EFFECTS OF LIGHT AVAILABILITY AND TREE SIZE ON THE ARCHITECTURE OF ASSIMILATIVE SURFACE IN THE CANOPY OF PICEA-ABIES - VARIATION IN SHOOT STRUCTURE

Dependence of shoot morphology on relative irradiance (diffuse site factor, a(d)) and total tree height (TTH) was studied in a natural canopy of Picea abies (L.) Karat. Surface area of foliar elements was characterized by total needle area (TLA, all-sided needle surface), projected needle area (PLA, area of detached needles laid flat on a horizontal surface), and vertical and horizontal silhouette shoot areas (SSA(0) and SSA(90), defined as shadows cast by shoots with fixed horizontal inclinations of 0 degrees and rotation angles of 0 degrees and 90 degrees, respectively). With increasing irradiance and tree size, packing of needles (needle number per unit shoot length, L(s)) and packing of TLA (TLA/L(s)) increased, but packing of SSA(0) (SSA(0)/L(s)) was invariant. Accordingly, the SSA(0) to TLA ratio (STAR(0)) decreased, signifying that the proportion of TLA exposed to direct solar radiation declined, and thus the cost of SSA(0) in terms of TLA increased with increasing a(d) and TTH. Factors causing costlier SSA(0) at high irradiances and in tall trees included increases in needle packing, changes in needle morphology (increases in needle dry weight per TLA and decreases in TLA/PLA), and changes in needle position and angle of needle inclination. The weight of the shoot axis (M(a)) was used as an estimate of biomass costs for mechanical support of the needles and water supply to the needles. Foliage-area-based needle-support costs (NSCa), defined as M(a) per unit foliage surface (TLA, PLA or SSA(0)) increased with a(d) and were not related to TTH, whereas foliage-weight-based needle-support costs (M(a)/M(D)) decreased with increasing TTH and were independent of a(d).