Analysis of “Tasman” poplar’s (Populus deltoides x Populus nigra) root systems for the quantification of bio-engineering services in New Zealand pastoral hill country.

Poplar (Populus sp.) is an important species for preventing shallow, rainfall-triggered landslides and hydraulic bank erosion in New Zealand. However, quantifying the spatial root distribution pattern and reinforcement remains challenging. This study aimed to find the Root Bundle Model with the Weibull survival function (RBMw), a root distribution model (RDM), and a root reinforcement model for the implementation in models such as BankforMAP and SlideforMAP. Our study was conducted within a 26-year-old “Tasman” poplar stand at Ballantrae Hill Country Research Station in the North Island of NZ. We measured root distribution at distances of 1.5, 2.5, 3.5, and 4.5 m from the stem of four poplar trees whose diameters ranged from 0.41 to 0.56 m and from eleven soil profiles along a transect located in a sparse to a densely planted poplar stand. This created a unique database of root distribution. 124 laboratory tensile tests and 66 field pullout tests on roots with diameters up to 0.04 m were carried out. The root distribution model well predicted spatial root partition in trenches of single tree root systems with R² = 0.78 and in the transect with R² = 0.85. The model tends to overestimate root distribution when planting density was higher than 200 stems per hectare. The maximum lateral root reinforcement model tends to underestimate forces in single tree root systems with R² = 0.64, but it well performs along the transect within the stand with different planting densities. The basal root reinforcement model performed well in predicting its vertical distribution as a function of soil depth. In conclusion, our study provided a detailed dataset for the quantification of root distribution and reinforcement of poplars on a hillslope for the purpose of increasing slope stability and mitigating hydraulic bank erosion. The implementation of these data in models for the simulation of shallow landslides and hydraulic bank erosion is fundamental for the identification of hazardous zones and the prioritization of bio-engineering measures in NZ catchments. Moreover, the results are used to formulate a general guideline for the planning of bio-engineering measures considering the temporal dynamics of poplar’s growth and their effectiveness in sediment and erosion control.