Fire-induced Compound Disturbance - Species Replacement and Ecohydrologic Response in South East Australian Forests

Climate changes and associated consequences are beginning to manifest in forested landscapes (eg. Mountain Pine Beetle, increased fire frequency and intensity). At the severe end of forest ecological (and hydrologic) responses are species replacement. A warming and drying climate predicted for south eastern Australia is likely to bring increased wildfire activity to a region that is already prone to highly intensive fire. Fire frequency in the highly productive, wet mountain forests, typified by Eucalyptus regnans, has been in the order of 70 years or greater. E.regans is an obligate seeder: medium/hot fire kills the trees, which then regenerate from seed as single aged strands. However there have been several large short interval fire events in mountain forests in the past decades that overlap in area (eg. 1926-1939, 2003-2006-2009). E.regnans, and other ash-type species, require 15-20 years to develop seed. If re-burnt, the stands cannot naturally regenerate. Frequently acacia species colonise these twice burnt areas, resulting in the transformation of tall mountain forests to acacia shrublands. Around 80% of the water for the city of Melbourne (> 4 Mill. pop.) is sourced from these forests. A widescale change to the forest species composition could have a significant impact on water supply. Further, such a dramatic species change has profound eco-hydrologic implications with a system perturbed from a long term equilibrium to a new state. Little is known about the eco-hydrologic implications of a climate/fire driven radical species shift resulting from a compound disturbance.

Plot-based water balance (sapflow/throughfall/stemflow/ microclimate) studies across age sequences have been combined with dynamic stand modelling to build a quantitative understanding of the multi-decadal eco-hydrology of these altered states. Stand sapwood area and sap velocity as a function of vapour pressure deficit were used to drive the hydrologic part of the model. Measured ET showed a divergence in water use between E.regnans and A. dealbata after the age of 20 years. This ET change is driven by differences in sap velocity and sapwood area. These differences increase as the stands age, resulting in A.dealbata transpiring around 60% less than E.regnans stands at age 80. This represents a fundamental change in system eco-hydrology.