Three years of free-air CO2 enrichment (POPFACE) only slightly affect profiles of light and leaf characteristics in closed canopies of Populus

Modelling is used to predict long‐term forest responses to increased atmospheric CO₂ concentrations. Although productivity models are based on light intercepted by the canopy, very little experimental data are available for closed forest stands. Nevertheless, the relationships between light inside a canopy, leaf area, canopy structure, and individual leaf characteristics may be affected by elevated CO₂, affecting in turn carbon gain. Using a free‐air CO₂ enrichment (FACE) design in a high‐density plantation of Populus spp., we studied the effects of increased CO₂ concentrations on transmittance (τ) of photosynthetic photon flux density (Q_p), on ratios of red/far‐red light (R/FR), on leaf area index (LAI), on leaf inclination, on leaf chlorophyll (chl) and nitrogen (N) concentrations, and on specific leaf area (SLA) in the 2nd and 3rd years of treatment. Continuous measurements of τ were made in addition to canopy height profiles of light and leaf characteristics. Two years of Q_p measurements showed an average decrease of canopy transmittance in the FACE treatment, with very small differences at canopy closure. Results were explained by an unaffected LAI in closed canopies, without a FACE‐induced stimulation of relative crown depth. In agreement, leaf inclination and extinction coefficients for light were similar in control and FACE conditions. Ratios of R/FR were not significantly affected by the FACE treatment, neither were leaf characteristics, with the exception of leaf N, which allows speculation about N limitation. In general, treatment differences in canopy profiles resulted from an initial stimulation of height growth in the FACE treatment. P. × euramericana differed from P. alba and P. nigra, but species did not differ significantly in their response to the FACE treatment. By the time fast‐growing high‐density forest plantations have passed the exponential growth phase and reached canopy closure, the likely effects of elevated atmospheric CO₂ concentration on canopy architecture and absorption of Q_p are minor.     

Partitioning net ecosystem carbon exchange with isotopic fluxes of CO2

Because biological and physical processes alter the stable isotopic composition of atmospheric CO₂, variations in isotopic content can be used to investigate those processes. Isotopic flux measurements of ¹³CO₂ above terrestrial ecosystems can potentially be used to separate net ecosystem CO₂ exchange (NEE) into its component fluxes, net photosynthetic assimilation (F_A) and ecosystem respiration (F_R). In this paper theory is developed to partition measured NEE into F_A and F_R, using measurements of fluxes of CO₂ and ¹³CO₂, and isotopic composition of respired CO₂ and forest air. The theory is then applied to fluxes measured (or estimated, for ¹³CO₂) in a temperate deciduous forest in eastern Tennessee (Walker Branch Watershed). It appears that there is indeed enough additional information in ¹³CO₂ fluxes to partition NEE into its photosynthetic and respiratory components. Diurnal patterns in F_A and F_R were obtained, which are consistent in magnitude and shape with patterns obtained from NEE measurements and an exponential regression between night‐time NEE and temperature (a standard technique which provides alternate estimates of F_R and F_A). The light response curve for photosynthesis (F_A vs. PAR) was weakly nonlinear, indicating potential for saturation at high light intensities. Assimilation‐weighted discrimination against ¹³CO₂ for this forest during July 1999 was 16.8–17.1‰, depending on canopy conductance. The greatest uncertainties in this approach lie in the evaluation of canopy conductance and its effect on whole‐canopy photosynthetic discrimination, and thus the indirect methods used to estimate isotopic fluxes. Direct eddy covariance measurements of ¹³CO₂ flux are needed to assess the validity of the assumptions used and provide defensible isotope‐based estimates of the component fluxes of net ecosystem exchange.     

Solar ultraviolet-B and photosynthetically active irradiance in the urban sub-canopy: A survey of influences

Stratospheric ozone loss in mid-latitudes is expected to increase the ultraviolet-B (UVB) radiation at the earth's surface. Impacts of this expected increase will depend on many factors, including the distribution of light in other wavelengths. Measurements of the photosynthetically active radiation (PAR) and UVB irradiance were made under clear skies at an open field and under the canopy of scattered trees in a suburban area in W. Lafayette, Indiana, USA (latitude 40.5°). Results showed that when there was significant sky view, the UVB penetration into sub-canopy spaces differs greatly from that of PAR. The UVBT _canopy (transmittance; irradiance below canopy/irradiance in open) was inversely related to sky view. The UVB irradiance did not vary as greatly between shaded and sunlit areas as did PAR. Analysis of measurements made near a brick wall indicated that the leaf area of a canopy and the brick wall primarily acted to block fractions of the sky radiance and contributed little scattered UVB to the horizontal plant. A model was developed to predict the UVB and PART _canopy based on diffuse fraction, sky view, and porosity of the crown(s) through which the beam is penetrating. The model accounted for the UVB and PART _canopy to within 0.13 and 0.05 root mean squared error (RMSE), respectively. Analysis of the errors due to model assumptions indicated that care must be taken in describing the sky radiance distribution, the porosity of trees, the penetration of diffuse radiation through porous trees, and the location of sky-obstructing trees and buildings.     

Modelling the discrimination of 13CO2 above and within a temperate broad-leaved forest canopy on hourly to seasonal time scales

Fluxes and concentrations of carbon dioxide and ¹³CO₂ provide information about ecosystem physiological processes and their response to environmental variation. The biophysical model, CANOAK, was adapted to compute concentration profiles and fluxes of ¹³CO₂ within and above a temperate deciduous forest (Walker Branch Watershed, Tennessee, USA). Modifications to the model are described and the ability of the new model (CANISOTOPE) to simulate concentration profiles of ¹³CO₂, its flux density across the canopy–atmosphere interface and leaf‐level photosynthetic discrimination against ¹³CO₂ is demonstrated by comparison with field measurements. The model was used to investigate several aspects of carbon isotope exchange between a forest ecosystem and the atmosphere. During the 1998 growing season, the mean photosynthetic discrimination against ¹³CO₂, by the deciduous forest canopy (Δ_canopy), was computed to be 22·4‰, but it varied between 18 and 27‰. On a diurnal basis, the greatest discrimination occurred during the early morning and late afternoon. On a seasonal time scale, the greatest diurnal range in Δ_canopy occurred early and late in the growing season. Diurnal and seasonal variations in Δ_canopy resulted from a strong dependence of Δ_canopy on photosynthetically active radiation and vapour pressure deficit of air. Model calculations also revealed that the relationship between canopy‐scale water use efficiency (CO₂ assimilation/transpiration) and Δ_canopy was positive due to complex feedbacks among fluxes, leaf temperature and vapour pressure deficit, a finding that is counter to what is predicted for leaves exposed to well‐mixed environments.     

Use of digital webcam images to track spring green-up in a deciduous broadleaf forest

Understanding relationships between canopy structure and the seasonal dynamics of photosynthetic uptake of CO₂ by forest canopies requires improved knowledge of canopy phenology at eddy covariance flux tower sites. We investigated whether digital webcam images could be used to monitor the trajectory of spring green-up in a deciduous northern hardwood forest. A standard, commercially available webcam was mounted at the top of the eddy covariance tower at the Bartlett AmeriFlux site. Images were collected each day around midday. Red, green, and blue color channel brightness data for a 640 × 100-pixel region-of-interest were extracted from each image. We evaluated the green-up signal extracted from webcam images against changes in the fraction of incident photosynthetically active radiation that is absorbed by the canopy (f _APAR), a broadband normalized difference vegetation index (NDVI), and the light-saturated rate of canopy photosynthesis (A _max), inferred from eddy flux measurements. The relative brightness of the green channel (green %) was relatively stable through the winter months. A steady rising trend in green % began around day 120 and continued through day 160, at which point a stable plateau was reached. The relative brightness of the blue channel (blue %) also responded to spring green-up, although there was more day-to-day variation in the signal because blue % was more sensitive to changes in the quality (spectral distribution) of incident radiation. Seasonal changes in blue % were most similar to those in f _APAR and broadband NDVI, whereas changes in green % proceeded more slowly, and were drawn out over a longer period of time. Changes in A _max lagged green-up by at least a week. We conclude that webcams offer an inexpensive means by which phenological changes in the canopy state can be quantified. A network of cameras could offer a novel opportunity to implement a regional or national phenology monitoring program.     

Mathematical models of the photosynthetic response of tree stands to rising CO2 concentrations and temperatures

Two published models of canopy photosynthesis, MAESTRO and BIOMASS, are simulated to examine the response of tree stands to increasing ambient concentrations of carbon dioxide (C_a) and temperatures. The models employ the same equations to described leaf gas exchange, but differ considerably in the level of detail employed to represent canopy structure and radiation environment. Daily rates of canopy photosynthesis simulated by the two models agree to within 10% across a range of CO₂ concentrations and temperatures. A doubling of C_a leads to modest increases of simulated daily canopy photosynthesis at low temperatures (10% increase at 10°C), but larger increases at higher temperatures (60% increase at 30°C). The temperature and CO₂ dependencies of canopy photosynthesis are interpreted in terms of simulated contributions by quantum-saturated and non-saturated foliage. Simulations are presented for periods ranging from a diurnal cycle to several years. Annual canopy photosynthesis simulated by BIOMASS for trees experiencing no water stress is linearly related to simulated annual absorbed photosynthetically active radiation, with light utilization coefficients for carbon of ?= 1.66 and 2.07g MJ^?1 derived for C_a of 350 and 700 μmol mol^?1, respectively.     

Exposure to an enriched CO2 atmosphere alters carbon assimilation and allocation in a pine forest ecosystem

We linked a leaf-level CO₂ assimilation model with a model that accounts for light attenuation in the canopy and measurements of sap-flux-based canopy conductance into a new canopy conductance-constrained carbon assimilation (4C-A) model. We estimated canopy CO₂ uptake (A_nC) at the Duke Forest free-air CO₂ enrichment (FACE) study. Rates of A_nC estimated from the 4C-A model agreed well with leaf gas exchange measurements (A_net) in both CO₂ treatments. Under ambient conditions, monthly sums of net CO₂ uptake by the canopy (A_nC) were 13% higher than estimates based on eddy-covariance and chamber measurements. Annual estimates of A_nC were only 3% higher than carbon (C) accumulations and losses estimated from ground-based measurements for the entire stand. The C budget for the Pinus taeda component was well constrained (within 1% of ground-based measurements). Although the closure of the C budget for the broadleaf species was poorer (within 20%), these species are a minor component of the forest. Under elevated CO₂, the C used annually for growth, turnover, and respiration balanced only 80% of the A_nC. Of the extra 700 g C m⁻² a⁻¹ (1999 and 2000 average), 86% is attributable to surface soil CO₂ efflux. This suggests that the production and turnover of fine roots was underestimated or that mycorrhizae and rhizodeposition became an increasingly important component of the C balance. Under elevated CO₂, net ecosystem production increased by 272 g C m⁻² a⁻¹: 44% greater than under ambient CO₂. The majority (87%) of this C was sequestered in a moderately long-term C pool in wood, with the remainder in the forest floor–soil subsystem.     

Ecophysiological controls over the net ecosystem exchange of mountain spruce stand. Comparison of the response in direct vs. diffuse solar radiation

Cloud cover increases the proportion of diffuse radiation reaching the Earth's surface and affects many microclimatic factors such as temperature, vapour pressure deficit and precipitation. We compared the relative efficiencies of canopy photosynthesis to diffuse and direct photosynthetic photon flux density (PPFD) for a Norway spruce forest (25‐year‐old, leaf area index 11 m² m⁻²) during two successive 7‐day periods in August. The comparison was based on the response of net ecosystem exchange (NEE) of CO₂ to PPFD. NEE and stomatal conductance at the canopy level (G_canopy) was estimated from half‐hourly eddy‐covariance measurements of CO₂ and H₂O fluxes. In addition, daily courses of CO₂ assimilation rate (A_N) and stomatal conductance (G_s) at shoot level were measured using a gas‐exchange technique applied to branches of trees. The extent of spectral changes in incident solar radiation was assessed using a spectroradiometer. We found significantly higher NEE (up to 150%) during the cloudy periods compared with the sunny periods at corresponding PPFDs. Prevailing diffuse radiation under the cloudy days resulted in a significantly lower compensation irradiance (by ca. 50% and 70%), while apparent quantum yield was slightly higher (by ca. 7%) at canopy level and significantly higher (by ca. 530%) in sun‐acclimated shoots. The main reasons for these differences appear to be (1) more favourable microclimatic conditions during cloudy periods, (2) stimulation of photochemical reactions and stomatal opening via an increase of blue/red light ratio, and (3) increased penetration of light into the canopy and thus a more equitable distribution of light between leaves. Our analyses identified the most important reason of enhanced NEE under cloudy sky conditions to be the effective penetration of diffuse radiation to lower depths of the canopy. This subsequently led to the significantly higher solar equivalent leaf area compared with the direct radiation. Most of the leaves in such dense canopy are in deep shade, with marginal or negative carbon balances during sunny days. These findings show that the energy of diffuse, compared with direct, solar radiation is used more efficiently in assimilation processes at both leaf and canopy levels.     

苹果密植园与间伐园树冠层内叶片光合潜力比较

通过对成龄苹果密植园和间伐园树冠不同层次和部位叶片光合潜力及辐射通量密度、叶片N含量和比叶重等指标的比较分析,研究了苹果园改造前后辐射能和氮素利用效率差异及其与产量品质的关系.结果表明：间伐显著改善了冠层内的辐射环境,间伐园冠层内的辐射分布明显比密植园均匀,相对辐射通量密度小于30%的无效光区接近0,而密植园冠层内的最低相对辐射通量密度为17%,在相对高度03以下均为无效光区;间伐园内冠层叶片的光合效率显著提高,间伐园树冠中、下部叶片的光合速率比密植园分别提高了78%和102%;叶片的最大羧化速率和最大电子传递速率也有较大幅度的提升.苹果园冠层叶片的光合效率与叶片N含量存在显著的相关关系,而叶片N含量又与辐射通量密度存在显著的相关关系,因此,可根据冠层叶片相对N含量的垂直分布间接和定量地判断叶片的光合效率或相对辐射通量密度的空间分布.     

Seasonal soil‐surface carbon fluxes from the root systems of young Pinus radiata trees growing at ambient and elevated CO2 concentration

Elevated atmospheric CO₂ concentration may result in increased below‐ground carbon allocation by trees, thereby altering soil carbon cycling. Seasonal estimates of soil surface carbon flux were made to determine whether carbon losses from Pinus radiata trees growing at elevated CO₂ concentration were higher than those at ambient CO₂ concentration, and whether this was related to increased fine root growth. Monthly soil surface carbon flux density (f) measurements were made on plots with trees growing at ambient (350) and elevated (650 μmol mol^?1) CO₂ concentration in large open‐top chambers. Prior to planting the soil carbon concentration (0.1%) and f (0.28 μmol m^?2 s^?1 at 15 °C) were low. A function describing the radial pattern of f with distance from tree stems was used to estimate the annual carbon flux from tree plots. Seasonal estimates of fine root production were made from minirhizotrons and the radial distribution of roots compared with radial measurements of f. A one‐dimensional gas diffusion model was used to estimate f from soil CO₂ concentrations at four depths. For the second year of growth, the annual carbon flux from the plots was 1671 g y^?1 and 1895 g y^?1 at ambient and elevated CO₂ concentrations, respectively, although this was not a significant difference. Higher f at elevated CO₂ concentration was largely explained by increased fine root biomass. Fine root biomass and stem production were both positively related to f. Both root length density and f declined exponentially with distance from the stem, and had similar length scales. Diurnal changes in f were largely explained by changes in soil temperature at a depth of 0.05 m. Ignoring the change of f with increasing distance from tree stems when scaling to a unit ground area basis from measurements with individual trees could result in under‐ or overestimates of soil‐surface carbon fluxes, especially in young stands when fine roots are unevenly distributed.     

Gross primary production is stimulated for three Populus species grown under free-air CO2 enrichment from planting through canopy closure

How forests will respond to rising [CO₂] in the long term is uncertain, most studies having involved juvenile trees in chambers prior to canopy closure. Poplar free‐air CO₂ enrichment (Viterbo, Italy) is one of the first experiments to grow a forest from planting through canopy closure to coppice, entirely under open‐air conditions using free‐air CO₂ enrichment technology. Three Populus species: P. alba, P. nigra and P. x euramericana, were grown in three blocks, each containing one control and one treatment plot in which CO₂ was elevated to the expected 2050 concentration of 550 ppm. The objective of this study was to estimate gross primary production (GPP) from recorded leaf photosynthetic properties, leaf area index (LAI) and meteorological conditions over the complete 3‐year rotation cycle. From the meteorological conditions recorded at 30 min intervals and biweekly measurements of LAI, the microclimate of leaves within the plots was estimated with a radiation transfer and energy balance model. This information was in turn used as input into a canopy microclimate model to determine light and temperature of different leaf classes at 30 min intervals which in turn was used with the steady‐state biochemical model of leaf photosynthesis to compute CO₂ uptake by the different leaf classes. The parameters of these models were derived from measurements made at regular intervals throughout the coppice cycle. The photosynthetic rates for different leaf classes were summed to obtain canopy photosynthesis, i.e. GPP. The model was run for each species in each plot, so that differences in GPP between species and treatments could be tested statistically. Significant stimulation of GPP driven by elevated [CO₂] occurred in all 3 years, and was greatest in the first year (223–251%), but markedly lower in the second (19–24%) and third years (5–19%). Increase in GPP in elevated relative to control plots was highest for P. nigra in 1999 and for P. x euramericana in 2000 and 2001, although in 1999 P. alba had a higher GPP than P. x euramericana. Our analysis attributed the decline in stimulation to canopy closure and not photosynthetic acclimation. Over the 3‐year rotation cycle from planting to harvest, the cumulative GPP was 4500, 4960 and 4010 g C m^?2 for P. alba, P. nigra and P. x euramericana, respectively, in current [CO₂] and 5260, 5800 and 5000 g C m^?2 in the elevated [CO₂] treatments. The relative changes were consistent with independent measurements of net primary production, determined independently from biomass increments and turnover.     

Will the Three Gorges Dam affect the underwater light climate of <Emphasis Type="Italic">Vallisneria spiralis</Emphasis> L. and food habitat of Siberian crane in Poyang Lake?

Almost 95% of the entire population of the Siberian crane (Grus leucogeranus) winter in Poyang Lake, China, where they forage on the tubers of the submerged aquatic macrophyte Vallisneria spiralis. The Three Gorges Dam on the Yangtze River may possibly affect this food source of the Siberian crane by affecting the light intensity reaching the top of the V. spiralis canopy. In this study, the photosynthetically active radiation at the top of the V. spiralis canopy (PAR_tc) in Lake Dahuchi was modeled from 1998 to 2006, and the potential impacts of changes in water level and turbidity on the underwater light climate of V. spiralis were analyzed. PAR_tc was calculated from incident irradiance while the losses due to reflection at the water surface, absorption, and scattering within the water column were taken into consideration. The results indicated significant differences in PAR_tc between years. Six years of water level and Secchi disk depth records revealed a seasonal switching of the lake from a turbid state at low water levels in autumn, winter, and spring to a clear state at high water levels during the monsoon in summer. The highest PAR_tc occurred at intermediate water levels, which were reached when the Yangtze River forces Lake Dahuchi out of its turbid state in early summer and the water becomes clear. The intended operation of the Three Gorges Dam, which will increase water levels in May and June, may advance the moment when Lake Dahuchi switches from turbid to clear. We suggest that this might increase production of V. spiralis and possibly improve the food habitat conditions for wintering Siberian crane in Poyang Lake.

Potential and limitations of inferring ecosystem photosynthetic capacity from leaf functional traits

The aim of this study was to systematically analyze the potential and limitations of using plant functional trait observations from global databases versus in situ data to improve our understanding of vegetation impacts on ecosystem functional properties (EFPs). Using ecosystem photosynthetic capacity as an example, we first provide an objective approach to derive robust EFP estimates from gross primary productivity (GPP) obtained from eddy covariance flux measurements. Second, we investigate the impact of synchronizing EFPs and plant functional traits in time and space to evaluate their relationships, and the extent to which we can benefit from global plant trait databases to explain the variability of ecosystem photosynthetic capacity. Finally, we identify a set of plant functional traits controlling ecosystem photosynthetic capacity at selected sites. Suitable estimates of the ecosystem photosynthetic capacity can be derived from light response curve of GPP responding to radiation (photosynthetically active radiation or absorbed photosynthetically active radiation). Although the effect of climate is minimized in these calculations, the estimates indicate substantial interannual variation of the photosynthetic capacity, even after removing site‐years with confounding factors like disturbance such as fire events. The relationships between foliar nitrogen concentration and ecosystem photosynthetic capacity are tighter when both of the measurements are synchronized in space and time. When using multiple plant traits simultaneously as predictors for ecosystem photosynthetic capacity variation, the combination of leaf carbon to nitrogen ratio with leaf phosphorus content explains the variance of ecosystem photosynthetic capacity best (adjusted R² = 0.55). Overall, this study provides an objective approach to identify links between leaf level traits and canopy level processes and highlights the relevance of the dynamic nature of ecosystems. Synchronizing measurements of eddy covariance fluxes and plant traits in time and space is shown to be highly relevant to better understand the importance of intra‐ and interspecific trait variation on ecosystem functioning.     

18O composition of CO2 and H2O ecosystem pools and fluxes in a tallgrass prairie: Simulations and comparisons to measurements

In this paper we describe measurements and modeling of ¹⁸O in CO₂ and H₂O pools and fluxes at a tallgrass prairie site in Oklahoma. We present measurements of the δ¹⁸O value of leaf water, depth‐resolved soil water, atmospheric water vapor, and Keeling plot δ¹⁸O intercepts for net soil‐surface CO₂ and ecosystem CO₂ and H₂O fluxes during three periods of the 2000 growing season. Daytime discrimination against C¹⁸OO, as calculated from measured above‐canopy CO₂ and δ¹⁸O gradients, is also presented. To interpret the isotope measurements, we applied an integrated land‐surface and isotope model (ISOLSM) that simulates ecosystem H₂¹⁸O and C¹⁸OO stocks and fluxes. ISOLSM accurately predicted the measured isotopic composition of ecosystem water pools and the δ¹⁸O value of net ecosystem CO₂ and H₂O fluxes. Simulations indicate that incomplete equilibration between CO₂ and H₂O within C₄ plant leaves can have a substantial impact on ecosystem discrimination. Diurnal variations in the δ¹⁸O value of above‐canopy vapor had a small impact on the predicted δ¹⁸O value of ecosystem water pools, although sustained differences had a large impact. Diurnal variations in the δ¹⁸O value of above‐canopy CO₂ substantially affected the predicted ecosystem discrimination. Leaves dominate the ecosystem ¹⁸O‐isoflux in CO₂ during the growing season, while the soil contribution is relatively small and less variable. However, interpreting daytime measurements of ecosystem C¹⁸OO fluxes requires accurate predictions of both soil and leaf ¹⁸O‐isofluxes.     

Temporal dynamics and spatial variability in the enhancement of canopy leaf area under elevated atmospheric CO2

Increased canopy leaf area (L) may lead to higher forest productivity and alter processes such as species dynamics and ecosystem mass and energy fluxes. Few CO₂ enrichment studies have been conducted in closed canopy forests and none have shown a sustained enhancement of L. We reconstructed 8 years (1996–2003) of L at Duke's Free Air CO₂ Enrichment experiment to determine the effects of elevated atmospheric CO₂ concentration ([CO₂]) on L before and after canopy closure in a pine forest with a hardwood component, focusing on interactions with temporal variation in water availability and spatial variation in nitrogen (N) supply. The dynamics of L were reconstructed using data on leaf litterfall mass and specific leaf area for hardwoods, and needle litterfall mass and specific leaf area combined with needle elongation rates, and fascicle and shoot counts for pines. The dynamics of pine L production and senescence were unaffected by elevated [CO₂], although L senescence for hardwoods was slowed. Elevated [CO₂] enhanced pine L and the total canopy L (combined pine and hardwood species; P<0.050); on average, enhancement following canopy closure was ～16% and 14% respectively. However, variation in pine L and its response to elevated [CO₂] was not random. Each year pine L under ambient and elevated [CO₂] was spatially correlated to the variability in site nitrogen availability (e.g. r²=0.94 and 0.87 in 2001, when L was highest before declining due to droughts and storms), with the [CO₂]‐induced enhancement increasing with N (P=0.061). Incorporating data on N beyond the range of native fertility, achieved through N fertilization, indicated that pine L had reached the site maximum under elevated [CO₂] where native N was highest. Thus closed canopy pine forests may be able to increase leaf area under elevated [CO₂] in moderate fertility sites, but are unable to respond to [CO₂] in both infertile sites (insufficient resources) and sites having high levels of fertility (maximum utilization of resources). The total canopy L, representing the combined L of pine and hardwood species, was constant across the N gradient under both ambient and elevated [CO₂], generating a constant enhancement of canopy L. Thus, in mixed species stands, L of canopy hardwoods which developed on lower fertility sites (～3 g N inputs m^?2 yr^?1) may be sufficiently enhanced under elevated [CO₂] to compensate for the lack of response in pine L, and generate an appreciable response of total canopy L (～14%).     

Temperature-independent diel variation in soil respiration observed from a temperate deciduous forest

The response of soil respiration (R_s) to temperature depends largely on the temporal and spatial scales of interest and how other environmental factors interact with this response. They are often represented by empirical exponential equations in many ecosystem analyses because of the difficulties in separating covarying environmental responses and in observing below ground processes. The objective of this study was to quantify a soil temperature‐independent component in R_s by examining the diel variation of an R_s time series measured in a temperate deciduous forest located at Oak Ridge, TN, USA between March and December 2003. By fitting 2 hourly, continuous automatic chamber measurements of CO₂ efflux at the soil surface to a Q₁₀ function to obtain the temperature‐dependent respiration (R_t) and plotting the diel cycles of R_t, R_s, and their difference (R_i), we found that an obvious temperature‐independent component exists in R_s during the growing season. The diel cycle of this component has a distinct day/night pattern and agrees well with diel variations in photosynthetically active radiation (PAR) and air temperature. Elevated canopy CO₂ concentration resulted in similar patterns in the diel cycle of the temperature‐independent component but with different daily average rates in different stages of growing season. We speculate that photosynthesis of the stand is one of the main contributors to this temperature‐independent respiration component although more experiments are needed to draw a firm conclusion. We also found that despite its relatively small magnitude compared with the temperature‐dependent component, the diel variation in the temperature‐independent component can lead to significantly different estimates of the temperature sensitivity of soil respiration in the study forest. As a result, the common practice of using fitted temperature‐dependent function from night‐time measurements to extrapolate soil respiration during the daytime may underestimate daytime soil respiration.     

Canopy radiation‐ and water‐use efficiencies as affected by elevated [CO2]

This study used an environmentally controlled plant growth facility, EcoCELLs, to measure canopy gas exchanges directly and to examine the effects of elevated [CO₂] on canopy radiation‐ and water‐use efficiencies. Sunflowers (Helianthus annus var. Mammoth) were grown at ambient (399 μmol mol^?1) and elevated [CO₂] (746 μmol mol^?1) for 53 days in EcoCELLs. Whole canopy carbon‐ and water‐fluxes were measured continuously during the period of the experiment. The results indicated that elevated [CO₂] enhanced daily total canopy carbon‐ and water‐fluxes by 53% and 11%, respectively, on a ground‐area basis, resulting in a 54% increase in radiation‐use efficiency (RUE) based on intercepted photosynthetic active radiation and a 26% increase in water‐use efficiency (WUE) by the end of the experiment. Canopy carbon‐ and water‐fluxes at both CO₂ treatments varied with canopy development. They were small at 22 days after planting (DAP) and gradually increased to the maxima at 46 DAP. When canopy carbon‐ and water‐fluxes were expressed on a leaf‐area basis, no effect of CO₂ was found for canopy water‐flux while elevated [CO₂] still enhanced canopy carbon‐flux by 29%, on average. Night‐time canopy carbon‐flux was 32% higher at elevated than at ambient [CO₂]. In addition, RUE and WUE displayed strong diurnal variations, high at noon and low in the morning or afternoon for WUE but opposite for RUE. This study provided direct evidence that plant canopy may consume more, instead of less, water but utilize both water and radiation more efficiently at elevated than at ambient [CO₂], at least during the exponential growth period as illustrated in this experiment.     

Wood CO2 efflux in a primary tropical rain forest

The balance between photosynthesis and plant respiration in tropical forests may substantially affect the global carbon cycle. Woody tissue CO₂ efflux is a major component of total plant respiration, but estimates of ecosystem‐scale rates are uncertain because of poor sampling in the upper canopy and across landscapes. To overcome these problems, we used a portable scaffolding tower to measure woody tissue CO₂ efflux from ground level to the canopy top across a range of sites of varying slope and soil phosphorus content in a primary tropical rain forest in Costa Rica. The objectives of this study were to: (1) determine whether to use surface area, volume, or biomass for modeling and extrapolating wood CO₂ efflux, (2) determine if wood CO₂ efflux varied seasonally, (3) identify if wood CO₂ efflux varied by functional group, height in canopy, soil fertility, or slope, and (4) extrapolate wood CO₂ efflux to the forest. CO₂ efflux from small diameter woody tissue (<10 cm) was related to surface area, while CO₂ efflux from stems >10 cm was related to both surface area and volume. Wood CO₂ efflux showed no evidence of seasonality over 2 years. CO₂ efflux per unit wood surface area at 25° (F_A) was highest for the N‐fixing dominant tree species Pentaclethra macroloba, followed by other tree species, lianas, then palms. Small diameter F_A increased steeply with increasing height, and large diameter F_A increased with diameter. Soil phosphorus and slope had slight, but complex effects on F_A. Wood CO₂ efflux per unit ground area was 1.34±0.36 μmol m^?2 s^?1, or 508±135 g C m^?2 yr^?1. Small diameter wood, only 15% of total woody biomass, accounted for 70% of total woody tissue CO₂ efflux from the forest; while lianas, only 3% of total woody biomass, contributed one‐fourth of the total wood CO₂ efflux.     

Scaling CO2-photosynthesis relationships from the leaf to the canopy

Responses of individual leaves to short-term changes in CO₂ partial pressure have been relatively well studied. Whole-plant and plant community responses to elevated CO₂ are less well understood and scaling up from leaves to canopies will be complicated if feedbacks at the small scale differ from feedbacks at the large scale. Mathematical models of leaf, canopy, and ecosystem processes are important tools in the study of effects on plants and ecosystems of global environmental change, and in particular increasing atmospheric CO₂, and might be used to scale from leaves to canopies. Models are also important in assessing effects of the biosphere on the atmosphere. Presently, multilayer and big leaf models of canopy photosynthesis and energy exchange exist. Big leaf models — which are advocated here as being applicable to the evaluation of impacts of global change on the biosphere — simplify much of the underlying leaf-level physics, physiology, and biochemistry, yet can retain the important features of plant-environment interactions with respect to leaf CO₂ exchange processes and are able to make useful, quantitative predictions of canopy and community responses to environmental change. The basis of some big leaf models of photosynthesis, including a new model described herein, is that photosynthetic capacity and activity are scaled vertically within a canopy (by plants themselves) to match approximately the vertical profile of PPFD. The new big leaf model combines physically based models of leaf and canopy level transport processes with a biochemically based model of CO₂ assimilation. Predictions made by the model are consistent with canopy CO₂ exchange measurements, although a need exists for further testing of this and other canopy physiology models with independent measurements of canopy mass and energy exchange at the time scale of 1 h or less.Abbreviations LAI leaf area index - NIR near infrared (700–3000 nm) radiation - PAR photosynthetically active (400–700 nm) radiation - PI photosynthetic irradiance (400–700 nm) - PPFD photosynthetic photon flux area density (400–700 nm) - PS I Photosystem I - PS II Photosystem II - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuP₂ ribulose-1,5-bisphosphate     

Photosynthesis, light and nitrogen relationships in a young deciduous forest canopy under open-air CO2 enrichment

Leaf photosynthesis (Ps), nitrogen (N) and light environment were measured on Populus tremuloides trees in a developing canopy under free‐air CO₂ enrichment in Wisconsin, USA. After 2 years of growth, the trees averaged 1·5 and 1·6 m tall under ambient and elevated CO₂, respectively, at the beginning of the study period in 1999. They grew to 2·6 and 2·9 m, respectively, by the end of the 1999 growing season. Daily integrated photon flux from cloud‐free days (PPFD_day,sat) around the lowermost branches was 16·8 ± 0·8 and 8·7 ± 0·2% of values at the top for the ambient and elevated CO₂ canopies, respectively. Elevated CO₂ significantly decreased leaf N on a mass, but not on an area, basis. N per unit leaf area was related linearly to PPFD_day,sat throughout the canopies, and elevated CO₂ did not affect that relationship. Leaf Ps light‐response curves responded differently to elevated CO₂, depending upon canopy position. Elevated CO₂ increased Ps_sat only in the upper (unshaded) canopy, whereas characteristics that would favour photosynthesis in shade were unaffected by elevated CO₂. Consequently, estimated daily integrated Ps on cloud‐free days (Ps_day,sat) was stimulated by elevated CO₂ only in the upper canopy. Ps_day,sat of the lowermost branches was actually lower with elevated CO₂ because of the darker light environment. The lack of CO₂ stimulation at the mid‐ and lower canopy was probably related to significant down‐regulation of photosynthetic capacity; there was no down‐regulation of Ps in the upper canopy. The relationship between Ps_day,sat and leaf N indicated that N was not optimally allocated within the canopy in a manner that would maximize whole‐canopy Ps or photosynthetic N use efficiency. Elevated CO₂ had no effect on the optimization of canopy N allocation.