Vegetation Inventory Project: Whitman Mission National Historic Site

Erixson, John; Cogan, Dan

Whitman Mission National Historic Site (WHMI) is located in southeast Washington near the town of Walla Walla. Established in 1939, the 100 acre site (along with a 40-acre Living Trust Area to the west of WHMI) memorializes the tragic killing of 13 Merthodist missionaries by local Native Americans at the historic Waiilatpu Mission in 1847. Located near the confluence of the Walla Walla River and Mill Creek, WHMI today commemorates the United States westward expansion along the Oregon Trail while preserving many of the original mission building foundations, gravesites and memorials associated with the massacre. The vegetation at WHMI is heavily influenced by moisture and human activity. Starting at the rivers edge long riparian corridors of cottonwood (Populus spp.) and willow (Salix spp.) forests give way to broad floodplain terraces containing a mix of tall grasses, planted trees and mowed lawns. The flat nature of WHMI’s landscape is interrupted along the east boundary by the prominent Memorial Shaft Hill and its steep slopes. The dry and steep nature of the hillslopes support unique pockets of native (Pseudoroegneria spicata) bluebunch wheatgrass and Idaho fescue (Festuca idahoensis) prarrie. Revegetation of native plants at WHMI has been an important objective for park management for over fifty years and has been on-going and sustained since 1985. Recent restoration efforts strive to return the entire site to a historic condition similar to the 1800’s Mission era. Restoration goals include replacing cheatgrass (Bromus tectorum) and other non-native grasses with Great Basin wildrye (Leymus cinereus) and similar tall grass species and planting native riparian plants along a natural waterway that was diverted in the past by an irrigation canal. To better understand the native, non-native and restored plant community dynamics at WHMI the NPS Vegetation Inventory Program (NVIP) funded a vegetation inventory and mapping project in 2006 as part of the larger Upper Columbia Basin Inventory and Monitoring Network (UCBN) network-wide inventory program. A three-year, three-phase program was initiated to complete the tasks of sampling, classifying, and mapping the plant associations (i.e. communities) and vegetation alliances within WHMI. Phase-one, directed by UCBN staff in conjunction with Northwest Management Incorporated (NMI), Idaho State University (ISU), and S.M. Stoller Corporation (Stoller) resulted in the plant association and vegetation alliance classification using the National Vegetation Classification System (NVCS). Vegetation classification was based on the 2010 field sampling of 71 representative classification plots located within the boundary of WHMI and the adjoining Land Trust Area. Analysis of the classification plot data by ISU and Stoller ecologists occurred in 2010. Through this process 13 distinct plant associations were identified as were four vegetation alliances. Phase-two, directed by NMI and Cogan Technology, Inc. (CTI) resulted in creation of the digital vegetation and land use map and project geodatabase. Mapping was conducted primarily by interpreting vegetation-specific and non-vegetated site signatures from the 2009 and 2011 National Aerial Imagery Program (NAIP) ortho-photo products. CTI reliably discerned 24 vegetation and14 land use/land cover map units. The mapping process relied on manual digitizing of homogenous photo-signatures supported by on-site verification. All interpreted data were converted to Geographic Information System (GIS) databases. The final map layer contains vegetation and land-use polygons with corresponding map class, NVCS, and modifier attributes.