Vegetation Change and the Role of Atmospheric CO2Enrichment on a Relict Site in Central Oregon: 1960–1994

Abstract

Atmospheric CO2 enrichment is increasingly implicated as a contributory cause of vegetation change on North American rangelands. To explore the role of the rising CO2 content of the earth's atmosphere as an agent of vegetation change, we revisited (June 1994) the Island Research Natural Area (IRNA), a relict site of semiarid vegetation in central Oregon, to conduct a replicate survey of Driscoll's (1962) 1960 work. IRNA is one of the best non-laboratory settings to examine CO2 -induced vegetation change because human activities have been limited there. The physiological effects of increased levels of atmospheric CO2 include increased photosynthetic rates, reduced transpiration rates, and increased water-use efficiency. While all species benefit from atmospheric CO2 enrichment, the increase in CO2 alters competitive interactions, typically favoring those species that outcompete others for light, nutrients, habitat, and other resources necessary for reproductive success. Our results show that between 1960 and 1994 IRNA experienced large increases in woody-species cover and density, no change in perennial herbaceous cover, and decreases in annual cover. Western juniper density and cover increased by 41 percent and 113 percent, respectively. Similarly, big sagebrush density and cover approximately doubled. These findings are similar to findings reported in other long-term studies conducted at sites with a variety of land-use activities. We found, however, that none of the traditional mechanisms typically invoked to explain these changes were compatible with the biotic changes that we observed. On the IRNA site, no organized fire suppression had occurred, grazing had been absent since the 1920s, logging was non-existent, variabilities in seasonal water-year precipitation totals were not statistically significant between the 2-, 10-, and 34-year periods prior to each vegetation survey, and disease or pest infestation was not evident. We conclude that the vegetation changes observed are consistent with laboratory and controlled field experiments that enrich the air with CO2.