A warmer world means warmer plants, but not a runaway greenhouse effect

Researchers measure leaf respiration while undertaking fieldwork in Spain. Photo: Owen Atkin

Plants are not entirely green. While photosynthesis is obviously limited to their chlorophyll-containing (green) organs, all of their living tissues will also respire, and thus evolve carbon dioxide (CO₂). Indeed, such biological oxidation of carbon is both substantial and temperature sensitive; these are two issues of consequence for climate change, and focal points for research by Associate Professor Owen Atkin and colleagues in the Functional Ecology Group at the School of Biology.

Putting carbon exchange into a global perspective, humans generate about six Giga tonnes per year from burning fossil fuels, whereas plant respiration is responsible for ten times this, at about 60 Gt/yr. This is potentially worse in a warmer world if (as is widely reported) physiologically-active plants double their respiratory rate with each 10°C rise. Will this lead to a runaway greenhouse effect? Or, instead, are there systematic differences in plant respiration between functional groups that need to be accommodated in a global model? This important question forms part of the research of the Functional Ecology Group.

Through extensive experimentation with grasses, forbs, shrubs and trees the Functional Ecology group has confirmed the likelihood of a short-term rise in plant respiratory activity with increased temperature. However, with longer-term monitoring, the group noticed that plants acclimatised to their new temperature regime, and in so doing, constrained their long-term respiratory loss of carbon during growth and survival.

For example, an arctic plant, grown at sub-optimal low temperature, shows a heightened response whereas a tropical plant held under supra-optimal high temperature, shows a diminished response to further warming. In both cases, respiratory costs of growth and survival eventually return to a homeostasis where carbon costs (respiration) become scaled to rates of net carbon gain (growth) - a relationship linked with functional type, and thus the carbon cost of sustaining growth in diverse environments.

Consequently, in a generally warmer world, the relative importance of carbon losses from arctic versus tropical biota will change, but not in a generic fashion that would otherwise lead to a runaway greenhouse effect. From this work a global mosaic of respiratory response to increased temperature seems more likely. Unfortunately for climate modellers, this will mean added complication in having to accommodate these functional groups in their predictions for changes in global climate during coming decades.

Further Info Online:

• http://www.rsbs.anu.edu.au/ResearchGroups/EDG/Profiles/Owen_Atkin