Optimal canopy light-use strategy shapes global greenness dynamics

"Greenness" is a key indicator of the functional state of vegetation. However, the physiological processes behind seasonal patterns in greenness are diverse and incompletely understood, hindering the predictability of climate-driven shifts in global foliage phenology. Optimality principles suggest that plants invest in canopy architecture to maximize light capture. Therefore, we hypothesize that, irrespective of specific physiological mechanisms, greenness (f_APAR: fractional canopy light absorption) commonly tracks the seasonal dynamics of potential production (A₀: theoretical canopy carbon uptake with all light absorbed). In other words, plants tend to display foliage when it is most productive. We show that observations confirm this hypothesis and develop a model predicting f_APAR from the seasonal cycle of A₀, with a phenological lag that increases (from 2 weeks to 3 months) with increasing moisture. This model captures 81% of observed variations in f_APAR and shows that light and environmentally regulated biophysical constraints shape global patterns of vegetation greenness, its seasonal cycle, and its recent increase.