Uncertainty, Emergence, and Statistics in Dendrochronology - Dendroclimatology - page 83

based on the Kalman filter (Visser and Molenaar 1988 ; Cook and Johnson 1989 ) .
Thus, one does not necessarily have to abandon the linear model to demonstrate a
change in the way that trees respond to climate. However, the clear strength of the
process-based model is that it provides insights into why the change occurred. The
Kalman filter cannot do this in any obvious way.
The existence of crossdating between trees is likewise remarkable when it is
viewed from microenvironmental and tree physiological perspectives. A schematic
from Harold C. Fritts' seminal book Tree Rings and Climate (Fritts 1976 ) nicely
illustrates the complex ways that high temperature and low precipitation affect phys-
iological processes within the tree to produce a narrow annual ring (Fig. 4.2 ) . This
schematic applies to a tree of a given species within a stand, but the individu-
als of the same species within the stand undoubtedly experience different levels
of environmental forcing on growth. Each tree, situated on a variable landscape,
will experience different levels of available soil moisture; different levels of inso-
lation and evapotranspiration demand; different levels of soil nutrient status; and
in closed-canopy forests, different levels of competition with neighboring trees for
available resources for growth. These varying external growth-limiting factors result
in variable absolute growth rates between trees in any given year. Yet, the resulting
secondary growth on the bole of each tree produces a common pattern of annual ring
width over time; i.e., the trees crossdate, and this crossdating among the same and
closely related tree species can extend over remarkably large regions (Fritts 1965 ;
Cropper and Fritts 1982 ) . Admittedly, crossdating is not universal among all tree
Fig. 4.2 A schematic showing how low precipitation and high temperature jointly contribute to
the formation of a narrow annual ring in trees. Reproduced from Fritts ( 1976 ) with kind permission