RECONSTRUCTING RICHNESS IN THE PLANT FOSSIL RECORD

A comparison of species richness of leaf litter samples derived from a variety of modern forest types provides a means for estimating diversity of source forests in the fossil record. Single samples of temperate forest litter relatively consistently record about three-quarters of the source tree species larger than 10 cm diameter at breast height within the surrounding hectare. Tropical and subtropical samples, in contrast, contain a very small proportion of leaves of the tree species in the surrounding hectare of source forest, and demonstrate that the forest sampled by a single collection is about 0.1 to 0.125 hectare. Five combined samples from a tropical forest hectare with about 25-m intervals between adjacent samples can be used to extrapolate to hectare-based richness estimates, provided tha t sufficient specimens are recovered from each site. The major differences in methods needed for reconstruction of source forest richness from forests in different climatic zones indicates that a climatic filter should be applied to estimates of plant diversity in the past.