Solar radiation is more important than rainfall to Amazon Rainforest leaf production

Study led by scientists at Brazil’s National Space Research Institute and published in PLOS ONE shows that increased insolation is the key driver of Amazon forest greening.

By Peter Moon  |  Agência FAPESP – An aspect of the Amazon Rainforest that has been known for many decades is that trees produce new leaves and the forest becomes greener with the onset of the wet season. In the case of temperate forests, most models used in research show a decrease in photosynthesis during the dry season, and scientists have long believed that the same was true of tropical forests.

However, according to a hypothesis first proposed in 2006, new leaf production (leaf flushing) in the Amazon does not follow the same pattern as that in temperate forests. The factor that leads to greening in most of the region is not the increase in rainfall but growing insolation (solar radiation).

The hypothesis has just been proven thanks to a study conducted by researchers at the National Space Research Institute (INPE) in Brazil, in collaboration with colleagues from France, Cameroon, the United Kingdom, Finland, and the United States.

The study, which is described in an article published by PLOS ONE, began with an analysis of satellite images and field data to show that in most of the Amazon, the key driver of greening is not a shortage or superabundance of water but an increase in insolation, as explained by the main authors of the study: Frenchman Fabien Hubert Wagner and Brazilian Luiz Eduardo Aragão, both from the INPE.

The goal was to prove that satellite data can be used to measure Amazon Rainforest greening. The study was performed with support from FAPESP and the National Council for Scientific & Technological Development (CNPq). Wagner is currently the recipient of a regular grant from FAPESP and a fellowship under its Young Investigator Awards program.

“Tropical cycles are usually defined in terms of alternating dry and wet seasons, but we’ve succeeded in demonstrating that seasonal variation in visible leaf production is largely triggered by increased insolation. We identified this in 70.4% of the area studied,” Wagner said.

The study covered three entire Brazilian states – Amazonas, Roraima and Amapá – as well as western Pará; the northern Peruvian Amazon; the Amazon regions of Ecuador, Colombia and Venezuela; and Guyana, French Guiana, and Surinam.

“Our current explanation of this phenomenon is that in most of the Amazon region, plants never suffer water stress. There is always enough water, even in the dry season. In the absence of water scarcity, increasing insolation is the factor that triggers leaf flushing,” Wagner said.

In addition, the study also showed that rainfall determines the production of a new generation of leaves by trees in the Amazon Rainforest over 29.6% of the area analyzed. In this case, during the dry season, plants suffer water stress and stop producing leaves. Flushing resumes in the wet season. This is true of the southern portion of the Peruvian Amazon; northern Bolivia; the Brazilian states of Acre, Rondônia and Mato Grosso; and the midwestern portion of Pará. “No one had ever succeeded in modeling the phenomenon in this way before,” Wagner said.

Areas with sun and water limitations 

The researchers obtained their results by establishing a correlation between monthly insolation and precipitation data from the Amazon and the expansion or contraction of the green area of forest observed from space by the National Aeronautic and Space Administration’s (NASA) Terra and Aqua remote sensing satellites, which orbit Earth at an altitude of 705 km. 

They used enhanced vegetation index (EVI) imagery obtained by the Moderate Resolution Imaging Spectroradiometers (MODIS) on board Terra and Aqua.

The instruments map the entire planet every two days and acquire data on global-scale dynamics, including maps of vegetation cover and changes in cloud cover over the oceans and continents or in the lower atmosphere, for example. 

The images used by the researchers spanned an area between 10° N and 20° S latitude and between 80 °W and 40° W longitude. The datasets analyzed were from observations by the Terra and Aqua satellites collected between 2000 and 2012 at a 1-km spatial resolution.

Monthly precipitation measurements from 2000-13 were obtained from the joint NASA/Japan Aerospace Exploration Agency (JAXA) Tropical Rainfall Measuring Mission (TRMM) satellite. Although the TRMM ceased when the spacecraft ran out of fuel in 2015, after 17 years of orbit, the data provide monthly precipitation estimates for tropical and subtropical regions at a 0.25°-spatial resolution.

Maximum temperatures were obtained from a global climate database available from 1901-2012. The researchers used monthly surface solar radiation data at a 0.5°-spatial resolution from 2000-12. These values were estimated by the paired “Clouds and the Earth’s Radiant Energy System” (CERES) instruments on board Terra and Aqua. All data from these two satellites, as well as TRMM data, are freely available online.

“One of the study’s key contributions was its successful separation of areas with solar radiation and water limitations,” Aragão said. The dry season in the Amazon runs from July to October, with some variations in certain parts of the region.

“Plants begin producing leaves in the middle of the dry season and continue doing so until the end of the season, reaching maximum leaf cover and photosynthesis when the rains begin,” Aragão said. “However, to produce leaves in the dry season, they need access to water. This occurs in 70% of the Amazon region, where the soil has the best water storage capacity.”

According to Aragão, who is also an associate professor at the University of Exeter in the UK, the study is valuable for much more than the statistical correlation. Notably, it establishes between the effects of insolation and precipitation on Amazon Rainforest greening.

“It shows how the Amazon biome is reacting to global climate change. Next, steps will include attempting to replicate the model in other areas of tropical vegetation,” he said.

The article “Climate drivers of the Amazon forest greening” by Fabien Hubert Wagner, Bruno Herald, Vivien Rossi, Thomas Hiker, Eduardo Fiji Maeda, Abler Sanchez, Alexei I. Lyapustin, Lênio Soares Galvão, Yujie Wang and Luiz E. O. C. Aragão can be read at: doi.org/10.1371/journal.pone.0180932.