We grew cuttings of two early (mid Oct.) and two late (early Nov.)
leaf-fall Populus tremuloides Michx. genotypes
(referred to as genotype pairs) for c. 150 d in open-top chambers
to understand how twice-ambient (elevated) CO2
and soil N availability would affect growth and C allocation. For the study,
we selected genotypes differing in leaf
area duration to find out if late-season photosynthesis influenced C allocation
to roots. Both elevated CO2 and high
soil N availability significantly increased estimated whole-tree photosynthesis,
but they did so in different ways.
Elevated CO2 stimulated leaf-level photosynthesis rates, whereas
high soil N availability resulted in greater total
plant leaf area. The early leaf-fall genotype pair had significantly higher
photosynthesis rates per unit leaf area than
the late leaf-fall genotype 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 season.
Across both genotype pairs, high soil N availability significantly increased
fine root length production and
mortality by increasing both 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 did not affect fine root life span. The early leaf-fall genotype pair
had 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 availability and will increase under elevated atmospheric CO2.
In addition, this study reinforces the need for
better understanding of the variation in tree responses to elevated CO2,
within and among species.