Effects of light availability and tree size on the architecture of assimilative surface in the canopy of Picea abies: variation in shoot structure

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Tree Physiology, 15:791–798

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Effects of light availability and tree size on the architecture of assimilative surface in the canopy of Picea abies: variation in shoot structure

Ülo Niinemets (1, 2) and Olevi Kull (1)

1. Institute of Ecology, Estonian Academy of Sciences, 40 Lai Street, EE2400 Tartu, Estonia / 2. LS Pflanzenökologie, BITÖK, Universität Bayreuth, Postfach 10 12 51, 95448 Bayreuth, Germany / Received February 2, 1995

Summary

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.) Karst. 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 (SSA₀ and SSA₉₀, defined as shadows cast by shoots with fixed horizontal inclinations of 0° and rotation angles of 0° and 90°, 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 SSA₀ (SSA₀/L_s) was invariant. Accordingly, the SSA₀ to TLA ratio (STAR₀) decreased, signifying that the proportion of TLA exposed to direct solar radiation declined, and thus the cost of SSA₀ in terms of TLA increased with increasing a_d and TTH. Factors causing costlier SSA₀ 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 (NSC_a), defined as M_a per unit foliage surface (TLA, PLA or SSA₀) increased with a_d and were not related to TTH, whereas foliage-weight-based needle-support costs (M_a/M_n) decreased with increasing TTH and were independent of a_d.

Keywords: conifers, foliage area, LAI, leaf morphology, needle packing, silhouette area, STAR.