Academic papers released in July 2010 have indicated that previous assumptions about the impact of increased temperatures (due to climate change/global warming) on the rate of photosynthesis may not be the full story.  Rather, it appears that for many plants, water availability is much more important than temperature in determining how photosynthesis in plants changes in a changing environment.

FLUXNET is a "network of regional networks" coordinating observations of the exchanges of carbon dioxide, water vapour and energy between terrestrial ecosystems and the atmosphere.  In Australia, the CSIRO is a participant through its OzFlux network - see http://www.cmar.csiro.au/ozflux/  The Australian ecosystems covered by observation stations include wet/dry savanna, rainforest, wet sclerophyll eucalyptus forest, pasture and sugar cane.

OzFlux Group 2008Photo from http://www.cmar.csiro.au/ozflux/news/index.html

International researchers led by scientists from the Max Planck Institute for Biogeochemistry have been looking at the reaction of plants to variations in temperatures.  They have found that the rate at which plants fix carbon dioxide from the atmosphere (photosynthesis) in most ecosystems does not alter significantly as temperatures change.  On the other hand, for over 40% of the Earth's vegetated surface plants, photosynthesis increases as water availability rises.  This effect was most marked in temperate grasslands and shrublands (69%) and lowest in tropical rainforests (29%).

For more details see http://www.mpg.de/english/illustrationsDocumentation/documentation/pressReleases/2010/pressRelease201007041/index.html 

The second study, also led by the Max Planck Institute for Biogeochemistry in Jena, Germany, showed that there was little difference in the sensitivity in the terrestrial ecosystem respiration to air temperature across 60 FLUXNET sites around the world, covering a number of different ecosystems.

See http://www.sciencemag.org/cgi/content/abstract/science.1189587v1 

These results suggest that more work is required on global climate models, while confirming what many farmers already knew - water is critical.