Mycorrhizal structure and global change in a Mediterranean-type ecosystem.

ALLEN, MICHAEL F., CARA HINKSON, OLA KAREN, MICHELLE DUGGAN, JOHN KLIRONOMOS, WALTER C. OECHEL & STEVE HASTINGS

Department of Biology, San Diego State University, San Diego, CA 92182, USA.

Increasing atmospheric levels of CO₂ are a consistent change in the global environment. Mediterranean-type ecosystems represent important study systems both because they are among the most impacted by human development and because they have a variety of mycorrhizal types. We are examining the effects of elevated CO₂ in the field using FACE facilities and natural CO₂ springs. Increasing CO₂ results in reduced foliar N concentrations. Thus, we postulated that elevated CO₂ should favor ectomycorrhizae over endomycorrhizae and increase the utilization of organic forms of N. Data from the California FACE facility showed an increasing predominance of ectomycorrhizal plants with elevated CO₂ through time. This is associated with increasing basidiomycete hyphal length with no change in AM or saprophytic hyphae, and no change in the bacterial food web. These data suggest more rapid mineralization. However, data from both the California FACE facility and high CO₂ springs in Italy show a decreasing d¹⁵N value, indicating more scavaging of inorganic N, especially nitrates. There was no measurable change in the d¹⁵N of AM plants although there was a shift in the spore densities of AM fungi. We suggest that these data indicate that the shift in mycorrhizal activity is toward decreasing N use efficiency at the ecosystem level associated with shifts in the composition of the mycorrhizal fungi and host plants.