Cellulose-based toughness, but not silica- or phenolic-based defense, explains species differences in herbivory and leaf lifespan of tropical tree seedlings

Background/Question/Methods

It is widely recognized that herbivory resistance and leaf lifespan are enhanced by carbon-based physical defense, such as fiber-based toughness, and carbon-based chemical defense, such as phenolics and tannins. In contrast, silica-based defense is expected to enhance hardness, but its relationship with herbivory resistance and leaf lifespan is yet to be examined in most angiosperms except for grasses. Main questions: How do cell-wall fiber contents, toughness, phenolics, tannin and silica concentrations correlate with species difference in herbivory rates and leaf lifespan among tropical tree species? How are these traits affected by leaf age and light environments?  Methods: Seedlings of 24 tropical tree species were grown from seeds in field common gardens established in gaps and understory of a tropical moist forest in Panama. Leaf lifespan was determined from monthly census of marked leave for 4 years (median leaf lifespan 140-867 days in gap; longer in shade). Leaf toughness was quantified with a cutting test. Also determined were lamina dry mass density, lamina thickness, concentrations of cell wall fibers (hemicellulose, cellulose, lignin), phenolics, tannins and silica. 

Results/Conclusions

Lamina density, but not toughness, exhibited plastic response to gaps vs. shade, while neither trait was affected by leaf age. Leaf lifespan was positively correlated with cellulose and toughness in shade-tolerant species but only weakly in gap-dependent species. The relationship between cellulose per unit mass and density-corrected toughness was universal across gap-dependent and shade tolerant species. Leaf lifespan was independent of lamina thickness, concentrations of lignin, phenolics, tannin and silica. Dry mass density and cellulose contents were the only traits correlated with proportion of leaves showing herbivory damage. Path analysis shows that leaf lifespan was directly enhanced by density and toughness, and indirectly by cellulose via its direct effect on toughness. Different suits of leaf traits were correlated with early seedling survival in gaps vs. shade. Conclusion: Cellulose and lamina density synergistically enhance leaf fracture toughness, and this, rather than phenolics, tannins and silica concentrations explains species differences in herbivory, leaf lifespan and shade tolerance of tropical tree species.