Forest succession on alluvial landforms of the McKenzie River Valley, Oregon

Using association tables and a two-dimensional ordination procedure, two topo-edaphic climaxes(associations) have been identified. These are: (1) the Tsuga heterophylla/Polystichum munitum-Oxalis ()regatta association occurring on terraces with fine, sandy loam to silt loam soils derivedfrom silty river alluvium; and (2) the Tsuga heterophylla/Berberis nervosa-Gaultheria shallonassociation occurring on rockier soils with coarser alluvium or glacial outwash. Two early seralassocies, each dependent on sedimentation, plus later seral associes, at least one dependent on recentfires, are also identified. A major factor leading to vegetation differences appears to be moistureavailability in late summer. (The soils in the Tsuga heterophylla/Berberis nervosa-Gaultheriashallon habitat type are shallower, coarser, and rockier than those in the Tsuga heterophylla/Polystichum tnunitum-Oxalis oregana habitat type). This hypothesis is supported by plant-moisture stress-measurements on saplings growing in each of the topo-edaphic associations.