Characterizing Strain and Load Transfer in the Root Flare

2014 | Gregory Dahle, West Virginia University

Trees are subjected to various loading events throughout their lives. In addition to self-loading, they are subjected to wind, ice and snow loads. Excessive loading can cause damage to trees leading to failure. Much of the research dealing with whole failure has focused on bending stress in the trunk, or with root system stability. Researchers have suggested that a key area of interest is the root-soil plate complex, as the root-soil plate is thought to be the most important region in terms of root stability. We believe that the industry would benefit from a better understanding of how loads are carried from the stem through the root flare into the root-soil plate. Measuring loading in the root flare is difficult, yet the use of stereo photogrammetry allows the measurement of strain (wood deformation) in urban trees, which is a direct response to loading. This project seeks to determine the pattern of strain distribution in the root flare to determine how load transfers from the trunk to the root-soil plate. Learning how strain, and therefore load, is carried through the root flare will help understand how trees resist overturning. The impact from this research will inform how trees distribute loading into the root-soil plate which will increase our knowledge of tree stability during tree risk assessments. This knowledge will help tree risk assessors and arborists better evaluate the likelihood of failure of individual trees.

Study Results

This research project was conducted at West Virginia University to determine how strain builds up along the first order roots of urban trees. We employed static load trials on 15 pin oak (Quercus palustris) trees, inducing a 0.1° tilt at the base of the trunk. Tissue deformation (strain) was captured using a stereo cameras and an ARAMIS digital image correlation system. The magnitude of strain was found to be the same between the windward and leeward roots. It appears that the load and strain is equally distributed in the root system during static load trials, at least in terms of the direction of the loading.

This information is important to practitioner as it demonstrates that both the leeward and windward roots are equally important in terms of tree stability. As trees are increasing important in the urban landscape, arborist should take care to preserve the roots systems to maintain the health and structural stability of individual trees. The knowledge gained from this study suggests that arborists should consider all structural roots important in terms of load distribution, regardless of dominant wind loading regimes.

Year: 2014

Funding Duration: 1-3 years

Grant Program: John Z. Duling

Grant Title: Characterizing Strain and Load Transfer in the Root Flare

Researcher: Gregory Dahle

Key words:

Peer Reviewed Publications from Grant:

General Audience/Trade Publications:


  • Beezley, K. 2016. Determination of strain patterns across the root-stem transition zone in trees. Masters of Science Thesis. West Virginia University.
  • Beezley, K, G. Dahle. 2015. Strain measurement within trees across the root-stem transition zone. International Society of Arboriculture 91th Annual Conference, Orlando FL. August 2015. Presentation to AREA Student Travel Grant.

For more information on this project, contact the researcher via TREE Fund at