The Untold Story of the Understory: Direct Emission of Volatile Organic Compounds from Understory Species of a Ponderosa Pine Forest and Their Potential Role in Atmospheric Chemistry

Volatile organic compounds (VOCs) from terrestrial ecosystems can be oxidized in the atmosphere and drive regional atmospheric chemistry in terms of both reactivity and secondary organic aerosol formation. The ecological drivers behind these emissions are essential to understand to predict their atmospheric impact under changing climate conditions. Studies often focus on canopy-level emissions over forests or other terrestrial ecosystems using tower measurements to investigate fluxes of both the primary biogenic hydrocarbons and their secondary reaction products. Forest tower measurements can be coupled to leaf-level studies of dominant plant species to gain insight on the chemical processes occurring in the canopy atmosphere. These ecosystem-scale emissions of forests are typically attributed only to dominant tree species. However, the forest floor remains an important part of forest ecosystems, and understory plants are typically ignored by atmospheric chemistry studies, and their impacts on atmospheric chemistry may be underestimated.

The understory is notoriously challenging to measure volatile emissions. While many forest floor flux studies include emissions from both vegetation and soil, our approach observes direct emissions of volatile compounds using a small-volume chamber which encloses only the aerial parts of the plants. We conducted measurements at a semi-arid, montane ponderosa pine forest near Woodland Park, Colorado, where we collected volatile emissions from more than 10 representative understory species. Temperature, humidity, carbon dioxide, and light were all maintained using the chamber, ensuring that the collected emissions from each understory species are at comparable environmental conditions. VOCs were collected onto sorbent tubes for analysis with thermal desorption gas chromatography mass spectrometry. Between analytical standards and the NIST spectral library, we identified over 25 unique compounds emitted from the pine forest understory, including terpenes and oxidized species. These terpenoid compounds are distinct from those emitted by Ponderosa pines, representing an overlooked contribution to atmospheric reactivity.

In addition to the catalogue of emissions, we further highlight differences in the temperature dependence of various compounds from sage (Artemisia frigida), a prominent understory species at this field site. Using a randomized site selection approach, we conducted a ground-based survey of 10 patches of the forest located within the footprint of an on-going eddy covariance flux tower measurement. We calculate the understory fluxes of multiple terpenoid compounds and estimate the understory’s contribution to ecosystem-level monoterpene and total reactive organic carbon fluxes. With this, we provide one of the first quantitative assessments of understory flora contribution to organic carbon emissions at a forest site.