Brown is the new green: Grassland responses to heat and drought
Grasslands cover approximately 40% of the global land surface and provide a variety of ecosystem services including carbon sequestration and biodiversity. In Australia, grasslands and grazing lands cover approximately 70% of the continent and support a livestock industry that contributes ~$17 billion to the economy annually.
Severe water deficit, coupled with climate variability, is a major threat to the livestock industry due to its effect on the productivity of grasslands. Grasslands are typically more sensitive to drought than forests, as they lack the deep root systems which can buffer forest ecosystems from the water deficit associated with drought. Many previous studies have demonstrated the susceptibility of grassland ecosystems to drought, based on the relationship between annual net primary productivity and precipitation.
However, these studies also show that there are differences among grasslands in their sensitivity to drought. These differences are determined by differential drought sensitivities of the plant species, which are determined by multiple plant functional traits. Several mechanisms, including stomatal regulation of carbon assimilation, hydraulic regulation of plant water status, and leaf browning and shedding, play important roles in determining the drought response of plant species. In trees, a number of studies have recently shown that carbon assimilation and hydraulic traits are linked and help to explain variation in drought response among species, but there are fewer data available for grasses. I aim to test whether drought sensitivity of stomatal conductance, hydraulic conductance and leaf browning are co-ordinated in Australian grass species.
In addition, I also aim to use a trait-based approach to scale up species-specific drought responses to predict grassland responses to future drought events at ecosystem scale. Since grassland ecosystems are highly diverse, measuring carbon assimilation and hydraulic traits of all important grass species would be very difficult in terms of time and cost constraints. On the other hand, previous studies have shown that, for tree species, mechanistic traits are correlated with both morphological traits and climatic origin. Further, these correlative relationships between mechanistic and morphological traits of tree species have offered an alternative for model parameterisation, by using morphological traits as proxies for mechanistic traits of those tree species. Similar relationships may be identifiable for grass species.
The goals of my thesis are to identify the functional traits of grasses that determine their drought sensitivity, in order to develop mechanistic understanding at species level, and study how to scale these functional traits to predict ecosystem level drought responses.
My thesis will have the following components:
- Investigate the underlying mechanisms related to differential drought responses of grasses. I will quantify how stomatal regulation, hydraulic dysfunction and leaf browning are affected by soil moisture availability for a range of Australian grass species from differing environments.
- Measure morphological traits of a range of grasses and test whether correlations with mechanistic traits and with home climate can be used to estimate mechanistic traits at large scales.
- Predict drought responses of grasslands based on underlying physiological and phenological mechanisms of grass species to soil water deficit and the links between those mechanistic and morphological traits of grasses.
Professor Belinda Medlyn, Professor David Tissue, Professor Sally Power and Professor Elise Pendall.