Tall trees in the Amazon can suffer from warmer temperatures
Central Amazonia’s tall trees are impacted by periods of high maximum temperatures and then shed their leaves and branches. With the increase in deforestation, the smaller, fragmented remaining forests have higher temperatures, which in turn exacerbate the negative warming effects on the tall trees.

A new study published in Nature Communications led by researchers in the University of Helsinki, in cooperation with scientists across the globe, elaborates on the impact rising temperatures have for the Amazon rainforest. The study shows that tall trees in Central Amazonia are impacted by maximum temperatures of the understory above 35 degrees.  

If very high greenhouse gases (GHG) and CO2 emissions double from current levels by 2050, maximum temperatures in the Amazon will likely exceed 35 degrees Celsius at least 150 days a year by end of century, according to the IPCC’s Sixth Assessment Report.

The canopies of old-growth, intact forests usually buffer the high temperatures observed in the understory to some extent. However, when the temperatures in these underlying layers reach 35 degrees, as they usually do in August and September in many regions, the tall trees then shed their leaves and branches.

– If the number of days registering these very high temperatures inside the forests increases, we might see that the tall trees will suffer considerably, says postdoctoral researcher Matheus Nunes, the lead author of the study.

The “breathe” of the Earth is in flux

In tropical forests, there is still a lot of uncertainties in the timing and causes of seasonal events, such as leaf shedding and burst. Understanding these patterns is crucial to understanding how tropical ecosystems will respond to climate change.

– We designed an experiment using repeated surveys using a modern laser scanner to investigate the seasonal dynamics of Amazonian forests, says Eduardo Maeda, the project coordinator funded by the Academy of Finland.

In the past decades, there has been a debate whether plants in the Amazon are more limited by light or by water. This study provides evidence that the problem is more complex, as it demonstrated a high variability across the forest vertical layers. In other words, trees occupying the lower layers, or strata, were more light-limited while tall trees occupying the upper strata were mostly affected by climatic variations.

Forest fragmentation

To complicate matters further, the study shows that deforestation in the region exacerbates the negative warming effects.

– The small, fragmented remaining forests tend to have hotter temperatures in the understory, because of more light penetrating the forest, says José Luís Camargo, co-author of the study and director of the “Biological Dynamics of Forest Fragments Project”, the world’s longest-running study of habitat fragmentation in Central Amazonia.

The hotter temperatures in these remaining forest fragments add further pressure on the tall trees, which cause trees to shed their leaves and branches for a prolonged time. Currently, it is estimated that 176,555 km2 of Amazonian forests are under the influence of edge effects. If deforestation continue and forests become more fragmented, we are likely to see severe consequences and a large-scale shift in the way tropical ecosystems breathe.

History of the study

Scientists have long sought to understand whether degraded forests can maintain important ecosystem functions like trapping carbon from the atmosphere to reduce global warming, preserve water cycling to maintain rainfall and cool temperatures, and preserve biodiversity.

One of the largest and longest running experiments on the effects of forest fragmentation was started in 1979 in the Central Amazon (near Manaus, Brazil) by a team of scientists led by the pioneering conservation biologist Thomas Lovejoy, who passed away last December. For the ‘Biological Dynamics of Forest Fragments Project’ (BDFFP), Lovejoy and colleagues worked with local farmers – who were clearing forest for cattle ranching – so that a series of forest fragments were left intact. Hundreds of scientific papers have since been published from the project, contributing to our understanding of the effects of habitat fragmentation on tropical forests and their ability to persist and support biodiversity.