Document: RAU-3-76-2

Long-term dynamics of riparian willow on the elk winter range of Rocky Mountain National Park (Colorado).

PEINETTI, R.* and M.BCOUGHENOUR

Colorado State University, Fort Collins, CO 80523, USA ¹

Abstract:
We analyzed changes in willow (Salix sp.) cover and relationships with probably causative factors. The study area comprised one valley of the elk winter range of Rocky Mountain National Park (Colorado). Low (LR) and high (HR) resolution digital maps of vegetation and hydrology were created. Maps where incorporated into a spatial, process-based simulation model (SAVANNA) to estimate willow productivity. LR maps were generated by photo-interpretation and were used to conduct a change analysis, from historic (1946) to recent (1996) conditions. Riparian shrub cover and river features were digitized on-screen using a GIS. HR maps were created from a 1998 field survey of the valley to quantify heterogeneity within willow stands. Homogeneous vegetation patches were visually identified, and a GPS was used to delimit their boundaries. We classified patches based on willow density, percentage dead, and dominant morphotypes - young (colonization), short (>1.5 m), intermediate (1.5-2.0 m), and tall (>2.0 m). A decrease in willow cover (21 ha or 21%) was determined using the LR maps. During this 50 year period, elk increased 8-fold and beaver declined to near disappearance. The beaver decline caused a reduction in river length (56%) and surface water area (69%). The upper valley, where less river changes were observed, was dominated by tall willow, while short and dead willows dominated areas of river reduction. Willow cover was heterogeneous, comprised of: colonization (12%), high density short (10%), intermediate (7%) and tall (27%) willow, high density dead willow (17%), and areas with low willow density (27%). A reduction of more than 50% in total willow aboveground productivity was estimated using 1946 and 1998 vegetation maps (132 and 63 ton). The percentage reduction in productivity largely surpassed the percentage willow cover reduction. Productivity was also decreased by degradation of cover - reductions in density and increases in the proportion of short vs. tall willow. The willow decline was apparently caused by a number of factors, including drier and warmer climate, increased elk herbivory, and possibly human activities. However, the beaver decline appeared to play the most significant role, as the total area of riparian habitats, where water table depths are sufficiently high for willow growth, was reduced.

Keywords: Salix - willow - riparian vegetation

This abstract is being presented at: 11:00 AM in session:
Oral Session #20: Riparian Ecology.