Climate dependent variations in leaf respiration

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Plants play a vital role in regulating the concentration of atmospheric carbon dioxide (CO2, an important greenhouse gas). Critical in determining atmospheric CO2 concentrations and magnitude of global warming is the rate of respiration exhibited by plants; globally, near 60 Gt C per year is released into the atmosphere by plant respiration. This is a large flux compared with the relatively small release of CO2 from the combustion of fossil fuels (< 6 Gt C per year). To predict future atmospheric CO2 concentrations and global surface temperatures, we need to accurately model how plant respiration responds to climate. At present, most climate models assume that plant respiration responds to climate in a simplistic, predictable manner. For example, respiration is assumed to double in rate for every 10oC rise in temperature. Moreover, rates of leaf respiration are assumed to be the same in the light as in darkness, when measured at the same temperature. There is, however, growing evidence that neither assumption is correct. For example, we know that the temperature response of respiration is highly dynamic and adjusts to long-term changes in temperature (i.e. respiration 'acclimates'). Moreover, we know that light inhibits leaf respiration, particularly at high daytime temperatures. Failure to account for such dynamic responses of respiration results in large errors in predicted rates of net ecosystem C exchange; such errors are likely particularly important in low productivity ecosystems where plant respiration represents a large proportion of overall C exchange. It is vital, therefore, that such dynamic variations in respiration be accounted for in ecosystem gas exchange models predicting the impacts of future climate change on the biosphere. To do this, we need to first carefully quantify how plant respiration responds to variations in climate (both in darkness and in the light). This data can then be used to develop mathematical equations that allow the effects of variations in plant respiration to be incorporated into C exchange models that accurately predict current and future rates of plant C exchange, both in individual ecosystems and globally. Our research will focus on the effects of climate on leaf gas exchange of three low productivity forest ecosystems that are globally widespread (Mediterranean dryland, boreal evergreen conifer and boreal deciduous conifer forests), often experience high day-time leaf temperatures during the summer and operate close to the threshold of positive growth; minor changes in leaf respiration can result in such forests switching from net absorbers of atmospheric C to net emitters of C. At present, our understanding of how climate dependent variations in leaf respiration impact on rates of net CO2 exchange of low productivity forests is limited. In particular, the role light inhibition of leaf respiration in determining rates net CO2 exchange on hot summer days is unknown. Our research will quantify the importance of variations in leaf respiration due to seasonal acclimation and inhibition in the light on overall plant carbon balances of trees growing in these low productivity ecosystems. Thus, in addition to providing the data necessary for improving the predictive power of climate models, our research will establish for the first time the quantitative importance of leaf respiration in determining the C economy, and thus growth, of these globally widespread low productivity ecosystems. Because the knowledge obtained will be based on general principles, we will also be able to apply the findings to other ecosystems where leaf respiration potentially represents an important determinant of net ecosystem CO2 exchange (e.g. light-limited under-storey plants in tropical forests). In such environments, small changes in whole plant carbon balances may have important consequences for seedling growth and survival.