We took advantage of the distinctive system-level measurement capabilities of the Biosphere 2 Laboratory (B2L) to examine the effects of prolonged exposure to elevated [CO 2 ] on carbon flux dynamics, above-and belowground biomass changes, and soil carbon and nutrient capital in plantation forest stands over 4 years. Annually coppiced stands of eastern cottonwoods (Populus deltoides) were grown under ambient (400 ppm) and two levels of elevated (800 and 1200 ppm) atmospheric [CO 2 ] in carbon and N-replete soils of the Intensive Forestry Mesocosm in the B2L. The large semiclosed space of B2L uniquely enabled precise CO 2 exchange measurements at the near ecosystem scale. Highly controllable climatic conditions within B2L also allowed for reproducible examination of CO 2 exchange under different scales in space and time. Elevated [CO 2 ] significantly stimulated whole-system maximum net CO 2 influx by an average of 21% and 83% in years 3 and 4 of the experiment. Over the 4-year experiment, cumulative belowground, foliar, and total aboveground biomass increased in both elevated [CO 2 ] treatments. After 2 years of growth at elevated [CO 2 ], early season stand respiration was decoupled from CO 2 influx aboveground, presumably because of accelerated fine root production from stored carbohydrates in the coppiced system prior to canopy development and to the increased soil carbohydrate status under elevated [CO 2 ] treatments. Soil respiration was stimulated by elevated [CO 2 ] whether measured at the system level in the undisturbed soil block, by soil collars in situ, or by substrate-induced respiration in vitro. Elevated [CO 2 ] accelerated depletion of soil nutrients, phosphorus, calcium and potassium, after 3 years of growth, litter removal, and coppicing, especially in the upper soil profile, although total N showed no change. Enhancement of aboveand belowground biomass production by elevated [CO 2 ] accelerated carbon cycling through the coppiced system and did not sequester additional carbon in the soil.
Image sequence processing methods were applied to study the effect of elevated CO 2 on the diel leaf growth cycle for the first time in a dicot plant. Growing leaves of Populus deltoides, in stands maintained under ambient and elevated CO 2 for up to 4 years, showed a high degree of heterogeneity and pronounced diel variations of their relative growth rate (RGR) with maxima at dusk. At the beginning of the season, leaf growth did not differ between treatments. At the end of the season, final individual leaf area and total leaf biomass of the canopy was increased in elevated CO 2 . Increased final leaf area at elevated CO 2 was achieved via a prolonged phase of leaf expansion activity and not via larger leaf size upon emergence. The fraction of leaves growing at 30-40% day À1 was increased by a factor of two in the elevated CO 2 treatment. A transient minimum of leaf expansion developed during the late afternoon in leaves grown under elevated CO 2 as the growing season progressed. During this minimum, leaves grown under elevated CO 2 decreased their RGR to 50% of the ambient value. The transient growth minimum in the afternoon was correlated with a transient depletion of glucose (less than 50%) in the growing leaf in elevated CO 2 , suggesting diversion of glucose to starch or other carbohydrates, making this substrate temporarily unavailable for growth. Increased leaf growth was observed at the end of the night in elevated CO 2 . Net CO 2 exchange and starch concentration of growing leaves was higher in elevated CO 2 . The extent to which the transient reduction in diel leaf growth might dampen the overall growth response of these trees to elevated CO 2 is discussed.
We examined the effects of atmospheric vapor pressure deficit (VPD) and soil moisture stress (SMS) on leaf-and stand-level CO 2 exchange in model 3-year-old coppiced cottonwood (Populus deltoides Bartr.) plantations using the large-scale, controlled environments of the Biosphere 2 Laboratory. A short-term experiment was imposed on top of continuing, long-term CO 2 treatments (43 and 120 Pa), at the end of the growing season. For the experiment, the plantations were exposed for 6-14 days to low and high VPD (0.6 and 2.5 kPa) at low and high volumetric soil moisture contents (25-39%). When system gross CO 2 assimilation was corrected for leaf area, system net CO 2 exchange (SNCE), integrated daily SNCE, and system respiration increased in response to elevated CO 2 . The increases were mainly as a result of the larger leaf area developed during growth at high CO 2 , before the short-term experiment; the observed decline in responses to SMS and high VPD treatments was partly because of leaf area reduction. Elevated CO 2 ameliorated the gas exchange consequences of water stress at the stand level, in all treatments. The initial slope of light response curves of stand photosynthesis (efficiency of light use by the stand) increased in response to elevated CO 2 under all treatments. Leaf-level net CO 2 assimilation rate and apparent quantum efficiency were consistently higher, and stomatal conductance and transpiration were significantly lower, under high CO 2 in all soil moisture and VPD combinations (except for conductance and transpiration in high soil moisture, low VPD). Comparisons of leaf-and stand-level gross CO 2 exchange indicated that the limitation of assimilation because of canopy light environment (in well-irrigated stands; ratio of leaf : stand 5 3.2-3.5) switched to a predominantly individual leaf limitation (because of stomatal closure) in response to water stress (leaf : stand 5 0.8-1.3). These observations enabled a good prediction of whole stand assimilation from leaf-level data under water-stressed conditions; the predictive ability was less under well-watered conditions. The data also demonstrated the need for a better understanding of the relationship between leaf water potential, leaf abscission, and stand LAI. NomenclatureA net 5 leaf net photosynthetic CO 2 assimilation B2L 5 Biosphere 2 Laboratory E 5 leaf transpiration ECW 5 eastern cottonwoods g s 5 stomatal conductanceCorrespondence: Ramesh Murthy, LAI 5 leaf area index PPF 5 photosynthetic photon flux SGCA 5 system gross CO 2 assimilation SGCA L 5 system gross CO 2 assimilation per unit leaf area at light saturation SMS 5 soil moisture stress SNCE L or D 5 system net CO 2 exchange (soil area basis), L or D as subscripts refer to light or dark VPD 5 atmospheric vapor pressure deficit
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