VEGETATION CHANGE AND THE ROLE OF ATMOSPHERIC CO2 ENRICHMENT ON A RELICT SITE IN CENTRAL OREGON - 1960-1994

Atmospheric CO2 enrichment is increasingly implicated as a contributor y cause of vegetation change on North American rangelands. To explore the role of the rising CO2 content of the earth's atmosphere as an age nt of vegetation change, we revisited (June 1994) the Island Research Natural Area (IRNA), a relict site of semiarid vegetation in central O regon, to conduct a replicate survey of Driscoll's (1962) 1960 work. I RNA is one of the best non-laboratory settings to examine CO2-induced vegetation change because human activities have been limited there. Th e physiological effects of increased levels of atmospheric CO2 include increased photosynthetic rates, reduced transpiration rates, and incr eased 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, nut rients, habitat, and other resources necessary for reproductive succes s. Our results show that between 1960 and 1994 IRNA experienced large increases in woody-species cover and density, no change in perennial h erbaceous cover, and decreases in annual cover. Western juniper densit y and cover increased by 41 percent and 113 percent, respectively. Sim ilarly, big sagebrush density and cover approximately doubled. These f indings are similar to findings reported in other long-term studies co nducted at sites with a variety of]and-use activities. We found, howev er, that none of the traditional mechanisms typically invoked to expla in these changes were compatible with the biotic changes that we obser ved. On the IRNA site, no organized fire suppression had occurred, gra zing had been absent since the 1920s, logging was non-existent, variab ilities in seasonal water-year precipitation totals were not statistic ally significant between the 2-, 10-, and 34-year periods prior to eac h vegetation survey, and disease or pest infestation was not evident. We conclude that the vegetation changes observed are consistent with l aboratory and controlled field experiments that enrich the air with CO 2.