Growth patterns and climate sensitivity differ for understory and canopy trees.

While vertical forest structure is known to influence microclimatic conditions and tree growth, detailed knowledge of differences in seasonal growth dynamics between understory and canopy trees, especially at fine temporal scales, is still lacking. In this study, we compared growth dynamics of five temperate tree species (Picea abies, Fagus sylvatica, Carpinus betulus, Acer campestre, Ulmus laevis) based on sub-daily measurements from automatic band and pivotal dendrometers across four old-growth forests. Hourly growth probabilities were modelled using Generalized Linear Mixed-Effects Models (GLMMs) in relation to climatic conditions and day phases. Understory trees exhibited shorter growing seasons, accumulated fewer growing hours and displayed a less distinct diel growth rhythm than canopy trees. Nonetheless, when growth occurred, their growth rate per growing hour, normalized by stem diameter, was higher. However, overall growth remained constrained, likely because light availability limited the frequency or duration of such favorable periods, preventing consistent utilization of the buffered microclimatic conditions typical of the forest understory. The GLMMs successfully captured growth dynamics in canopy trees but performed poorly for understory trees, indicating lower dependency of their growth patterns on climatic conditions. Overall, our results show that understory tree growth is constrained primarily by limited light availability rather than by atmospheric conditions, leading to fewer growth periods that account for a larger proportion of the annual increment compared with canopy trees.