Fertility and Disturbance Gradients: A Summary Model For Riverine Marsh Vegetation

Abstract

Although freshwater shorelines occupy extensive areas of the temperate zone, we still have few conceptual models for pattern and process in shoreline vegetation. This study uses multivariate vegetation data to describe vegetation—environment relationships in a set of riverine wetlands and then explores general relationships between pattern and process. Samples were collected from five marshes along the Ottawa River (eastern Canada) (n = 94 sample units). Detrended correspondence analysis was used to described major gradients, and TRWINSPAN was used to classify vegetation types. TWINSPAN produced four major classes dominated by Sparganium eurycarpum, Eleocharis smallii, Scirpus americanus, and Typha latifolia. Within each class, two associations could be recognized, differing in the degree to which one species managed to dominate the vegetation. Ordination showed that these vegetation types were arranged along two major axes: a standing crop and litter gradient, and a water depth gradient. Species richness was greatest just above the late August waterline in Eleocharis smallii vegetation that had low fertility, intermediate total biomass (250g/m 2) and low littermass (30 g/m 2). Very high biomass (>400g/m 2) was observed where indices of high fertility and low disturbance coincided. Low species richness in this Typha—dominated vegetation is thought to be a result of competitive exclusion. Exposure to waves, ice, and flowing water produced a fertility gradient. The least fertile sites had small evergreen species such as Eriocaulon septangulare and Ranunculus flammula. These species possessed traits associated with Grime's "stress tolerator" strategy. The three main factors controlling vegetation competition were water depth, the effects of spring flooding in removing litter, and the fertility gradient produced by waves and flowing water. These were incorporated into a conceptual model including both patterns and processes observed along the Ottawa River.