A century of riparian forest expansion following extreme disturbance: Spatio-temporal change in Populus/Salix/Tamarix forests along the Upper San Pedro River, Arizona, USA

Populus-Salix forests are a valued riparian vegetation type in western North America. These pioneer, obligate phreatophytes have declined on some rivers, raising conservation concerns and stimulating restoration plantings, but have increased on others. Understanding patterns and causes of forest change is essential for formulating conservation, restoration and management plans. Our goal was to assess spatio-temporal patterns of vegetation change on the Upper San Pedro River in semiarid Arizona, USA, one of the few undammed rivers in the region. Over 100 years ago, intense floods initiated channel incision and substantially altered hydrogeomorphology. Pioneer trees began to establish in the widening post-entrenchment zone as the surfaces began to stabilize. Using a time-series of aerial photographs (1955-2003) we quantified recent change in area of riparian cover types. Analysis indicated that wooded area in the post-entrenchment zone nearly tripled from 1955 to 2003, at the expense of bare ground, and the active channel narrowed appreciably. This forest expansion represents a long-term response to river entrenchment, with the temporal pattern influenced by recent flood cycles and biogeomorphic feedbacks. Populus-Salix have established episodically during the infrequent years with high winter flood runoff, sequentially filling available recruitment space. Older cohorts cover wide swaths of the floodplain while young trees form narrow bands lining the channel. Barring extreme flooding, the pioneer forests are expected to senesce over the coming century. An additional factor that has shaped the pattern of post-entrenchment forest expansion is anthropogenic water withdrawal. Populus-Salix forest increase has been greatest within a conservation area, where stream flows are largely perennial. In drier, agricultural sectors, Populus-Salix have declined while the more deeply-rooted Tamarix has increased. Overall, the study reveals that long-term fluctuations in pioneer forest area and age Structure are common on dryland rivers, and shows how past events Such as extreme floods can interact with recent environmental practices such as freshwater withdrawal to influence riparian forest patterns. This underscores the necessity of a long-term perspective for forest conservation and management. (C) 2010 Elsevier B.V. All rights reserved.