Morphotype community structure of ectomycorrhizas on Douglas fir (Pseudotsuga menziesii Mirb. Franco) seedlings grown under elevated atmospheric CO2 and temperature

Mycorrhizas alter the acquisition of carbon and nutrients, thereby affectin g numerous plant and ecosystem processes. It is important, therefore, to de termine how mycorrhizal populations will change under possible future clima te conditions. Individual and interactive effects of elevated atmospheric C O2 concentration and atmospheric temperature were assessed in a 2 x 2 facto rial design [ambient and elevated (200 ppm above ambient) CO2 concentration s, and ambient and elevated (4 degrees C above ambient) temperatures]. In J une 1993, 2-year-old Douglas fir (Pseudotsuga menziesii Mirb. France) seedl ings were planted in 12 environment-tracking chambers (n = 3) containing re constructed, low-nitrogen, native for est soil. Climate treatments were imp osed shortly thereafter, and the seedlings grew until June 1997. Soil cores were taken twice per year during the exposure period. We present findings on changes in the community structure of ectomycorrhizal (ECM) root tips, c ategorized into morphotypes using gross morphological traits. A diverse and stable community of morphotypes (a total of 40) was encountered; no more t han 30 of which were seen at any sampling time. In the first sample, there were only 15 morphotypes found in the 12 chambers. Morphotype numbers incre ased during the first half of the experiment, remaining fairly constant the reafter. Near the end of the exposure, elevated-temperature treatments main tained more morphotypes than ambient treatments. However, overall, absolute measures (number of ECM tips) were affected primarily by CO2 treatment, wh ereas proportional measures (e.g., Simpson's index) were affected primarily by temperature. While some morphotypes were negatively affected seasonally by higher temperatures (putative Rhizopogon group), others (Cenococcum) se emed to thrive. Underlying the dominant patterns of change in diversity, dr iven by the Rhizopogon group, subdominant populations responded slightly di fferently. Community diversity through time tended to increase at a greater rate for all subdominant populations compared with the rate when dominant populations were included.