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要旨:
A model is presented which solves simultaneously for leafscale
stomatal conductance, COj assimilation and the
energy balance as a function of leaf position within
canopies of well-watered vegetation. Fluxes and conductances
were calculated separately for sunlit and shaded
leaves. A linear dependence of photosynthetic capacity on
leaf nitrogen content was assumed, while leaf nitrogen
content and light intensity were assumed to decrease exponentially
within canopies. Separate extinction coefHcients
were used for diffuse and direct beam radiation. An efficient
Gaussian integration technique was used to compute
fluxes and mean conductances for tbe canopy. Tbe multilayer
model syntbesizes current knowledge of radiation
penetration, leaf pbysiology and tbe pbysics of evaporation
and provides insigbts into tbe response of wbole
canopies to multiple, interacting factors. Tbe model was
also used to explore sources of variation in tbe slopes of
two simple parametric models (nitrogen- and ligbt-use
efficiency), and to set bounds on tbe magnitudes of tbe
parameters.
For canopies low in total N, daily assimilation rates are
-10% lower wben leaf N is distributed uniformly tban
wben tbe same total N is distributed according to tbe exponentially
decreasing profile of absorbed radiation.
However, gains are negligible for plants witb bigb N concentrations.
Canopy conductance, G^, sbould be calculated
as Ge = AX(/',,g,,+/,hgsh), wbere A is leaf area index,/,,
and/sh are tbe fractions of sunlit and sbaded leaves at eacb
level, and g^i and g^y, are tbe corresponding stomatal conductances.
Simple addition of conductances witbout tbis
weigbting causes errors in transpiration calculated using
tbe 'big-lear version of tbe Penman-Monteitb equation.
Partitioning of available energy between sensible and
latent beat is very responsive to tbe parameter describing
tbe sensitivity of stomata to tbe atmospberic bumidity
deficit. Tbis parameter also affects canopy conductance,
but bas a relatively small impact on canopy assimilation.
Simple parametric models are useful for extrapolating
understanding from small to large scales, but tbe complexity
of real ecosystems is tbus subsumed in unexplained
Correspondence: R. Uuning, CSIRO, Centre for Environmental
Mechanics, PO Box 821, Canberra, ACT2601, Australta.
© 1995 Blackwell Science Ltd
variations in parameter values. Simulations witb tbe multilayer
model sbow tbat botb nitrogen- and radiation-use
efficiencies depend on plant nutritional status and tbe diffuse
component of incident radiation, causing a 2- to 3-fold variation in tbese efficiencies.
