The Fremont cottonwood is a dioescious, desiduous tree. Their simple, single toothed leaves which are alternately located (Shaw 1989) provide sufficient light to surrounding leaves, but may collectively be very shading. The Fremont cottonwood life history has multiple affects on the ecosystem function of the Green River. Cottonwood seeds are engulfed by a tuft of hair that allows them to be transported via wind or water during mid-June. Their process of seed release historically coincides with spring flows which provide fresh sediment and soil moisture for the recruits. Cottonwoods remain dormant in the fall and will begin their act of leaf senescence in late September (Taylor 2000). This process may be important for the Green River ecosystem as cottonwoods are some of the largest riparian plants found on the river banks and may therefore be the largest contributors of woody debris to the Green River. Woody debris provides habitat for many aquatic invertebrates in addition to reintroducing organic carbons which together may trophically impact the Green River ecosystem.
Cottonwoods also serve as important habitat for many avian species. Certain bird species are dependent on cottonwoods for breeding; cottonwood abundance and population size are inextricably linked for these species. There are two main mechanisms at play here: shading and size. For some species, such as the western yellow-billed cuckoo, the broad, flat nature of a cottonwood leaf provides the shade that is essential in preventing egg overheating during the summertime breeding season (Hunter et al. 1985). For other species, such as the northern flicker (Figure 1), the size of the tree is more important than the amount of shading it provides. The northern flicker is a type of woodpecker; it excavates cavities into large trees and subsequently lays and incubates its eggs inside of the cavity. Small trees or shrubs such as tamarisk do not constitute a suitable breeding habitat alternative for most cavity-nesters. Tree size is also an important factor for large birds of prey such as osprey and eagles, which require large trees in which they can perch and nest.
On the middle Green River, we observed several avian species that appeared to have clear associations with the Fremont cottonwoods. From Flaming Gorge Dam to Split Mountain Canyon, mature and/or dead cottonwoods were the only vegetation type in which we observed northern flickers, ospreys, and turkey vultures. Western kingbirds (Figure 2) also seemed to be closely tied to the cottonwoods throughout the river reaches. The consistent associations that we observed were probably all due to tree size, and not necessarily the amount of shading provided, although this may be an additional benefit for certain species. The northern flicker and osprey require large cottonwood trees for breeding habitat, while the turkey vulture (and also osprey) are dependent on the cottonwoods because they provide posts from which these species can search for prey. The western kingbird could be associated with cottonwoods because they require relatively large trees in which to construct their nests; however, they can also nest in willow trees, and were not observed utilizing this habitat on the middle Green River. We speculate that the cottonwoods might provide additional thermal refugia for the western kingbird in this system. Consequently, the lack of cottonwood recruitment in the reach below Flaming Gorge Dam to the confluence of the Yampa River and a continued mortality rate of pre-existing trees may become a conservation concern for many birds.
Figure 1. Northern flicker on nest.
Figure 2. Western kingbird in a cottonwood.
References
Hunter, W. C., Anderson, B. W., and Omhart, R. D. 1985. Summer avian community composition of Tamarix habitats in three southwestern desert riparian systems. Pp. 128-134 in Riparian ecosystems and their management: reconciling conflicting uses. U. S. Department of Agriculture, Forest Service General Technical Report RM-120.
Taylor, Jennifer L. 2000. Populus fremontii. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [2006, June 26].
Shaw, R.J. 1989. Vascular Plants of Northern Utah: an identification maual. Pg. 243-244. Utah State University Press.