For three forest canopies (a sparse, boreal needleleaf; a temperate broadleaf; and a dense, tropical, broadleaf stand) light-use efficiency (LUE) is found to be 6-33% higher when sky radiance is ...dominated by diffuse rather than direct sunlight. This enhancement is much less than that reported previously for both crops (110%; Choudbury, 2001 ) and moderately dense temperate woodland (50-180%). We use the land-surface scheme JULES to interpret the observed canopy response. Once sunflecks and leaf orientation are incorporated explicitly into the scheme, our simulations reproduce convincingly the overall level of canopy gross photosynthetic product (GPP), its enhancement with respect to diffuse sunlight and the mean 15% reduction in productivity observed during the afternoon due to stomatal closure. The LUE enhancement under diffuse sunlight can be explained by sharing of the canopy radiation-load, which is reduced under direct sky radiance. Once sunflecks are accounted for the advantage of implementing more sophisticated calculations of stomatal conductance (e.g. Ball-Berry and SPA submodels) is less obvious even for afternoon assimilation. Empirical relations are developed between observed carbon flux and the environmental variables total downwelling shortwave radiation (SW), canopy temperature (T) and the fraction of diffuse sky radiance (fDIF). These relations allow us to gauge the impact of increased/reduced insolation on GPP and net ecosystem exchange (NEE). Overall the three stands appear to be fairly stable within global trends and typical interannual variability (SW changing by <15%). Greatest sensitivity is exhibited by the boreal site, Zotino, where NEE falls by 9±4% for a 15% reduction in SW.
There is still no consensus on the impact of cloud on terrestrial carbon sequestration. Nevertheless, the fraction of sky irradiance which is diffuse (fDIF) is close to half as a global annual ...average, owing mainly to the presence of clouds. Furthermore, as a consequence of human-induced perturbations, the occurrence and opacity of cloud is changing both regionally (due to deforestation and drainage) and globally (shortwave “solar” dimming). In this study, we quantify the impact of cloud on carbon assimilation at an unprecedented number of FLUXNET sites (38) and for six plant functional types (PFTs). We compare results from previously established empirical and statistical methods with novel land-surface and three-dimensional (3D) radiative-transfer (RT) simulations that take explicit account of diffuse sunlight. We record a much lower enhancement in canopy light-use efficiency (LUE) under diffuse sunlight relative to direct sunlight (factor 1.12–1.80) compared to previous authors (factors 2–3). Increased radiation-sharing, due to varied leaf orientation within the canopy, is the primary cause of LUE-enhancement rather than beam penetration within an open crown structure. Under cloud, we consistently record a decrease in primary productivity (≥10–40%) and an unequivocal decline in daily carbon sequestration (60–80%), owing to the dramatic reduction in total (direct plus diffuse) irradiance that occurs when clouds obscure the solar disk (≥60% attenuation). A cooling-induced reduction in ecosystem respiration offsets the decline in primary productivity by about one third.
We use the radiation transfer simulation of Xilouris et al. (CITE) to constrain the quantity of dust in three nearby spiral galaxies (NGC 4013, NGC 5907 and NGC 4565). The predicted visual optical ...depth from the model is compared with the thermal continuum radiation detected from NGC 4013 and NGC 5907 at $850~{\mu}$m and from NGC 4565 at 1.2 mm. The former is based on SCUBA images of NGC 4013 and NGC 5907, reduced and presented for the first time in this work. The comparison of visual optical depth and $850~\mu$m (1.2 mm) emission yields the emissivity of dust grains in the submillimeter (millimeter) waveband. We infer a value of $1.2 \times 10^{4}$ for the emissivity at $850~\mu$m which is a factor 4 higher than the benchmark, semi-empirical model of Draine & Lee (CITE). At 1.2 mm our values are a factor 1.5 higher than this model. Our estimates are more closely aligned with recent measurements carried out in the laboratory on amorphous carbon and silicate particulates. A comparison between the distribution of $850~\mu$m (1.2 mm) surface brightness and the intensity levels in the 12CO(1–0) and 21 cm lines underlines the spatial association between dust detected in the submillimeter/millimeter waveband and molecular gas clouds. We suggest that the relatively high emissivity values that we derive may be attributable to amorphous, fluffy grains situated in denser gas environments.
For computational expediency, regional and global land‐surface models (LSMs), especially those coupled to climate simulations, adopt simple algorithms when calculating radiative transfer (RT) and ...canopy photosynthesis at the vegetated land surface. Nevertheless, the interaction of sunlight with vegetation is recognized as one of the most critical processes represented in LSMs. The present study calculates global, terrestrial Gross Primary Product (GPP) with a version of the land‐surface model JULES which has been modified to take explicit account of sunfleck penetration and leaf orientation within the canopy. A comparison with equivalent simulations adopting the Big Leaf (BL) or two‐stream (2ST) RT scheme, indicate that current regional/global LSMs may overestimate GPP by 10% globally and up to 25% regionally. Specifically, their use of average light profiles, and consequent neglect of the dispersion in leaf irradiance, at any given height in the canopy leads to both a general overestimation of canopy light‐use efficiency (LUE) and a failure to capture the LUE‐enhancement under diffuse sunlight (“diffuse fertilization effect”). We also examine the current limitations of regionally/globally implemented RT schemes with respect to canopy architecture. This is done by coupling JULES to the ray‐tracing numerical model FLIGHT, the latter simulating light transfer and photosynthesis in both uniform one‐dimensional (1‐D) canopies and 3‐D tree crowns. When the distribution of leaf nitrogen (N) is configured in a manner consistent with field measurements, output from the 3‐D and 1‐D FLIGHT simulations is fairly similar (predicted GPP differs by ≤5%). Similarly, both Leaf Angle Distribution (LAD), when restricted to its observed range, and leaf‐clumping appear to have a minor influence over canopy productivity. We conclude that current LSMs can radically improve their calculation of regional/global GPP by adopting a multilayer approach. This will allow the separate treatment of sunlit and shaded foliage, at discrete heights within the canopy, as well as the accurate representation of active leaf‐N.
We have expanded the Monte Carlo, ray‐tracing model FLIGHT in order to simulate photosynthesis within three‐dimensional, heterogeneous tree canopies. In contrast to the simple radiative transfer ...schemes adopted in many land‐surface models (e.g., the Big Leaf approximation), our simulation calculates explicitly the leaf irradiance at different heights within the canopy and thus produces an accurate scale‐up in photosynthesis from leaf to canopy level. We also account for both diffuse and direct sunlight. For a Siberian stand of Scots pine Pinus sylvestris, FLIGHT predicts observed carbon assimilation, across the full range of sky radiance, with an r.m.s. error of 12%. Our main findings for this sparse canopy, using both measurements and model, are as follows: (1) Observationally, we detect a light‐use efficiency (LUE) increase of only ≤10% for the canopy when the proportion of diffuse sky radiance is 75% rather than 25%. The corresponding enhancement predicted by our simulations is 10–20%. With such small increases in LUE, our site will not assimilate more carbon on overcast days compared to seasonally equivalent sunny days; (2) the scale‐up in photosynthesis from top‐leaf to canopy is less than unity. The Big Leaf approximation, based on Beer's law and light‐acclimated leaf nitrogen, overpredicts this scale‐up by ≥60% for low sky radiance (≤500 μmolPAR m−2 s−1); (3) when leaf nitrogen is distributed so as to maximize canopy photosynthesis, the increase in the canopy carbon assimilation, compared with a uniform nitrogen distribution, is small (≃4%). Maximum assimilation occurs when the vertical gradient of leaf nitrogen is slightly shallower than that of the light profile.
The assumed concentration and distribution of active leaf nitrogen (N) within vegetation canopies has a major influence on the gross primary product (GPP) predicted by land-surface models. We couple ...a Monte Carlo, three-dimensional (3D), ray-tracing simulation to the land-surface model JULES in order to compare the vertical profiles of light and active leaf-N in three forest stands of diverse composition and structure. Our simulations, which are driven by real climate data, strengthen the view that tree canopies are only partially light-acclimated. Notably, our computation demonstrates the importance of both diffuse sunlight and the manner in which the light profile is quantified. For example, when our temperate, broadleaf stand is subjected to diffuse sunlight, the mean leaf irradiance declines steeply in the upper third of crown (
k
ext
≳
1, where
k
ext is the exponential extinction coefficient) but is relatively shallow below that height (
k
ext
≤
0.75). Under direct sunlight, the foliage divides into sunlit and shaded fractions and a more appropriate measure of the light environment is probably the mode irradiance rather than the mean irradiance. Under direct sunlight, the optimal vertical distribution of Rubisco (i.e. that which maximises GPP for a fixed amount of active leaf-N in the canopy) is calculated to be shallow in both the upper and lower portions of the canopy (
k
rub
<
0.27, where
k
rub, is the exponential N-allocation parameter). Furthermore, an abrupt step, from high to low photosynthetic capacity, is predicted in the upper third of the crown. The theoretical gain in GPP for a stand that is fully light-acclimated compared to one that is only partially light-acclimated (i.e. with
k
rub
=
0.15, as measured in real tree canopies) is moderately important (8–13%). Our results are relatively insensitive to canopy architecture, i.e. crown structure, leaf-clumping and the leaf angle distribution.
We present Monte Carlo radiative-transfer simulations for spiral galaxies modelled as a stellar disc and a two-phase clumpy dust distribution. We divide the volume occupied by the dust into a ...three-dimensional grid and assign each cell a clump or smooth medium status. Cell dimension, clump dust mass and spatial distribution are derived from the observed properties of giant molecular clouds and molecular gas in the Galaxy. We produce models for several values of the optical depth and fraction of the interstellar medium residing in clumps. As a general result, clumpy models are less opaque than the corresponding homogeneous models. For the adopted parameters, the increase in the fraction of energy that escapes the disc is moderate, resulting in surface-brightness profiles that are less than one magnitude brighter than those of the homogeneous models. The effects of clumping are larger for edge-on views of the disc. This is in contrast with previous preliminary results for clumping in the literature. We show how differences arise from the different parametrization and clump distribution adopted. We also consider models in which a fraction of the stellar radiation is emitted within the clumps. In this case, galaxies are less transparent than in the case when only dust is clumped. The opacity can be even higher than in the homogeneous case, depending on the fraction of embedded stellar emission. We point out the implications of the results for the determination of the opacity and dust mass of spiral galaxies.
Abstract Imaging of in vivo gene expression using luciferase expression in various organs has been used for several years. In contrast to other organs, in vivo imaging of the lung, particularly after ...non-viral gene transfer has not been extensively studied. The aim of this study was to address several questions: (1) Does in vivo light emission correlate with standard tissue homogenate-based luciferase detection in a dose-dependent manner? Recombinant Sendai virus (SeV) transduces airway epithelial cells very efficiently and was used to address this question, (2) Is the sensitivity of the assay sufficient to detect non-viral gene transfer? We treated mice with SeV-Lux vector using our standard “sniffing” protocol, a method that predominantly results in lung deposition. Dose-related in vivo light emission was visible in all animals. Importantly, there was a significant correlation ( r > 0.90, p < 0.0001) between the in vivo and ex vivo assays in both the left and right lung. We next transfected the nasal epithelium via nasal perfusion or the lungs (“sniffing”) of mice with a luciferase plasmid (pCIKLux) complexed to the cationic lipid GL67 ( n = 25–27/group) and imaged luciferase expression in vivo 24 h after transfection. Gene expression was detectable in both organs. Correlation between the in vivo and ex vivo assays was significant ( r = 0.52, p < 0.005) in the left, but not the right lung. The correlation in the nose was weaker ( r = 0.45, p < 0.05). To our knowledge these studies show for the first time that this non-invasive method of assessing pulmonary gene transfer is viable for evaluating non-viral gene transfer agents.
SCUBA imaging of the NGC 7331 dust ring Bianchi, S.; Alton, P. B.; Davies, J. I. ...
Monthly notices of the Royal Astronomical Society,
08/1998, Letnik:
298, Številka:
3
Journal Article
Recenzirano
Odprti dostop
We present observations of the spiral galaxy NGC 7331 using the Submillimetre Common User Bolometer Array (SCUBA) on the James Clark Maxwell Telescope. We have detected a dust ring of 45 arcsec ...radius (3.3 kpc) at wavelengths of 450 and 850 μm. The dust ring is in good correspondence with other observations of the ring in the mid-infrared (MIR), CO and radio continuum, suggesting that the observed dust is associated with molecular gas and star formation. A B − K colour map shows an analogous ring structure with an asymmetry about the major axis, consistent with the extinction being produced by a dust ring. The derived temperature of the dust lies between 16 and 31 K and the gas-to-dust ratio lies between 150 and 570, depending on the assumed dust emission efficiency index (β = 1.5 or 2).
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