Wind and Water


A windthrown Picea abies (Norway spruce) with an exceptionally shallow root system, which simply lifted off the underlying soil. Hochharz, Germany.

Tree failure

“Tree failure” refers to large woody parts or an entire tree failing mechanically and usually falling to the ground.  Wind can damage and destroy trees catastrophically.  Strong wind, perhaps interacting with snow and ice on the canopy or saturated soil, can break sound branches and stems at any height, or cause uprooting of the tree. This is most common in unusual, catastrophic wind events.

In the vast majority of cases in many forests, the breaking, snapping, and uprooting of trees is associated with decay in the stem, butt, or roots ​[4, 8, 17]​.  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 there 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 (tracheids).
  • 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.

The loss of strength (or “moment capacity” in physics) due to decay is related to size and location of the decayed area ​[1, 5, 13]​. This becomes quite important in evaluating the safety of trees in developed areas, which is discussed more thoroughly in the Hazard Tree pages.

When an entire tree goes over, foresters often apply the term windthrow.  Strictly speaking, though, real windthrow is uprooting – most of the larger roots come out of the soil, and where roots do break, they are mostly sound, not decayed. Again, strictly speaking, this is a soil failure, not a root failure, and is generally less common than failures associated with decay.

Tree failures, caused by decay pathogens and/or wind, play important ecological roles in forests ​[2, 9, 11, 16, 18, 19]​. They create canopy gaps that present opportunities for establishment of new individuals, often different species. They create heterogeneity in forest age and size structure and structural diversity, with patches of differing ages and species. Forest development and maturation create conditions favorable for these pathogens, and wind, snow, and ice function as secondary mortality agents ​[8]​.

Chronic Wind Stress

Crown shyness in Abies balsamea (balsam fir) in the White Mountains of New Hampshire, northeastern USA. Photo courtesy of Dr. Tom Harrington.
Rime ice on Abies balsamea (balsam fir) on Wildcat Mtn., New Hampshire, northeastern USA. Photo courtesy of Dr. Tom Harrington.

In some areas of the world, forests at high elevations experience very strong and frequent winds, causing chronic wind stress. In the mountains of New England, USA, 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 ​[3, 12]​:

A fir wave followed by the killing front of a second one. Arrows represent the direction of movement (generally direction of prevailing wind) and point to the killing front. Regeneration rises following death of the overstory. After Sprugel 1976.
Witches’ brooms caused by rime ice leading to broken twigs (can also be caused by winter injury). Repeated killing of tips stimulates branching behind the dead tips. White Mountains, New Hampshire, USA. Photo courtesy of Dr. Tom Harrington.
  • rocking of trees in the wind, which snaps smaller roots off and causes injuries to roots as they rub on rocks. This may be the most important mechanism;
  • collisions between branches in the crown between two trees, breaking off frozen twigs and leading to a phenomenon called “crown shyness;”
  • crown damage due to ice storms and rime ice.

In high subalpine forests, this mortality may be organized into waves that move about 1.25 m yr-1 up the mountain ​[7, 10, 14]​. Such forests are generally pure Abies spp. and the phenomenon is often called “fir waves” in English. The featured image at top shows fir waves in Abies balsamea (balsam fir) on Mt. Moosilauke, as seen from the Appalachian Trail in New Hampshire, USA.

In Japan, they are called “Shimagare”, meaning dead tree strips. They are prominent in several mountain ranges of central Honshu. Mt Shimagare in the Yatsugatake Mountains is named after them ​[7]​. They have been studied there since 1933. They were recognized as moving waves and attributed primarily to wind and stand dynamics.

At lower elevations, in spruce-fir forests, the damage and mortality are scattered and less organized. However, when an opening occurs in a stand, such as a road, ski trail, or clear-cut, the forest on the lee side of the opening may experience simultaneous overstory mortality due to chronic wind stress, much as in a fir wave ​[3, 6, 7, 12, 15]​. The mechanisms of damage at such artificial forest edges appear to be the same as those operating in natural Shimagare forests.


Water – Not enough

When insufficient water is a chronic problem for trees, it 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

Water is certainly one of those good things that you can have too much of.  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.

More general information is available on abiotic diseases and injury.


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