A thermal tolerance trait distinguishes microclimatic niches of tropical trees

A third of tropical tree species emit isoprene, a trait associated with enhanced leaf thermal tolerance. We have previously shown that isoprene emitting tropical trees (IE) photosynthesize to higher temperatures than non-emitters (NE), and that IE increase in relative abundance toward warmer climates. Isoprene may affect tree composition shifts under climate change, altering forest function and directly affecting climate via the strong influence of tropical forest isoprene emissions on atmospheric aerosol and cloud formation.

We test whether the capacity to emit isoprene differentiates tree species' demographic rates and microenvironmental niche associations in a tropical forest. We combine new species measurements of isoprene emissions with long-term forest inventory data from Barro Colorado Island (BCI), Panama. Individual tree crown light environments (1=high light to 3=deep shade) are estimated using a 'perfect plasticity approximation' approach, whereby relative crown positions are modeled from stem coordinates and sizes. Contrary to common expectation, we find that IE are significantly associated with partial-illumination (layer 2) crown environments (chi-squared, p < 0.001), and midstory maximum tree sizes (multinomial regression, p < 0.05), and are underrepresented among large, canopy-surface trees (layer 1). This finding is consistent with the hypothesis that isoprene emission, as an on-demand stress-response trait, is selective for dynamically variable environments, where leaf temperatures vary rapidly with intermittent direct sun exposure. IE are concentrated in intermediately mesic habitats, compared to swamp or dry plateau, and did not differ in drought-related mortality. Across the 35 yr time series, IE had lower sapling recruitment rates (to 1 cm diameter) than NE, and even lower relative mortality rates, resulting in a 16% increase in the proportion of IE trees at BCI since 1980.

Understanding the ecology of isoprene emission informs the role of photosynthetic stress responses in tropical forest demography. We conclude that tropical IE are defined by slow demographic rates and tolerance to environmental variability. Variation in the proportion of emitters—and thereby forest emission capacity—may depend not only on mean climate conditions, but also on forest structure.