GCB | 观察并模拟植被和生态系统水分利用效率的历史趋势

摘要

植物水分利用效率 (WUE，通过光合作用每单位蒸腾损失的水分所获得的碳) 是有关植物生理上控制陆地生态系统与大气之间水和二氧化碳交换的跟踪指标。在叶片层面，CO₂浓度升高往往会增加碳的吸收 (在没有其他限制的情况下)，并降低气孔导度，这两种影响都会导致叶片WUE的增加。在生态系统层面，间接影响 (如叶面积指数增加、土壤节水) 可能会放大或减弱CO₂的直接影响。因此，叶片WUE的变化在多大程度上转化为生态系统规模的变化并不清晰。多项研究报告指出了叶片与生态系统WUE的增长幅度差异远远大于目前对树木生理和尺度的理解，这是个有待解决的问题。此外，目前的植被模型在叶片和生态系统WUE方面产生了不一致的、往往是不现实的变化幅度和模式，需要更好地评估潜在的方法。本文章中，我们综述了叶片到生态系统的连续尺度上WUE观测和模拟的历史趋势变化的原因，包括方法问题，目的在于阐明空间尺度内和跨空间尺度上观测到差异的原因。我们强调，尽管对环境驱动因素变化的生理反应需根据观察尺度进行不同的解释，但每个数据集都存在很大的不确定性，这些不确定性往往被低估。由植被模型对影响WUE的主要过程所作的假设强烈影响模拟的历史趋势。我们提出了改善基于长期观测的WUE估算的建议，这将更好地反映植被模型中的WUE。

表1  WUE的常用公式以及用于推断趋势的测量方法

图1 a. 长期叶片气体交换策略 — 对水资源利用效率趋势的影响

b. 从TR和EC的历史数据或由文献中的植被模型模拟得到的数据观测，推断出的ΔWUE与同一时期的大气CO₂浓度变化 (ΔCa) 的对比

Abstract

Plant water-use efficiency (WUE, the carbon gained through photosynthesis per unit of water lost through transpiration) is a tracer of the plant physiological controls on the exchange of water and carbon dioxide between terrestrial ecosystems and the atmosphere. At the leaf level, rising CO₂ concentrations tend to increase carbon up-take (in the absence of other limitations) and to reduce stomatal conductance, both effects leading to an increase in leaf WUE. At the ecosystem level, indirect effects (e.g. increased leaf area index, soilwater savings) may amplify or dampen the direct effect of CO₂. Thus, the extent to which changes in leaf WUE translate to changes at the ecosystem scale remains unclear. The differences in the magnitude of increase in leaf versus ecosystem WUE as reported by several studies are much larger than would be expected with current understanding of tree physiology and scaling, indicating unresolved issues. Moreover, current vegetation models produce inconsistent and often unrealistic magnitudes and patterns of variability in leaf and ecosystem WUE, calling for a better assessment of the underlying approaches. Here, we review the causes of variations in observed and modelled historical trends in WUE over the continuum of scales from leaf to ecosystem, including methodological issues, with the aim of elucidating the reasons for discrepancies observed within and across spatial scales. We emphasize that even though physiological responses to changing environmental drivers should be interpreted differently depending on the observational scale, there are large uncertainties in each data set which are often underestimated. Assumptions made by the vegetation models about the main processes influencing WUE strongly impact the modelled historical trends. We provide recommendations for improving long-term observation-based estimates of WUE that will better inform the representation of WUE in vegetation models.

▉  原文信息

本文编辑：

温正 | 北京信息科技大学，硕士