David W. Stahle is a professor of Physical Geography and the Conservation of Natural Resources at the University of Arkansas. He received his B.A. in Anthropology from the University of Arizona in 1973, his M.A. in Archaeology from the University of Arkansas in 1978, and his Ph.D. in Geography from Arizona State University in 1990.

Stahle's research interests include all aspects of dendrochronology (the study of tree rings), particularly climate change and the proxy evidence for past variation in the El Nino/Southern Oscillation and other large scale atmospheric circulations. He has developed GIS-based predictive models for the location of ancient forests, and is conducting active research in the United States, Mexico and Africa. Stahle's research is funded by NOAA, NSF, NPS and the USGS and he has published in a variety of journals including, Science, Nature, Journal of Climate and Bulletin of the American Meteorological Society.

Stahle's response:
There are many sound arguments against the logging of our last remaining old-growth forests in the USA, but the threat of global climate change is probably not the strongest. The remaining old-growth forests constitute just a small fraction of the country, and the USA constitutes just a small fraction of the land area of earth. So logging what's left of old growth in the USA, bad though that would be, probably would not have a big impact on global climate, particularly if these ancient forest sites were allowed to regenerate into some kind of forest cover. It should be acknowledged, however, that net deforestation worldwide is a different matter. Climate simulations have indicated that replacing the triple canopy virgin rainforest in Amazonia with cattle pasture would probably indeed have a drastic impact on the regional climate, and these changes would surely alter the global atmospheric circulation as well.

Stahle's response:
Tree rings are shedding new light on several historical mysteries. White spruce trees from Alaska indicate that the coldest summer in 400 years occurred in 1783. This unusual cold was probably brought on by the 1783 eruption of the Laki volcano in Iceland, and appears to have been responsible for widespread starvation and population decline among the Inuit during the "Time summer Did Not Come" (Gordon Jacoby and others, Quaternary Science Reviews, 1999).

The 16th century "megadrought" across North America (roughly extending from 1540-1590) has been linked with the disappearance of the English Lost Colony of Roanoke Island (1587), the abandonment of the Spanish colony of Santa Elena at Parris Island, South Carolina (1587), the abandonment of Tewa, Keres, and other Puebloan villages in New Mexico (mid-16th century), and with two of the greatest human mortality events in New World history....the cocoliztli epidemics of 1545 and 1576 in Mexico when millions died from hemorrhagic fevers possibly associated with a rodent vector leveraged by extreme drought conditions (Stahle and others, Eos, Transactions of the American Geophysical Union, March 21, 2000).

Professor Mike Baille describes tree ring and other evidence for a climatic catastrophe in AD 536, which he postulates might have been due to a comet impact with earth (Exodus to Arthur: Catastrophic Encounters with Comets, Batsford Ltd., London, 1999). The evidence for a 536 climate extreme is impressive and includes record or near-record narrow tree rings in Fennoscandia, Ireland, North America, Chile, and Mongolia, frost rings in Mongolia, historical evidence for dry fogs, unusual cold, crop failure, and famine over Europe and the Middle East, and Chinese records of dim sun, frost damage to crops and famine. A cataclysmic volcanic eruption might also explain these global scale climate anomalies of 536, but undisputed ice core evidence for a major eruption at 536 or shortly before has yet to be produced.

Val LaMarche (deceased) and Katie Hirschboeck describe tree-ring evidence for a massive volcanic eruption in 1627B.C., which is believed by many analysts (but by no means all) to have been the catastrophic explosion of Santorini on the island of Thera, which may have played a role in the collapse of the Minoan Civilization.

Tree rings and other annually-resolved paleoclimatic proxies such as glacial ice layers and coral bands record detailed environmental histories and will no doubt provide many interesting new insights into history and prehistory.

Stahle's response:
Radiocarbon dating is an outstanding method for age determination covering the late glacial and post-glacial period. In any dating problem, it is always preferable to have multiple dating methods providing concordant results. But there is a special relationship between dendrochronology and radiocarbon dating, because wood samples that have been exactly dated to the calendar year by dendrochronology have then been analyzed for radiocarbon content. It was found early on in the development of radiocarbon dating that carbon dates systematically deviated from true calendar age determined from tree rings. This systematic deviation is due to long-period variations in the solar magnetic field that would alternately shield the earth from incoming cosmic radiation and thereby alter the production rate of atmospheric radiocarbon. This systematic error in carbon dating is now well recognized and routinely corrected. So it is entirely true to say that dendrochronology has helped improve the accuracy of radiocarbon dating over the last 10,000 years or so during which we have continuous tree-ring chronologies.

Tree-ring dating is the most accurate and precise dating method in geochronology, capable of dating determinations to the season of a specific calendar year. However, the oldest known trees on earth are just a shade less than 5000 years old, so tree-ring dating is really only possible during the post-glacial period. Dendrochronologists have been able to extend exact tree-ring chronologies back to 10,000 years in rare circumstances by including older wood found in archaeological or subfossil deposits, but most tree-ring chronologies are less than 1000 years long.

There are many other natural timekeepers. The horizontal sedimentary rock strata seen in the sidewalls of the Grand Canyon of the Colorado River have to be among the most famous pages of earth history, recording deep geological time with the evolutionary changes in life witnessed in the beautiful rock strata of western North America. But there are also many excellent natural archives that record annual changes, and my personal favorites include annually banded corals (the dating of which is referred to as sclerochronology), varved sediments found at the bottom of proglacial lakes and in a few other lacustrine and marine settings, and annually banded ice found in some glaciers worldwide.

Stahle's response:
In my opinion, we are not doing enough to protect ancient climate sensitive forest trees, banded coral heads of the tropical oceans, or even the annually layered ice found in tropical glaciers such as Quelccaya in Peru or Kilimanjaro in East Africa. Coral reefs need more serious protection from overfishing and water pollution. And it may be ironic, but coral reefs in the tropical oceans and ice sheets at the summits of tropical mountains are both threatened by global warming, and yet many political leaders are in denial about our wasteful overconsumption of fossil fuels and the harmful consequences of that waste to our environment, economy, and even national security.

Ancient forests with giant trees get a good deal of press, deservedly so, but not the dwarfed, gnarly, old-growth trees found on harsh sites throughout the United States and really around the world. Our public vision of old growth tends to perceive redwoods or other huge trees of temperate rainforests, and not the stubby, stunted, and most ancient of trees such as the bristlecone pine, the blue oaks of the California foothills, or the craggy chestnut oak still found along exposed positions of the Blue Ridge Parkway. Out of ignorance for the true antiquity of these sorts of super senescent trees, they continue to be destroyed for very mundane reasons. You can see an example of this type of low-stature noncommercial woodland at our website on the Ancient Cross Timbers in the south-central USA (http://www.uark.edu/xtimber). I believe that if people realized that many trees in noncommercial woodlands were old at the birth of American democracy, then they might very well have a higher opinion of these austere survivors and the intricate natural histories they preserve in their centuries-long tree-ring series. If we realized that these ancient woodlands on stressful sites scattered throughout the United States were indeed one part of the complex vegetation mosaic that made up the presettlement natural landscape, then we might think twice about scraping away these authentic Americans for chip mills, parking lots, or new homes landscaped with still more Bermuda grass and ornamental pear trees. --