Story | 09/25/2018 11:52:55

Forests are changing along with the climate

Global warming makes trees grow faster but also causes storms, heavy snow, pests and drought.

The climate has already warmed in recent decades and is expected to warm further. Climate researchers also predict that the northern hemisphere will warm quicker than the equator. This means that we can expect the growing season to be longer and tree growth to accelerate.

“There is more and more carbon dioxide in the atmosphere, which also accelerates growth. After all, it is nutrition for trees,” says Mikko Peltoniemi, Senior Scientist from the Natural Resources Institute Finland (Luke).

At first glance, this could be seen as positive development for Finnish forests, but it also has its downsides.

Heavy snow and dry summers

As oceans warm, they release steam into the atmosphere, which falls back to earth as rain. Because of this, climate change is expected to increase rain and snowfall.

“Warm, humid winters increase moisture in the snow, making it heavy. This increases the risk of snow damage to the forests”, says Hilppa Gregow, Head of Unit from the Finnish Meteorological Institute. She says that snow loads have grown and may continue growing in the southern and middle parts of the country for 30 years more, and even longer in the north.

“However, summers are becoming drier in Northern Europe, which increases the risk of forest fires. There can be periods of drought and periods of torrential rain. The weather is becoming more and more unpredictable.”

Frost-free ground increase the risk of wind damage

In addition to rainfall, winds will also increase.

“Air currents in the atmosphere have changed and storms last longer even in continental areas. They can also reach Finland, which has so far been reasonably safe from storms,” says Gregow.

Rising temperatures also cause shorter frost seasons, which further increases the risk of wind damage.

“Frost anchors roots to the ground. If there is no frost, winds can cause more damage to trees, even if there is no increase in the number of storms”, says Peltoniemi. Harvesting fallen and broken trees can be difficult on frost-free, wet ground.

“However, damaged trees should be harvested quickly, especially when the next summer is going to warm, dry and full of insects,” Gregow adds.

Biodiversity reduces risk

As scientists predict the future, forest owners can prepare for the changes. According to Gregow, current good forestry practices are the best measures. She emphasises the need to acknowledge risks right from the outset, with forest renewal.

“We need to consider carefully what to plant in each soil type and determine where the wind will hit hardest once the crown layer gets higher,” says Gregow. She also recommends a diverse selection of tree species.

“Conditions largely determine the risks but the risks are reduced with greater biodiversity of tree stands. For example, pests are usually specific to a certain tree species,” Peltoniemi says. She reminds us that biodiversity is essential to the proper functioning of nature.

Harvest timing

Gregow emphasises the importance of thinning to prevent trees from growing too tall and thin and becoming prone to strain from snow and wind. With the increased growth rate, thinning is becoming increasingly important.

Forest cycles may be 5–10 years shorter in the coming years and as much as 10–20 years shorter by 2050,” Gregow estimates. In addition to thinning, she believes that early felling is good for reducing risks from snow, wind, fire and pests.

“Natural Resources Institute Finland is drafting a map for storm damage risks. This will allow everyone to check the estimated risks for their own forest area”, tells Peltoniemi.

When forest is felled, the carbon bound in trees is released into the atmosphere over years or even weeks. If forests were allowed to grow more robustly than usual, they would bind more carbon and delay its release. According to Peltoniemi, sturdier forests could mitigate climate change in the short term.

“To mitigate climate change, all forest owners should ensure that their forest is regenerated swiftly after harvesting and that new forest begins to grow quickly,” Peltoniemi concludes.

Photo: Adobe Stock


Marianna Salin