Ring-width time series obtained from Engelmann Spruce trees growing at high altitude sites in the Rocky Mountains, Colorado, exhibit dominant 11-year spectral periodicities. A significant linear cross-correlation also exists between these series and the Zurich series of annual sunspot numbers. A regression model based on these proxy data has been developed for the annual variation of the 11-year solar cycle. It is established that over the calibration period a very high percentage of the variance (40%) in growth patterns, contributed by a single source, can be explained by solar variation. The model correlates with the Zürich series of sunspot numbers at the 99% significance level post 1870 AD. However, over the total period 1700–1870 AD the comparison was found to be not statistically significant at lag 0. Some possible reasons for this are considered.