Soil bioengineering in Iran: Lessons from the past and prospects for the future

 
Background and objectives: Soil bioengineering techniques offer multifaceted benefits and find applications across diverse landscapes. These approaches are notable for their high sustainability, adaptability to environmental changes, and effective reestablishment. While existing bioengineering studies focus largely on assessing root systems for soil stabilization, it remains pivotal to estimate key biotechnical characteristics, such as root distribution and mechanical strength, for various plant species. These insights serve as crucial inputs for developing robust soil reinforcement models.
Methodology: Root system distribution and mechanical strength data are integral for accurate reinforcement estimation. In Iran, conventional methods such as profile trenching and soil core sampling were employed for destructive sampling to investigate root system distribution and density. This focused on several tree species, including Fagus orientalis Lipsky, Carpinus betulus L., Alnus glutinosa (L.) Gaertn., Acer velutinum Boiss., Quercus castaneifolia C.A.Mey., Parrotia persica (DC.) C.A.Mey., Fraxinus excelsior L., Picea abies (L.) H.Karst., Quercus brantii Lindl., Quercus libani Oliv., Haloxylon persicum Bunge ex Boiss. & Buhse, Salix alba L., Tamarix hispida Willd., and Avicennia marina (Forssk.) Vierh. The mechanical properties of root samples from various species and habitats were investigated in a laboratory setting using a universal device. A novel in-situ pullout device with a 10-kilonewton capacity was employed in Iran to measure mechanical resistance in mangrove forests.
Results: Findings indicated that the mean root area ratio, a density indicator, ranged from 0.15% to 2.6%. Various factors significantly influenced root area ratio values, including species, slope gradient, health status, human intervention, stress direction, altitude, trunk diameter, age, and horizontal and vertical distances from the trunk. Mechanical resistance values were impacted by factors such as health status, pollarding, slope gradient, wind direction, season, altitude, age, and trunk diameter. Generally, higher slope gradients, trunk diameters, elevations, and ages increased root density, while diseases, pests, and pollarding treatments decreased it. Mechanical resistance values varied with factors such as season, trunk diameter, elevation, and the presence of stressors like slope and prevailing wind. In Iran, mean tensile force, tensile strength, and pullout resistance ranged between 17-145 N, 3-45 MPa, and 1.19 kN, respectively.
Conclusion: Iran possesses essential data for applying classical models of soil reinforcement across various plant species. To enhance models, the use of in-situ pullout apparatus for assessing root mechanical characteristics in their natural environment is recommended. Despite the wealth of Iranian bioengineering literature, knowledge gaps persist, particularly in root architecture, distribution, and density, with a need for data on whole tree extraction and the development of allometric models for soil bioengineering applications.