Abdi Ehsan, PhD.

Shallow landslides are a frequently recurring problem in some parts of Iran, including the
Hyrcanian forest. In addition to traditional civil engineering measures, a potential solution
for this problem is the application of soil bioengineering techniques. The mechanical reinforcement
effect of plant roots is one of the major contributions of vegetation to the mitigation of
shallow landslides. Given the lack of information on the mechanical properties of common
Hyrcanian forest species, the present study assessed the root strength of 10 common species
of this forest. Eight tree species occurring in natural regeneration sites (Carpinus betulus,
Fagus orientalis, Parrotia persica and Quercus castaneifolia) and plantations (Acer velutinum,
Alnus glutinosa, Fraxinus excelsior and Picea abies) and two shrub species
(Crataegus microphylla and Mespilus germanica) were selected. Fresh roots were collected
and mechanical tests were carried out on 487 root samples. The ranges of root diameter,
tensile force, and root resistance were 0.29–5.90 mm, 3.80–487.20 N, and 2.41–224.35 MPa,
respectively. Two different algorithms, including the nonlinear least square method and logtransformation,
were used to obtain power regressions for diameter-force and diameter-resistance
relationships. The results of the two algorithms were compared statistically to choose
the optimal approach for soil bioengineering applications. The nonlinear least square method
resulted in lower Akaike information criteria and higher adjusted R2 values for all species,
which means that this model can more efficiently predict tensile force and resistance based on
root diameter. Log-transformation regressions generally underestimate tensile force and resistance.
Significant differences were found among mean root tensile force (ANCOVA;
F=37.36, p<0.001) and resistance (ANCOVA; F=34.87, p<0.001) of different species. Also,
root diameter was significant as a covariate factor in tensile force (F=1453.77, p<0. 001) and
resistance (F=274.26, p<0.001). Shrub species and trees in natural regeneration sites had
higher tensile force and resistance values, while trees from plantation stands had lower values.
The results of this study contribute to the knowledge on the root force and resistance characteristics
of several shrub and tree species of the Hyrcanian forest and can be used in evaluating
the efficiency of different species for bioengineering purposes.