Ra. Vertessy et al., Factors determining relations between stand age and catchment water balance in mountain ash forests, FOREST ECOL, 143(1-3), 2001, pp. 13-26
There is a well-documented empirical relationship between stand age and wat
er yield for mountain ash (Eucalyptus regnans) forested catchments in the C
entral Highlands of Victoria, Australia. Catchments covered with old-growth
stands of mountain ash yield almost twice the amount of water annually as
those covered with re-growth stands aged 25 years. In this paper, we provid
e a mechanistic hydro-ecologic explanation for this phenomenon. We measured
leaf area index (LAI), sapwood area index (SAI) and various water balance
components in several mountain ash stands, ranging in age between 5 and 240
years. Sap flow measurements show that sap velocity does not vary apprecia
bly amongst stands of different ages, but a systematic decline in SAI with
age produces a concomitant decrease in stand transpiration. The decline in
overstory SAI is accompanied by a decline in overstory LAI. Understory LAI
increases as the overstory LAI decreases, but this layer transpires at only
about 63% of the mountain ash rate on a per unit leaf area basis. Hence, w
hile total stand LAI decreases are modest over time, the trend is for a sig
nificant decline in total stand transpiration as the forest ages. Rainfall
interception also declined over time and there was some indication that int
erception per unit leaf area also declined. Such reductions can be explaine
d by lesser turbulent mixing and elevated humidity around the bulk of the l
eaf area in the mature forest. There were small decreases in forest floor e
vaporation through time, though this only accounted for about 5-8% of the s
ite water balance. Our water balance measurements agree qualitatively with
empirical water yield relationships developed for mountain ash forests, tho
ugh the magnitudes of change differ, (C) 2001 Elsevier Science B.V. All rig
hts reserved.