How does ecosystem water balance affect net primary productivity of woody ecosystems?
Eamus, Derek
Australia is a very dry continent, with low levels of rainfall and high temperatures and evaporative demand for much of the continent for much of the year. Net primary productivity (NPP) of Australia is also low. These two observations are presumably related, and, at a catchment scale, correlations between ecosystem water balance and NPP surely exist. This paper shows that these assumptions are probably correct, and then reviews the mechanisms by which ecosystem water balance may determine NPP. The measurement and quantification of ecosystem water balance is not always easy, and several measures, involving various ratios of rainfall, potential evapotranspiration, soil water content and related parameters, have been used. This topic is briefly discussed. Ecosystem water balance influences NPP through effects at several scales. Ecosystem water balance influences several leaf attributes, especially specific leaf area and foliar nitrogen content. In addition, carbon and nitrogen partitioning between photosynthetic and non-photosynthetic tissues, rates of photosynthesis and stomatal conductance are affected. At the larger scale, whole-tree hydraulic conductance and allocation of biomass to root, stem and leaf mass are affected by ecosystem water balance and influence NPP. Respiration rates and the relative contributions of root, stem and leaf respiration vary with ecosystem water balance and therefore influence NPP. Finally, attributes at the canopy and stand scales also respond to catchment water balance and influence NPP. These mechanisms are discussed in relation to woody ecosystems. In addition, I propose that stands of young trees on xeric sites behave functionally and mechanistically the same as old stands on mesic sites.
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