Thursday, July 4, 2013

The foliage on native plants in Australia's arid zones is boosted by CO2

Increased levels of carbon dioxide (CO2) have helped boost green foliage across the world’s arid regions over the past 30 years through a process called CO2 fertilisation, according to CSIRO research.

In findings based on satellite observations, CSIRO, in collaboration with the Australian National University (ANU), found that this CO2 fertilisation correlated with an 11 per cent increase in foliage cover from 1982-2010 across parts of the arid areas studied in Australia, North America, the Middle East and Africa, according to CSIRO research scientist, Dr Randall Donohue.

"In Australia, our native vegetation is superbly adapted to surviving in arid environments and it consequently uses water very efficiently," Dr Donohue said. "Australian vegetation seems quite sensitive to CO2 fertilisation.

The fertilisation effect occurs where elevated CO2 enables a leaf during photosynthesis, the process by which green plants convert sunlight into sugar, to extract more carbon from the air or lose less water to the air, or both.
This, along with the vast extents of arid landscapes, means Australia featured prominently in our results."

"While a CO2 effect on foliage response has long been speculated, until now it has been difficult to demonstrate," according to Dr Donohue.

"Our work was able to tease-out the CO2 fertilisation effect by using mathematical modelling together with satellite data adjusted to take out the observed effects of other influences such as precipitation, air temperature, the amount of light, and land-use changes."

The fertilisation effect occurs where elevated CO2 enables a leaf during photosynthesis, the process by which green plants convert sunlight into sugar, to extract more carbon from the air or lose less water to the air, or both.

If elevated CO2 causes the water use of individual leaves to drop, plants in arid environments will respond by increasing their total numbers of leaves. These changes in leaf cover can be detected by satellite, particularly in deserts and savannas where the cover is less complete than in wet locations, according to Dr Donohue.

"On the face of it, elevated CO2 boosting the foliage in dry country is good news and could assist forestry and agriculture in such areas; however there will be secondary effects that are likely to influence water availability, the carbon cycle, fire regimes and biodiversity, for example," Dr Donohue said.

"Ongoing research is required if we are to fully comprehend the potential extent and severity of such secondary effects."

This study was published in the US Geophysical Research Letters journal and was funded by CSIRO's Sustainable Agriculture Flagship, Water for a Healthy Country Flagship, the Australian Research Council and Land & Water Australia.

Reprinted from CSIRO

Donohue, R. J., M. L. Roderick, T. R. McVicar, G. D. Farquhar. 2013 "Impact of CO2 fertilization on maximum foliage cover across the globe's warm, arid environments. Geophys. Res. Lett. 40, doi:10.1002/grl.50563

Non-deciduous perennials are responding better than deciduous, annual and ephemeral plants

Here is the abstract from a previous paper by Dr Donohue (my paras):

Using Advanced Very High Resolution Radiometer data spanning 1981–2006 and calibrated for long-term analyses of vegetation dynamics, we examine whether vegetation cover has increased across Australia and whether there has been a differential response of vegetation functional types in response to changes in climatic growing conditions. Trends in vegetation cover are interpreted within Budyko's energy – water limitation framework.

Results from an Australia-wide analysis indicate that vegetation cover (as described by the fraction of Photosynthetically Active Radiation absorbed by vegetation; fPAR) has increased, on average, by 0.0007 per year – an increase of ∼8% over the 26 years. The majority of this change is due to a 0.0010 per year increase in persistent fPAR (representing nondeciduous perennial vegetation types; up 21%).

In contrast, recurrent fPAR (representing deciduous, annual and ephemeral vegetation types) decreased, on average, by 0.0003 per year (down 7%), the trends of which are highly seasonal.

Over the same period, Australian average annual precipitation increased by 1.3 mm yr−2 (up 7%).

A site-based analysis using 90 long-term meteorological stations with minimal localized land-cover changes showed that energy-limited sites where total fPAR increased generally experienced decreases in precipitation, and water-limited sites that experienced decreases in cover were almost always associated with decreases in precipitation.

Interestingly, where vegetation cover increased at water-limited sites, precipitation trends were variable indicating that this is not the only factor driving vegetation response. As Australia is a generally highly water-limited environment, these findings indicate that the effective availability of water to plants has increased on average over the study period. Results also show that persistent vegetation types have benefited more than recurrent types from recent changes in growing conditions.

Regardless of what has been driving these changes, the overall response of vegetation over the past 2–3 decades has resulted in an observable greening of the driest inhabited continent on Earth.