Gregory, K.M., submitted, Paleoclimatic Implications of Tree-ring Growth Characteristics of 35 Ma Sequoia affinis from Florissant, Colorado as Compared to Modern Coast Redwood and Giant Sequoia, in Evanoff, E., and Gregory, K.M., eds. GSA Special Paper on the Florissant paleovalley: Denver Natural History Museum

Tree-ring width and variability are functions of the interplay between climate, site conditions, and genotype. In studies of Holocene wood, ring widths can be calibrated with year-to-year instrumental records of climate to identify the climate signal, and can then be used to retrodict past climate. A similar method can be used with pre-Holocene wood; descriptive statistics of fossil ring-width series are compared to those from the nearest living relatives to determine the climate signal. The general paleoclimate can then be qualitatively estimated. Studies on modern trees suggest that the most useful paleoclimatic indicator is mean ring width, which is a measure of overall growth. Interannual variability, total variability, intercorrelation within and between trees, and percent missing rings are indicators of environmental stress and the genera's position within its ecological range. Comparison of ring-width series from 34.2 Ma Sequoioxylon pearsallii from Florissant, Colorado to series from modern coast redwood (Sequoia sempervirens) and giant sequoia (Sequoiadendron giganteum) indicates that the Florissant trees had higher mean ring widths than their modern counterparts, and similar ecological statistics. Thus the Florissant redwood experienced a more favorable climate. Two of the fossil stumps crossdated, which supports the suggestion, based on surveying, that they are from a single forest. Comparison of the modern climate for coastal California and foliar physiognomic-based estimates of the Florissant paleoclimate from the literature indicates that at Florissant, mean annual precipitation was less, though a greater proportion fell during the growing season, mean annual temperature was similar, and the mean annual range of temperature was greater than at the modern coast. The increased growth of the Florissant trees suggests that either (1) the higher growing season precipitation at Florissant created a more favorable soil moisture balance than at the modern coast, or (2) both sites had similar effective growing season precipitation because of the higher summer temperatures at Florissant; however the valley bottom sites of the fossil trees were more favorable than any of the modern sites.