- Facilitative succession in a boreal bryophyte community - 65 Journal of Vegetation Science 17: 65-76, 2006 © IAVS; Opulus Press Uppsala. Abstract Question: What are the drivers of bryophyte succession in paludifying boreal Picea mariana forests? Location: The Clay-Belt of Québec and Ontario, Canada. Methods: The bryophyte community and habitat variables (forest floor thickness, water table, stand density, canopy openness micro-climate and presence of ericaceous species) were analysed in a chronosequence of 13 stands from 50 to more than 350 years since fire. Results: Across the chronosequence, feathermosses were re- placed by shade and desiccation tolerant slower growing hum- mock Sphagna and then by faster growing hollow Sphagna. These changes were linked with both increasing light avail- ability and the movement of the water table into the forest floor. Conclusions: As water table rise is dependent on forest floor thickness, which is in turn influenced by the presence of Sphagna, this successional sequence represents an example of facilitation. Furthermore, it emphasizes the importance of water table rise in determining stand level, and landscape level variables such as carbon balance. Keywords: Black spruce; Clay Belt; Feathermoss; Habitat variables; Picea mariana; Species replacement; Sphagnum; Water table. Nomenclature: Anderson (1990) for Sphagnum, and Crum & Anderson (1981) for other mosses. Abbreviations: CWD = Coarse woody debris; DCA = Detrended Correspondence Analysis; LCR = Live crown ra- tio; PAR = Photosynthetically active radiation; pCCA = Par- tial Canonical Correspondence Analysis; TSF = Time since fire; VPD = Vapour pressure deficit. Facilitative succession in a boreal bryophyte community driven by changes in available moisture and light Fenton, Nicole J. 1* & Bergeron, Yves 1,2 1 UQAT-UQAM National Science and Engineering Research Council Industrial Chair, Université du Québec en Abitibi- Témiscamingue, 445 Boul. de l’Université, Rouyn-Noranda, Québec J9X 1C5, Canada; 2 E-mail yves.bergeron@uqat.ca; * Corresponding author; E-mail nicole.fenton@uqat.ca Introduction Boreal forests are distinguished from many other biomes by the importance of the bryophyte layer in ecosystem functioning. The bryophyte layer constitutes an important component of the biomass, and influences total net primary production (Gower et al. 1997; Bisbee et al. 2001) and soil respiration (O’Connell et al. 2003). An example of the influence of the bryophyte layer is the paludification of boreal forests, a phenomenon by which a forest on mineral soil is transformed into a treed peatland (Crawford et al. 2003) via the accumulation of a thick forest floor and a rising water table (Glebov & Korzukhin 1992). Associated with these changes in the soil there is an establishment and subsequent expansion of Sphagnum spp. mosses into the previously bryophyte layer dominated by feathermosses (Pleurozium schreberi, Ptilium crista-castrensis, Hylocomium splendens), oc- curring ca. 100 years after a fire. The ecosystemic consequences of this change in bryophyte functional groups can be dramatic; as compared to feathermosses, Sphagnum has a higher carbon fixation rate (Bisbee et al. 2001; Swanson & Flanagan 2001), a greater buffer- ing effect on soil temperature (Dioumaeva et al. 2002) and a slower decomposition rate (Swanson & Flanagan 2001; Turetsky 2003). Furthermore, wetter Picea mariana-Sphagnum stands have a longer fire cycle and because of their greater humidity they lose less carbon when they are burned (only 12% vs. 33% of the net primary production according to Harden et al. 2000). The replacement of species along a successional gradient can be viewed as being driven either by the population life span of individual species, or by interac- tions among species and habitat conditions (Bazzaz 1990). Bryophyte colonies in forest floors may have almost unlimited life spans due to their continual upward growth as long as conditions remain un- changed, therefore it is unlikely that the shift ob- served is due to the death of feathermoss colonies. In light of this, the shift in bryophyte functional groups