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Root-eating insects to undermine CO2 sinks


Trees need healthy roots to take advantage of extra CO2 for photosynthesis (Source: casadaphoto/iStockphoto)

Root nibblers Insects nibbling away at tree roots could prevent the increase in plant biomass scientists expect as CO2 levels increase, suggests new research.

The findings have implications for the capacity of forests to act as carbon sinks, says researcher, Dr Scott Johnson, an ecologist from the University of Western Sydney.

Johnson and colleague Dr Markus Riegler report their findings in a recent issue of PLOS ONE.

"If something is knocking out the essentials like root function that the plants need to capitalise on elevated CO2, then that's quite important," says Johnson.

Plants grow by photosynthesising, taking up CO2 and water to produce glucose and oxygen.

As atmospheric CO2 levels rise, scientists predict that this will increase the rate of photosynthesis and boost the amount of plant biomass on the planet.

This CO2 "fertilisation effect" is the rationale behind carbon sinks - the idea that plants will help soak up CO2.

But, says Johnson, scientists have tended to neglect the impact of root-feeding insects, which can damage roots, preventing plants from taking up water and limiting the amount of photosynthesis.

Glasshouse experiments

Johnson and Riegler carried out a set of glasshouse experiments to study the impact of different CO2 levels and root-feeding insects on the growth of bluegum (Eucalyptus globulus) seedlings.

The researchers compared what happened in growth chambers of 400 parts per million CO2 and 600 parts per million of CO2 (representing current day concentrations and those expected in the next 30 to 40 years).

They also looked at the impact of the soil-dwelling insect Xylotrupes Gideon australicus - a rhinoceros beetle - which is one of many scarab beetles whose larvae lives in the soil and feeds on roots.

In the absence of the beetle larvae, the trees grew much faster under elevated CO2, with shoot and root biomass increasing by 46 and 35 per cent respectively, but that changed once the insects were introduced.

"The plants basically stopped that elevated growth and just behaved as though they were under ambient CO2 conditions," says Johnson.

"These plants weren't growing as well as they should have been," he says. "Essentially they're not able to take advantage of the elevated CO2."

Johnson and Riegler also found that when the insects were present the leaves of these trees had 9 per cent less water content compared to those of other trees.

The researchers also examined the roots and found they had been eaten and damaged due to the larvae moving around in the pots.

The percentage of total plant mass damaged was greater in the plants grown under elevated CO2, says Johnson.

Carbon sequestration

Johnson says the findings are relevant to our understanding of carbon sequestration because root-feeding insects are "quite big players" in the Australian landscape.

"If the plants are not growing as big or not doing as well then that's going to reduce carbon sequestration," he says.

In related research presented at an ecological meeting in New Zealand this week, Johnson reports on the role root-feeding insects play in grasslands.

He says under elevated CO2, carbon to nitrogen ratios increase in C3 grasses (normal pasture grasses) and because insects are limited in nitrogen, this triggers a "compensatory feeding response".

"They've got to eat much more plant tissue to get enough nitrogen under elevated CO2," says Johnson.

The same does not occur in native C4 grasses, he says.

Tags: climate-change, rain-forests-and-forest