ENRICHED ATMOSPHERIC CO2 AND SOIL P EFFECTS ON GROWTH AND ECTOMYCORRHIZAL COLONIZATION OF JUVENILE PONDEROSA PINE

Interactive effects of atmospheric CO2 enrichment and soil P fertility on above- and below-ground development of juvenile ponderosa pine (Pi nus ponderosa Dougl. ex Laws.) were examined. Seedlings were reared fr om seed in atmospheres with 700 mu l l(-1), 525 mu l l(-1), or ambient CO2 concentrations, and in a potting mix with 68, 43, or 18 mu g g(-1 ) soil P, and all were inoculated with the mycobiont Pisolithus tincto rius (Pers.) Coker and Couch shortly after emergence. At 4-month inter vals over the 1-year duration of the study, three whole seedlings of e ach combination of CO2 and P treatments were harvested to permit detai led assessment of shoot and root growth and ectomycorrhizal colonizati on. After 4 months, shoot volume, root dry weight, and total root leng th of seedlings grown in 700 mu l(-1) CO2 were greater than those of s eedlings grown in the other atmospheres regardless of P treatment, and shoot/root ratios decreased as the CO2 concentration increased within each P treatment as well. After 8 months, the smallest shoot volumes and root weights and lengths within each P treatment were those of see dlings grown in ambient CO2. Root weight and total length increased as the CO2 concentration increased in high soil P, but the greatest root weights and lengths within the medium and low P treatments were those of seedlings reared in the 525 mu l(-1) CO2 atmosphere. Nevertheless, shoot/root ratios decreased with increasing CO2 in both high and medi um soil P at the second harvest, and the highest shoot/root ratio in l ow P was that of seedlings grown in ambient CO2. After 1 year, the lar gest shoot and root volumes within the high and medium P treatments we re those of seedlings grown in intermediate CO2, while the reverse was true in low P. The effects of CO2 concentration on dry weights, total root length, and shoot/root ratio at the final harvest were nonsignif icant. As proved true with seedling growth, CO2 effects on ectomycorrh izal colonization varied temporally, as mycorrhizal development was no t affected by the atmospheric treatments after 4 months, while seedlin gs grown in ambient CO2 exhibited the highest percent infections withi n each P treatment at the second harvest but those grown in 700 mu l l (-1) CO2 had the highest percentages after 1 year. These results sugge st that elevated CO2 exerts stimulatory effects on shoot and root deve lopment of juvenile ponderosa pine which may be dependent on P availab ility to some degree, but these effects are somewhat transient and var y in magnitude over time.