EFFECTS OF ELEVATED CO2 AND NITROGEN ON THE SYNCHRONY OF SHOOT AND ROOT-GROWTH IN PONDEROSA PINE

We monitored effects of elevated CO2 and N fertilization on shoot and fine root growth of Pinus ponderosa Dougl. ex P. Laws. and C. Laws. gr own in native soil in open-top field-exposure chambers at Placerville, CA, over a 2-year period. The experimental design was a replicated 3 x 3 factorial with the center treatment missing; plants were exposed t o ambient (similar to 365 mu mol mol(-1)) air or ambient air plus eith er 175 or 350 mu mol mol(-1) CO2 in combination with one of three rate s of N addition (0, 100 or 200 kg ha(-1) year(-1)). All CO2 by N inter actions were nonsignificant. Both the CO2 and N treatments increased p lant height, stem diameter and leaf area index (LAI). Elevated CO2 inc reased fine root area density and the occurrence of mycorrhizae, where as N fertilization increased coarse root area density but had no effec t on fine root area density. Spring flushes of shoot height and diamet er growth were initiated concurrently with the increase in new root ar ea density but height and diameter growth reached their maxima before that of fine roots. The temporal patterns of root and shoot growth wer e not altered by providing additional CO2 or N. Greatest root loss occ urred in the summer, immediately following the period of greatest new fine root growth. Elevated N initially reduced the fine root area dens ity/LAI ratio independently of CO2 treatment, indicating that the rela tionship between fine roots and needles was not changed by CO2 exposur e.