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 50

1990 , 1994 ) . For example, the earlier beginning and fast rate of xylan synthesis,
mainly included in radial walls, can increase radial growth and result in formation
of large tracheids or vessels. On the contrary, the synthesis of cellulose reduces
an initial nonuniformity of the wall and so hinders radial growth (Catesson 1990 ,
1994 ) .
All mature xylem elements (except parenchyma cells) have a thick secondary
cell wall. However, thickness can noticeably differ in tracheids formed at different
times in the growth season. In earlywood tracheids, the cell wall is noticeably thin-
ner (1.5-3.0

μ μ

m. As
well as tracheid sizes, the thickness of a cell wall can vary greatly, especially in late-
wood cells in various tree rings, and also in various parts of the tree (Zahner 1968 ;
Larson 1969 ; Creber and Chaloner 1984 ; Vaganov et al. 1985 ) . The greater cell wall
thickness compared with tracheids differentiated at the beginning of the growing
season is one of the main criteria of definition of latewood. As of now the issue has
not been solved as to how the transition from formation of earlywood to formation
of latewood is regulated. In any case such a transition is connected with the control
of synthesis of the secondary wall. External and internal factors can influence for-
mation of the secondary wall. Experimental research shows that seasonal change
of external factors, such as light exposure, photoperiod, water deficit, nutrients,
or temperature influence both the quantity of latewood cells and the thickness of
their cell walls (see Brown and Sax 1962 ; Wilson 1964 ; Zahner 1968 ; Larson 1969 ;
Denne and Dodd 1981 ; Creber and Chaloner 1984 ; Vaganov et al. 1985 ; Carlquist
1988 a , b; Downes and Turvey 1990 ; Antonova and Stasova 1993 ; Lev-Yadun and
Aloni 1995 ; Savidge 1996 ) .
In early works conducted by Wodzicki ( 1971 ) , Skene ( 1972 ) , and Denne ( 1971 )
it was pointed out that the rate of cell wall deposition varied relatively little within a
growing season. For example, in spite of differences in the growth rates of individ-
ual trees (different tree vigor), the actual rate of deposition of cell wall material was
about 0.1-0.2

m). In latewood tracheids, cell wall thickness can reach 7-8

m 2 per day and seemed to show little change during the course of μ

the season in suga canadensis (Skene 1972 ) . The time period required for lysis

of the cytoplasm was about 4 days, with no evidence of any changes with tree
vigor (Skene 1972 ) . For tree rings of Douglas-fir ( Pseudotsuga menziesii )itwas
4-5

T

m 2 /day, and in trees with a well-developed crown at the beginning of a sea-
son, 6-7

μ

m 2 /day (Dodd and Fox 1990 ) . These authors have compared rates of
radial growth and formation of the cell wall in young trees distinguished by devel-
opment of the crown, and for different heights in the stem. The differences are
not marked, except in the case of the growth rate in the second half of a season
in trees with a well-developed crown. The experimental data of Wodzicki ( 1971 )
have shown that the rate of maturation has no clear influence on the radial size
and thickness of the cell wall, and the main role is played by the duration of these
stages of differentiation. The average value of the rate of cell wall deposition in
Pinus sylvestris was a little more than in Tsuga and Pseudotsuga, reaching 5-
10

μ

m 2 /day. Similarly, Sviderskaya ( 1999 ) obtained a rate of cell wall deposition
of 5-7

μ

m 2 /day in observations of seasonal tree-ring formation in three coniferous
species ( Pinus sylvestris, Picea obovata, Abies sibirica ). So, as in the case of the

μ