Cottonwoods, Elk, and Wolves in the Lamar Valley of Yellowstone National Park

Abstract

In September 2001, the diameter at breast height (dbh) of all cottonwood (Populus spp.) Ն5 cm in diameter was measured within a 9.5-km 2 section of the Lamar Valley (elevation ϳ2000 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 Ն5-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.

Water Tables Constrain Height Recovery of Willow on Yellowstone's Northern Range

“…Treatment effects on H were analyzed using repeatedmeasures ANCOVA in SAS PROC MIXED (SAS Institute 2002-2003 for a randomized complete block design with a repeated-measures structure. An autoregressive (lag ¼ 1) covariance parameter was significant and therefore retained in the model.…”

“…Accumulating evidence suggests that winter browsing by an abundant elk (Cervus elaphus L.) population caused a fundamental change in the state of the northern elk wintering range of Yellowstone National Park (YNP; Kay and Wagner 1994, Beschta 2003, Larsen and Ripple 2005, Wolf et al 2007). In particular, elk browsing has been implicated in the disappearance of historically abundant stands of woody deciduous plants such as willow (Salix spp.…”

“…), and cottonwood (P. angustifolia James) during the past 80 years (Kay and Chadde 1991, Ripple and Larsen 2000, Larsen and Ripple 2005. Elk browsing of woody riparian vegetation likely was less intense during the early 1900s when wolves (Canis lupus L.) were abundant, increased in intensity after wolves were extirpated from the northern range (Beschta 2003, Larsen and Ripple 2003, and decreased again following wolf reintroduction in 1994 (Ripple and Beschta 2004a). However, it is unclear whether woody deciduous plant communities will be restored across the entire northern range by the recent moderation of elk browsing (Smith et al 2003).…”

“…All but one of the studies to date documenting wolfinduced willow recovery (Ripple et al 2001, Beschta 2003, Ripple and Beschta 2003, 2004b have been conducted along the few large, low-gradient streams that were never hydrologically influenced by beaver, because they are too wide for beavers to dam. Rivers of this size account for ;23% of the total length of streams larger than first order on the northern range.…”

Hydrologic Regime and Herbivory Stabilize an Alternative State in Yellowstone National Park

“…1A). Demographic studies and comparisons of modern and historical photographs illustrate that striking reductions in the abundance and stature of willows (Kay 1990), cottonwoods (Populus angustifolia James; Keigley 1997, Beschta 2003, and aspen (Populus tremuloides Michaux; Ripple and Larsen 2000) occurred during the 20th century, particularly on the northern range. These riparian woodlands have been replaced by grasslands in many areas (Houston 1982, Engstrom et al 1991, Singer et al 1994 leading to an alternative state that we refer to as the elk-grassland state (Fig.…”