The first comprehensive study of treelines around the world has shown they are not advancing as much as scientists thought in the face of global warming.

Treelines are where trees stop growing because climatic conditions become cold enough that the local environment can’t support them. The study, by the Bio-Protection Research Centre in New Zealand, looked at the changes in treelines at both high altitudes, in mountainous regions, and at high latitudes, or the southern and northern extremities of the globe.

The researchers looked at temperature records going back to 1900 at or near 166 tree fringes around the world. The results were published in the journal Ecology Letters and showed that only just over half of the treelines had advanced even though temperatures had risen noticeably at two-thirds of the sites. At two sites, treelines had receded.

"Surprisingly these results reveal that treelines are not universally responding to climate warming by advancing, as expected," said Melanie Harsch of the BPRCNZ. Treelines had been thought to be more sensitive to summer temperatures rather than winter and general advance of treeline positions expected on that basis,” she said.

But warming winter temperatures were found to be a significant factor at sites where trees were advancing, particularly in the northern hemisphere. In northern climes, climate change has been found on average to have a bigger impact on winter temperatures than summer.

"These results show that treelines are responding to warming, but are not consistent in that only half of the sites showed signs of advance despite most sites experiencing warming. Several studies on plant species' responses to climate warming have shown mixed results and this study provides a possible explanation – both winter and summer conditions control treeline position," concluded Harsch. "Our expectations of response depend upon which factors are limiting the current treeline distribution. Where summer temperature is the primary limiting factor we can expect to continue seeing advance, but at other sites treeline advance is unlikely to occur until other limiting factors are first lessened."

Harsch said the study also showed that other factors such as disturbance, scale, and distance to the ocean did not demonstrate strong relationships with treeline advance.

ScienceAlert/Eureka Alert, 18/8/09