Bert Buizer, from VU University in Amsterdam, the Netherlands, and colleagues, investigated the way that plants respond to higher temperatures using specially designed "greenhouses" to speed up global warming. They studied a common sub-Arctic and Arctic heathland plant called the black crowberry (Empetrum nigrum) which currently grows between 52°N and 79°N (roughly from the Netherlands to Svalbard).
Previous studies from the Netherlands, UK and Ireland have shown that this plant is already retreating northwards. Modelling studies indicate that the march northwards will cover tens of kilometres over the coming years. Meanwhile, in Sweden the black crowberry is expected to move to higher altitudes, and in Svalbard it will move swiftly north in the more temperate regions. But are these range shifts all a response to warmer temperatures?
To answer this question Buizer and the team constructed "open top chambers" and placed them around a number of black crowberry clusters at both the southernmost extent of its range (in the Netherlands) and the northernmost extent (in Svalbard). The open-top chambers were large Perspex domes, with their tops chopped off.
"In the Netherlands the use of open-top chambers enhanced the mean summer (June, July, August) temperatures by 2°C and on Svalbard by 1.7°C," explained Buizer.
The researchers then observed the effect of these enhanced temperatures over a number of growing seasons (2005 to 1010 in the Netherlands and 2008 to 2010 in Svalbard). To their surprise Buizer and his colleagues did not see any evidence of heat stress for black crowberry growing in chambers in southern locations; in fact they found that the reverse was true.
"In both locations the plants increased growth rate, shoot length, biomass, berry weight and size," Buizer told environmentalresearchweb. "This is what we expected to see at the northern range margin in Svalbard, but it is the opposite of what we expected at the southern range margin in the Netherlands."
What’s more, the scientists noted that the enhanced temperatures didn’t appear to increase the competitiveness of other plant species – another effect that could have explained some of the crowberry’s retreat at the southern end of its range.
Their results (published in Environmental Research Letters (ERL)) suggest that increased temperatures do help to explain the northward expansion of the black crowberry at the limit of its northern range. But the retreat in the Netherlands, UK and Ireland still remains a mystery. "The causes of the contraction of the southern range margin cannot be answered by the results of our temperature-manipulating experiments and additional research will be necessary," said Buizer.
Now the team is investigating other environmental factors, to see if they can get to the bottom of the problem.