Comment on essd-2022-136

<strong class="journal-contentHeaderColor">Abstract.</strong> The maximum rate of Rubisco carboxylation (<span class="inline-formula"><i>V</i>_cmax</span>) determines leaf photosynthetic capacity and is a key parameter for estimating the terrestrial carbon cycle, but its spatial information is lacking, hindering global ecological research. Here, we convert leaf chlorophyll content (LCC) retrieved from satellite data to <span class="inline-formula"><i>V</i>_cmax</span>, based on plants' optimal distribution of nitrogen between light harvesting and carboxylation pathways. We also derive <span class="inline-formula"><i>V</i>_cmax</span> from satellite (GOME-2) observations of sun-induced chlorophyll fluorescence (SIF) as a proxy of leaf photosynthesis using a data assimilation technique. These two independent global <span class="inline-formula"><i>V</i>_cmax</span> products agree well (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow><msup><mi>r</mi><mn mathvariant="normal">2</mn></msup><mo>=</mo><mn mathvariant="normal">0.79</mn><mo>,</mo><mi mathvariant="normal">RMSE</mi><mo>=</mo><mn mathvariant="normal">15.46</mn><mspace width="0.125em" linebreak="nobreak"/><mrow class="unit"><mi mathvariant="normal">µ</mi></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="129pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="1cb08b9869c7d3facff26f397ef438e1"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-14-4077-2022-ie00001.svg" width="129pt" height="15pt" src="essd-14-4077-2022-ie00001.png"/></svg:svg></span></span>mol m<span class="inline-formula">^−2</span> s<span class="inline-formula">^−1</span>, <span class="inline-formula"><i>P</i><i>&lt;</i>0.001</span>) and compare well with 3672 ground-based measurements (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><msup><mi>r</mi><mn mathvariant="normal">2</mn></msup><mo>=</mo><mn mathvariant="normal">0.69</mn><mo>,</mo><mi mathvariant="normal">RMSE</mi><mo>=</mo><mn mathvariant="normal">13.8</mn><mspace linebreak="nobreak" width="0.125em"/><mrow class="unit"><mi mathvariant="normal">µ</mi></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="123pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="e4b1cd6460782f3283fc52e81199344e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-14-4077-2022-ie00002.svg" width="123pt" height="15pt" src="essd-14-4077-2022-ie00002.png"/></svg:svg></span></span>mol m<span class="inline-formula">^−2</span> s<span class="inline-formula">^−1</span> and <span class="inline-formula"><i>P</i><i>&lt;</i>0.001</span> for SIF; <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow><msup><mi>r</mi><mn mathvariant="normal">2</mn></msup><mo>=</mo><mn mathvariant="normal">0.55</mn><mo>,</mo><mi mathvariant="normal">RMSE</mi><mo>=</mo><mn mathvariant="normal">18.28</mn><mrow class="unit"><mi mathvariant="normal">µ</mi></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="128pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="1e7590bac7c2176af37541ae765f8518"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-14-4077-2022-ie00003.svg" width="128pt" height="15pt" src="essd-14-4077-2022-ie00003.png"/></svg:svg></span></span>mol m<span class="inline-formula">^−2</span> s<span class="inline-formula">^−1</span> and <span class="inline-formula"><i>P</i><i>&lt;</i>0.001</span> for LCC). The LCC-derived <span class="inline-formula"><i>V</i>_cmax</span> product is also used to constrain the retrieval of <span class="inline-formula"><i>V</i>_cmax</span> from TROPical Ozone Mission (TROPOMI) SIF data to produce an optimized <span class="inline-formula"><i>V</i>_cmax</span> product using both SIF and LCC information. The global distributions of these products are compatible with <span class="inline-formula"><i>V</i>_cmax</span> computed from an ecological optimality theory using meteorological variables, but importantly reveal additional information on the influence of land cover, irrigation, soil pH, and leaf nitrogen on leaf photosynthetic capacity. These satellite-based approaches and spatial <span class="inline-formula"><i>V</i>_cmax</span> products are primed to play a major role in global ecosystem research. The three remote sensing <span class="inline-formula"><i>V</i>_cmax</span> products based on SIF, LCC, and SIF<span class="inline-formula">+</span>LCC are available at <a href="https://doi.org/10.5281/zenodo.6466968">https://doi.org/10.5281/zenodo.6466968</a> (Chen et al., 2022), and the code for implementing the ecological optimality theory is available at <span class="uri">https://github.com/SmithEcophysLab/optimal_vcmax_R</span> and <a href="https://doi.org/10.5281/zenodo.5899564">https://doi.org/10.5281/zenodo.5899564</a> (last access: 31 August 2022) (Smith et al., 2022).