1. Introduction
Forest ecosystems serve as biodiversity hotspots and plays pivotal roles in climate regulation, soil and water conservation, etc. [
1,
2]. However, global warming is profoundly reshaping the structure and function of forest ecosystems [
3]. The IPCC [
4] report highlights that rising global temperatures have increased the frequency of extreme climate events, significantly affecting the tree growth, biodiversity, and carbon storage capacity of forests. Coniferous forests, as typical vegetation in cold regions, are particularly sensitive to changes in temperature and precipitation [
5,
6,
7,
8]. Devi et al. [
9,
10] pointed out that continuous climate change could impact plant growth and disrupt the functions of ecosystems. Similarly, Linder et al. [
11] found that warming can affect phenology, alter photosynthesis, and consequently influence plant growth, species composition, forest distribution, and even the overall stability of forest ecosystems. In cold semi-arid regions, forests are now under considerable stress due to their inherent vulnerability and sensitivity to environmental changes. Given these climate-driven pressures, understanding stand structural dynamics—particularly size, age, and regeneration patterns—becomes critical to assessing forest structure and guiding adaptive management.
Stand groups constitute structural entities where conspecific cohorts coexist through niche-mediated associations, forming the fundamental demographic units for population persistence and community succession. Stand structure attributes involve characteristics such as tree size, diameter, height, diversity, etc. [
12,
13,
14,
15,
16], while variations reveal the temporal and spatial changes in the composition and abundance of stand groups [
17,
18,
19,
20]. Diameter at breast height (DBH) distribution serves as a critical indicator of forest stand structure, with variations across DBH classes revealing insights into regeneration status and ecological stability [
21]. Research has highlighted the importance of trees with large and extra-large DBH in carbon storage and habitat provision within forest ecosystems [
22]. Conversely, a higher proportion of trees with small DBH often signifies active regeneration capacity, with forests undergoing recovery or expansion [
23]. Tree height distribution further reflects species competition and forest stratification [
24]. Forests composed of mature trees are characterized by complex tree height distributions with multi-layered vertical structures, enhancing resource efficiency and biodiversity [
25]. In contrast, forests composed of seedlings and saplings exhibit concentrated lower height distributions, reflecting rapid growth potential and intensified resource competition [
26]. The ecological succession theory of Odum [
27] underscores the linkage between forest stability and tree age composition. A higher proportion of young stands typically indicates strong regeneration capacity, while a dominance of mature and old stands reflects stability and ecological maturity [
22].
Static life table and dynamic index analysis provide further quantitative tools for understanding changes in the survival and mortality of species [
23]. Previous studies on species structure and dynamics have commonly utilized DBH class data to construct static life table and develop tools such as quantitative dynamic indices and survival curves [
16,
28,
29,
30]. These methods allow for the analysis of current survival status and environmental adaptability while shedding light on natural regeneration patterns and responses to disturbances. However, although DBH data are easy to obtain and operate, they cannot accurately reflect tree age information, which may lead to misunderstandings of the structure and variations in stand groups. Using tree age data by tree-ring analysis via tree core sampling can provide a more precise basis for examining the structure and variations in forest stand groups [
13].
The Altay Mountains in Northwest China is an important forest ecosystem, with an average elevation of 2140 m. However, with the intensification of global climate change and human activities, the natural forests in the Altay Mountains of Northwest China face severe ecological pressures, including warming temperatures, shifts in precipitation patterns, etc. Larix sibirica Ledeb., Picea obovata Ledeb., and Abies sibirica Ledeb. are the dominant coniferous species in the Altay Mountains in Northwest China. A. sibirica is an evergreen species that prefers sunlight and is adapted to cool and wet climatic conditions. It is mainly distributed between 1400 and 2400 m in the Altay Mountains of Northwest China. P. obovata is also an evergreen tree species that prefers sunlight and is adapted to cool climate, with an elevation range of about 1200–2500 m. In contrast, L. sibirica is a typical positive pioneer species in the natural distribution area (1300–2600 m in elevation), growing rapidly and exhibiting strong drought resistance. These dominant coniferous species coexist through niche differentiation, reflecting certain similarities (such as adaptation to cool climate) while also exhibiting ecological distribution and functional differences. Research on coniferous species in the Altay Mountains of Northwest China has primarily focused on ecological adaptation, biomass estimation, climatic responses, and anthropogenic disturbance impacts, lacking in-depth analyses of stand structure and regeneration capacity, particularly regarding static life tables and dynamic quantitative analyses, resulting in limitations in assessing population stability.
This study aims to analyze the structure and regeneration of coniferous stand groups in the representative montane forests of the Altay Mountains in Northwest China through survey data from sampling plots. The purpose of this study aims to address the following two questions: (1) How do structural heterogeneity and regeneration capacity differ among dominant coniferous stand groups in the representative montane forests, and what do these differences reveal about their future ecological management? (2) Does the species’ complementarity effect in the mixed stand group (A. sibirica-P. obovata-L. sibirica) enhance the radial growth rate and disturbance resistance? The findings are of significant guiding importance for formulating effective forest management and conservation plans, ensuring the sustainable development of the forest ecosystem of the Altay Mountains.
4. Discussion
A. sibirica and
P. obovata are heliophilous, evergreen coniferous tree species, while the
L. sibirica is a heliophilous and deciduous coniferous species. These characteristics determine differences in ecological functions, structures, and regeneration capabilities [
7]. Both the
A. sibirica and
P. obovata are shade-tolerant species, often occupying lower canopy positions in the stand groups, and maintain regeneration in understory environments through strong regenerative capacity.
L. sibirica exhibits strong drought resistance, often serving as a pioneer species in establishing communities in arid and nutrient-poor environments [
8].
The DBH, tree height, age, and density variations in the
L. sibirica and
P. obovata stand groups all showed a unimodal distribution, indicating a certain stability in the stand structure. Pretzsch [
22] noted that the concentration of tree height and the height range of mature trees often reflect the coordinated development of trees in terms of light exposure, spatial competition, and resource acquisition. The distribution patterns of DBH, tree height, and age indicate that the mixed stand group possesses strong regeneration ability, while the regeneration ability of the
L. sibirica stand group is weaker [
41]. The mutualistic competition between
A. sibirica and
P. obovata in the mixed stand group may promote regeneration through resource allocation optimization, while the monospecific stand group is limited by ecological niche overlap of a single species (such as light competition) [
42]. However, the lack of large DBH trees within the mixed stand group will also impact its stability. From a forest development perspective, the three stand groups represent a complete successional sequence: the
L. sibirica stand group corresponds to a mature community, the mixed stand group reflects an early secondary successional stage, and the
P. obovata stand group represents an intermediate successional phase. Thus, the observed spatial heterogeneity in stand structures is essentially a spatial manifestation of temporal succession, making age differences an inherent and meaningful component of the natural forest dynamics.
The survival curves of the three stand groups all exhibit Deevey-II type, which is consistent with the findings of Liu et al. [
30] regarding
Taxus cuspidata in Northeast China, indicating that the seedling mortality is high in the early stage, while the growth in the following stage tends to stabilize. As time progresses, the mortality rate decreases, leading to a more stable structure in the mid-to-late-stage. Hu et al. [
43] indicated that the factors affecting the mortality rate of evergreen coniferous species shift from abiotic to biotic as size increases, whereas the mortality rate of deciduous broadleaf species is primarily influenced by biotic factors. The stability of the number of surviving individuals in different life stages of the
L. sibirica stand group further demonstrates its lower mortality rate and higher survival rate, which may be key factors allowing this species to maintain sustainable vegetation development in the face of competition and environmental changes. A study conducted by Kharuk et al. [
44] in the Sayan Mountain showed that
L. sibirica has strong adaptation potential under future climate conditions due to its drought tolerance. The mixed stand group shows high densities of saplings and seedlings, but also high mortality rates, reflecting its sensitivity to environmental stress and inter- and intra-species competition, a phenomenon closely related to density-dependent mortality mechanisms [
27,
45]. For heliophilous species, neighboring competition is the main driving force behind trees’ mortality throughout their lifespans [
43]. Davis and Condit [
46] also noted that tree growth and survival are influenced by neighboring plants, including both resource competition and facilitation, highlighting the importance of neighboring plants in shaping structure. As tree age increases, the density of the three stand groups shows a declining trend. In the late stages, the significant decrease in density poses greater mortality risks, especially under conditions of insufficient resource supply [
23,
47,
48].
The standardized number of dead individuals, individual mortality rate, and disappearance rate in the
P. obovata stand group exhibited negative values in the early stage. According to Proctor [
49], while negative values in the static life table are inconsistent with mathematical assumptions, they can still provide useful ecological information. This phenomenon may be attributed to density effects. The observed maximum density, reaching 2388 individuals/ha, may be linked to positive density dependence effects, such as group dynamics or local seed dispersal aggregation [
50,
51]. Although an increase in density may lead to increased competition for resources, resulting in a negative density-dependent effect (such as a decrease in individual survival due to resource scarcity), a positive density-dependent effect can promote the survival and reproduction of species under certain conditions. The high density and robust regeneration capacity of young
P. obovata individuals likely explain the emergence of negative values. Despite challenges from resource competition and environmental stress, the species sustains itself through high seed germination rates and rapid replenishment. Thus, it can be inferred that the characteristics of
P. obovata in the early stage are characterized by a “rapid replenishment—high elimination” model. Zhang et al. [
16] conducted a study on the population of
Pinus koraiensis in the Changbai Mountain and found that the population shows a growth type, and the high mortality rate of juvenile individuals, limited living space and resource conditions, and the obvious physiological aging of older individuals are the main reasons restricting the growth of
P. koraiensis population, as is consistent with the changes observed in the
P. obovata stand group in this study.
The mixed stand group has a significant advantage in radial growth, while the monospecific stand group may perform poorly due to lower resource utilization efficiency or weaker ecological adaptability. In the mixed-species stand group, different tree species exhibit differentiated strategies in light, nutrient, and water utilization, effectively utilizing resources at different levels through niche differentiation, reducing resource competition, and optimizing resource allocation [
42]. Compared to monospecific forests, the interactions between different tree species in mixed forests lead to higher growth rates and biomass, especially in resource-limited environments [
52,
53]. According to the study by Loreau and Hector [
54], biodiversity positively impacts the productivity of ecosystems, and the complementarity effects between species significantly increase with species richness. Mixed stand groups enhance the ecosystem’s adaptability to environmental changes by increasing species diversity and niche differentiation, thereby promoting the radial growth of species. The mixed stand group in this study exhibits a variation in “growth-stable-growth”, showing the most pronounced overall growth trend regardless of external disturbances. This stability can be attributed to the species diversity of the mixed-species stand group and its more effective resource utilization and niche differentiation abilities [
55]. The growth potential of the monospecific stand group (
L. sibirica or
P. obovata) is weak, particularly under disturbance conditions, which has a more pronounced negative impact on their species quantity [
42]. However, the maximum risk probability of random disturbances shows that the mixed stand group has the highest maximum risk probability, indicating that it is more sensitive to random disturbances. This is because the species composition and structure within the mixed stand group are more complex, and in the face of external disturbances, the responses of different species may have conflicting synergistic and competitive effects [
54]. The
L. sibirica stand group has the lowest maximum risk probability, indicating a strong ability to withstand random disturbances, which can be attributed to its advantages in niche adaptability in unfavorable environmental conditions [
7,
56].
Although this study employed a systematic sampling design to capture the characteristics of dominant coniferous stand groups in the representative montane forests of the Altay Mountains in Northwest China, the representativeness of the sample data remains subject to certain limitations. Additionally, due to the absence of long-term, repeated measurements, the static life table analysis provides results that approximate the probability of occurrence for trees within specific size classes rather than direct estimates of survival or mortality rates. To enhance the robustness of future research, efforts should focus on expanding the number of sample plots through comprehensive field surveys, supplemented by remote sensing technology for broader spatial coverage and long-term monitoring [
57]. Such an approach should encompass a wider range of ecological gradients and environmental conditions to more accurately elucidate the structural and dynamic characteristics of the dominant coniferous species in the Altay Mountains region.
Furthermore, this study prioritizes stand-scale structural heterogeneity and regeneration dynamics as immediate drivers of conservation planning. However, future research could integrate two complementary axes to deepen actionable insights. First, linking stand complexity patterns (e.g., canopy gaps, species mixtures) with individual tree growth responses across climatic gradients may reveal how structural diversity balances ecosystem stability (e.g., carbon sequestration) and regeneration capacity (e.g., post-disturbance recruitment). Second, coupling these ecological assessments with species-specific climatic thresholds (e.g., drought tolerance, thermal optima) could identify priority areas where targeted interventions (e.g., density management, assisted migration) would most effectively buffer against climate-driven regeneration failures. For instance, high-resolution projections of thermal stress zones, overlaid with stand structural maps, might pinpoint vulnerable slopes where assisted migration of drought-tolerant genotypes could prevent recruitment collapse. Such cross-scale syntheses would align restoration targets with physiological and climatic realities, ensuring that management strategies are both ecologically viable and climate-responsive.