Climatic variation and tree-ring structure in conifers: empirical and mechanistic models of tree-ring width, number of cells, cell size, cell-wall thickness and wood density

Variations in tree-ring structure are simulated with (1) an empirical model using monthly climatic data as statistical predictors and (2) a mechanisbc model using daily climatic data as growth limiting conditions. The empirical model calibrates standardized ring-width and density chronologies of tree-rings with monthly temperature, precipitation or Palmer Drought Severity Index. The empirical model can be used with any tree-ring and climatic data set to evaluate linear, curvilinear or interactive relationships, to examine different types of calibration procedures, to calculate growth response to climatic factors, to compare the responses of different data sets, to examine a number of environmental questions about climatic change and to detect possible effects of altered environmental conditions on growth such as those caused by air pollution. The empirical model is applicable to any species and site where there is a tree-ring chronology and a climatic record of sufficient length. The mechanistic model uses mathematical equations to simulate processes affecting cell size, cell-wall thickness and wood density variations for individual rings using precipitation, temperature and humidity measurements. Limiting conditions are estimated from temperature, day length and a calculated water balance. The mechanistic model has been validated for Pinus sylvestris L. from dry sites in south Siberia and for P. ponderosa Laws. from southern Arizona. The results from both models are instructive as to what factors are important to the growth of different ring structures and when these factors become limiting. While the models will be developed further, they have already contnbuted to an understanding of tree-nng and climate relationships and have provided a number of answers to certain environmental questions. The empirical model is used in tandem with the mechanistic model to obain important insights on relationships that the mechanistic model calculations lack.