Understanding the drivers of forest aboveground biomass (AGB) is essential to further understanding the forest carbon cycle. In the upper Yangtze River region, where ecosystems are incredibly fragile, the driving factors that make AGB changes differ from other regions. This study aims to investigate AGB’s spatial and temporal variation of Pinus densata in Shangri-La and decompose the direct and indirect effects of spatial attribute, climate, stand structure, and agricultural activity on AGB in Shangri-La to evaluate the degree of influence of each factor on AGB change. The continuous sample plots from National Forest Inventory (NFI) and Landsat time series were used to estimate the AGB in 1987, 1992, 1997, 2002, 2007, 2012, and 2017. The structural equation model (SEM) was used to analyze the different effects of the four factors on AGB based on five scales: entire, 1987–2002, 2007–2017, low population density, and high population density. The results are as follows: (1) The AGB of Pinus densata in Shangri-La decreased from 1987 to 2017, with the total amount falling from 9.52 million tons to 7.41 million tons, and the average AGB falling from 55.49 t/ha to 40.10 t/ha. (2) At different scales, stand structure and climate were the drivers that directly affect the AGB change. In contrast, the agricultural activity had a negative direct effect on the AGB change, and spatial attribute had a relatively small indirect effect on the AGB change. (3) Analyzing the SEM results at different scales, the change of the contribution of the agricultural activity indicates that human activity is the main negative driver of AGB change in Shangri-La, especially at the high population density region. In contrast, the change of the contribution of the stand structure and climate indicates that the loss of old trees has an important influence on the AGB change. Forest resources here and other ecologically fragile areas should be gradually restored by adhering to policies, such as strengthening forest protection, improving forest stand quality, and limiting agricultural production activities.

Soil compaction under high-density pavements creates a root restriction in tree pit that restricts urban forest vitality. Structural soil is a Nature-based solution balancing load-bearing capacity with root development; however, its three-dimensional (3D) biological mechanisms remain unclear. We quantified structural soil's regulation of root architecture in eight-year-old Liquidambar styraciflua in Shanghai using dual-frequency ground-penetrating radar (400 and 900 MHz). Comparisons between the Structural Soil Group (SSG) and the Original Soil Group (OSG) revealed that structural soil significantly improved tree growth. Average diameter at breast height (DBH) increased by 37.52% (25.73 vs. 18.71 cm), and total root density (400 MHz, root diameter [RD] ≥ 2 cm) was 1.7 times higher. Structural soil remodeled root configuration across three dimensions: runoff capture, deep anchoring, and horizontal exploration. Surface root density (900 MHz, RD ≥ 1 cm) in the 0–20 cm layer increased 11.6-fold, enhancing runoff interception. Additionally, continuous macropore networks increased deep root density (101–400 cm depth) by 1.8 times, shifting roots from shallow entrenchment to stable 3D penetration. Roots also broke physical boundaries by abandoning uniform proximal attenuation for directional exploration, forming high-density functional zones 2.50–4.50 m from the trunk. By inducing deep rooting and reinforcing distal roots, structural soil transforms root systems from 2D restricted expansion to full 3D space utilization. This reconstruction significantly enhances the biological vitality and 3D architecture of urban trees, providing a scientific paradigm for green infrastructure governance in megacity regeneration.