Can tree management reduce trees overturning during storm events?

Some trees fall during storm events in strong wind and rain (these trees were termed as windthrow or wind snap in the paper below) but some trees managed to stay upright. So what factors contributed to these trees to stand firm while the others to fall? For this entry, the following paper would be referred to heavily.

Source: Moore GM 2014, Wind-thrown tree: Storms or management? Arboriculture & Urban Forestry 40(2):53-69

There is a Chinese saying that "big trees attract more wind", and that is true but there is also the assumption (not just by the Chinese) that tree crowns act like sails on a boat/ship, and the total surface area of the foliage were taken in the past. But doing that overestimated the wind force exerted on the tree. Rather, tree crowns behave more like a perforated sieve.

I do not know if calculation were made to estimate wind forces exerted on "sails" or "sieves" for the trees locally, but crowns were reduced or tree were removed I suppose to show that something is being done. It is not clear that crown reduction to reduce the foliage surface area would necessarily reduce the wind load, because depending on which branches are removed, the capacity for mass damping (see previous blog post) may also be diminished.

Most tree roots are near the surface and spreads 3-4 times the spread of the tree crown (also known as the drip line). There are different types of roots in the root system - descending (or vertical, sinker, or oblique) roots, spreading roots, tap roots, and fine roots. Fine roots bind closely to the soil and consolidate the root plate as they are large in number and surface area.

Structural roots are important to tree stability and have to be protected within a critital root zone (Matheny and Clark 1998, Anomymous 2009), or structural root zone (SRZ) - same meaning.
Anonymous, 2009. Protection of trees on development sites, Australian Standard # 4970, Standards Australia, Sydney.

Roots that contribute to the anchorage are the resistance of leeward roots to bending (25%) and the resistance of tap roots and decending roots to uprooting (75%). Therefore the windward roots, which is pulled upwards during toppling, is the most important component in resisting windthrow. When they do get pulled up, the descending roots, if present, often follow intact. If there are no descending roots in the exposed windward side, it is often an indication that they have not been present, which may be important in diagnosing the causes of failure. Failure usually occurs closer to the trunk in wet soils.

Other professions (geotechnical, civil engineering, etc.) have noted the important of descending roots in stabilising slopes, with deeper rooted trees stabilising slopes only to the depth to which descending roots can penetrate (Gray and Sotir 1996).
Gray DH and RB Sotir. 1996. Biotechnical and soil bioengineering slope stabilisation. Wiley and Son, New York City, New York, U.S.

Root plates are often depicted as being circular but that is a simplified approximation. A study suggested that an elliptical root plate may be a better approximation to reflect root systems (based on two Pinus species).

Figure 5 below was flipped horizontally to match the directions in Figure 4. It was the other way around in the original paper, which is strange. The root plate is more likely to extend to the windward side of the trunk, and damage to anchoring roots at that side, by trenching or construction, is more likely to lead to the root plate tilting (Figure 4). Especially if the roots are damaged or severed close to the trunk. Mowing could have also repeatedly damaged the lateral root that had come to the soil surface. While the wounds created may not be structurally imparing, they may provide access for pests and diseases into the tree. A number of them would start to decay and desending roots would also die along, causing the tree more prone to toppling by wind.

Construction work such as adding backfill soil or re-contouring surfaces could divert water flows away from the root systems, and pose sudden water deficits to the trees, which would show signs of wilting and dieback. However, many trees with significant dieback could remain standing while trees with healthy crown could be wind-thrown. Hence caution should be applied when observing the crown in visual assessments. Signs of dieback and significant deadwood in the crown may also indicate that the trees may have been stressed for some time and that there had been a loss of root mass.

A study have shown that depth of root plate is not significant in tree failure. In addition, urban soil profiles are altered and compacted such that penetration by tree roots is limited to no more than 1000mm depth. In the case where soils are compacted near the surface - at heavily trafficked areas of a park, sports activities, people grouping under trees for shade - reduces aeration and water penetration. This in turn inhibit descending and lateral roots, which can affect tree stabililty. However, compaction increases soil strength.

Heavy rains would saturate soil and reduce the strength of the connection between soil and tree roots, resulting in trees overturning and the descending rotos of the root system of the windward side would be intact. Soil may also be waterlogged due to poor drainage or altered subterranean water flows. Due to "high water table", root developement could be restricted to depth of 200mm and above.  In waterlogged soils, desending roots would die back over time. After the tree had been overturned by wind in such a case, the exposed soil would have a pungent odour and there would be a large number of small blackened roots. 

Management may have contributed to forest tree failure after forest thinning operations, which has been well-researched because of the economic losses that results. Due to heavy rains that reduce soil strength and accompanying strong winds. I would argue that this applies to urban trees and shrubs thinning as well.

Standard protection systems cannot deal with the nuances of every tree and root system that develop in response to particular environments.

Arborists should include as part of their inspection protocols, trees that show damaged or decayed lateral roots and the loss of descending roots, evidence of site or trenching work close to the trunk, and whether trees are growing in compacted soil, waterlogged soil and backfilled soil. However in my opinion, I do not think an arborist could observe loss of descending root and waterlogged soil.

The question is raised as to whether the storm was the final trigger in a lengthy chain of events relating to tree management, leading to wind-thrown and whole-tree failure.