Showing papers by "Eva Falge published in 2000"

On Measuring Net Ecosystem Carbon Exchange Over Tall Vegetation on Complex Terrain

Abstract: To assess annual budgets of CO2 exchange between the biosphere and atmosphere over representative ecosystems, long-term measurements must be made over ecosystems that do not exist on ideal terrain. How to interpret eddy covariance measurements correctly remains a major task. At present, net ecosystem CO2 exchange is assessed, by members of the micrometeorological com- munity, as the sum of eddy covariance measurements and the storage of CO2 in the underlying air. This approach, however, seems unsatisfactory as numerous investigators are reporting that it may be causing nocturnal respiration flux densities to be underestimated. A new theory was recently published by Lee (1998, Agricultural and Forest Meteorology91: 39- 50) for assessing net ecosystem-atmosphere CO2 exchange (Ne) over non-ideal terrain. It includes a vertical advection term. We apply this equation over a temperate broadleaved forest growing in undulating terrain. Inclusion of the vertical advection term yields hourly, daily and annual sums of net ecosystem CO2 exchange that are more ecologically correct during the growing season. During the winter dormant period, on the other hand, corrected CO2 flux density measurements of an actively respiring forest were near zero. This observation is unrealistic compared to chamber measurements and model calculations. Only during midday, when the atmosphere is well-mixed, do measurements ofNe match estimates based on model calculations and chamber measurements. On an annual basis, sums of Ne without the advection correction were 40% too large, as compared with computations derived from a validated and process-based model. With the inclusion of the advection correction term, we observe convergence between measured and calculated values ofNe on hourly, daily and yearly time scales. We cannot, however, conclude that inclusion of a one-dimensional, vertical ad- vection term into the continuity equation is sufficient for evaluating CO 2 exchange over tall forests in complex terrain. There is an indication that the neglected term, u.@c=@x/, is non-zero and that CO2 may be leaking from the sides of the control volume during the winter. In this circumstance, forest floor CO 2 efflux densities exceed effluxes measured above the canopy.

Modelling age- and density-related gas exchange of Picea abies canopies in the Fichtelgebirge, Germany.

Abstract: Differences in canopy exchange of water and carbon dioxide that occur due to changes in tree structure and density in montane Norway spruce stands of Central Germany were analyzed with a three dimensional microclimate and gas exchange model STANDFLUX. The model was used to calculate forest radiation absorption, the net photosynthesis and transpiration of single tree s, and gas exchange of tree canopies. Model parameterizations were derived for six stands of Picea abies (L.) Karst. differing in age from 40 to 140 years and in density from 1680 to 320 trees per hectare. Parameterization included information on leaf area dist ribu- tion from tree harvests, tree positions and tree sizes. Gas exchange was modelled using a single species-specific set of physio logical parameters and assuming no influence of soil water availability. For our humid montane stands, these simplifying assumptions appeared to be acceptable. Comparisons of modelled daily tree transpiration with water use estimates from xylem sapflow measure - ments provided a test of the model. Estimates for canopy transpiration rate derived from the model and via xylem sapflow measur e- ments agreed within ± 20%, especially at moderate to high air vapor pressure deficits. The results suggest that age and density dependent changes in canopy structure (changes in clumping of needles) and their effect on light exposure of the average needle lead to shifts in canopy conductance and determine tree canopy transpiration in these managed montane forests. Modelled canopy net photosynthesis rates are presented, but have not yet been verified at the canopy level. norway spruce / xylem sapflow / canopy transpiration / canopy light use efficiency / biosphere-atmosphere interactions