From suppressed to dominant: 3D crown shapes explain the “to grow or wait” growth behavior in close-to-nature forests

Crown plasticity enables trees to acclimate to competition and environmental stress, but how social strata and species-specific behavior modulate crown structure–secondary growth relationships remains poorly understood. We, therefore, tested three hypotheses in close-to-nature forests across wet (Germany) and dry (Spain) sites using terrestrial laser scanning and tree-ring width data of Norway spruce (Picea abies (L.) H. Karst.) and Silver fir (Abies alba Mill.) in Germany, and Stone pine (Pinus pinea L.) in Spain. We examined (HⅠ) crown structure and radial growth are positively correlated and modulated by social strata and species-specific behavior; (HⅡ) local competition and canopy space filling modulate the relationships across sites, (HⅢ) space-use efficiency varies across social strata and species behavior across sites. We observed that crown shape metrics and ring width variability were significantly correlated (p < 0.05), modulated by species and social identity. Dominant and codominant trees showed stronger relationships, while suppressed trees exhibited greater crown plasticity, allowing adaptive responses to canopy gaps. Spruce and fir adjusted crown structure under suppression, while pines maintained static crowns, decoupling structure from growth. Crown-growth relationships were influenced by neighborhood competition and canopy space filling (p < 0.05). Site-specific patterns emerged: wet sites had greater light competition, increasing canopy space filling and growth irregularity; dry sites were dominated by competition for water. Crown space- use efficiency also varied by species and social stratum. Dominant trees occupied more canopy space, but suppressed trees showed higher space-use efficiency especially by spruce and fir, by maintaining plastic crowns and waiting for more favorable light conditions. Despite similar biomass investment, dominant spruce and fir had higher space-use but lower biomass-use efficiency, while suppressed trees invested biomass more efficiently, signaling growth potential. These results show that asymmetric crown plasticity drives a grow or wait strategy across strata, informing precision forestry interventions for climate-adaptive forest management.