Factors associated with wetwood intensity of Populus fremontii (Fremont cottonwood) in Arizona

Wetwood is a condition of Populus fremontii and other tree species characterized by staining and water soaking in the heartwood, bleeding from wounds and stem junctions, and leaf and branch dieback. A field survey indicated that wetwood symptoms were present in populations of Populus fremontii at all 17 riparian sites surveyed in Arizona. However, incidence and severity of bleeding symptoms varied within and among sites. Within sites, incidence and severity increased with tree size. In the smallest size class of trees (1-32 cm dbh), incidence (% of trees with wetwood bleeding symptoms) ranged among sites from 14% to 83%, and mean severity ranged from 1.1 to 2.6 (on a 5-point scale). Closer host spacing (i.e., greater density), more homogeneous stand composition, finer substrate texture, and channel instability all were significantly correlated with increased expression of wetwood symptoms. Bleeding symptoms also were significantly correlated with canopy effects. As the severity of bleeding symptoms increased, so did canopy decline independent of changes in host density.