Importance of air and soil temperatures for bioclimatic delimitations in Norwegian mountains

Abstract

Distribution limits of plants in relation to climatic variables have long interested ecologists. High latitudes and altitudes house distribution limits of forests, treelines, treeline ecotones and vascular plants in alpine and Arctic areas, which are all considered to be climatically sensitive. Separation of Arctic, alpine and boreal biomes have mostly been defined by forest distribution limits. The biomes are bioclimatically defined, consequently it should assumed that they are defined both in terms of biota and climatic variables. Temperatures (air and soil) have been considered as one of the most important factor in limiting tree growth and determining the position of treeline; beyond treelines we come across open areas usually dominated by heath communities. Earlier studies have focused on air temperatures and rarely included measured soil temperatures. Recent studies have however, shown that plant distribution limits are often better explained by soil than by air temperatures. Also, soil characteristics are highly affected by temperature variables.

The main focus of this thesis has been to study vegetation, air and soil temperature conditions with increasing latitude and altitude in southern and northernmost birch forest limits in Norway. The forest limit position in northernmost Norway has been used as the main criteria for including the narrow coastal heath fringe in the Arctic biome. If this criteria is useful, it should be assumed that the forest limit is climatically defined. In addition, coastal heaths of northernmost Norway have been an issue of debate since decades on whether it belongs to Arctic biome. Data on temperature and soil properties have been collected from the northernmost coastal heaths along with adjacent heaths located south of polar treeline, and compared with Arctic heaths from Bear Island and Spitsbergen. Plants and plant communities found growing on high mountain summits often represent limits for plant distribution, we have studied relationships between air- and soil temperatures and plants at summits from south to north Norway. We have also tried to answer if we could use air temperatures as proxy for soil temperatures.

Norway along with adjoining Islands spanning from 57 °N to 81 °N latitude; with its undulating and rugged topography, high mountains (0 - 2469 m) and huge fjords, offers a perfect study area. For this study, sampling was conducted in alpine treeline ecotone, forests, forest limits, treeline and treeless heaths. Data on vegetation communities, soil temperature, air temperature, soil properties (only for Paper II and III), tree-ring (only for Paper II) and topography have been gathered. Study plots (2 X 2 m) were randomly selected in homogeneous vegetation stands for data collection. Air temperatures at 2 m height above ground were interpolated from gridded temperature dataset. Soil temperatures were obtained by using temperature data-loggers dug down at 10 cm below ground in each plot.

The southern limits of birch forests are located at low elevations near coastal areas and the elevation increase as we move inland. Warmer temperature conditions are recorded in coastal areas as compared to inland areas. The temperature conditions in northernmost birch forests located far north are slightly warmer than the southern forests. Additionally, the temperature conditions from northernmost forests were comparatively warmer than other such transitional zones. From our results it was clear that these northernmost forests are not limited by temperature but rather due to presence of Barents Sea, wind and topography.

We studied these northernmost coastal heaths along with heaths located immediate south of treeline, heaths from Bear Island and Spitsbergen. Our results suggests that based on vegetation, temperature and soil properties; heaths south and north of treeline were similar and it is unjustifiable to group them in to two separate zones.

Study of air and soil temperatures of vegetation plots on 19 mountain summits revealed that air temperature is generally a poor proxy for soil temperature in cold areas, except during July. From our results it was clear that vegetation composition and lichen cover influenced soil temperatures; lichen abundance and degree of soil frost were strongly correlated.