About the thesis:
Global warming causes large areas of lichen heaths to become overgrown and outcompeted by shrub vegetation in alpine and Arctic areas all over Norway and other countries.
We found that this decrease in lichen heaths leads to further heating of the atmosphere, but (surprisingly) to cooling of the underlying soil. We measured that lichens reflect more sunlight than shrubs because of the lighter colour of the lichens. This stronger reflection of sunlight results in less energy being available to heat the direct environment of lichens compared to shrubs. Therefore, the atmosphere will heat up more when shrubs overgrow and outcompete the lichen heaths.
It was expected that the soil temperature below lichens would also be lower in comparison to shrubs. However, we found the opposite, as the soils below shrubs were cooler than below lichens. We relate this to the shading of the soil by the leaves and branches of the shrubs and to the thicker litter layer below shrubs, consisting of dead leaves and roots, which is insulating the soil.
This cooling effect of shrubs on the soil might have important consequences for the permafrost and the carbon storage in the soil after lichen heaths shift into shrub vegetation.
The measurements that led to these findings were conducted at Imingfjell, a mountain area in Norway at the border between the counties Vestfold og Telemark and Viken during the summers of 2018 and 2019.
We measured the incoming and reflected solar radiation as well as the thermal radiation from the Earth and the atmosphere simultaneously at lichen heaths and shrub vegetation. At the same time, we measured the soil temperature and thermal energy that flows into the soil below the same lichens and shrubs.
During the summer of 2019, we applied several treatments on some of the lichen and shrub plots in order to gain knowledge on the reasons for the difference in soil temperature between lichens and shrubs.
Future studies should quantify the increase of the air temperature when large areas of lichen heaths will turn into shrub vegetation over the entire northern hemisphere. Moreover, the consequences of the cooling effect of the shrubs on the soil compared to the lichen heaths should be studied.