Beschta, R. L. and W. J. Ripple.
2005. Rapid assessment of riparian cottonwood recruitment: Middle John Day River, northwestern Oregon. Ecological Restoration 23:150-156.

The authors developed a rapid method to visually enumerate black cottonwood by size class along a 41-mile length of the Middle Fork of the John Day River in Oregon. They viewed sample sites where cottonwood trees were visible from a county road that more or less paralled the river. A study size was one in which at least one cottonwood could be observed that was at least 4 m in height. Study sites were classified as open to domestic and wild ungulate browsing or not. Cottonwood trees that were visible were classified as pole sized (from 4-8 m in height), and mature (> 8 m in height). A total of 41 pole-sized and 916 mature black cottonwoods were observed along the entire study area. The proportion of pole-sized cottonwoods was much higher in sites where ungulate browsing was not occurring compared to sites where it was occurring (85% and 5%, respectively). Inspection of flow records for the river revealed that a five-year flow was about 5 cubic feet per second, which was exceeded ten times between 1970 and 2000. This indicated that adequate high-flow events were occurring to produce suitable conditions for black cottonwood germination. However, ungulate browsing was sufficiently heavy to prevent young cottonwoods from reaching maturity over most of the study area.

Lite, S. J. and J. C. Stromberg.
2005. Surface water and ground-water thresholds for maintaining Populus-Salix forests, San Pedro River, Arizona. Biological Conservation 125:153-167.

Ground-water and surface flow depletions are altering riparian ecosystems throughout the southwestern United States, and have contributed to the decline of forests of the pioneer trees Populus fremontii (Fremont cottonwood) and Salix gooddingii (Goodding willow). On some rivers, these forests have been replaced by shrublands ofTamarix ramosissima (tamarisk), a drought-tolerant species from Eurasia. The physiological response of these three riparian plant species to decreases in water availability is well studied, but little attention has been given to shifts in community and population structure in response to declines in surface flow and ground-water levels. Based on study of 17 sites spanning a hydrologic gradient, this research identified hydrologic thresholds above which P. fremontii–S. gooddingii maintain tall dense stands with diverse age classes, and above which they are more abundant than T. ramosissima stands along the San Pedro River in Arizona. Surface flow permanence was the hydrologic variable that explained most of the variance in species abundance and relative importance, with inter-annual ground-water fluctuation and depth to ground water also contributing. P. fremontii and S. gooddingii were dominant over T. ramosissima at sites where surface flow was present more than 76% of the time, inter-annual ground-water fluctuation was less than 0.5 m, and average maximum depth to ground water was less than 2.6 m, during a two-year period of data collection. Because T. ramosissima is a shrub species with smaller leaves and lower canopy heights than P. fremontii and S. gooddingii, these shifts in species composition corresponded to decreases in maximum canopy height and upper stratum (above 8 m) vegetation volume as site water availability declined. As well, sites with deeper water tables and more intermittent flows had greater areal coverage of shrublands and less of woodlands. The altered vegetation structure along dewatered rivers may lower wildlife habitat quality. This study provides river managers with quantitative hydrologic guidelines for conserving tall dense P. fremontii–S. gooddingii stands and emphasizes the importance of maintaining long-term hydrologic conditions that are favorable to these species. The study also has implications for riparian restoration and invasive species management by suggesting that desired outcomes may be achieved through restoration of physical processes.

Colorado Riparian Association