Australia only has a tiny portion of it’s landmass which is sub alpine or alpine. We know that climate change is already impacting on mountain environments, and without meaningful action to reduce greenhouse emissions, this will only continue.

The true alpine zone, that area above the treeline, is tiny relative to the landmass. The tree line is the highest elevation that sustains tree growth and is around 1,800 metres above sea level in mountain areas on the mainland (lower in Tasmania). The tree line is mainly defined by the gradual disappearance of snow gums (Eucalyptus pauciflora), which are a type of Eucalyptus that can withstand the severe cold and dry conditions of the mountains. The tree line is defined by temperature, not altitude, which explains why Australia has a lower tree line than most other countries.

As the climate warms, it can be expected that snow gums will be able to colonise the open alpine terrain above. This will lead to the loss of the true alpine vegetation, as these communities are ‘pushed off the top’ of the mountains and replaced by snow gum woodland. New research sheds light on this process, and has shown the role that fire plays in how snow gums encroach of alpine zones.

A research paper titled ‘Alpine treeline ecotone stasis in the face of recent climate change and disturbance by fire’ (available here) and authored by Aviya Naccarella, John W. Morgan, Seraphina C. Cutler, and Susanna E. Venn considers the interaction between fire, climate change and the treeline. In short, and as you would expect, this research suggests that more frequent fire slows the rate of colonisation of trees above treeline.

They say that while climate is considered a ‘key determinant of alpine treeline position, and hence treeline position is likely to be responsive to climate warming, other disturbance may also play a critical role. These other disturbances are, at present, poorly understood’.

Their research looked at sections of the alpine treeline in Australia where fire has been both extensive and recurrent during the last few decades. They found a ‘limited number’ of saplings of the alpine treeline species Eucalyptus pauciflora growing above the treeline, but the number of young trees ‘depended on the frequency of fire’.

‘In the absence of fire, short-distance (<10 m) regeneration was observed between 2002 and 2018. After two fires in this period, there was no sapling regeneration above treeline despite extensive recruitment in the period before fire (pre-2002). Fire frequency also affected the stand structure of subalpine woodlands. Few changes in structure occurred in the unburnt and once-burned forests; twice-burned stands, however, lost smaller-sized trees (<~20 cm basal girth) and this may be evidence of a fire ‘interval squeeze’. While changes in the environment, including climate warming, may allow saplings to grow above the current treeline, as has been observed in many alpine regions of the world, this capacity may depend on disturbance type and frequency. Hence, there is potential for long time lags in treeline movements in the Australian Alps because tree persistence and recruitment is affected by drivers other than just climate. There is a clear need to frame alpine treeline establishment processes beyond just being a response to climate warming’.

Key findings:

Without fire, trees will move up into alpine zones. In the absence of fire, ongoing climate warming (a ~0.4°C increase in mean annual temperature since 1992) would promote the upslope migration of the alpine treeline

‘We did observe ongoing sapling recruitment above treeline at Mt Hotham, the only mountain in our study that escaped wildfire between 2002 and 2018’.

Movement into alpine zones is slow, with most new trees establishing within 10 m of the treeline. This is because Eucalyptus pauciflora trees are generally small in height, less than 5 metres, at the treeline, and because the seeds have no appendages to aid wind-dispersal, the modelled mean maximum dispersal distance is about 16 metres from the parent tree.

Many of the saplings above treeline are young: low height (<~60 cm) and age (<~10 years) ‘The age of individuals indicates high turnover and thus low longer-term survival above treeline. The height of individuals we observed suggests that a height threshold determined by freezing stress may exist. Although mature E. pauciflora are relatively resistant to frost, individuals less than 30 cm tall can be easily killed by substantial shoot dieback’.

Younger trees are more at risk of dying. Mortality of E. pauciflora germinants is likely highest during the first growing season (being dependent on summer soil water availability) and the first winter (being dependent on the depth and duration of snow cover).

Multiple bushfires change snow gum woodlands. In short, more frequent fire will see fewer small trees, as the fire will kill them without necessarily killing the older trees, creating a more even-aged stand.

‘In the absence of disturbance by fire, woodland stand structure below treeline changed with new plants being recruited into the population. Models indicate that woodland structure has remained stable at single burn sites like Mt McKay, with a higher proportion of saplings to mature individuals. This suggests treeline populations are resilient to a single bushfire. Models at twice-burned treelines indicate a scarcity of saplings, leading to a more even- aged stand. Prior to being twice-burnt, saplings were prominent, suggesting a shift in treeline dynamics over time with bushfire occurrence.

‘How E. pauciflora treeline responds to fire disturbance appears to be influenced by fire frequency. Once-burned areas show no evidence of pulsed seedling regeneration, but there is little evidence of tree or sapling mortality. Twice-burned areas (within a decade), however, lose small-sized trees and saplings both above the treeline and within the subalpine woodland. The occurrence of two fires within a decade is an unprecedented event in the Australian Alps. Although mature tree mortality was low, and as such there was no evidence of treeline recession, woodland structure had shifted substantially. The substantial reduction in saplings above- and below-treeline suggests younger and smaller individuals are more susceptible to fire. Despite E. pauciflora capacity to resprout within 6 months of germination, the lack of soil protection for the lignotuber could result in increased vulnerability, particularly during high-intensity fires. Additionally, the low height of individuals above treeline likely increased their susceptibility to both (i) fire due to their close proximity to the ground layer and surface fuels and (ii) frost due to the loss of groundlayer vegetation that may protect them from cold temperatures.

Fires, particularly at short-intervals, have been found to cause treeline depression across a range of global treelines. Conversely, fires have also been found to accelerate the effect of rising temperatures through reducing competition with surrounding vegetation. Fire may assume an even greater importance in the Australian Alps in the future, with temperatures predicted to increase by 1.4–3.8°C over the next century for south-eastern Australia, increasing the frequency of high fire danger days by as much as 70% by 2050. Additionally, understanding how fire and climate may differentially or similarly affect lower elevation woodlands, including the lower montane E. pauciflora boundary, will inform how E. pauciflora ecotones could change in the future. Abrupt changes to the fire regime has already caused the landscape-wide loss of obligate seeder mountain forests in south-eastern Australia, suggesting there is the potential for shifts across many ecotone boundaries’.

There is a growing body of research that shows that snow gum forest does not respond well to repeated fire.

Naccarella A, Morgan JW, Cutler SC, Venn SE (2020) Alpine treeline ecotone stasis in the face of recent climate change and disturbance by fire. PLoS ONE 15(4): e0231339. https://doi.org/10.1371/journal.pone.0231339