HIGHER levels of atmospheric carbon dioxide may be contributing to the woody weed invasion that has taken over much of Australia's rangelands, and other grasslands across the world.
Concentrations of atmospheric CO2 are today about a third higher than when Europeans first drove their sheep and cattle onto the abundant grasslands of inland eastern Australia during the early 1800s.
Australian and US scientists have found that on average, woody plants are profiting more from the historically high levels of CO2 than grasses. Rising levels of CO2 may have thus helped turn many of those former grasslands into the shrublands they are today.
"Most rangeland pastoral areas across the world have seen a thickening of woody vegetation," said Dr Chris Stokes, a research scientist with CSIRO Sustainable Ecosystems at Townsville.
"Some of that has been ascribed to grazing practices, but some scientists also believe that it has been helped along by higher levels of CO2. I suspect very strongly that CO2 is a contributing factor, along with management."
The US Department of Agriculture Agricultural Research Service (ARS) has grown key American rangeland species in a series of closed greenhouses that each mimic a different CO2 concentration, representative of a range from the last Ice Age to 2050.
The ARS team at Fort Collins, Colorado, found that high CO2 levels favoured cool-season grasses over warm-season grasses (see below) and weedy shrubs over native forage grasses.
When CO2 concentrations in the greenhouse chambers were double today's levels, at about 720 parts per million (ppm), the growth of fringed sage, a small weedy shrub, increased 40-fold.
The ARS researchers also found that plants are less sensitive to changes in today's levels of CO2 - at 384ppm, the highest for 800,000 years, and possibly the last 15 million years - than they were when CO2 levels were lower.
In 1842, when Europeans began to fully exploit Australia's rangelands, CO2 levels were at about 284ppm.
Dr Stokes cautioned against extrapolating from greenhouse trials, because increasing levels of atmospheric CO2 were likely to bring about a range of other factors, including increased temperatures and changed rainfall regimes, that muddied the picture.
CSIRO research near Townsville also established the woody plants were better equipped to benefit from higher levels of CO2, but not for reasons that were self-evident.
All plants take in CO2, which when combined with water in the presence of light results in photosynthesis and the creation of sugars and energy - a process that literally powers the living world.
Plants take in CO2 through leaf pores called stomata. Opening stomata to allow CO2 to enter the plant can be a costly business for the plant: as a rule of thumb, Dr Stokes said, for each CO2 molecule it captures, a plant loses 1000 water molecules.
When CO2 concentrations are higher, the process becomes more efficient. Each opening of a stomata captures more CO2, and the plant can be more economical about opening its pores and so lose less water.
"The biggest response to higher CO2 levels is that all plants can use water more efficiently," Dr Stokes said.
"Some of the water being saved drains deeper into the soil, to where the grass roots can't reach, so some of the water that grasses used to use was instead accessed solely by the deeper-rooted trees. Even tree seedlings were able to do really well on that extra water."
"Some of our modelling suggests that because everything is using water more efficiently, there will be a small increase in grass production - but there will be a bigger increase in tree production."
CO2, C3 and C4 plants
Most of the world's plants can be divided into two groups, C3 and C4, that relate to the levels of atmospheric carbon dioxide their ancestors evolved in.
That evolution will determine how these two groups respond to increasing levels of CO2 in the modern atmosphere, CSIRO scientist Dr Chris Stokes (pictured) says.
Plants that evolved in an era of low atmospheric CO2 have a C4 metabolic pathway that includes a CO2 concentrator mechanism and high water efficiency.
Increasing atmospheric CO2 levels are unlikely to be of direct benefit to C4 plants, which include tropical grasses like sorghum, and maize and millet.
Plants that evolved under conditions of high CO2--cool seasons grasses like wheat, soybean and cotton, and most trees--developed the C3 pathway, a metabolic process that employs an enzyme, RuBisCo, to channel CO2 into the photosynthetic process.
RuBisCo has a weakness, Dr Stokes says: because it evolved under conditions of high CO2 and low oxygen (O2), it is susceptible to being oxidised by O2.
Climbing CO2 levels play to RuBisCo's strengths. Today it has more chance of bumping into a CO2 molecule over an O2 molecule than it did in 1750, at the start of the Industrial Revolution, and as a result, Dr Stokes said, elevated CO2 is likely to offer a small but as-yet unquantified efficiency advantage to C3 species.
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