Direct and Indirect Effects of Wildfire Smoke on Leaf-Level Biogenic Volatile Organic Compound Emissions in a Montane Coniferous Forest

As extreme weather events increase in severity and frequency, so do wildfires; our understanding of these events is vital to predicting future climate conditions more accurately. Previous work has characterized emissions of chemical constituents of wildfire smoke and the role of ecosystem biomass burning emissions of biogenic volatile organic compounds (BVOC), but the direct and indirect effects of wildfire smoke on leaf-level emissions remain largely unexplored. Here we investigate changes in photosynthesis and BVOC emissions for ponderosa pine needles resulting from several smoke events over the summers of 2020 and 2021. During these field studies, this montane coniferous forest was exposed to varying degrees of smoke intensity and extent. We investigate those leaf-level emissions by coupling a portable photosynthesis system with three mass spectrometric instruments: proton-transfer reaction, iodide chemical ionization, and thermal desorption gas chromatography. We find that following a prolonged period of intense smoke where photosynthesis remains negligible, stomates can be forced open, creating a burst of BVOC emissions on a leaf level (e.g., formic acid and methyl salicylate), and a sudden release of smoke constituents including nitrophenol. Additional findings show that smoke-exposed leaves can also exhibit altered emission behavior - for example with monoterpene fluxes decreasing at higher temperature. We explore several potential mechanisms behind these findings, including stomatal and non-stomatal uptake, surface deposition, and defensive behaviors. Our findings show that the presence of transported wildfire smoke has substantial effects on leaf-level emissions of reactive organic carbon.