Revista Chilena de Historia Natural 79 (3): 321-328, 2006
RESEARCH ARTICLE
Ontogenetic variation in light interception, self-shading and biomass distribution of seedlings of
the conifer Araucaria araucana (Molina) K. Koch
One of the factors thought to contribute to ontogenetic declines in plant growth rates is
diminishing light interception efficiency, as a result of the difficulties of avoiding self- shading among a growing number of leaves, and
by stems. The effects of plant size on self-shading and light interception have rarely been quantified, however. We used a three-
dimensional digitising system to construct virtual models of the architecture of Araucaria araucana seedlings 71 to 358
mm tall, and modelled their light interception in the forest understorey using the program YPLANT. We also analyzed seedling
allometry, to determine the combined effects of biomass distribution and self-shading on total light interception. Average light
interception efficiencies calculated for A. araucana (29 %) were the lowest reported for rainforest tree seedlings, reflecting
the limitations imposed by short leaves, lack of petioles and an inability to develop planar foliage geometry on branches. Total light
interception was related to seedling leaf area by an exponent of 0.735, reflecting increasing self-shading as seedlings grew bigger.
However, because leaf area was related to seedling mass by an exponent of 1.24, light interception scaled nearly isometrically (0.91
power) with seedling mass. This resulted from taller plants having proportionally thinner stems, and a smaller fraction of their biomass
in roots. Thus, an ontogenetic increase in self-shading in A. araucana is largely offset by allocation changes which
increase leaf area ratio as seedlings grow bigger. These mechanisms conserving the relationship of light interception with plant mass
seem likely to be restricted to species with long-lived leaves, growing in humid situations protected from wind stress. In open habitats,
where wind and drought stress likely make such allocation patterns less feasible, the role of self-shading in ontogenetic declines in
relative growth rate may be more evident.
Key words:
allocation, allometry,
FLORADIG, leaf area ratio, virtual plants, YPLANT
