Crown-based growth strategies explain the distribution and co-occurrence of tree functional groups along environmental gradients
Elucidating interspecific variation in growth strategies across environments is critical for understanding tree diversity and distribution. However, a systematic analysis across diverse forests remains scarce due to limited large-tree crown measurements. We analysed 3,814 trees from 107 canopy species in 23 natural forests using forest census data and unmanned aerial vehicle. Relative growth rate (growth rate/biomass) was decomposed into growing space efficiency (growth rate/crown area) and crown area ratio (crown area/biomass). Relative growth rates were similar among functional groups (deciduous broadleaf, evergreen broadleaf, evergreen conifer), but their underlying components differed. Deciduous trees exhibited 34-43% higher crown area ratio but 24-27% lower growing space efficiency than evergreen broadleaf and conifer trees. Across environments, growing space efficiency correlated positively with mean annual temperature, likely from extended growing seasons while crown area ratio correlated negatively, particularly in deciduous species. Snow loading limited crown area ratio in evergreen species, while deciduous achieved higher values through avoiding snow loads. Correlation between crown area ratio and specific leaf area indicated a coordinated strategy between crowns and leaves. Different functional groups achieve comparable growth efficiency either through crown expansion or biomass production, explaining their distinct distribution as well as co-occurrence across broad environments.