Allometric relations and growth in Pinus taeda: the effect of elevated CO2, and changing N availability

Abstract

summaryLoblolly pine (Pinus taeda L.) seedlings were grown for 138 d at two CO2 partial pressures (35 and 70 Pa CO2) and four N solution concentrations (0.5, 1.5, 3.5 and 6.5 mM NH4NO3). Allometric regression analysis was used to determine whether patterns of biomass allocation among functionally distinct plant‐parts were directly controlled by CO2 and N availability or whether differences between treatments were the result of size‐dependent changes in allocation. Both CO2 and N availability affected growth of loblolly pine. Growth stimulation by CO2 at nonlimiting N solution concentrations (3.5 and 6.5 mM NH4NO3) was c. 90%. At the lowest N solution concentration (0.5 mM NH4NO3), total plant biomass was still enhanced by 35% under elevated CO2. Relative growth rates were highly correlated with net assimilation rates, whereas leaf mass ratio remained unchanged under the wide range of CO2 and N solution concentrations. When differences in plant size were adjusted apparent CO2 effects on biomass allocation among different plant parts disappeared, indicating that CO2 only indirectly affected allocation through accelerated growth. N availability, by contrast, had a direct effect on biomass allocation, but primarily at the lowest N solution concentration (0.5 mM NH4NO3). Loblolly pine compensated for N limitation by increasing specific lateral root length and proportional biomass allocation to the lateral root system. The results emphasize the significance of distinguishing size‐dependent effects on biomass allocation from functional adjustments made in direct response to changing resource availability.