Physical defence traits enhance seedling survival of neotropical tree species

1. Physical defence traits of stems and leaves should enhance biomechanical strength and survival of seedlings. For eight neotropical tree species that differ widely in life-history strategies, we compared stem and leaf biomechanical traits of 1 and 7-month-old seedlings grown in the shaded forest understorey and in the laboratory. 2. Material traits (biomechanical traits per unit volume, mass or cross sectional area) were positively associated with seedling survival across species. Shade tolerant species that survive well in the forest understorey had stems and leaves with greater modulus of elasticity (stiffness), fracture toughness (resistance to tear), tissue density and fibre contents, compared to less shade tolerant species. 3. Seedling survival was most strongly correlated with stem tissue density at both 1 and 7 months (Spearman's correlation coefficient r(s) = 0.93 and 0.90), but was also strongly correlated with leaf density and stem toughness at 7 months (r(s) = 0.93 and 0.89, respectively). 4. Multiple material traits were strongly and positively correlated with each other in both stems and leaves. However, these traits varied independently of seed and seedling size among species, indicating the unique importance of physical defence as functional traits. 5. Structural traits of stems that integrate size with material traits, including % critical buckling height, flexural stiffness, work-to-bend and stem flexibility, showed no significant interspecific correlation with seedling survival. 6. Modulus of elasticity and fracture toughness of stems generally increased as seedlings aged from 1 to 7 months, especially in species with high tissue density. In contrast, fracture toughness of leaf mid-vein and lamina showed inconsistent ontogenetic changes across species. 7. These results demonstrate that biomechanical traits including tissue density and fracture toughness should be considered as important functional correlates of seedling survival and overall life-history strategies of tree species.