Sources of variability in isoprene emission and photosynthesis in two species of tropical wet forest trees

Isoprene (2-methyl-1,3-butadiene), a volatile organic compound produced by many plants, is the principal source of photochemically active reduced compounds in the troposphere. Emission from tropical forest trees accounts for > 70 percent of the annual global flux of isoprene, and under certain environmental conditions, trees may lose a large fraction of their fixed carbon to isoprene production. It is not known, however, if the production and emission of isoprene serve an adaptive role, or are factors that control isoprene emission from tropical trees well understood. We present results from a study investigating patterns of variability in isoprene emission and photosynthesis in two tropical wet forest tree species, Brosimum utile (H. B. K.) Pitt (Moraceae) and Dussia munda (Leguminosae/Fabaceae). We used leaf-level measurements to investigate the within- and among-individual variability in isoprene emission and photosynthetic rates, and looked at how these rates changed with light intensity. Leaves of both species showed similar responses to increases in light intensity, with photosynthesis appearing closer to saturation than isoprene emission at high light intensities. There was a large difference in both the photosynthetic and isoprene emission patterns of canopy and subcanopy leaves, with canopy leaves consistently showing much greater isoprene flux and photosynthetic rates than subcanopy leaves. This difference was smaller, although still discernible, when differences in specific leaf weight (SLW) were considered. These results suggest that tropical trees exhibit biochemical as well as structural responses to canopy position, with patterns of vertical variability that resemble those seen in temperate trees. These data have important implications for understanding the possible functional significance of isoprene emission and for creating accurate canopy-level isoprene flux models.