Connections between leaf traits, phenological types, and isoprene emissions

The Amazon rainforest is the world’s largest producer of isoprene, a gas emitted mostly by tree leaves. Isoprene protects trees from various sources of stress. It also affects our atmosphere and climate by contributing to the production of particles that can form clouds.

Trees in the central Amazon forest have different ways of producing leaves, known as leaf phenological types. Some trees keep their leaves all year and slowly and continuously produce new ones. They are called evergreen trees. Others massively drop their leaves – remaining bare for a few weeks – and produce new leaves at a given time of the year. These are called brevideciduous trees.

For a new study, Michelle Robin and her colleagues wanted to see if these differences in leaf phenological types influence whether or not trees emit isoprene, and how much isoprene they emit. To investigate this, the team measured leaf isoprene emissions at standard light and temperature conditions. They also recorded other leaf morphological, physiological and chemical characteristics, called traits, for 175 trees from 124 different tree species. This is not an easy task, considering that most of the trees they measured were higher than 20 m! But the help of an incredible team of tree climbers and field assistants made it possible.

With their measurements, the scientists could find a couple of interesting connections:

  • in evergreen trees isoprene emissions were associated with leaves that invest in mechanical defenses: Tougher leaves that do not photosynthesize as much and do not store monoterpenes (a chemical defense-related compound).
  • in brevideciduous trees higher isoprene emissions were associated with leaf traits that indicate investments in chemical defenses: High isoprene emissions occurred coupled with higher amounts of sesquiterpenes and phenolic compounds.

These findings suggest that the way isoprene emissions are associated with mechanical or chemical defenses varies between the different leaf phenological types in the central Amazon rainforest. Understanding these relationships can help scientists improve current and future predictions of isoprene fluxes from the Amazon forest to the atmosphere. This is extremely important considering the importance of isoprene emissions. They play major roles in tree survival (by protecting against many stresses) and climate regulation (by participating in the formation of clouds). Thereby, these results help to further assess how current global climate changes will have different effects on isoprene emitters and non-emitters, and how this will consequently impact the Earth’s climate.

Robin et al. published the study “Interactions between leaf phenological type and functional traits drive variation in isoprene emissions in central Amazon forest trees” Open Access in the journal Frontiers in Plant Science.

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