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Open Journal of Forestry, 2020, 10, 124-134

https://www.scirp.org/journal/ojf

ISSN Online: 2163-0437

ISSN Print: 2163-0429

DOI:

10.4236/ojf.2020.101009 Jan. 6, 2020 124 Open Journal of Forestry

Investigating the Effect of Weather and

Seasonal Factors on Root Stability Using

Dynamic Measurements

Shadabeh Fathi

, Laszlo Bejo

, Ferenc Divos

Institute of Wood Based Products and Technologies, University of Sopron, Sopron, Hungary

Head, FAKOPP Enterprise, Sopron, Hungary

Abstract

The dynamic tree stability assessment technique allows trees to be measured

efficiently, under realistic weather conditions. In this study, the

stability of

four trees, including two conifers and two broad-leaved species was asses

sed

in the Botanical Gardens of the University of Sopron, Hungary. The ex-

amined trees included Horse chestnut, Japanese zelkova, Douglas fir and

Giant sequoia. Each tree was measured in various weather and seasonal con-

ditions. Results show that the seasons affected the stability of br

oadleaved

trees significantly, due to considerable changes in the crown surface area,

while this difference was much lower in softwoods. Rai

ny weather loosens the

topsoil, which adversely affects the stability of trees with relatively shallow

roots. Lower layers take longer to saturate, and therefore trees with very deep

roots are usually unaffected by the looser topsoil, while the increased w

eight

of the top layer compacts the lower layers and improves stability, as evi-

denced by results measured on Sequoia. Snow accumulation on the branches

increases the inertia of the tree, which imposes higher torque on the root col-

lar, decreasing stability. In the meantime, the increased resistance offered by

frozen ground stabilizes the tree, which more-or-less counterbalances this ef-

fect. A more extensive database of tree stability under different conditions is

being built to allow for more comprehensive a

nalysis of various factors, like

wind direction, tree health and morphology, shading, etc.

Keywords

Tree Stability, Dynamic Stability Assessment, Safety Factor, Wind Direction,

Precipitation, Foliage, Critical Wind Pressure

1. Introduction

Many factors affect the root system (soil conditions, weather, age of tree, biolog-

How to cite this paper: Fathi, S., Bejo, L.,

& Divos, F.

(2020).

Investigating the Effect of

Weather and Seasonal

Factors on Root Sta-

bility Using Dynamic Measurements

Open

Journal of Forestry

, 10,

124-134.

https://doi.org/10.4236/ojf.2020.101009

Received:

December 6, 2019

Accepted:

January 3, 2020

Published:

January 6, 2020

20 by author(s) and

Scientific

Research Publishing Inc.

This work is licensed under the Creative

Commons Attribution International

License (CC BY

4.0).

http://creativecommons.org/licenses/by/4.0/

Open Access

S. Fathi et al.

DOI:

10.4236/ojf.2020.101009 125 Open Journal of Forestry

ical factors). Root length, root angle, number of roots, and root diameter vary

greatly between species. In trees, as in engineered structures, if the normal ser-

vice loads create stresses that are just below the strength of the material, they

have practically no strength reserve, and the smallest accident may lead to brea-

kage. The safety factor in trees is defined as the material strength divided by the

service load (Matteck et al., 1993).

Wood in trees is flexible, and behaves as neither an ideal solid nor an ideal

fluid (Vogel, 1996). Trees should be able to safely withstand high winds. The

safety factor is calculated relative to the high wind level typical to a geographical

area (Cullen, 2002). Many studies on conifer seedlings show that root deflection

in propagation containers can contribute to long-term growth problems after

planting in the forest (Krasowski, 2003). Wood and most materials that come

from plants are described as viscoelastic because their mechanical behaviour

contains both elastic and viscous elements (Miller, 2005). These properties result

in nonlinear behavior. Evidence of the influence of tree architecture on wind

firmness has also been shown by Fourcaud et al. (1999) who carried out me-

chanical studies on two rubber tree clones that had similar wood properties but

dissimilar crown structures (Cilas et al., 2004). The shape and structure of trees

have an important impact on their mechanical stability under dynamic loading.

As trees grow, the added biomass translates into greater dead weight, and the

upper parts of the tree are exposed to higher wind speeds, creating larger bend-

ing moments at its base (Niklas & Spatz, 2000).

Yang et al. (2016) explored the influence of root moisture content on tensile

resistance and strength with different root diameters and for different tree spe-

cies. The results showed that root moisture content affected the tensile proper-

ties. A slight loss of root moisture content could enhance tensile strength, but

too much loss of water resulted in weaker capacity for root elongation with ten-

sile resistance. The main factors contributing to slope stability include soil shear

strength, soil-root interactions, the quantity and distribution of roots, as well as

root tensile properties (Genet et al., 2005).

Several ways exist for measuring root stability. One of them is the pulling test,

which has about 25 years of background in Germany (Wessolly & Erb, 2016).

More recently, a dynamic method was developed that could take advantage of

actual wind loading, despite the chaotic relationship between momentary wind

intensity and inclination (Bejo, Divos, & Fathi, 2017).

The aim of this study is to investigate the parameters that affect the stability of

the trees. In this study, mostly external parameters, like seasonal effects, precipi-

tation and wind direction are considered. Some of the intrinsic characteristics

(like crown shape and root structure) are also considered in the interpretation of

the results.

2. Factors Influencing Tree Stability

There are many factors that influence tree stability (i.e. the tree’s resistance

S. Fathi et al.

DOI:

10.4236/ojf.2020.101009 126 Open Journal of Forestry

against uprooting). Some of these, like crown shape and surface area, age, etc.

are intrinsic properties of the tree, and are partly taken into consideration when

assessing the stability of the trees. There are also external factors that impact the

stability significantly. The most important of these factors are discussed in this

chapter.

2.1. Seasonality

In temperate climates, the seasons affect the growth, metabolism and general ac-

tivity of trees. This also influences the stability of trees in several different ways,

as follows:

1) Foliage changes

The changing of the seasons has a major effect on the foliage of broadleaved

species. This also has a major impact on tree stability, since leaves significantly

increase the crown surface, and transfer much of the wind loads to the system of

twigs and branches, and eventually, through the crown, to the root system. This

means that winds of the same intensity will impose much higher loads on the

crown (and therefore much higher torque on the root collar) in the summer,

when peak foliage is present, then in the winter, when leaves are absent.

In conifers, this effect is much less pronounced, or may be all together absent,

since their crown surface area does not change dramatically in the winter (except

for some rare exceptions, like larch, which sheds its nedles in the winter.

2) Biological activity

In general, trees are biologically much more active in the spring and summer,

and tend to decrease their activity, and eventually go dormant in the winter. This

affects the root system, which tends to swell and be more firmly anchored in the

soil in the spring and summer, due to the increased sap flow, and become

somewhat looser in the autumn and winter. This affects the stability of all trees

adversely, albeit not nearly as strong as the foliage change in broadleaved trees.

3) Other seasonal factors

Seasonal changes also affect tree stability through changes in temperature.

Particularly, the frozen soil in the winter may become much more resistant

against uprooting, which will positively influence the stability of the tree. The

nature of precipitation also tends to change in the winter, but this will be dis-

cussed in the next chapter.

2.2. Precipitation

Precipitation will also affect the stability of trees. The effect is markedly different

depending on the form of precipitation.

1) Rain

Rainy weather—especially when it’s prolonged—affects both the tree and the

soil. Rain-covered foliage will have a somewhat increased inertia, but the effect is

much more pronounced in the soil. Rainwater penetrates the ground, and loo-

sens the soil, which will therefore allow more movement, and become less resis-

tant to uprooting. Both of these effects will act towards decreasing tree stability.

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