Optimal light use strategy explains seasonal dynamics and trends in vegetation greenness

“Greenness” is a key indicator of the functional state of vegetation. However, 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 plants invest in canopy architecture to maximize light capture. Therefore, we hypothesize, irrespective of specific physiological mechanisms, greenness (f_APAR: fractional canopy light absorption) tracks seasonal dynamics of potential production (A₀: theoretical canopy carbon uptake with all light absorbed). In other words, plants everywhere 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 time-lag increasing (from two weeks to three months) with moisture. This model captures 81% of observed variations in f_APAR and shows that light and environmentally regulated biophysical constraints drive global patterns of vegetation greenness, its seasonal cycle, and its recent increase.