Shedding of older needle age classes does not necessarily reduce photosynthetic primary production of Norway spruce

The hypothesis that the frequently observed shedding of older needle age classes in stands of Norway spruce suffering from atmospheric pollutants has minimal effect on photosynthetic primary production was assessed. Using structural parameters of young Norway spruce Picea abies (L.) Karst.] trees, an existing 3-dimensional canopy photosynthesis model was adapted and validated for conifer canopies. This model was employed to quantitatively demonstrate that, depending on the stand density, the loss in photosynthetic active foliage area may be compensated by higher photosynthetic rates of the remaining younger foliage age classes due to higher levels of photosynthetically active radiation (PAR) within the thinned tree crowns. Under certain circumstances, beneficial effects on the tree's water use efficiency may be expected. Extrapolating the model results to mature tree stands is addressed. The potential effects of higher PAR levels on abundance and species diversity of the epiphytic and understory vegetation, as well as the general importance of the older foliage age classes for spruce trees on a long term scale, are discussed.