Rising Temperatures Threaten Photosynthesis in Tropical Rainforests

Tropical rainforests, such as those in North Queensland’s Daintree rainforest, have long played a key role in moderating the world’s climate. Called the “lungs” of Earth, they are believed to be responsible for consuming carbon dioxide and producing oxygen by photosynthesis. Scientists now show that continued global warming will prevent tropical forests from performing efficient photosynthesis. This will hasten climate change and reduce the forests’ role in mitigating global warming.

Tropical forests and their crucial role in the global climate

The Daintree rainforest, a tropical North Queensland, is home to ancient plant families dating back to the supercontinent of Gondwana. Stepping into this dense, humid forest can be likened to stepping into a time warp, with the thick green canopy of leaves, the ferns, and the wet soil. Rainforests around the globe, from the Amazon to Southeast Asia, absorb carbon dioxide. In return, they release water vapor and oxygen, which are key ingredients for maintaining Earth’s climate.

However, at higher global temperatures, tropical rainforests may not be able to perform this important function efficiently. Recent studies show that warming temperatures will reduce the photosynthetic capacity of tropical forests. This decrease in their ability to absorb carbon dioxide could potentially accelerate climate change.

Challenges Facing by Tropical Trees

Plants, in the form of tropical trees, also have the acclimation effect so that even under moderately raised temperatures, it changes the photosynthesis in order to survive. But tropical trees lack this ability because it is not built up during such stable climatic environments.

Scientists have established an experiment involving a canopy crane to reach treetops and research how tropical trees in the Daintree react to warming. Specifically, the study selected four mature species of trees, whose leaves were heated by 4°C—this temperature increase is predicted for the end of the century in tropical systems. Notably, this is one of the longest such experiments, having lasted eight months.

Warming Cuts Photosynthesis

The results were worrying. For all species, warming depressed photosynthesis. Photoproduced values decreased on average by 35% for all leaves subjected to the high-temperature conditions than their controls under non-warming treatments. This lowering of photosynthesis occurred through two fundamental causes:

Stomatal Closure: The pores on the leaves, known as stomata, were less open due to warmer, drier air. It meant that fewer carbon dioxide molecules entered the leaf and more water vapor left the leaf.

Interference of Enzymes: Higher temperatures affected the enzymes which were involved in photosynthesis; they became less efficient in fixing carbon.

The trees even after eight months have very little adaptability to elevated temperatures. It might be suggesting that the tropical trees are near the thermal limits, and therefore cannot even survive under moderate increase in temperature.

Consequences on Global Water Cycle

Apart from diminished carbon uptake, the study points to possible interference with the global hydrological cycle as a result. Warming initiates stomatal closure, which not only reduces the amount of water transpired by trees but also increases the extent to which the surrounding atmosphere draws transpiration from the trees. This interaction involving release and absorption by the atmosphere could signify effects significant to the global hydrological cycle.

Little Leeway for Adaptation

Tropical forests are already at the physiological limits of photosynthesis. Given the expectation of continued warming and drying, there is little scope for trees to adapt to these changes. This lack of resilience could undermine the role of forests in regulating the climate, making it more difficult to combat climate change.

This research underscores the urgent need for action in the face of climate change. As tropical forests reach their limits, they will lose their capacity to absorb carbon dioxide, potentially accelerating global warming. Preserving these vital ecosystems is crucial for ensuring that tropical rainforests continue to play their role as Earth’s climate regulators. Reducing global emissions is also essential in supporting the health and function of these rainforests.