The means by which growth CO2 concentration ([CO2]) affects anatomy and water relations responses to drought and vapour pressure deficit (VPD) were studied for yearly coppiced, 4-year-old Populus deltoides clones that were grown in either 400 mu mol mol(-1) (ambient) or 800 mu mol mol(-1) (elevated) CO2 for 3 years. It was hypothesized that, during drought, trees growing in elevated [CO2] would have a lower volume flux density of water (J(V)), stomatal conductance (g(s)) and transpiration per leaf area (E), as well as a lower stomatal density and a greater stomatal response to drought and changes in VPD than would trees in ambient [CO2]. Trees in elevated [CO2] actually had higher J(V) values throughout the study, but did not differ from trees in ambient [CO2] with respect to g(s) or E under saturating light or E scaled from J(V) (E-scaled), all of which indicates that the higher J(V) in elevated [CO2] resulted from those trees having greater leaf area and not from differences in g(s). Furthermore, although plants in elevated [CO2] had greater absolute leaf loss during the drought, the percentage of leaf area lost was similar to that of trees in ambient [CO2]. g(s) and E under saturating light were affected by changes in VPD after the first 9 days of the experiment, which coincided with a large decrease in water potential at a soil depth of 0.1 m. Trees in elevated [CO2] had a greater stomatal density and a lower wood density than trees in ambient [CO2], both traits that may make the trees more susceptible to xylem cavitation in severe drought. Drought and VPD effects for the P. deltoides clone were not ameliorated by long-term growth in elevated [CO2] compared with ambient [CO2], and plants in elevated [CO2] possessed anatomical traits that may result in greater stress associated with long-term drought.

Alterations in temperature (T) and vapor pressure deficit (VPD) strongly influence gas exchange, but because VPD is highly influenced by T, the effects of these two factors are difficult to separate. Here, the concomitant effects of T and VPD on CO(2) uptake, stomatal conductance, and transpiration at leaf- and canopy-levels were examined for a stand of trees (Populus deltoides) enclosed within large mesocosms. T and VPD were independently altered to yield a factorial combination of treatments of low (24 degrees C) or high (30 degrees C) T and low (0.75) or high (1.75 kPa) VPD. Traditional leaf-level gas exchange measurements were compared with whole-canopy exchange to verify typical scaling methods. Elevated T resulted in an average 40% and 14% increase in midday leaf-level and canopy-level net CO(2) uptake, respectively. Other physiological responses to elevated T and VPD were similar at both scales, but the magnitude of change was usually less pronounced at the canopy-level. Surprisingly, only minimal interactions between T and VPD were found to influence responses of CO(2) uptake and stomatal conductance at either level.

INTRODUCTION Terrestrial ecosystem respiration releases approximately 60 Pg carbon into the atmosphere anually (Schimel et al 1995). This is a relatively large amount in comparison to the 7.1 Pg carbon released per year from fossil fuel combustion, cement production and land use changes. Terrestrial ecosystem respiration thus plays a critical role in determining net CO2 fluxes into the atmosphere. Understanding the factors that regulate ecosystem respiration (Re) is therefore a prerequisite for understanding how future changes in climate will affect atmospheric CO2 concentrations. The response of Re to elevated CO2 is unclear. However, there is growing evidence that climate dependent variations in photosynthesis, and thus substrate supply, influence the rate of ecosystem respiration (Re). Furthermore, there is evidence that variations in substrate supply can affect the temperature-sensitivity of respiration (Azcón-Bieto & Osmond 1983, Atkin et al 2002, Covey-Crump et al 2002). Consequently, climate-dependent changes in photosynthesis might result in concomitant changes in both the rate, and temperature-sensitivity, of Re. Re is the sum of several respiratory processes, which, for the purposes of simplicity, can be divided into aboveand below-ground respiration. If we are to better understand the impact of a changing climate on Re, it will be necessary to understand the response of these aboveand below-ground processes to changes in climate, and the extent to which they determine the respiratory response at the whole ecosystem level. In this study we investigated the impact of elevated atmospheric CO2 on leaf, soil and whole ecosystem respiration in order to determine (1) the extent to which photosynthetic CO2 uptake dictates the rate and temperature-sensitivity of respiratory CO2 release, and (2) the degree to which changes in leaf and soil respiration determine the ecosystem level response to changes in climate.

There is little current understanding of the relationship between soil microbial community composition and soil processes rates, nor of the effect climate change and elevated CO(2) will have on microbial communities and their functioning. Using the eastern cottonwood (Populus deltoides) plantation at the Biosphere 2 Laboratory, we studied the relationships between microbial community structure and process rates, and the effects of elevated atmospheric CO(2) on microbial biomass, activity, and community structure. Soils were sampled from three treatments (400, 800, and 1200 ppm CO(2)), a variety of microbial biomass and activity parameters were measured, and the bacterial community was described by 16S rRNA libraries. Glucose substrate-induced respiration (SIR) was significantly higher in the 1200 ppm CO(2) treatment. There were also a variety of complex, nonlinear responses to elevated CO(2). There was no consistent effect of elevated CO(2) on bacterial diversity; however, there was extensive variation in microbial community structure within the plantation. The southern ends of the 800 and 1200 ppm CO(2) bays were dominated by beta-Proteobacteria, and had higher fungal biomass, whereas the other areas contained more alpha-Proteobacteria and Acidobacteria. A number of soil process rates, including salicylate, glutamate, and glycine substrate-induced respiration and proteolysis, were significantly related to the relative abundance of the three most frequent bacterial taxa, and to fungal biomass. Overall, variation in microbial activity was better explained by microbial community composition than by CO(2) treatment. However, the altered diversity and activity in the southern bays of the two high CO(2) treatments could indicate an interaction between CO(2) and light.

The use of rooted stem cuttings of loblolly and slash pines offers an opportunity to capture additional genetic gain in plantations. Using rooted cuttings with high genetic performance values as planting stock is becoming increasingly common with many species around the world. Rooted cuttings are being utilized to capture genetic gain at two levels. The first multiplies seed from previously tested, outstanding crosses. This approach delivers genetic gain equivalent to mass controlled pollination and could be important when seed is in short supply, such as when superior crosses are first identified, but before they can be established in seed orchards and grown to sufficient size to meet reforestation objectives. The ultimate utility of both full-sib deployment methods will depend on the cost effectiveness and amount of genetic gain delivered in a given period of time. The second level of genetic gain to be exploited is that which comes from propagation of individual clones. This will deliver the most genetic gain, provided an efficient system can be developed for within-family selection and propagation.

We examined the effects of atmospheric vapor pressure deficit (VPD) and soil moisture stress (SMS) on leafand stand-level CO2 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 CO2 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 CO2 assimilation was corrected for leaf area, system net CO2 exchange (SNCE), integrated daily SNCE, and system respiration increased in response to elevated CO2. The increases were mainly as a result of the larger leaf area developed during growth at high CO2, before the short-term experiment; the observed decline in responses to SMS and high VPD treatments was partly because of leaf area reduction. Elevated CO2 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 CO2 under all treatments. Leaf-level net CO2 assimilation rate and apparent quantum efficiency were consistently higher, and stomatal conductance and transpiration were significantly lower, under high CO2 in all soil moisture and VPD combinations (except for conductance and transpiration in high soil moisture, low VPD). Comparisons of leafand stand-level gross CO2 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.

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 [CO2] 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 [CO2] in carbon and N‐replete soils of the Intensive Forestry Mesocosm in the B2L. The large semiclosed space of B2L uniquely enabled precise CO2 exchange measurements at the near ecosystem scale. Highly controllable climatic conditions within B2L also allowed for reproducible examination of CO2 exchange under different scales in space and time. Elevated [CO2] significantly stimulated whole‐system maximum net CO2 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 [CO2] treatments. After 2 years of growth at elevated [CO2], early season stand respiration was decoupled from CO2 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 [CO2] treatments. Soil respiration was stimulated by elevated [CO2] whether measured at the system level in the undisturbed soil block, by soil collars in situ, or by substrate‐induced respiration in vitro. Elevated [CO2] 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 above‐ and belowground biomass production by elevated [CO2] 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 CO2 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 CO2 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 CO2. Increased final leaf area at elevated CO2 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(-1) was increased by a factor of two in the elevated CO2 treatment. A transient minimum of leaf expansion developed during the late afternoon in leaves grown under elevated CO2 as the growing season progressed. During this minimum, leaves grown under elevated CO2 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 CO2, 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 CO2. Net CO2 exchange and starch concentration of growing leaves was higher in elevated CO2. The extent to which the transient reduction in diel leaf growth might dampen the overall growth response of these trees to elevated CO2 is discussed.

Isoprene is the most abundant biogenic hydrocarbon released from vegetation and it plays a major role in tropospheric chemistry. Because of its link to climate change, there is interest in understanding the relationship between CO 2 , water availability and isoprene emission. We explored the effect of atmospheric elevated CO 2 concentration and its interaction with vapour pressure deficit (VPD) and water stress, on gross isoprene production (GIP) and net ecosystem exchange of CO 2 (NEE) in two Populus deltoides plantations grown at ambient and elevated atmospheric CO 2 concentration in the Biosphere 2 Laboratory facility. Although GIP and NEE showed a similar response to light and temperature, their responses to CO 2 and VPD were opposite; NEE was stimulated by elevated CO 2 and depressed by high VPD, while GIP was inhibited by elevated CO 2 and stimulated by high VPD. The difference in response between isoprene production and photosynthesis was also evident during water stress. GIP was stimulated in the short term and declined only when the stress was severe, whereas NEE started to decrease from the beginning of the experiment. This contrasting response led the carbon lost as isoprene in both the ambient and the elevated CO 2 treatments to increase as water stress progressed. Our results suggest that water limitation can override the inhibitory effect of elevated CO 2 leading to increased global isoprene emissions in a climate change scenario with warmer and drier climate.

Isoprene is the most abundant volatile hydrocarbon emitted by many tree species and has a major impact on tropospheric chemistry, leading to formation of pollutants and enhancing the lifetime of methane, a powerful greenhouse gas. Reliable estimates of global isoprene emission from different ecosystems demand a clear understanding of the processes of both production and consumption. Although the biochemistry of isoprene production has been studied extensively and environmental controls over its emission are relatively well known, the study of isoprene consumption in soil has been largely neglected.

We measured night-time respiration and daytime photosynthesis of leaves in canopies of 4 m tall cottonwood (Populus deltoides) trees to investigate the link between leaf respiration and photosynthetic capacity.Trees were grown at three CO2 partial pressures [p(CO2)(a)] (42, 80, 120 Pa) and experimentally exposed to differing nocturnal temperatures (15, 20 or 25degreesC), but constant daytime temperatures (30-32degreesC), in a short-term whole-ecosystem environmental manipulation.Rates of night-time leaf dark respiration (R-d) increased significantly at all growth CO2 partial pressures when nocturnal temperatures were increased from 15 to 25degreesC. Predawn leaf nonstructural carbohydrate (soluble sugars and starch) content was significantly lower at the higher night temperatures. Photosynthetic capacity (A(max)) during the day increased significantly between 15 and 25degreesC at 42 and 80 Pa, but not at 120 Pa.These findings indicate that the previously determined relationships between elevated night-time temperature, dark respiration and increased photosynthetic capacity may also hold at elevated p(CO2)(a). This response may have a significant influence on plant and ecosystem carbon exchange under global change scenarios.

Cottonwood (Populus deltoides Bartr. ex Marsh.) trees grown for 9 months in elevated carbon dioxide concentration ([CO2]) showed significant increases in height, leaf area and basal diameter relative to trees in a near-ambient [CO2] control treatment. Sample trees in the CO2 treatments were subjected to high and low atmospheric vapor pressure deficits (VPD) over a 5-week period at both high and low soil water contents (SWC). During these periods, transpiration rates at both the leaf and canopy levels were calculated based on sap flow measurements and leaf-to-sapwood area ratios. Leaf-level transpiration rates were approximately equivalent across [CO2] treatments when soil water was not limiting. In contrast, during drought stress, canopy-level transpiration rates were approximately equivalent across [CO2] treatments, indicating that leaf-level fluxes during drought stress were reduced in elevated [CO2] by a factor equal to the leaf area ratio of the two canopies. The shift from equivalent leaf-level transpiration to equivalent canopy-level transpiration with increasing drought stress suggests maximum water use rates were controlled primarily by atmospheric demand at high SWC and by soil water availability at low SWC. Changes in VPD had less effect on transpiration than changes in SWC for trees in both CO2 treatments. Transpiration rates of trees in both CO2 treatments reached maximum values at a VPD of about 2.0 kPa at high SWC, but leveled off and decreased slightly in both canopies as VPD increased above this value. At low SWC, increasing VPD from approximately 1.4 to 2.5 kPa caused transpiration rates to decline slightly in the canopies of trees in both treatments, with significant (P = 0.004) decreases occurring in trees in the near-ambient [CO2] treatment. The transpiration responses at high VPD in the presence of high SWC and throughout the low SWC treatment suggest some hydraulic limitations to water use occurred. Comparisons of midday leaf water potentials of trees in both CO2 treatments support this conclusion.

To further our understanding of the influence of global climate change on isoprene production we studied the effect of elevated [CO2] and vapour pressure deficit (VPD) on isoprene emission rates from leaves of Populus deltoides Bartr. during drought stress. Trees, grown inside three large bays with atmospheres containing 430, 800, or 1200 μmol mol-1 CO2 at the Biosphere 2 facility, were subjected to a period of drought during which VPD was manipulated, switching between low VPD (approximately 1 kPa) and high VPD (approximately 3 kPa) for several days. When trees were not water-stressed, elevated [CO2] inhibited isoprene emission and stimulated photosynthesis. Isoprene emission was less responsive to drought than photosynthesis. As water-stress increased, the inhibition of isoprene emission disappeared, probably as a result of stomatal closure and the resulting decreases in intercellular [CO2] (Ci). This assumption was supported by increased isoprene emission under high VPD. Drought and high VPD dramatically increased the proportion of assimilated carbon lost as isoprene. When measured at the same [CO2], leaves from trees grown at ambient [CO2] always had higher isoprene emission rates than the leaves of trees grown at elevated [CO2], demonstrating that CO2 inhibition is a long-term effect.

Because of scaling problems point measurements of soil CO2 efflux on a small volume of soil may not necessarily reflect an overall community response. The aim of this study was to test this hypothesis in the Biosphere 2 facility and achieve the following broad goals: (1) investigate soil net CO2 exchange–temperature relationship at the community level; (2) compare soil net CO2 exchange at the community level to the traditional sample point estimates of CO2 efflux scaled up to the community level; (3) evaluate the usefulness of a facility such as Biosphere 2 for conducting community level experiments for studying response to a climatic perturbation under controlled environmental conditions. A 550 m3 volume of soil with 282, 15 cm tree stumps was enclosed at the Biosphere 2 Center and warmed from 10 to 25 °C over a period of 34 days. Net CO2 exchange from this community was measured at various points on the soil surface with 78.5 cm2 chambers and for the whole community using each of the three bays at Biosphere 2 Center as a closed system. Soil CO2 efflux rates obtained by point measurements showed tremendous variability from location to location. At the community level and with point measurements, net CO2 exchange increased exponentially with increasing soil temperatures. Q10 values from both the point and community level measurements ranged from 1.7 to 2.5. Scaling of point measurements by soil surface area and time overestimated community rates by 36% revealing some of the limitations of point measurements. This experiment demonstrates how Biosphere 2 facility could be used to study behavior of individual components and measure responses at the community level and test our capacity to scale point in time and space measures of community processes to the community level.

Plant respiration is an important physiological process in the global carbon cycle serving as a major carbon flux from the biosphere to the atmosphere. Respiration is sensitive to temperature providing a link between environmental variability, climate change and the global carbon cycle. We measured leaf respiration in Populus deltoides after manipulating the air temperature surrounding part of a single leaf, and compared this to the temperature response of the same leaves after manipulating the temperature of the stand. The short-term temperature response of respiration (Q(10) - change in the respiration rate with a 10 degreesC increase in leaf temperature) was 1.7 when the leaf temperature was manipulated, but 2.1 when the stand-level temperature was changed. As a result, total night-time carbon release during the five-day experiment was 21% lower when using the Q(10) estimates from the tradition leaf manipulation compared to the stand-level manipulation. We conclude that the temperature response of leaf respiration is related to whole plant carbon and energy demands, and that appropriate experimental procedures are required in examining respiratory CO2 release under variable temperature conditions.

• Leaf respiration and its temperature response were measured in 4-m-tall, 1-yr-old Populus deltoides trees to assess the effect of within-canopy distribution of respiratory physiology on total foliar C exchange of a model ecosystem at Biosphere 2. • Over the course of five nights, air temperature was varied over a 10°C range and the steady-state rate of leaf respiration was measured. These data were then modeled to calculate the temperature response of leaf and canopy respiration. • Results indicate that there is considerable within-canopy variation in both the rate of respiration and its temperature response and that these variables are most strongly related to leaf carbohydrate and leaf N. Scaling these results to the ecosystem level demonstrates the importance of quantifying the vertical distribution of respiratory physiology, particularly at lower temperatures. • Simplifying assumptions regarding the variation in respiration and its temperature response with canopy height tend to result in an underestimation of the actual C loss if the assumptions are based on lower- or mid-canopy leaf physiology, but overestimate C loss if the model assumptions are based on upper-canopy physiology.

In order to investigate the relative impacts of increases in day and night temperature on tree carbon relations, we measured night-time respiration and daytime photosynthesis of leaves in canopies of 4-m-tall cottonwood (Populus deltoides Bartr. ex Marsh) trees experiencing three daytime temperatures (25, 28 or 31 degreesC) and either (i) a constant nocturnal temperature of 20 degreesC or (ii) increasing nocturnal temperatures (15, 20 or 25 degreesC). In the first (day warming only) experiment, rates of night-time leaf dark respiration (R-dark) remained constant and leaves displayed a modest increase (11%) in light-saturated photosynthetic capacity (A(max)) during the day (1000-1300 h) over the 6 degreesC range. In the second (dual night and day warming) experiment, R-dark increased by 77% when nocturnal temperatures were increased from 15 degreesC (0.36 mumol m(-2) s(-1)) to 25 degreesC (0.64 mumol m(-2) s(-1)). A(max) responded positively to the additional nocturnal warming, and increased by 38 and 64% in the 20/28 and 25/31 degreesC treatments, respectively, compared with the 15/25 degreesC treatment. These increases in photosynthetic capacity were associated with strong increases in the maximum carboxylation rate of rubisco (V-cmax) and ribulose-1,5-bisphosphate (RuBP) regeneration capacity mediated by maximum electron transport rate (J(max)). Leaf soluble sugar and starch concentration, measured at sunrise, declined significantly as nocturnal temperature increased. The nocturnal temperature manipulation resulted in a significant inverse relationship between Amax and pre-dawn leaf carbohydrate status. Independent measurements of the temperature response of photosynthesis indicated that the optimum temperature (T-opt) acclimated fully to the 6 degreesC range of temperature imposed in the daytime warming. Our findings are consistent with the hypothesis that elevated night-time temperature increases photosynthetic capacity during the following light period through a respiratory-driven reduction in leaf carbohydrate concentration. These responses indicate that predicted increases in night-time minimum temperatures may have a significant influence on net plant carbon uptake.

Two experiments were conducted to determine theeffect of handling, short-term storage, andinitial water stress on cutting water potential (ΨW) and rooting of loblolly pine(Pinus taeda L.) stem cuttings. First,stock plants and cuttings were measured forΨW at predawn (04:00 a.m.) and earlymorning (09:00 a.m.). Cuttings were thensevered, wrapped in wet paper towels, andplaced in insulated containers for 2 or 7 h atapproximately 30 °C or for 21 h in coldstorage (4 °C). Water potentials ofcuttings were measured at the end of eachstorage period. Second, effects of initialwater stress on rooting performance of cuttingswere tested by withholding water from dormant(winter) and succulent (summer) cuttings forvarying periods of time. After each dryingtreatment, ΨW was measured on asample of cuttings and the remainder of thecuttings were transferred to a greenhouse withintermittent mist for 12 weeks.

Response of Carbon Dioxide Efflux from a 550m Soil Bed to a Range of Soil Temperatures R. Murthy, K. L . Griffin, S. J. Zarnoch, P.M. Dougherty, B. Watson, J. van Haren, R. L. Patterson, and T. Mahato 1 Columbia Earth Institute, Lamont-Doherty Earth Observatory, Biosphere 2 Center, Columbia University, Oracle, AZ 85623(Email: rmurthy@bio2.edu, Tel: 520-896-6422, fax: 520-896-5034); 2 Department of Earth and Environmental Sciences, Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964; 3 USDA Forest Service, Southern Research Station, Asheville, NC 28802; 4 Westvaco Corp., Forest Research, P.O. Box 1950, Summerville, SC 29484; 5 Biosphere 2 Center, Columbia University, Oracle, AZ 85623