Peatland plant communities surrounding small (<200 ha) boreal ponds were characterized by indicator species, water and peat chemistry, and diversity. Peatland–pond complexes are common in boreal Alberta and are found in three different landforms (clay-till plain, outwash plain, and moraine). Pond area and perimeter were larger in the clay-till plain than in other landforms, although not significantly different. Across the three landforms, cluster analysis detected five peatland communities: marshes, wet open fens, dry open fens, treed fens, and bogs. The zonation pattern of communities surrounding the ponds varied at all sites, and there was no typical pattern, except that marshes were always found closest to water. Based on the bryophyte species, most communities are considered moderate-rich fens. Nonmetric multidimensional scaling indicated communities fell along a wet-to-dry gradient and a bare-to-vegetated gradient. Water depth and temperature, peat depth, and peat C/N ratios differed between open and treed peatland communities, and pH was similar in all communities. Alpha and gamma diversity was highest in the treed fen and lowest in the marsh community in both total species and bryophyte species. The total number of plant species, some of which are considered rare, found in all communities was 139.Key words: marsh, fen, bog, classification, vegetation ecology, diversity.

. 2006. Inferred history of a boreal pond from sediment and vegetation characteristics. Can. J. Soil Sci. 86: 335-347. Shallow ponds associated with peatlands in boreal Alberta contain large quantities of stored carbon in their peaty sediments, and exhibit spatial changes in response to dry or wet conditions. Peatland ponds around Utikuma Lake experienced a partial to nearly complete drawdown in 2002, in response to drought. A fourfold investigation was begun to assess the natural drought and flood cycles, assess the current C stocks along the shore, determine what species were colonizing the newly exposed shorelines, and to place the magnitude of the current drought into historical perspective. Ten vegetation communities were found to have colonized the exposed sediments. Paleostratigraphy shows that the surface communities differ markedly from paleocommunities. Stratigraphy of the wetland sediments demonstrates that only two changes occurred, one a successional switch from a sedge-dominated marsh to a forested bog, the other a much more recent disturbance that caused water levels near the pond to rise. No evidence of constantly changing paleocommunities was found, suggesting that previous droughts have not left a visible paleorecord. The more recent disturbance has likely redistributed peat into the pond basins and subsequently it has broken down to detritial peat, altering its physical structure, and perhaps its rate of decomposition. Continued exposure of the peat is likely to enhance decomposition. In this study both flooding and drought may have impacted the wetland in ways likely to result in higher rates of decomposition and enhanced CO 2 emissions under global warming. , les étangs tourbeux de la région du lac Utikuma se sont presque entièrement asséchés consécutivement à la sécheresse. Les auteurs ont entrepris une étude en quatre volets afin d'évaluer les cycles naturels de sécheresse et d'inondation, de mesurer les réserves actuelles de carbone sur les rives, d'identifier les espèces colonisant les fonds exposés et de classer la gravité de la nouvelle sécheresse dans son contexte historique. Dix populations végétales colonisent les sédiments exposés. La paléostratigraphie révèle que les populations en surface diffèrent nettement des communautés plus anciennes. La stratigraphie des sédiments tourbeux ne révèle que deux changements : une modification dans la succession des plantes durant laquelle le marais de carex s'est transformé en tourbière boisée et une perturbation beaucoup plus récente qui a entraîné une élé-vation du niveau de l'eau près de l'étang. Les auteurs n'ont relevé aucun indice d'une constante modification des paléocommu-nautés, signe que les sécheresses précédentes n'ont pas laissé de signes visibles derrière elles. La perturbation la plus récente a sans doute donné lieu à une nouvelle répartition de la tourbe dans l'étang. La tourbe est devenue détritique, ce qui en a modifié la structure et éventuellement le taux de décomposition. Si la tourbe reste exposée, sa décomposition s'accél...