Warmer climate yields more heat-loving trees

Trees are at the mercy of the local conditions they grow in. They need sun and warmth to germinate and thrive, and summer temperatures affect how far north and how high up the mountains they can grow. However, some types of trees, such as dwarf birch, are not so picky about temperature. As long as the mean temperature of June, July, August, and September is above 4.3 degrees, dwarf birch will grow.

The dwarf birch has the lowest summer temperature requirement of all Norwegian tree species. More heat-loving tree species like elm, ash, beech, oak, hazel, lime, maple, and black alder demand higher summer temperatures. Precisely this high temperature requirement is the reason why species like ash, oak, and beech are only found in certain areas in the southern part of Norway.

But what happens if the average summer temperatures increase?

Increased temperatures over the past century have allowed different tree species to grow further into the country and higher up in the mountains than before.

Therefore, when forest owners plan for the future, knowledge is needed about how temperature changes affect the growth and development of trees. The tree species we choose to plant today – whether it is spruce, pine, birch, black alder, or beech – will have to grow and thrive in an entirely different climate than today.

Climate researchers at NVE and NIBIO have thus produced maps showing changes in mean summer temperatures over the last 60 years. Changes in mean temperature of the four summer months are relevant for forestry, in that heat-loving trees like beech, for example, might extend further into the country -- if other important requirements are also met.