On plots of pasture at Pontville in southeastern Tasmania, grasslands were exposed to increased levels of carbon dioxide (CO2) and an expected two-degree temperature rise to mimic global warming over the next 50 years.
The plantlife on the plots was measured for flower and seedlings produced, growth rate, productivity and nutrient levels. By the end of the experiment, the research team was able to predict what would happen to productivity with global warming during different seasons of the year and with varying rainfall.
While summer rainfall boosted plant growth in response to elevated CO2 levels, spring and autumn rainfall reduced the fertilisation effect.
The increase in productivity caused by elevated CO2 was greatest in years where summer rainfall approached or exceeded the amount of spring and autumn rainfall - and smallest in years with abundant autumn and spring rainfall or little summer rainfall.
They found one reason for these differences may be the effect of rainfall on nutrient availability, partly because high rainfall during cool, moist seasons resulted in nitrogen limitation, reducing or even preventing biomass stimulation by elevated CO2.
"Projections that rain could reduce the stimulation by higher CO2 held true in autumn and spring, mainly because regular rainfall leads to nitrogen limitation but also because the water is plentiful in the soil," Professor Hovenden explained. "But summer's typically isolated, heavy rainfall events would maximise growth because the higher CO2 concentrations would help with water retention. To optimise the benefit, farmers will have to be careful about soil nutrient levels because there is also a link there and results could be further improved if fertiliser is applied at particular times of the year."
Being able to predict how the ecosystem will react to climate change, he said, was the "holy grail" for plant scientists.
Under present climate change predictions, extreme rainfall events will become more frequent and intense in New Zealand by the end of the century, while drought risk would increase substantially.
According to a report released last year by the Prime Minister's Chief Science Adviser, Sir Peter Gluckman, increasing yields from rising CO2 fertilisation was likely to benefit the forestry industry by 2040 and beyond.
Pastoral farmers would have to adapt, with pasture growth rates changing regionally and seasonally.
In horticulture, shifting rainfall and extreme heat events could change characteristics of products from different regions.
