“Tree failure” refers to large woody parts or an entire tree failing mechanically and usually falling to the ground. Everyone knows that wind can damage and destroy trees catastrophically. Unusually strong wind, perhaps interacting with snow and ice on the canopy or saturated soil, can break branches and stems at any height, or cause uprooting of the tree.
When an entire tree goes over, foresters often apply the term windthrow. Strictly speaking, though, real windthrow is uprooting – most of the larger root system comes out of the ground, and where roots break, they are mostly sound, not decayed.
In many cases, the vast majority in many forests, the breaking, snapping, and partial uprooting of trees is associated with decay in the stem, butt, or roots. Start looking at downed trees when you spot them. You can even do this post-mortem exam with trees that went down years ago. Carefully examine places where wood broke from wood. These are the failure surfaces. Several things to look for:
- Is there obvious, advanced decay at these surfaces? If you see it right on the failure surface, you can be pretty sure it was decayed before the failure. Decay of exposed surfaces looks quite different from decay that develops inside the wood and then is exposed by failure.
- Is the failure surface splintery, indicating failure of sound wood, or is the more breakage across the grain, known as a brash failure and indicating failure of decayed wood? Remember, decay need not be advanced or obvious to significantly decrease wood strength. Splintery vs. brash failures are more difficult to distinguish in hardwoods, with short fibers, than in conifers, which have long fibers.
- Do you see any signs of a decay pathogen around the failure surface? You may see rhizomorphs and/or fans of an Armillaria species if it is a root or butt failure, or mycelium associated with advanced decay. Together with noting what type of decay is present, if any, these are clues to diagnosing the disease.
Chronic Wind Stress
In some areas of the world, forests at high elevations experience very strong and frequent winds. In the mountains of New England and in Japan, the tree line is thought to be determined largely by wind rather than other ecological determinants such as growing season. There are apparently several mechanisms:
- one is rocking of trees in the wind, which snaps smaller roots off and causes injuries to roots as they rub on rocks;
- another is collisions between branches in the crown between two trees, breaking off frozen twigs and leading to a phenomenon called “crown shyness;”
- another is crown damage due to ice storms and rime ice. Probably the root damage is most important.
At the highest forested elevations, this mortality is organized into waves that progress slowly up the mountain. The forests there are fir and the phenomenon is called “fir waves.” At lower elevations, in spruce-fir forests, the mortality is less organized and scattered around.
Water – Not enough
Can be a chronic problem for trees that can be characterized as a disease. Acute symptoms include wilting, but over time slower growth, then twig and branch dieback can be major symptoms. Often, weak secondary pathogens can be involved in killing back twigs and branches of drought-stressed trees.
Moisture stress of trees is a function not only of precipitation, but also soil properties, temperature, humidity, and wind. For most plants, a soil with balanced particle sizes and lots of organic matter is ideal, partly because of moisture. The finer particles and organic matter increase the moisture-holding capacity of the soil. Soil that is high in sand and low in clay and organic matter could lead to drought stress right after soil drains.
Water – Too much
The other extreme is just as bad. Actually, the problem is insufficient O2. And just like not enough water, there can really be two causes: excessive rainfall or flooding, and soil structure that impedes drainage. Just as a sandy soil drains too quickly, holding little moisture, a soil high in clay (often called “heavy” soil) tends to have poor drainage and limited air spaces. Rootlet mortality and attack by oomycetes are common consequences. Soil compaction and hardpans have a similar effect. Trees that do well in wetlands usually tolerate low-O2 environments and consequently do well in cities, where pavement and compaction reduce soil aeration. The U.S. Forest Service has online info on flooding and its effects on trees.