With climate change, the ability of rainforests to store carbon can diminish. This is due to a decrease in photosynthesis rates in the leaves of rainforest species as temperatures rise and the failure of the trees’ natural cooling mechanisms during droughts. Species that store the most carbon are most at risk from rising temperatures. A new thesis from the University of Gothenburg demonstrates this.
Some tree species are able to withstand rising temperatures in the tropics by absorbing large amounts of water through their leaves and exhaling through pores that are widely spaced. Primarily fast-growing trees that establish themselves early in the development of a rainforest. The same cannot be said for the trees that comprise the canopy of old growth rainforests. They grow more slowly, but become larger and taller, and their leaves lack the same capacity for transpiration-based cooling.
The leaves’ “air conditioning” is powered by water.
“Since the tropics have not experienced Ice Ages, their climate has been historically and seasonally stable. As a result of climate change, temperatures have begun to rise, and we have observed that certain species of trees are dying at a higher rate than we had previously understood “The thesis’ author, Maria Wittemann, states the following:
She has studied several tree species that can be roughly divided into early successional species, which establish themselves early in a new rainforest, and late successional species, which grow more slowly but become much larger over time and are thus a more effective carbon sink. There is a clear distinction between how the two groups of trees respond to heat. Similar to an air conditioning system, the early successional species expand the pores in their leaves so that they can transpire large amounts of water, lowering the temperature in their leaves. Late successional species do not open their pores as much, making it more difficult for them to maintain body temperature.
More vulnerable to drought
“Our measurements revealed significant temperature differences between the leaf surfaces. The temperature difference between late successional species and early successional species growing in the same location may be 10 degrees Celsius. Late successional species had a harder time adapting to abnormally high temperatures. These trees had a higher rate of mortality “says Maria Wittemann.
However, the abundant transpiration of the early successional species’ leaves requires a great deal of water. During a period of drought, researchers observed that early successional species became more heat-sensitive and shed their leaves. Due to their lower water consumption, late successional species were more resistant to drought.
“Our findings indicate that photosynthesis rates in rainforest trees decrease when leaf temperatures rise, which is primarily the case for late successional species. The proteins and membranes in their leaves, which are essential for photosynthesis, fail, and the trees eventually perish from carbon starvation because they are unable to convert enough carbon dioxide from the air. This impacts the ecosystem as a whole. For instance, we know that some animals consume the fruits of late successional species “says Maria Wittemann.
Cooperation with a nearby college
According to previous research, the situation is most dire in the Amazon. According to estimates, this carbon sink will become a carbon source as early as 2035. In African rainforests, the effects of climate change have not been as severe.
The University of Gothenburg is conducting research in Rwanda’s high-elevation forests in collaboration with the University of Rwanda. The trees have been studied in situ, but seeds have also been planted in Gothenburg climate chambers to examine their development at various temperatures.
“We collaborate with various Rwandan stakeholders. When planting new trees in Rwanda, it is important to know which indigenous species can survive in a warmer climate “says Maria Wittemann.
The sensitivity of trees to climate change was investigated by planting tree species adapted to a cooler climate in Rwanda’s elevated tropical rainforests at three sites with distinct climates and altitudes. One step downward on the elevation gradient represents a potential future climate.
