Forests facing an uncertain future
Although climate change is already clearly under way, the many complicating factors make it hard to predict its ultimate scale, according to Professor Eero Nikinmaa of Helsinki University, who studies interactions between forests and the climate.Since warmer air can hold more moisture, the future may bring scarcer but heavier rains. In combination with longer intervening droughts this is very bad news. If forecasts are based on an average global temperature rise limited to two degrees – the target many hope will be set at the coming Copenhagen climate summit – this would mean an average warming in Finland’s latitudes of 4-5 degrees, with warming most pronounced during the coldest seasons.
Southern Finland will lose its traditional snowy winters, and in Northern Finland winter will start to loosen its icy grip much earlier.
“But rising trends in emissions over the last ten years have already exceeded the worst scenarios forecasted in the 1990s, and people have started talking about a possible rise in global temperatures of as much as five degrees,” Nikinmaa says.
Growing seasons to start sooner
Changing temperatures will alter the annual growth cycles of plants and trees. Tree growth begins and ends according to mechanisms triggered by temperatures and light conditions. "The growing season will undoubtedly lengthen. But the extreme natural variations in light conditions in the north won't change at all.
Finland already has dark but mild autumns, with growth ending when light becomes insufficient. Forest growth rates in autumn are only about half the summer rates," explains Nikinmaa. In the springtime, climate change will contrastingly allow trees to exploit more light energy, when warmer temperatures make it possible for them to start growing sooner.
"Some signs that growth is commencing earlier can already be seen, such as the appearance of buds, and shifts in the tree line. But the greatest rises in temperatures have happened in the autumn, which does not increase trees' productivity. It does mean, however, that forests will release more carbon dioxide sooner."
Trees in competition
Within ecosystems, climate change affects the prospects of different species in diff erent ways, changing their ability to compete with other species. Versatile deciduous trees might benefi t more than coniferous species.
Pines may cope with droughts better than spruce trees, and maintain their dominance in many areas. The Norway spruce may be one of the losers, and some experts think it could even ultimately vanish from Southern Finland. Recent investments in the planting of young spruce stands might prove to be futile.
"So far tree varieties of local provenance have grown best in trials, since more southerly species are more sensitive to damage. But in future we will have to look harder to see whether spruce varieties originating from neighbouring regions such as Southern Sweden and the Baltic Countries might do well in Finland."
By Jussi-Pekka Aukia
Illustration by Vuokko Isoherranen