Abstract
Abstract. Lichen heaths are declining in abundance in alpine and Arctic areas partly due to an increasing competition with shrubs. This shift in vegetation types might have important consequences for the microclimate and climate on a larger scale. The aim of our study is to measure the difference in microclimatic conditions between lichen heaths and shrub vegetation during the growing season. With a paired plot design, we measured the net radiation, soil heat flux, soil temperature and soil moisture on an alpine mountain area in southern Norway during the summer of 2018 and 2019. We determined that the daily net radiation of lichens was on average 3.15 MJ (26 %) lower than for shrubs during the growing season. This was mainly due to a higher albedo of the lichen heaths but also due to a larger longwave radiation loss. Subsequently, we estimate that a shift from a lichen heath to shrub vegetation leads to an average increase in atmospheric heating of 3.35 MJ d−1 during the growing season. Surprisingly, the soil heat flux and soil temperature were higher below lichens than below shrubs during days with high air temperatures. This implies that the relatively high albedo of lichens does not lead to a cooler soil compared to shrubs during the growing season. We predict that the thicker litter layer, the presence of soil shading and a higher evapotranspiration rate at shrub vegetation are far more important factors in explaining the variation in soil temperature between lichens and shrubs. Our study shows that a shift from lichen heaths to shrub vegetation in alpine and Arctic areas will lead to atmospheric heating, but it has a cooling effect on the subsurface during the growing season, especially when air temperatures are relatively high.
Highlights
Lichen heaths are one of the most dominant vegetation types across alpine and Arctic areas (Cornelissen et al, 2001)
Our study shows that a shift from lichen heaths to shrub vegetation in alpine and Arctic areas will lead to atmospheric heating, but it has a cooling effect on the subsurface during the growing season, especially when air temperatures are relatively high
Considering the higher albedo of the lichen plots compared to the shrub plot that we reported in an earlier study (Aartsma et al, 2020) and the insulating properties of terricolous lichens (Porada et al, 2016), we hypothesize that (1) lichen heaths have a lower net radiation than shrubs, (2) the soil heat flux is lower below lichens than below shrubs, and (3) the soil temperature is lower below lichens than below shrubs
Summary
Lichen heaths are one of the most dominant vegetation types across alpine and Arctic areas (Cornelissen et al, 2001). The lichen cover has decreased in alpine and Arctic areas during the last decades (Cornelissen et al, 2001; Joly et al, 2009; Elmendorf et al, 2012; Lang et al, 2012; Fraser et al, 2014; Maliniemi et al, 2018). A long-term study reported a decrease in lichen cover of up to of 25 % in northern Fennoscandia (Maliniemi et al, 2018). This indicates that similar declining trends have been observed throughout alpine and Arctic areas. The lichen decline is attributed to grazing by reindeer (Joly et al, 2009; Bernes et al, 2015) and an increased competition with vascular plants that benefit from climate change, especially shrubs
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