Trees in tropical rainforests are dying faster01.06.2018 • Articles
Cristina Amorim, IPAM
As if deforestation wasn’t bad enough, a series of other threats kills, at an ever-increasing pace, the trees of the Amazon and other rainforests on Earth.
An expert review of scientific articles, which included Brazilian researcher Paulo Brando, from IPAM (Amazon Environmental Research Institute), indicates that the mortality rate of these trees shows signs of acceleration in recent years. The reasons are the increase in temperature, more prolonged and worse droughts, stronger winds, more extensive fires, more vines, and even the abundance of carbon dioxide in the atmosphere — one of the causes of the greenhouse effect and a fundamental element of photosynthesis.
Climate change is related to all of the problems pointed out. “The review shows that there is strong evidence linking the mortality of tropical rainforest trees to the changes expected for these regions, at global and regional scales,” says Brando.
The study focused on intact forests, both primary and old, in South America, Africa, and Southeast Asia. However, the Brazilian Amazon is more represented in it than the other rainforests because it is the most studied place of all, with more data available.
“In the Amazon, all of these causes of tree mortality are present,” says Brando. “But it’s hard to say that one is more relevant than the other because they all have a role to play. Droughts cause mortality peaks, while the increase of CO2 causes background changes. Windstorm events, for example, affect fragmented areas more, and fire causes a lot of damage in the southeastern region of the Amazon.”
The mathematics of death
It is impossible to establish which of these attacks is worse. Droughts, for example, have become increasingly long and severe — in the Amazon, anomalous episodes occurred in 1997, 2005, 2010, and 2015. As an immediate defense, trees take extreme actions, such as closing the stomata (a pore, found in the epidermis of leaves, stems, and other organs, that facilitates gas exchange) and losing more leaves.
These leaves, in turn, accumulate in abundance in the soil and serve as fuel for forest fires, which spread quickly and last longer.
Droughts and higher temperatures can still make trees starve, which is also a defense mechanism that ends up killing trees. By closing the stomata to save water inside, trees stop capturing the carbonic gas from the air, its source of food, while consuming what is inside.
The forced regime leaves them more susceptible to pest attacks, such as insects, or the competition for food with vines – which, in turn, have proliferated in these environments. And even if such diet doesn’t happen, excess carbon dioxide in the air does not mean they will grow abundantly.
“When there is a lot of carbon dioxide, some trees can dominate the area and steal resources from its neighboring individuals. Thus, there is an expected increase in tree mortality but not necessarily drastic changes in carbon stocks”, explains the IPAM researcher. “Another explanation is that the forest becomes more dynamic with more CO2; it grows faster and dies faster, both by metabolism and changes in forest structure.”
Not even the fact that they are close to the equator brings advantage for tropical rainforests in a warmer planet: a new temperature regime, expected in the coming years due to climate change, can change the metabolism of trees.
The study authors open a discussion about the scenarios that may reverse the picture, such as an increase in annual precipitation, but do not address how human action can reverse the picture.
According to Brando, reducing the rate of climate change and stabilizing the process as soon as possible, which involves cutting CO2 emission levels but also deforestation, are vital factors in maintaining the world’s rainforests. “The smaller the forest edge area, common in fragmented landscapes, the lower the impact of droughts, fires, and winds.”
He also notes the importance of deepening the analysis. “Observation networks are essential to understanding if our forests are healthy, and Brazil has made important advances. We need to know what’s really going on to close holes in the observations that still exist and prepare for the effects of climate change.”
The review was led by Nate McDowell of the Pacific Northwest National Laboratory (USA) and published in the February 2018 issue of the New Phytologist.