Chronology of the spread of tamarisk in the central Rio Grande

Like many dryland rivers of the southwestern United States, the central Rio Grande suffered a collapse of its native cottonwood forests and an expansion of tamarisk (Tamarix spp.) in the early 20th century, A paramount example of an opportunistic colonizer, tamarisk occupied land made available by the plow, the bulldozer and the shrinking of a channel depleted of flow by upstream water development. Changes in both the physical environment and the native vegetation were well underway by the time tamarisk became widespread. There is no evidence that it actively displaced native species nor that it played an active role in changing the hydraulic or morphologic properties of the river. Its present dominance in the Presidio Valley is due to the chance conjunction in 1942 of a large summer flood, seed source, and declining cotton prices that fostered abandonment of farm fields. The history of tamarisk on the central Rio Grande demonstrates the complex nature of vegetation change, The passive role of tamarisk in landscape change holds the hope that its response to geomorphic and hydrographic variables can be understood and predicted.