Growth, nutrition, and water relations of ponderosa pine in a field soil as influenced by long-term exposure to elevated atmospheric CO 2

Abstract

Atmospheric CO 2 enrichment effects on growth, nutrition, and water relations of ponderosa pine ( Pinus ponderosa Dougl. ex Laws.) were investigated in a field study of 5 years duration which included a soil N fertility variable in order to assess interactions of CO 2 and N on tree responses. Open-top chambers permitted creation of atmospheres with 700 μl l −1, 525 μl l −1, or ambient CO 2 concentrations. Trees were reared from seed in a mid elevation Sierra Nevada forest soil with a total N concentration of 856 μg g −1 or in soil amended with sufficient (NH 4) 2SO 4 to increase total N by 100 μg g −1 or 200 μg g −1. The intermediate CO 2 treatment within the intermediate N treatment was omitted from the study. Height and diameter measurements at the conclusions of each of the five growing seasons revealed significant growth enhancement by atmospheric enrichment, initially more pronounced in 525 μl l −1 than in 700 μl l −1 CO 2. Final measurements indicated that the twice-ambient atmosphere ultimately proved the most stimulatory, however, but the magnitude of the growth responses to elevated CO 2 was somewhat dependent on soil N availability throughout the study. Foliar analysis revealed reductions in N, P, and S concentrations during the second season and that of Mg in the third season in the above-ambient CO 2 treatments, while P was increased by high CO 2 in high soil N but decreased by high CO 2 in low N during the third season. Among micronutrients, foliar Fe was elevated in high CO 2, a response that also extended to Al, while Mn and B concentrations were reduced in this atmosphere, but significant treatment effects on micronutrients and Al were found in the second season only and were evident primarily in low soil N. During the fourth growing season, foliar N was initially higher but then later become lower in the above-ambient atmospheres. Predawn and midday measurements of xylem water potential made at intervals during the fifth season indicated substantial variability in responses to treatment, but increased moisture stress in trees reared in the enriched atmospheres was periodically evident.