Water Use by Perennial Evergreen Plant Communities in Australia and Papua New Guinea

As the first step in an attempt to paint a broad picture of the growth of perennial evergreen plant communities in Australia and Papua New Guinea, a model of water use by these ecosystems has been deveioped with use of monthly climatic data. The leaf area index of such communities in Australia appears to remain relatively constant throughout the year. Monthly water balance observations on seven perennial evergreen plant com- munities - in regions ranging from arid to humid in southern Australia - reveal a linear relationship between the ratio of community evapotranspiration to pan evapora- tion (Ea/Eo) and the water store (W) for that month (monthly precipitation plus extractable soil water at the beginning of the month (P + Sext)): Ea/Eo = k(P + Sext) . As the community matures towards the climax condition, the coefficient k in this relationship is shown to approach a maximal value in which extractable soil water (Sext) approaches zero but rarely, if ever, equals zero during the driest time of the year. The coefficient k provides an index of the maximal complexity of structure and physiology possible in plant communities growing in the particular region. The linear relationship appears to hold until the water store (W) approaches saturation, when Ea/Eo tends to equal or slightly exceed 1.0. These water use equations have been applied to the climatic data of 347 climatic stations in Australia and 10 in Papua New Guinea, on the assumptions that: (1) all precipitation enters the soil; (2) the topography is level; (3) the plant community is evergreen and perennial with a relatively constant leaf area index. Maps have been prepared showing the maximal values of k ; the mean annual value of Ea/Eo (when drainage results as soon as Sext exceeds 5.0 or 30.0 cm, arbitrarily chosen low and high values respectively), and the optimal amount of extractable soil moisture which may be held in the soil of the region. The influence of various environmental and biotic factors on the water balance operating in the ecosystem is explored. The effects of topography and insolation, run-off/run-on, microdistribution of precipitation, rainfall interception and stem flow, variation in distribution and amount of extractable soil water, waterlogging, root distribution, and seasonal changes in foliage projective cover have been investigated from ecological studies reported in the literature. The foliage projective cover of climax evergreen plant communities is shown to increase linearly as the maximal value of k increases from region to region. Provided internal drainage does not occur, the maximal value of the coefficient k for any climatic station is not affected by the addition or subtraction of water by run-on or run-off; the mean annual value of Ea/Eo increases linearly from zero as the water entering the soil increases. In contrast, the effect of topography on insolation induces an inverse response in both the maximal value of k and the mean annual value of Ea/Eo as radiation (and pan evaporation) increases. The feasibility of using the monthly values of Ea/Eo to estimate growth of perennial evergreen plant communities is discussed.