<jats:title>Abstract</jats:title><jats:sec><jats:title>Aims</jats:title><jats:p>Light availability varies drastically in forests, both vertically and horizontally. Vertical light heterogeneity (i.e., patterns of light attenuation from the forest canopy to the floor) may be related to light competition among trees, while horizontal light heterogeneity (i.e., variations in light intensity at a given height within forests) may be associated with light‐niche partitioning among tree species. However, light heterogeneity in vertical and horizontal directions and their associations with forest structure are rarely studied to date. Here we report the first comprehensive study to compare the vertical and horizontal light heterogeneity in differently aged forests in two forest types.</jats:p></jats:sec><jats:sec><jats:title>Location</jats:title><jats:p>Twelve forest stands of different ages in cool‐temperate forests (consisting of deciduous broad‐leaved trees) and five of different ages in warm‐temperate forests (evergreen conifer and deciduous broad‐leaved trees) in Japan.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>We measured vertical light profiles at 1‐m intervals from the understorey (1 m above the ground) to the top canopy (12–22 m depending on stands) at 16 locations for each stand (20 m × 20 m). We also measured structural parameters (diameter at breast height, height, and crown dimensions) for all major trees in these stands.</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>Along the secondary successional gradients, the vertical and horizontal light heterogeneity changed in a systematic manner in both forests. The vertical light attenuation rate was steeper in early succession and more gradual in late succession, and the horizontal light heterogeneity was relatively small in early succession and more pronounced in late succession. The vertical light attenuation rate was different between the two forest types; the light intensity dropped more sharply from the canopy surface in the cool‐temperate forests due to the crown being vertically shorter and denser (i.e., higher leaf density per unit volume).</jats:p></jats:sec><jats:sec><jats:title>Conclusion</jats:title><jats:p>In early succession, a steeper light attenuation rate is likely related to the strong light competition among co‐occurring trees and thus a self‐thinning process. In late succession, the high spatial light heterogeneity in forests (i.e., larger horizontal light heterogeneity and gradual light attenuation rate) may allow more species to partition light, and thus may enhance species coexistence and diversity.</jats:p></jats:sec>