Estimating Forage Production and Winter Severity on the National Elk Refuge, Jackson, WY

Each October through November, elk and bison migrate to the National Elk Refuge (NER) near Jackson, WY to feed on native forage produced on the Refuge. Usually by late January, standing forage has been utilized or becomes unavailable because of snow and ice, and NER staff then supplies supplemental feed in the form of alfalfa pellets. For the past 34 years, NER staff has estimated total annual forage produced on the Refuge by clipping vegetation within different plant community types. Keetch Byram Drought Index (KBDI), and growing degree-days (GDD) based on daily average temperature threshold of 5 deg C have been calculated daily for Jackson and Moose climatological stations since 1949. Also, daily snow water equivalent (SWE) has been estimated for these stations using climatological records. Critical temperatures (-18 deg C for elk) have been summarized using departure from daily minimum temperatures. SWE, forage production, and critical temperatures are weighted and combined to determine Index of Winter Severity (IWS) for elk and bison. Methods for calculating the IWS for elk and bison and how the IWS is used for Refuge operations will be presented. Average KBDI for different accumulations of GDD and spring precipitation are compared to production of grass and forbs on 34 different plant community groups.