Quantifying the dynamics of Root Distribution and Root Reinforcement of Forest Tree Species and their Applications in Forest Management

Forest plays a vital role in ecosystem services, including soil stabilization, erosion control, biodiversity conservation, and carbon sequestration. In mountainous regions, tree roots significantly contribute to the prevention of rainfall-induced shallow landslides and soil erosion by reinforcing the soil and increasing its shear strength. However, quantifying root distribution, root reinforcement, and belowground biomass across spatial scales remains a major scientific challenge, given the complexity of root systems, high variability in site conditions, and the lack of spatially explicit field data. This thesis addresses these challenges by combining field measurements, laboratory mechanical tests, and advanced modeling techniques to evaluate the ecological role of tree roots in both stabilizing slopes and sequestering carbon. The study focuses on two important tree species: Populus deltoides × nigra "Tasman" poplar in a study case in New Zealand and Cryptomeria japonica in Switzerland and Japan. These species are widely used in bioengineering and afforestation due to their deep root systems, rapid growth, and adaptability to diverse site conditions. For the Tasman poplar, extensive field excavations were carried out around 4 individual trees of various sizes, with four trenches (360o) at different distances around the stem. Additionally, root distribution in 10 transects locating from sparse to dense planting zones was recorded. Root mechanical properties were assessed through 124 tensile and 66 pullout tests with root diameters up to 0.04 m, conducted within a 26-year-old stand on a pastoral hill slope in New Zealand. The Root Distribution Model (RDM), Root Bundle Model with Weibull survival function, and Root Reinforcement Model (RRM) were calibrated and validated. RDM showed strong performance in predicting root spatial patterns, particularly in sparse stands (< 200 sph) and mature individuals whereas overestimated number of roots in dense zone > 200 sph. RRM underestimates root force within single tree root systems but performed better in the transects locating in sparse zone. In all cases, RRM predicted well basal root reinforcement. The thesis work revealed that "Tasman" poplars need at least 20 years to reach minimum values of lateral root reinforcement at the stand scale and at least 30 years are needed to reach root reinforcement sufficient for shallow landslide stabilization depending on their disposition. RDM was utilized to estimate root biomass for Tasman poplars. The measurements revealed that root biomass is positively correlated with tree sizes and negatively correlated with soil depth and distance from the stem. RDM underestimated root distribution and root biomass in poplar trees, could be due to missing data of root quantities in the dense zone from 0 to 1.5 m from the stem and at distance further than 4.5 m. A generalized allometric equation was also developed by integrating our field data with existing data from previous studies at same location for estimating root dry weight across a wide DBH range, allowing accurate, non-destructive estimation of root biomass and carbon stocks. A single mature tree can store over 170 kg of carbon in its root system, highlighting its significant potential for climate change mitigation. In the case of Cryptomeria japonica, data from forest stands in China, Japan and Switzerland were used to quantify and compare root distribution and root mechanical properties. Integrating laboratory tensile tests and field pullout tests for roots ≤10 mm proved to be effective in predicting a representative mechanical profile for RBMw calibration. Coarse root distribution was generally consistent between Switzerland and Japan while fine root patterns and basal reinforcement differed with Swiss stands exhibited stronger reinforcement in upper soil layers (0 - 0.15 m) while Japanese stands showed deeper root anchorage (0.15 - 0.45 m). [continue]