On the role of lichens in vegetation-climate interactions

Abstract

Current climate change is amplified over northern high latitudes, affecting the biogeochemical interactions and feedback between vegetation and climate. One step towards a more complete understanding of vegetation-climate interactions at high latitudes is to study the contribution from nonvascular photoautotrophs (NVPs), that is lichens, bryophytes, and biological soil crust communities. While NVPs have received less attention than their vascular counterparts, their contribution to boreo-arctic ecosystems is considerable. Mat-forming species often dominate vegetation and NVPs encompass great variation in physiology and adaptions that enables them to persist in a wide range of environmental conditions. This thesis focuses on the ecological and climatological interactions of the group of bright macrolichens commonly known as reindeer lichens. These relations are approached through three research objectives, where each objective is explored in a corresponding research paper. The first objective was to quantify the effect of reindeer lichens on surface albedo, i.e., solar reflectance, and investigate how surface albedo varies between vegetation types and along environmental gradients. This was investigated through collecting and analysing field observations on a vegetation type scale. The second objective was to estimate current and historic reindeer lichen volumes across the pan-Arctic and relate changes in lichen volumes to drivers of environmental change. This was investigated with a remote sensing approach on a landscape scale, looking at how lichen abundance varies spatially and temporally. The last objective was to investigate the effects of winter heatwaves on growing season ecophysiology in lichen rich vegetation, and how the effects may differ between species and type of winter stress. This was done through a field experimental approach, studying the responses on a species scale. The results show how albedo of bright reindeer lichens is likely the highest albedo of common, naturally occurring vegetation, approaching 0.4. The high albedo is significant for vegetation-climate interactions at high latitudes, especially as carbon sequestration rates are relatively low in these regions. Across the pan-Arctic, I found an overall decline in reindeer lichens in the last 40 years. The decline was largest in the first half of the study period (1984–2001) and driven by a rapid increase in Rangifer population. From 2001–2020, Rangifer herbivore pressure was released in many areas, but the increased competition from trees and shrubs likely halted the lichen’s recovery. While excessive herbivory, increase in wildfires, and summer warming benefit herbs and shrubs at the expense of lichens, winter heatwaves represent an opposite effect. Lichens prove to be much more robust against the ecophysiological stress of ice encapsulation and midwinter thaw and freeze than vascular plants and bryophytes. However, two of the three lichen study species showed some susceptibility to ground icing, and this infers different ecophysiological responses depending on the characteristics of future winter climate change. The results presented here gives opportunities for better modelling of surface albedo regulation in lichen vegetation, updated knowledge on the state of pan-arctic lichen populations, and novel insight into the responses to winter stress in lichens.