Modelling age- and density-related gas exchange of Picea abies canopies inthe Fichtelgebirge, Germany

Differences in canopy exchange of water and carbon dioxide that occur due t o 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 trees, and gas exchange of tree canopies, Model parameterizations were derived for six stands of Picea abies (L.) Karst. differing in age from 30 to 140 years an d in density from 1680 to 320 trees per hectare. Parameterization included information on leaf area distribution from tree harvests, tree positions an d tree sizes. Cas exchange was modelled using a single species-specific set of physiological parameters and assuming no influence of soil water availa bility. For our humid montane stands, these simplifying assumptions appeare d to be acceptable. Comparisons of meddled daily tree transpiration with wa ter use estimates from xylem sapflow measurements provided a test of the mo del. Estimates for canopy transpiration rate derived from the model and via xylem sapflow measurements agreed within +/- 20%, especially at moderate t o high air vapor pressure deficits. The results suggest that age and densit y dependent changes in canopy structure (changes in clumping of needles) an d their effect on light exposure of the average needle lead to shifts in ca nopy conductance and determine tree canopy transpiration in these managed m ontane forests. Modelled canopy net photosynthesis rates are presented, but have not yet been verified at the canopy level.