Evaluating The Effect Of Climate In A Sierran Mixed Conifer Forest

Forest growth and yield models such as CACTOS and PROGNOSIS have typically modeled future growth dynamics using unchanging climatic data. These models, applied to a future where the climate is changing, can lead to important discrepancies between actual and predicted growth rates. Yeh and Wensel (2000) first examined these discrepancies in CACTOS performance. They identified summer temperature and winter precipitation as key climatic variables influencing tree growth and developed indices to adjust growth based on the climate. However, the relationships established by Yeh and Wensel were formulated using the CACTOS growth simulator, an input-intensive growth and yield model. No independent validation of the climate-growth indices was conducted. This study examines the reliability of Yeh and Wensel’s climate adjustments to the growth model for forest management applications. Biological growth signals were removed from long-term annual growth records. These empirical records were derived from tree ring analyses. A smoothing spline was used to quantify the biological growth signal (i.e., low-frequency variation in increment). A climate signal (i.e. the residual) was derived by subtracting this spline from the long-term record. The resulting residuals were correlated against the predicted climate growth signal as given by Yeh and Wensel’s model. The correlation of climatic influence on growth for different species and crown classes was also considered. The results of this study indicate that Yeh and Wensel’s model provides growth estimates that reliably inform management decisions. However, the relationship between climate and growth is stronger in dominant tree classes, indicating that climate more predictably affects more dominant trees. For sub-dominant trees, climatic variables had little correlation. It does not appear that competition between dominant and sub-dominant trees is confounding the correlation between sub-dominant tree growth and climate. Suppressed or understory tree growth may be more dependent on edaphic and/or microclimate gradients