Inter-specific variation of isoprene emissions in a Southeast Asian tropical rainforest: Links to light environments and atmospheric implications

Tropical forests are primary global sources of isoprene, a reactive volatile organic compound (VOC) influencing atmospheric chemistry and climate. Southeast Asian tropical forests, characterized by the dominance of dipterocarp species, play a significant role in regional emissions. However, a mechanistic understanding of how these unique forest ecosystems regulate emissions remains limited due to a scarcity of species-specific isoprene emission data. This data gap is crucial as these distinct ecosystems face increasing threats from disturbances. Such disturbances can alter microenvironments, notably light environments, leading to shifts in species composition that potentially modify regional VOC fluxes and complicate emission predictions. To address this knowledge gap, we measured leaf-level isoprene emissions from 10 ecologically important tree saplings in a lowland dipterocarp forest (Pasoh Forest Reserve). Our selection prioritized species inhabiting diverse light environments to explore relationships between emission capacity, photosynthetic traits, and in situ light conditions. Only 2 light-demanding Macaranga species (M. gigantea and M. hypoleuca) at disturbed forest edges consistently emitted isoprene. The remaining 8 species, including 2 canopy-dominant dipterocarps (Shorea leprosula and Neobalanocarpus heimii), showed no detectable emissions, a finding may help explain the relatively low regional isoprene fluxes previously observed at this site. The identification of these Macaranga species as isoprene emitters is relevant given that disturbances often create favorable light environments for such pioneers. Our case study, using historical tree survey data from a fire-disturbed area, suggests that Macaranga dominance following such disturbances could transiently create hot spots of elevated isoprene emissions. This further implies the importance of species-specific emission data, particularly for ecologically dominant or disturbance-responsive pioneer species, in assessing how forest composition and its shifts influence regional isoprene fluxes and atmospheric chemistry.