The study was conducted in the middle and lower reaches of the Yarlung Zangbo River in Nyingchi City (93°4^′25.3^″~94°23^′1.5^″E, 20°0^′14.1^″~39°0^′14.6^″N), Tibet Autonomous Region, China, from Lang County to Milin County, with an elevation of 2991~3156 m Fig. 1. It has a temperate semi-humid monsoon climate, with abundant sunshine and average annual precipitation of more than 8°C, and the annual rainfall is about 600 mm which are known to contain high populations of C. gigantea. The other sub-counties regularly experience harsh natural habitat, making survival of C. gigantea a challenge. By conducting a belt survey of C. gigantea plant communities along the Yarlung Zangbo River from Lang County to Milin County, a total of 76 species of 36 families and 68 genera of plants were recorded.

A reconnaissance survey was carried out in September 2015 to identify key areas where C. gigantea grows within the Yarlung Zangbo River area, Tibet Autonomous Region, China, as well as for the natural habitat study. It was found that the C. gigantea grew majorly in the highest densities in the middle and lower reaches of the Yarlung Zangbo River in Nyingchi City, from Lang County to Milin County, which formed the basis for selecting the area for this study. Representative sites of the growth points of each C. gigantea from Lang County to Milin County were selected through a desktop study using Google Maps, which were later verified on the ground to be true.

According to the topography and the survival status of C. gigantea, the overall 28 typical sample plots (20 m × 20 m) were investigated. In each plot, the first corner was established using a Global Positioning System (GPS), and the other three corners were laid using bearings in the four cardinal directions orientation. A total of 28 plots, respectively, three shrub sample plots (5 m × 5 m) and three herb sample plots (1 m × 1 m) were set up along the diagonal of each sample plot.

In every plot, measure the base diameter (tree height ≤ 1.5 m) with a vernier caliper or measure the diameter at breast height (DBH) of the stem (tree height > 1.5 m) with a tape measure [19]. Record the number of plants in each plot and their crown diameter measured with a linear tape.

C. gigantea were divided into seedlings (tree height ≤ 1.3 m), juveniles (1.3 m < tree height ≤ 1.5 m), and mature (tree height > 1.5 m) as the survey results. The breast height diameter age (y) of each C. gigantea (tree height > 1.5 m) was calculated using the equation y = 58.143e^0.026x [20]. In every plot, C. gigantea trees were classified into the following 10 age classes using the height plus diameter at breast height method. (1) Class Ⅰ is for seedlings, with tree height ≤ 1.3 m, (2) Class II is for juveniles, with 1.3 m < tree height ≤ 1.5 m, (3) For trees with tree height > 1.5 m, 8 age classes were divided into 20 years as one age class. Class III is for the breast diameter ages of 50–70, Class IV is for the breast diameter ages of 70–90, Class V is for the breast diameter ages of 90–110, Class VI is for the breast diameter ages of 110–130, Class VII is for the breast diameter ages of 130–150, Class VIII is for the breast diameter ages of 150–170, Class IX is for the breast diameter ages of 170–190, Class X is for the breast diameter ages of 190–210. For age class distribution graphs, count the number of trees of each age class in each plot separately. Combine the trees of each age class from all plots within the same population to form the basic age structure data for each population. Plot an age structure diagram with age class as the x-axis and trees/hm² as the y-axis.

Drawing on the method of Guo et al. (2019) [21], the importance values of species in the tree, shrub, and grass layers of the C. gigantea plant community were calculated according to the importance value = (relative multiplicity + relative height + relative cover)/3, respectively. The community species importance value matrix was constructed, and the systematic cluster method (Hierarchical cluster method) was used for the classification of community types. Referencing the method of Wang et al. (2016) [22], the Species with >15% of the species importance value of each layer in the community is the dominant species of the layer for community naming [21].

Static life tables and survival curves were developed based on age-structured data for each population [23]. According to the following equations, the future development trend of C. gigantea populations was predicted for the next 20, 40, and 80 years, respectively [24]. Mt(1)=Mt−1(1)+Xt−X(t−n)n

To determine the impact of natural environmental conditions on C. gigantea growth, we conducted separate surveys of the environmental conditions of various populations [25]. Soil sampling and vegetation surveys were conducted simultaneously. During the reconnaissance survey, record the basic information of the sample plots and randomly collect three soil samples from each sample plot at a depth of 0–20 cm to analyse their physical and chemical properties (Fig. 2). At the same time, information on the species composition, height, and cover of shrubs and herbaceous plants in each sample plot was also obtained. To quantify the relative importance of the eight largely independent environmental variables depicted in Fig. 2 for the renewal and expansion of C. gigantea populations, and to visualize their inter-correlations within the study area, principal component analysis (PCA) was performed on the environmental data set using Origin 2024.

Data entry was performed using Microsoft Excel 2019, analyzed using SPSS 27, and plotted using Origin 2024. The hierarchical cluster method was used to classify community types. Principal component analysis (PCA) was used to explore the relationship between natural growth environments and various populations of C. gigantea. Mapping the geographical distribution of C. gigantea using ArcGIS 10.8.

The species composition is relatively complex, with few tree and shrub species. Among them, C. gigantea is the main species in the tree layer. Through the systematic cluster analysis, integrated C. gigantea habitat conditions, the survey sample site was divided into four populations with C. gigantea as the main species, respectively: Population A (C. gigantea − Ceratostigma minus − Cymbopogon distans + Artemisia wellbyi + Tribulus terrestris), Population B (C. gigantea − Sophora moorcroftiana + Ceratostigma minus − Orinus thoroldii + Artemisia wellbyi), Population C (C. gigantea + Pinus densata − Leptodermis xizangensis + Sophora moorcroftiana − Orinus thoroldii + Artemisia wellbyi), and Population D (C. gigantea − Isodon oresbius + Leptodermis xizangensis + Ceratostigma minus − Artemisia wellbyi) clusters. The importance values for the tree layer, shrub layer, and herb layer are shown in Table 1. The structure of the communities is detailed in Fig. 2.

Population A included six sample plots, with elevations ranging from 3070 to 3181.47 m, C. gigantea plant densities ranging from 36 to 125 individuals/hm², and C. gigantea seedlings were missing individuals, and there were fewer plant species with lower importance values. Population B had five sample plots, mainly located at an elevation of 3060 m, and 58.3% of plants in the sample plots had importance values lower than 10%. Population C contains 9 sample plots, mainly distributed at the altitude of 2991~3103 m, with rich species composition, among which there are two kinds of trees, C. gigantea and Pinus densata, but the importance value of Pinus densata is only 3.87%, and both of them show that there are more young individuals and fewer middle-aged and old individuals. Population D had 8 sample plots, mainly located at 3056~3156 m above sea level, with C. gigantea plant density of 60~475 individuals/hm², and a rich species composition of 63 species, including 48 species in the herb layer, but only Artemisia wellbyi had an importance value greater than 15%. The environmental factors of the community are shown in Fig. 2.

The age structure of populations can objectively reflect the survival status of populations as well as renewal strategies [26]. Fig. 3 shows that the age structures of different C. gigantea populations are basically more young and middle-aged plants and less old-aged plants, with an overall declining trend. Both population A and population B exhibit generational gaps, with a lack of middle-aged and elderly individuals. Population A also exhibits a lack of young individuals. Population C and Population D have C. gigantea individuals of all ages, with more young and middle-aged individuals and fewer old individuals, with Population C having the youngest individuals. It is foreseeable that after 80 years, the current young and middle-aged individuals will enter the middle and old ages, respectively. If there are not enough young individuals to replenish them, the overall stability of the basic populations will be seriously undermined.

Fig. 5 shows that the maximum value of strain trees of each age class of various groups of C. gigantea shifted backward sequentially in the predicted sequence, and the older individuals increased in comparison. However, the number of elderly individuals remains relatively small, and the overall trend continues to show a decline [24].

As shown in Table 3, PC1 contributes more to the variance among the two principal components, reaching 24.8%, with the main significantly correlated indicators being soil organic matter content and soil moisture content. PC2 accounts for 22.9% of the variance. Altitude and soil thickness were the main significantly correlated indicators, with factor loadings of 0.870 and 0.619, respectively. Indicating that altitude and soil thickness were also dominant factors affecting the renewal and expansion of the C. gigantea population.

As shown in Fig. 6, the factors with longer indicator arrows include soil organic matter content, soil moisture content, altitude, and soil thickness. Indicating that these four factors have high loadings and significantly contribute to the ranking. In PC1, Soil Organic Matter Content and Soil Moisture Content have the greatest influence on the C. gigantea population in the study area, and the angle between them is small, indicating a strong correlation between the two. In PC2, Altitude and Soil Thickness also have a significant influence on the C. gigantea population.

C. gigantea is widely used in afforestation, medicine, construction, and the production of Tibetan incense for religious and cultural activities [29]. But it is endangered due to geographic isolation, extreme climates, anthropogenic disturbances, and difficulties in natural regeneration [30,31]. C. gigantea seeds lack powerful dispersers. It has a low reproductive rate, with male and female plants flowering at different times, low seed germination rates, and cones that are susceptible to insect damage. At the same time, the rapid climate change induced by human activities warrants close attention as well [32].

Yang et al.’s [33] survey of the Milin County–Lang County section of the Yarlung Zangbo River revealed that the arid-hot valley characteristics result in overall low species diversity throughout this stretch. Microclimate conditions transition from arid-hot to humid-cold with increasing elevation, while soil types gradually shift from sandy to loamy. Among these factors, influenced by altitude, slope aspect, soil moisture content, soil nutrients, and soil density, C. gigantea is predominantly distributed within the 0–200 m elevation range above the river surface, at an altitude of 3000–3400 m [33]. In this paper, we investigated the relationship between the distribution of C. gigantea communities and environmental factors. The results also showed that the environmental factors affecting the process of renewal and expansion of C. gigantea populations mainly include soil organic matter content, soil moisture content, altitude, and soil thickness, etc.

Moreover, for plants, the root system plays a pivotal role in the uptake and transport of soil moisture and mineral nutrients. It influences plant growth by extracting soil moisture, thereby sustaining transpiration and maintaining the plant’s hydration status, which in turn supports growth and physiological responses [34–37]. Simulation studies indicate that surface soil moisture will decrease significantly in certain regions, whilst deep-seated (or even bedrock) moisture levels will show a lesser decline or even a slight increase. This pattern of shallow dryness and deep moisture is driving plants to extend their water-absorption depth to greater depths during drought periods, thereby maintaining transpiration and water balance [38–42]. However, for C. gigantea seedlings, this strategy may prove counterproductive. As key nutrients such as nitrogen and phosphorus are predominantly concentrated in the topsoil layer, their availability is highly dependent on diffusion and mass transport processes driven by soil moisture [43]. Should the upper layers become water-deficient, nutrient migration is impeded, leading to a concurrent decline in mineralisation, litter decomposition, and mycorrhizal activity. Nutrients become “locked” within the parched topsoil, rendering them inaccessible to seedlings with shallow root systems [44–46]. Although deep roots can alleviate water stress, they cannot compensate for the disruption of nutrient supply in the upper layers. Consequently, seedlings face nutrient deficiency even when water remains accessible, thereby inhibiting their early growth and survival. Therefore, the redistribution of soil moisture from shallow aridity to deep hydration may become a critical bottleneck limiting seedling establishment and population renewal by diminishing the availability of nutrients in the topsoil.

At the same time, although certain higher-altitude regions possess suitable soil depth to provide C. gigantea with a stable growing medium, high elevations are susceptible to environmental fluctuations. This results in shallow soil layers with limited water retention capacity, causing fertility to plummet abruptly with shifts in moisture levels. Naturally regenerated C. gigantea seedlings are predominantly concentrated in areas adjacent to rivers. Although low-altitude regions readily accumulate thick soil layers with abundant water retention and high organic matter content, they are frequently subjected to erosion through surface runoff and lateral scouring by river currents. This process carries away C. gigantea seedlings along with surface nutrients, encompassing individuals of all age classes within the species, and has led to frequent generational gaps within the population. Of course, the occurrence of generational gaps within populations of C. gigantea may be attributable to a variety of complex factors, including human and livestock disturbance, natural disasters, and other such circumstances.

Subsequent research may investigate the root systems of C. gigantea across different age classes in relation to the physicochemical properties of soil at various depths. Further studies may also explore the effects of light, temperature, heat, and human and livestock disturbance on the survival and reproduction of C. gigantea populations.

Research findings indicate that among the various populations of C. gigantea, the number of extant individuals is greater in the juvenile and subadult age classes, with fewer individuals in the adult and old age classes. Research findings also indicate that populations of C. gigantea possess the capacity to attain considerable longevity. However, frequent generational gaps occur within various C. gigantea populations, accompanied by high mortality rates during the transition from juvenile to middle-aged and elderly stages. Overall, these populations exhibit a severe trend towards ageing and have even reached a critically endangered state. Therefore, given the unique habitat requirements of C. gigantea, it is imperative to implement corresponding conservation measures and restoration strategies. (1) Enclose the natural habitats of all populations of C. gigantea to eliminate threats from humans and livestock, thereby restoring its natural ecosystem. (2) Establish an artificial breeding research base to enhance studies on the resistance of C. gigantea across all age groups, alongside breeding and domestication efforts. (3) Implement appropriate relocation conservation measures to seek environments more suitable for the survival of C. gigantea. (4) Strengthen management and conservation awareness campaigns, and undertake the collection and preservation of germplasm resources. (5) Conduct timely and appropriately scaled replanting in populations lacking seedlings, while strengthening protection for individuals of all age classes within C. gigantea, particularly those of middle to advanced age.