Geoscience Reference
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Summarizing published data on the variability of radial tracheid dimension
within conifer tree rings, we can say (Vaganov 1996 a ):
1. Radial tracheid dimension shows a clear seasonal trend (from earlywood to
latewood) except in some subtropical and tropical trees (Vaganov et al. 1985 ) .
2. Radial tracheid dimension shows variability due to climatic factors operating
within a growing season that combine with the seasonal trend (a typical example
of this is the formation of 'false' rings caused by intraseasonal drought) (Fritts
1976 ; Schweingruber 1988 , 1996 ) .
3. The average radial tracheid dimension is more or less constant over a long
period of tree growth. This constancy results in the close linear relationship
between tree-ring width and the number of cells produced annually (Gregory
1971 ; Vaganov et al. 1985 , 1992 ) .
4. The range of variation in radial tracheid dimension in tree rings of different
conifer species is usually limited—from 8 to 70
μ
m (Vaganov et al. 1985 ) .
What indirect data confirm the suggestion that the final radial tracheid dimension
cannot be effectively controlled by external influences during enlargement? They
come mainly from measurements of tracheidograms (Vaganov et al. 1985 , 1992 ) .
Narrow layers including only two to three tracheids with small radial dimension and
thin cell walls can often be observed in wide tree rings from dry conditions (Fritts
1976 ; Shashkin and Vaganov 1993 ) . The appearance of those cells in the earlywood
zone may, in most cases, be the result of periods of moisture deficit lasting only
from several days to a very few weeks. Obviously, the existence of such a layer of
small, thin-walled cells cannot be due to the effect of water stress on cells that were
enlarging at that time. On the other hand, if water stress affects the cells that have
only just started to enlarge, then why are the other cells in the enlargement zone not
affected?
There are several lines of evidence that apparently conflict with the idea that the
environmental control of the final radial dimension achieved by a tracheid acts on the
enlargement stage. It is, however, possible to explain this evidence if this control acts
on cell production; that is, it is effective in the cambial zone, not in the enlargement
zone. The evidence for the relationship between the growth rate of cambial cells
and the final radial dimension of the tracheids they produce is obtained by means
of a kinetic approach. It needs to be tested by other direct and indirect sets of data,
because not all the questions that arise have been answered. Most questions come
from the enormous volume of research on hormonal control of wood formation
(see Zimmermann 1964 ; Barnett 1981 ; Creber and Chaloner 1984 ; Savidge 1996 ;
Kozlowski and Pallardy 1997 ) . There are many examples of specific and nonspecific
effects of hormones and other plant growth regulators (auxin, IAA, gibberellin, ethy-
lene, and others) on the production rate of tracheids as well as on their size. Some
of this work is based on saplings, and may be of limited applicability to mature
trees in natural stands. Furthermore, we do not believe these data are necessarily
contradictory to the statements made above.
We do not know the precise mechanism by which the kinetics of cell production
and growth in the cambial zone determine the ultimate radial dimension of a
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