Isoprene emissions differentially mediate defense responses in brevideciduous and evergreen central Amazon Forest trees

The Amazon basin contains over half of the world’s tropical forests and stores up to 200 Pg of carbon, yet increasing climate-driven stress events are shifting it from a carbon sink to a net carbon source. Plant volatile isoprenoids (i.e., isoprene, monoterpenes, and sesquiterpenes) play key roles in tree stress tolerance, defense strategies, and forest-atmosphere interactions. These compounds also influence atmospheric chemistry by promoting secondary organic aerosol and cloud formation, thereby affecting climate processes. This doctoral thesis investigated how volatile isoprenoid emissions vary with leaf functional traits, phenological strategies, and environmental drivers in central Amazon Forest trees, and whether emission potential can be predicted using leaf spectroscopy. Leaf-level measurements revealed a chemical-mechanical defense tradeoff across phenological strategies. In brevideciduous trees, higher isoprene emission factors were linked to greater diversity of stored sesquiterpenes and higher phenolic content, indicating coordinated chemical defenses. In evergreen trees, isoprene emission was associated with higher leaf toughness and carbon-to-nitrogen ratios, alongside reduced probability of terpene storage, suggesting a predominance of mechanical defenses. Hyperspectral reflectance models predicted isoprene emission and mono- and sesquiterpene storage with accuracies of 0.65-0.72. Emissions of all compounds increased with leaf temperature, but brevideciduous trees showed stronger sesquiterpene increases. Current global emission models substantially overestimated canopy-level isoprene fluxes, especially for evergreen trees. Overall, the results demonstrate that incorporating species- and phenology-specific variability is essential for improving volatile isoprenoid flux estimates. The findings support the hypothesis that stress responses in central Amazon trees arise from coordinated strategies of leaf turnover and volatile production.