A. Ibrom

Energy balance closure at FLUXNET sites

A comprehensive evaluation of energy balance closure is performed across 22 sites and 50 site-years in FLUXNET, a network of eddy covariance sites measuring long-term carbon and energy fluxes in contrasting ecosystems and climates. Energy balance closure was evaluated by statistical regression of turbulent energy fluxes (sensible and latent heat (LE)) against available energy (net radiation, less the energy stored) and by solving for the energy balance ratio, the ratio of turbulent energy fluxes to available energy. These methods indicate a general lack of closure at most sites, with a mean imbalance in the order of 20%. The imbalance was prevalent in all measured vegetation types and in climates ranging from Mediterranean to temperate and arctic. There were no clear differences between sites using open and closed path infrared gas analyzers. At a majority of sites closure improved with turbulent intensity (friction velocity), but lack of total closure was still prevalent under most conditions. The imbalance was greatest during nocturnal periods. The results suggest that estimates of the scalar turbulent fluxes of sensible and LE are underestimated and/or that available energy is overestimated. The implications on interpreting long-term CO2 fluxes at FLUXNET sites depends on whether the imbalance results primarily from general errors associated

Flux partitioning between understorey and overstorey in a boreal spruce/pine forest determined by the eddy covariance method

The eddy covariance method was used to determine turbulent heat fluxes and carbon dioxide fluxes inside a boreal spruce ‐ pine forest (2.5 m above the forest floor) during the growing season in 1994 and 1995. Different data quality tests and spectral analysis were applied, confirming that most of the data collected inside the forest canopy, can be used to determine fluxes. Results of hourly averaged water- and carbon fluxes are compared to flux data measured continuously above the canopy. Large nonstationarities in sensible heat flux can be explained by nonlocal transport phenomena. Latent heat and carbon dioxide fluxes were more stationary because the sink/source strengths of water and carbon dioxide at the soil surface are more homogeneous compared to sources/sinks of sensible heat. Turbulent transport in the trunk space is caused by large intermittent eddies of 5‐100 m size, deduced from spectral analysis. Evaporation from soil and soil vegetation accounts for 10% of the total stand evaporation with rates between 0.1 and 0.6 mm per day. In the daytime, the carbon loss from the soil is partly compensated by carbon uptake from the soil vegetation, resulting in flux rates of 0.45‐0.9 mmol m ˇ2 s ˇ1 . During the night, carbon fluxes of 0.1‐3.6 mmol m ˇ2 s ˇ1 (mean 2 mmol m ˇ2 s ˇ1 ) were observed under the canopy. Above the canopy, daily carbon uptake varied between 15 and 22 mmol m ˇ2 s ˇ1 near noon (daytime mean 9.5 mmol m ˇ2 s ˇ1 ). # 1999 Elsevier

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.

Carbon balance gradient in European forests: should we doubt ‘surprising’ results? A reply to Piovesan & Adams

This paper responds to the Forum contribution by Piovesan & Adams (2000) who criticized the results obtained by the EUROFLUX network on carbon fluxes of several European forests. The major point of criticism was that the data provided by EUROFLUX are inconsistent with current scientific understanding. It is argued that understanding the terrestrial global carbon cycle requires more than simply restating what was known previously, and that Piovesan & Adams have not been able to show any major conflicts between our findings and ecosystem or atmospheric-transport theories.

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.

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.

Eddy-correlation measurements of fluxes of CO2 and H2O above a spruce stand

Abstract Atmospheric fluxes of CO2 and H2O above a mature spruce stand (Picea abies (L.) Karst.) have been investigated using the eddy- correlation technique. A closed path sensor adapted to the special requirements of long-term studies has been developed and tested. Field measurements have been performed since April 1995. Estimates of fetch showed a very narrow source area dimension under instable stratification (≤ 200 m). Fetch requirements at night are not met in some directions. Energy balance closure was influenced systematically by the wind direction indicating a substantial attenuation of the vertical wind motion by the tower (up to 40 %). Even for optimal flow directions, energy balance closure was about 88%. Intercomparison of the used ultra sonic anemometer (USAT-3) with a GILL - anemometer showed systematically lower values of vertical wind speed fluctuations (13 %). Average CO2-fluxes ranged between -13 at noon to 3 μmol m−2, s−1 at night in summer. In November and December the stand released CO2 on a daily basis. A preliminary estimate of the cumulative net carbon balance over the observed period of 9 months is 4–5 t, C ha−1.

Energy and water fluxes above a cacao agroforestry system in Central sulawesi, indonesia, indicate effects of land-use change on local climate

Rapid conversion of tropical rainforests to agricultural land-use types occurs throughout Indonesia and South-East Asia. We hypothesize that these changes in land-use affect the turbulent heat exchange processes between vegetation and the atmosphere, and the radiative properties of the surface, and therefore, induce an impact on local climate and water flows. As part of the international research project (SFB 552, Stability of Rainforest Margins in Indonesia, STORMA) the turbulent heat fluxes over a cacao agroforestry system (AFS) were investigated, using the eddy covariance technique. These first heat flux observations above a cacao AFS showed an unexpectedly large contribution of the sensible heat flux to the total turbulent heat transport, resulting in an averaged day-time Bowen ratio of /3 = H/λE 1. Seasonality of β did mainly coincide with the seasonal course of precipitation, which amounted to 1970 mm yr -1 during the investigated period. The findings are compared to invastigations at four neotropical rain forests where daytime β were substantially smaller than 1. All discussed sites received similar incident short wave radiation, however, precipitation at the neotropical sites was much higher. Our first observations in a nearby Indonesian upland rain forest where precipitation was comparable to that at the cacao AFS showed an intermediate behaviour. Differences in β between the cacao AFS and the tropical forests are discussed as a consequence of differing precipitation amounts, and albedo. From these comparisons we conclude that conversion from tropical forests to cacao AFS affects the energy fluxes towards increased heating of the day-time convective boundary-layer.

Effects of land-use change on matter and energy exchange between ecosystems in the rain forest margin and the atmosphere

Greenhouse gas and energy fluxes between the land surface and the atmosphere are important aspects for the evaluation of land-use options in tropical areas. Changes in vegetation cover alter the capacity to absorb carbon dioxide and solar radiation from the atmosphere and influence the magnitudes of latent and sensible heat flows to the atmosphere. If happening at a larger spatial scale, land-use change can lead to significant local feedbacks like drought, flooding, soil erosion or shifts in local climate.

Stomatal and surface conductance of a spruce forest: Model simulation and field measurements

Abstract Canopy surface conductances of a spruce forest in the Solling hills (Central Germany) were derived from LE and H fluxes measured (by eddy correlation technique and the Bowen ratio method) and modelled (by an one-dimensional non-steady-state SVAT model (SLODSVAT)) using a rearranged Penman-Monteith equation (“Big-leaf” approximation) during June 1996. They were compared with canopy stomatal conductances estimated by consecutive integrating the stomatal conductance of individual leaves over the whole canopy (“bottom-up” approach) using SLODSVAT model. The results indicate a significant difference between the canopy surface conductances derived from measured and modelled fluxes (“top-down” approach) and the canopy stomatal conductances modelled by the SLODSVAT (“bottom-up” approach). This difference was influenced by some non-physiological factors within the forest canopy (e.g. aerodynamic and boundary layer resistances, radiation budget, evaporation from the forest understorey). In general, canopy surface conductances derived from measured and modelled fluxes exceeded canopy stomatal conductance during the whole modelled period. The contribution of the understoreys evapotranspiration to the total forest evapotranspiration was small (up to 5–9% of the total LE flux) and was not depended on total radiation balance of forest canopy. Ignoring contribution of the understoreys evapotranspiration resulted in an overestimation of the canopy surface conductance for a spruce forest up to 2 mm/s (up to 10–15%).