EFFECTS OF STAND STRUCTURE AND PHYSIOLOGY ON FOREST GAS-EXCHANGE - A SIMULATION STUDY FOR NORWAY SPRUCE

The process-based simulation model STAND-FLUX describes canopy water v apor and carbon dioxide exchange based on rates calculated for individ ual trees and as affected by local gradients in photon flux density (P FD), atmospheric humidity, atmospheric carbon dioxide concentration, a nd air temperature. Direct, diffuse, and reflected PFD incident on fol iage elements within compartments of individual trees (defined by vert ical layers and a series of concentric cylinders centered on the trunk ) is calculated for a 3-dimensional matrix of points. Foliage element gas exchange rates are based on estimates of carboxylation, RuBP regen eration, and respiratory capacities as well as the correlated behavior found between stomatal conductance and assimilation rate. Because of the difficulties associated with effective sampling and description of spatial variation in structure and leaf level gas exchange parameters for trees comprising the forest canopy, the significance for canopy w ater and carbon dioxide exchange of varied representations of tree fol iage distribution and of physiology is examined. The additional intera ctive effects encountered due to changes in tree density and, thus, sp atial aggregation or disaggregation of foliage is also studied. The an alysis is conducted within the context of observed structural and phys iological variation encountered in Norway spruce (Picea abies) stands in the Fichtelgebirge region of central Germany. Potentials for simpli fying the three-dimensional canopy gas exchange model without sizable influence on canopy flux rates were small. A relatively large number o f sample points within the tree crowns is necessary to obtain consiste nt calculations of flux rates because of the nonlinear relationship be tween PFD and net photosynthesis. Transpiration and net photosynthesis for stands with a low leaf area index (LAI) may be obtained from sing le tree estimates for each tree class weighted by class frequency, whi le 30 or more trees per class in differing relation to neighboring tre es may be necessary to calculate reliable estimates of net photosynthe sis in canopies with high LAI. The complexity in structure assumed for modeled trees was important, especially when overall canopy foliage a rea was either high or low due to spatial heterogeneity in clumping, e .g., potential canopy overlaps or side-lighting. Effects were greater for calculated net photosynthesis than for transpiration, reflecting h igher sensitivity of net photosynthesis to differences in light distri bution within individual trees. Accuracy in estimates of physiological parameters is equally important, and these characteristics have profo und effects on estimated canopy gas exchange rates. While one-dimensio nal representations of canopy structure or approximations of tree phys iological characteristics from other canopies or species may often be necessary in assessing vegetation/atmosphere exchanges, especially in the study of water balance of landscapes or regions, STANDFLUX provide s a tool that can aid in evaluating the limitations of these simpler a pproaches.