G. Gravenhorst

HETEROGENEOUS SO2-OXIDATION IN THE DROPLET PHASE

Abstract In order to determine the rate controlling step for sulfate formation in a heterogeneous droplet system. SO2-transfer within the gasphase towards and within droplets are calculated, equilibrium between SO2 in the gasphase and sulfur (IV) in cloud and fog droplets is reached within less than 1 second. Oxidation of sulfur (IV) to sulfate in the droplet phase proceeds slower by orders of magnitutde. Three mechanisms of SO2-oxidation are discussed: 1. (a) SO2-oxidation by O2 in the absence of catalysts; 2. (b) SO2-oxidation by O2 in the presence of catalysts and 3. (c) SO2-oxidation by strongly oxidizing agents. Mechanism 1. (a) contributes only to a negligible extent to sulfate formation in droplets even in the presence of typical concentrations of ammonia. Indications are strong that the major function of the SO2-NH3-H2O-system is not the oxidation of SO2 to sulfate. Oxidation mechanism (b) may contribute to a significant extent to sulfate formation in urban fogs in which case the concentrations of catalysts can be sufficiently high. For clouds in remote areas with much lower catalyst concentrations SO2-oxidation by mechanism 2. (b) seems to be of little importance. The oxidation by strongly oxidizing agents (mechanism 3. (c)) appears to be the dominant mechanism although some experimental discrepancies have to be resolved.

The role of canopy structure in the spectral variation of transmission and absorption of solar radiation in vegetation canopies

This paper presents empirical and theoretical analyses of spectral hemispherical reflectances and transmittances of individual leaves and the entire canopy sampled at two sites representative of equatorial rainforests and temperate coniferous forests. The empirical analysis indicates that some simple algebraic combinations of leaf and canopy spectral transmittances and reflectances eliminate their dependencies on wavelength through the specification of two canopy-specific wavelength-independent variables. These variables and leaf optical properties govern the energy conservation in vegetation canopies at any given wavelength of the solar spectrum. The presented theoretical development indicates these canopy-specific wavelength-independent variables characterize the capacity of the canopy to intercept and transmit solar radiation under two extreme situations, namely, when individual leaves 1) are completely absorptive and 2) totally reflect and/or transmit the incident radiation. The interactions of photons with the canopy at red and near-infrared (IR) spectral bands approximate these extreme situations well. One can treat the vegetation canopy as a dynamical system and the canopy spectral interception and transmission as dynamical variables. The system has two independent states: canopies with totally absorbing and totally scattering leaves. Intermediate states are a superposition of these pure states. Such an interpretation provides powerful means to accurately specify changes in canopy structure both from ground-based measurements and remotely sensed data. This concept underlies the operational algorithm of global leaf area index (LAI), and the fraction of photosynthetically active radiation absorbed by vegetation developed for the moderate resolution imaging spectroradiometer (MODIS) and multiangle imaging spectroradiometer (MISR) instruments of the Earth Observing System (EOS) Terra mission.

Influence of small-scale structure on radiative transfer and photosynthesis in vegetation canopies

The use of Beers law to describe the radiation regime in plant canopies is valid for a sufficiently large volume filled densely with phytoelements. This set a limit to the scale at which models, based on Beers law, can account for structural features of vegetation canopies and provide an adequate prediction of the radiation regime. The aim of our paper is to analyze radiation interaction in vegetation canopies and consequent photosynthetic rates at a scale at which Beers law loses its validity. We use fractals to simulate the structure of vegetation canopies at this scale. It is shown that both the radiation regime and the photosynthesis depend on the fractal dimension of the plant stand. The development of radiative transfer models in fractal-like media as well as measurements and modeling of fractal characteristics of trees and tree communities are essential for better understanding and scaling of radiative transfer and photosynthetic processes from an individual leaf to the canopy.

The ocean as source or sink of reactive trace-gases

SummaryAfter a brief review on recent experimental work on the uptake resp release of SO2 and NH3 by the ocean, new results on the distribution of these reactive trace-gases in the marine atmosphere are presented. In the case of SO2 there is no doubt that the ocean acts as a sink. In the case of NH3 new observations and calculations show that under certain conditions the ocean may be a NH3 source.

Small-sclae study of three-dimensional distribution of photosynthetically active radiation in a forest

Abstract We use the transport theory to simulate three-dimensional radiation distribution in a vegetation canopy of a small area (ca. 0.1–0.3 ha). This theory is based on two contradictory assumptions (Ross, 1981). On the one hand, the model resolution have to be so high that input variables for the transport equation can approximate the given forest stand with necessary degree of accuracy. On the other hand, the transport theory is based on the assumption that Beers law can be locally applied to plant canopies that is valid for sufficiently large volumes filled with phytoelements. This sets a limit to the resolution and to the predicting accuracy, not only of our model, but also of any other using Beers law. The aim of our paper is to estimate these limits as a function of input variables. A detailed analysis of input variables (canopy structure, optical properties of foliage elements and soil, radiation input at the canopy boundary) and of their effect on the radiative field underlie our investigations. A comparison of our three-dimensional simulation results with field measurements is also included in our paper, not only to test the model, but also to illustrate the specification of a model resolution and the accuracy of predicted radiative field in a real small heterogeneous experimental site. The forest albedo is an important ecological variable characterising the forest scattering capacity. To measure this variable, two hemispherical sensors are usually mounted above the forest canopy. The first one records a downward energy flux from the atmosphere, and the second, an upward irradiance reflected by the forest. The ratio of their responses is usually interpreted as the forest albedo. As an example, the model is used to quantify an inadequacy of this interpretation by simulating both the sensor response and the three-dimensional distribution of the radiation reflected, and by comparing these results with measurements in the field.

The response of the water fluxes of the boreal forest region at the Volga's source area to climatic and land-use changes

Abstract The project “Volgaforest” was focused on a study of the water budget of the forested Upper Volga catchment in Russia in order to describe: • the terrestrial water balance of the Upper Volga catchment as a function of external factors, such as climate and land-use, and • the response of forest ecosystems to these external factors. Future changes of water budget of the Upper Volga catchment area were estimated from: past and present dynamics of the atmospheric, water and forest conditions, different climatic scenarios and SVAT (Soil–Vegetation–Atmosphere Transfer) and hydrological models. Analysis of past climatological and hydrological data showed a large atmospheric and hydrological variability of the Upper Volga catchment. During the last 50–60 years the mean annual air temperature increased by 1.2 °C, and annual precipitation increased by 140 mm. However, no significant trend of annual runoff during the last 20 years could be found. Air temperature and precipitation changes were significant during winter and spring but very small in summer. Coniferous and mixed coniferous-broadleaf forests cover at present about 72% of the catchment area. During the last 30 years the area of natural coniferous forests (spruce, pine) decreased from 8.4% to 7% and the area of mixed forests increased from 52% to 59% of the total land area. Results of field measurements at a forest site showed a large variability of energy and water fluxes during the entire year. Transpiration of the boreal forest ecosystem measured using a sap flow method during the dry summer 1999 was limited by very dry soil water conditions, especially for spruce trees, and during the rainy summer 2000 probably by lack of oxygen in the rooting zone. Transpiration was about 10–20% larger for broadleaf trees (birch, aspen) than for spruce trees. Model estimations of possible changes in the hydrological regime of the Upper Volga catchment area for climatic scenarios suggest changes of evapotranspiration, surface runoff and soil moisture storage. Reduced snow accumulation, earlier melting, increased runoff reaction on precipitation in autumn and winter and drier soils in summer are the principal impacts on water resources of predicted future climatic changes. Surface runoff during the spring will be higher but summer and autumn runoff can be slightly suppressed by higher transpiration of deciduous tree species. Decreased summer precipitation and increased transpiration will result in decreasing ground water discharge, and lowering water levels of Volga river and of the Upper Volga lakes.

Determination of heat and water vapour fluxes above a spruce forest by eddy correlation

Heat and water vapour fluxes have been measured by eddy correlation at three levels above a spruce forest (tree height 31 m) in central Germany. A number of methodical problems are discussed, and necessary data corrections are presented. Stationarity of the fluxes is proven by cospectral analysis, confirming that averaging times of 10–30 min are adequate to describe the transport processes. Sensible and latent heat fluxes do not vary with height in the layer from (z − d) = 6.5z0 to (z − d) = 11.7z0. Based on footprint estimates it is concluded that, in general, the measured fluxes may be taken as representative of the spruce canopy. Even in the worst-case situations, when the wind comes from a beech-spruce transition 200 m away, no height dependence of the fluxes is detected. On the selected days, which were mainly bright, a Bowen ratio near one is found during the hours of highest available energy, whereas at night no evapotranspiration is observed. The turbulent diffusion coefficients of heat and water vapour follow a distinct diurnal pattern, with a maximum of about 10 m2 s−1 shortly after noon. The heat diffusion coefficient tends to exceed that of water vapour by a factor of about 1.2, showing daytime-dependent variations. Owing to some unexplained deficits in the energy balance this value is not very certain, but the results are consistent with some other published data. At least they demonstrate that uncritical application of the similarity hypothesis (Dh = Dv) above high vegetation may lead to erroneous flux calculations.

Application of a Six-Layer SVAT Model for Simulation of Evapotranspiration and Water Uptake in a Spruce Forest

Abstract The One-Dimensional non-steady-state Six-Layer SVAT model (SLODSVAT) was applied to a quasihomogeneous stand of spruce trees (Picea abies [L.] Karst) in the Solling hills (Germany) in order to describe the water transport from the soil into the atmosphere through the roots-stem-shoots-needles system of the trees and to predict the possible response to changes of soil water conditions on transpiration rate of the forest. The modelled water uptake and evapotranspiration rates were compared with long-term sap flow, eddy correlation and gradient flux measurements for a one-week test period (01-08.07.1995) which provided a variety of weather conditions including clear as well as partly cloudy and rainy days. Moreover, for this period the sensitivity of response of the transpiration rate and water uptake to changes of environmental conditions is estimated. The results show, that the SLODSVAT can describe and simulate the short-term variability of water uptake by the roots and evapotranspiration in the spruce forest adequately under different environmental conditions. For the selected period the SLODSVAT explained about 94% of the variation of water uptake (r2=0.940), and 88% and 78% of variation of evapotranspiration measured by Bowen ratio - energy balance (r2=0.881) and eddy correlation (r2=0.785) methods, respectively. Thus, these results give evidence that it is possible to estimate and predict evapotranspiration and transpiration rates for spruce forest ecosystems in the stand-scale during one vegetation period if appropriate input parameters for the soil and canopy structure and the atmospheric conditions are available.

Modelling three-dimensional distribution of photosynthetically active radiation in sloping coniferous stands

Abstract Solar irradiance is a major environmental factor governing biological and physiological processes in a vegetation canopy. Solar radiation distribution in a canopy and its effect are three-dimensional in nature. However, most of the radiation models up to now have been one-dimensional. They can be successfully applied to large-scale studies of forest functioning. The one-dimensional modelling technique, however, does not provide adequate interpretation of small scale processes leading to forest growth. In this article we discuss a modelling strategy for the simulation of three-dimensional radiation distribution in a vegetation canopy of a small area (about 0.25–0.3 ha). We demonstrate its realisation to predict the three-dimensional radiative regime of phytosynthetically active radiation in a real coniferous stand located on hilly surroundings. Our model can be used to investigate the influence of different climatic conditions, forest management methods and field sites on the solar energy available for forest growth in small heterogeneous areas. Further, a three-dimensional process-oriented model helps to derive global variables affecting bio-physiological processes in a vegetation canopy shifting from small scale studies of the functioning of forests to regional, continental, and global scale problems.

Input of Atmospheric Particles into Forest Stands by Dry Deposition

In this study the dry input of atmospheric particles into a forest stand is quantified. A wash-off-method using the natural leaf surfaces as collectors of the dry deposition was chosen. The direct on-site-measurement on living branches were achieved in a spruce stand (Picea abies (L.) Karst) at Solling, Germany. The ion exchange processes occurring on natural branches can reliably be quantified through immediate sequential washings. In order to calculate also the gas dry deposition of those trace elements which occur in both particle and gas phases, a resistance model was used. From these results the deposition velocity of particulate aerosol components into the forest stand was calculated. Dry particle deposition constitutes an important part of the total matter input into the forest ecosystem. Just the nitrogen input into Solling only by dry deposition (from particle-, mist-, and gas-deposition) with about 30 kg N ha−1 a−1 already exceeds the critical load of 20 kg N ha−1 a−1 by far, and this is without even considering the additional load by wet deposition which amounts to 15 kg ha−1 a−1. These findings are of greatest ecological importance, as the damage to the stability of the forest ecosystem caused by increased nitrogen input is considerable. Only a quick and drastic reduction of sulphur and nitrogen emissions could stop the further increase of the nutritient imbalance and the progressing acidification of this ecosystem.