The principle of site selection developed by A.E. Douglass and E. Schulman has guided dendrochronologists in the development of long climate sensitive tree-ring chronologies worldwide. The longest and most sensitive tree-ring records of drought are often found in arid-site conifers grown on steep exposed positions where only precipitation can recharge the soil moisture reservoir essential for tree growth. The discovery of the longevity and direct precipitation sensitivity of swamp-grown baldcypress (Taxodium distichum) in the southeastern United States was a surprising contradiction to the site selection concept, and we now realize that climatic sensitive tree-ring chronologies can be developed for a wide variety of species and microenvironments.

Results/Conclusions

Long chronologies of baldcypress have been developed throughout the southeastern United States and Mexico, and they all exhibit a direct correlation between growing season precipitation and ring width, in spite of frequently flooded site conditions. This counterintuitive response can be explained by the stratification of the fine root system in response to the vertical gradient in dissolved oxygen, redox potential, and moisture flux. The feeder root system often becomes perched in the near-surface water column and soils, rendering the trees vulnerable to root drydown during drought. Excellent reconstructions of precipitation and drought indices have been derived from baldcypress, and the estimates have been validated against instrumental observations. Baldcypress was heavily exploited for lumber production, but remnant populations of very old trees survive across the Southeast and have provided a network of millennium-long tree-ring chronologies for climate and environmental reconstruction.