Hummock Vegetation at the Arctic Tree-line near Churchill, Manitoba

Résumé

Hummocks, small earth or peat mounds, are widely distributed in the arctic and develop as a consequence of biomass accumulation and cryoturbation in the active layer. There is general agreement that the type of vegetation covering peat hummocks may alter the accumulation rate of organic material and thus hummock growth and local carbon sink dynamics. Studies on hummock plant community compositions from the arctic are very scarce. Here, I present results of a case study from the arctic tree-line near Churchill, Manitoba (Canada). Vegetation composition, hummock height and soil moisture content were recorded in 40 peat hummocks located along a tree-line gradient from open forest to tundra. Based on a cluster analysis I found three moss-dominated types of hummock vegetation, according to (1) a Tomenthypnum nitens (golden fuzzy fen moss) type on low hummocks, (2) a Hylocomium splendens (stair-step moss) type on medium-sized hummocks, and (3) a Pleurozium schreberi (red-stemmed feathermoss) type on hummocks higher than 60-70 cm. I found hummock height to increase towards the forest interior with decreasing water content of the upper organic layer on the hummock top. This is indicated by a significant change in vegetation composition towards drought resistant moss species on higher hummocks. Furthermore, species richness decreased with increase in hummock height. Based on evidence from historical tree-line invasion the overall results suggest that hummock height increases due to peat accumulation over the course of time resulting in a typical change in plant community composition.