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 45

xylem mother cells), but a higher rate of cell production during a season must
also be associated with an increase in the rate of cell division or faster cycling of
xylem mother cells.
4. There is a typical distribution of measured mitotic index along the cambial zone:
moving inward from the last phloem cells produced, the mitotic index increases
to a maximum approximately one-third the distance across the cambial zone and
then decreases, either slowly or rapidly depending on whether the cambial zone
is wide or narrow (that is, depending on the number of cells in the cambial zone)
(Bannan 1957 ; Wilson 1964 ) .
5. There are differences in the rates of cell production even if the relationship
between the number of cells produced during a season and the number of cells
in the cambial zone is the same (Gregory and Wilson 1968 ) . These differences
are associated with higher or lower mitotic index.
6. In conifers, the first cell divisions (in the growing season) are evident in cambial
cells near to or often adjoining the last differentiated tracheids of the previ-
ous year's growth ring (Bannan 1955 ; Grillos and Smith 1959 ; Bannan 1962 ;
Zimmermann and Brown 1971 ; Savidge 1993 ) .
What can we say about the possible control of each characteristic? One possibil-
ity is that the specific growth rate distribution within the cambial zone depends on
tree species as well as the geographical zone of growth (regional climate). There is
some evidence for this from observations (Gregory and Wilson 1968 ; Sviderskaya
1999 ) . From the dendrochronological point of view, these characteristics are more
or less stable through a long period of growth and do not affect interannual varia-
tions of cell production. The dormant and especially the starting size of the cambial
zone may be closely related to the previous growth and tree vigor (Dodd and Fox
1990 ; Sviderskaya 1999 ) . Dendroclimatic analysis based on 'cell chronologies' of
larch and spruce near the northern timberline shows the significant effect of start-
ing conditions (temperature and soil melting) on production of cells and tree-ring
width (Hughes et al. 1999 ; Vaganov et al. 1999 ) . These characteristics affect the
interannual variability in tree-ring width and cell production. However, the starting
number of cells in the cambial zone may also be affected by growth conditions in
the previous year and, so, might be responsible for autocorrelation in tree-ring series
that appear, at first sight, to have no climate cause (Fritts 1976 ) . The main possible
environmental control is related to intraseasonal variations of size of the cambial
zone and the specific growth rate of xylem mother cells. This thesis is supported by
practically all experimental data, which show close correlation between average and
maximal size of the cambial zone and annual xylem increment (see Wilson 1964 ;
Gregory 1971 ; Skene 1972 ; Vaganov et al. 1985 ) . In combination with cell cycle
distribution across the cambial zone, these seasonal variations may explain much of
the interannual deviation in total cell production and tree-ring width.
Analysis of the data shows that there are two determinants of variability in the
sizes of the cambial zone: one that is more or less constant over many years, deter-
mined by the condition of the tree as a whole (growth class, vigor, energy of growth,
age, position in stand, etc.); and a second, an intraseasonal component, which is