HYDROLOGICAL INFLUENCES ON VEGETATION AND MARL FLAT PATTERNS IN MINEROTROPHIC FENS

The water balance and hydrological flowpaths through three calcareous fens in close proximity to one another in southern Ontario, Canada were studied to examine the effect of hydrology on vegetation patterns. Networks of piezometers and wells, discharge and meteorological data, water quality measurements, and vegetation surveys were used to meet this objective. The largest fen (~ 2.8 ha) studied comprised four distinct fen vegetation zones and an area of marl formation and derived the majority of its water from a spring-origin stream that bisected the site. The vegetation patterns and carbonate precipitation were related to the water availability and chemical composition, which were controlled by the stream. A long, narrow fen (~ 0.5 ha) received the bulk of its water during spring snowmelt seepage from an upslope pool and bank overflow from an adjacent stream. The vegetation pattern followed the long axis of this site, which was coincident with the hydraulic and nutrient gradients. A small, egg-shaped fen (~ 0.28 ha) received comparatively small water inputs from groundwater upwelling, subsurface contributions from an upslope cedar thicket, and precipitation, and had the largest water table fluctuation. The vegetation of this fen responded to the water sources by the establishment of small but tall (up to 60 cm) hummocks and interconnected hollows that dried-up by late June. This research suggests that there is large variability in peatland form and function within a given hydrogeologic region and that local, site-scale hydrology has a tremendous influence on fen vegetation composition, patterns, and diversity.