Niels Otto Jensen

Respiration as the main determinant of carbon balance in European forests

Carbon exchange between the terrestrial biosphere and the atmosphere is one of the key processes that need to be assessed in the context of the Kyoto Protocol. Several studies suggest that the terrestrial biosphere is gaining carbon, but these estimates are obtained primarily by indirect methods, and the factors that control terrestrial carbon exchange, its magnitude and primary locations, are under debate. Here we present data of net ecosystem carbon exchange, collected between 1996 and 1998 from 15 European forests, which confirm that many European forest ecosystems act as carbon sinks. The annual carbon balances range from an uptake of 6.6 tonnes of carbon per hectare per year to a release of nearly 1 t C ha -1 yr-1, with a large variability between forests. The data show a significant increase of carbon uptake with decreasing latitude, whereas the gross primary production seems to be largely independent of latitude. Our observations indicate that, in general, ecosystem respiration determines net ecosystem carbon exchange. Also, for an accurate assessment of the carbon balance in a particular forest ecosystem, remote sensing of the normalized difference vegetation index or estimates based on forest inventories may not be sufficient.

Airborne multispectral data for quantifying leaf area index, nitrogen concentration, and photosynthetic efficiency in agriculture

Abstract Airborne multispectral data were acquired by the Compact Airborne Spectral Imager (CASI) for an agricultural area in Denmark with the purpose of quantifying vegetation amount and variations in the physiological status of the vegetation. Spectral reflectances, vegetation indices, and red edge positions were calculated on the basis of the CASI data and compared to field measurements of green leaf area index (LAI; L) and canopy nitrogen concentrations (Nc) sampled at 16 sites. Because of the variety of the samples with respect to vegetation type, leaf age, and phenological developmental stage, the data of L and Nc were uncorrelated. The scattering effect of leaves effectuated a robust linear relationship between L and near-infrared (NIR) reflectance (r=.93), whereas the Nc (vegetative period) was significantly correlated with the spectral reflectance in the green (r=−.88) and far-red wavebands (r=−.94). The correlations between vegetation indices and L were also important, in particular, for the enhanced vegetation index (EVI; r=.88), whereas the red edge position correlated less significantly with Nc (r=.78). Assuming L and Nc to be responsible for most of the spatial variability in the CO2 assimilation rates, remote sensing-based maps of these variables were produced for use in a coupled sun/shade photosynthesis/transpiration model. The predicted rates of net photosynthesis and transpiration compared reasonably with eddy covariance measurements of CO2 and water vapour fluxes recorded at four different crop fields. The results allowed evaluation of the spatial variations in the photosynthetic light, nitrogen, and water use efficiencies. While photosynthesis was linearly related to the transpiration, the light use efficiency (LUE) was found to be dependent on nitrogen concentrations.

Nitric oxide emission from a Norway spruce forest floor

Although the available data on soil NO emission are increasing, only few measurements have been made in temperate forests. Emissions from these forests are generally low, but recent studies have shown that NO emission from N-affected forests can be much higher. In this study the emission of NO from the floor of a moderately N-affected Norway spruce forest in Denmark was measured using dynamic chambers during two seasons (April 1995 and September 1995). At the same time, fluxes of CO2 and NO2, soil water content inside and outside the chambers, soil temperature at 10 cm depth, soil surface temperature and air temperature were recorded. The emission of NO in April was low (< 0.3 ng N m−2 s−1), whereas it was significantly higher in September (21 ng N m−2 s−1 with a range 0–82 ng N m−2 s−1). In April the soil surface temperature was low (< 6°C) and the soil moisture was high (26% vol/vol). In September the soil surface temperature was high (16°C) and soil moisture was significantly lower (14% vol/vol) than in April. There was a high correlation between CO2 emission and NO emission. The fluxes of NO2 were very small and not related to NO emission. Generally, the variation within a single plot was small, whereas the variation between plots was large. About half of the total variation of NO emission in September could be explained by a combination of soil moisture, distance to tree trunks, and moss cover. No significant relationship could be found between NO emission and soil surface temperature or soil temperature. Other parameters such as the C/N dynamics in the soil must be taken into account to provide a better mechanistic understanding of the NO emission. An annual emission of 3.15 kg NO-N ha−1 yr−1 was estimated by the amount of NH4+ in throughfall. Compared to most of the previous published measurements of NO emission from temperate forest soils, the values presented here are high and of the same magnitude as found in N-affected forests in Central Europe.

Ozone uptake by an evergreen forest canopy : temporal variation and possible mechanisms

Patterns of ozone concentration ([O(3)]), O(3) deposition velocity (v(d)) and O(3) flux (F(c)) over an evergreen forest canopy are shown in relation to measuring method, physiological activity of the trees, and time of year. The gradient and eddy correlation methods were compared and showed similar diel v(d) patterns. Daytime F(c) was correlated with CO(2) and water vapour fluxes, while no correlation between [O(3)] in the range 10-70 ppb (nl l(-1)) and F(c) was seen in this study. F(c) was primarily driven by stomatal conductance, reactions with surfaces, particles and gases, and not by [O(3)]. On a monthly basis, [O(3)] was always highest in the afternoon while v(d) was typically higher in the morning, resulting in an equal F(c) over the day. Night-time F(c) was more than half of the daytime O(3) flux. The data reveal the importance of emissions of nitric oxide and terpenes as O(3) removal factors in evergreen forest dominated by Norway spruce.

Determination of the surface drag coefficient

This study examines the dependence of the surface drag coefficienton stability, wind speed, mesoscale modulation of the turbulent flux and method of calculation of the drag coefficient. Data sets over grassland, sparse grass, heather and two forest sites are analyzed. For significantly unstable conditions, the drag coefficient does not depend systematically on z/L but decreases with wind speed for fixed intervals of z/L, where L is the Obukhov length. Even though the drag coefficient for weak wind conditions is sensitive to the exact method of calculation and choice of averaging time, the decrease of the drag coefficient with wind speed occurs for all of the calculation methods. A classification of flux calculation methods is constructed, which unifies the most common previous approaches.The roughness length corresponding to the usual Monin–Obukhovstability functions decreases with increasing wind speed. This dependence on wind speed cannot be eliminated by adjusting the stability functions. If physical, the decrease of the roughness length with increasing wind speed might be due to the decreasing role of viscous effectsand streamlining of the vegetation, although these effects cannot be isolated from existing atmospheric data.For weak winds, both the mean flow and the stress vector often meander significantly in response to mesoscale motions. The relationship between meandering of the stress and wind vectors is examined. For weak winds, the drag coefficient can be sensitive to the method of calculation, partly due to meandering of the stress vector.

A functional HAZOP methodology

Abstract A HAZOP methodology is presented where a functional plant model assists in a goal oriented decomposition of the plant purpose into the means of achieving the purpose. This approach leads to nodes with simple functions from which the selection of process and deviation variables follow directly. The functional HAZOP methodology lends itself directly for implementation into a computer aided reasoning tool to perform root cause and consequence analysis. Such a tool can facilitate finding causes and/or consequences far away from the site of the deviation. A functional HAZOP assistant is proposed and investigated in a HAZOP study of an industrial scale Indirect Vapor Recompression Distillation pilot Plant (IVaRDiP) at DTU-Chemical and Biochemical Engineering. The study shows that the functional HAZOP methodology provides a very efficient paradigm for facilitating HAZOP studies and for enabling reasoning to reveal potential hazards in safety critical operations.

Lateral dispersion of pollutants in a very stable atmosphere—the effect of meandering

Abstract A model based on single particle diffusion is introduced to account for the effect of “meandering” on lateral plume dispersion in a very stable atmosphere. It is assumed that small scale atmospheric turbulence is absent, so that only large horizontal eddies are effective. A formula for the lateral standard deviation σ y as function of observation time, distance from source, mean wind speed, lateral turbulence intensity, and scale of the atmospheric motion is derived. Climatological time series of temperature lapse rates, wind speeds, and wind directions can be used as input to calculate σ y . Meteorological data from Riso and the small island Sprogo have been analysed in order to identify all situations in which the atmosphere is so stable that small scale turbulence cannot exist. The purpose is to see in how many of these situations meandering is also absent. The results show that, as a rule, meandering will be present in a strongly stable atmosphere with low wind speeds. If the dispersion by meandering is not taken into account, estimates of mean concentrations can easily be a factor of 4–6 too high.

Comparisons of Measured and Modelled Ozone Deposition to Forests in Northern Europe

The performance of a new dry deposition module, developedfor the European-scale mapping and modelling of ozone flux to vegetation, was tested against micrometeorological ozone and water vapour flux measurements. The measurement data are for twoconiferous (Scots pine in Finland, Norway spruce in Denmark) and one deciduous forest (mountain birch in Finland). On average, themodel performs well for the Scots pine forest, if local inputdata are used. The daytime deposition rates are somewhat over-predicted at the Danish site, especially in the afternoon. The mountain birch data indicate that the generic parameterisationof stomatal responses is not very representative of this northernspecies. The module was also tested by using modelled meteorological data that constitute the input for a photochemical transport model.

Effective Roughness Calculated from Satellite-Derived Land Cover Maps and Hedge-Information used in a Weather Forecasting Model

In numerical weather prediction, climate and hydrologicalmodelling, the grid cell size is typically larger than the horizontal length scales of variations in aerodynamicroughness, surface temperature and surface humidity. These local land cover variations give rise to sub-gridscale surface flux differences. Especially the roughness variations can give a significantly differentvalue between the equilibrium roughness in each of the patches as compared to the aggregated roughness value,the so-called effective roughness, for the grid cell. The effective roughness is a quantity that secures thephysics to be well-described in any large-scale model. A method of aggregating the roughness step changesin arbitrary real terrain has been applied in flat terrain (Denmark) where sub-grid scale vegetation-drivenroughness variations are a dominant characteristic of the landscape. The aggregation model is a physicaltwo-dimensional atmospheric flow model in the horizontal domain based on a linearized version of theNavier Stoke equation. The equations are solved by the Fast Fourier Transformation technique, hence the codeis very fast. The new effective roughness maps have been used in the HIgh Resolution Limited Area Model(HIRLAM) weather forecasting model and the weather prediction results are compared for a number of casesto synoptic and other observations with improved agreement above the predictions based on currentstandard input. Typical seasonal springtime bias on forecasted winds over land of +0.5 m s-1 and-0.2 m s-1 in coastal areas is reduced by use of the effective roughness maps.

Stomatal acclimation influences water and carbon fluxes of a beech canopy in northern Germany

Summary The response of beech leaves to leaf-to-air vapour pressure deficit (VPD) is not constant throughout the growing season. In situ measurements of leaf gas exchange showed that this intra-annual variation of stomatal sensitivity to VPD cannot be described adequately by a simple annual course of phenology alone. Reduction in stomatal aperture in dry air was negatively correlated with the sum of photosynthetic photon flux density (PPFD) of the previous three weeks, but was positively correlated with the mean VPD of the previous month. These acclimation effects contribute significantly to explaining variances in stomatal sensitivity across the growing season. The observed mid-term reaction to PPFD is in agreement with recent findings regarding the blue light perception of stomatal guard cells. A mathematical formulation representing these mid-term variations in stomatal sensitivity was incorporated into an ecosystem flux model describing the carbon and water exchange between forests and the atmosphere. Model results were in good agreement with independent eddy covariance measurements above the beech canopy. The model, therefore, is useful for investigating the importance of stomatal acclimation for ecosystem carbon and water balances. At an annual scale, acclimation appears to influence the fluxes moderately (less than 10%). The model suggests, however, that stomatal acclimation optimises the seasonal course of carbon gain and water loss of beech forests significantly with respect to the varying availabilities of water and energy. Die Reaktion von Buchenblattern auf das Wasserdampfdefizit zwischen Blatt und Luft ist nicht das ganze Jahr uber konstant. Die in-situ gemessene intraannuelle Variabilitat der stomataren Sensibilitat gegenuber der Luftfeuchte kann daher mit einer einfachen phanologischen Kurve nicht adaquat beschrieben werden. Daten, die uber drei klimatisch sehr unterschiedliche Jahre an Buchenblattern im Hauptforschungsraum des Bornhoved-Projektes aufgenommen wurden, zeigten vielmehr, das das Ausmas, in dem die stomatare Leitfahigkeit als Reaktion auf trockene Luft verringert wird, einerseits negativ korreliert ist mit der mittleren Tagessumme der photosynthetisch aktiven Quantenstromdichte der letzten drei Wochen, andererseits positiv korreliert ist mit dem mittleren Wassersattigungsdefizit der Luft des jeweils letzten Monats. Eine Berucksichtigung dieser als stomatare Akklimatisation interpretierten Befunde erklart die Varianz der stomataren Sensibilitat signifikant besser. Die beobachtete mittelfristige Reaktion der Stomata auf die Witterung stimmt mit kurzlich publizierten biochemischen Beobachtungen uberein, nach denen die Blaulichtrezeptoren in Schlieszellen von Spaltoffnungen ebenfalls auf das Lichtklima reagieren. Eine mathematische Formulierung fur die mittelfristige Anpassung der stomataren Sensibilitat wurde in ein Okosystemflus-Modellsystem, das die Wasser- und CO2-Flusse zwischen Waldern und der Atmosphare berechnet, integriert. Die Ergebnise von Simulationslaufen mit dem erweiterten Modell zeigten gute Ubereinstimmung mit Eddy Kovarianz Messungen, die unabhangig uber dem Buchenwald durchgefuhrt wurden. Weitere Simulationlaufe zeigten, das die Akklimatisation die jahrlichen Bilanzen zwar nur moderat (weniger als 10%) beeinflust, aber tagliche Abweichungen von bis zu 60% auftraten. Es konnte gezeigt werden, das die stomatare Akklimatisation zur Optimierung des Kohlenstoffgewinns der Baume bei wechselnder Verfugbarkeit von Wasser und Energie beitragt.