. Die kanadische Feld - naturforscher. Natural History. 1997 Ruder und Collins: Vegetation Nachfolge- und Störung 555 Auflösung im Vergleich zu den Offeneren erreicht mit geflochtenen und Split. Der Bereich unmittelbar unter dem Chulitna Fluss hatte mehr Kopfsteinpflaster als die anderen beiden erreicht, aber die besten Materialien wurden in der niedrigsten erreichen. Reduzierte Gradienten und größere Breite unterhalb des Zusammenflusses führte zu langsamer Wasser Geschwindigkeiten und mehr Sand Deposition in die untersten beiden erstreckt. Die Susitna River flow ist häufig höchste im Juli und August im Sommer regnet, wenn seine Con-tributory Gletscher ein

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. The Canadian field-naturalist. Natural history. 1997 Helm and Collins: Vegetation Succession and Disturbance 555 breakup compared to the more open reaches with braided and split channels. The section immediately below the Chulitna River had more cobbles than the other two reaches, but the finest materials were in the lowest reach. Reduced gradients and greater width below the confluence resulted in slower water velocities and more sand deposition in the lowest two stretches. The Susitna River flow is frequently highest in July and August during summer rains when its con- tributory glaciers are also melting, but high flows also occur during breakup (personal observations). Lowest water levels occur during freezeup, but stag- ing behind ice may cause river flows almost to sum- mer levels. Additionally, overflow may refreeze and encase some plants in ice. Methods Vegetation was stratified into successional stages based on site visits in 1980 and early summer 1981 and on black-and-white aerial photographs of the river in 1951 and 1980 (1:48 000). These stages were identified as Early Shrub (Dryas, Juvenile Poplar, Willow, Horsetail), Intermediate (Alder, Young Poplar), and Late (Old Poplar, Birch - Spruce). Sample sites of relatively homogeneous vegetation were selected in each successional stage in the three reaches of the river and sampled in 1981 (Figure 1). Eleven (1, 5, 6, 8, 9, 13, 14, 16, 21, 22, 25) of the thirteen Early Shrub sites were resampled in 1984, one (20) had been completely or partially eroded between 1981 and 1984 and one (18, Yellow Dryas) was not resampled because it was a minor type. A former Birch - Spruce stand that had been logged was examined in 1981 and a Balsam Poplar - White Spruce Forest was examined in 1984 to understand the transition between Old Poplar Forest and Paper Birch - White Spruce Forest but the site was not sampled quantitatively. Sites were identified according to the Alaska Vegetation Classification System (Viereck et al.

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