Paleotemperature estimation from dicotyledonous wood anatomical characters

Following an earlier study documenting the relationships between wood anato mical features and climate, we examine the utility of dicotyledonous wood a ssemblages in assessing paleoclimate. The wood anatomy of dicotyledons from modern forest sites in North, Central, and South America, England Africa, Malaysia, and the Pacific islands was used to derive equations to predict s ite mean annual temperature. The best equations gave estimates within 5 deg rees C at validation sites. Because trees are physiologically active over a wide range in temperature throughout the year, it would be surprising if w ood anatomical characters were more finely tuned to temperature. Mean annua l temperature (MAT) was estimated from five fossil wood assemblages. Yellow stone, Wyoming (early Eocene) Clarno Nut Beds, Oregon (middle Eocene) Post, Oregon (late Eocene); Vantage, Washington (middle Miocene); and Fejej, Eth iopia (Miocene). We used the two best equations to estimate paleotemperatur e at each site; the MAT estimates differed by 0.1 degrees C at Fejej and 5. 7 degrees C at Yellowstone, with differences intermediate to these at the o ther fossil sites. Compared to present-day values, the paleotemperature est imates indicate that, at middle latitudes of the Northern Hemisphere, mean annual temperatures were warmer in the Eocene and Miocene, whereas the near -equatorial Fejej site was the same temperature in the Miocene as it is tod ay. Where possible, we compared the MAT estimates obtained using fossil woo ds with those obtained using fossil leaves, and found discrepancies of up t o 13 degrees C. These differences may, in part, reflect the complicating ef fect of evolutionary trends in wood anatomical characters that may have bee n independent of climate change.