A new model for predicting understorey leaf area from biomass in eucalypt forest to test the ecohydrological equilibrium theory

1. Ecohydrological equilibrium theory predicts that vegetation leaf area approaches a maximum dictated by climate and available soil moisture, and that changes in the overstorey and understorey layers compensate such that combined leaf area is maintained. Thus, rapid recovery of understorey following overstorey thinning and burning can diminish catchment water yield increases expected as a result of reductions in evapotranspiration losses. However, the understorey layer is rarely considered in management decisions, in part because methods to estimate understorey leaf area are poorly developed. Improved methods are needed to advance our understanding of ecological and hydrological processes involving forest understorey. 2. We developed models for accurately estimating leaf area of the biodiverse understorey layer of eucalypt forests. Models based on biomass as the independent variable were simpler and more general than models based on above-ground nitrogen or N/C ratios which were sensitive to the relative proportions of N-2-fixing plants. We applied our model to compare understorey leaf area with overstorey leaf area in regrowth eucalypt forest, and to derive estimates of leaf area for the world's forests. 3. In thinned and prescribed burnt forest, understorey leaf area was inversely proportional to overstorey leaf area such that combined stand leaf area after only 4 5 years was unaffected by treatment. This represents the first reported confirmation of the ecohydrological equilibrium theory that is described in terms of the leaf area of the two layers. In contrast, understorey leaf area did not respond to thinning in plots with no follow-up burning. We conclude that more attention should be paid to strategies for managing understorey recovery when manipulating regrowth eucalypt forests for increased catchment water yields. 4. Synthesis and applications. Recovery in vegetation leaf area after thinning in eucalypt forest can be rapid due to the response of the understorey layer. We show that understorey leaf area can completely compensate for reductions in overstorey leaf area, consistent with ecohydrological equilibrium theory, within 5 years of disturbance if thinning is followed by prescribed burning. Applying our model to published biomass data for the world's forests, we also show that forest understorey can have a leaf area comparable with some forest overstorey. Without this new model, these increases in biological understanding would be much more difficult to obtain. By simplifying the estimation of understorey leaf area, the new model will enable researchers to progress rapidly towards greater understanding of understorey and overstorey interactions in natural and disturbed forest ecosystems.