The carbon supply of Picea abies trees at a Swiss montane permafrost site

Background: In search for a mechanistic explanation of climatic treeline formation, permafrost sites in montane forests provide ideal conditions to compare soil temperature impacts on tree growth, as dwarfed trees on pockets of cold soil grow in juxtaposition with tall forest on markedly warmer soil. Aim: The carbon supply status of dwarfed Norway spruce trees on a low elevation permafrost site in Switzerland was investigated by analysing their non-structural carbon pools. According to the sink-limitation hypothesis of alpine treeline formation, the low soil temperatures over permafrost should restrict growth processes more than photosynthesis, causing an oversupply of carbon. Methods: Needles and sapwood of mature spruces were sampled from dwarfed trees over permafrost and from tall trees at an adjacent forest on several dates across the growing season. All tissues were analysed for non-structural carbohydrates (NSC). Branch wood was additionally analysed for lipids (acylglycerols). Results: Comparing NSC of dwarfed trees with that of surrounding tall trees, indicated higher concentrations in trees over permafrost throughout the season, except for needles sampled in May, which likely reflects the delayed start of the growing season on permafrost. On average, NSC concentrations were about 10% higher in needles, 25% higher in stem wood and 40% higher in branch wood in dwarfed compared to tall trees. Low lipid concentrations in branch wood (< 1% d. m.) suggest a minor role for lipids as carbon reserves in spruce. Nevertheless, lipid concentrations were about 30% higher in dwarfed than in tall trees. Conclusions: The enrichment with carbon reserves of dwarfed spruce on permafrost confirms previous findings from climatic alpine treelines. The results are therefore in line with the sink-limitation hypothesis of treeline formation, and suggest that the same mechanism applies to situations where only roots experience low temperatures.