Estimating Historical Sage-Grouse Habitat Abundance Using a State-and-Transition Model

Use of reference conditions to compare current conditions what managers believed represented healthy and functioning systems has become a common approach to evaluate vegetation and habitat conditions and aid development of land management plans. Often reference conditions attempt to describe landscapes as they existed and functioned prior to about 1850, and often largely rely on expert opinion. We developed reference conditions for sagebrush (Artemisia spp. L.) ecosystems in eastern Oregon based on ecological site descriptions, soil surveys, climate data, wildfire records, expert opinion, and literature using a state-and-transition (STM) modeling framework. Using ecological site descriptions for the Malheur High Plateau Major Land Resource Area (MHP), we divided sagebrush communities into four groups based on grass productivity in low, average and high productivity years. Literature helped us determine which disturbance factors to include, the community phases for each model, and associated seasonal habitat for greater sage-grouse (Centrocercus urophasianus). We developed successional timelines in the absence of disturbance, and determined the probable outcomes of a given type of disturbance event. We used fire records and climate data to develop disturbance event probabilities and periodicities. Contrary to our expectations, fire did not appear to be the most important factor influencing sagebrush ecosystems under reference conditions in our models. The modeled historical abundance of sage-grouse breeding and brood-rearing habitat was within range of or greater than the amount recommended by sage-grouse biologists, but the abundance of wintering habitat was less. By using objective criteria as much as possible, our approach should also be repeatable in other locations. Since we used climate criteria to define most disturbance probabilities, our models provide an opportunity to examine how changes in climate could affect plant communities, disturbance regimes, and the quality and quantity of sage-grouse habitat in future modeling efforts