Splits or cross-sectional cuts in tree trunks often reveal interesting markings in the wood. These markings might be caused by pathogens, insect activity, fractures or cracks in the wood, or other factors; without guidance from an expert, it’s easy to misinterpret what you’re seeing. Thanks to Matt Bertone, diagnostic entomologist with the NC State University Plant Disease and Insect Clinic; and Mike Munster, plant disease diagnostician, for assistance with the following two examples.
Below is a cross sectional view of a branch from a dead swamp bay (Persea palustris). The dark squiggly lines are known by plant pathologists as zone lines. These lines mark the advance of decay fungi, specifically the demarcation between two distinct fungal colonies. Wood containing zone lines is known as spalted wood by woodworkers, and is highly valued for artistic effect in specialty wood products. The dark marking towards the lower center of the stem indicates the cross-section of an ambrosia beetle tunnel.
Next, we have the side view of the split trunk of a loblolly pine (Pinus taeda) showing a relatively straight tunnel made horizontally into the wood by a species of ambrosia beetle. The black staining is from the fungus the beetles introduce and tend to as a food source. In contrast, pine beetles (such as the famous Southern Pine Beetle) burrow under the bark along the trunk, without tunneling directly into the wood.