Treelines are Not Quite Responding to Climate Warming

A general premise is that treelines are thought to be more temperature sensitive, and so the rise in summer temperatures due to a warming climate should result in an advance of the treeline position.

A new study from the Bio-Protection Research Centre in New Zealand put that premise to test.

A treeline in Alaska

The researchers looked at 166 treeline sites with temperature data taken from the closest climate station to each site during the 20th century.

Results..........

---52% of the treeline sites advanced while the overall temperature increased during the long-term period.
--47% of the treeline sites remained stable.
--1% receded.

"Surprisingly these results reveal that treelines are not universally responding to climate warming by advancing, as expected," said Melanie Harsch, "However they demonstrate the importance of temperature on treeline advance over other factors such as disturbance, latitude, scale, elevation and distance to the ocean; none of which demonstrated strong relationships with the probability of treeline advance."

These results provide no evidence of the prevailing view that high altitude and latitude treelines are controlled only by summer temperatures. Instead they show that treelines are more likely to advance at sites that had warmed during the winter months, according to the ScienceDaily article.

Summer temperature is widely considered to be the primary control of treeline formation and maintenance, whereas winter temperatures have previously been considered less critical because of the insulative effects of snow.

"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," said Harsch.