As the planet warms, it’s not just humans who feel the heat – the trees too. Rising temperatures disrupt the main engine of life on Earth: photosynthesis.
“The disproportionate warming in the Cerrado belt is driven not only by rising global temperatures, but also by local deforestation and fragmentation of forest areas. As grasslands and cropland have eaten away at forests, their cooling effects are muted. “
A recent study from Brazil adds to concerns that climate change is changing the face of the planet. Literally. Tropical forests could look more and more like deciduous forests or savanna in the future, according to research based on Brazil’s Cerrado biome. This ecoregion bordering the Amazon Rainforest is a melting pot of savannahs, grasslands and forests.
That paper, published on Environmental Research Letter in March, it focused on four tree species found in the Amazon rainforest and savannas: Qualea parviflora, known as pau – terra in Portuguese; Pseudobombax longiflorum, or Brazilian shaving brush tree; Hymenaea stigonocarpa (Cerrado jatobá); and Macrocarp blobs, also known as angelim.
“This article extends the knowledge of heat tolerance in tropical species in extremely hot regions, testing species that have not been tested before,” said Gotthard Heinrich Krause, professor of plant physiology at Heinrich Heine University Düsseldorf, Germany.
Krause, who was not involved in the research, noted that the paper underscores how “an increase in the number of extreme heat waves, often combined with drought stress,” adds to the heat stress facing trees.
The maximum temperature in the Cerrado region and the surrounding forest can reach 45 ° Celsius (113 ° Fahrenheit). The area has warmed markedly in recent decades, and the heat waves that regularly sweep the region are becoming hotter and drier.
How much a leaf warms depends on how much solar radiation it absorbs and what is lost through conduction and longwave radiation. Leaves facing the harsh tropical sun heat up faster than the surrounding air.
“Prolonged heat exposure can cause damage to leaf tissue, impair photosynthetic efficiency and, consequently, tree fitness,” said Igor Araújo, first author of the new paper and an ecologist at Mato Grosso State University, Brazil.
The disproportionate warming in the Cerrado belt is driven not only by rising global temperatures, but also by local deforestation and fragmentation of forest areas. As grasslands and cropland have eaten away at forests, their cooling effects are muted.
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Trees cool themselves and the surrounding area by passing water through the stomata that decorate the surface of their leaves. This process, together with evaporation, is also the reason why forest areas cause rainfall. “One mature tree in the Amazon can reach 1000 L [264 gallons] water per day, functions as a natural air conditioner for the environment. This process is called the biotic pump, which is reduced by deforestation, “said Araújo.
When faced with scorching heat and dry seasons, the pores of the leaves close tightly to conserve water. “This will reduce or prevent transpirational cooling of the leaves, causing a substantial increase in leaf temperature above air temperature,” says Krause.
The study predicted, for the first time, what temperatures the leaves could tolerate. To do this, the authors calculated the temperature at which critical components of the photosystem were damaged and the temperatures that the leaves experienced. The difference between these two levels is called the thermal safety margin (Tsm).
Krause hopes that the temperatures at which scientists observe permanent damage to leaf tissue will be higher than the researchers expected. This means a wider safety net, but still not sufficient to protect most of the species considered in the study, given the current rate of warming.
“It’s not just the tree’s health that’s declining; other scientists have pointed out that heat stress also affects absorption of carbon dioxide by trees. According to Krause, there is a reduction in CO2 absorbed by plants even at temperatures lower than those of short-circuit photosynthetic temperatures. “
Such a hostile environment is dangerous for the leaves and trees. The tree does not shed its leaves as part of a seasonal cycle, but because they do not perform their function of harnessing energy.
The authors found that in some species the maximum leaf temperature already exceeded this threshold. But if the average temperature rose by even 2.5 ° C (4.5 ° F), this would be true for most tree species, they estimate. With an increase of 5 ° C (9 ° F), all tree species studied will suffer from leaf burn.
It’s not just the tree’s health that’s declining; other scientists have pointed out that heat stress also affects absorption of carbon dioxide by trees. According to Krause, there is a reduction in CO2 absorbed by plants even at temperatures lower than the temperature of short circuit photosynthesis. “Such reduction will contribute to the reduction of carbon sinks in tropical forests,” he said.
Currently, savanna species are better adapted to higher temperatures than rainforest species. “Our results thus indicate an expected shift in future decay and thus a trend towards savanna vegetation replacing forest in an area in Southern Amazonia characterized by mostly deforestation,” write the authors. What is happening on the Amazon-Cerrado border may be a precursor to tropical jungle fever around the world. Unlike humans, this forest does not have air conditioning or sunscreen to protect it.