How well understood are the processes that create dendroclimatic records? A mechanistic model of the climatic control on conifer tree-ring growth dynamics

We develop the observational and conceptual basis for modeling conifer cambial processes as a direct but nonlinear and multivariate response to external environmental conditions. The model, here termed Vaganov-Shashkin (VS), reproduces the critical features linking climate variability to tree-ring proxy formation. We discuss recent test simulations of tree-ring width data from a variety of sites and spatiotemporal scales. Our experiments demonstrate that the model skillfully reproduces observed patterns of tree-ring growth across a range of environments, species, and scales. Model performance is found to be robust to parameter estimation. We discuss present and future applications of the VS model, including exploration of the biological basis of emergent phenomenon and prediction of the influence of climate change on conifer tree growth dynamics. Introduction Climatic variation is a major factor affecting tree ring growth and wood formation. Accordingly, there has been a significant amount of dendroclimatology research focused on the extraction and validation of climatic variability from tree ring data series (Fritts 1966; Cook and Jacoby 1977; Fritts et al. 1979; Hughes et al., 1984; more recently, see for example Stahle and Cleaveland 1992; Grissino-Mayer 1996; Villalba et al. 1998; Briffa et al. 2001, 2004; Cook et al. 2002; Pederson et al. 2004; Watson and Luckman 2004; Salzer and Kipfmueller 2005). The main morphological and anatomical characteristics of tree rings bearing the climatic signal have been described: tree ring width, density profile, cell diameter, and cell wall thickness (Vaganov 1996). Although stable isotope methods and models are increasingly valuable (McCarroll and Loader 2004), the most widely-used tree ring characteristics for climate reconstructions have been tree ring width and maximum latewood density. Such reconstructions have relied almost entirely on statistical covariation of the proxy observations and climate data during the modern period. But H.C. Fritts saw the vital importance of concurrent research into the anatomical pattern of tree ring formation in response to changing environment. He was one of the first to make a careful