Factors determining relations between stand age and catchment water balance in mountain ash forests
Introduction
Forests of the Victorian Central Highlands yield almost all of the water for the city of Melbourne, Australia (Fig. 1). The mountain ash species (Eucalyptus regnans F. Muell.) covers just over half of this area but yields about 80% of the total water yield because it is situated in the wettest areas. Hence, these forests are of great strategic importance to the catchment managers, Melbourne Water. Since the 1960s they have conducted and sponsored several studies focussed on the ecologic and hydrologic function of these forests and the manner in which they respond to disturbances such as logging and fire (Langford, 1976; Moran and O′Shaughnessy, 1984; Kuczera, 1985, Kuczera, 1987; Jayasuriya et al., 1993; Haydon et al., 1996; Vertessy et al., 1998; Watson et al., 1999).
The ecology of mountain ash forests is now well understood (Ashton, 1975, Ashton, 1976). They are confined to the wetter parts of the highlands of Victoria and Tasmania, and generally grow at altitudes between 200 and 1000 m, where mean annual rainfall exceeds 1200 mm. Fire is an infrequent but vital component in the life cycle of these forests. Tree seedlings only survive and grow in exposed soil with direct sunlight. In nature, these conditions are only created by bushfires. Immediately after fire in a mature mountain ash forest, seed stored in woody capsules in the crowns of the trees is released onto the exposed soil surface. Hundreds of thousands of seeds germinate per hectare. The intense competition between the plants for light results in rapid tree growth in the young stand and the weaker trees are soon shaded out and die. This natural thinning of the stand proceeds quickly at first and continues for the life of the stand but at an ever-decreasing rate. Gaps formed by thinning are not occupied by new mountain ash trees but instead fill with shrubs and small trees such as Acacia spp. By the time the forest is over 200 years old, the trees exceed 80 m in height and large gaps of up to 80 m in width are formed in the canopy. Significant understory appears after about 15 years of age and is strongly developed by age 80 years. In the absence of fire, mountain ash would disappear from a site within 500 years.
A significant body of empirical evidence shows that the amount of water yield from mountain ash forest catchments is related to stand age, suggesting inter alia, changes in evapotranspiration (ET) over time (Langford, 1976; Kuczera, 1987; Watson et al., 1999). An idealised curve was developed by Kuczera (1985) to describe the relationship between mean annual water yield and stand age for mountain ash forest (Fig. 2). The curve combines the known hydrologic responses of eight large (14–900 km2) basins to a major fire in 1939, and assumes a pure mountain ash forest cover. The ‘Kuczera curve’ is characterised by the following features:
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The mean annual water yield from large catchments covered by pure mountain ash forest in an old-growth state (>200 years) is about 1195 mm for a region with mean annual rainfall of about 1800 mm.
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After burning and full regeneration of the mountain ash forest with young trees, the mean annual water yield reduces rapidly to 580 mm by age 27 years.
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After age 27 years, mean annual water yield slowly returns to pre-disturbance levels, taking as long as 150 years to recover fully.
Silvicultural treatment experiments entailing clearfelling and natural regeneration of mountain ash-forested small catchments have revealed water yield responses to disturbance which are qualitatively similar to the Kuczera curve, at least for the first 30 years after treatment (Watson et al., 1999). One difference is that water yields are elevated above the pre-treatment average for the first five to eight years after treatment as the forest regenerates. However, after this time, water yields decline below pre-treatment levels by a factor of almost half, as predicted by the Kuczera curve. The small catchments analysed by Watson et al. (1999) exhibit a range of responses due to the fact that not all of the treatments were imposed on old-growth forest, and because of extraneous influences such as insect defoliation which affected some catchments but not others. However, the longest and most relevant experiment of these, the Coranderrk experiment, clearly shows a response similar to the Kuczera curve (Fig. 3). The magnitude of the runoff changes at Coranderrk are smaller than those predicted by the Kuczera curve, because Coranderrk has a mean annual rainfall of <1200 mm, as opposed to the region considered by Kuczera (1985) which had a mean annual rainfall of about 1800 mm.
Various studies have examined age-dependent patterns of rainfall interception (Langford and O′Shaughnessy, 1978; Haydon et al., 1996), fog drip (O′Connell and O′Shaughnessy, 1975), xylem water potential and stomatal conductance (Legge, 1980), sap flow (Dunn and Connor, 1993), sapwood area development (Jayasuriya et al., 1993), and leaf area development (Watson and Vertessy, 1996) in these forests. Only Haydon et al. (1996) have attempted to draw such data together into a time-varying water balance for mountain-ash forest. However, their study did not account for soil/litter evaporation, nor transpiration from understory vegetation. In this paper, we build upon their efforts by reporting additional data, and by relating water balance changes to changes in leaf area index in the forest. We conclude by presenting a time series for all water balance components for the 1800 mm isohyet in the study area. Our water balance estimates follow a trend qualitatively consistent with the Kuczera curve, though the magnitudes of runoff differ.
Section snippets
Site description
All field measurements were undertaken in the Maroondah catchments, located approximately 80 km northeast of the city of Melbourne, Australia (Fig. 1). Mean annual rainfall across these sites ranges between 1200 and 2800 mm, depending on elevation, and to a lesser extent, aspect. The rainfall is mainly of low intensity and is evenly distributed throughout the year, though there is a slight spring maximum. Mean monthly temperatures vary between: 2 and 6°C in winter, and 15 and 18°C in summer. Mean
Leaf area index
Total leaf area index LAItot, mountain ash LAI (LAIash), and understory leaf area index LAIund all change appreciably as the mountain ash forest ages (Fig. 4). LAItot rapidly rises to 5.4 by age 7 years, then declines steadily to 3.6 by age 240 years. LAIash rises to about 4.0 at age 15 and decreases to about 1.3 at age 235 years, indicating a roughly threefold difference between old-growth and re-growth stands. LAIund was not calculated for the first five years as Eq. (2) erroneously predicts
Discussion
Major hydro-ecologic changes occur during the life cycle of mountain ash forests which have significant consequences for water yield, and the water supply systems dependent on runoff from these forests. We have demonstrated that old-growth mountain ash forests yield significantly more water than young re-growth forests of the same species because of lower ET. We have estimated the difference in ET for 15- and 240-year old forests to be 460 mm per year for sites with 1800 mm annual rainfall.
Acknowledgements
Funding for this study was provided by the Cooperative Research Centre for Catchment Hydrology. Melbourne Water kindly provided us with access to their field sites and data sets, as well as valuable logistic support. We thank D. McJannet, J. Buckmaster, J. Beringer, S. Davis, R. Campbell, R. Benyon, S. Haydon, J. Snodgrass, I. Watson, C. Pfeiffer, T. Hatton, P. Gribben, J. Morris, N. Tapper, D. Dunkerley, H. Elsenbeer, J. Margules and J. Brophy for helping us unlock some of the hydrologic
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