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

Loblolly 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.