Contrasting species composition between common and rare woody species in the intake area of the South-to-North Water Diversion Project

Understanding the species composition and structural dynamics of both common and rare species in forest communities is fundamental to comprehending forest ecosystem functioning. The intake area of the South-to-North Water Diversion Project, recognized as an ecologically sensitive region, harbors forest communities that play a pivotal role in water resource conservation and regional environmental stability. A thorough investigation into the species composition and distributional patterns of common and rare species within this area provides critical scientific insights into biodiversity conservation and community assembly mechanisms. To address these questions, we established 36 permanent plots of 20 × 20 m in the intake area of Nanyang City, Henan Province. Within each plot, we systematically surveyed all woody species and subsequently analyzed their species composition, diameter- and height-class structure, as well as the sexual system composition of both common and rare species. Our results revealed that: (1) A total of 2, 210 woody individuals were recorded in the intake area, belonging to 26 families, 33 genera, and 39 species. Common species exhibited absolute dominance in both individual abundance and importance value. (2) Species accumulation curves showed a sharp increase in species number when plot counts were below 20, followed by a marked deceleration thereafter. (3) Common species dominated across all diameter and height classes, while rare species were largely confined to diameter class II (5 cm ≤ DBH < 10 cm) and height class III (6 m ≤ H < 9 m), indicating a conspicuous growth-stage bottleneck. (4) Common species were predominantly monoecious plant, whereas hermaphroditic species occurred mainly as rare species. Collectively, these findings demonstrate that common and rare species jointly underpin the stand structural integrity, species diversity, and long-term successional potential of the forest community in intake area.

Spatial variation in species richness per unit area represents one of the most prominent patterns in biodiversity (Jetz and Rahbek, 2002). It arises from the spatial overlap of species across a continuum from extremely rare to common taxa, reflecting the inherently uneven distribution of abundances among species within local communities (van Schalkwyk et al., 2019; Bregović et al., 2019). This implies that local species richness is generated through the additive contributions of both rare species and common species, representing the spatial superimposition of these two contrasting groups (Lennon et al., 2004). Given the marked differences between rare and common species in geographic range, occupancy, abundance, habitat breadth, and biomass, their respective contributions to ecosystem functions and processes are also likely to diverge considerably (Lyons et al., 2005; Chapman et al., 2018). Understanding the ecological differences between common and rare species at the local community scale may thus serve as a critical gateway to elucidating the mechanisms underlying biodiversity maintenance.

Common and rare species differ systematically across multiple ecological axes (Lyons et al., 2005). At the local scales, common species are widely distributed, numerically dominant, and evenly spread across species richness gradients, thus contributing more to overall richness patterns (White et al., 2022). They also tend to possess higher reproductive output and dispersal capacity, which confer them with competitive advantages during community assembly (White et al., 2022). In contrast, rare species have smaller populations, restricted ranges, and narrower niches, making them more vulnerable to disturbance, environmental change, and competitive exclusion (Pérez-Quesada and Brazeiro, 2013; Zhang et al., 2022). These ecological differences also apply to plant sexual systems, reflecting divergent reproductive strategies under varying population densities (Wang et al., 2020).

However, rare species should not be underestimated: they often occupy unique ecological niches in community functional space and enhance community adaptability to diverse environments via niche complementarity (Liu et al., 2024). For instance, rare species contribute disproportionately to community trait space relative to their numerical abundance (Zhang et al., 2022). Their distinctive functional trait combinations mean that declines in rare species abundance can profoundly affect forest function, functional redundancy, and resilience (Leitão et al., 2016; Jain et al., 2014). Tang et al. (2023) further demonstrated that rare species drive α-diversity and contribute more to forest multifunctionality than common species. Thus, clarifying the ecological traits of common versus rare species and understanding the differentiation in their population characteristics is critical for uncovering the mechanisms that sustain forest communities.

The intake area of the South-to-North Water Diversion Project, designated in 2025 as a national practice and innovation base for the concept that “lucid waters and lush mountains are invaluable assets”. The structure, composition, and functioning of its forest communities largely determine the water quality safety and ecological stability of the water source (Li et al., 2022, 2024). Located in the transitional zone from the northern subtropical to the warm-temperate climatic region, this area harbors diverse vegetation types and marks a key junction for the Floristic Region of Central and North China. Xichuan County within the area, the core water source of the Middle Route project, possesses rich forest resources and has developed a positive feedback loop of forest-based water conservation (Liang et al., 2026). Nevertheless, poor understanding of local species diversity and successional patterns has hindered science-based biodiversity conservation in the region.

To address these knowledge gaps, this study focused on the woody plant communities in the intake area of the South-to-North Water Diversion Project. Based on 36 established forest plots, we analyzed the differences among common, occasional, and rare species in terms of community composition, diameter-class and height-class structures, and sexual system composition. Specifically, we addressed three questions: (1) what is the compositional structure of common, occasional, and rare species in the plant communities of the intake area? (2) what is the diameter-height class distribution characteristics of common, occasional, and rare species? and (3) what are the sexual system characteristics of the plant communities? Based on the ecological differences between common and rare species documented in previous studies, we formulated three a priori hypotheses corresponding to these questions: (1) common species would dominate community composition in terms of individual abundance and importance value; (2) common species would dominate across all diameter and height classes and shape the structural framework of the forest community, whereas rare species would lack large-sized individuals; and (3) monoecious species would predominate among common species, whereas hermaphroditic taxa would be overrepresented among rare species, reflecting divergent reproductive strategies under contrasting population densities. This study aims to provide theoretical foundations and data support for the management and sustainable utilization of regional forest resources.

Site description

Our study focused on the tree communities starting hub of the Middle Route of the South-to-North Water Diversion Project, located in Nanyang, Henan, China. The topography descends from northwest to southeast, shaped by the Han River system into a stepped landscape of alternating mountains and valleys. The region has a transitional zone from subtropical to warm-temperate climates, with annual sunshine duration of 1, 800~2, 100 h, mean annual temperature of 14.4~15.7 °C, and a frost-free period of 230d. Mean annual precipitation ranges from 800 to 1, 000 mm, of which the rainy season (June–August) contributes ~56 %, and mean annual relative humidity is about 75% (Liang et al., 2026).

The vegetation in the study area is classified as forest – deciduous broad-leaved forest – temperate deciduous broad-leaved forest, dominated by Quercus variabilis and Pinus massoniana population. The region harbors over 80 species of nationally and provincially protected plants, i.e., Quercus acutissima, Ginkgo biloba, Metasequoia glyptostroboides, Glyptostrobus pensilis, Camellia oleifera, Castanea mollissima, and others.

Sample plot establishing

We conducted a forest community survey in the intake area of the South-to-North Water Diversion Project in Xixia, Neixiang, and Xichuan counties, Nanyang, Henan, China. Notably, Xixia and Xichuan counties were designated in 2025 by the Ministry of Ecology and Environment as national practice innovation bases for the “lucid waters and lush mountains are invaluable assets”. Following the minimum-area criterion, we established 12 permanent 1-ha forest plots. Within each plot, three representative 20 m × 20 m subplots were randomly selected, a total of 36 subplots. All plots were placed in areas with weak human disturbance, relatively uniform slope and aspect for field vegetation surveys (Figure 1).

Within each subplots, we measured all woody stems with diameter at breast height (DBH) ≥ 3 cm, and recorded the species name, DBH, and tree height for each individual. In this study, we analyzed only individuals with DBH ≥ 3 cm, as smaller stems were predominantly shrubby and clonal, often representing multiple branches of the same genet, which would have inflated stem counts and obscured structural patterns. All woody plants were identified to species using the electronic version of the Flora of China (available at https://www.iplant.cn/frps2019/), from which we also recorded family, genus, and sexual system for each species.

We calculated α-diversity indices including species richness, Shannon-Wiener index, Simpson diversity index, and Pielou’s evenness index. Considering the existence of singleton species and uneven sampling effort across subplots, rarefaction resampling was performed prior to diversity calculation. Each subplot was randomly resampled to the minimum number of individuals (21 individuals), and the resampling procedure was repeated 100 times to reduce stochastic variation. For Pielou’s evenness, only replicates containing more than one species were used for averaging; plots with zero valid replicates were treated as not applicable (—). All diversity analyses were implemented using the “vegan” package in R (R Development Core Team, 2020).

Based on the presence or absence of pistils and stamens in flowers (floral morphology), sexual systems were classified into the monoecy, dioecy, and hermaphroditism. Species that could not be assigned to these three categories were grouped as “others”, including taxa with polymorphic sexual expression, ambiguous floral morphology, and other cases that cannot be reliably determined.

Data analysis

Species classification

To distinguish the functional roles of common and rare species in the community, we utilized local abundance to separate common, occasional, and rare species within each plot. Species with a density ≤ 1 ind./ha were defined as rare species; those with a density between >1 and ≤10 ind./ha were considered occasional species; and species with a density >10 ind./ha were classified as common species (Hubbell and Foster, 1986).

Importance value

Importance value (IV) is a comprehensive quantitative metric that reflects the overall dominance and functional contribution of each species in the community, calculated as the sum of relative abundance, relative frequency, and relative dominance (based on basal area).

Importance value (IV) of each species was calculated as the sum of its relative basal area (RB), relative abundance (RA), and relative frequency (RF). These three relative indices were computed as: RB = (basal area of the species i/ total basal area of all species) × 100%; RA = (number of individuals of the species i/ total number of individuals of all species) × 100%; and RF = (frequency of the species i /sum of frequencies of all species) × 100%.

Species - area relationship

Species accumulation curves (SAC) serve as an effective approach for assessing species diversity and identifying rare species within a given region (Liang et al., 2025). To characterize the variation in species richness with increasing sampling area, we used 20 m × 20 m subplots as the basic sampling units. We performed repeated random sampling without replacement, progressively increasing the number of units, and constructed accumulation curves by plotting cumulative species richness against sample size using a random accumulation algorithm. All analyses were implemented with the vegan package in R 4.0.1 (R Development Core Team, 2020).

Cumulative rank - abundance curve

Rank-abundance curves (RAC) were constructed by ranking species in decreasing order of abundance, with rank on the x-axis and species abundance on the y-axis. The curves reflect both species richness and evenness: a steeper curve indicates dominance by few species, whereas a flatter curve suggests more equitable individual distribution. To better illustrate abundance patterns, we annotated the RAC with the abundance ranges of common species, occasional species, and rare species, highlighting their contributions to community structure.

Diameter and height class structure

For diameter class structure, we used 11 classes with a 5-cm class width, starting from a minimum diameter of 3 cm (e.g., I: < 5 cm; II: 5 cm ≤ DBH < 10 cm; and so forth). For height class structure, we applied the upper-boundary exclusion method (i.e., intervals defined as [lower, upper)), with a starting height of 1.8 m and a 3-m interval, which resulted in six height classes (e.g., I: H < 3 m; II: 3 m ≤ H < 6 m; and so forth).

Species composition and diversity of woody plants

In 36 subplots (20 m × 20 m) in the intake area of the South-to-North Water Diversion Project, we recorded 2, 210 woody plants with DBH ≥ 3 cm, representing 26 families, 33 genera, and 39 species (Supplementary Tables S1, S2). The families contributing the largest number of individuals include Fagaceae, Pinaceae, Juglandaceae, Cupressaceae, and Fabaceae (Supplementary Table S2).

There are differences in species richness, species abundance, Shannon–Wiener index, Simpson index, Pielou’s evenness among plots (Supplementary Table S3). Species richness rose rapidly with sample number, and although the accumulation rate declining after 20 plots, a continued upward trend was still evident. Notably, 24 sample plots captured 90% of species, indicating the selected plots adequately represented the forest communities in the intake area (Figure 2).

The rank–abundance curve revealed pronounced dominance of a few species within the sampled plots (Figure 3). Among the 39 recorded species, 11 were common, 18 occasional, and 10 rare, accounting for 28.2%, 46.2%, and 25.6% of total species richness, respectively. The cumulative abundance of the top 11 species reached 2, 117 individuals, or 95.8% of the total, indicating a high degree of community dominance. Occasional and rare species, despite representing most of the species’ pool, contributed only marginally to overall abundance because of their small population sizes.

Quantitative characteristics of common, occasional, and rare species

The top five species in terms of importance value across the study plots were, in descending order: Quercus variabilis, Pinus massoniana, Pistacia chinensis, Albizia kalkora, and Platycarya strobilacea (Table 1). Common species consistently exhibited higher relative abundance, relative frequency, relative dominance, and importance value than occasional and rare species. Importance values ranged from 3.31% to 55.55% and collectively accounted for 76.5% of the total importance value across all species in sample plots. In contrast, the top five rare species had equal values of relative abundance and relative frequency, with their importance values remaining consistently at 0.25%, whereas importance values of occasional species ranged from 0.65% to 1.07%.

Diameter and height class distributions

Common species dominated across all diameter and height classes, with the number of individuals in small- to medium-diameter classes (Class II) and the corresponding height classes being markedly higher than in other classes (Figure 4). Occasional and rare species lacked large-diameter mature individuals, with their populations concentrated primarily in the small- to-medium diameter classes. This pattern was especially pronounced for rare species, which were absent from both the smaller diameter classes and the lower height classes.

Sexual system characteristics of common, occasional, and rare species

Among the three sexual systems (Table 2), monoecious plants were numerically dominant, with 1, 972 individuals (89.2% of total stems), followed by dioecious (142 individuals) and hermaphroditic (16 individuals) species. In terms of species richness, monoecious and hermaphroditic species each comprised 13 species, whereas dioecious species accounted for 11 species, representing a relatively lower proportion of the total species pool.

The sexual system composition differed markedly among common, occasional, and rare species (Figure 5). In common species, monoecious plants accounted for the largest proportion in terms of both number of individual and species abundance. In occasional species, dioecious plants had higher number of individual than monoecious and hermaphroditic species. In contrast, rare species were predominantly characterized by hermaphroditic sexual systems.

Species composition of woody plants

Based on the survey, the woody plant community in the sample plots comprised 39 species, belonging to 33 genera and 26 families (Supplementary Tables S1, S2). Among the genera with relatively high individual abundances, most exhibited temperate floristic affinities, whereas only Pinaceae, Cupressaceae, Sapindaceae, and Pittosporaceae were characterized as subtropical elements. In terms of species composition, temperate deciduous species accounted for 34 of the total species, representing the dominant component, while subtropical species contributed a considerably lower proportion. This compositional pattern in the study area indicates that the forest community exhibits transitional characteristics between subtropical and warm-temperate floristic regions. Analysis of the abundance accumulation curve revealed that the woody plant community in the intake area comprised 11 common species, 18 occasional species, and 10 rare species, with no significant difference in species numbers between common and rare species. This classification indicates that occasional species constituted a larger proportion of the local species pool. The steep ascending phase of the accumulation curve reflects a high degree of dominance by a few dominant species, whereas the flattened segment corresponding to rare species suggests a relatively even distribution of individual abundance among species (Matthews et al., 2026). Nevertheless, although rare species contributed only marginally to total individual abundance (accounting for just 25.6% of species richness but far less in stem numbers), their presence substantially elevated the overall species richness of the community. This pattern underscores the importance of rare species as a critical component of the local species pool, despite their limited numerical representation, and highlights that community diversity assessments based solely on abundance would significantly underestimate the contribution of these sparsely distributed taxa.

In this study, the top five species in terms of importance value were, in descending order, Quercus variabilis, Pinus massoniana, Pistacia chinensis, Albizia kalkora, and Platycarya strobilacea. This compositional pattern is indicative of a forest community shaped by a few dominant taxa, a typical structural feature of forests in the transitional zone between warm-temperate and subtropical climates. Notably, the relatively high importance values of Quercus variabilis and Pinus massoniana indicate that these species possess broad ecological amplitudes and strong competitive abilities, enabling them to persist across diverse microhabitats (Wang et al., 2023), and thus form the fundamental structural framework of the forest community in the intake area. Meanwhile, the importance values of rare species were significantly lower than those of common species, which does not mean that the role of rare species in forest ecosystems can be ignored (Mouillot et al., 2013). Rare species contribute to functional diversity through their functional uniqueness, exceeding expectations based on their limited abundance, occupancy, or distribution range (Leitão et al., 2016). Previous studies have shown that rare species have lower functional redundancy (FR) but higher functional vulnerability (FV) compared to common species (Liu et al., 2024). This indicates that the loss of rare species is more likely to lead to the decline of community ecological functions, potentially affecting the functionality and resilience of ecosystem (Liu et al., 2024). Occasional species accounted for the largest proportion of the local species pool in this study (18 species, 46.2%), yet their importance values ranged only from 0.65% to 1.07%. This pattern may reflect the influence of local environmental conditions or historical disturbance events on species abundance distributions (He et al., 2024). Despite constituting the largest proportion of the species pool, occasional species exhibited consistently relative lower importance values, persisting as minor components under current stand conditions. They may represent a reservoir of opportunistic species whose expansion is likely contingent upon specific disturbance events (He et al., 2024), a hypothesis requiring long-term demographic monitoring for verification.

Variation of diameter and height class from common, occasional, and rare species

The distribution of individuals across diameter and height classes offers a spatial perspective on the developmental stages of the community, providing theoretical insights into forest population dynamics and community assembly. Common species dominated all diameter and height classes, with the highest individual count recorded in class II (5 cm ≤ DBH < 10 cm), which corresponded predominantly to height class III (6 m ≤ H < 9 m). This pattern suggests a stable regeneration status within the community, where sapling recruitment appears sufficient to sustain intergenerational replacement of common species. However, the relatively low abundance in class I (3 cm ≤ DBH < 5 cm) may be primarily associated with the high density and diversity of understory shrubs, which likely impose constraints on early-stage recruitment (Chen et al., 2026). In the early stages of regeneration, saplings often suffer from limited resource acquisition, placing them at a competitive disadvantage against both adult trees and the shrub layer (Sinclair et al., 2020). Notably, common species, particularly Quercus variabilis, persist steadily across large diameter classes, a pattern that strongly attests to their prolonged life cycles and superior competitive ability. By virtue of their continuous and concentrated distribution across diameter classes, coupled with their numerical dominance, these common species form the structural backbone of the community, thereby securing the demographic foundation necessary for long-term forest stability.

Occasional and rare species exhibited distribution patterns in both diameter and height classes that were notably different from those of common species. In terms of diameter structure, rare taxa were mostly confined to classes II (5 cm ≤ DBH < 10 cm) and III (10 cm ≤ DBH < 15 cm), with a complete absence of individuals in the largest size class (≥30 cm). Correspondingly, in height class structure, rare species were also restricted predominantly to classes II (3 m ≤ H < 6 m) and III (6 m ≤ H < 9 m), lacking individuals in the upper height strata (≥12 m). This consistency between the upper limits of diameter and height distributions suggests that rare species face dual constraints in vertical space occupation, unable to progress into the upper canopy either through diameter increment or height growth. Rather than reflecting simple competitive suppression by canopy shading (Lu et al., 2021), this pattern is more consistent with the behavior of gap-dependent or mid-successional taxa that require canopy openings for successful regeneration. Under closed-canopy conditions, these species may persist as static remnants rather than through dynamic regeneration (Xu et al., 2023), maintaining a limited number of intermediate-sized individuals without producing new recruits or advancing into the upper canopy. The complete absence of rare species in both the smallest and largest size classes further support this conclusion.

The composition of sexual system

Plant sexual system governs individual fitness, stand dynamics, genetic diversity, and interspecific interactions, thereby shaping the ecological and evolutionary composition of plant populations (Etterson and Mazer, 2016; Carta et al., 2025). In terms of individual numbers, monoecious plants exhibited absolute dominance, accounting for 1, 972 individuals (89.2% of the total). This dominance indicates that monoecious species have developed notably large population sizes within the forest community.

A significant divergence in sexual system composition emerged among common, occasional, and rare species. For common species, monoecious plants dominated in both individual abundance and species richness. This pattern reflects a differentiated niche strategy in community assembly and species coexistence among sexual systems. The underlying mechanism likely lies in the reproductive strategy of monoecious plants, which improves population expansion capacity and environmental adaptability under intense resource competition, allowing them to maintain a dominant position and sustain large population sizes within the community (Peng et al., 2014; Käfer et al., 2017). Due to high self-compatibility, monoecious plants can rapidly established populations during the early colonization phase, a trait particularly critical for the initial assembly of forest community (Käfer et al., 2017). Meanwhile, common species act as the dominant taxa under environmental filtering and exhibit strong ecological adaptability to local habitat conditions (Antorán et al., 2026). Notably, the sexual system allocation observed for common species in our study is consistent with previous findings in temperate forests (Vary et al., 2011), indicating that such allocation patterns possess are generalizable across regions with similar climatic conditions.

The predominance of hermaphroditic taxa among rare species may be attributable to their intrinsically small population sizes and low local densities. Rare species are generally characterized by low population density, and any reproductive strategy depending on cross-pollination between distinct individuals therefore faces a substantially elevated risk of failure (Renner, 2014). For rare species with small population sizes, hermaphroditism represents a bet-hedging reproductive strategy (Busch and Delph, 2012). In habitats where conspecific pollen availability is insufficient, hermaphroditic species take advantage of their self-compatibility (Cardoso et al., 2018), even when pollinators are scarce of conspecific mates are absent. Meanwhile, this reproductive constraint is further compounded by limited understory light availability caused by canopy shading from overstory trees, which reduces photosynthetic efficiency and reproductive output (Reek et al., 2026). The combined effect of this light limitation and their reliance on specific pollinator guilds may impose potential constraints on population regeneration of these rare species (Soares et al., 2014). This interpretation is consistent with our observation that rare species are largely confined to intermediate diameter and height classes, indicating that their populations persist through limited but continuous recruitment rather than through expansive demographic growth.

The relative high proportion of dioecious individuals observed in occasional plant species is primarily attributable to sexual segregation, whereby individuals of different sexes occupy distinct ecological niches (Tonnabel et al., 2017), thereby mitigating intraspecific resource competition and ultimately enhancing the overall environmental adaptability of the population. In contrast, rare species are predominantly represented by hermaphroditic plants, a pattern that is intimately associated with their reproductive traits (Helmstetter et al., 2026). Characterized by small population sizes, narrow distribution ranges, and stringent habitat requirements, these species typically occur in communities with extremely low individual abundances, rendering them in a persistently vulnerable population state (Liu et al., 2024).

A systematic investigation on the species composition, diameter and height class structures, and sexual systems of forest communities in the intake area of the South-to-North Water Diversion Project was conducted. Common species dominated community composition, with their relative abundance and importance values exceeding those of occasional and rare species. Common species dominated across all diameter and height classes, with peak abundance in the II diameter class (5 cm ≤ DBH < 10 cm), whereas rare species occurred in the intermediate classes (5~15 cm DBH, 3~9, height). Regarding sexual system composition, monoecious plants accounted for the largest proportion of individuals among common species, while hermaphroditic species were predominated represented among as rare species. This pattern reflects the niche strategies adopted by different sexual systems in community assembly.

From a conservation perspective, the demographic vulnerability of rare species warrants particular attention in the South-to-North Water Diversion water source area. The confinement of rare species to intermediate size classes and their reliance on outcrossing reproductive strategies suggest that they may face heightened extinction risk under canopy closure or habitat fragmentation, which could in turn compromise the long-term functional stability of forest ecosystems critical for water conservation. To safeguard both biodiversity and water regulation services, management practices should prioritize the maintenance of canopy heterogeneity and the retention of rare species’ microhabitats, while future research should incorporate functional trait measurements and long-term demographic monitoring to elucidate the mechanisms linking plant sexual systems, community assembly, and ecosystem service provision in this strategic water source region.