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 42

model that integrates the primary environmental controls on conifer cambial activ-
ity is described, and we provide examples of its application in dendroclimatology.
A version of this model, here referred to as Vaganov-Shashkin (VS), also forms
the core of the cambial simulation portion of the more complete TREERING2000
model of conifer tree growth (Fritts et al. 1999 ) . We review recent applications of the
VS model to the interpretation of tree-ring observations in a number of regions, envi-
ronments, and climate change scenarios. Implicit in this review is the assumption
that the VS model includes the most important processes required to successfully
simulate observed characteristics of tree rings, if it can be shown that the simula-
tions are consistent with observations. Finally, we discuss the role of forward models
in the development, calibration, and interpretation of tree-ring-based estimates of
climate variability and change.
3.2 Cambial Activity
The formation of mature xylem cells is usually considered as a process that occurs
in three stages: the division of cells, the growth of cells by radial expansion, and
the maturation of tracheids, when cell walls thicken and the protoplast is autolysed.
All stages are divided in space and in time, although they may overlap partially
(Gamaley 1972 ) . In the majority of plants, xylem is a complex tissue consisting
of differentiated cells of more than one type, generated by a secondary meristem,
the cambium. Plant meristems and cambium in particular, by virtue of their acces-
sibility, became a subject of research with the appearance of the first microscope.
The history of cambium research is well described; for example, in Larson's book
(1994). However, until now our knowledge of the factors initiating and regulating
the origin of tissue from meristems and the physiology of meristem has been rather
fragmentary. This has led to the absence of a uniform nomenclature (Wilson 1966 ;
Schmid 1976 ; Catesson 1984 ; Larson 1994 ) .
Cambium has common features intrinsic to all meristematic tissues and has, as a
highly specialized secondary meristem, specific features:
1. It is a self-sustaining cell-like system; that is, it retains its various functions for
extended periods of time, frequently throughout the life of the plant, which may
last centuries or millennia.
2. In woody plants the cambium grows at the expense of the growth of the tree. An
increase in the number of cambial cells happens at the expense of the division
and differentiation of those cells and of the primary (apical) meristem.
3. Cambial derivatives can be differentiated into various types of xylem and phloem
cells.
4. Cambium has a strictly ordered spatial organization.
The cambial cells form a continuous layer, covering the trunk, branches, and
roots. Therefore, the cambium, on the one hand, is distributed in space, and, on the
other hand, it is a linked system, where the adjacent cells are in direct contact. The