Quantifying the effect on shallow landslide activity of actual and potential poplar and pine stands in New Zealand hill country.

In New Zealand shallow landslides are a prominent contributor to soil erosion in unvegetated slopes (hill country) and to water quality degradation. Selective well-planned re-vegetation of steep slopes can reduce shallow landslide hazard with comparatively low economic consequences.

The main non-native planted tree species that contribute to slope stability are Poplar species (Populus sp.) and Pine (Pinus radiata). We will present field data quantifying the root distribution and root strength of poplar and pine trees from New Zealand. 4 poplar trees with a medium Diameter at Breast Height (DBH) of 0.48 m are included. Circular trenches have been dug at fixed distances from stem and the roots counted and their diameter measured systematically. 64 root pull-out tests over varying soil depth and root diameter provide calibration for lateral root reinforcement (Gehring et al., 2019; equation 3). The combination of root counts and root reinforcement calibration enables the parametrization of root reinforcement on a single tree scale. The Pinus radiata calibration is the adopted from Giandrossich et al., 2020 which applied a similar methodology.

Using the slope stability model SlideforMap, we assess and compare (re)vegetation scenarios and their effect on slope stability. In addition to a detailed inclusion of vegetation, SlideforMap takes local soil and hydrology into account in the parametrization. Scenarios without poplar/radiata stands, dispersed trees and plantations are run and compared under varying precipitation conditions.

We believe this approach enables regional decision makers to optimize tree planting to significantly reduce slope instability at minimal economic costs.

Literature:

Gehring, E., Conedera, M., Maringer, J., Giadrossich, F., Guastini, E., & Schwarz, M. (2019). Shallow landslide disposition in burnt European beech (Fagus sylvatica L.) forests. Scientific Reports, 9(1), 1–11. https://doi.org/10.1038/s41598-019-45073-7

Giadrossich, F., Schwarz, M., Marden, M., Marrosu, R., & Phillips, C. (2020). Minimum representative root distribution sampling for calculating slope stability in pinus radiata d.Don plantations in New Zealand. New Zealand Journal of Forestry Science, 50, 1–12. https://doi.org/10.33494/nzjfs502020x68x