Trees plus temperature change is not a simple equation
It seems obvious that warmer temperatures are good for trees. After all, trees tend be smaller, the further north or the higher up the mountains they grow. Eventually, at treeline, they stop growing altogether.
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Lodgepole pines produce new pollen cones, like these, every growing season.
(photo: Claire Eamer)
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"We know temperature is a limiting factor," says Scott Green, a forestry researcher from the University of Northern British Columbia. "In colder climates, trees don't grow very fast and they don't grow very big."
In the past, the general assumption has been that warmer temperatures associated with climate change will increase the growth and reproduction of northern trees, resulting in the Yukon's forests moving both further north and further up the mountains.
But it's not that simple, says Green. In fact, it's not at all clear how climate change will affect Yukon forests.
"The interactions are so complex it's really hard to get a handle on it."
Green is trying to get a handle on it. Within a year or two, he hopes to set up a detailed study in the Yukon of how climate affects trees. In the meantime, he's studying seedlings in a greenhouse to see how several factors affect their life histories.
An important aspect of a tree's life history is coping with the end of the growing season. Like the rest of us, trees make preparations for surviving the cold of winter, and it's important to make those preparations at the right time.
It's particularly important for trees at the edge of their range, like the evergreens of northern forests, says Green.
Trees invest their energy and their future in the yearly growth of buds. If the buds remain unset and unprotected too late in the season, the entire season's growth can be lost -- and the tree itself might not survive.
Something tells the trees when to start preparing, but it's not necessarily temperature, says Green. The main variables he's been looking at this summer are temperature and photoperiod, or the length of the day.
What's clear at this point, he says, is that there is "a tremendous amount of species variation."
Green's seedlings are typical northern species: spruce, subalpine fir, and lodgepole pine. All three have large geographical ranges approaching both northern and elevational treelines.
He has kept the seedlings cozy, much cozier than they would be outside the greenhouse, to see how they respond. Will they continue growing or will they begin shutting down for the winter, just as they would in the open?
"What I'm finding is that different species cue in to different signals," he says.
His spruce seedlings have shown little sensitivity to photoperiod at all. The seedlings are thriving in the comfortable warmth of the greenhouse, with no sign of shutting down for the winter despite rapidly shortening days.
"Spruce is just chugging right along, happy as a clam," says Green.
The subalpine fir seedlings, on the other hand, are ignoring the temperature and making preparations for winter. Subalpine fir has shown a strong sensitivity to photoperiod, despite the favourable growing conditions maintained in the greenhouse, Green says.
Lodgepole pine seems to fall somewhere in the middle, responding to some degree to both factors.
Green hastens to point out that his greenhouse experiment provides no more than a hint about how forests will respond to climate change. It touches on only a couple of variables and a few species. Trees and forest systems are vastly more complex than that, and so is climate.
"One of the problems is that we're trying to understand a change that hasn't occurred yet, and the system is so complex that it's hard to mimic all the factors involved."
The Yukon study Green is planning will take a more complex and realistic look at how the trees of northern forests respond to temperature. By studying plots at different elevations within the same geographical area, he hopes to sort out the various climatic factors that affect different tree species' life histories.
Green isn't expecting to get all the answers about trees and climate change from his Yukon study, but a few answers would help.
"It's very difficult to definitively look at tree response to climate," he says. "It's just a really hard nut to crack.
- For more information about forests and climate change, go to forest.c-ciarn.ca on the internet or contact Scott Green at greens@unbc.ca.
