ESTIMATING PALEOCLIMATE BASED ON CONTEMPORARY FOREST INVENTORIES: A SEARCH FOR THE MODERN ANALOG OF THE FLORISSANT FOSSIL FLORA

We used 493 tenth ha forest inventories from North, Central, and South America, compiled from the online database SALVIAS, to model the relationship between higher taxonomic composition and climate in modern plant communities. We applied this model to estimate climate for the Eocene Florissant fossil flora of Colorado, and compared our results to groupings obtained from cluster analysis of combined fossil and contemporary plant assemblages.

Canonical correspondence analysis of site-by-taxon and site-by-climate matrices from a training set of inventories was used to develop taxonomically-based predictions of key climatic variables for the remaining sites. Site-by-taxon matrices were constructed using presence-absence of genera and families to permit comparison with the fossil flora. Predicted values of total annual precipitation, length of the dry season, mean annual temperature, and variance of mean monthly temperature corresponded closely to actual values for contemporary samples. For the Florissant flora, our model estimated values of temperature and precipitation equivalent to a modern-day wet middle-elevation forest at subtropical latitudes. UPGMA cluster analysis based on genus presence-absence placed Florissant in a position intermediate between the cloud forests of northeastern Mexico and the deciduous broad-leaved forests of the eastern United States, in agreement with results of the taxonomically-trained climate model.

Although the well-documented existence of “non-analog” plant communities suggests caution in predicting past from present taxon-climate associations, the effectiveness of higher taxa as predictors of climate for modern assemblages suggests that physiological and evolutionary traits related to climatic tolerance are highly conserved within lineages. Thus, associations between environment and taxa among contemporary biotic assemblages may shed light on those of past assemblages as well. Our identification of the cloud forest of northeastern Mexico as the closest modern analog of Florissant should be viewed as tentative given the large number of Florissant taxa also known from modern Asian forests. Future analyses will be improved by the planned addition of Asian inventories to the SALVIAS database.