Tropical forests, and the Amazon rainforest in particular, are the most important source of biogenic volatile organic compounds (BVOCs) to the atmosphere. The species isoprene dominates these emissions. Isoprene is a reactive BVOC species that only live minutes to hours in the atmosphere before it reacts chemically with other compounds, contributing to the formation of ozone and aerosols. Thereby, isoprene plays a key role in atmospheric processes and may have a significant impact on global climate. However, we still know very little about the biological and environmental factors that regulate the emission of isoprene. Thus, model estimates are highly uncertain. Better understanding these factors is challenging not least because of the immense plant diversity of the Amazon region. It is still unclear, which plant species emit isoprene at all, and how these emissions vary seasonally and under extreme climatic conditions like El Niño events.
Measuring isoprene emissions
To address these uncertainties, Eliane Gomes Alves and her colleagues measured isoprene emissions at the ATTO 80-meter tower at eight heights in and above the canopy. They also observed 194 trees with a camera installed at the tower to track leaf phenology changes over time. Out of those 194 trees, the team observed 36 trees along a canopy walkway in detail with respect to leaf ages and phenology, and partially with respect to isoprene emissions of individual leaves. Overall, the team collected data across three years, including 2015, a year characterized by a strong El Niño with a heatwave and drought in the Amazon region.
The researchers found that isoprene emissions varied seasonally, with increased emissions during the dry season and dry-to-wet transition season, compared to the wet season and wet-to-dry transition season.
In addition, the team looked at the effect of the 2015 El Niño on isoprene emissions. Because tropical plants typically emit more isoprene at higher temperatures, they expected emissions to increase significantly during this period of extreme heat. However, the scientists found that the values were only slightly higher than during normal dry season. They argue that this is likely due to the stress plants experience due to the El Niño induced drought that likely offset the expected increase in isoprene emissions.
The effect of leaf age
As a next step, they looked at the isoprene emission capacity of leaves of different ages. They found that growing and mature leaves (1 to 6 months old) have the highest emissions, while young (up to 1 month old) and old (over 6 months old) leaves emit on average one third less isoprene. These results are in line with those from prior studies, but Eliane Gomes Alves and her colleagues were able to confirm it for more species.
Even though leaf phenology in the evergreen Amazon does not follow a well-defined seasonal pattern as in mid and high latitudes, many tree species in the Amazon still have a seasonal cycle. Trees tend to flush new leaves in the wet-to-dry transition season, leading to a substantially higher fraction of mature leaves in the dry season and dry-to-wet transition season. Since mature leaves emit more isoprene, the emission capacity of Amazonian leaves is highest during this time, peaking in October and November.
Finally, the scientists included their findings in the widely used Model of Emissions of Gases and Aerosols from Nature (MEGAN) to test if they could improve it, and what factors are most relevant. They found that the key factor in improving model predictions is to include the seasonal variation of isoprene emissions related to leaf age.
Eliane Gomes Alves et al. published the study Intra- and interannual changes in isoprene emission from central Amazonia Open Access in Atmospheric Chemistry and Physics.
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