Human agency, environmental drivers, and western juniper establishment during the late holocene

Western juniper (Juniperus occidentalis var. occidentalis) is undergoing rapid rates of expansion, and human activities (domestic livestock grazing, fire suppression) are typically presented as the primary agents of change. To determine the importance of active disturbance as a vehicle for these major ecosystem changes (e.g., a near doubling of western juniper cover at some locales over a recent 30-yr period), we conducted a comparative study of western juniper establishment at matched disturbed and less-disturbed sites in Oregon (USA). We used dendroecological techniques to randomly Sample and cross-date 160-200 trees per site from five Research Natural Areas (RNAs) or proposed RNAs, and from areas adjacent to these RNAs that are actively disturbed. For each location we determined the density of adult and juvenile western juniper and created a timeline of tree establishment. We discuss the probable causes of these establishment pattern changes., Recent increases in establishment could best be described as geometric on the disturbed sites. While trees on the less disturbed sites are generally older and have a more consistent establishment history, they also are experiencing increasing rates of establishment. Disturbance does appear to accelerate rates of establishment of western juniper, especially with domestic livestock grazing on sites that are downslope from established woodlands. Climatic changes, a lack of high-severity fires, an increasing seed rain, and atmospheric carbon dioxide enrichment are more recent causal mechanisms contributing to establishment. The driving forces proffered to explain the late 1800s to early 1900s pulse of establishment for western juniper (e.g., favorable climatic conditions, domestic livestock grazing) appear to be operative at the majority of our study sites. Conversely, a second pulse of establishment (post 1940s) occurred during a period that was not characterized by climatic conditions favorable for above-average radial growth of western juniper. Because our results show that rates of establishment are generally accelerating regardless of the disturbance regime, we suggest that active human disturbance is not a required element for these ecosystem changes to occur, and that other agents of change have either appeared or become more dominant in recent decades.