Clearly, the historic records for this species do not necessarily reflect the historic (climatic) distribution of the species, and any changes thereafter may reflect ongoing recovery as population numbers increase and expand back across KNP.

Attribution of causes of change in plant species distributions will remain problematic when land use legacies are at play. While it is logical to think that alpine plants are primarily responding to climate drivers (and appear to be doing so in the northern hemisphere), the situation is likely more complex. Many of the examples of elevational range shifts in the northern hemisphere are species moving into a contracting nival zone where new habitat is becoming available for species to establish. In Australia, mountains are covered in vegetation due to deep soils and opportunities for new species establishment outside of their range, due to climate change, may be more limited. Interestingly, it may be that disturbances from feral ungulates, fire and people are the factors that accelerate these changes in the future.  Hence, understanding the interplay between climate and other drivers becomes essential to better forecast the future of alpine plants in Australia. This is best achieved by permanent plot studies, ongoing systematic collection of herbarium specimens, careful consideration and of interpretation of landscape history, and well-designed experiments that can account for the milieu of biotic factors that affect species distributions. 

References

Auld, J., Everingham, S. E., Hemmings, F. A., & Moles, A. T. (2022). Alpine plants are on the move: quantifying distribution shifts of Australian alpine plants through time. Diversity and Distributions 28, 943– 955. https://doi.org/10.1111/ddi.13494

Costin, A.B., Gray, M., Totterdell, C.J. & Wimbush, D.J. (2000) Kosciuszko alpine flora. CSIRO Publishing, Clayton South.

Carr, SG.M. & Turner, J.S. (1959) The ecology of the Bogong High Plains. II. Fencing experiments in grassland C. Australian Journal of Botany 7, 34-63.

Good, R.B. (1992) Kosciusko heritage. National Parks and Wildlife Service, Hurstville.

Scherrer, P. & Pickering, C.M. (2005) Recovery of alpine vegetation from grazing and drought: data from long-term photoquadrats in Kosciuszko National Park, Australia. Arctic, Antarctic, and Alpine Research 37, 574-584.

Wahren, C-H.A. , Papst, W.A. & Williams, R.J. (1994) Long-term vegetation change in relation to cattle grazing in sub-alpine grassland and heathland on the Bogong High-Plains: an analysis of vegetation records from 1945 to 1994. Australian Journal of Botany 42, 607-639.