Bogs and their laggs in coastal British Columbia, Canada: Characteristics of topography, depth to water table, hydrochemistry, peat properties, and vegetation at the bog margin

The transition zone at the margin of raised bogs (the lagg) is rarely studied, yet it can be important for maintaining a high water table in the peat mound. Where the lagg has been damaged or lost to agriculture, industry, or residential development, it may be necessary to restore a functional lagg inside the historic bog boundary to maintain the ecological health of the bog. Seventeen laggs from raised bogs in coastal British Columbia (BC) were studied to determine the natural range of lagg characteristics in this region. The laggs could be separated into two hydrotopographic forms: Marginal Depression (with mean early summer depth to water table of 12 cm and mean tree basal area of 2.8 m2/ha) and Flat Transition to forest (with mean early summer depth to water table of 34 cm and mean tree basal area of 26.3 m2/ha). These hydrotopographic forms were further classified into four vegetative lagg types: 1) Spiraea Thicket, 2) Carex Fen, 3) Peaty Forest, and 4) Direct Transition to forest (no lagg ecotone). The Carex Fen and Direct Transition lagg types were generally found in the Pacific Oceanic wetland region (cool, wet climate), while the Spiraea Thicket and Peaty Forest lagg types were more common in the Pacific Temperate wetland region (relatively warmer and drier climate). Regional differences in bog and lagg characteristics appear to be related to mean annual precipitation and mean annual temperature. The timing of seasonal fluctuations in depth to water table were similar for bogs and laggs, but the amplitude was generally greater in the lagg. Near-surface pore-water chemistry varied across the bog expanse – bog margin transition: pH, Ca2+ concentrations, and pH-corrected electrical conductivity generally increased from bog to lagg, although not consistently for individual study transects. Mg2+ and Na+ concentrations increased from bog to lagg for less than half of the studied transects. The most consistent indicators of the lagg, which may be of greatest use for delineation of lagg conservation zones include: topography, depth to water table, tree basal area, ash content of the peat, and dominant species.