A novel feature shaping ecosystems in the Anthropocene is rapid climatic change. Forests and climate are altering each other through biophysical feedbacks. Tropical forests are key players: they are exceptionally diverse, store significant amounts of carbon, and influence atmospheric processes via year-round physiological activity. How tropical trees will respond to novel climates is highly uncertain. Understanding their response requires knowledge of the plant traits that mediate climatic stress and how those traits vary across species and environments.
A potentially key plant trait that is understudied in the field is the emission of volatile isoprenoids (VI, a class of secondary metabolites) from leaves. Emission of VI is responsive to light and heat. Laboratory and greenhouse studies show that VI production extends photosynthetic activity to greater temperatures and water stress—putatively due to an antioxidant role—and influences the plant carbon balance. Efforts to study this trait in the field have been limited in part by instrumentation. We developed a new high-precision, field-mobile system for standardized quantification of leaf-level VI emissions. The new measurement system was used to determine covariation of VI emissions with other axes of forest function at a tropical primary forest site in the eastern Amazon.
Results/Conclusions
Our new detection system demonstrated clear emission-rate distinction between plant species, with measurement precision and detection limit both better than 0.5 nmolVI m-2leaf s-1. Standardized leaf emission rates were quantified alongside determination of taxa, tree heights, light environments, wood traits, and leaf traits. Preliminary results from over 200 leaves and 80 trees show that taxonomy was the most important predictor of emissions. The strongest emitters were found in the mid canopy, supporting the putative role of VI in mitigating intermittent stress (e.g., sun and heat flecks). Some shaded understory trees showed unexpectedly high emission potentials. The relationship between leaf emission potential and specific leaf area (SLA; fresh leaf area / dry mass) describes an envelope with a decreasing upper bound on emission potential as SLA increases (quantile regression: 85th quantile, p<0.01). That result is consistent with a carbon economics constraint on leaf emissions.
This work provides a foundation for a predictive model of isoprenoid emission capacity among tropical trees that depends on environmental factors, plant traits, and phylogeny. Integrating this new axis into our knowledge of plant strategies could help us understand differences in plant responses to climate variability, and improve our predictions of forest resilience and climate feedbacks.