Extreme flood events can shape riparian vegetation patterns for decades. Near the turn of the 19^th century in southern Arizona, intense floods triggered downcutting of the Upper San Pedro River and substantially altered hydro-geomorphology. Formerly abundant marshlands were largely replaced by pioneer riparian trees which began to establish in the widening floodplain in the early 1900s. These forests now are valued by stakeholders and are the target of conservation efforts. Our goal was to better understand the magnitude and causes of variability in the spatio-temporal patterns of forest development along this semi-arid region river. To this end, we quantified recent (1955 to 2003) change in vegetation over a 100-km length of the river. Using a time-series of aerial photographs, seven cover types were mapped within three geomorphic zones.

Results/Conclusions

Since 1955, the Upper San Pedro River channel has narrowed by 50% and wooded cover on the floodplain has increased by over 150%. The vegetation increase at the southern end of the river, where stream flows are largely perennial, is a result of Populus/Salix forest expansion; the increase in the drier northern sector largely reflects expansion of drought-tolerant Tamarix. Trees have established episodically during periods with favorable flow conditions for recruitment. We interpret the expansion of these pioneer forests to be part of a long-term, biogeomorphic response to historic river entrenchment, with the temporal pattern shaped by climatic cycles and the spatial pattern shaped by anthropogenic water withdrawals and associated water availability gradients. Other cases of progressive expansion of pioneer riparian trees following extreme disturbance exist on western North American rivers, suggesting that long-term fluctuations in pioneer forest area are the norm in dryland regions. Vegetation changes on the pre-entrenchment surfaces (river terraces) differ from those in the active floodplain. On these surfaces, hydrogeomorphic changes including increased depth to water table have contributed to expansion of deep-rooted Prosopis woodlands, and agricultural land conversion has contributed to sharp declines in riparian Sporobolus grasslands. Overall, the study indicates that riparian vegetation patterns are a product both of recent cultural and natural phenomenon and of past extreme events that set in motion long-term trajectories of change.