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 46

determined by change of the specific growth rate of xylem mother cells and on
which their number also simultaneously depends (Vaganov 1996 b ). The first deter-
minant can be considered as a constant on the long-term scale of the life span of
a tree (some years, some decades); the second depends on current environments
(climatic conditions within a season).
The assumption that the cell cycle is equal along the cambial zone throughout the
season leads to the following conclusion: in order to control division it is necessary
to control the size (or width) of the cambial zone (the number of dividing cells) and
their specific growth rate. This means that two mechanisms are involved in control.
One is clearly positional, and the other may have dependence on concentration or
have some other nature. Formally, the unequal (and, in the case of the hypothesis
presented here, increased) cell growth rate across the cambial zone needs only one
control—positional, which is easily described in mathematical terms and the corre-
sponding equations. Of course, nature does not always conform to the simplicity of
its mathematical description.

3.3 Cell Expansion

Expansion of the radial size of cells after they leave the cambial zone is the next
main stage of the cytodifferentiation of xylem (Gamaley 1972 ; Roberts 1976 ) .
Roberts ( 1976 ) noted that at this stage, 'the cells exhibit high variability in the
extent and regulation of expansion. The deposition of primary wall material dur-
ing expansion requires the synthesis of primary wall monomers. Protein synthesis
occurs. DNA replication involving endoreplication and gene amplification may
occur' (Roberts 1976 , pp 36-37). The visible result of the enlargement is greatly
increased radial cell size. In earlywood the radial dimension of tracheids reaches
50-60

m. So, during the formation of early-
wood the radial size of tracheids increases up to 7-8 times, in latewood, up to 2-3
times in comparison with the starting size of a cell in a cambial zone, which is about
7-8

μ m; in latewood it is about 15-25 μ

m on the average.
Most experimental results indicate that final tracheid size is not determined by
the rate of radial expansion (Wodzicki 1971 ; Skene 1972 ; Vaganov et al. 1985 ; Dodd
and Fox 1990 ; Sviderskaya 1999 ; but see Antonova et al. 1995), thus we must look
elsewhere. It is obvious, however, that such a search must focus on the period prior
to radial cell expansion; namely, on cell production. Sviderskaya ( 1999 ) estimated
the specific growth rate of cells within the cambial zone using cell production data
and the size of the cambial zone over the course of the growing season (Fig. 3.1 ) .
A good relationship was found between the intraseasonal changes in the specific
growth rate and changes in the size of tracheids leaving the cambium with a given
growth rate. This means that the radial tracheid dimension is primarily determined
by the average radial growth rate in the cambial zone or by the average rate of
cell division within the cambial zone. In other words, radial tracheid dimension
corresponds to the magnitude of cell production in the cambial zone.

μ