In a warming world, will plants hold more carbon... or less?

An international team of scientists from Australia, the United States, Sweden and New Zealand, have found that a wide range of different kinds of plants respond to temperature changes in strikingly similar ways.

Over a three-year period, a diverse range of plant species – from herbs and grasses to towering trees – were surveyed at 18 locations across the planet, including temperate rainforests in Far North Queensland, boreal forests in Sweden, tropical forests in Costa Rica and the Alaskan tundra. The plants were specifically chosen to represent seven different types of ecosystems.

Researchers measured the respiratory response of the plants to temperature changes, and despite the wide range of plant types and diversity of habitats, found that the “sensitivity of respiration” to temperature, decreases as plants get warmer, and was consistent across all 231 species examined. This challenges current assumptions on how plant respiration responds to changes in temperature.

According to Owen Atkin from the ARC Centre of Excellence in Plant Energy Biology at ANU: “Our findings highlight the need to better understand the response of plants to temperature changes if we are to predict how natural ecosystems will respond in future climate scenarios.

“We have a reasonably good understanding of plant biology, but we haven’t translated that fully to current climate models.”

The research, published online in the journal PNAS, is the most comprehensive study of plant respiration responses to temperature yet conducted, and has important implications for predicting climatic responses to increased global temperatures as well as our understanding of how plants may respond to a warming planet.

Climate models that simulate the interactions of global cycles of carbon with land and oceanic systems, currently assume that plant respiration doubles for every 10 degree Celsius rise in temperature. This assumption, however, has now found to be incorrect.

“We clearly demonstrate there is a fundamental response of respiration to temperature,” explains Mark Tjoelker of the Hawkesbury Institute for the Environment at Western Sydney University, and a co-author of the paper, “and an over-simplification of this response function in current climate models.”

Plants play a major role in the global carbon cycle – the movement of carbon between the atmosphere, oceans and  land – by using the Sun’s energy to convert carbon in atmospheric carbon dioxide (CO₂) into organic compounds, such as sugars and fats. We know this as photosynthesis, and this process results in a flow of CO₂ from the atmosphere to the biosphere, causing plants to act as carbon sinks that soak-up atmospheric CO₂. In fact, plants account for about 120 gigatonnes of carbon being removed from the atmosphere every year.

But plants do breathe, or rather, their cells consume oxygen and give off CO₂. Cellular respiration by plants returns around half of the carbon they capture, back to the atmosphere, and so global rates of respiration - how fast plants are breathing - have a significant influence on the size of the land carbon sink and in turn the levels of atmospheric CO₂.

Climate models to date, however, have poorly represented the respiration response function to temperature in their simulations. 

“What’s exciting about the research is they appear to have found a convergence in the temperature response of plant respiration, which represents a significant step forward in improving the accuracy of climate models.” says Martin De Kauwe from the Climate and Forest Ecosystem Modelling Group at Macquarie University.

The research indicates that in a warmer world plants may retain more of the carbon made available to them by photosynthesis and, therefore, possess greater capacity to act as carbon sinks, which our climate models should reflect.

Also, by retaining more carbon, plants increase their biomass, though this may not necessarily mean biomass humans find useful; we mainly use seeds or fruits. Indeed, work carried out under the Australian Grains Free Air CO₂ Enrichment (AGFACE) program, has reported reductions in the nutritional content of many staple food  crops as higher amounts of CO₂ are made available for plant growth, with lower levels of protein and micronutrients like iron and zinc observed.

According to Tjoelker, current research is “just the tip of the iceberg” in our understanding of plant respiration and how plants will function in a warming world.

This article was first published in Australian Popular Science Issue #90 May 2016.