The Expansion of Moso Bamboo (Phyllostachys edulis) Forests into Diverse Types of Forests in China from 2010 to 2020

Abstract

Moso bamboo (Phyllostachys edulis) forests, characterized by their rapid growth and clonal reproduction, have emerged as a significant threat to adjacent forest ecosystems. However, in China, the area, speed, and spatial distribution of moso bamboo forest expansion into other types of forests remains poorly understood. In this study, we present a case analysis of moso bamboo forests, employing a decade-long dataset from the forest second type inventory (FSTI) that utilizes transition matrices, neighboring ratio analysis, and spatio-temporal autocorrelation. This comprehensive investigation focuses on the spatio-temporal expansion of moso bamboo forests into diverse types of forests, with the aim of providing science-based recommendations for effective moso bamboo forest management. Our findings reveal that areas of moso bamboo forests have been expanding at an approximate annual rate of 2%, with an average expansion speed (including moso bamboo forests manually planted) of approximately 8 m per year. The length of moso bamboo–woodland ecotones (BWEs) increases as a consequence of moso bamboo forest expansion, indicating a sustained escalation in the extent of this expansion. Coniferous forests and evergreen broad-leaved forests are mainly invaded, accounting for around 58% of all invaded forests. The rate of moso bamboo forest expansion into different types of forests varies, although the rate remains fairly consistent within the same forest type. Moso bamboo forest expansion exhibits significant spatial heterogeneity. Furthermore, the area of moso bamboo forest intrusion into various types of forests in different provinces is notably influenced by the presence of moso bamboo forests and the proportional distribution of different forest types. The factors contributing to bamboo forest expansion encompass stand characteristics, soil attributes, light intensity, moso bamboo afforestation, forestry practices, and human disturbances.

1. Introduction

It is generally accepted that a species within its native range is not considered “invasive”. Nevertheless, some native species display behaviors akin to non-native species. Charles Elton, in his seminal 1927 book Animal Ecology, used the term “invasion” to refer to the spread of native species. For the same phenomenon, others have used various terminologies. In 1993, Garrott described the increasing dominance of native species in their original range as overabundance [1]. The overabundance or expansion of native species can diminish natural diversity by consolidating resources, introducing or spreading infectious diseases and parasites, altering the species composition or relative abundance of sympatric species, and even causing local extinctions, leading to significant damage similar to that caused by non-native invasive species. Although Garrott recognized the severe negative impact of overabundant native animals on other native species, there has been limited attention given to these species. Valéry (2013) reaffirmed that some native species benefiting from a change in their environment can meet both characteristic traits of a biological invasion. The rapid growth and clonal reproduction of bamboo enhance its ability to compete for space and form dense stands [2]. Griscom’s study indicates that the mean percent mortality of tree seedlings was over twice as high in forest plots dominated by bamboo versus forest plots without bamboo [3]. In this way, bamboo-dominated areas significantly inhibit the development of tree seedlings, profoundly impacting the structure and dynamics of forest ecosystems [4,5].

Bamboo is one of the grass plants native to forests [6]. The expansion of bamboo is occurring worldwide, especially in Asia, owing to its rapid growth and extensive clonal reproduction [7]. In China, bamboo expansion into neighboring forests is more serious than in other Asian countries. The area of bamboo forests in China increased from 3.868 million ha to 4.6778 million ha during the ten-year period from 2008 to 2018 [8,9]. Among bamboo species, moso bamboo is one of the most widely distributed varieties, comprising over 70% of the total area of all bamboo species. Moso bamboo is a typical example of clonal reproduction, relying on its robust underground rhizomes to continually expand into neighboring communities, resulting in the continuous enlargement of moso bamboo forests [10,11]. Furthermore, in many districts of China, the management of moso bamboo forests has been extensive due to factors such as poor transportation, high costs in terms of labor and materials (land preparation, weeding, fertilization), and low economic returns. In some regions, the management of moso bamboo forests has even been abandoned [12,13]. The lack of proper management has exacerbated the unregulated growth of moso bamboo. On the other hand, conservation policies in China have led to the prohibition of timber harvesting in certain areas, as influenced by national regulations. Banning bamboo harvesting will intensify the encroachment of moso bamboo into the surrounding areas and the process of replacing other forest types [14,15]. As moso bamboo continues its invasive encroachment, it progressively influences national ecological security by modifying regional soil properties, nutrient composition [7,16,17], simplifying forest community structure and function [18,19], diminishing species diversity [20,21], and altering forest biomass [22,23]. Thus, the investigation into the characteristics of moso bamboo expansion, particularly when it encroaches upon other forest types, assumes a critical role in safeguarding national ecological security. It is essential to gain an understanding of moso bamboo expansion through dedicated research.

“Scale” as a paradigm in ecology plays an equally vital role in expansion ecology research [24]. Current studies on moso bamboo forest expansion into other forests primarily focus on plot and protected area scales [25]. At this scale, the research into moso bamboo expansion predominantly employs plot experiments to investigate the impact of bamboo forests on the local environmental conditions, such as soil elements, soil organic matter, and microbial communities. This approach further delves into the mechanisms underlying the expansion of bamboo forests into other forest types [26]. In their research, some colleagues explored the correlation between soil nitrogen mineralization and fungal decomposition capacity in broad-leaved forests invaded by moso bamboo within the Wuyi Mountain [27]. Some investigated the alteration of assembly processes and interaction patterns within the soil microbial community of subtropical broad-leaved forests due to moso bamboo expansion in Lingfeng Mountain, Anji County, Zhejiang Province, China [25]. Several studies concerning the encroachment of bamboo forests into other forested landscapes have been undertaken, primarily centered on Japan [28,29,30,31]. Japan, positioned between 31° N–46° N, displays considerable differences in climate features in contrast to China. Climatic characteristics serve as the foremost factors influencing the distribution of moso bamboo. Investigating the distribution features, alterations in the area, and the expansion rate of moso bamboo forests in China will help to understand the discrepancies in moso bamboo forest encroachment across diverse regions, thus strengthening their management.

Explorations into distinct forest regions carry strong regional imprints. The encroachment of bamboo forests is profoundly shaped by the geographical layout of forests and the prevailing regional environmental conditions [10]. Given the wide distribution of moso bamboo in China’s subtropical regions, the strategies adopted by moso bamboo for infiltrating various forest types exhibit significant diversity. Furthermore, these strategies are bound by the geographical dispersion of plant landscapes and the distinct environmental features in various regions. Consequently, conducting a macro-scale analysis of moso bamboo expansion into diverse types of forests in China allows for a comprehensive understanding of the overall characteristics and trends in moso bamboo expansion, offering a comprehensive interpretation for the disparities in moso bamboo expansions.

A survey of the literature concerning factors closely linked to bamboo expansion underscores the significance of forest characteristics as the primary driving forces behind bamboo’s successful spread into adjacent ecosystems [32,33]. Okutomi [5] suggests that moso bamboo has a proclivity for infiltrating forests with simpler structural compositions and lower species diversity. Moreover, Suzuki and Nakagoshi [34] have reported that the pace of bamboo expansion is intertwined with the slope and orientation of neighboring forests. Additionally, the role of natural disturbances, including wind, snowfall, and wildfires, in hastening the clonal expansion of bamboo is well recognized [35,36]. Furthermore, certain investigations indicate that soil attributes exert a notable influence on bamboo expansion. Dong [37], for instance, has observed a positive correlation between the extent of expansion and the number of new culms with soil fertility and porosity. In a complementary vein, Liang [38] have posited that adequate soil moisture acts as an impediment to rhizome growth. Consequently, it is apparent that bamboo expansion is entwined with the characteristics of aboveground forest stands and subterranean soils at the bamboo–woodland ecotone (BWE), which serves as a transition zone between bamboo forests and woodlands. The key to understanding bamboo forest expansion lies in scrutinizing the ecotone, where the forest type, soil attributes, and microbial environment dictate the pace of moso bamboo expansion. Meanwhile, fluctuations in the ecotone’s movements and length imply shifts in the features of bamboo forest expansion [39]. This study endeavors to grasp the overarching trends and the velocity of bamboo forest expansion through an exploration of ecotone dynamics. Given that moso bamboo is a species with limited dispersal capabilities, natural expansion predominantly transpires at the edges of existing bamboo forests, while non-edge expansion typically results from human cultivation. An examination of anthropogenic interference is pivotal for discerning variations in artificial plantings across different regions and for curtailing the further encroachment of bamboo forests. To date, few studies have simultaneously addressed changes in the ecotones. Historically, remote sensing interpretation has served as the primary data source for macro-level research [40,41,42]. Nevertheless, interpreting data at this scale presents challenges, such as difficulties in distinguishing bamboo species and achieving adequate accuracy [43].

The basic data are from the forest inventory for planning and design, named the forest second type inventory (FSTI), which aims to inventory the distribution, quantity, quality, and ownership of forest and provide an objective portrayal of the forest conditions covering the whole of China, conducted by the China National Forestry and Grassland Administration [44]. The FSTI integrates plot-based field surveys and remote sensing interpretation to ensure the accuracy of forest inventory data. Remote sensing interpretation is mainly applied to delineate the boundaries of a subcompartment based on the texture and color of the image in the early stage. The forest subcompartment is the basic unit for this inventory, referred to as a patch with the same forestry characteristics inside, different in adjacent areas. Forestry characteristics encompass ownership, forest category and species, ecological public welfare forest ownership and protection level, forestry engineering category, land type, origin, dominant tree species, age class, crown closure, site type, and other factors. Typically, the area of a subcompartment does not exceed 25 hectares. Plot-based field surveys are used to verify the boundaries of a subcompartment and collect forest attributes of the subcompartment, including compartment number, subcompartment number, forest type, site attributes such as ownership and terrain, tree species, tree size, overall tree condition, etc. The inventory is conducted every 10 years, with a five-year frequency in areas or units with high management levels. FSTI field protocols are described in the national core field guide. This document describes the standards, codes, methods, and definitions for FSTI field data that are consistent and uniform across China. The current version of the Standard is GBT 26424-2010 [45], technical regulations for inventory for forest management planning and design. The outcomes constitute a database for the FSTI, including a vector polygon feature with graphics and attribution table. The smallest unit on the map is the stand patch, and the combination of patches forms a map of the FSTI. The recordings in the table correspond to a single subcompartment and each recording has all field attributions. This study mainly uses the field named “Dominant tree species” and the spatial information of the subcompartment.

Within this study, we broaden the scope of our research to include moso bamboo expansion extending from protected areas to the nation. With forest second type inventory data spanning from 2010 to 2020, we assess the area, average rate, and spatial distribution differences in moso bamboo expansion into other forests. Leveraging a decade’s worth of forest second type inventory data from China, this research aims to address a set of crucial inquiries: (1) Has moso bamboo consistently encroached into different forest types, and if so, what varying expansion rates are observed? (2) What trends characterize the transformation of the critical moso bamboo-woodland ecotone during the expansion process? (3) What disparities exist in terms of expansion speed and spatial distribution as moso bamboo spreads across different forests?

2. Materials and Methods

2.1. Study Area

China is located in eastern Asia and bordered by the western coast of the Pacific Ocean to the east. Its latitude ranges from 3°51′ N to 53°33′ N, and the longitude ranges from 73°33′ E to 135°05′ E. It encompasses a diverse range of climatic zones, including tropical, subtropical, warm-temperate, temperate, cold-temperate, and plateau regions. China’s landscape features plateaus, mountains, hills, and plains. China hosts an extensive distribution of plant species, horizontally extending across tropical, subtropical, and temperate zones, and vertically covering a wide array of terrains, including plains, hills, mountains, and plateaus. It boasts a rich array of forest types, with coniferous forests predominantly found in the northern and mountainous regions, while evergreen and deciduous broadleaved forests dominate the southern areas [46]. Bamboo forests are widespread in the tropical and subtropical regions of central and southern China (Figure 1). Bamboo has been cultivated in China for over 3000 years, solidifying its status as an indigenous plant. Currently, China maintains approximately 7.01 million hectares of bamboo forests, housing 39 genera and 837 species of bamboo, which constitute 51% of the world’s 1642 bamboo species [47]. China holds the distinction of being the world’s foremost source of bamboo resources, leading in bamboo variety, forest area, and stock volume. Notably, moso bamboo comprises approximately 70% of China’s total bamboo resources. As a native invasive species, moso bamboo poses challenges to other types of forests during its expansion.

2.2. Data Processing

The ArcGIS pro3.1, a geo-information system software package, was applied to select and analyze the data from the forest second type inventory database. Firstly, we obtained the vector layers of all second administration regions in 2010, 2015, and 2020 from the China National Forestry and Grassland Administration. Then, we selected the records for which the field “Dominant tree species” = “66000” to ensure that the land where moso bamboo is planted has been identified. We combined the data chosen in the three years together to obtain the region where moso bamboo forest is present from 2010 to 2020. The output layer is used to extract data from this combined region within the forest resources survey databases for these three years, resulting in the creation of a moso bamboo forest type database covering the period from 2010 to 2020. These forest stands were categorized into nine primary forest types based on the dominant tree species, including coniferous forests, evergreen broad-leaved forests, deciduous broad-leaved forests, other bamboo forests, non-forests, shrubbery, non-timber forests, other forests, and moso bamboo forests (Figure 2). Moso bamboo forests in Tibet are mainly concentrated in the Linzhi region, which, due to its small area, was virtually negligible and subsequently excluded from the datasets for further analysis.

2.3. The Area Transition Matrix of Expansion Analysis

Employing the feature to raster tool in ArcGIS Pro, the vector layers in moso bamboo forest type database were converted into raster layers with dimensions of 200 m × 200 m. The coordinate system of the resulting raster layers was then established as Albers, thereby yielding raster layers depicting forest types. Subsequent inputting of these raster into Fragstats enabled the generation of transition matrix tables depicting the alterations in bamboo forest area between the periods 2010–2015 and 2015–2020.

2.4. Analysis of the Change in BWE

2.4.1. Average Expansion Rate

Calculating the average expansion rate involved subtracting the total area of patches in the former year from that of the latter, divided by the length of the intersect edge in the two years (clip the previous year’s boundary by the latter year’s region in ArcGIS). The average expansion rate of different forest types by bamboo forest involved dividing the area invaded by bamboo forest for each forest type by the length of the expansion edge.

2.4.2. Neighboring Ratio

Using the nearest neighbor tool in ArcGIS Pro, the identification of neighboring expanding patches was carried out. Patches with a nearest neighbor distance of 0 were classified as neighboring invading patches for two consecutive years. Utilizing the reproductive characteristics of moso bamboo forests, this study proposes the concept of the neighboring ratio to gain a clearer understanding of the proportion of man-made plantations. The neighboring ratio is defined as the ratio of the area (S_n) of patches that underwent neighboring expansion compared to the preceding period, to the sum of the area of neighboring expansion (S_n) and the area of non-neighboring expansion (S_m). [49] This indicator is primarily employed to contrast the total area of expansion in proximity with that of non-neighboring expansion. A higher ratio indicates more pronounced neighboring expansion and, by extension, a reduced extent of non-neighboring areas, implying a lesser extent of human-planted moso bamboo forests.

(1)$NR={\displaystyle \frac{S_n}{S_n+{ S}_m}}\times 100\%$

Patches with a nearest neighbor distance of 0 were classified as neighboring invading patches, while others were non-neighboring patches.

2.5. Spatial Heterogeneity Analysis

Local indicators of spatial association (LISA) is used to measure whether the distribution of spatial variables has agglomeration, including global spatial autocorrelation and local spatial autocorrelation [50]. Spatial autocorrelation analysis is an important tool for assessing the extent of spatial aggregation for variables of interest. It can reveal correlations among the attributes of study units, at either a global or local scale [51]. Here, GeoDa 1.6.7 was used for the spatial analysis of the independent variables. Spatial autocorrelation was assessed using global Moran’s I and local Moran’s I. The degree of spatial association was visualized in a scatter plot, and local indicators of spatial association (LISA) were calculated and visualized on the study area map [52].

3. Results

3.1. Dynamic Encroachment Process of Moso Bamboo

Overall, from 2010 to 2020, there was a remarkable increase in the extent of moso bamboo forests in China, expanding from 44,238.32 km² to 53,549.96 km² (Figure 3), at an average annual growth area rate of 2.1%. During the 2010–2015 period, the net area of moso bamboo forest encroachment into other forest types (defined as the area that moso bamboo forests invade other forest landscapes minus the area that other forest types encroach on moso bamboo forests) amounted to 3365.63 km², with an average annual growth area rate of 1.6%. Subsequently, during the 2015–2020 period, the net area of moso bamboo forests encroaching on other forest types was 5946.01 km², with an average annual growth rate of 2.4%. During the 2015–2020 period, the area of moso bamboo forests increased by 0.77 time compared to the 2010–2015 period.

As shown in Figure 3, during the 2010–2015 period, the predominant forest types affected by moso bamboo forest expansion were coniferous forests, evergreen broadleaved forests, non-forests, and deciduous broadleaved forests, with expansion areas measuring 7882.34 km², 3612.02 km², 2253.38 km², and 1638.25 km², respectively, collectively constituting 83.72% of the total invaded area by bamboo forests. The net area of moso bamboo forest encroachment on other forest ranked in the order coniferous forests > deciduous broad-leaved forests > shrubbery > evergreen broad-leaved forests > other bamboo forests > other forests> non-timber forests > non-forests, with transfer areas of 2366.79 km², 675.83 km², 425.93 km², and 367.84 km², respectively. Furthermore, non-forested areas encroached upon a portion of moso bamboo forests, accounting for 34.56% of the net expansion area by bamboo forests.

In the 2015–2020 period, the forest types affected by the expansion of moso bamboo forests, ranked by area in descending order, included coniferous forests, non-forests, evergreen broad-leaved forests, non-timber forests, other bamboo forests, deciduous broad-leaved forests, shrubbery, and other forests. The primary forest types impacted were coniferous forests, non-forests, evergreen broad-leaved forests, and non-timber forests, with expansion areas of 7936.27 km², 4908.01 km², 4905.03 km², and 1656.81 km², respectively, collectively representing 85.86% of the total invaded area by bamboo forests. The net area of moso bamboo forest expansion into other forest ranked as non-forests > coniferous forests > non-timber forests > other bamboo forests > deciduous broadleaved forests > evergreen broadleaved forests > other forests > shrubbery. The primary types impacted were non-forests, coniferous forests, non-timber forests, and other bamboo forests, with transfer areas of 2473.50 km², 1779.23 km², 610.44 km², and 530.44 km², respectively, cumulatively exceeding 90.71% of the net expansion area by bamboo forests.

As shown in Figure 4, when examining the expansion data for moso bamboo forests between the 2010–2015 and 2015–2020 periods, it is clear that coniferous forests, evergreen broad-leaved forests, and non-forests represent the largest area, collectively accounting for over 75% of all invaded forests. Coniferous forests and evergreen broad-leaved forests are the primary targets of moso bamboo forest expansion, with their combined expansion ratio exceeding 55% of all types. In terms of the net expansion data spanning these two stages, coniferous forests appear as the most frequently invaded forest type, with a nearly equivalent net expansion area in both periods. The most significant difference lies in non-forests. During the 2010–2015 period, moso bamboo forests encroached upon non-forested areas by 1163.04 km², whereas in the 2015–2020 period, moso bamboo forests invaded non-forested areas covering 2473.50 km². Despite a relatively large invaded area in evergreen broadleaved forests, they exhibit comparatively greater stability, resulting in a lower overall expansion ratio. The proportional areas of forest types invaded by moso bamboo forests in these two stages remain relatively consistent, with coniferous forests, evergreen broad-leaved forests, and non-forests collectively representing approximately 80% of the invaded moso bamboo forest.

3.2. Dynamic Characteristics of Moso Bamboo Expansion in Bordering Other Forests

Over the 2010–2015 period, moso bamboo forests expanded in adjacent areas by 10,921.55 km², while non-adjacent areas witnessed an expansion of 7458.93 km². During the 2015–2020 period, the expansion in adjacent areas covered 12,467.30 km², with non-adjacent expansion encompassing 10,139.09 km². In the 2010–2015 period, the average expansion extended by 8.02 m/year, while in the 2015–2020 period, the expansion rate was 7.62 m/year.

The total length of the invaded border in 2010 was 26,681.36 km, which increased to 32,065.88 km in 2015 and further to 35,391.36 km in 2020. Moso bamboo forests exhibited varying expansion rates across different forests, with the fastest expansion observed in other bamboo forests and the slowest in non-forested areas. In the 2010–2015 period, the expansion rate of moso bamboo forests (Figure 5) into other forests was as follows: other bamboo forests > shrubbery> coniferous forests > non-timber forests > other forests > deciduous broad-leaved forests > evergreen broad-leaved forests > non-forests. In the 2015–2020 period, the expansion rate of moso bamboo forests was as follows: other bamboo forests > coniferous forests > non-timber forests > evergreen broad-leaved forests > shrubbery > deciduous broad-leaved forests > other forests > non-forests.

The neighboring rate of moso bamboo forests is 0.8 across the entire China. As shown in Figure 6, in the 2010–2015 period, provinces with higher bamboo forest expansion NR included Guangdong, Fujian, and Jiangxi, while in the 2015–2020 period, Anhui, Hubei, and Hainan exhibited larger NR. The main provinces where the NR increased were Hainan, Yunnan, and Anhui Provinces, while Guangdong, Chongqing, and Hubei Provinces experienced decreases in it.

3.3. Spatial Heterogeneity of Moso Bamboo Expansion into Different Types of Forests

From 2010 to 2020, the analysis reveals a significant positive correlation between the expansion area of moso bamboo forests in each province and the total area of moso bamboo forests in each province (Figure 7a,b). This suggests that the expansion of bamboo forests occurred within existing bamboo forest areas, consistently with the phenomenon of bamboo forests expanding outward through rhizome sprouting.

A study of spatial auto-correlation in moso bamboo forests across provinces shows that while the expansion area of moso bamboo forests is positively associated with the total area of bamboo forests in each province, there are marked regional variations in the expansion area of bamboo forests. Jiangxi province and its surrounding areas are identified as core regions with high-high clustering, whereas Henan and Shandong provinces exhibit low–low clustering characteristics (Figure 7a,b).

The spatial auto-correlation analysis reveals a notable heterogeneity in bamboo forest expansion across provinces in various forest types. The expansion of moso bamboo forests in evergreen broad-leaved forests tends to cluster in specific regions, primarily centered around Jiangxi and its neighboring provinces. Significantly, during two distinct periods, 2010–2015 and 2015–2020, the regions with the most extensive moso bamboo forest expansion areas were Jiangxi and Fujian provinces. In the 2010–2015 period, Fujian accounted for the largest expansion area, comprising approximately 21.65% of the total expansion area in evergreen broad-leaved forests in 2015. In the 2015–2020 period, Jiangxi held the largest expansion area, representing about 20.88% of the total expansion area in evergreen broad-leaved forests in 2020. Moso bamboo forest expansion in deciduous broad-leaved forests primarily occurred in the middle and lower reaches of the Yangtze River.

As shown in Figure 8, during the 2010–2020 period, the provinces with significant expansion areas were Anhui and Hunan Province, collectively constituting approximately 39% of the total. In the 2010–2015 period, Zhejiang Province held the largest expansion area, covering about 33.4% of the total expansion area in deciduous broad-leaved forests. In the 2015–2020 period, Guangdong Province had the largest expansion area, accounting for approximately 31.94% of the total expansion area. The expansion of moso bamboo forests in other bamboo-rich regions appeared more dispersed, with primary occurrences in southwestern China, Guangdong Province, and Zhejiang Province. Notably, during the 2010–2015 and 2015–2020 periods, Zhejiang Province was the province with significant expansion areas. In the 2010–2015 period, Yunnan Province held the largest expansion area in other bamboo-rich regions, accounting for approximately 32.32% of the total expansion area in 2015. In the 2015–2020 period, Guangdong Province had the largest expansion area in other bamboo-rich regions, comprising approximately 79.33% of the total expansion area in 2020. Bamboo forest expansion in shrubbery was primarily concentrated in Hunan Province, with a lower proportion in the 2015–2020 period compared to the 2010–2015 period. The expansion of moso bamboo forests into coniferous forests was primarily concentrated in Jiangxi Province and its surrounding regions. Among these regions, during both periods (2010–2015 and 2015–2020), the provinces with significant expansion areas were Fujian Province and Hunan Province (Table 1). In the 2010–2015 period, Hunan Province held the largest expansion area, accounting for approximately 41.74%, while in the 2015–2020 period, Jiangxi Province had the largest expansion area, representing about 17.30%. The expansion of moso bamboo forests in non-forest areas was primarily distributed in Fujian Province, Guangdong Province, and Jiangxi Province. Notably, during both periods, the provinces with significant expansion areas were Fujian Province and Jiangxi Province. In the 2010–2015 period, Fujian Province accounted for the largest expansion area, comprising approximately 36.90%, while in the 2015–2020 period, Guangdong Province had the largest expansion area, accounting for about 22.82%. In summary, the most extensive erosion occurred in Jiangxi Province and its surrounding regions, but the distribution of erosion areas varied across different forest landscape types.

4. Discussion

4.1. The Strength and Speed of Moso Bamboo Expansion and the Changes in BWE and NR

This study has revealed a 2.1% annual average increasing area in moso bamboo forests over a 10-year period. A study conducted by Suzuki et al. similarly indicates a 2% annual expansion rate for moso bamboo forests in Japan [34], highlighting a consistent trend in bamboo forest expansion across Asia. Nevertheless, it is crucial to recognize that the growth rate of moso bamboo is not always constant and can be subject to various influences, including non-biological factors and human activities [48]. The average speed of bamboo forest expansion into other forests is 8 m/yr, which sharply contrasts with findings in Mount Tianmu, Zhejiang Province [53], where the expansion rate is only 3 m/yr. This also differs from the review stating that “the elongation speed of bamboo whips generally reaches 4–5 m/yr” [32]. The primary reason for this disparity lies in the scale of the research. In Jiangxi Province, within nature reserves, bamboo forest expansion is a consequence of natural succession, minimally disturbed by human socio-economic activities. On a national scale, where forest types are more diverse, regions undergoing bamboo forest expansion may be influenced by human cultivation of bamboo as an economic crop. The planted bamboo patches are included in the calculation of bamboo expansion growth rates, resulting in an average growth rate higher than the annual growth rate of bamboo whips.

Over the two periods, the NR of the national moso bamboo forests averaged approximately 0.8, indicating that moso bamboo forests primarily expand naturally. Zhejiang, Fujian, Jiangxi, and Hunan Provinces exhibited relatively higher NRs in both periods. However, Jiangxi and Fujian Provinces experienced a further decrease in NR in the latter period, while Hunan and Zhejiang observed an increase. This is largely related to the bamboo forestry policies of each province. Economic incentives in Fujian and Jiangxi Provinces, driven by the bamboo industry, led to a decrease in NR due to the artificial planting of bamboo forests.

Between 2010 and 2020, the length of the critical bamboo forest ecotone has significantly increased, with the contact area between bamboo forests and other forests gradually expanding. This has led to an increase in the shape index over the three periods, signifying the growing complexity of the critical bamboo forest ecotone [44]. This suggests that moso bamboo forest expansion into other forest types will intensify in the future.

4.2. The Differences in Moso Bamboo Expansion into Various Forests

Moso bamboo forest expansion predominantly targets coniferous forests, followed by evergreen broad-leaved forests. An analysis of the expansion rates of moso bamboo forests into coniferous and evergreen broad-leaved forests confirms that the expansion rate into coniferous forests exceeds that into evergreen broad-leaved forests [54]. Bamboo forests share the same ecological niche with coniferous and evergreen broad-leaved forests, all of which are typical forest types in subtropical regions, covering extensive areas in such regions [55]. Moso bamboo, classified as a heliophile and shade-tolerant species, thrives in forests with small, sparse canopies, and low density [56]. Coniferous forests, with their narrower canopy widths, are more susceptible to moso bamboo expansion, leading to the gradual formation of mixed bamboo and fir forests, ultimately culminating in pure moso bamboo forests. Furthermore, coniferous forests generally exhibit lower biodiversity compared to evergreen broad-leaved forests. Consequently, evergreen broad-leaved forests maintain higher stability, resulting in a slower expansion rate during moso bamboo intrusion [57,58]. Over the ten-year study period, significant disparities were observed in moso bamboo forest expansion into non-forested land. In the earlier period, a substantial portion of moso bamboo forests succumbed to erosion by non-forest landscapes, while the later period witnessed an increased occurrence of moso bamboo expansion into non-forest landscapes. The variations between these two periods were primarily influenced by the bamboo industry’s life cycle and adjustments in nature conservation policies. The period from 2004 to 2014 marked the mature phase of the bamboo industry’s life cycle, characterized by stable market demand and a consistent increase in moso bamboo forest harvesting. This resulted in extensive felling of moso bamboo forests, converting them into farmland and barren land, leading to significant erosion between 2010 and 2015. However, after 2015, due to rising labor costs, changes in supply and demand dynamics, and enhancements in the natural conservation system, a substantial prohibition on bamboo forest logging was enforced, resulting in a distinct trend in moso bamboo forest expansion into other types of forests.

4.3. Spatial Heterogeneity in Moso Bamboo Forest Expansion in Different Provinces

The area of moso bamboo forest expansion into other forest types in each province exhibits a significant positive correlation with the existing moso bamboo forest area. This correlation aligns with the reproductive characteristics of moso bamboo (Phyllostachys edulis), a typical clonal plant species whose propagation and renewal primarily occur through the division of rhizomes. This growth pattern diverges from the typical “fixed growth” mode observed in many other woody plants, featuring rapid growth, sustainable strength, and a propensity for spreading.

Spatial heterogeneity in moso bamboo forest expansion is evident across different forest types in various regions. The expansion of evergreen broad-leaved and coniferous forests is prominent in Jiangxi and surrounding provinces. The expansion of deciduous broad-leaved forests mainly occurs in the middle and lower reaches of the Yangtze River. The expansion of other bamboo forests is scattered and primarily distributed in southwestern China, Guangdong, and Zhejiang Province. The expansion of shrubbery is mainly observed in Hunan province. A significant correlation is apparent between provinces experiencing moso bamboo expansion in different forest types and in the province’s moso bamboo forest area, and the distribution proportion of the forest landscape in that province. The top three provinces in terms of moso bamboo-invaded area for each landscape type belong to the top ten provinces in moso bamboo forest area ranking (percentage-wise), and the proportion of this forest type is among the highest nationwide. Although Sichuan and Hubei Province have a substantial proportion of deciduous broadleaf forests, the expansion primarily occurs in Hubei due to the smaller area of moso bamboo forests in Sichuan. Despite the larger area of shrub forests in Sichuan compared to Hunan province, moso bamboo expansion into shrub forests is most extensive in Hunan due to the significantly smaller area covered by moso bamboo forests in Sichuan.

4.4. The Possible Reasons for the Expansion of Moso Bamboo Forest

The expansion of bamboo forests can be attributed to both natural and human factors. The area of moso bamboo forest expansion due to natural factors is greater than that due to human factors. In terms of natural expansion, bamboo propagates through the outward growth of its rhizomes [11,59]. However, the rate of expansion varies significantly across different forest types, indicating that certain factors in these forests inhibit bamboo expansion. These factors differ from those influencing bamboo distribution mentioned by Yu [48], since local bamboo forests and adjacent forests share the same climate and geographic location but differ in stand characteristics, soil properties, and light intensity [60]. And various studies have shown that changes in soil physical and chemical properties (including depth, moisture content, organic carbon content, total nitrogen, total phosphorus, pH, and microorganisms), stand structure (canopy closure, canopy height, stand density, species diversity), and light intensity [56] facilitate bamboo growth. Therefore, these factors are crucial in the expansion of bamboo forests. Regarding human factors, the main causes of bamboo expansion include moso bamboo afforestation, forest management, and human disturbance [35]. The growth of bamboo forests alters soil properties and microbial composition, leading to a pattern of bamboo invasion that reduces biodiversity, changes community structure and composition, weakens ecosystem functions, and inhibits community regeneration.

4.5. Suggestions for the Management of Moso Bamboo Forests

The expansion of moso bamboo into coniferous and broad-leaved forests carries adverse repercussions for the ecosystem, encompassing decreased species diversity in both the tree and shrub layers [9], and a more homogenous forest structure. In the realm of strictly protected area management, a paramount focus should be placed on bamboo control. This can be achieved through practices such as bamboo shoot harvesting and bamboo thinning, complemented by physical barriers like stone paths and water systems. These measures are imperative to deter moso bamboo expansion [61] and to preserve the integrity of natural forest vegetation. In areas of stringent protection, clear-cutting provides a comprehensive solution to the challenge posed by bamboo expansion. As supported by the literature, clear-cutting leads to a secondary succession process characterized by increased species diversity and enhanced ecosystem stability. To curb future expansions of moso bamboo into coniferous forests, it is recommended to plant high-density native broadleaved species around the periphery of moso bamboo forests. These plantings can effectively impede the rate of expansion. To safeguard against the expansion of moso bamboo forests into non-native regions due to climate change, the establishment of a moso bamboo forest database is advised. The integration of this database into forest inspections will facilitate the continuous monitoring of moso bamboo forest introductions and natural expansion. This monitoring will prevent bamboo from spreading beyond its native areas as climate conditions evolve. Additionally, as moso bamboo is an economically significant plant, its extent is subject to considerable human influence. Governments can introduce policies such as “bamboo as a substitute for plastic initiative” to stimulate the bamboo industry, enhance the consumption of bamboo products, and help mitigate bamboo forest expansion. In future moso bamboo forest management, individual provinces can implement tailored management strategies, taking into account the varying expansion rates of moso bamboo forests and the distinctive characteristics of the bamboo industry within each province. This approach addresses the challenges posed by bamboo forest expansion from a socio-economic perspective.

4.6. Contributions and Limitations

This study broadens the research scope of moso bamboo expansion from protected areas to the national region. The choice of different research scales can impact the results of moso bamboo forest invasion studies, which will be advantageous for scale selection on moso bamboo forest invasion in future research. In contrast to the fact that the concept of BWEs is merely studied, this paper, for the first time, computes BWEs for varying temporal lengths and changes in various forest types. Delineating the trend in moso bamboo forest expansion based on length fluctuations promots a more comprehensive exploration of changes in this forest ecotone. The conceptual formula for NR is presented, establishing the theoretical foundation and methodology for studying the expansion of local species into surrounding ecosystems. The application of the forest second type inventory data to bamboo forest expansion is employed for the first time, with the research results indicating the high reliability and validity of the forest second type inventory data.

The research at hand delves into the spatio-temporal changes in the encroachment of moso bamboo forests into other forest types between 2010 and 2020, investigating spatial characteristics, variations in tree species, and changing trends. It delivers valuable insights into the development of moso bamboo forests in China over the past decade and provides recommendations for addressing the ecological impacts of moso bamboo forest expansion. It is important to acknowledge the limitations of this study. Due to constraints in data acquisition, this study is confined to a ten-year time-frame, which is relatively brief. Subsequent research endeavors could explore longer time series and more precise data analysis by incorporating high-resolution remote sensing imagery and national forest inventory data. Moreover, human disturbances exert a significant influence on bamboo forest development in China. In comparison to natural drivers, human-induced factors are unpredictable. To study the natural development of bamboo forests, it is essential to select bamboo forests free from human interference as research subjects. Predicting the future development of bamboo forests and monitoring ecological security under conditions devoid of human interference are also crucial. Additionally, bamboo forests exhibit high sensitivity to environmental changes, making them suitable indicator plants to monitor environmental alterations in the absence of human disturbances.

5. Conclusions

This study indicates a clear positive correlation between the area invaded by moso bamboo forest and the existing moso bamboo forest area. Moso bamboo forests are expanding area at an average annual rate of 2%. As the complexity and length of bamboo–woodland ecotones (BWEs) increase, moso bamboo forests will further encroach into other forest. At different research scales, the speed of bamboo forest expansion into different forest types varies significantly. In particular, at the national scale, the average expansion rate of bamboo forests (including moso bamboo forests manually planted) is 8 m per year. The area and speed of bamboo forest expansion into different forests are not consistent. Tt is contingent upon the proportion of bamboo forest area at the location extent and the forest types being invaded. Coniferous forests and evergreen broad-leaved forests are the primary forests expanded by moso bamboo forests. However, coniferous forests and other bamboo forests are most rapidly invaded by moso bamboo. Expansion areas of bamboo forests in China exhibit pronounced spatial heterogeneity; the areas of different forest types expanded by moso bamboo forests have a substantial correlation with the bamboo forest areas in the province and the distribution proportion of the forest landscape in the province. The factors contributing to bamboo forest expansion encompass stand characteristics, soil attributes, light intensity, moso bamboo afforestation, forestry practices, and human disturbances.

Author Contributions

Conceptualization, D.L. and Y.Z.; methodology, J.W. (Juan Wei); software, D.L.; validation, J.W. (Jiangzhou Wu), Z.C. and J.H.; formal analysis, Y.Z.; data curation, S.Z.; writing—original draft preparation, D.L.; writing—review and editing, J.W. (Juan Wei) and L.Y.; visualization, D.L.; supervision, Y.Z.; project administration, Y.Z.; funding acquisition, Y.Z. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Dataset available on request from the authors.

Conflicts of Interest

We declare no conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Table

Figure 1. Distribution map of all species of bamboo stands in China [48].

Figure 2. Diagram of data processing workflow.

Figure 3. The area of moso Bamboo forests from 2010 to 2020.

Figure 4. The transfer area of moso bamboo forests into other forests during the 2010–2015 and 2015–2020 periods.

Figure 5. The average rate of bamboo expansion across different forest types and the overall average rate of expansion.

Figure 6. The neighboring rate of moso bamboo forest in every province.

Figure 7. (a) The correlation between the area of the moso bamboo expanding and the existing extent in every province and the spatial auto-correlation map of the moso bamboo expanding in 2010–2015. (b) The correlation between the area of the moso bamboo expanding and the existing extent in every province and the spatial auto-correlation map of the moso bamboo expanding in 2015–2020. *** stands for the significance level >0.99.

Figure 8. The area of moso bamboo forest expanding into the following areas: (a) evergreen broad-leaved forest (2010–2015); (b) evergreen broad-leaved forest (2015–2020); (c) deciduous broad-leaved forest (2010–2015); (d) deciduous broad-leaved forest (2015–2020); (e) non-timber forest (2010–2015); (f) non-timber forest (2015–2020); (g) other bamboo forest (2010–2015); (h) other bamboo forest (2015–2020); (i) shrubbery (2010–2015); (j) shrubbery (2015–2020); (k) coniferous forest (2010–2015); (l) coniferous forest (2015–2020); (m) other forest (2010–2015); (n) other forest (2015–2020); (o) non-forest (2010–2015); (p) non-forest (2015–2020).

Figure 8. The area of moso bamboo forest expanding into the following areas: (a) evergreen broad-leaved forest (2010–2015); (b) evergreen broad-leaved forest (2015–2020); (c) deciduous broad-leaved forest (2010–2015); (d) deciduous broad-leaved forest (2015–2020); (e) non-timber forest (2010–2015); (f) non-timber forest (2015–2020); (g) other bamboo forest (2010–2015); (h) other bamboo forest (2015–2020); (i) shrubbery (2010–2015); (j) shrubbery (2015–2020); (k) coniferous forest (2010–2015); (l) coniferous forest (2015–2020); (m) other forest (2010–2015); (n) other forest (2015–2020); (o) non-forest (2010–2015); (p) non-forest (2015–2020).

Table 1

A comparison of the major provinces in terms of invaded areas during different stages of bamboo forest expansion.

Forest Types	The Province with the Largest Invaded Area in the 2010–2015 Period	The Province with the Largest Invaded Area in the 2015–2020 Period	The Provinces Consistently Ranking within the Top Three in Invaded Areas in the Two Periods
Evergreen broad-leaved forest	Fujian Province	Jiangxi Province	Fujian Province, Jiangxi Province
Deciduous broad-leaved forest	Zhejiang Province	Guangdong Province	Anhui Province, Hunan Province
Non-timber forest	Zhejiang Province	Fujian Province	Fujian Province, Jiangxi Province
Other bamboo forest	Yunnan Province	Guangdong Province	Zhejiang Province
Shrubbery	Hunan Province	Hunan Province	Hunan Province
Coniferous forest	Hunan Province	Jiangxi Province	Fujian Province, Hunan Province
Other forest	Hunan Province	Jiangxi Province	Fujian Province, Zhejiang
Non-forest	Fujian Province	Guangdong Province	Fujian Province, Jiangxi Province

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