© 1998 Heron Publishing—Victoria, Canada
Stoichiometry of foliar carbon constituents varies along light gradients in temperate woody canopies: implications for foliage
morphological plasticity
Ülo Niinemets (1) and Olevi Kull (1)
1. Estonian Institute of Ecology, Riia 181, Tartu EE 2400, Estonia / Received March 17, 1997
Summary
Foliar morphology and chemical composition were examined along a light gradient in the canopies of five deciduous temperate
woody species, ranked according to shade-tolerance as Populus tremula L. < Fraxinus excelsior L. < Tilia cordata Mill. = Corylus avellana L. < Fagus sylvatica L. Foliar carbon was divided between structural (cell-wall polysaccharides, lignin) and nonstructural (proteins, ethanol-soluble
carbohydrates, starch) fractions. Foliar morphology of all species was strongly affected by irradiance. Both leaf dry mass
per area (MA), a product of leaf density and thickness, and leaf dry to fresh mass ratio (Dw), characterizing the apoplastic leaf fraction, increased with increasing relative irradiance (Isum, calculated as the weighted mean of fractional penetration of diffuse and direct irradiance). Though the relationships were
qualitatively identical among the taxa, more shade-tolerant species generally had lower values of MA than shade-intolerant species, and their morphological relationships with irradiance were curvilinear; however, there were
no signs of saturation even at the highest irradiances in shade-intolerant species. In all species, lignin concentrations
increased and cell-wall polysaccharide concentrations decreased with increasing irradiance. Consequently, biomass investment
in structural leaf components appeared to be relatively constant along light gradients. The relationship between irradiance
and structural compounds tended to be asymptotic in the more shade-tolerant species, whereas MA was linearly correlated with concentrations of structural leaf components, suggesting that similar factors were responsible
for the curvature in the morphological and chemical relationships with irradiance. Because lignin increases tissue elastic
modulus thereby rendering leaves more resistant to low leaf water potentials, we conclude that changes in stoichiometry of
cell wall components were related to foliage acclimation to the gradients of water deficit that develop in the canopy and
inherently accompany light gradients. We also conclude that increased lignification decreased leaf expansion growth, and that
species differences in lignification were partly responsible for the observed interspecific variability in morphological plasticity.
Analysis of structural leaf compounds provided no indication of how shade-intolerant species with low investments in lignin
acclimated to gradients of water availability in the canopy. Because shade-intolerant species generally had higher capacities
for photosynthesis than shade-tolerant species, we postulated that they should also have a greater ability for osmotic adjustment
of leaf water potential with photosynthates. The concentrations of soluble carbohydrates increased with increasing irradiance
in all species; however, the osmotic adjustment achieved in this way was similar in all species, except for shade-intolerant
F. excelsior, which had a lower potential for osmotic adjustment with carbohydrates than the other taxa. Although we did not determine
whether the gradients of stem water potential and leaf water deficits were similar in canopies of different species, we demonstrated
that water relations play a central role in determining foliar structure and composition along light gradients in the canopy.
Keywords:
foliar morphology, irradiance, lignin, nitrogen, nonstructural carbohydrates, osmotic adjustment, shade-tolerance, structural
carbohydrates, water requirement.
