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Warming climate could mean thinner northern forests, UW researchers say

Anita Weier  —  7/14/2008 4:07 pm

UW-Madison scientist David Mladenoff has been warning for years that some trees common to northern Wisconsin -- balsam fir, spruce and jack pine -- could disappear from the state as the climate warms.

But now Mladenoff and fellow UW forest ecologist Robert Scheller are adding that it will be difficult for southern Wisconsin species -- oaks and hickories for instance -- to move northward to replace them.

Why?

Not only is warming expected to outpace the speed at which southern trees can migrate, but barriers to dispersal such as agricultural lands and urban areas also will delay progress, Mladenoff said.

Consequently, the standing amount of forest up north could decrease. Currently filled-out forests could thin.

"The trees that are there now will be experiencing less than optimal conditions, and the southern species aren't going to fill in as quickly as we'd like," Mladenoff said.

Trees move into new areas by producing seeds, which are carried over short distances by wind, birds or mammals. Under the right conditions, dispersed seeds then grow into seedlings and eventually mature trees, which produce their own seeds.

It's a slow process, but dispersal becomes even slower when forests are fragmented -- broken up by farms, cities or suburbs. It can be difficult for seeds to cross such gaps. A wide band of agricultural land that runs across the middle of the state would be a major obstacle, he warned.

Inter-species competition also might be a factor, with hemlock reducing dispersal of less shade-tolerant southern species.

Scheller and Mladenoff used satellite information and forest inventory data to predict how landscapes will respond to climate shifts. They used climate predictions to examine probable forest succession, seed dispersal and tree growth during the 200 years since 1990, and their findings were published in the current issue of Climate Research.

But despite their gloomy report, there may be ways to speed the migration.

Woodland managers might start testing certain southern Wisconsin species in the north on a trial basis, Mladenoff suggested.

The state might even consider bringing back the field trials that were done in the 1950s and 1960s, when researchers collected genetic variants of individual tree species all over the state and planted them in many locations to see where they did best, he said.

"A lot of this is about our incomplete knowledge of how genetically diverse some species are, and how adaptable they may be in different climates," Mladenoff explained.

Anita Weier  —  7/14/2008 4:07 pm