Forest floor VOC emissions in a mountain pine–juniper stand: magnitudes, composition, and implications for ecosystem-scale fluxes

Volatile organic compound (VOC) emissions from forests are typically attributed to tree canopies, while the forest floor remains comparatively understudied despite its complex mixture of litter, soil microbes, and understory vegetation. Here we report first results from a pilot study, in which we measured forest-floor VOC fluxes in a mountain pine–juniper (Pinus sylvestris, Juniperus communis) stand, at the Forest Atmosphere Interaction Research (FAIR) site of the University of Innsbruck in Mieming, Austria. The site is characterized by a relatively open tree canopy dominated by pines, and a forest floor that is almost completely covered by vegetation, including dominant juniper individuals. We compared mean soil emissions with ecosystem-scale VOC fluxes obtained at the same site during a period shortly after the soil measurements, to assess the relative contribution of the forest floor to whole-ecosystem VOC exchange.

Using a dynamic chamber approach coupled to online VOC detection using Proton Transfer Reaction Mass Spectrometry (PTR-MS), we quantified forest floor emissions at two locations: (i) a site dominated by pine needle litter and mosses, and (ii) a site where moss and litter co-occurred with heather (Erica herbacea), some Polygala chamaebuxus and various grasses (Sesleria ssp., Carex ssp.) understory.

Both sites emitted substantial amounts of terpenoid compounds, such as monoterpenes and sesquiterpenes, but also isoprene, as well as oxygenated compounds such as methanol, acetaldehyde and acetone. The emissions were temporally variable and differed between the two micro-sites, consistent with differences in biological composition, substrate and meteorological conditions. While the exact sources cannot be resolved from these measurements alone, plausible contributors include microbial activity within soil and the litter–moss layer, as well as root and shoot emissions from understory shrubs.

A comparison of the forest floor VOC fluxes with the total ecosystem exchange revealed that the mean soil fluxes of many VOCs were on the order of a few to about 40 % of the respective ecosystem fluxes. Strikingly, for sesquiterpenes the soil emissions at both microsites exceeded ecosystem-scale fluxes by about a factor three and seven, respectively. This discrepancy suggests substantial within-canopy loss processes (e.g., rapid oxidation or deposition) and/or differences in temporal representativeness between the datasets.

These findings expand the known role of the forest floor as an active VOC source and suggest that bottom-up budgets that focus solely on canopy emissions may underestimate ecosystem-scale fluxes, especially under conditions favorable to microbial or understory vegetation activity.