Willow resilience on Yellowstone's Northern Elk Range : a function of environmental gradients

Abstract

Willow, a deciduous, woody shrub, is a characteristic and often dominant riparian species (Amlin and Rood 2002) that has been unable to successfully regenerate throughout much of its western range, and Yellowstone National Park (YNP) is no exception (Singer et al. 1994, NRC 2002a). The primary objective of this study was to understand growth and maintenance of established willow stands as a community and as individual species following winter browse. These were based on the premises that (1) different levels of herbivory produce varying levels of compensatory growth (Brookshire et al. 2002), (2) different channel types provide diverse hydrologic conditions for vegetation establishment and maintenance (Patten 1998, Castelli et al. 2000), (3) riparian biodiversity is a function of fluvial dynamics and is increased by the degree of hydrologic connectivity of the system (Amoros and Bornette 2002), and (4) the possibility of willow species being either generalist (showing water source shifts) or specialist (availability doesn't influence water source) (Dawson and Ehleringer 1991, Busch et al. 1992, Schwinning and Ehleringer 2001). Site selection and design, and sampling scheme were designed to evaluate biophysical gradients both within and between sites over time. Gradients of biophysical parameters were quantified throughout the growing season. Regressions were used to identify relationships among physical and biological parameters or characteristics. Vegetative communities were compared using Sorenson's similarity index. Ecosystem functions that influence willow presence on the Northern Range include establishment, browse pressure, and maintenance and resilience or their ability to recover. Establishment of willow was a result of availability of their preferred water source while winter decline was a function of location and herbivore preferential selection. Maintenance and resilience were dominated by soil water use in the early season and groundwater use later in the season but with distinct variations between hydrologic systems. Nutrient availability and hyporheic connectivity, essential to dispersing those nutrients among the plant communities, also may influence growth and resilience of willow plants. However, excess or even "acceptable" levels of only one of the physical factors was not enough alone to control dominant plant growth and response to herbivory.