Research shows some trees become overheated in warmer rainforests

The capacity of tropical forests to store carbon may decline at the rate of climate change. This is due to the photosynthetic rates in the leaves of rainforest species falling at higher temperatures and the trees’ natural cooling systems failing during droughts. The increase in heat mainly threatens the species that store the most carbon. This has been demonstrated in a new thesis from the University of Gothenburg.

Some tree species are able to withstand the increasing heat in the tropics by drawing large amounts of water to their leaves and transpiring through the wide-open pores of their leaves. They are mainly fast-growing trees that become established as soon as a rainforest grows. The same cannot be said for the trees that make up the rainforest canopy in old-growth forests. They grow more slowly, but get bigger and taller, and their leaves don’t have the same ability to cool through transpiration.

Water powers the “air conditioning” of the leaves

“The tropics haven’t had ice ages so they’ve had a relatively stable climate historically and seasonally. With climate change it started to get warmer, and then we saw that some tree species are showing mortality rates increased, but we didn’t really know why before,” explains Maria Wittemann, who wrote the thesis.

She studied several tree species that can be roughly divided into early successional species, which establish early in a new rainforest, and late successional species, which grow more slowly but gain considerable size, and are therefore a plus large long-term carbon sink. . A clear difference is how trees in the two groups handle heat. Early succession species open the larger pores in their leaves, through which they transpire large amounts of water, thus maintaining the temperature in their leaves – similar to an air conditioning system. Late-successional species do not open their pores as much and therefore find it more difficult for them to stay cool.

More sensitive to drought

“We found large temperature differences in the leaves in our measurements. There could be a difference of 10 degrees Celsius between late successional species and early successional species growing in the same location. Late successional species had more difficulty coping with abnormally high temperatures.These trees had a higher mortality rate,” explains Maria Wittemann.

However, the profuse transpiration of early-successional species through their leaves also requires a lot of water. During a period of drought, the researchers noted that early-successional species became more vulnerable to heat and lost their leaves. Their reduced water consumption meant that late-successional species were more drought-tolerant.

“Our results show that the photosynthetic rates of rainforest trees drop when the temperature increases in their leaves, which occurs mainly in late-successional species. The proteins and membranes of their leaves, which are essential for the photosynthesis, fail and eventually trees die because they cannot convert enough carbon dioxide from the air. This affects the whole ecosystem. We know, for example, that some animals eat the fruits of late-successional species,” explains Maria Wittemann.

Cooperation with a local university

Previous research shows that the situation is worse in the Amazon. It is estimated that this carbon sink will be transformed into a carbon source by 2035. In the African tropical forests, climate change has not gone that far.

Research at the University of Gothenburg is conducted in high altitude forests in Rwanda in collaboration with the University of Rwanda. The trees were studied in situ, but seeds were also planted in climate chambers in Gothenburg to study their development at different temperatures.

“We work with various stakeholders in Rwanda. There isn’t much rainforest left in Rwanda and when they plant new trees, they want to know which native species will be able to survive in a warmer climate,” explains Maria Wittemann.