Comparative Rates of Western Juniper Afforestation in South-Central Oregon and the Role of Anthropogenic Disturbance∗

∗This study was funded by the National Science Foundation Grant ♯SBR-9809245, and by an Appalachian State University Research Council Grant. We thank Harry Blount, Stacy Carnine, Joel Davis, Betsy Herrmann, Kimberly Eldridge, Allison Miller, Paul Mitchell, Andrew Paul, and Mark Pelfrey for field and/or laboratory assistance. We also thank Chris Larson for image analysis, Jeffery McMichael of the Georgia State University Cartography Laboratory for production of Figures 1 and 5, and Truman Hartshorne and three anonymous reviewers for their constructive comments.

Abstract

We examine changes in canopy cover for adult western juniper from the 1960s to 1994 in central Oregon using repeat aerial photography. We compare changes at four sites with a land-use history of minimal anthropogenic disturbance to changes on adjacent sites that have a disturbance history more typical of central Oregon rangelands. Canopy cover increased at all sites, but afforestation on sites with domestic livestock grazing was greater. The potential driving forces common to all sites include a long fire-free interval, early twentieth-century favorable climatic conditions, biological inertia, and atmospheric CO₂ enrichment.

∗This study was funded by the National Science Foundation Grant ♯SBR-9809245, and by an Appalachian State University Research Council Grant. We thank Harry Blount, Stacy Carnine, Joel Davis, Betsy Herrmann, Kimberly Eldridge, Allison Miller, Paul Mitchell, Andrew Paul, and Mark Pelfrey for field and/or laboratory assistance. We also thank Chris Larson for image analysis, Jeffery McMichael of the Georgia State University Cartography Laboratory for production of Figures 1 and 5, and Truman Hartshorne and three anonymous reviewers for their constructive comments.

Key Words:

• afforestation
• canopy-cover increases
• repeat aerial photography
• western juniper

Notes

∗This study was funded by the National Science Foundation Grant ♯SBR-9809245, and by an Appalachian State University Research Council Grant. We thank Harry Blount, Stacy Carnine, Joel Davis, Betsy Herrmann, Kimberly Eldridge, Allison Miller, Paul Mitchell, Andrew Paul, and Mark Pelfrey for field and/or laboratory assistance. We also thank Chris Larson for image analysis, Jeffery McMichael of the Georgia State University Cartography Laboratory for production of Figures 1 and 5, and Truman Hartshorne and three anonymous reviewers for their constructive comments.

Note: RNA, research natural area; N.A., not applicable.

Note: Numbers shown parenthetically are mean Z-scores based on October-June precipitation.

^aFor all figures in this column, significance between index values based on one-tailed difference of means t-test with unequal variance based on 196x–1977 versus 1978–1994 periods. ∗p<0.05. ∗∗p<0.01.