For three forest canopies (a sparse, boreal needleleaf; a temperate broadleaf; and a dense, tropical, broadleaf stand) light-use efficiency (LUE) is found to be 6–33% higher when sky radiance is dominated by diffuse rather than direct sunlight. This enhancement is much less than that reported previously for both crops (110%; Choudbury, 2001) and moderately dense temperate woodland (50–180%). We use the land-surface scheme JULES to interpret the observed canopy response. Once sunflecks and leaf orientation are incorporated explicitly into the scheme, our simulations reproduce convincingly the overall level of canopy gross photosynthetic product (GPP), its enhancement with respect to diffuse sunlight and the mean 15% reduction in productivity observed during the afternoon due to stomatal closure. The LUE enhancement under diffuse sunlight can be explained by sharing of the canopy radiation-load, which is reduced under direct sky radiance. Once sunflecks are accounted for the advantage of implementing more sophisticated calculations of stomatal conductance (e.g. Ball–Berry and SPA submodels) is less obvious even for afternoon assimilation. Empirical relations are developed between observed carbon flux and the environmental variables total downwelling shortwave radiation (SW), canopy temperature (T) and the fraction of diffuse sky radiance (fDIF). These relations allow us to gauge the impact of increased/reduced insolation on GPP and net ecosystem exchange (NEE). Overall the three stands appear to be fairly stable within global trends and typical interannual variability (SW changing by <15%). Greatest sensitivity is exhibited by the boreal site, Zotino, where NEE falls by 9±4% for a 15% reduction in SW.