The lagg ecotone: an integrative part of bog ecosystems in North America

Abstract

Ecological gradients have always been a central theme of plant ecology. Yet, very little is known on the bog–lagg–mineral land gradient. Improved knowledge of this gradient is important to the understanding of wetland plant ecology and wetland delineation alike. In this study, the relation between vegetation composition and peat thickness is analysed in 20 raised bogs of eastern Canada using split moving window dissimilarity analysis. Four different transition types are identified: (1) an abrupt transition without an ecotonal community; (2) a narrow transition with a lagg-swamp ecotonal community (most common transition type); (3) a narrow transition with two ecotonal communities (a lagg-fen and a lagg-swamp); and (4) a broad transition with a large wetland adjacent to the bog. Laggs that could be defined as separate habitat (type 1) occured, on average, on peat soil over 30 cm thick (defined as > 30 % dry mass of organic matter), making the lagg an integral part of the peatland complex. Other unique features of the lagg are: (1) a very densely structured habitat where trees, shrubs, herbs and Sphagnum mosses are abundant; (2) soils characterised by relatively high contents of total nitrogen and total phosphorus, but a particularly low N:P ratio (8.7 ± 2.5); and (3) a relatively high water table, which fluctuates substantially (from −33 to +10 cm in site A; from −12 to +10 in site B).