Effects of elevated CO(2) and nitrogen on the synchrony of shoot and root growth in ponderosa pine.

Abstract

We monitored effects of elevated CO(2) and N fertilization on shoot and fine root growth of Pinus ponderosa Dougl. ex P. Laws. and C. Laws. grown 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 to ambient (~365 micro mol mol(-1)) air or ambient air plus either 175 or 350 micro mol mol(-1) CO(2) in combination with one of three rates of N addition (0, 100 or 200 kg ha(-1) year(-1)). All CO(2) by N interactions were nonsignificant. Both the CO(2) and N treatments increased plant height, stem diameter and leaf area index (LAI). Elevated CO(2) increased fine root area density and the occurrence of mycorrhizae, whereas N fertilization increased coarse root area density but had no effect on fine root area density. Spring flushes of shoot height and diameter growth were initiated concurrently with the increase in new root area density but height and diameter growth reached their maxima before that of fine roots. The temporal patterns of root and shoot growth were not altered by providing additional CO(2) or N. Greatest root loss occurred in the summer, immediately following the period of greatest new fine root growth. Elevated N initially reduced the fine root area density/LAI ratio independently of CO(2) treatment, indicating that the relationship between fine roots and needles was not changed by CO(2) exposure.