The role of mesophyll CO2 diffusion in modulating the response of photosynthetic carbon uptake to CO2 enrichment of a mature temperate forest

陆地生物圈捕获了人类活动释放的每10个二氧化碳分子中的3个。北半球成熟的温带森林的重要性突出，因为它们占全球陆地碳吸收量的约40%。光合作用作为绝大多数二氧化碳吸收的驱动因素，预计将随着大气二氧化碳浓度的增加而增加（e[CO2]）。然而，试图预测二氧化碳吸收率未来趋势的大规模模型是高度不确定的，因为我们对光合作用对二氧化碳的敏感性缺乏清晰的认识。这种不确定性在很大程度上可归因于叶片内二氧化碳扩散的未知性质。例如，将叶肉扩散过程纳入全球模型使预测的二氧化碳吸收率提高了约16%，这表明以前的估计可能大大低估了未来的碳吸收率。因此，对当前和未来陆地生物圈受二氧化碳可得性限制程度的估计，需要建立在对扩散过程、它们如何随环境条件变化以及它们如何适应环境变化的扎实理解的基础上。因此，掌握扩散过程的机制知识和准确预测它们在自然环境中的行为的工具，对于指导我们理解最重要的气候-碳循环反馈的规模至关重要。考虑到温带森林的全球重要性，伯明翰森林研究所于2017年在一个有160年历史的落叶林场建立了一个自由空气二氧化碳浓缩（FACE）实验（投资1500万英镑）。这是第一次在成熟的温带森林中进行这样的实验，因此提供了一个独特的机会来测试一个关键问题：成熟树木的光合作用在多大程度上受到二氧化碳供应的限制，以及我们必须在多大程度上考虑未来气候下二氧化碳扩散的适应？通过这个项目，我们将创造二氧化碳扩散的新知识，可以应用于一般的植物。我们将用它来检验一个广泛的假设，即在e[CO2]下，温带落叶森林的成熟树木将使它们的二氧化碳扩散能力与其碳需求相匹配。本研究将初步探讨在温室中生长的模式植物叶肉扩散的基本机制。我们将利用这些自身对全球碳循环做出巨大贡献的作物植物，来描绘叶肉扩散的个体特性，并确定它们对实验明确定义的环境条件（如二氧化碳浓度和温度）的响应。这将为我们提供二氧化碳扩散对这些环境参数的瞬时响应机制。然后，我们将使用二氧化碳控制的温室来测试在个别环境参数可以特定修改的条件下重要温带森林物种的适应能力。在最后一步，我们将测试森林树木在自然环境中适应其扩散特性的能力，这不仅包括e[CO2]的直接影响，还包括所有间接影响。总之，我们将使用尖端技术和数学模型来推进我们对植物碳吸收的定义过程的理解。通过在成熟的温带森林中进行的第一次FACE实验，我们将确定在当前和未来的环境下，树木的光合作用有多少是受二氧化碳限制的，从而解决碳循环模型中的一个主要不确定性。这对社会的影响是巨大的：如果我们的研究结果表明，由于叶肉扩散，光合作用继续保持二氧化碳的限制，这可以为我们减少温室气体排放赢得更多的时间。另一方面，如果植物能够完全适应这些过程来吸收[二氧化碳]，那么森林作为碳汇的贡献将受到更大的限制，因此我们将面临气候变化带来的更严重的后果。

The terrestrial biosphere captures about 3 in every 10 molecules of CO2 released by human activities. Mature temperate forests of the northern hemisphere stand out in their importance as they are responsible for ~40% of the global terrestrial carbon uptake. Photosynthesis, the driver for the vast majority of this CO2 uptake, is expected to increase in response to increased atmospheric CO2 concentrations (e[CO2]). However, large-scale models that attempt to predict future trends in CO2 uptake rates are highly uncertain because we lack a clear understanding of the sensitivity of photosynthesis to e[CO2]. Much of this uncertainty can be attributed to the unknown nature of CO2 diffusion inside the leaf. For example, incorporating mesophyll diffusion processes into global models increased predicted CO2 uptake rates by ~16%, which shows that previous estimates may have substantially underestimated future carbon uptake. Estimates of how much the current and future terrestrial biosphere is limited by the availability of CO2 therefore need to be grounded in a solid understanding of diffusion processes, how they vary with environmental conditions and how they may acclimate to changes in their environment. Thus, a mechanistic knowledge of diffusion processes and tools to accurately predict their behaviour in the natural environment are essential to guide our efforts to understand the magnitude of the most important climate-carbon cycle feedback.Given the global significance of temperate forests, the Birmingham Institute of Forest Research established a free air CO2 enrichment (FACE) experiment in a >160-year-old deciduous forest stand in 2017 (>£15M investment). This is the first such experiment in a mature temperate forest and thus provides a unique opportunity to test a key question: how much is photosynthesis in mature trees limited by the supply of CO2 and to what degree do we have to account for acclimation of CO2 diffusion under future climates? With this project we will create new knowledge of CO2 diffusion that can be applied to plants in general. We will use it to test the broad hypothesis that under e[CO2] mature trees of a deciduous temperate forest will match their CO2 diffusion capacity to their carbon requirement. The research will initially investigate the basic mechanism of mesophyll diffusion in model plants grown in a glasshouse. We will use these crop plants, which make a large contribution to the global carbon cycle in their own right, to delineate individual properties of mesophyll diffusion and determine their responses to experimentally well-defined environmental conditions, such as CO2 concentration and temperature. This will give us the mechanisms of the instantaneous responses of CO2 diffusion to these environmental parameters. We will then use CO2-controlled glasshouses to test the acclimation capacity of important temperate forest species under conditions where individual environmental parameters can be specifically modified. In a final step, we will test the capacity of forest trees to acclimate their diffusion properties in a natural environment, which includes not only direct but also all indirect effects of e[CO2].In summary, we will use cutting-edge technology and mathematical modelling to advance our understanding of a defining process in plant carbon uptake. Using the first FACE experiment in a mature temperate forest we will determine how much photosynthesis in trees is CO2-limited, both under current and future environments, addressing a major uncertainty in carbon cycle modelling. The implications for society are large: If our results suggest that photosynthesis continues to stay CO2-limited due to mesophyll diffusion this could buy us more time to reduce greenhouse gas emissions. If, on the other hand, plants can fully acclimate these processes to e[CO2] the contribution of forests as a carbon sink will be more restricted and we will thus face more severe consequences of climate change.