Stem growth phenology, not canopy greening, constrains deciduous tree growth.

This article investigates the temporal relationship between canopy phenology and stem growth phenology in four temperate deciduous tree species—three diffuse-porous species (Fagus sylvatica, Acer pseudoplatanus, Carpinus betulus) and one ring-porous species (Quercus robur × petraea)—at the Swiss-Canopy-Crane II site over four climatically contrasting years. The study found that in diffuse-porous species, stem growth resumption lagged several weeks behind spring canopy greening, whereas in the ring-porous species, the two events were mostly synchronous. Additionally, individual trees with earlier and shorter main growth periods exhibited higher annual stem increments than those with later and longer growth periods, challenging the assumption that longer growth duration leads to greater annual growth. These findings suggest that canopy greening alone is an unreliable predictor of stem growth phenology and that intra-specific variation in growth timing complicates the use of canopy phenology for modeling forest productivity.