What Are the Causes of a Leaning Tree on a Hillside?

Soil Creep

• Soil creep is the slowest mass movement process that can occur on an unstable slope. Affected by various climate and geological factors, unconsolidated or loose soils gradually slide or creep down the slope at rates generally equal to 1cm or less per year. Over time, the creeping soil forces trees, fences, utility poles and other structures into leaning or angled postures. Trees often develop a curved trunk when affected by soil creep as they attempt to grow vertically while adjusting to downward soil movement.

Causes of Soil Creep

• Soil creep is fairly common in hilly and mountainous geological areas. Seasonal wet-dry climate cycles can induce soil creep as moisture regularly infiltrates then drains from soil on slopes, especially if the soil is underlain by an impervious type of bedrock. Freeze-thaw cycles also contribute to soil creep. Continual contraction and expansion or heaving cycles destabilize soil. Contributing geological factors also include nearby, low-grade seismic activity. Human activity also can cause soil creep. Road construction and building processes that fail to stabilize soil on nearby hillsides can lead to soil creep and landslides.

Slumping and Landslides

• Slumping, sliding and flows are types of mass wasting phenomena that advance more quickly than soil creep. Hillside soil that is beginning to slump or slide can force trees into leaning angles but without the characteristic curved trunks seen with soil creep. Slumping and slides occur when soil on a slope becomes oversaturated due to heavy rains, construction grades were not equalized at the slope's base, the slope's base is undercut by erosion from a river or ocean tides and seismic activity occurs. Hillsides prone to slump are also at risk for rapidly developing mudflows and landslides. These processes present dangerously unstable conditions.

Practical Considerations

• Soil creep is usually a slow enough process that sometimes mitigation strategies can be put in place to prevent property and infrastructure damage. Careful consideration from geologists or engineers, however, should be included in all building plans. The same is true for slump-, slide- and flow-prone areas. Unlike soil creep, those situations pose imminent threats to human life as well as to buildings and roads. Hiking along slump- and slide-prone natural areas is a risky activity. If a mass wasting event occurred previously on a hillside, it probably will occur again.