by Whit Gibbons

January 12, 2014

Q: My understanding is that longleaf pine was preferred over loblolly pine for turpentine production. Is it because longleaf pines produce more sap? Also, what explanation did you once give for why a tree that lives in a habitat that frequently catches fire under natural conditions would produce a highly flammable substance like turpentine? Seems like that would be self-destructive for the species.

A: Turpentine, a substance characteristic of pine trees and other conifers, is composed of a mixture of resins and volatile oils. The by-products have been used in a wide variety of applications including caulking for wooden ships, solvent for paint and varnish, and as an ingredient in insecticides, cleaning agents, and shoe polish. Turpentine products were once even used for medicinal purposes. A great turpentine industry once flourished in the South, where pine trees, especially longleaf and slash pine, were tapped for turpentine, the way sugar maples are tapped for sap to produce maple syrup.

The turpentine industry took advantage of pine trees' natural response to injury. If the bark is broken, the tree begins to ooze a sticky, yellowish sap that eventually dries and seals the wound with a covering of resin. The material is resistant to most wood-eating insects that might further damage the tree. The liquid can be distilled to produce turpentine. When ship building was at its height, longleaf pine forests were abundant in the Coastal Plain and therefore nearest the coast. Also, longleaf reportedly produced more and thicker resin than other pine trees. Slash pine was more commonly used in Florida but, when available, longleaf was the preferred choice for turpentine production.

Both longleaf and slash pines have a characteristic that makes turpentine production seem counterintuitive. They live in what is known as a fire climax community. This means that, historically, trees and other plants that persisted in a longleaf pine community had to survive periodic fires that swept naturally through the forests, primarily as a result of summer lightning strikes. In fact, many ecologists criticize the prescribed burning by some forest management programs in winter because natural fires would usually have occurred in summer. Presumably, plants and animals in regions that experienced frequent fires evolved to tolerate warm weather fires.

Longleaf pines are well adapted to survive fires at intervals of only a few years. The early so-called grass stage of a young longleaf can be burned back to the ground and then, unharmed, resprout the same season. The thick bark of the pine tree is resistant to fire (and has no turpentine in it) so that a larger tree is also immune to a fast-burning forest fire.

Why would a pine tree that under natural conditions was sure to be subjected to numerous fires during its lifetime be saturated with readily flammable turpentine? One explanation is that turpentine works to the tree's advantage after it has died.

Here's how. A pine tree dies, and months or years later a fast-burning fire sweeps through the area. The dead tree, especially the standing stump where the turpentine has collected, becomes an inferno and burns up completely, along with any dead needles or limbs on the ground. Thus nutrients that were bound inside the dead tree are returned to the soil and once again become available for other pine trees.

Why would this be of advantage to the dead pine tree? The simplest answer is that most nearby pine trees would be descendants of the burned tree; thus the tree would be returning the nutrients to its own kin. In addition, periodic fires would eliminate other trees that were not fire-tolerant species and that might otherwise compete with the pine trees. So instead of engaging in self-destructive turpentine production, pine trees have worked out an efficient and effective mechanism to deal with periodic fires over evolutionary time. In one of Mother Nature's paradoxes, being saturated with a flammable material can be beneficial to a pine tree in the long run.

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