Defense varies during leaf development in tropical seedlings, shedding light into herbivory-mediated patterns of biodiversity

Background/Question/Methods

Many tropical forest tree species delay greening their leaves until full expansion, either by delayed chlorophyll synthesis or delayed development of the chloroplasts themselves. This strategy is thought to provide newly flushing leaves with protection against damage by herbivores by keeping young leaves devoid of nutritive value. Because young leaves suffer the greatest predation from invertebrate herbivores, delayed greening could prevent costly tissue loss. Many species that delay greening also produce anthocyanin pigments in their new leaves, giving them a reddish or bluish tint. These anthocyanins may be fungicidal, protect leaves against UV damage, make leaves cryptic to herbivores blind to the red part of the spectrum, or even attract herbivores, diverting them from more costly older leaves. Here, we undertake a comprehensive survey of seedlings, saplings and mature trees in a hyper-diverse forest in western Amazonia and ask the following questions: (i) Do species with seedlings that delay greening also delay greening as trees? (ii) Do species with delayed leaf greening have higher survival than those that do not delay greening their leaves?

Results/Conclusions

From the most comprehensive survey of juvenile leaf coloration in tropical trees to date we found that out of 588 identified seedling species, delayed greening was found in 37% of species (52% of individuals): 30% had red leaves and 22% had pale leaves. Among saplings, we determined juvenile leaf color for 380 species. Delayed greening was found in over 50% of these: 30% had red leaves and 20% had pale leaves. These proportions are toward the high end for forest communities in the Neotropics. Further, we found a significant effect of juvenile leaf color on 1-yr survival of first-year seedling recruits. Species with delayed greening had a significantly higher probability of survival to their second year of life than species without delayed greening. This survival advantage remained in more established seedlings also (>1 yr), but only for those species with red juvenile leaves, and the survival advantage was less. Defense against herbivory in newly expanded leaves exerts strong selective pressure on young seedlings, and a delayed greening strategy appears to improve seedling survival and influence the species composition of later stages