Solar‐Induced Fluorescence Does Not Track Photosynthetic Carbon Assimilation Following Induced Stomatal Closure

Since 2006, six satellites measuring solar‐induced chlorophyll fluorescence (SIF) have been launched to better constrain terrestrial gross primary productivity (GPP). The promise of the SIF signal as a proxy for photosynthesis with a strong relationship to GPP has been widely cited in carbon cycling studies. However, chlorophyll fluorescence originates from dynamic energy partitioning at the leaf level and does not exhibit a uniformly linear relationship with photosynthesis at finer scales. We induced stomatal closure in deciduous woody tree branches and measured SIF at a proximal scale, alongside leaf‐level gas exchange, pulse amplitude modulated (PAM) fluorescence, and leaf pigment content. We found no change in SIF or steady‐state PAM fluorescence, despite clear reductions in stomatal conductance, carbon assimilation, and light‐use efficiency in treated leaves. These findings suggest that equating SIF and photosynthesis is an oversimplification that may undermine the utility of SIF as a biophysical parameter in GPP models. Plain Language Summary Earth's vegetation plays a key role in storing carbon that would otherwise reside in the atmosphere. Recently, there has been increasing interest in measuring fluorescent light emitted by the chlorophyll in plant cells in order to track carbon uptake. Satellite fluorescence measurements show a strong, direct relationship with primary productivity. However, leaf‐level chlorophyll fluorescence studies have yielded insights into the origin of this signal as one of several pathways by which plants consume excess absorbed light. At finer scales, fluorescence emission may become inversely related to photosynthetic rate, due to the additional role of heat dissipation as an alternative pathway for plants to partition energy. To investigate the contradiction between measurements across scales, we experimentally manipulated tree branches, inhibiting photosynthesis by closing the stomata through which plants exchange water and carbon dioxide gases. We observed significant reductions in leaf‐level gas exchange in treated branches but found no similar change in fluorescence measured at the leaf level or from a proximal tower. While fluorescence offers physiological insights, we suggest that the close relationship with primary productivity at the satellite scale could result from a shared driver, such as chlorophyll content and that fluorescence data should be interpreted with care. Key Points Leaf‐level chlorophyll fluoresce