When do clouds and aerosols lead to higher canopy photosynthesis?

• Unexplained variation in canopy photosynthesis calls for a revised approach. • New guidelines enable consistent quantification of the diffuse fertilization effect. • First observation of how cloud type influences photosynthesis enhancement. • Aerosols and thin clouds enhance photosynthesis more frequently than thick clouds. • Site characteristics modulate photosynthesis enhancement at multiple scales. Clouds and aerosols can increase canopy photosynthesis relative to clear-sky values through changes in total and diffuse solar radiation: the diffuse fertilization effect (DFE). DFE varies across observational sites due to (a) inconsistent definitions and quantifications of DFE, (b) unexplored relationships between DFE and cloudiness type, and (c) insufficient knowledge of the effect of site characteristics. We showed that: DFE definitions vary, DFE quantifications do not connect to existing definitions or do not isolate the causal factor, and a systematic protocol to quantify DFE is lacking. A new theoretical framework served to clarify the relation between DFE definitions, and showed how DFE varies with cloudiness types and site characteristics. We proposed guidelines for a systematic DFE quantification across studies, and which aim to isolate the causal factor of DFE. Applying our framework to observations of canopy photosynthesis, solar radiation and cloudiness types we quantified DFE at daily and sub-daily time scales. We showed for the first time how DFE varies with cloudiness type, due to the varying trade-off between diffuse radiation and total solar radiation. Using an observation-driven canopy photosynthesis model, we showed that the DFE varies with site characteristics and time of day. The DFE responded strongly to leaf area index, canopy nitrogen distribution, leaf orientation and leaf transmittance, with leaf area index and leaf orientation driving DFE occurrences at our site. Our study emphasizes the importance of quantifying the DFE systematically and accurately across observational sites and highlights the need for information on cloudiness climatology and site characteristics.