Degree project in Biology, Master of Science (2 years), 2020
Examensarbete i biologi 60 hp till magisterexamen, Uppsala universitet, 2020 Biology Education Center and Murdoch University, WA
Supervisor: Robert Muscarella & Joseph Fontaine
Carbon Sink to Carbon Sources: Compound Disturbances Reduce Ecological Resilience
Shareen Sanders
Ecological resilience is the ability of an ecosystem to regain important ecological functioning, such as carbon sequestration, after a disturbance event. The south-west Australian floristic region (SWAFR), for example, is prone to drought and fire. Forest ecosystems in the SWAFR have evolved traits to tolerate and regenerate after these events such as fire-triggered germination and the ability to resprout. These traits allow forests of the SWAFR to retain ecological function.
However, future climate projections suggest an increase in average temperature as well as a decrease in average winter rainfall across the SWAFR. These adverse conditions are likely to amplify the intensity and frequency of disturbance events such as drought and fire. Shifts in the magnitude of these disturbance events could increase the recovery period required for recruitment and recovery of ecological functioning as well as a shift in forest structure.
To assess the effects of compound disturbances on ecosystem functioning, we investigated above-ground biomass (AGB) accumulation of understorey plants at sites in the Northern Jarrah Forest (NJF) of the SWAFR that experienced different degrees of drought and fire intensity. We used allometry to estimate the AGB of understorey plants and used these estimates to create generalised and species-specific allometric models to calculate the AGB of all understorey plants using height and canopy volume measurements.
Within a disturbance event, sites experiencing either more severe drought and fire intensities on average accumulated substantially more understorey AGB than sites subjected to both low drought and moderate fire intensities. These results suggest that understorey species within the SWAFR gain a competitive advantage in increased drought and fire severity conditions. This can be attributed to reduced functioning of tree species and canopy die-off, followed by increasing light, water and nutrient availability for understorey plants.
The increase in understorey AGB accumulation also suggests a shift in overall forest structure to more dense, compact, low-ground small stems, which are known to increase fire probability. An increase in fire probability shortens the time period between fire intervals and can detrimentally affect forest recovery, especially in drought conditions.
These changes may shift ecosystems within the SWAFR to a state of non-equilibrium, reducing resilience to future disturbance events and converting forests from carbon sinks to carbon sources.
