Cottonwoods, elk, and wolves in the Lamar Valley of Yellowstone National Park

In September 2001, the diameter at breast height (dbh) of all cottonwood (Populus spp.) greater than or equal to5 cm in diameter was measured within a 9.5-km(2) section of the Lamar Valley (elevation similar to2000 m), in northeastern Yellowstone National Park. A total of 700 trees were measured of which 71% were narrowleaf cottonwood (P. angustifolia) and 29% were black cottonwood (P. trichocarpa). Slightly more than half of the narrowleaf trees were growing on point bar landforms at three major meander bends of the Lamar River, with the remainder occurring in small groves spread across various floodplain surfaces within the valley. Almost all of the black cottonwood occurred on point bars at only two of the meander bend locations. Tree diameters for both species ranged mostly between 30 and 110 cm with a nearly total absence of cottonwoods between 5 and 29 cm in diameter. Age vs. diameter relationships were developed and used as a basis for estimating establishment dates for all narrowleaf cottonwoods. These relationships, in conjunction with the dbh data, indicated an absence of cottonwood recruitment (i.e., growth of seedlings/suckers into greater than or equal to5-cm diameters trees) over approximately the last 60 years. The paucity of cottonwood recruitment appears to have occurred independently of fire history, flow regimes, channel migrations, or factors affecting normal stand development, but over the same period of time that wolves (Canis lupus) had been extirpated from Yellowstone National Park. With the removal of this wide-ranging and keystone predator, elk (Cervus elaphus) populations were able to browse riparian plant communities unaffected by wolves.