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 61

3.8 Model Applications
Temporal variability in the relationship between climate and tree-ring-derived prox-
ies has been identified in a range of species and locations (Briffa et al. 1998 a , b;
Biondi 2000 ; Jacoby et al. 2000 ; Aykroyd et al. 2001 ; Wilmking et al. 2004 , 2005 ) .
Such instability might be particularly important in environments where both tem-
perature and precipitation can be important controls on tree growth (Anchukaitis
et al. 2006 ) , where the timing of the onset of growth or the length of the grow-
ing season strongly influences tree-ring proxies (Vaganov et al. 1999 ; Aykroyd et al.
2001 ; Vaganov et al. 2006 ; Evans et al. 2006 ) , and for high-latitude or high-elevation
temperature-sensitive trees under anthropogenic climate forcing (Jacoby et al. 2000 ;
Wilmking et al. 2004 , 2005 ) . The Vaganov-Shashkin model produces synthetic
chronologies that would be expected if climate, mediated by cambial processes,
were the only external control on tree growth. This characteristic potentially allows
dendroecologists to evaluate the importance of hypothesized ecological factors that
might be responsible for differences observed between actual tree-ring chronologies
and simulations. For instance, it can also be used to develop null hypotheses against
which to test theories about the influences of insects, disease, CO 2 enrichment,
carbon storage, pollution, and disturbance on tree growth. Furthermore, because
the Vaganov-Shashkin model has the ability to simulate nonlinear relationships
between tree-ring formation and the environment, it can be used to determine
whether observed variability in climate-tree growth relationships arise as a func-
tion of climate itself, as a stochastic feature without a determinant cause, or through
possibly unobserved influences by biological or ecological changes not related to
climate.
The Vaganov-Shashkin model has recently been applied to simulate tree-ring
proxies across a range of environments for a variety of species and using sev-
eral complementary approaches. The particular methodology for developing and
analyzing synthetic chronologies depends in part on the research questions posed
and the availability of meteorological and tree-ring data with which to drive and
evaluate the model. The simplest approach to modeling tree-ring chronologies is
to use single meteorological stations close to the actual tree-ring chronology site.
Several studies have demonstrated that using appropriately chosen local meteoro-
logical stations can produce simulations that skillfully reproduce actual tree-ring
width patterns (Vaganov et al. 1999 ; Evans et al. 2006 ; Vaganov et al. 2006 ) .
These studies target cases in which direct model-data intercomparisons are easily
made, but do not assess the extent to which model skill is general across environ-
ments and species. An intermediate approach exploits spatiotemporal techniques
like principal components analysis (PCA), which decompose a large set of time
series into a few low-order empirical functions that contain the primary modes of
robust common variance in networks of observed and simulated tree-ring data net-
works (Anchukaitis et al. 2006 ) . A third approach is to compare tree-ring datasets to
meteorological datasets on a large scale for assessing model robustness and the gen-
eral suitability of the tree-ring dataset for climate monitoring (Evans et al. 2006 ) .
These nonlocal approaches permit assessment of the suitability of the proxy data