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 divers ity of source forests in the fossil record. Single samples of temperat e forest litter relatively consistently record about three-quarters of the source tree species larger than 10 cm diameter at breast height w ithin the surrounding hectare. Tropical and subtropical samples, in co ntrast, 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. F ive combined samples from a tropical forest hectare with about 25-m in tervals between adjacent samples can be used to extrapolate to hectare -based richness estimates, provided tha t sufficient specimens are rec overed from each site. The major differences in methods needed for rec onstruction of source forest richness from forests in different climat ic zones indicates that a climatic filter should be applied to estimat es of plant diversity in the past.