Plants and soils, especially those in the Amazon rainforest, emit most of the biogenic volatile organic compounds (BVOCs) found in the atmosphere. There they affect many processes, such as the formation of secondary organic aerosols, which in turn influence the Earth’s radiative budget. However, few studies thus far looked into emissions from insects such as termites.
BVOCs in the Amazon
Nora Zannoni and her colleagues measured BVOC emissions at the ATTO tall tower in several heights. Specifically, they looked at one particular BVOC called α-pinene. This organic compound exists in two mirror image forms called (−)-α-pinene and (+)-α-pinene. Such chiral forms are identical in all physical properties. However, insects and plants can distinguish between these forms and may prefer to interact with one or the other. And indeed, few existing studies show that the ratio between the chiral compounds varies regionally.
This is also true for ATTO, where (+)-α-pinene are generally rarer. Now the scientists discovered further variations between those mirror image forms. At ATTO they measured how the BVOC emissions behaved over the course of the day. They found that when plants start photosynthesizing in the morning, BVOC emissions increase. To put it simply, above the canopy they reach their maximum abundance in the afternoon, when temperatures are highest and plants are most active. This is especially true for (−)-α-pinene, confirming that its main source is the forest.
But the story is a little different for (+)-α-pinene. Their concentration declines after a morning peak. Furthermore, the team found that (−)-α-pinene are the dominant form above 80 m height. The otherwise less common (+)-α-pinene, however, is more abundant at 40 m height, which coincides with the average canopy height. This shows that chiral BVOCs at ATTO are neither equally abundant nor is the ratio of the two forms constant over time, season or height.
Termites hold a surprise in store
But this change in the chiral ratio at 40 m height also indicates an addition source for BVOCs. Nora Zannoni and her co-authors considered many different explanations and performed additional measurements to solve the mystery. They found one other place, where (+)-α-pinene was more abundant: above nests of termites.
Around the ATTO tall tower is a rather large gap in the canopy. Here we find many fast growing plants, which continuously flush new leaves. Insects, such as termites, are known to like to feed on such new leaves and thus might be more abundant near the tall tower. Therefore, the emissions from the nest might affect the abundance of (+)-α-pinene at 40 m height.
To assess how significant a role the termites really play, the research team must study them in more detail in the future. For example, they plan on sampling isolated termite species. This will also be helpful to find out if the insects themselves are responsible for the change in the prevalent α-pinene form, or insect-plant interactions may also play a role.
Zannoni et al. published the study “Surprising chiral composition changes over the Amazon rainforest with height, time and season” Open Access in the new Nature Journal Communications Earth & Environment.