Ecology of Alpine Lichen Heaths: Relationships Between Vegetation, Soil Temperature, and Edaphic Factors in Southern Norway

Abstract

Alpine lichen heaths are found on nutrient-poor and deep soils in the mountain plateaus in southern Norway. These are generally characterized by terricolous or so-called “mat-forming” lichens dominated by those in the genera Cladonia and Flavocetraria. Lichens, technically speaking, are not plants; instead, they are a symbiosis between a fungus and a photosynthetic alga or cyanobacterium. They are an important component of the mountain vegetation community and are chiefly known for their importance as the winter forage for reindeer (Rangifer tarandus). Several studies have shown decreases in lichens throughout their range, and these declines are usually attributed to a combination of overgrazing, nitrogen deposition, and a mixture of direct and indirect effects of climate change. Lichens are also poor competitors and are quickly pushed aside by faster-growing, taller vascular species. As such they are usually found on sites with little to no competition from vascular plants.

This study was established to investigate the relationships between snow, soil temperatures, soil properties, and vegetation in areas with differing climates, with a particular emphasis on lichen-dominated communities. Three mountain areas were chosen along a climactic gradient from a semi-oceanic area in the southwest to a semi-continental area in the northeast. Transects of varying length were laid out across the landscape, following the local topography from windy, generally snow-free ridges down to early snowbed communities. Vegetation analyses were performed using 2X2 meter subplots randomly placed within subjectively chosen plots located in areas of heterogeneous vegetation along the transects. Soil temperatures were recorded at 5 and 30 cm below the surface in the plots. The soils in the subplots were also sampled to a depth of ca. 10 cm, and three soil profile pits were excavated at each study area to aid in characterizing the soils and growing conditions at these areas.

Daily average soil temperatures at 5 cm over a period of 33 months were used in the analyses and were compared to vegetation composition using both regression and ordination analysis. Several composite ecological variables based on soil temperatures were defined and compared to vegetation abundances. These included growing season start, end, and length; number of days with soil temperatures below 0° C; the amount of time it took for the soils to thaw completely, frost sum, and heat sum. The soil samples from profiles and from subplots were analyzed for pH, percent carbon (C), nitrogen (N), and organic matter (OM) along with concentrations of plant available phosphorus (P), potassium (K), magnesium (Mg), and calcium (Ca). These data were also compared to both soil temperature and vegetation data.

Lichen-dominated plots were generally characterized by seasonally frozen soils with low concentrations of N, P, K, and Mg. The analyses showed a strong correlation between lichen abundances and soil temperatures. Lichens were dominant on sites where the soils were coldest, and were scarcest where soil temperatures were above freezing throughout the year. Additionally, lichen-dominated sites had lower mean daily soil temperatures throughout the year. This is most likely due to a combination of their high albedo and the insulating effect of the lichen vegetation itself, which forms thick mats comprised of living tissues at the top and dead and decaying tissues at the bottom. There were also a number of correlations between vegetation abundances and the composite ecological variables. Differences were found at both the species and functional group level and shows the importance of soil temperatures for the vegetation distribution pattern.

Climate models for southern Norway predict increasing precipitation throughout the year, increased snow depths in mountain areas, and shortened snow seasons – a more oceanic climate. These changes could affect vegetation abundances primarily due to the effects of snow on soil temperature patterns. Lichen species and other vegetation types that rely on frozen soils could face increasing competition and a reduction of suitable habitat. This work has shown the importance of seasonally frozen soils to vegetation distribution patterns in alpine areas.