Fire history and understorey vegetation: Water and nutrient relations of Eucalyptus gomphocephala and E. delegatensis overstorey trees

Fire regimes in temperate forests and woodlands have changed significantly in Australia since European settlement. We hypothesised that an absence of fire leads to the increased development of woody understorey/midstorey and that this may be correlated with decreased water and/or nutrient availability in overstorey temperate eucalypts currently declining in health. Sites with a history of being long unburnt or recently (and frequently in the case of Eucalyptus gomphocephala woodland) burnt (relative to median fire intervals for the vegetation type) were established in E. gomphocephala woodland in Western Australia and in Eucalyptus delegatensis forest in Tasmania. In long unburnt sites in both E. gomphocephala woodland and E. delegatensis forest, there was greater percent cover of understorey/midstorey and eucalypts had higher water use efficiency, indicative of greater soil water limitation, as estimated by foliar carbon isotope ratios. In E. gomphocephala woodland foliar Cu and Zn were significantly lower in eucalypts of long unburnt, relative to frequently burnt, sites. In E. gomphocephala woodland, understorey/midstorey (shrub) cover was positively correlated, and foliar copper and zinc levels were negatively correlated to health of overstorey trees. In E. delegatensis forest foliar phosphorus (P) was significantly lower in eucalypts of long unburnt, relative to recently burnt, sites. In E. delegatensis forest moss cover was positively correlated and foliar P was negatively correlated to health of overstorey trees. The understorey/midstorey that develops in the long absence of fire may alter ecological processes that lead to less favourable water- and nutrient-relations in E. gomphocephala woodland and E. delegatensis forest that are associated with decline in crown health. However this study does not definitively show a link between understorey/midstorey vegetation and overstorey tree water- and nutrient-relations. This link will be investigated in future research.

► Long unburnt or recently burnt sites were established in eucalypt woodland and forest.
► In both long unburnt sites there was greater percent cover of understorey/midstorey.
► In both long unburnt sites eucalypts had higher water use efficiency.
► Foliar Cu and Zn were significantly lower in eucalypts of long unburnt woodland.
► Foliar phosphorus was significantly lower in eucalypts of long unburnt forest.