Forest Succession and Sedimentation on a Meandering-River Floodplain, Northeast British Columbia, Canada

Abstract

An examination of primary forest succession within two migrating bends along the Beatton River in northeast British Columbia provides information on the interrelationships between the geomorphic and biotic components of a meandering-river floodplain. Overbank sedimentation rates are estimated from changes in the wood structure of partially buried balsam poplars, and from distinct changes in floodplain slope. Particular emphasis is placed upon the changing age-structure of dominant tree species to describe successional changes within the floodplain forest. Rapid sedimentation on the youngest ridges favours the establishment of a dense, non-reproducing balsam poplar stand of uniform age, and prevents the establishment of white spruce seedlings. Following an abrupt decline in overbank sedimentation on surfaces approximately 50 years old, white spruce rapidly colonize the bare mineral soil beneath the poplar canopy and form a dense, relatively even-aged stand. With the death of mature poplars on surfaces 100-150 years in age, the spruce seedlings are released and develop into a mature, non-reproducing stand. This stand persists until its senescence on ridges 350-400 years old. Here, a second, similar, although less even-aged spruce stand establishes, maturing in a like manner to the flrst, to eventually decline on ridges 500-550 years in age. On this older surface there is the suggestion of yet a third generation of spruce seedlings. These oscillations are reflected in changes of the vascular understory-species during succession, and compare to the cyclic regeneration and climax instability described by Watt (1947) and Whittaker (1953). Tree densities and overbank sedimentation achieve minimum values on floodplain surfaces greater than approximately 200 years in age. A reasonable explanation to account for negligible sediment deposition on these older surfaces is that the more open trunk density there permits relatively rapid overland flow, preventing the deposition of suspended sediment load. Sedimentation and vegetation appear to be strongly interrelated components within the total floodplain system.