Plants play a key role in regulating Earth’s climate, but recent research suggests that rising temperatures could disrupt this balance, because plants are leaking more water than previously thought.
UBC assistant professor Dr. Sean Michaletz, a newly minted Sloan Research Fellow in the department of botany, studies how plants respond to heat. His findings challenge a long-standing assumption about plant water loss and could change how climate models predict future warming.
What do “leaky” plants have to do with climate change?
Our entire biosphere depends on plants. During photosynthesis, plants absorb carbon dioxide through tiny pores in their leaves and, using light, ‘breathe out’ water vapour and oxygen in an exchange. Since carbon dioxide is the main driver of global warming, understanding how temperature affects this process is crucial for predicting climate change.
It was previously thought that plants lose most of their water through their pores, which close in extreme heat to conserve water. But our research found that as temperatures rise, plants lose more water through their cuticle—the waxy layer on their leaves, which cannot close—than through their pores. The thinner the cuticle, the greater the water loss.
This means that in extreme heat, plants continue losing water but cannot take in carbon dioxide, limiting photosynthesis and reducing their role as a carbon sink. In extreme temperatures, they could even become carbon sources, accelerating climate change.
My back-of-the-envelope calculation suggests that a medium-sized leaf exposed to 50 °C could lose about one-third of a teaspoon of water per day through the cuticle. Scaled up to entire forests, this could alter global water and carbon cycles—an impact that our current climate change models may underestimate.
How hot is too hot?
In another study of 200 plant species in Vancouver, we found that photosynthesis starts to break down between 40 and 51 °C. During the 2021 heat dome, temperatures soared to 49.6 °C, pushing plants to their limits.
Our ongoing research suggests that 60 °C may be the highest temperature plants can survive—beyond this point, proteins break down, leading to cell injury and death. Only a few desert and tropical species have ever been observed surviving at such extreme temperatures.
Globally, researchers are working to determine the “tipping point” where Earth’s vegetation releases more carbon dioxide than it absorbs, switching from a carbon sink to a carbon source. Our estimates suggest this could happen around 30 °C, though key uncertainties remain—especially how microclimates and water availability affect photosynthesis under extreme heat.
With global temperatures already averaging 16°C, understanding these limits is critical for predicting climate feedback loops and the future of Earth’s ecosystems in a warming world.
What can we learn from human-made biospheres?
As a postdoctoral fellow, I worked at Biosphere 2, a research facility originally designed as a self-sustaining, closed ecological system. Researchers, called biospherians, were sealed inside for a planned two-year experiment to test whether humans could survive without external oxygen or supplies. The goal was to test this concept on Earth, with the idea of sending such domes into space someday. However, the experiment faced unexpected challenges: concrete curing led to a carbon dioxide buildup, while prolonged isolation triggered social and psychological stress among the biospherians.
Biosphere 2 later transitioned into a research and public education centre, where I studied how high temperatures affect plants in its experimental rainforest.
Plants have survived climate shifts for hundreds of millions of years, but all species face upper limits set by the laws of physics. While some plants can tolerate higher temperatures better than others, the precise breaking point – and how soon plants might reach it – remains uncertain. But based on recent measurements, we may be closer than we think.
Journal
New Phytologist