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 43

spatial organization of the cambial zone is important from the point of view of reg-
ulation of its activity, as it imposes a number of specific requirements on regulation
and control. Another, but not less significant, aspect of the spatial organization of
the cambium is that it is the basis of the spatial cell-like organization of xylem and
phloem. For example, it orders the radial tracheid files, the formation of the vessel
system, and so on.
The growth of a tree ring is the result of periclinal divisions of cells in the cambial
zone and of their differentiation. The growth rate depends on the number of cells in
the cambial zone and their rate of division. In coniferous species, the growth of a
tree ring during a season is always accompanied by a change in the number of cam-
bial cells, which has characteristic dynamics that are general for all species (Wilson
1966 ; Gregory 1971 ; Skene 1972 ; Kutscha et al. 1975 ; Vaganov et al. 1985 ) . In dor-
mancy, the size of the cambial zone reaches a minimum and usually includes 4-5
cells but can reach up to 10 (Larson 1994 ) . The radial diameter of cells in the cam-
bial zone is equal to 5-6 m (Bannan 1955 ;

Alfieri and Evert 1968 ; Vaganov et al. 1985 ) . Activation of the cambial zone starts
with a swelling of cells, and then the first divisions appear. After activation the size
of the cambial zone is increased, and the number of cells in it increases and reaches
maximum values up to 20 (15-16 on average for different species) (Larson 1994 ) .
There is evidence for a relationship between the number of cells in the cambial zone
during the dormant period (and at the starting date) and the total annual produc-
tion of xylem. So, Gregory ( 1971 ) found that this relationship for Alaskan white
spruce is described as N camb

μ m in average but does not exceed 10 μ

N ( R 2

0.001).
Sviderskaya ( 1999 ) obtained similar results from seasonal observations of tree-ring
formation in Scots pine ( Pinus sylvestris ) and Siberian fir ( Abies sibirica )inthe
Siberian taiga (Fig. 3.1 ) .
All the available data on the duration of the cell cycle, as well as of separate
phases of it, show significant variability between samples taken in different parts
of a tree during a growing season, especially for the size of the cambial zone. This
essential variability is determined by the weather conditions of the season and other
factors. Thus it is clear that the length of the cell cycle in the cambial zone changes
during the growth season. Combining this statement with the observed curvilinear
relationship between the number of cells in the cambial zone and annual xylem
increment, we are led to the following conclusion: the regulation of cell production
by the cambial zone can be achieved by increasing the number of cells in the cambial
zone as well as by increasing the rate of cell division in the cambial zone.
We can summarize the results concerning the kinetics parameters of cambial
activity in xylem cell production observed in different conifer species:

= 3.82 + 0.05 × = 0.75; n = 37; p <

1. The number of cambial cells in dormant cambium and active cambium is rather
different. There is a significant relationship between the number of dormant
cells in the cambial zone and subsequent annual xylem increment (Skene 1972 ;
Sviderskaya 1999 ) .
2. The number of cells in the cambial zone varies during a season due to internal and
external factors (Fig. 3.1 ) . The average duration of the cell cycle in the cambial