The peak of cold resistance

The peak of cold resistance

Australia is hot and dry. Ask any tourist on sun-drenched Bondi Beach and they will affirm this well-known paradigm. Yet although much of our flora has adapted to thrive in our baking interior and dry open woodlands, many unique plants have evolved in a very different setting: the frigid landscape of the Australian High Country. These alpine plants are found nowhere else on Earth. Our alpine regions may not be the coldest in the world, but winter months in the Australian Alps regularly see thermometers struggling to climb above zero.

Cold presents challenges for both plants and animals. Whereas many animals are mobile and can migrate to more favourable climes, plants cannot move to escape the cold. In some cases, the cold is too much for a plant to survive. Nowhere is this more apparent than at the treeline. The treeline is the altitudinal threshold above which conditions become too harsh for trees to survive. This threshold often appears remarkably abrupt. On steep hillsides, a thick forest of Snow Gums can transition to a herb-rich shrubland in the space of a few metres. Even at slightly lower elevations, landscape topography can create conditions that are beyond the tolerance threshold of trees. Cold air sinking down the slopes can pool in valleys and natural bowls – mimicking the frigid conditions experienced at higher altitudes. The exact reason treelines form has yet to be fully resolved. Factors such as mechanical damage from ice forming in cells, too few days with a temperature warm enough for sufficient energy to be produced by photosynthesis, or cold-induced restrictions on growth rate may all be at play.

Trees are absent from the bottom of this natural bowl near Falls Creek and cold air sinking down the surrounding hillsides and pooling in the bowl is likely a contributing factor to their absence. Image: Rowan Mott

Whereas trees are particularly sensitive to the harshest of alpine conditions, many plants not only grow above the treeline, but flourish there. This is thanks to a diverse array of traits that have evolved over countless generations. Alpine shrubs, such as Kunzea muelleri and Baeckea gunniana, typically have smaller leaves than related species growing at lower elevations. By reducing the size of their leaves, these shrubs limit the leaf surface area they expose to the winter chill, thus minimising heat loss. However, if the temperature drops below freezing, alpine plants protect their precious leaves by using physiological mechanisms to prevent ice crystals forming and rupturing cells from within. These include making specialised proteins and changing the concentration of various cell compounds, such as sugars, to stabilise cell membranes and enable super-cooling (keeping water in a liquid state well below zero degrees).

Along with thermal energy, water is another resource that alpine plants can scarcely afford to lose. When temperatures fall below freezing, much of the water in the environment is unavailable to plants because it is in its solid form – ice. So not only does winter mean alpine plants have to contend with cold, but it can also bring the added stress of water shortage. The small leaf size of alpine plants means that they have fewer pores (called ‘stomata’) through which water is lost and the leaves are often thick and leathery which further helps to retain water. At even finer scales, the leaves may be covered in tiny hairs. These hairs reduce the airflow over the leaf surface, which not only reduces the amount of air drawing water vapour away from the leaf, but has the added benefit of reducing the amount of warmth that is stripped from the leaf too.

Alpine shrubs, such as these Hovea montana, are small-statured to protect from wind chill. Image: Rowan Mott

In addition to adaptations of individual leaves, our alpine plants also possess whole-of-plant adaptations that increase their ability to withstand the cold. Alpine shrubs often have a compact growth form which helps trap pockets of air and reduce the flow of cold across leaves and stems. Herbs and grasses display growth forms such as tussocks and cushions, with dense foliage adapted for reducing air flow around the plant. And it isn’t just the living tissue that contributes to this insulative effect – amongst the living foliage of alpine grass tussocks, dead material can often accumulate and add to the heat-retention capacity.

Alpine plants also have a surprising ally in the battle to avoid being killed by the bitter cold: snow! The low stature of rosette herbs, such as Craspedia jamesii, and tussocks, such as Poa hiemata, means they are blanketed by snow in the winter. Yet far from being a problem, this snow cover actually benefits the plants beneath. The snow acts like an igloo and shields plants from extreme temperature fluctuations and exposure to biting, cold winds. Many shrubs have actually evolved flexible stems engineered to bend as snow accumulates on the plant. This prevents the woody stems from snapping under the weight of the snow above.

Once blanketed by snow, alpine plants are no longer exposed to excessive temperature fluctuations and chilling winds. Image: Rowan Mott

Although many of the adaptations of alpine plants are the result of persevering through periods of bitter cold, others are targeted towards capitalising rapidly when conditions are good. Alpine plants store considerable quantities of nutrients below ground in their root tissue. When spring arrives, they can rapidly mobilise these stored resources and begin producing new growth and flowers. Whereas many alpine plants opt for this ‘quick off the blocks’ approach, others opt for a ‘head start’ approach. Many of our alpine shrubs produce their flower buds in autumn and then allow these newly formed buds to sit dormant during winter. As soon as the warmth of the coming spring brings about snowmelt, flowering can commence instantly because no time is wasted on bud formation.

Similarly, herbs and grasses also adopt strategies to optimise their flowering and maximise reproduction. Although the rosette leaves of many herb species are low to the ground, they can send up a flower well above ground level. At this height, the flower becomes a beacon for pollinating insects, and the higher vantage point exposes any seed that is produced to windier conditions thereby increasing the likelihood that seeds will effectively disperse away from the parent plant.

A Spotted Alpine Xenica butterfly is drawn to the beacon-like flower heads of Craspedia jamseii which are held well above the leaves of the plant. Image: Rowan Mott

Although these adaptations have proven successful over countless generations, the flora of the Australian Alps faces an uncertain future. Despite making the most of the short growing season, alpine plants are very slow growing. This makes them poorly suited to replacing tissue lost during disturbances such as fire. As climate change wields its influence, fire frequency is likely to increase. Similarly, species adapted to slightly warmer locations will spread into alpine areas. If these species are plants that have a superior competitive ability under the new environmental conditions, or herbivores that damage alpine vegetation, then our highlands may become a real conservation low point. Australia does not have higher mountains to which alpine vegetation can retreat.

For this suite of species adapted to bitter cold, the sad irony is that many now face the cold reality of extinction. Although for most of us, our experiences of the majesty of the Australian Alps take place during the winter months, I encourage you all to make the trip up in summer. Not only will venturing to the High Country be a welcome escape from oppressive heat and sunburnt beach-goers, but you will also be greeted by a carpet of wildflowers. The beauty of these cold-hardy plants is enough to send a chill down your spine.

Banner image of Kunzea muelleri courtesy of Rowan Mott. 

Leave a Reply

Your email address will not be published.