Warming tundra and the emission of volatile organic compounds

Current climate warming in Arctic regions is driving changes in the structure and composition of tundra ecosystems. This is causing shifts in local plant communities and changes the physiological stress that plants experience during a growth season. One response to increasing temperatures is system wide increases in the amount of reactive gases, so-called volatile organic compounds (VOCs), that plants release to the atmosphere. The increasing VOC flux from the Arctic tundra to the atmosphere may have implications via climate feedbacks, for example, through particle and cloud formation in these regions with low anthropogenic influence. We know that the release of VOCs from vegetation is both temperature-dependent and controlled by vegetation composition because different plant species release a distinct blend of VOCs. Hence, the interplay between such pathways from climate warming to plant VOC emissions are important in our general understanding of how Arctic ecosystems are responding to a changing climate.

In a recent paper published in PNAS we outline the presence and relative importance of two causal path ways from local temperature to plant VOC emissions. Using both spatial hierarchical correlation models and ecosystem dynamics models we quantify the direct (plant stress) and indirect (structuring vegetation cover) effect of temperature on VOC emission in the Arctic.

The Arctic tundra vegetation at the islands of Disko in West Greenland is responding rapidly to climate warming. Causing a shift and increase in the volatile organic compounds released by the vegetation.

The study builds on several years of warming experiments and dynamic ecosystem modelling work done at the University of Copenhagen by Riikka Rinnan and Jing Tang. Using data from 11 years of monitoring at four Arctic sites we show that temperature is simultaneously changing VOC emissions rates directly and indirectly via vegetation composition. However, within individual groups of compounds the direct effect was in most cases larger compared the indirect.

Local correlations between temperature and volatile organic compound emissions. Top left: Camber measurements of VOC emission for a given vegetation type.  Top right: The conceptual model tested via a structural equation model, in which temperature and soil moisture affect VOC emissions directly or indirectly by structuring the vegetation cover. Bottom: Structural equation models representing direct and indirect linkages of environmental factors on VOC emission. An example of a structural equation model for monoterpene emission. Solid arrows represent significant linear paths supported by the model; dashed lines are omitted paths. Values represent standardized effect sizes.

These findings were mirrored at larger scales, using a process-based dynamic ecosystem model for the compounds in the isoprene and monoterpenes groups. By manipulating the presence of plant establishment, mortality, disturbance, and growth, as well as soil biogeochemical processes in response to input climate variability, we compared two different scenarios: One in which warming only affects the VOC production rate and emission, but without warming-induced vegetation changes (direct effects), and one in which warming affects both the VOC production and emission, as well as vegetation dynamics (direct + indirect effects). 

 From this we show that warming alone caused large increases in annual isoprene and monoterpene emissions averaged across the Pan-Arctic region, with larger increases for 4 °C than 2 °C warming. Including indirect temperature effects (e.g., via phenology, vegetation dynamics, and plant physiological processes under a warmer climate allowing for longer growing seasons) further enhanced this increase, but again with relatively smaller magnitude compared to the direct warming effects.

Relative changes in isoprene emission under direct and indirect effects of warming by 2 °C (A and B) and 4 °C (C and D). A and C show the direct effect on isoprene production and emission rate, and (B and D) show the direct + indirect effects mainly through changes in vegetation composition and vegetation-related processes averaged for the period 1999–2012. Notice the large local differences in B and D.

In summary, we show that ongoing warming has strong direct increasing effects on VOC emissions from Arctic ecosystems and also indirect effects resulting from alterations in vegetation composition and biomass. Exactly what this means for local-to-regional impacts on atmospheric composition is still to be understood. However, forecasting how plant communities will change in response to climate change is challenging and our work outline the complexity of the mechanisms driving Arctic VOC emissions.

Paper reference:

Rinnan R., Iversen L. L., Tang J., Vedel-Petersen I., Schollert M. & Schurgers G. (2020): Separating direct and indirect effects of rising temperatures on biogenic volatile emissions in the Arctic. PNAS. DOI: 10.1073/pnas.2008901117