How Well Understood Are the Processes that Create Dendroclimatic Records? A Mechanistic Model of the Climatic Control on Conifer Tree-Ring Growth Dynamics - Dendroclimatology - page 39

rings would arise as a result of optimal climatic conditions for photosynthesis, while
a narrow ring would signal a deficit of carbohydrates and environmental condi-
tions that restrict photosynthetic capacity. Clearly, given that a large quantity of
complex carbohydrates, in the form of cellulose, lignin, and hemicelluloses and
other polysaccharides, form the bulk of tree biomass, the availability of a source
of carbon is an obvious prerequisite for secondary growth. Consistent with a carbon
source-limited understanding of secondary growth, it has been observed that plant
resources allocated to reproduction can result in a reduction of overall basal growth
(Wheelwright and Logan 2004 ) . Defoliation can also induce basal growth suppres-
sion and narrow tree rings (Speer et al. 2001 ) , presumably through limitations on
available carbohydrates (Ericsson et al. 1980 ) .
Ecophysiological modeling of the environmental controls on the annual incre-
mental basal growth has generally made use of this carbon-balance, source-sink
approach (Bassow et al. 1990 ; Makela 1990 ) . In essence, this type of model focuses
on how the external environment controls carbon fixation and the production of
other metabolites by trees in the canopy (buds and leaves) and how these are then
allocated to various plant parts. Carbon-balance models may include calculations
of photosynthesis as a function of tree age, foliage and crown composition, height,
and root mass (Bassow et al. 1990 ) . Assimilated carbon is then partitioned into
branch, bole, root, and crown growth, as well as to respiration. Explicit models
linking environmental conditions to resultant tree-ring characteristics have been
developed.
Perhaps the most complete of these models (albeit still considered overly simplis-
tic by Fritts), incorporating processes linking environmental conditions to tree-ring
characteristics via photosynthesis, respiration, carbon storage, and cambial pro-
cesses, is TREERING2000 (Fritts et al. 1999 ) . This model was developed and
refined exhaustively using, in part, tree-ring data and meteorological observations
from the Santa Catalina Mountains in southern Arizona. Such models provide
explicit tests of our understanding of the processes governing tree-ring formation
and variation, and have been a critical component of the science of dendrochronol-
ogy. It is important to note that TREERING2000, is not purely ecophysiological.
It uses photosynthesis and assimilation to modify the rates of cellular processes
in the cambium (see next section), and is therefore a hybrid carbon-balance and
cellular process model (Fritts et al. 1999 ) . The MAIDEN model (Misson 2004 )
uses a stand-level ecophysiological approach, which simulates the environmental
control on photosynthesis and respiration, using both climate and allometric data
to determine carbon fixation and mean bole increment diameter, which can be
interpreted as analogous to tree-ring width for comparison with dendroecological
data.
A drawback to this approach is that modeling the partitioning of carbon within a
tree depends strongly on parameters that are not completely understood or described
(see LeRoux et al. 2001 for a review and critique). Further, there is evidence of age-
related trends in partitioning coefficients (Makela 1990 ) . Many allocation processes
will scale with tree height, a non-climatic, age-dependent metric that is itself at least
partially a function of stand-level competition for nutrients, sunlight, and water.