A time comparison study of vegetation on The Island Research Natural Area in Central Oregon
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This study on The Island, a Research Natural Area (IRNA) in the central Oregon, USA, western juniper (Juniperus occidentalis) zone, measured vegetation cover during the early 1990s and compared it to an earlier study conducted in the early 1960s. Two plant associations were studied: western juniper/big sagebrush (Artemisia tridentata)/bluebunch wheatgrass (Agropyron spicatum [Elymus spicatus]) and western juniper/bitterbrush (Purshia tridentata)/bluebunch wheatgrass. The same study design and measurement techniques were used in both studies to document percent cover of vegetation on study-site macroplots, microplots, and aerial photographs. Major differences noted by 1993 included greater juniper and shrub cover, primarily big sagebrush, a more even mix of grass species rather than a dominance of bluebunch wheatgrass and cheatgrass as noted in the 1960 study, and higher litter cover. Weather data from 1952-1993 were analysed, and no major events or trends were found in the 30-year period between studies. This study also compared present-day IRNA sites in both plant associations with sites located in the nearby grasslands that had experienced greater impact from livestock and humans. The comparison sites had higher tree and shrub cover than IRNA and differed in Idaho fescue (Festuca idahoensis) and bluebunch wheatgrass cover. Finally our study was compared to another recent study in the western juniper/big sagebrush/bluebunch wheatgrass plant association on IRNA. We established new representative macroplots whereas the other study used the exact plots of the 1960s study. Results were similar showing that either method may be satisfactory. A continuing bank of data sets of vegetation change and land use will assist us to understand the complexities of plant community dynamics over time.