Plant finding to change how we model changes in global atmospheric CO2

Understanding the movement of carbon dioxide (CO₂) into and out of ecosystems in response to warming global temperatures is a crucial part of predicting how Earth’s climate will change in the future.

Scientists have long expected that global warming will lead to plants increasing the amount of CO₂ they release into the atmosphere through respiration.

In turn, this would lead to even more warming in a positive feedback loop which would accelerate climate change.

“This is likely true, but this latest research reveals that carbon fluxes under warmer future climates will not increase as much as currently thought,” says Professor Ian Wright, chief scientist at Western Sydney University’s Hawkesbury Institute of the Environment and co-author of a new study which investigated how plant respiration will adapt to warming global temperatures.

Respiration is a critical process in which plants take the glucose they make from photosynthesis and use it to produce cellular energy to fuel growth and other cellular functions. However, it also releases CO₂ as a byproduct.

The respiration from their woody stems is a major contributor to the Earth’s annual carbon “flux” – the rate at which CO₂ is added or removed from the atmosphere.

“Short-term, temperature-driven changes in plant respiration rates are measured in seconds, minutes and hours,” says Wright.

“Due to the quick-acting enzymatic processes in plant tissues the changes in plant respiration are very fast, and predictable.”

In the short term, plant respiration jumps dramatically because respiratory enzymes have higher catalytic rates at higher temperatures.

“This contrasts with the long-term, temperature-driven changes in respiration rates that are measured in months, years and decades,” says Wright.

“Most global ecosystem models in the past have assumed that the same short-term behaviour in plants also applies over a longer time period.”

But the new research reveals that this is not the case.

The researchers predicted that the rate of respiration in woody stem tissues would differ compared to respiration in leaves.

Stems transport water taken up by the roots to the canopy to replace water lost through evaporation. Water becomes less viscous as the temperature increases, so the researchers hypothesised that it should take less energy to transport the water, therefore decreasing the respiration rate of stem tissues.

The team evaluated how stem respiration responds to changing temperatures using a new Global Stem Respiration Dataset, which includes data from 187 species sampled at 68 field sites spanning all climate zones.

These measurements revealed a strong reduction in stem respiration in response to higher temperatures across all sites globally.

Further experimentals will be needed to reveal the mechanisms that control this differing respiration in plant stems and leaves.

But the modelling indicates that the amount of CO₂ respired from tree trunks is not expected to increase as sharply as previous models, which overestimated stem respiration.

The research has significant impact, Wright says, as future climates are predicted to have more frequent and more intense events such as heatwaves, fires, droughts and floods.

“We’re already seeing that play out both here in Australia and around the world.

“However, these new findings suggest that, to some extent, ecosystems globally will slow the trends in one key driver of these changes – elevated atmospheric CO₂.”

The study has been published in the journal Science.