I analyzed two sets of the same 583 vegetation plots (0.25–10 m2) and 97 plant taxa sampled in 1993 and 2001 in the Peace-Athabasca Delta (PAD), northern Alberta, Canada. Cluster analyses and Bray-Curtis ordinations were used to examine relationships between environmental and biological factors and wetland community changes over a flood-drawdown cycle. While water regime was key to explaining variation in the vegetation, the vegetation gradients were complex. Vegetation structure, bison grazing, and landscape attributes such as distance to nearest major river, relative elevation, and geographic location within the PAD also played significant roles in explaining variation in the vegetation. Autocorrelations among physical and biological factors indicate that the physical and biological regimes were integrated. The importance of environmental and biological variables in explaining vegetation variation changed from pre- to post-flood. The 1993 and 2001 vegetation matrices were positively associated, indicating that by 2001 the vegetation had returned to a state similar to that of 1993. Overall, 35% of the plots remained within the same cluster type from 1993 to 2001. Eight of the 25 cluster groups and 53% of all plots occupied the middle of environmental and biological gradients. There was a higher diversity of wet graminoid communities than there was of dry woody communities. Species turnover, unpredictable processes, and overlap in ecological tolerances impart an irreducible stochasticity to the vegetation at the plot scale. The future of the PAD's vegetation and biota are uncertain due to the combined effects of climate change, wildfire, exotic species, water allocations, and discharge of contaminants and saline waters from the tar sands industry. Other than in areas of local infestation, weeds remain a minor cause for concern but without a transition to cooler, moister conditions, weeds may rise in dominance and affect the future of the ecosystem.
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Vol. 28 • No. 2