A. Oltchev

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.

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.

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%).

Forests as protection against airborne immissions

The effect of a spruce forest in the Solling-hills (Germany) on the concentration of airborne trace compounds in the atmospheric boundary layer is discussed. The discussion is based on field measurements of vertical concentration profiles and vertical fluxes of reactive trace gases and particles in and above a spruce forest and on numerical modelling. Measured SO 2 -, O 3 - and PAN-concentrations are 10 % to 20 % lower near the forest floor than just above the canopy. NO 2 is emitted from the forest into the atmospheric boundary-layer and NO is transported both from the atmosphere and from the forest floor into the spruce canopy air space. The net NO x -flux between the atmosphere and this spruce forest can be neglected compared quantitatively to other N-fluxes. Numerical experiments using two models show that a 1000 m long spruce forest reduces the near surface concentration of an airborne trace substance with a deposition velocity of about 0.7 cm/s by up to 6 % as compared to the upwind SO 2 -concentration. If a forest is replaced by a meadow the SO 2 -concentration at the former downwind side of a forest increases by about 10 %.