GROWTH AND C ALLOCATION OF POPULUS-TREMULOIDES GENOTYPES IN RESPONSE TO ATMOSPHERIC CO2 AND SOIL N AVAILABILITY

We grew cuttings of two early (mid Oct.) and two late (early Nov.) lea f-fall Populus tremuloides Michx. genotypes (referred to as genotype p airs) for c. 150 d in open-top chambers to understand how twice-ambien t (elevated) CO2 and soil N availability would affect growth and C all ocation. For the study, we selected genotypes differing in leaf area d uration to find out if late-season photosynthesis influenced C allocat ion to roots. Both elevated CO2 and high soil N availability significa ntly increased estimated whole-tree photosynthesis, but they did so in different ways. Elevated CO2 stimulated leaf-level photosynthesis rat es, whereas high soil N availability resulted in greater total plant l eaf area. The early leaf-fall genotype pair had significantly higher p hotosynthesis rates per unit leaf area than the late leaf-fall genotyp e pair and elevated CO2 enhanced this difference. The early leaf-fall genotype pair had less leaf area than the late leaf-fall genotype pair , and their rate of leaf area development decreased earlier in the sea son. Across both genotype pairs, high soil N availability significantl y increased fine root length production and mortality by increasing bo th the amount of root length present, and by decreasing the life span of individual roots. Elevated CO2 resulted in significantly increased fine root production and mortality in high N but not low N soil and di d not affect fine root life span. The early leaf-fall genotype pair ha d significantly greater fine root length production than the late leaf -fall genotype pair across all CO2 and N treatments. These differences in belowground C allocations are consistent with the hypothesis that belowground C and N cycling is strongly influenced by soil N availabil ity and will increase under elevated atmospheric CO2. In addition, thi s study reinforces the need for better understanding of the variation in tree responses to elevated CO2, within and among species.