The area of foliage absorbing solar radiation is often expressed as leaf area index (LAI). In this study, specific leaf area (SLA), leaf area at tree level (LA) and LAI at stand level were measured on eight experimental plots of Norway spruce (Picea abies) and Scots pine (Pinus sylvestris), together with tree and stand measurements from biomass harvest. Both projected area and half total surface area were measured, and a model of the relationship between the units was constructed. SLA was larger for pine (61.62 cm 2 g −1) compared to spruce (50.2 cm 2 g −1) and showed a trend of decreasing higher up in the crown. Leaf area was significantly higher for Norway spruce compared to Scots pine on both tree and stand level. Models were constructed using diameter at breast height, tree height and stand basal area to estimate LA. The models were general site-independent models that can be used for easy estimations of single tree leaf area. Indirect measurement of LAI (LAIe) was shown to underestimate LAI with on average 30-73% depending on species and measurement technique. Using the extensive data collected, conversion models were constructed for estimating LAI from LAIe together with basal area, stem number and stand height. These species-specific conversion models will allow for more accurate estimations of LAI that can be used in mechanistic models of forest growth and for future estimates of LAI from remote sensing data.
Admixtures of birch in Norway spruce plantations are being promoted as a means to increase habitat and species diversity. The implications of this mixture were analysed with regional survey data from southern Sweden. Permanent sample plots from the Swedish National Forest Inventory (NFI), with Norway spruce and admixture of birch, were used to describe the temporal trends in the admixture, regarding species composition and competitive strength. Observations from thinned plots show a higher harvest removal in birch (35%) than for Norway spruce (19%). Observations without thinnings in the period before measurement showed that individual birch tree growth was lower compared to Norway spruce and it decreased even more with increasing stand age and competition. In addition, a complementary field survey, with multiple distributed sample plots in each stand, was used to detect within-stand variation of species composition and density. Although within-stand heterogeneity was larger in mixed stands in terms of species composition, it was not different from Norway spruce monocultures in terms of stand density. These two surveys show that the admixture of birch, for several reasons, decreases over stand age and although birch increases tree species diversity, it does not necessary imply a change in density.
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