Knapp, P.A. and Soule, P.T. 1998. Recent Juniperus occidentalis (Western Juniper) expansion on a protected site in central Oregon. Global Change Biology 4: 347-357.
In the Third Annual Kuehnast Lecture of the Department of Soil, Water and Climate presented to students and faculty of the University of Minnesota on 12 October 1995, and entitled "CO2 and the Biosphere: The Incredible Legacy of the Industrial Revolution," Dr. Sherwood B. Idso cited over a hundred scientific studies that document the increasing growth enhancement and range expansion of trees and other woody plants that have paralleled the development of the Industrial Revolution over the past two centuries. On the basis of numerous other studies that suggest that trees may be more responsive to atmospheric CO2 enrichment than are herbaceous plants, he proposed that these phenomena were but the obvious consequences of the anthropogenically-driven increase in the air's CO2 content that has been experienced over this period. Since that time, many reports supportive of his hypothesis have been published; and in this review, we report on one of the most interesting of these recent studies.
What Was Done
The authors begin by reviewing several scientific studies that document a dramatic increase in the range of western juniper trees in the Pacific Northwest region of the United States over the last century. They note that the land area covered by these trees has more than doubled over this time period and that juvenile establishment rates at some locations have increased geometrically since the early 1960s. They also note that the rate of western juniper expansion in this region is "unprecedented" and that it is the first time within the current interglacial epoch that such a range expansion has occurred under relatively dry conditions.
Within the context of this ongoing phenomenon, the authors revisited the Horse Ridge Research Natural Area - a protected site in central Oregon that has experienced minimal anthropogenic disturbance over the past several decades - to see what vegetation changes have occurred since a detailed vegetation cover assessment of the area was conducted in 1972. From archived aerial photographs of the site, as well as a number of ground-based measurement programs, they evaluated several of the same parameters measured in the earlier study. They also investigated records of local climate, fire history and biotic influences in an effort to determine the likely cause or causes of the vegetation changes they documented.
What was learned
Over the 23-year period between this study and the earlier assessment of the area, total tree cover increased by 59% and shrub cover by 7%, while the area covered by perennial herbs and grasses declined by 38%. The number of clumps of western juniper, in particular, rose by 37%, with the number of juniper stems rising by 53%.
What it means
After studying all available evidence, the authors concluded that the expanding presence of western juniper at their study site "cannot be explained adequately by the traditional mechanisms commonly cited." In fact, the historical climate record of the area suggests that conditions there "have not been favorable for an expansion of woody species this century." The biological advantage conferred upon trees by the historical rise in the air's CO2 content was thus one of the few forces for vegetation change of the nature noted that the authors considered a viable explanation for their observations. Although they could not conclusively prove that rising levels of atmospheric CO2 were responsible for the growing importance of trees in this central Oregon nature preserve, their finding is an important piece of evidence that is bringing us closer to the time when we will be able to make such a determination, one way or the other.