Relationship between leaf flushing phenology and defensive traits of canopy trees of five dipterocarp species in a tropical rain forest

While the canopy layer shows the highest productivity in forests, it also has the highest herbivore population density. How do canopy trees cope with attack from herbivores under such conditions? We investigated the relationship between leaf flushing phenology, defensive and photosynthetic traits, such as leaf flushing frequency, the number of emerging leaves per flushing term, the leaf life span, total phenolic and condensed tannin contents, leaf toughness, leaf nitrogen content, the photosynthetic rate at light saturation (Pmax) and leaf mass per area (LMA), in canopy trees of five dipterocarp species in a Malaysian tropical rain forest. Interspecific variations were clearly observed in leaf flushing frequency, ranging from occasional to continuous patterns. The total phenolic content significantly increased with leaf flushing frequency. Leaf toughness showed no correlation with leaf flushing phenology, but species with occasional leaf flushing had greater leaf toughness than those with continuous leaf production. There was a negative correlation between leaf toughness and tannin content, and a positive correlation between the former and Pmax. In addition, the leaf herbivory rate significantly increased with a larger number of emerging leaves per term and a higher comprehensive index (PC1) from PCA analysis using leaf flushing frequency and the number of leaves. Therefore, our results suggest that tropical canopy species have various defensive strategies against herbivore attack by regulating the intensity of chemical, physical and phenological defenses; species with high leaf flushing frequency have more chemically defended leaves, while those with low leaf flushing frequency have tougher leaves with higher photosynthetic abilities.