Initiation of Sphagnum moss hummocks in bogs and the presence of vascular plants: Is there a link?

Establishment of specific vascular plants and Sphagnum species, as well as asymmetrical competition and facilitation between the two types of plants are apparently important in the development of microtopography in peatlands. To determine whether peatland vascular plants can facilitate the initiation of Sphagnum hummocks, and consequently the differentiation of bog microtopography, we investigated the effects of vascular plant life form and structure on Sphagnum stem length and biomass. We showed that Sphagnum stem length and biomass were enhanced by low density of vascular plants and other introduced structures, which thus favoured hummock formation. Dense covers of vascular plants also promoted moss height growth, but the Sphagnum stems were etiolated and fluffy, their densities were too low and biomass was too small to initiate clear hummocks. We also showed that vascular plants contributed to microhabitats with stable temperatures and high relative humidity favourable to Sphagnum growth. Stress-gradient hypothesis, predicting that the relative frequencies of facilitation and competition events will vary inversely along abiotic stress gradients, could explain the nature of the interaction between mosses and vascular plants. At the onset of microstructures formation in peatlands, abiotic stress is probably more important and facilitation events could be frequent. Microclimatic effects of vascular plants may be essential for Sphagnum growth. Then, the presence of ericaceous shrubs or young trees enhances the microtopography by physically reinforcing the hummocks. During accentuation of microtopography, the positive interactions between Sphagnum mosses and vascular plants may be replaced by competition as abiotic stress declines. This study introduces new evidence for the role of vascular plants in the formation and maintenance of hummocks, especially under drier growing conditions. Crown Copyright (C) 2011 Published by Elsevier Masson SAS. All rights reserved.