SHEDDING OF OLDER NEEDLE AGE CLASSES DOES NOT NECESSARILY REDUCE PHOTOSYNTHETIC PRIMARY PRODUCTION OF NORWAY SPRUCE - ANALYSIS WITH A 3-DIMENSIONAL CANOPY PHOTOSYNTHESIS MODEL

The hypothesis that the frequently observed shedding of older needle a ge classes in stands of Norway spruce suffering from atmospheric pollu tants has minimal effect on photosynthetic primary production was asse ssed. Using structural parameters of young Norway spruce [Picea abies (L.) Karst.] trees, an existing 3-dimensional canopy photosynthetic mo del was adapted and validated for conifer canopies. This model was emp loyed to quantitatively demonstrate that, depending on the stand densi ty, the loss in photosynthetic active foliage area may be compensated by higher photosynthetic rates of the remaining younger foliage age cl asses due to higher levels of photosynthetically active radiation (PAR ) within the thinned tree crowns. Under certain circumstances, benefic ial effects on the tree's water use efficiency may be expected. Extrap olating the model results to mature tree stands is addressed. The pote ntial effects of higher PAR levels on abundance and species diversity of the epiphytic and understory vegetation, as well as the general imp ortance of the older foliage age classes for spruce trees on a long te rm scale, are discussed.