The world's most iconic forests are under threat from climate change. Climate-firevegetation feedback mechanisms are altering the usual successional trajectories of forests. Many obligate seeder forests across the globe are experiencing regeneration failures and subsequent alterations to their recovery trajectories. For example, the persistence of Eucalyptus regnans F. Muell. forests in southeast Australia is highly vulnerable to the effects of climate-driven increases in wildfire frequency. Shortening of the wildfire return interval from >100 years to < 20 years would inhibit or entirely stop regeneration of E. regnans, leading to replacement with understorey species such as Acacia dealbata Link. In this study, it is hypothesised that following such replacement, forest overstorey structure and transpiration will diverge. An experiment was designed to test this hypothesis by measuring and comparing overstorey transpiration and structural properties, including sapwood area and leaf area, between E. regnans and A. dealbata over a chronosequence (10-, 20-, 35-and 75-/80-year-old forests). We found that overstorey structure significantly diverged between the two forest types throughout the life cycle of A. dealbata after age 20.The study revealed strikingly different temporal patterns of water use, indicating a highly significant eco-hydrologic change as a result of this species replacement.Overall, the results provide a strong indication that after age 20, overstorey transpiration in Acacia-dominated forests is substantially lower than in the E. regnans forests they replace. This difference may lead to divergence in water yield from forested catchments where this species replacement is widespread.

Climate‐induced fire regimes may change species abundance and species composition in affected forest types, potentially altering pyro‐eco‐hydrologic feedbacks. In some fire‐prone forests across the globe, eco‐hydrologic thresholds (changing points, or tipping points, in ecohydrology when vegetation shifts from one steady vegetation to another) are being exceeded due to changes in relationships between climate, fire and vegetation. Following compound disturbances, forests may fail to maintain ecological resilience. Under multiple burn conditions, Eucalyptus regnans F. Muell. forests in south east Australia are highly vulnerable to ecological tipping points. In Victoria, over 189 000 ha of obligate seeder forests have been burned two or more times within 18 years. These short return‐interval fires allow Acacia dealbata to become the dominant overstorey species. Such a dramatic species replacement may result in a new evapotranspiration (ET) regime, leading to a new hydrologic state. Stand scale dynamic models were combined with field estimated ET in E. regnans and A. dealbata forests aged 10, 35 and 75/80 years. We found that long‐term forest structure, ET and water yield significantly diverge between E. regnans and A. dealbata forests with increasing age. These divergences imply a non‐equilibrium state after A. dealbata replaces E. regnans under high‐frequency fire conditions. In senescing A. dealbata, understorey transpiration contribution of 29.8% to system ET was similar to that of overstorey transpiration (31.2%), indicating the understorey and overstorey contribute equally to total ET at the final stage of Acacia forests. In contrast, in 75‐year‐old E. regnans forests, understorey contribution to the total system evapotranspiration is about 16%. This suggests that, after the Acacia life cycle finishes, the ET regime will transit into a new state that will be dominated by shrubby understorey species. Our findings suggest that this climate‐induced species replacement would decrease long‐term ET, inferring an increase in streamflow.

&lt;p&gt;Most of the water that ends up in Melbourne&amp;#8217;s water supply catchments originates from wet Eucalyptus forests that are dominated by Eucalyptus regnans, the tallest known angiosperm on earth. Studies had shown that catchments that are dominated by these forests can experience a significant long-term (&gt;100 years) reduction in streamflow after a stand-replacing fire, which was attributed to higher water-use of the dense overstory regrowth. However, despite several lines of evidence, the direction, extent and duration of post-fire hydrological behaviour vary significantly between catchments and between fire events. Here we propose that this variability is caused by initial stocking density and species composition after the fire, and the climatic conditions that prevail during forest regeneration that affect tree growth and mortality rates. In order to test the hypothesis, we formulated an ecohydrological model that simulates hydrology, growth and forest dynamics of E. regnans and Acacia dealbata, which are known to compete for resources during the initial stages of vegetation recovery. The new model shows high skill in predicting long-term streamflow when compared to observations using multiple sources of data. Simulation analysis shows that the direction, extent and duration of post-fire hydrological behaviour are sensitive to initial stocking density and to the relative abundance of species that regenerate after the fire, which influence the rate of self-thinning during stand development. Furthermore, simulation results show that the observed long-term reduction in streamflow is less likely to occur when the forest would have been less dense before the fire, which theoretically could only occur when a high proportion of the short-lived A. Dealbata regenerated after the previous fire. This highlights the importance of including mechanisms that control the effect of species composition on forest dynamics when modelling the effect of possible future climatic scenarios on water yield.&lt;/p&gt;

&lt;p&gt;Fire as a hydrologic agent has been most frequently examined in terms of erosion and water quality, with studies on the ecohydrology expressed as evapotranspiration/streamflow often focussing on short term perturbation that relaxes with vegetation recovery. Far more dramatic ecohydrologic impacts are possible if repeated fire disturbance leads to species change. Such a scenario occurs in some forests in south-eastern Australia, a region that is among the most flammable in global terms due to the confluence of climatic and stand productivity factors. The most vulnerable of these forests are the &amp;#8220;ash&amp;#8221; type &amp;#8211; mainly Eucalyptus regnans and E. delegatensis. The E.regnans ecology&amp;#160; has evolved with long fire intervals as medium/hot fire kill the trees, which then regenerate as single aged strands. However there have been several large short interval fire events in mountain forests (eg. 1926-1939, 2003-2006-2009-2019) in the past decades that overlap in area. E.regnans, and the other ash-type species, require 15-20 years to develop seed. If re-burnt, the stands cannot naturally regenerate. Frequently acacia and other understorey species colonise the sites, resulting in a dramatic change in forest structure and biomass.&lt;/p&gt;&lt;p&gt;The implications of this change are significant, with potentially high magnitude changes in ecohydrologic functioning. Further, these areas are the principal water supply catchments the city of Melbourne (&gt; 4 M pop.) and a number of other towns. The impact of high frequency fire that is predicted to increase under climate change therefore has the potential to change ecology, hydrology and essential ecosystem services, in this case, water supply.&lt;/p&gt;&lt;p&gt;An extensive field experimentation and modelling program set out to (a) investigate the climatic conditions under which these wet forests burn and the sensitivity of these drivers to predicted climate change; and (b) evaluate the eco-hydrologic impact of a species change from E.regnans to acacia species over an age sequence of 80 years.&lt;/p&gt;&lt;p&gt;Results revealed there is an envelope of dry surface soil and maximum vapour pressure deficit (VPD) within which there is a 50% chance of uncontrolled fire. The most damaging fires occurred when VPD was within the upper 0.01% of values and available surface soil water below 55%. Modelling suggests this conjunction of drivers will increase significantly in the future.&lt;/p&gt;&lt;p&gt;Stand structure, particularly sapwood area, diverged between the eucalypts and acacias at age 10-20 years, with the difference increasing until acacia death at age 80. This structural parameter scales with ET, with acacias exhibiting a marked decline over time relative to E. regnans. This ET change is principally driven by sapwood area. These differences increase as the stands age, resulting in A.dealbata using around 30% of an E.regnans stand at age 80. This represents a fundamental change in eco-hydrology, and suggests a system pushed to a state of disequilibrium. The stand structural attributes over the age sequence indicate a large change in carbon stocks, resulting in significant alteration of both carbon and water cycles under this disturbance. The results have significant implications for water supply, forest ecosystem services, and system feedbacks of flammability-fire-ecohydrology.&lt;/p&gt;