12 Days of Conifers: Coring the Prickly Ponderosa

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Ponderosa pines have long needles in bunches of three, and medium-sized cones with spines that point out and can feel prickly when you pick one up (some people refer to them as “prickly ponderosas”). They can be quite big trees, reaching heights over 200 feet and diameters over 8 feet. The oldest known ponderosa was thought to be 600 years old.

Ponderosa pines are widely distributed across the Western United States. In California, they are found in the Coast Range, Klamath, Cascades, and Sierra Nevada mountain ranges, from 500 to 3,500 feet in Northern California and 5,300 to 7,300 feet in Southern California. Like many California ecosystems, forests in this range have a long history of fire.

In particular, ponderosa pines are adapted to a fire regime of frequent low-intensity surface fires. Ponderosa seedlings might be killed by this type of fire, but larger trees are well equipped to survive a surface fire. They have thick, fire resistant bark, and they tend to self-prune limbs, making it harder for a surface fire to spread to the treetops. Though ponderosa pines can survive surface fires, fire damage may influence its growth, and fire scars will be visible among the tree’s rings.

Today’s photos show USGS scientist and Humboldt State graduate student Zach Wenderott taking tree cores of ponderosa pines in Lassen National Park as part of a dendrochronological (tree ring) study of how prescribed fire affects tree growth. Using a T-shaped tool called an increment borer, he can cut narrow cylindrical samples from the tree (they can fit neatly in a straw). These cores allow scientists to study the tree rings with little damage to living trees. Back in the lab, cores are sanded down so that the rings are clearer and often scanned and analyzed digitally.

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Tree growth patterns are detectable in tree cores because the wood looks different depending on how fast the tree was growing when it was formed. When the tree is growing quickly, the cells are big and thin-walled and the wood appears lighter. When it is growing slowly, the cells are small and densely packed, and the wood looks darker. This is why you see annual rings—there’s a contrast between the wood laid down during the growing season (winter and spring in California) and the dry season (summer and fall in California). Rings appear wider when the tree is growing quickly and conditions are good and narrower when they are poor, as during a drought.

The Lassen study is just one of many examples of USGS scientists using dendrochronology to answer many different questions in forest ecology.

Learn more about some of USGS’s forest research here.

Learn more about tree ring research at USGS here.

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