Minimum representative root distribution sampling for calculating slope stability in Pinus radiata D.Don plantations in New Zealand

Abstract

Background: Rainfall-triggered shallow landslides on steep slopes cause significant soil loss and can be hazards for property and people in many parts of the world. In New Zealand’s hill country, they are the dominant erosion process and are responsible for soil loss and subsequent impacts on regional water quality. Use of wide-spaced trees and afforestation with fast growing conifers are the primary land management tools in New Zealand to help control erosion and improve water quality. To decide where to implement erosion controls in the landscape requires determining the most susceptible places to these processes and models that incorporate how trees reinforce soils to understand if, and when, such treatments become effective.
 Methods: This paper characterises the mechanical properties of Pinus radiata D.Don roots (the common tree species used for afforestation in New Zealand) by means of field pullout tests and by measuring the root distribution at 360 degrees around trees. The Root Bundle Model (RBM) was used to calculate the root reinforcement. Statistical analysis was carried out to assess the statistical reduction coefficients of root reinforcement that depend on the number of measurements, used in geotechnical analysis to reduce the mean value of a parameter to a so-called characteristic value.
 Results: We show that to reach an effective level of root reinforcement, trees of 0.5 m DBH require a density of about 300 trees per hectare. Trees of this size are about 30 years of age across many sites and have generally reached the recommended conditions for clear-fell harvesting. The analysis of variance shows that 4 trees are the minimum number to be excavated to obtain sufficient root information to obtain less than 5% of error with a 95% of probability on the estimation of a design value of root reinforcement in accord with geotechnical standards.
 Conclusions: We found that the variability of lateral and basal root reinforcement does not limit the implementation of vegetation in slope stability models for Pinus radiata. We adopt for the first time the concept of a minimum sampling requirement and characteristic value, similarly to what is assumed for the value of effective soil cohesion in geotechnical guidelines for slope stability calculations.

Root age influences failure location in grass species during mechanical testing

“…Roots are exposed adjacent to the parent tree through a trench and pulled out of soil (Giadrossich et al 2017). For instance, Giadrossich et al (2020) combined laboratory tensile tests (root diameters: 1-4 mm) and field pull-out tests (root diameters: 5-57 mm) to quantify mechanical properties of Pinus radiata roots.…”

A Novel Integrated Design Methodology for Nature-Based Solutions and Soil and Water Bioengineering Interventions: The Tardio&Mickovski Methodology

“…Root system morphology and properties can be studied by field techniques [59][60][61][62] or indirectly estimated from theoretical root distribution models [6,63,64]. Published literature [65][66][67][68] includes data of root systems of the most common living material used in SWB solutions.…”

Shallow landslides and vegetation at the catchment scale: A perspective

“…This problem reflects the integration of historical land management and long-term geomorphic processes, in particular estimating the spatio-temporal patterns of root strength, pore pressure, regolith depth and hydraulic properties across the landscape (e.g. Cislaghi et al, 2017;Schmaltz and Mergili, 2018;Hales, 2018;Giadrossich et al, 2020;Masi et al, 2021). These represent the key epistemic uncertainties driven by vegetation and require better understanding how much field data (and generating them) are needed to calibrate and validate existing and future models across a range of realistic management situations, and…”