Combined Effects of Air Humidity and Temperature on Biogenic Volatile Organic Compound Emissions and Photosynthesis

Abstract Temperature and air humidity are critical environmental factors regulating photosynthesis and biogenic volatile organic compound (BVOC) emissions from plants, which influence air quality and climate change. While previous research has demonstrated the impacts of temperature on photosynthesis and BVOCs, the effects of humidity and the combined effects of temperature and humidity remain understudied. Here we discuss the discrete and synergistic impacts of temperature and humidity on ponderosa pine trees. We used a portable photosynthesis system coupled to a proton‐transfer‐reaction mass spectrometer to quantify leaf‐level changes in photosynthesis, stomatal conductance, and emissions as a function of both temperature and humidity. Results demonstrate that all BVOC emissions investigated increased with temperature, regardless of humidity, in agreement with literature. To our knowledge, we present the first direct observation of temperature‐dependent methyl vinyl ketone and methacrolein emissions from ponderosa pines. We find that elevated humidity enhances the baseline emissions of many BVOCs. Increasing relative humidity from 30% to 50% resulted in basal emission rates increasing between 1.7‐fold for sesquiterpenes and 2.9‐fold for 2‐methyl‐3‐buten‐2‐ol. Our results may help explain field observations where BVOC emissions are decoupled from temperature under some conditions, and we clearly illustrate the need for further investigations on BVOC humidity sensitivity over larger (e.g., ecosystem) scales and on other plant species to improve chemical transport model predictions.