Institute for Space Sciences, Free University of Berlin, Berlin, 12165, Germany.
Glob Chang Biol. 2014 Dec;20(12):3727-42. doi: 10.1111/gcb.12664. Epub 2014 Aug 1.
Photosynthesis simulations by terrestrial biosphere models are usually based on the Farquhar's model, in which the maximum rate of carboxylation (Vcmax ) is a key control parameter of photosynthetic capacity. Even though Vcmax is known to vary substantially in space and time in response to environmental controls, it is typically parameterized in models with tabulated values associated to plant functional types. Remote sensing can be used to produce a spatially continuous and temporally resolved view on photosynthetic efficiency, but traditional vegetation observations based on spectral reflectance lack a direct link to plant photochemical processes. Alternatively, recent space-borne measurements of sun-induced chlorophyll fluorescence (SIF) can offer an observational constraint on photosynthesis simulations. Here, we show that top-of-canopy SIF measurements from space are sensitive to Vcmax at the ecosystem level, and present an approach to invert Vcmax from SIF data. We use the Soil-Canopy Observation of Photosynthesis and Energy (SCOPE) balance model to derive empirical relationships between seasonal Vcmax and SIF which are used to solve the inverse problem. We evaluate our Vcmax estimation method at six agricultural flux tower sites in the midwestern US using spaced-based SIF retrievals. Our Vcmax estimates agree well with literature values for corn and soybean plants (average values of 37 and 101 μmol m(-2) s(-1) , respectively) and show plausible seasonal patterns. The effect of the updated seasonally varying Vcmax parameterization on simulated gross primary productivity (GPP) is tested by comparing to simulations with fixed Vcmax values. Validation against flux tower observations demonstrate that simulations of GPP and light use efficiency improve significantly when our time-resolved Vcmax estimates from SIF are used, with R(2) for GPP comparisons increasing from 0.85 to 0.93, and for light use efficiency from 0.44 to 0.83. Our results support the use of space-based SIF data as a proxy for photosynthetic capacity and suggest the potential for global, time-resolved estimates of Vcmax .
陆地生物圈模型的光合作用模拟通常基于 Farquhar 模型，其中羧化作用的最大速率 (Vcmax) 是光合作用能力的关键控制参数。尽管 Vcmax 已知在空间和时间上会因环境控制而发生很大变化，但它通常在模型中用与植物功能类型相关联的表格值进行参数化。遥感可用于生成对光合作用效率的空间连续和时间分辨的视图，但基于光谱反射率的传统植被观测缺乏与植物光化学过程的直接联系。相反，最近基于卫星的太阳诱导叶绿素荧光 (SIF) 的空间测量可以为光合作用模拟提供观测约束。在这里，我们表明，来自太空的冠层顶部 SIF 测量值对生态系统水平的 Vcmax 敏感，并提出了一种从 SIF 数据反演 Vcmax 的方法。我们使用土壤-冠层观测光合作用和能量 (SCOPE) 平衡模型得出季节性 Vcmax 和 SIF 之间的经验关系，这些关系用于解决反问题。我们使用基于空间的 SIF 反演在位于美国中西部的六个农业通量塔站点评估我们的 Vcmax 估计方法。我们的 Vcmax 估计值与玉米和大豆植物的文献值（分别为 37 和 101 μmol m(-2) s(-1) 的平均值）吻合较好，并显示出合理的季节性模式。通过与使用固定 Vcmax 值的模拟进行比较，测试了更新的季节性变化 Vcmax 参数化对模拟总初级生产力 (GPP) 的影响。与通量塔观测的验证表明，当使用 SIF 从时间分辨的 Vcmax 估计值进行模拟时，GPP 和光能利用效率的模拟显著改善，GPP 比较的 R(2) 从 0.85 增加到 0.93，光能利用效率从 0.44 增加到 0.83。我们的结果支持将基于卫星的 SIF 数据用作光合作用能力的代理，并表明有可能进行全球、时间分辨的 Vcmax 估计。
