[PCA] Trees advance in a warming world

Patricia_Ford at fws.gov Patricia_Ford at fws.gov
Tue Aug 25 08:15:49 CDT 2009


Trees advance in a warming world
Matt Walker 
Editor, Earth News 
Trees around the world are colonising new territories in response to 
higher temperatures.
>From the US west coast to northern Siberia and south-east Asia, trees are 
growing at higher elevations, and at higher latitudes as the climate 
warms.
Of 166 sites studied, trees are advancing at more than half, while they 
are receding at just two sites.
The shift is revealed by the first global analysis of treelines published 
in the journal Ecology Letters.
However, the trees aren't responding quite how scientists expected.
Instead of advancing as summer temperatures rise, the trees' ability to 
colonise new areas appears to be more dependent on whether winter 
temperatures warm.
Hospitable territory
Treelines tend to form wherever conditions for growth become too harsh.
For example, at high altitudes and latitudes, the climate often becomes 
too cold for trees to survive. At this boundary, a treeline occurs, with 
forest on one side and shorter, hardier plants such as shrubs and plants 
on the other.
However, around the world, average air temperatures have risen during the 
past century.
This warming has been most pronounced at high altitudes and latitudes, the 
exact places where treelines form.
So in theory, trees should take advantage of these warmer, more hospitable 
climates, allowing treelines to advance higher and closer towards the 
poles.
In reality though, the picture has been more complicated. In some places, 
trees have advanced higher up mountains or further north, but in others 
they appear not to have moved.
To better understand what is going on, Melanie Harsch of the 
Bio-Protection Research Centre at Lincoln University in New Zealand and 
colleagues conducted a meta-analysis of a global dataset of 166 sites 
around the world at which treeline dynamics have been recorded since 1900 
AD.
When they analysed this data, the researchers found that air temperatures 
had increased at 111 of 166 sites, at an average rate of 0.013 degrees 
Celsius per year.
Summer warming occurred at 117 of the sites at a mean of 0.0189 degrees 
Celsius per year, more often than winter warming which occurred at 77 
sites at an average of 0.0199 degrees Celsius per year.
Most important, they found that treelines had advanced into previously 
inhospitable habitat at 87 sites.
The treelines remained stable at 77 sites, while trees had retreated at 
just two locations.
Crucially, the trees do not seem to be responding to warmer summer 
temperatures.
"We expected growing season warming to be the dominant driver," says 
Harsch.
"But we found that it was not, winter temperature was."
That could be because trees that have advanced during warm summers can 
more easily survive the odd cooler summer. Whereas those that advanced 
during warmer winters may not survive a particularly cold winter, making 
winter temperatures the limiting factor.
Also, the researchers found that diffuse treelines, where the number of 
trees growing gradually peters out, are much more likely to advance than 
abrupt treelines, which form when trees suddenly stop growing at a certain 
altitude or latitude.
That could be because other stress factors, such as wind abrasion, snow 
and ice damage limit the spread of species which create abrupt treelines.
Story from BBC NEWS:
http://news.bbc.co.uk/go/pr/fr/-/earth/hi/earth_news/newsid_8218000/8218335.stm


Published: 2009/08/25 09:37:57 GMT

© BBC MMIX

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