Impacts of fire on forest age and runoff in mountain ash forests

Abstract. Runoff from mountain ash (Eucalyptus regnans F.Muell.) forested catchments has been shown to decline significantly in the few decades following fire-- returning to pre-fire levels in the following centuries-- owing to changes in ecosystem water use with stand age in a relationship known as Kuczera’s model. We examined this relationship between catchment runoff and stand age by measuring whole-ecosystem exchanges of water using an eddy covariance system measuring forest evapotranspiration (ET) combinedwith sap-flowmeasurements of treewater use,withmeasurementsmade across a chronosequence of three sites (24, 80 and296years sincefire).At the 296-year old site eddy covariance systemswere installed above the E. regnans overstorey and above the distinct rainforest understorey. Contrary to predictions from the Kuczera curve, we found that measurements of whole-forest ET decreased by far less across stand age between 24 and 296 years. Although the overstorey tree water use declined by 1.8mmday1 with increasing forest age (an annual decrease of 657mm) the understorey ET contributed between 1.2 and 1.5mmday1, 45 % of the total ET (3mmday1) at the old growth forest