A quantitative top-down view of interactions between stresses: theory and analysis of nitrogen-water co-limitation in Mediterranean agro-ecosystems

The multiple factors constraining the growth, reproduction, and survival of diverse organisms are often non-additive. Research of interacting factors generally involves conceptual models that are specific for target organism, type of stress, and process. As a complement to this reductionist, bottom-up view, in this review I discuss a quantitative top-down approach to interacting stresses based on co-limitation theory. Firstly, co-limitation theory is revised. Co-limitation is operationally identified when the output response of a biological system (e.g. plant or population growth) to two or more inputs is greater than its response to each factor in isolation. The hypothesis of Bloom, Chapin, and Mooney, that plant growth is maximised when it is equally limited by all resources, is reworded in terms of co-limitation and formulated in quantitative terms, i.e. for a given intensity of aggregate stress, plant growth is proportional to degree of resource co-limitation. Emphasis is placed on the problems associated with the quanti. cation of co-limitation. It is proposed that seasonal indices of nitrogen and water stress calculated with crop simulation models can be integrated in indices accounting for the aggregated intensity of water and nitrogen stress (S-WN), the degree of water and nitrogen co-limitation (C-WN), and the integrated effect of stress and co-limitation (SCWN = C-WN/S-WN). The expectation is that plant growth and yield should be an inverse function of stress intensity and a direct function of co- limitation, thus proportional to SCWN. Secondly, the constraints imposed by water and nitrogen availability on yield and water use efficiency of wheat crops are highlighted in case studies of low-input farming systems of south-eastern Australia. Thirdly, the concept of co-limitation is applied to the analysis of (i) grain yield responses to water-nitrogen interactions, and (ii) trade-offs between nitrogen- and water-use efficiency. In agreement with theoretical expectations, measured grain yield is found to be proportional to modelled SCWN. Productivity gains associated with intensification of cropping practices are interpreted in terms of a trade-off, whereby water-use efficiency is improved at the expense of nitrogen-use efficiency, thus leading to a higher degree of resource co-limitation.