Partner profile: Trees tell the history of fire

Issue 8 > Partner profile page 1 > page 2

Slice of Shortleaf pine.

This slice of tree shows a wealth of historical information about forest growth and fire.

NPS photo.

As it turns out, trees have a lot to say about fire history. After all, they’ve been there for years--sometimes centuries--to see it! Researchers Dr. Henri Grissino-Mayer, Dr. Sally Horn, and their graduate students from the Department of Geography at the University of Tennessee have spent years figuring out just where, when, and how fire burned in these mountains in the past. Whether living or dead, trees are the key: as they grow they put on wood in rings, similar in some ways to how we grow taller and put on weight. Each tree ring consists of a layer of thin-walled cells formed early in the growing season (called earlywood) and thicker-walled cells produced later in the growing season (called latewood), Professor Grissino-Mayer explains. The distance between the beginning of the earlywood formation and the end of the latewood formation is one annual ring.

Fire scars in a Shortleaf pine.

These fire scars show regular fires in this Shortleaf pine's life.

NPS photo.

These rings can tell us stories: their width tells us how much a tree grew each year (wide rings mean lots of growth), their shape and evenness from year to year tells us about disturbance (for example, if a gap opened and they were able to burst into the canopy), and their patterns overall compared with other trees’ rings tell us about events such as fires or damaging insects at the landscape level.

In particular, Henri Grissino-Mayer and his research team are interested in fire history. Knowing when, where, and how fire happened in the past helps us plan for—and, in the case of the national park, manage—fires in our forests’ future. The study of tree rings to understand the past is called dendrochronology: dendron = tree, chronos = time, logos = the science of. The study of tree rings to understand fire history has an added piece: “pyro,” to make it dendropyrochronology. You can read more about these terms and much more about tree ring research at The Ultimate Tree Ring Web Pages.

How do scientists see the rings inside trees? We don’t usually cut down living trees just to see inside, although that was pretty common in the past (read about the martyred bristlecone pine). Instead, scientists can take samples of trees in two ways: (1) if a tree has died, they can cut a slice out of the standing snag (trunk) or fallen log, or (2) if the tree is alive, they can extract a narrow core from the tree in a way that doesn’t harm it. Tree coring, as the second option is called, is very common. It results in a pencil-shaped section of tree that shows rings from its outer edge, at the bark, to its inner core.

What materials would you need to core a tree? Click here to find out.

A fire-dependent pine-oak forest in the western part of the park.

Pine-oak forests such as this on a high ridge in the western part of the park depend on fire. Even with fire suppression, lightning ignited fires here through the 20th century.

NPS photo.

What have tree rings told us about fire? While the story is pretty complex, tree rings from the oldest trees in the park—those that were saplings in the early 1700s—tell us that fire used to be common, especially on ridges, on south-facing slopes, and in the western side of the park. All of these locations are drier than the areas surrounding them. Every five to seven years or so, small fires used to burn away the underbrush, leaf litter, and small trees that couldn’t tolerate the flames. These left tall, open stands of yellow pines (such as Shortleaf pine) and oak, all fire-tolerant species. They also left tell-tale evidence of their regular occurence in fire scars on the lower portion of pine trees. In most areas of the park, it’s very clear when this pattern of regular fires stopped: in the early 1930s, the park began suppressing fire, and tree rings changed. It's hard to find fire scars from the mid-20th century on trees in many areas (the tree pictured above, showing fires in the 1940s and 1960s being an exception). The forests’ fire-intolerant species, such as red maple, white pine, and black gum, began to thrive. Stands of these species have taken over some areas since fire disappeared, which changes the types of animals that live there, too. With the reintroduction of some controlled fire, the stands start to return to the mix of pines and oaks that existed for centuries before.

Go to page 2: Fire and climate research in the Smokies to read about research to uncover the history of fire in these mountains.

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