DISTRIBUTION PATTERNS OF FOLIAR CARBON AND NITROGEN AS AFFECTED BY TREE DIMENSIONS AND RELATIVE LIGHT CONDITIONS IN THE CANOPY OF PICEA-ABIES

Variations in the partitioning of foliar carbon and nitrogen in combin ation with changes in needle and shoot structure were studied in trees of Picea abies along a vertical gradient of relative irradiance (RI). RI was the major determinant of needle morphology, causing all needle linear parameters - width, thickness and length - to increase. Due to the different responsiveness of needle thickness and width in respect of RI, the ratio of total to projected needle area increased with RI. Furthermore, shoot structure was also influenced by RI, and the ratio of shoot silhouette area to total needle area, which characterises th e packing of needles and needle area within the shoot, was greater at lower values of irradiance. Needle dry weight per total needle area (L WA(t)) was also increased by RI. Similarly, irrespective of the measur e for surface area, needle nitrogen content per area. as the product o f needle dry weight per area and nitrogen content per needle dry weigh t (Nm), scaled quasi-linearly with needle weight per area. Thus, the c hanges in needle and shoot morphology made it possible to invest more photosynthesising weight per unit light-intercepting surface there, wh ere the pay-back due to elevated irradiances was the highest. However, N-m behaved in an entirely different manner, decreasing hyperbolicall y with LWA(t). Since non-structural (carbon in non-structural carbohyd rates), and structural (total minus non-structural) needle carbon per dry weight also increased with LWA(t) N-m was inversely correlated wit h both non-structural and structural carbon. Total tree height, increa sing significantly LWA(t), also influenced needle structure. It appear ed that total height did not affect needle thickness or width, but lar ger trees had greater needle density (dry weight per volume). Because needle density was positively correlated with needle carbon content pe r dry weight, it was assumed that the greater values of needle carbon content can be attributed to increased lignification and thickening of needle cell walls. Thus, it appeared that the proportion of supportin g structures was greater in needles of larger trees. Inasmuch as an in creased fraction of supporting structures dilutes other leaf substance s, including also leaf compounds responsible for CO2-assimilation, enh anced requirement for supporting structures may be responsible for low er rates of carbon assimilation per foliage dry weight observed in lar ge trees. Increasing water limitation with increasing tree size is dis cussed as a possible cause for increased needle supporting costs in la rge uses.