Treelines are the elevation or latitudinal limits where trees are capable of growth or survival and are considered to be early indicators of climate warming because they are constrained primarily by cold temperatures. Summer temperature is widely considered the primary control of treeline formation and maintenance, whereas winter temperatures have previously been considered less critical because of the insulative effects of snow. This study reveals how winter warming has overturned this prevailing view.
"Average temperatures have risen over the last century, with a more pronounced and rapid change at high altitudes and latitudes", said Ms. Melanie Harsch from the Bio-Protection Research Centre in New Zealand. "Within these zones, treelines are thought to be more temperature sensitive and so the rise in summer temperatures should result in an advance of treeline position."
"Surprisingly our results reveal that treelines are not universally responding to climate warming by advancing, as expected," said Harsch... "Our 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."
Read complete news at: http://www.sciencedaily.com/releases/2009/08/090812202047.htm#
Harsch M, Hulme P, McGlone M, Duncan R. Are Treelines Advancing? A global meta-analysis of treeline response to climate warming. Ecology Letters, 2009; DOI: 10.1111/j.1461-0248.2009.01355.x
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