A project undertaken at Deakin University, and supervised by Dr Susanna Venn.
Australia’s alpine regions are generally considered to be cold, wet and snowy places. However, the last decade of climate warming has led to declines in snowfalls and some extended periods over summer without rain, with similar trends predicted for the future.
Although Australian alpine vegetation has the capacity to survive mild fluctuations in climate, reduced snow and lower than average rainfall has led to sustained soil moisture deficits, with soil water values dropping well below plant wilting point. Such events in the recent past have led to widespread and severe dieback of some alpine plant species, however the physiological effects of drought on alpine plants, especially seedlings, are poorly understood.
In this project, we investigated the patterns between snow cover, soil moisture and plant water stress by undertaking a series of drought experiments in the field and in the glasshouse. We aimed to assess how well common alpine plants can cope with drought and reduced water availability, and to identify the best drought-tolerant candidates for future rehabilitation projects in alpine areas. In the field, we correlated snow depth, density and duration with soil moisture, and evaluated water stress in 13 common alpine plant species growing under and around the Australian Mountain Research Facility’s rainout shelters on the Bogong High Plains in Victoria and in the Snowy Mountains in New South Wales. These shelters intercept rainfall by 60-80% and thereby provide a severe drought treatment to the plants growing underneath, and the sites encompassed a range of elevations, soil depths and exposure. We used a pressure chamber to measure plant water potential at pre-dawn (baseline plant water content, no transpiration) and at midday (high water stress, peak transpiration), in spring, summer and autumn over three years. To complement the field experiment, we also undertook watering experiments on seedlings and young plants in the glasshouse to evaluate their responses to experimental drought.
Collecting leaf material from under an Australian Mountain Research Facility rainout shelter pre-dawn (when plants are not transpiring) for leaf water potential measurements.
Over winter, the rain-out shelters reduced snow depth compared to the controls and the snow keeps soil moisture conditions relatively stable with no large differences between the treatments. Despite the wet La Niña conditions that prevailed over the first year of our field study, the rain-out shelters were effective and provided drier conditions in spring, summer and autumn compared to the non-sheltered plots. However, the drought treatment effects were species-specific and not all species showed a significant treatment impact. The season and the time of day when the plants were sampled (pre-dawn or midday), strongly affected plant water relations in all species. Some of the grass-like species (graminoids) such as Poa hiemata (grass), Luzula modesta (rush) and Carex breviculmis (sedge), showed the strongest responses to the drought treatment and were able to adjust their internal water relations well tolerate very negative water potentials (water stressed at midday), suggesting that these species have tough leaves and are very drought tolerant.
Collecting leaf material for water potential measurements at midday (during peak transpiration and possible water stress) in summer at a high alpine site on the Bogong High Plains, Victoria
The three species used in the glasshouse experiments, Grevillea australis (shrub), Carex breviculmis (sedge) and Podolepis robusta (forb) showed varied responses to drought. Grevillea showed a low physical response to water stress by maintaining stable growth across all watering treatments. There was high variation in water potential and growth response in Carex. The drought treatment in Podolepis caused high water-stress at both pre-dawn (before photosynthesis and growth) and at midday (peak photosynthesis and growth) sampling times. The drought treatment also significantly reduced stomatal conductance and electron transport rate in Podolepis, reflecting strict stomatal regulation and reduced photosynthetic activity under water stress.
An Australian Mountain Research Facility rainout shelter in winter. We measured snow depth and density under the shelters and in adjacent control plots each year of the project.
Together, our field and glasshouse experiments have revealed that the majority of common Australian alpine plants are well adapted to a range of soil moisture conditions across seasons, even in summer at high elevation and exposed positions. Some species of grasses and shrubs are well adapted to drought and would make excellent candidates for rehabilitation projects. Other species, such as herbaceous forbs, may be better suited to protected sites with deep soils and reliable soil moisture. Our results also point to the underlying physiological mechanisms that may be driving vegetation change in Australia’s high mountain areas; general increases in the abundance of shrubs and graminoids.