Effects of clearcutting on species composition and community renewal of Rhododendron shrubs in northwest Guizhou Province, China

Abstract

Tree base sprouting is the main reproduction and expansion mode of Rhododendron plants. By leveraging the plot survey method, the species composition, community renewal, and species diversity in three Rhododendron shrub communities in control, and before and after clearcutting (CK, D_3yr, and D_6yr, respectively) were studied. Moreover, the dissimilarity of Rhododendron communities in CK, D_3yr and D_6yr were analyzed. The results showed that there were 26 plant species belonging to 14 families and 22 genera, in 3 communities in total, with 19 species of shrub plants and 7 species of herbaceous plants. The number of species increased from 13 in CK to 23 in D_3yr and then decreased to 20 in D_6yr. The height and coverage of D_3yr and D_6yr reached 39.3% and 58.9% of that of CK, respectively. The relative height of CK, D_3yr, and D_6yr was 43.79%, 65.4%, and 58.54%, respectively. The coverage of D_3yr and D_6yr reached 60.8% and 114.70% of that of CK, respectively. The relative coverage of CK, D_3yr, and D_6yr was 19.05%, 83.24%, and 77.32%, respectively. The important value of Rhododendron plants in the communities increased from 0.42 in CK to 0.74 in D_3yr, and then decreased to 0.67 in D_6yr. The α diversity in the shrub layer of D_3yr and D_6yr were generally lower than those of CK except Pielou evenness index. The β diversity indicates that the similarity between CK and D_3yr was lower, that between CK and D_6yr was moderate, and that between D_3yr and D_6yr was higher. The sprouting height and coverage of Rhododendron plants was significantly correlated with age and sprouting time. The sprouting ability of Rhododendron plants increased first and then decreased with age, while the sprouting ability of Rhododendron plants with age of 10–12 years was the strongest. Clearcutting measures can improve the dominance of Rhododendron plants in the communities, promote the sprouting and renewal of Rhododendron population, and accelerate the succession rate of communities.

1 Introduction

Rhododendron is one of the largest genera of angiosperms with more than 1200 species worldwide, which have important ecological and socio-economic applications of ornamental, cultural, scientific, economic, and medicinal value (MacKay and Gardiner, 2017; Li et al., 2018; Ahmad et al., 2021). The genus is mostly distributed in subtropical evergreen broad-leaved mountain forests, mixed coniferous broad-leaved forests, coniferous forests, and dark coniferous forests at altitudes varying from 1000 to 3800 m (Fang and Min, 1995). It only forms a single species of Rhododendron shrub or Rhododendron dwarf forest in certain high mountains above the tree line (Sun, 2002). Guizhou Province is located at the edge of the distribution center of modern Rhododendron (Hengduan Mountain region in China) and its transition zone along the eastern distribution range (Dai et al., 2020). More than 110 species of Rhododendron plants are distributed naturally in Guizhou Province (Dai et al., 2020). Northwest Guizhou Province is a vital area for the distribution of Rhododendron, with six subgenera and more than fifty species accounting for approximately 50% of the total Rhododendron population in Guizhou. The Baili Rhododendron National Nature Reserve at the junction of Qianxi and Dafang counties is the most representative, with 43 species of Rhododendron plants (Wang et al., 2010; Chen et al., 2013). It is a typical representative of the largest and contiguously distributed natural wild Rhododendron communities in the world (Rong et al., 2009) which may play an important role in understanding Rhododendron shrub ecosystem processes, succession, and shrub management. However, both the quantity of seedlings and the renewal rate of Rhododendron plants are low (Yang et al., 2020a). Meanwhile, it is easy for Rhododendron plants to be replaced by highly competitive trees and other shrubs because of succession (Bian et al., 2006). Coupled with global changes in recent years, Rhododendron plants, particularly some narrowly distributed Rhododendron species, have begun to decline or even die (Ma et al., 2014; Liu et al., 2019; Yu et al., 2019; Bitayan et al., 2021).

Sprouting renewal primarily refers to dry base sprouting and underground stem sprouting (Lu et al., 2021). New plants are formed via dormant or adventitious buds from underground stems and stubble of trees to achieve forest renewal (Vesk and Westoby, 2004). Sprouting plants can use nutrients in the soil more effectively through their strong root system, which usually grows faster than seedlings and has stronger adaptability (Kauffman, 1991; Chen et al., 2019). Owing to the weak competitiveness and high canopy density of Rhododendron shrub communities in northwest Guizhou, seed renewal of the Rhododendron population is difficult, and tree-based sprouting renewal is the main mode of reproduction and expansion (Kong et al., 2019). At present, the primary measures to promote tree-based sprouting renewal are controlled forest fires, clearcutting and other natural disasters such as hurricane, mudslides (Li, 1992; Luoga et al., 2004; Subedi et al., 2019). However, forest fires are harmful as these reduce the sprouting ability of shrub communities (Tang et al., 2001). In contrast, planned clearcutting promotes the growth of shrub tillering branches, regenerates and rejuvenates plant clusters, and increases the canopy width and biomass (Shang et al., 2020). Following clearcutting, sprouting plants usually grow faster than seedlings (Vesk and Westoby, 2004).

Research on sprouting renewal of Rhododendron plants in global started late at present; further, research on sprouting and stress tolerance of wild Rhododendron plants is limited, decreasing the promotion and utilization of wild Rhododendron plants resources (MacKay and Gardiner, 2016). Although several studies have reported renewal of Rhododendron shrubs by disturbance, altitude, and climate changes (Singh et al., 2019; Choudhary et al., 2021; Jia et al., 2021), there is limited knowledge regarding the sprouting renewal of Rhododendron plants by clearcutting. In this study, the effects of rational clearcutting on the sprouting renewal of Rhododendron communities were studied to determine the effect of clearcutting–sprouting on the role of Rhododendron plants in Rhododendron shrub communities. The objectives were to (1) Can clearcutting promote the community renewal of Rhododendron shrubs? (2) Does age affect the response of Rhododendron shrubs to clearcutting? (3) Can the sprouting and regeneration of Rhododendron be predicted after clearcutting?

2 Material and methods

2.1 Overview of the study area

The study area is in the Tiaohuapo Scenic Spot in the Baili Rhododendron National Nature Reserve, Guizhou Province, China (27°23′22′′N and 105°51′52′′E), near the Jiaozi Mountain, at an altitude of 1700–1900 m. The climate of the region can be characterized as subtropical humid monsoon climate, with an annual average temperature and precipitation of 11.8°C, and 1150.4 mm, respectively, a frost-free period of 257 days, and an annual sunshine duration of 1335.5 h. Zonal vegetation is dominated by the evergreen broad-leaved mountain forest. The existing vegetation is primarily Rhododendron shrub species exhibiting succession and transitional characteristics (Li and Chen, 2005; Jiang et al., 2015). Among Rhododendron shrubs, R. delavayi, R. annae, and Lyonia ovalifolia are the main dominant species. The soil type is primarily yellow soil with an acidic pH (4.61–5.32).

2.2 Plot setting and survey

In August 2015, three 20 m × 20 m fixed quadrats were set in the study area to represent the control community (CK), and three 5 m × 5 m survey quadrats were set diagonally in each quadrat. The species, plant number, branch number, height, and canopy width of the shrubs as well as the species and number of the herbaceous plants in the quadrats were recorded. Each Rhododendron shrub in the fixed quadrat was labelled.

In conjunction with the guidelines of Management Office of the Baili Rhododendron National Nature Reserve, all plants in the fixed quadrat were deforested and recovered. The shrubs in the fixed quadrat were deforested in December 2015 and the stubble height was recorded as approximately 15 cm. The number of stubbles was also recorded. The age of shrubs was determined based on the rings of the main pile of shrubs. All other plants were deforested and eliminated.

The same method was used to survey and record the sprouting number, height, and canopy width of Rhododendron shrubs in August 2018 and August 2021. Moreover, the species number, height, and canopy width of other shrubs, as well as the species and number of herbaceous plants in the quadrats, were also surveyed. The communities surveyed in 2018 and 2021 were denoted as D_3yr (Community 3 years after clearcutting) and D_6yr (Community 6 years after clearcutting), respectively.

2.3 Data analyses

2.3.1 Importance value

IV of the shrub layer for plant species in the three annual communities (CK, D_3yr, and D_6yr) was calculated using the equation (Wang et al., 2023)

where RH denotes the relative height, RC denotes the relative coverage, and RD denotes the relative density.

2.3.2 α-Diversity

The Margalef richness index (R), Simpson diversity index (D), Shannon–Wiener diversity index (H), and Pielou evenness index (J) were determined to measure the α-diversity of the three communities using the equations (Zhang et al., 2017)

where P_i denotes the relative IV of the i^th species, S denotes the number of species, and N denotes the number of individual plants in the quadrat.

2.3.3 β-Diversity

The Cody index (β_c), Whittaker index (β_ws), Jaccard index (C_j), and Sorenson index (C_s) were determined to measure the β-diversity of the three communities before and after clearcutting using the equations (Baselga, 2010)

where β_c and β_ws denote the dissimilarity measures, C_j and C_s denote the similarity measures, g(H) denotes the number of species increased along the time gradient H, L(H) denotes the number of plant species existing in the previous community but lost in the next community, ms denotes the total number of species recorded in the three communities, ma denotes the average number of species in each community, a denotes the number of species in community A, b denotes the number of species in community B, and j denotes the number of species shared by communities A and B.

2.3.4 Sprouting status

The ratio between the number of Rhododendron sprouts and stumps after clearcutting was determined to measure the ability of Rhododendron plant to rejuvenate and renew using the equation

where RS denotes sprouting ability, ES denotes the number of sprouts, and NS denotes the number of stubbles.

2.4 Data processing and statistical analyses

SPSS 26.0 software was used to measure the correlation (Pearson correlation) and linear regression analysis between the height and coverage as well as clearcutting time and age of Rhododendron. Analysis of variance was performed (p < 0.05 indicates significant differences) to determine the difference in IV of community species and biodiversity indices between the different stages of Rhododendron shrubs. All plots were prepared using SigmaPlot 14.0.

3 Results

3.1 Effects of clearcutting on Rhododendron shrub community characteristics

The three communities comprised 26 species, including nineteen species of shrubs and seven species of herbaceous plants, belonging to 14 families and 22 genera (Tables 1, 2). Ericaceae, Rosaceae, and Theaceae accounted for the highest number of species, specifically 19.23%, 15.38%, and 11.54% of the total, respectively. Three species of Rhododendron plants accounted for 11.54% of the total species.

CK comprised 13 species belonging to seven families (including Ericaceae, Lauraceae, Theaceae, and Poaceae) and 11 genera. Four species were identified in the herbaceous layer and nine in the shrub layer of which five viz., R. delavayi, L. ovalifolia, R. annae, and P. japonica of Ericaceae and L. pungens of Lauraceae exhibited IVs > 0.1 (0.31, 0.17, 0.12, 0.12, and 0.12, respectively). R. delavayi was the dominant species in the shrub layer of CK, and L. ovalifolia was the subdominant species.

D_3yr comprised the highest number of species, with 23 species from 12 families (including Ericaceae, Rosaceae, Theaceae, and Poaceae) and 19 genera. There were seven species in the herbaceous layer and 16 in the shrub layer of which two, R. delavayi and R. annae of Ericaceae, exhibited IVs of > 0.1 (0.53 and 0.19, respectively). R. delavayi was the dominant species in the shrub layer of D_3yr, and R. annae was the subdominant species.

D_6yr comprised 20 species belonging to nine families (including Ericaceae, Rosaceae, Theaceae, and Poaceae) and 16 genera. There were three species in the herbaceous layer and seventeen in the shrub layer. Consistent with D_3yr, two species in the shrub layer, R. delavayi and R. annae, exhibited IVs > 0.1 (0.46 and 0.18, respectively). R. delavayi was the dominant species in the shrub layer of D_6yr, and R. annae was the subdominant species.

In the CK→D_3yr→D_6yr community sequence, the height and coverage of Rhododendron shrubs first decreased and then increased (Figure 1). The height decreased significantly from 191.87 cm in CK to 75.61 cm in D_3yr and then recovered to 116.71 cm in D_6yr. The coverage decreased significantly from 68.46% in CK to 37.89% in D_3yr and then increased significantly to 73.28% in D_6yr. The relative height increased from 43.79% in CK to 65.40% in D_3yr and then decreased to 58.54% in D_6yr. The relative coverage increased from 49.05% in CK to 83.24% in D_3yr and then decreased to 77.32% in D_6yr. The height of Rhododendron species in the three communities were significantly different (p < 0.05). The coverage in D_3yr was significantly different from that in CK and D_6yr (p< 0.05). The relative height and coverage in CK were both significantly different from those in D_3yr and D_6yr (p < 0.05).

Figure 1

3.2 Effects of clearcutting on the diversity of species in Rhododendron shrub communities

The Margalef richness index values of species in the shrub layers of the three communities were significantly different (p < 0.05) (Figure 2). However, the differences in the Simpson diversity, Shannon–Wiener diversity, and Pielou evenness indices were not significant (p > 0.05). The values of all four indices were the highest in CK and lowest in D_3yr. The values of the Margalef richness, Simpson diversity, Shannon–Wiener diversity, and Pielou evenness indices of species in D_3yr were 61.15%, 80.30%, 84.79%, and 96.61% of that of species in CK, respectively.

Figure 2

As CK→D_3yr→D_6yr progressed, the dissimilarity coefficients of the three communities showed a decreasing trend (Table 3). Both Cody and Whittaker indices were the highest in CK and D_3yr (3.83 and 1.65, respectively) and the lowest in D_3yr and D_6yr (0.83 and 1.20, respectively). The similarity indices, the Jaccard and Sorenson indices, were the highest in D_3yr and D_6yr (0.87 and 0.93, respectively) and the lowest in CK and D_3yr (0.38 and 0.55, respectively). The similarity between CK and D_3yr was low, that between CK and D_6yr was moderate, and that between D_3yr and D_6yr was high.

3.3 Effects of clearcutting on the renewal of Rhododendron shrub communities

The height and coverage as well as the sprouting time and age of Rhododendron shrubs were significantly and positively correlated (p < 0.01). The correlation degree of sprouting time was higher than that of the age of Rhododendron shrubs (Table 4).

The sprouting time (t) and age (a) are the two main factors that affect clearcutting sprouting of Rhododendron shrubs. The linear relationships between sprouting height (h), sprouting coverage (c), sprouting time (t), and age (a) of Rhododendron shrubs after clearcutting were well fitted (R² = 0.65, p < 0.01 and R² = 0.50, p < 0.01, respectively) (Table 5).

The height of Rhododendron shrubs in CK, D_3yr, and D_6yr showed a gradual increasing trend with age (Figure 3). Compared with CK, the height of Rhododendron shrubs in D_3yr and D_6yr was lesser and increased slowly. The coverage rates of Rhododendron shrubs in the three communities also increased gradually with increasing age. The coverage rates of Rhododendron shrubs in D_3yr and D_6yr at the early stage were higher than those in CK.

Figure 3

To determine the sprouting ability at different ages, the age of Rhododendron shrubs was divided into three classes (I, II, and III). The sprouting ability of Rhododendron shrubs first increased and then decreased with age (Figure 4). Class II Rhododendron shrubs had the strongest sprouting ability. Moreover, the sprouting ability of class III Rhododendron shrubs was significantly different from that of classes I and II shrubs (p < 0.05).

Figure 4

4 Discussion

4.1 Effects of clearcutting on species composition and community characteristics of Rhododendron shrubs

Species composition reflects the structure, dynamic changes, and succession characteristics of a community and plays a decisive role in community diversity (Gilliam, 2007; Liu et al., 2020). In this study, 26 species belonging to 14 families and 22 genera were identified in the three communities studied (Table 1). The number of species in CK was the lowest probably because the canopy density of Rhododendron shrubs was high and the litter layer was thick, which reduced the migration and colonisation of Rubus trianthus, Cotoneaster horizontalis, Eurya loquaiana, Populus adenopoda, belonging to the Rosaceae, Theaceae, Salicaceae, as well as herbaceous plants. However, clearcutting weakened interspecies competition by reducing canopy density (Jones et al., 2019). Consequently, a large number of herbaceous plants, and species belonging to Rosaceae, Theaceae, and Salicaceae migrated; thus, D_3yr had the highest number of species. Subsequently, Rhododendron shrubs gradually occupied the upper space, the canopy density increased, and herbaceous plants gradually disappeared. Therefore, the number of species in D_6yr was lower compared with that in D_3yr.

The height and coverage of plants in communities are important indicators for evaluating the plant growth status (Chen et al., 2014). In this study, the height and coverage of plants in communities after clearcutting were significantly higher than those of plants in the seed renewal communities (Figure 1), primarily because the large number of sprouting branches produced by Rhododendron shrubs could occupy more space by utilizing the resources, hence promoting rapid formation of community hierarchy. Post clearcutting, the sprouting coverage of Rhododendron shrubs in D_6yr was higher than that of shrubs in CK, primarily because clearcutting promoted shrub tillering and expanded individual canopy width. After clearcutting, the relative height of Rhododendron shrubs in D_3yr and D_6yr also increased significantly, probably because Rhododendron shrubs occupied the upper space of the whole shrub community after clearcutting, playing a dominant role in light absorption and utilization. The absence of other tall trees and shrub vegetation enabled Rhododendron plants roots to utilize soil nutrients more effectively. Therefore, clearcutting resulted in the domination of Rhododendron plants in the community, with vigorous population renewal and a stronger self-sustaining ability of the population.

4.2 Effects of clearcutting on the diversity of Rhododendron shrub communities

As a basic feature of a community, biodiversity is a key factor driving the dynamics and processes of an ecosystem and can indicate the habitat status, composition structure, and distribution pattern of a community (Liu and Bra, 1998; Tilman et al., 2012). In this study, the evenness index of the communities changed after clearcutting (Figure 2). The index value was relatively large, indicating that the heterogeneity of the study area was poor. This was also determined by the characteristics of the sprouting branches of Rhododendron shrubs. Although clearcutting increased the number of species, the α-diversity was still lower than that in CK before clearcutting. This may be because Rhododendron plants has a strong sprouting ability. After clearcutting, Rhododendron plants is likely to occupy space in the form of sprouting branches to inhibit the migration and colonisation of foreign species (Grant and Loneragan, 1999; Kruger and Midgley, 2001). Therefore, Rhododendron plants with its strong sprouting abilities can reduce species diversity and species turnover in the community to a certain extent.

β-Diversity is usually expressed as the turnover rate of biological species in different habitats. A high β-diversity index indicates a low level of species similarity between different habitats or ecosystems (Yang et al., 2020b). The results showed that the similarity between CK and D_3yr was low, that between CK and D_6yr was moderate, and that between D_3yr and D_6yr was high (Table 3). The Cody and Whittaker indices were higher before and after clearcutting, whereas the Jaccard and Sorenson indices were lower, indicating that clearcutting changed the species composition and community structure of Rhododendron. The similarity between CK and D_3yr was low but that between CK and D_6yr was moderate, indicating that with the progression of CK→D_3yr→D_6yr, the number of common plant species first decreased and then increased. Subsequently, the similarity among species composition also first decreased and then increased, indicating that Rhododendron plants could recover to the pre-clearcutting community level in a short time through clearcutting–sprouting renewal.

4.3 Effects of clearcutting on the renewal of Rhododendron shrub communities

In Rhododendron shrub communities, both seedlings and sprouting seedlings can undergo population renewal. Nevertheless, sprouting seedlings are an important source of population dynamics (Vesk and Westoby, 2004). Due to the strong sprouting ability of Rhododendron plants, deforested communities rapidly self-substitute, which not only affects the community structure but also has an important impact on population dynamics. Clearcutting–sprouting renewal avoids the problems of very low sprouting and survival rates as well as weak seedling competitiveness. The results of this study similarly showed that the dominance of the Rhododendron population in D_3yr and D_6yr after sprouting was greater than that in CK before clearcutting. The growth recovery of Rhododendron plants after clearcutting is completely dependent on the amount of stubble, root tiller, and rhizome, whereas the number of sprouts is a common index to characterize the strength of sprouting (Chi et al., 2019). Although the sprouting ability of Rhododendron plants was strong in this study, the recovery ability first increased and then decreased with the age of the Rhododendron shrubs. The sprouting ability of Rhododendron shrubs of age 10–12 years was the strongest (Figure 4). This may be associated with the aging of plants, an excessively large root system, and a lack of rhizome growth, which led to partial necrosis of the underground root system.

The community succession of Rhododendron plants does not deviate far from the original direction because of its strong sprouting ability and the effect of the ecological niche after clearcutting (Ahmad et al., 2021). However, the communities in this study had a high number and proportion of Rhododendron shrubs initially, which were in situ substituted with a similarly high number, accelerating the recovery to the original structure and succession rate of the community. Therefore, the shrub layer of Rhododendron communities after clearcutting was primarily composed of sprouting branches of Rhododendron, and the final direction of succession was subtropical evergreen broad-leaved forest. Compared with seed renewal and succession, the clearcutting–sprouting succession of Rhododendron plants takes a shorter time to achieve maturity (Loucks, 1970; Chen et al., 2019).

5 Conclusions

In total, 26 species from 22 genera and 14 families were found in the three communities. Clearcutting promoted the migration and colonisation of the families Rosaceae, Theaceae, and Salicaceae and other herbaceous plants, whereas the relative height and coverage of shrubs in the communities increased significantly. After clearcutting, there was a slight change in community evenness and a decrease in the α-diversity. The β-diversity showed that clearcutting improved the dominance of Rhododendron plants in the community and promoted sprouting renewal of Rhododendron populations. Moreover, the sprouting ability of Rhododendron shrubs of age 10–12 years was the strongest. Clearcutting did not affect the direction of community succession, but could accelerate the succession rate. Owing to the short observation period of clearcutting and sprouting of Rhododendron communities in this study, it was not possible to accurately predict the rate of population renewal and community succession of Rhododendron shrubs. In subsequent studies, multiple clearcutting modes and Rhododendron population succession stages will be established in combination with the competitive ability of the Rhododendron population.

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