Leaf Volatile Emissions Structure Tree Community Assembly and Mediate Climate Feedbacks in Tropical Forests

Abstract

The biochemistry of leaves merges the fates of trees and the atmosphere. Leaf primary metabolism cycles carbon and indirectly drives atmospheric circulation via the latent heat of transpiration. Tropical forests contain half of global forest carbon, and actively cycle carbon and energy year round, making them critical components of the coupled biosphere-climate system. Climate change threatens tropical forests with rising temperatures and increasing variability of precipitation. Their response will influence future biodiversity as well as the fate of the climate. Understanding the physiological attributes that define tropical tree responses and feedbacks to climate is a current research priority. The emission of isoprene gas from plant leaves has been demonstrated to enhance leaf tolerance to high temperatures and drought. Isoprene is a volatile secondary metabolite produced in the chloroplast by approximately one-third of plant species. While the benefits of isoprene are supported by extensive laboratory and greenhouse-based research, work has only begun to explore how the trait is integrated in plant functional strategies. Whether isoprene influences differential species performance and survival across environments has yet to be tested. An impediment to filling this clear ecological research gap has been a lack of instrumentation capable of quantifying isoprene emissions from leaves in remote field settings. The first study presented here tests the hypothesis that isoprene emission influences plant community assembly shifts across environmental gradients and through time in tropical forests. The capacity for a species to produce isoprene was associated with increased relative abundance at higher temperatures and following drought anomalies. A negative relationship with the length of seasonal drought suggests a trade-off between isoprene emission and other plant traits, such as deciduous leaf habit. The second study presents the development of a new instrument that is uniquely optimized for field-based ecological research on leaf volatiles. The new system, named PORCO (Photoionization of Organic Compounds), utilizes custom leaf cuvettes, precision light control, and an optimized commercial photoionization detector to achieve real-time detection of leaf emissions with detection limits better than 0.5 nmol m⁻² leaf s⁻¹. The third study utilizes PORCO to test hypotheses about the structuring of isoprene within plant functional strategies and across forest microenvironments in an eastern Amazonian evergreen tropical forest. The results support the role of isoprene—and potentially other volatile isoprenoids—in mitigating effects of intermittent sun exposure in the sub-canopy. Emissions are structured in a complex, multivariate manner that depends on taxonomy, leaf and wood characteristics, tree height, and light environment. The results from this dissertation work demonstrate that isoprene emission from leaves affects plant responses to climate at ecologically relevant scales. Isoprene influences climate not only by its effect on primary leaf functions, but also by directly altering atmospheric chemistry, and contributing to aerosol and cloud properties. Understanding isoprene's role in forest responses to increasing temperatures and drought will help to predict the feedbacks between forest ecosystems and climatic change.