Could drought stress in trees affect air quality?

With climate change, trees will experience an increased frequency of stress events (i.e. drought, heat stress) causing both direct effects (e.g. CO₂ uptake through photosynthesis) and indirect effects (e.g. isoprenoid emissions) on atmospheric chemistry. Stress effects on photosynthesis are relatively well-studied when compared to plant isoprenoid emissions. Isoprenoids are carbon-rich compounds, and many tree species, such as conifers, can release large quantities of volatile isoprenoids. Volatile isoprenoids can then react with other molecules in the atmosphere, altering atmospheric composition and chemistry, which can contribute to issues in air quality and pollution, cloud formation, and changes in the amount of heat energy retained in the atmosphere.

The emission of isoprenoids is quite variable, and it is relatively unknown whether trees show variability in isoprenoid emissions across populations in response to stress. In a recent (open access) article in Tree Physiology, Lüpke and colleagues sought to determine just that: they looked at whether different populations of Scots pine (Pinus sylvestris), a widely distributed conifer, show variability in isoprenoid emissions under non-stress conditions and in response to drought stress and recovery. They found that different populations of Scots pine have distinct isoprenoid ‘fingerprints’. These fingerprints are related to the types of isoprenoids that the trees emit (or do not emit), and which compounds make up a greater proportion of emissions.

What does this mean? Since the type and amount of isoprenoids emitted by trees affects regional atmospheric chemistry, any predictions on how a given forest will affect air chemistry and quality would need to take into account the isoprenoid fingerprint of trees in the forest, since even trace amounts can have disproportionate effects on air chemistry. The actual causes of these distinct fingerprints are unclear, however isoprenoids can function as a defense against insect pests, especially when emissions are high. So it may be possible to predict which forests will have the greatest effect on atmospheric chemistry based on whether and how well the tree species are adapted to contend with insect attack.