Climate dependent variations in leaf respiration

植物在调节大气中二氧化碳（CO2，一种重要的温室气体）浓度方面起着至关重要的作用。决定大气CO2浓度和全球变暖程度的关键是植物的呼吸速率；在全球范围内，植物呼吸每年向大气中释放近60亿吨碳。与化石燃料燃烧产生的相对较小的二氧化碳释放（每年< 6亿吨摄氏度）相比，这是一个很大的通量。为了预测未来的大气二氧化碳浓度和全球表面温度，我们需要准确地模拟植物呼吸如何对气候做出反应。目前，大多数气候模型假设植物呼吸以一种简单的、可预测的方式对气候作出反应。例如，温度每升高10摄氏度，呼吸速率就会增加一倍。此外，假定在相同温度下测量的叶片呼吸速率在光照下与在黑暗中是相同的。然而，越来越多的证据表明，这两种假设都不正确。例如，我们知道呼吸的温度反应是高度动态的，并根据温度的长期变化进行调整（即呼吸“适应”）。此外，我们知道光线会抑制叶片呼吸，尤其是在白天温度较高的情况下。如果不能考虑到呼吸的这种动态响应，在预测净生态系统碳交换率时就会出现很大的误差；在植物呼吸占总碳交换很大比例的低生产力生态系统中，这种误差可能特别重要。因此，在预测未来气候变化对生物圈的影响的生态系统气体交换模型中考虑呼吸的这种动态变化是至关重要的。要做到这一点，我们首先需要仔细量化植物呼吸如何对气候变化做出反应（无论是在黑暗中还是在光明中）。这些数据可以用来建立数学方程，将植物呼吸变化的影响纳入碳交换模型，从而准确预测单个生态系统和全球范围内当前和未来的植物碳交换速率。我们的研究将集中于气候对三种低生产力森林生态系统（地中海旱地、北方常绿针叶林和北方落叶针叶林）叶片气体交换的影响，这三种森林生态系统在夏季经常经历较高的日间叶温，并且接近正生长的阈值；叶片呼吸的微小变化可导致这些森林从大气C的净吸收者转变为C的净排放者。目前，我们对气候依赖的叶片呼吸变化如何影响低生产力森林的净CO2交换率的了解有限。特别是，在炎热的夏季，光抑制叶片呼吸在决定净CO2交换速率中的作用尚不清楚。我们的研究将量化由于季节性适应和光照抑制而导致的叶片呼吸变化对这些低生产力生态系统中树木整体植物碳平衡的重要性。因此，除了为提高气候模型的预测能力提供必要的数据外，我们的研究还将首次确定叶片呼吸在确定全球普遍存在的低生产力生态系统的碳经济和增长方面的定量重要性。由于所获得的知识将基于一般原则，我们也将能够将这些发现应用于其他生态系统，在这些生态系统中，叶片呼吸可能是生态系统净二氧化碳交换的重要决定因素（例如，热带森林中光照有限的下层植物）。在这样的环境中，整个植物碳平衡的微小变化可能对幼苗的生长和存活产生重要影响。

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.