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 41

of water deficit, which results in an overall decrease in cell production as cambial
activity subsequently declines. Cellular processes in the cambium can therefore be
directly inhibited by climate, through temperature or pressure (via water potential)
influences on the cell cycle. The impetus to avoid cavitation and cell embolism in
vascular tissues would not require mediation through carbohydrate availability and
could act as a developmental determinant on the cambium itself.
Hormones and other substrates clearly play an important role in cellular growth
and differentation in the cambium, although the exact mechanisms through which
they influence these processes continue to be investigated (Savidge 1983 , 1996 ,
2000 a ; Dengler 2001 ) . Recent research on the genetic controls on xylem differ-
entiation by Schrader et al. ( 2003 ) has shown that hormone gradients across the
cambial zone are associated with differential gene expression and the specific stage
of cellular development. Because of this, cambial processes and development of new
xylem cells are inherently related to their position in the cellular file. Schrader et al.
( 2003 ) also concluded that the hormonal gradients across the cambium could change
rapidly, potentially in response to external environmental influences. Similarly,
Uggla et al. ( 2001 ) found concentration gradients of sucrose and indole-3-acetic
acid (IAA) across developing vascular plant tissue that corresponded to different
stages of cellular development, and moreover that the onset of latewood formation
did not correspond to any seasonal change in overall carbohydrate availability. The
importance of positional information and the rate of cellular processes, particularly
division, in the developing xylem will be discussed below in the context of the phys-
iological and observational basis for our model. Certainly, xylogenesis cannot occur
without the provisioning of sucrose and hormones from the canopy, which in turn
depends on access to water and nutrients provided by the roots. However, cellular
processes in the meristem and developing xylem directly regulate cambial activ-
ity, and secondary xylem development involves spatiotemporal gradients of cellular
growth and regulation, which can reflect the direct influence of climate on these
processes and patterns.
For dendroclimatology, it would be desirable to create a tractable forward
model that mechanistically includes only the critical processes that are minimally
necessary to link climate variables to tree-ring formation, but whose application
worldwide is not sensitive to the choice of a limited set of tunable parameters. Such a
class of models can be used to validate statistically based reconstructions of local cli-
mate variations, the assumptions behind which may not always be valid (Cook and
Pederson, Chapter 4 , this volume). For example, such models could predict a change
in the dominant environmental influence on tree-ring width variations, which can
be compared against actual observations. Such phenomena can only be diagnosed
from ad hoc statistical analyses, and Cook and Pederson ( Chapter 4 , this volume)
caution against a priori interpretation of tree-ring data when statistical uncertainty
and emergent behavior cannot be distinguished from one another.
Here, we introduce and briefly detail the processes that control ring development
in the context of the seasonal kinetics of cambial activity. We discuss the mecha-
nisms and controls on the production, expansion, and maturation of xylem cells, and
show how these may be externally influenced by climate. A mechanistic tree-ring