Petrified Wood as Climate Archive
When a tree is rapidly buried in volcanic ash or mineral-rich sediment, silica gradually replaces organic cell walls molecule by molecule, preserving the cellular anatomy in exquisite detail. Growth rings in this petrified wood — alternating bands of large thin-walled earlywood cells and small thick-walled latewood cells — record the rhythm of ancient seasons just as clearly as living trees record modern ones.
Reading Ring Width Patterns
Wide rings indicate years of abundant moisture and warmth; narrow rings indicate drought or cold stress. By measuring ring widths across a transverse section, paleobotanists extract a time series of growing-season quality. The mean sensitivity statistic quantifies how variable this series is — high sensitivity means the tree was growing near its environmental limits, making it a sensitive climate recorder.
Earlywood, Latewood, and Seasonality
The proportion of earlywood (large cells, formed during spring growth flush) to latewood (small cells, formed during summer/autumn slowdown) indicates the character of the growing season. A sharp earlywood-latewood transition signals an abrupt change from wet/warm to dry/cold — strong seasonality. A gradual transition suggests a more equable climate with a long, gentle growing season. Ring-porous versus diffuse-porous wood types further distinguish seasonal from aseasonal climates.
False Rings and Frost Rings
Not all ring-like features represent annual boundaries. False rings form when mid-season drought causes a temporary growth pause followed by resumption — creating an intra-annual density fluctuation. Frost rings contain a band of collapsed and deformed cells caused by a hard freeze during the growing season. Both features are valuable paleoclimate indicators: frost rings in Cretaceous polar wood, for example, demonstrate that even greenhouse climates experienced occasional freezing at high latitudes.