Classification of Oak Vegetation in the Willamette Valley

A plant community classification was developed describing the comprehensive variation of oak vegetation currently occupying the Willamette Valley Ecoregion of western Oregon. Multivariate statistical analyses were used to classify field collected floristic and habitat data. Field sampling targeted minimally managed, homogenous vegetation stands with a significant oak component occupying at least 0.5 ha. Potential habitat locations were identified from local expert knowledge, data from The Nature Conservancy’s previous conservation planning efforts, and an interpretation of high resolution NAIP imagery. Potential oak habitats were digitized in a GIS and a random selection of sites stratifying significant environmental gradients was generated in order capture the range of oak habitat variability. Precise sample plot locations were predetermined and established at digitized stand centroids. Field sampling was based on relevé plot data collection methods and included ocular estimates of species cover. A total of 350 stands of oak vegetation were sampled over two field seasons. Two-way indicator species analysis (TWINSPAN) was used as an exploratory tool to identify potential patterns in species cover data and Detrended Correspondence Analysis (DCA) was used to delimit final plant community types. Nine vegetation classes were described from ordinations including seven forest / woodland types and two savannas.