China’s massive wave of urbanization may be threatened by land subsidence. Using a spaceborne synthetic aperture radar interferometry technique, we provided a systematic assessment of land subsidence in all of China’s major cities from 2015 to 2022. Of the examined urban lands, 45% are subsiding faster than 3 millimeters per year, and 16% are subsiding faster than 10 millimeters per year, affecting 29 and 7% of the urban population, respectively. The subsidence appears to be associated with a range of factors such as groundwater withdrawal and the weight of buildings. By 2120, 22 to 26% of China’s coastal lands will have a relative elevation lower than sea level, hosting 9 to 11% of the coastal population, because of the combined effect of city subsidence and sea-level rise. Our results underscore the necessity of enhancing protective measures to mitigate potential damages from subsidence.

Populus euphratica Oliv. (Euphrates poplar), as the dominant tree species of desert riparian forests along the Central Asian inland rivers, plays a critical role in protecting arid land ecosystems. In recent decades, climate change and excessive water resources utilization activities have led to the environmental degradation of desert riparian forests along the Tarim River in northwest China. Understanding the forest stand structure and spatial distribution pattern provide important guidance for monitoring forest dynamics in support of sustainable management. However, few studies have examined how riparian forests stand attributes differ in response to environmental heterogeneity. In this study, terrestrial laser scanning (TLS) was applied to acquire a total of 1648 individual P. euphratica tree’s 3D structure attributes within 18 plots along the upper, middle, and lower reaches of the Tarim River, which included tree height (TH), diameter at breast height (DBH), crown diameter (CD), crown projection area (CPA), stand density index (SDI), age structure ratios, and spatial pattern. The results showed that the average tree segmentation and structure determination accuracies of TLS were 93.2 and 94.6%. From the upper to the lower reaches, the average TH and CD decreased by 3.8 and 0.3 m, while the DBH increased by 4.2 cm. The SDI and CPA exhibited the following order: upper reaches (454 n ha^–1, 82.3%) > middle reaches (382 n ha^–1, 67.3%) > lower reaches (263 n ha^–1, 39.1%), the differences were significant at 0.05 level. The population age structure changed from growing population in the upper reaches to stable population in the middle and a temporarily stable population in the lower reaches. The pair correlation g(r) function determined random distribution pattern in the upper reaches [g(r) = 1.2], an aggregated pattern in the middle [g(r) = 3.1], and lower reaches [g(r) = 9.7]. The decline in groundwater depth and soil moisture increased aggregated distribution pattern (R = 0.67 and 0.56, P < 0.05) of the P. euphratica along the mainstream of Tarim River. The results enrich our understanding of the current development stage of P. euphratica, which is important for optimizing management strategies and realizing the sustainability of floodplain ecosystems.

The Populus euphratica desert riparian forest, predominantly distributed along the Tarim River in northwestern China, has experienced significant degradation due to climate change and anthropogenic activities. Despite its ecological importance, systematic assessments of P. euphratica stand structure across the entire Tarim River remain scarce. This study employed terrestrial laser scanning (TLS) to capture high-resolution 3D structural data from 2741 individual trees across 30 plots within six transects, covering the 1300 km mainstream of the Tarim River. ANOVA, PCA, and RDA were applied to examine tree structure variation and environmental influences. Results revealed a progressive decline in key structural parameters from the upper to lower reaches of the river, with the lower reaches showing pronounced degradation. Stand density decreased from 440 to 257 trees per hectare, mean stand height declined from 9.3 m to 5.6 m, mean crown diameter reduced from 4.1 m to 3.8 m, canopy cover dropped from 62% to 42%, and the leaf area index fell from 0.51 to 0.29. Age class distributions varied along the river, highlighting population structures indicative of growth in the upper reaches, stability in the middle reaches, and decline in the lower reaches. Abiotic factors, including groundwater depth, soil salinity, soil moisture, and precipitation, exhibited strong correlations with stand structural parameters (p < 0.05, R² ≥ 0.69). The findings highlight significant spatial variations in tree structure, with healthier growth in the upper reaches and degradation in the lower reaches, enhance our understanding of forest development processes, and emphasize the urgent need for targeted conservation strategies. This comprehensive quantification of P. euphratica stand structure and its environmental drivers offer valuable insights into the dynamics of desert riparian forest ecosystems. The findings contribute to understanding forest development processes and provide a scientific basis for formulating effective conservation strategies to sustain these vital desert ecosystems, as well as for the monitoring of regional environmental changes.

Populus euphratica Oliv., an indicator species for eco-environmental change in arid areas, plays a key role in maintaining the stability of fragile oasis–desert ecosystems. Owing to human interference as well as to the harshness of the natural environment, P. euphratica forests have suffered severe damage and degradation, with trunk cavities (i.e., hollows) becoming increasingly pronounced, and thus posing a great threat to the growth, health, and survival of the species. Currently, there is a gap in our understanding of cavity formation and its distribution in P. euphratica. Here, cavities in the trunks and branches of a P. euphratica in a typical transect (Arghan) along the lower Tarim River were studied based on field positioning observations combined with laboratory analysis. The results revealed a large number of hollow-bearing P. euphratica stands in the study area; indeed, trees with hollows accounted for 56% of the sampled trees, with approximately 159 trees/ha. Sixty-six percent of hollow trees exhibited large (15 cm ≤ cavity width (CW) < 30 cm) or very large (CW > 30 cm) hollows. The main types of cavities in the trees were trunk main (31.3%), trunk top (20.7%), branch end (19.5%), and branch middle (19.5%). Tree parameters, such as diameter at breast height (DBH), tree height (TH), east–west crown width (EWCW), height under branches (UBH), and crown loss (CL) were significantly different between hollow and non-hollow trees. Both cavity height and width were significantly and positively correlated with DBH and CL, as well as with average crown width (ACW) (p < 0.001) and the distance from the tree to the river. The proportion of P. euphratica trees with cavities showed an overall increasing trend with increasing groundwater depth. Our findings show that cavities in P. euphratica varied with different tree architectural characteristics. Water availability is a major environmental factor influencing the occurrence of hollowing in desert riparian forests. The results provide scientific support for the conservation and sustainable management of existing desert riparian forest ecosystems.