Microclimatic comparison of lichen heaths and shrubs: shrubification generates atmospheric heating but subsurface cooling during the growing season

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.

Tundra vegetation change and impacts on permafrost

“…Ground surface layers such as plant litter and moss and lichen understories also exert significant controlling influence on thaw depths 109,119,142 , as has been illustrated in moss and litter manipulation experiments [109][110][111][112] . Mosses often form the understory of tundra vegetation, particularly in wetter tundra regions, and can form thick mats with low thermal conductivity, thus effectively insulating the permafrost [110][111][112]143 .…”

“…The insulation depends on the thickness of the moss mat and its moisture status, where moss thermal conductivity has a positive linear relationship with moss moisture content 111 , similar to soil organic layers 4,115,144 . In contrast to mosses, lichens do not contribute much to the attenuation of ground heat fluxes despite having low thermal conductivity, due their low thermal capacity 45,142 . Spatiotemporal patterns of organic soil layers such as peat, and thus thermal properties of the soil, are strongly controlled by microtopography, permafrost characteristics and hydrology 4,29,47 .…”

The application of dendrometers to alpine dwarf shrubs – a case study to investigate stem growth responses to environmental conditions

“…In general, shrubs are considered one of the most responsive plant functional groups to climate variability (Elmendorf et al, 2012). Their expanding trend, in turn, has been associated with climatic feedbacks, such as influence on surface albedo and frozen-ground processes (Sturm et al, 2001;Chapin et al, 2005;Blok et al, 2011;Aartsma et al, 2021). Therefore, an understanding of shrub growth physiology and its environmental controls is of crucial importance.…”

Equilibrium in soil respiration across a climosequence indicates its resilience to climate change in a glaciated valley, western Himalaya

“…Over centuries, continued warming and soil mineralization allowed colonization by shrubs like Artemisia spp., Juniperus spp., Rosa sericea , Lonicera spp. (Supplementary Table S1 online) and transformation of alpine meadow to scrub (3500–3800 m) 30 , 31 . High temperature-induced low moisture in the scrub further limited the increase in SR, resulting in similar SR rates than that in the meadow 32 .…”