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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) CO # 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 CO # and high soil N availability significantly increased estimated whole-tree photosynthesis, but they did so in different ways. Elevated CO # 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 CO # 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 CO # 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 CO # 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 CO # . In addition, this study reinforces the need for better understanding of the variation in tree responses to elevated CO # , within and among species.

It is uncertain whether elevated atmospheric CO will increase C storage in terrestrial ecosystems without concomitant increases in plant access to N. Elevated CO may alter microbial activities that regulate soil N availability by changing the amount or composition of organic substrates produced by roots. Our objective was to determine the potential for elevated CO to change N availability in an experimental plant-soil system by affecting the acquisition of root-derived C by soil microbes. We grew Populus tremuloides (trembling aspen) cuttings for 2 years under two levels of atmospheric CO (36.7 and 71.5 Pa) and at two levels of soil N (210 and 970 µg N g). Ambient and twice-ambient CO concentrations were applied using open-top chambers, and soil N availability was manipulated by mixing soils differing in organic N content. From June to October of the second growing season, we measured midday rates of soil respiration. In August, we pulse-labeled plants with CO and measured soil CO respiration and the C contents of plants, soils, and microorganisms after a 6-day chase period. In conjunction with the August radio-labeling and again in October, we usedN pool dilution techniques to measure in situ rates of gross N mineralization, N immobilization by microbes, and plant N uptake. At both levels of soil N availability, elevated CO significantly increased whole-plant and root biomass, and marginally increased whole-plant N capital. Significant increases in soil respiration were closely linked to increases in root biomass under elevated CO. CO enrichment had no significant effect on the allometric distribution of biomass or C among plant components, totalC allocation belowground, or cumulative (6-day) CO soil respiration. Elevated CO significantly increased microbial C contents, indicating greater availability of microbial substrates derived from roots. The near doubling of microbialC contents at elevated CO was a relatively small quantitative change in the belowground C cycle of our experimental system, but represents an ecologically significant effect on the dynamics of microbial growth. Rates of plant N uptake during both 6-day periods in August and October were significantly greater at elevated CO, and were closely related to fine-root biomass. Gross N mineralization was not affected by elevated CO. Despite significantly greater rates of N immobilization under elevated CO, standing pools of microbial N were not affected by elevated CO, suggesting that N was cycling through microbes more rapidly. Our results contained elements of both positive and negative feedback hypotheses, and may be most relevant to young, aggrading ecosystems, where soil resources are not yet fully exploited by plant roots. If the turnover of microbial N increases, higher rates of N immobilization may not decrease N availability to plants under elevated CO.

This study related age structure and composition of forests in southeastern Pennsylvania to edaphic properties caused by the activities of a local charcoal iron furnace from the late 18th to the late 19th century. Forests surrounding charcoal hearths were harvested on 20- to 30-year rotations to supply wood for charcoal production in earthmound kilns. Charcoal hearths were created by a recurring cycle of disturbance, including the removal of established vegetation, intense heat, and the deposition of charcoal. Thirty-eight hearths of Hopewell Furnace National Historic Site were examined to describe differences in vegetation relative to surrounding forests that have developed since the cessation of charcoal production in 1883. Hearth forests were significantly younger than surrounding forests, with lower tree density and basal area, decreased sapling and shrub abundance, and higher herbaceous cover. Compositional differences on charcoal hearths included decreased importance of several Quercus species and ericaceous shrubs and increased importance of Liriodendrontulipifera L. Results of this study indicated that the initial effect of charcoal production was the prevention of stump sprouting and a delay in recolonization following disturbance due to extreme soil physical and chemical properties. Failure of hearth seedlings to recruit into larger size classes, age–diameter relationships of Liriodendrontulipifera L. trees, and the failure of calcifuge species to reestablish similar cover on charcoal hearths suggested that unfavorable properties of charcoal soils continue to negatively affect long-term productivity. Relative to nonhearth soils, large, significant differences in soil pH, cation exchange capacity, base saturation, and exchangeable Ca, Mg, and K indicated that continuing inhibition of woody plants on charcoal hearths may be related to alterations in soil chemistry that have persisted for 110 years.