Monoterpene and sesquiterpene emissions increase with forest degradation and land use change in the Amazon Arc of Deforestation

Biogenic Volatile Organic Compounds (BVOCs) are primarily emitted into the atmosphere by plants. These compounds serve various functions, including cellular protection and defense at the leaf level, chemical signaling between and within plants, and regulation of large-scale biogeochemical processes, such as influencing atmospheric chemistry and contributing to aerosol formation. The Amazon Forest is the major source of BVOCs to the global atmosphere.  Over the past four decades, multiple studies have measured BVOC concentrations in the air, mainly focusing on central Amazonia. However, while there is much to be investigated in undisturbed forests, the Amazon is already undergoing changes in land use and climate, particularly in the Amazon Arc of Deforestation. These changes may affect BVOC emissions and associated processes at the biosphere-atmosphere interface in ways that are not yet fully understood. In this light, this study aimed to identify and quantify the main BVOCs emitted by trees and crops in a changing Amazon region. We measured the above-canopy BVOC concentrations and leaf-level BVOC emissions from crops (cotton and corn) and dominant tree species in a mosaic of disturbed forest fragments and agricultural fields in southeastern Amazonia during the wet and dry seasons of 2023. Surprisingly, our results revealed that monoterpene and sesquiterpene emissions were higher than isoprene emissions for most trees and crops. When we compared the same tree species across a gradient of forest degradation, we found that monoterpene and sesquiterpene emissions were up to three times higher in the most degraded forest areas. Furthermore, with a leaf temperature curve experiment, we observed that at 45°C, the amount of recently assimilated carbon emitted in the form of isoprene, monoterpenes, and sesquiterpenes were up to 35%, 5%, and 23%, respectively - suggesting that plants were losing a high amount of carbon to cope with the heat stress. In contrast to leaf-level measurements, our ambient air measurements indicated that monoterpene and sesquiterpene concentrations were significantly lower than isoprene concentrations during both the wet and dry seasons, indicating that the atmosphere in this region is very reactive and that only leaf-level measurements are likely to give us a true measure of monoterpene and sesquiterpene emissions. Yet, interestingly, sesquiterpene concentrations were higher in the dry season than in the wet season, supporting the leaf-level results showing that increased heat and drought may lead to higher emissions of sesquiterpenes. This may have occurred either because plants emit more monoterpenes and sesquiterpenes in response to stress or due to changes in plant species composition resulting from forest degradation and land use changes. This study presents the first observations of BVOCs conducted in the Amazon Arc of Deforestation at both the leaf and canopy levels. The observed shift in emissions towards monoterpenes and sesquiterpenes is likely modifying atmospheric chemical and physical processes and the carbon balance in this already changing Amazon region. This makes it crucial to include these changes in air quality and Earth system modeling.