Effects of elevated CO2 and nitrogen on the synchrony of shoot and root growth in ponderosa pine

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Abstract
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. 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 × 3 factorial with the center treatment missing; plants were exposed to ambient (∼365 μmol mol−1) air or ambient air plus either 175 or 350 μmol mol−1 CO2 in combination with one of three rates of N addition (0, 100 or 200 kg ha−1 year−1). All CO2 by N interactions were nonsignificant. Both the CO2 and N treatments increased plant height, stem diameter and leaf area index (LAI). Elevated CO2 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 CO2 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 CO2 treatment, indicating that the relationship between fine roots and needles was not changed by CO2 exposure.

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