Tree roots play a crucial role in hillslope stability, but quantifying their reinforcement remains challenging. This study aims to quantify the root reinforcement provided by Cunninghamia lanceolata across varying slope gradients based on in-situ pullout experiments. A total of 120 soil profiles were excavated to map root distribution across four slope gradients. Subsequently, 304 in-situ pullout experiments were conducted encompassing root diameters ranging from 1 to 8 mm. The Root Bundle Model Weibull was calibrated and coupled with root distribution data to quantify reinforcement contributions from a single tree to stands. It was found slope gradient significantly influences root distribution, with steeper slopes harboring coarser and more widely distributed roots. In-situ experiments revealed substantial variability in pullout stiffness and peak displacement for roots of the same diameter, with thicker roots exhibiting higher stiffness and greater displacement. Calculations indicate that root reinforcement exhibits an exponential decline with increasing distance from the stem but shows a marked positive association with slope gradient due to the influence on root distribution. Statistical analysis reveals that the area experiencing root reinforcement exceeding 10 kPa on a 40 degrees slope is roughly double that of 0 degrees and 20 degrees stands.

This study used triaxial tests to examine the impact of the root diameter of Cunninghamia lanceolata (Chinese fir) on the mechanical behavior of sand, including stress-strain development, strength, volumetric strain, and failure envelope. It also revealed the reinforcement mechanisms of roots with different diameters based on root-soil interactions. The results showed the following: (1) The addition of roots significantly enhanced sand strength and reduced volumetric deformation. The average peak strength increased by 31.8%, while the average peak volumetric strain decreased by 34.3%. (2) Roots provided additional cohesion and increased the friction angle of the sand, causing the failure envelope to shift upward and deflect. (3) Smaller-diameter roots improved the mechanical properties of sand more significantly, leading to higher peak strength, shear strength parameters, and smaller volumetric deformation. As the root diameter increased from 1 mm to 5 mm, the peak strength ratio decreased from 1.78 to 1.13, and the peak volumetric strain increased from 0.48 to 0.79. (4) Smaller-diameter roots, which form denser networks, allowing more roots to resist loads, and have a higher elastic modulus providing greater tensile stress, also possess higher tensile strength and critical sliding tensile stress, making them less likely to fail, thereby making the mechanical reinforcement of sand more significant.

Uncertainties about the ecophysiological response of plants to elevated temperature limit our ability to predict the impact of climate change on plants, especially in tropical and subtropical forests. One important source of the uncertainties is that the vast majority of warming studies manipulated only aboveground or only belowground temperature when in the real word warming takes place both aboveground and belowground. We used a full factorial design of air warming and soil warming with four temperature treatments: (1) unwarmed, (2) soil warming, (3) air warming, (4) soil plus air warming to explore the effects of warming on ecophysical processes/ characteristics of leaves and fine roots of Chinese-fir saplings. We measured photosynthesis, concentrations of oxidant substances, activity of antioxidant enzymes, and osmoregulatory substances in leaves and fine roots. We found that the soil warming increased photosynthetic rate by 68.9%, but air warming and soil plus air warming treatments did not. The concentrations of oxidant compounds, superoxide anion (O2- ), hydrogen peroxide (H2O2) and malondialdehyde (MDA) were higher in leaves than in fine roots under all treatments, possibly due to their differences in the degree of oxidative damage. Soil warming increased leaf catalase (CAT) activity by 58.5%, soil warming and air warming increased leaf peroxidase (POD) activity by 31% and 42.3%, respectively, and soil plus air warming increased leaf ascorbic acid peroxidase (APX) activity by 31%. These increases in antioxidant enzyme concentrations indicated that warming activated leaf antioxidant systems. The CAT activity was lower in leaves than in fine roots, while the POD activity and concentrations of osmoregulatory substances were higher in leaves than in fine roots across all treatments. Our study clearly illustrated that different warming treatments (aboveground and belowground) had different effects on plant growth and physiological processes. The differences in oxidant compounds and activities of antioxidant enzymes between leaves and fine roots indicated that warming affect different organs differently. This study provides insights into how climate warming may affect important physiological and biochemical processes in subtropical forests.