Eamus Derek
Institute for Water and Environmental Resource Management, University of Technology, Sydney, PO Box 123, Broadway, NSW 2007, Australia. Email:
Funct Plant Biol. 2003 Feb;30(2):187-205. doi: 10.1071/FP02084.
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.
澳大利亚是一个非常干旱的大陆，一年中大部分时间里，该国大部分地区降雨稀少、气温较高且蒸发量大。澳大利亚的净初级生产力（NPP）也很低。这两个现象可能存在关联，而且在流域尺度上，生态系统水平衡与净初级生产力之间肯定存在相关性。本文表明这些假设可能是正确的，然后回顾了生态系统水平衡可能决定净初级生产力的机制。生态系统水平衡的测量和量化并不总是容易的，人们已经采用了几种测量方法，涉及降雨、潜在蒸散、土壤含水量及相关参数的各种比率。本文将简要讨论这一主题。生态系统水平衡通过在几个尺度上产生影响来影响净初级生产力。生态系统水平衡会影响叶片的几个属性，特别是比叶面积和叶片含氮量。此外，光合组织和非光合组织之间的碳和氮分配、光合作用速率和气孔导度也会受到影响。在更大的尺度上，整棵树的水力传导率以及生物量在根、茎和叶质量之间的分配会受到生态系统水平衡的影响，并进而影响净初级生产力。呼吸速率以及根、茎和叶呼吸的相对贡献会随着生态系统水平衡而变化，因此也会影响净初级生产力。最后，林冠层和林分尺度上的属性也会对流域水平衡做出响应并影响净初级生产力。本文将结合木本生态系统来讨论这些机制。此外，我认为干旱地区的幼龄林分在功能和机制上与湿润地区的老龄林分相同。
