Relationship between Plant Distribution Patterns and the Process of River Island Formation

ABSTRACT

We examined the distribution of trees and shrubs on 14 mid-channel islands in the Swan River, northwest Montana. We observed the general trend that plants were distributed in a linear sequence of seral stages. Pioneer successional phase plants dominated the downstream ends of islands, mid-successional vegetation dominated the middle of the islands, and late successional vegetation and the oldest trees were largely restricted to the upstream portions of the largest islands. The observed pattern was most distinct on islands farthest upstream in the river channel (i.e., smallest catchment area) and least distinct on islands farthest downstream in the catchment basin. We conclude the consistent pattern of seral organization and the influence of catchment size would be unlikely if the development of mi-channel river islands were controlled by site-specific factors influencing individual islands independently. We suggest that all islands are responding similarly to the shared influence of fluvial dynamics (i.e., timing, duration, and magnitude of rise and fall of river discharge), but smaller scale variability in plant distribution among individual islands suggests site-specific variables and/or random events modify the action of fluvial dynamics.