Landscape Features, Vegetation and Developmental History of a Patterned Fen in South-Eastern Labrador, Canada

Abstract

U.SA. SUMMARY (1) A general description of landforms, vegetation and environmental characteristics of Leech Lake Peatland, a patterned mire complex located on the Eagle Plain, south-eastern Labrador, is presented. Particular emphasis is given to the developmental history of a minerotrophic mire, Leech Fen, and to the process of pattern (string-flark) formation. (2) Nine floristic assemblages are recognized by the releve method. The noda are characteristic of specific landforms related to a gradient of substratum height above the water table. (3) The water chemistry is typical of poor fens: pH 4.7-6.1; Ca2+ < 2.0 mg l-1; conductivity 4-5-11 uS cm-l. (4) Lateral water movement down the slope of patterned fens occurs through four pathways: sheet flow across the fen surface; surface flow in narrow channels along fen margins; surface flow in channels in strings; and percolation through near-surface layers of peat. (5) The development of surface patterns is largely dependent on topography and water movement. Steep slopes with large inputs of water have terraces of alternating strings and deep pools, whereas gentle slopes or areas with little water movement are generally unpatterned. Substratum topography and the distribution of surface features are unrelated. (6) The lower part of Leech Fen developed in a shallow bay of Leech Lake with the present shoreline stabilized by a forested ice-push ridge. In the upper part of the fen, peat growth progressed slowly up the slope. Most of the initial peat accumulation in the fen occurred after 4500 B.P. Sedges dominated the early vegetation, with shrubs increasing in importance later. Pattern formation did not occur until after the shrub vegetation had become established. (7) A proposed process for the development of the surface pattern of strings and flarks includes three parts: (i) development of an irregular surface of hummocks and hollows; (ii) gradual expansion and joining of depressions across the slope controlled by differential rates of peat accumulation; (iii) expansion and coalescence of pools through active peat degradation, which can be observed in Leech Lake Peatland at present. Once formed, pools are permanent features on the mire surface in which degradative processes remove all allochthonous and autochthonous materials.