Effects of Enclosure Succession on the Morphological Characteristics and Nutrient Content of a Bamboo Whip System in a Moso Bamboo ( Phyllostachys edulis ) Forest on Wuyi Mountain, China

: To investigate the morphological characteristics and nutrient content of bamboo whip systems in the Wuyi Mountain Moso bamboo ( Phyllostachys edulis ) forest in response to enclosure succession. The mixed Moso bamboo forests in the Wuyi Mountain Nature Reserve with 0, 4, 6, 11, 16, and 41 enclosure years were taken as the object of investigation. All the bamboo whips in the 2 m × 2 m sample plots were excavated layer-by-layer according to the soil layers of 0–20 cm, 20–40 cm, and 40–60 cm, and a total of 54 plots were dug. The morphological characteristics and nutrient contents of the Moso bamboo whips in the different soil layers were analyzed and evaluated. Enclosure management measures can not only effectively improve vegetation coverage, biodiversity, and biomass, but also improve soil moisture and nutrient status, indirectly affecting the vegetation, which is of great signiﬁcance for preventing soil erosion. The results showed that the whip number, whip diameter, ﬂagella number, whip length, and whip weight in the 0–20 cm soil layer were signiﬁcantly increased by 169.2%, 11.0%, 197.5%, 113.7%, and 109.0% ( p < 0.05), respectively, compared with CK after 41 years of enclosure. The average internode length was signiﬁcantly decreased by 27.9% ( p < 0.05) compared to CK after 41 years of enclosure. In the 20–40 cm soil layer, the whip diameter increased by 9.7% after 41 years of enclosure compared with CK, but the whip number, ﬂagella number, whip length, and whip weight were signiﬁcantly reduced after 16 and 41 years of enclosure compared to CK ( p < 0.05). In the 40–60 cm soil layer, the whip number, number of ﬂagella, and whip length increased signiﬁcantly after 6 and 11 years of enclosure compared with CK ( p < 0.05). In the 0–20 cm soil layer, the contents of total nitrogen, total phosphorus, and total potassium in CK were higher than those in other enclosure years, and the soluble sugar content in CK was signiﬁcantly higher than that in the enclosures of


Introduction
Enclosure succession is the process of community succession during forest closure.Currently, enclosures are widely used in the restoration and management of degraded grassland and forest vegetation in China [1,2].Enclosures can not only effectively improve vegetation cover, biodiversity, and biomass [3,4], but also improve soil moisture and nutrient conditions [5,6].This indirectly affects the root traits of vegetation [7], and is of considerable importance in preventing soil erosion.Yuying et al. [8] found that enclosures can effectively improve the ecological environment of the Baishitougou watershed in Daqingshan, increase the forest area, and improve the benefits of forests in water and soil conservation.Jishuai et al. [9] found that with an increase in the number of enclosure years, the soil water and fertilizer conditions of the grassland on Yunwu Mountain significantly improved.The root biomass and morphological characteristics of long manzanita (Stipa bungeana), large needlegrass (Stipa grandis), and sweet green needlegrass (Stipa przewalskyi) also significantly improved.Studies have shown that the root biomass of herbaceous plants increased after enclosure management began in larch plantations in North China [10].Peng et al. [11] found that the root biomass of plants significantly increased during the enclosure and restoration of degraded vegetation in sandy areas.Yanmin et al. [12] found that the N, P, and C content of the root system of grassland vegetation increased after enclosure management in the Yili Sericeous Desert.The effect of enclosures on the plant root system is concentrated in biomass, morphological characteristics, and nutrient content.However, to date, most studies have focused on the analysis of the root traits of grassland vegetation [13,14].Meanwhile, there have been relatively few studies on bamboo plants.Bamboo plants have a high ecological value, and their specific biological characteristics play an important role in environmental improvement and soil and water conservation.
Moso bamboo (Phyllostachys edulis) has high yield, fast growth, and strong sustainability.The strong reproduction and spatial expansion ability of the Moso bamboo whip provide prerequisites for its expansion into neighboring broadleaf forests [15][16][17].To date, local and international studies on bamboo whips in Moso bamboo forests have focused on the dynamic changes in bamboo whip systems in the distribution pattern of different vertical soil layers [18,19].Meanwhile, there have been relatively few reports on the effects of enclosure succession on the morphological characteristics and nutrient content of bamboo whips in the natural state of Moso bamboo forests.Therefore, in this study, a Moso bamboo mixed forest in the Wuyishan Nature Reserve of Nanping City, Fujian Province, was used as the experimental object.We hypothesized that the morphological characteristics and nutrient content of Moso bamboo whips in Moso bamboo mixed forest changed dynamically with the increase of enclosure years.The enclosure time was set at 4, 6, 11, 16, and 41 years and compared with the conventional operation of the Moso bamboo forest.The results of this study provided a reference for the successional progress of closed mangosteen forests and their management.

Overview of Experimental Conditions
Wuyi Mountain National Park is located in Nanping City, Fujian Province (longitude 117 • 24 13 -117 • 59 19 , latitude 27 • 31 20 -27 • 55 49 ).The altitude is 700 m below red soil, the altitude is 700-1100 m yellow red soil, and the altitude is 1100-1900 m above meadow soil.The protected area covers an area of 1280 km 2 and has a central subtropical monsoon climate.The average annual precipitation is 1684-1780 mm, the average air temperature is 17-19 °C, and the frost-free period is 252-272 d.The main vegetation in the study area Forests 2023, 14, 2193 3 of 14 includes the Luofu cone (Castanopsis faberi), quebracho (Castanopsis fargesii), and green fescue (Calamagrostis brachytricha).

Experimental Materials
In September 2022, a detailed survey and exploration of Wuyi Mountain National Forest Park was conducted.Mixed Moso bamboo forests in the Aotou, Tongmu, and Daan villages of Wuyi Mountain City with enclosure years of 4, 6, 11, 16, and 41 years, respectively, were selected as the objects of the survey.The control (0 years) was selected for conventional operation (i.e., digging Moso bamboo shoots during the Moso bamboo shoot period in the Moso bamboo forest; cutting down all of the Moso bamboo regularly), which was recorded as CK.Three sample plots were set up for each enclosure year, and the sample plots totaled 18.Each sample plot was set up with 25.8 m × 25.8 m sample squares, and three standard bamboos were screened in each standard plot.The 2 m × 2 m small sample squares were dug around them, totaling 54 small sample squares.Within each 2 m × 2 m sample plot, there were three different soil layers from top to bottom, namely, 0-20 cm, 20-40 cm, and 40-60 cm.The physical and chemical properties of the soil and the growth conditions of Moso bamboo and forest trees were investigated in each sample plot (Tables 1 and 2).The main tree species in each plot were: in the 4 years of enclosure plot, there were Symplocos congesta, Manglietia fordiana, Castanopsis eyrei, etc.; in the 6 years of enclosure plot, there were Photinia bodinieri, Liquidambar formosana, Cunninghamia lanceolata, etc.; in the 11 years of enclosure plot, there were Engelhardia roxburghiana, Schima superba, Machilus thunbergii, etc.; in the 16 years of enclosure plot, there were Cunninghamia lanceolata, Castanopsis faberi, Machilus thunbergii, etc.; in the 41 years of enclosure plot, there were Castanopsis faberi, Pinus massoniana, Machilus thunbergii, etc.; and CK was a pure bamboo forest.

Assessing Indicators of the Characteristics and Nutrient Content of Moso Bamboo Whips
The whip number, whip diameter, mean internode length, flagellar number, whip length, and whip weight in the different soil layers were measured to explore the distribution of bamboo whips in the different vertical soil layers.
In each small sample plot, bamboo whips and their corresponding whip roots were dug out to a depth of 60 cm using the bamboo whip tracking method.The whip number was recorded in different soil layers.The flagella number and whip length were measured with vernier calipers (Shanghai Xiusong Co., Ltd., Shanghai, China) from the tip of the whip to the proximal end of the mother bamboo in each layer of the soil to calculate the mean internode length.The whip diameter was measured with the breast tape measure (Beijing Pacific Tape Factory Store, Beijing, China) at a distance of 10 cm from the tip of the whips.The whip weight was weighed with the analytical balance (Ohaus Instruments Shanghai Co., Ltd., Shanghai, China).Whips with a length of 10 cm were weighed, put into a sealed bag with labels, and brought back to the laboratory.After weighing the whips with an analytical balance, 10 cm of bamboo whips were weighed and put into a sealed bag, labeled, and brought back to the laboratory to be dried in an oven (Shanghai Boxun Industry Co., Ltd., Shanghai, China) at 70-80 • C for 48 h to a constant weight.They were then crushed in a pulverizer (DC-500A model, Zhejiang Wuyi Dingzang Daily Metal Products Factory, Jinhua, China) and sieved through a 100-mesh sieve to determine their nutrient content.

Statistical Analysis
Data were processed and analyzed using Excel 2019 and SPSS 25.0.One-way analysis of variance (ANOVA), independent samples t-test, and least significant difference (LSD) multiple comparisons were used for one-way analysis of variance and multiple comparisons.Pearson's correlation analysis and principal component analysis were used for factor screening and detailed evaluation.Redundancy analysis was performed using the Canoco 5 software.

Morphological Characteristics of Moso Bamboo Whips
As shown in Figure 1, in the soil layer from 0 to 20 cm, the whip number, whip diameter, flagella number, whip length, and whip weight increased, then decreased, and then increased again.The mean internode length decreased, then increased, and then decreased again, with the increase in enclosure years.The whip number increased significantly by 44.4%, 82.0%, 25.6%, and 169.2% after 4, 6, 16, and 41 years of enclosure compared with that of CK, respectively (p < 0.05).The whip diameter increased significantly by 11.0% after 41 years of enclosure compared with that of CK (p < 0.05).After 4, 6, 11, and 41 years of enclosure, the flagellar number increased significantly by 44.8%, 51.3%, 32.9%, and 197.0%, and the whip length increased significantly by 35.7%, 22.1%, 19.6%, and 113.7%, compared with that of CK, respectively (p < 0.05).The whip weight increased significantly by 18.7% and 109.0%compared with CK after 4 and 41 years of enclosure, respectively (p < 0.05).The mean internode length was significantly reduced by 27.9% (p < 0.05) after 41 years of enclosure compared with CK (p < 0.05).
In the soil layer from 20 to 40 cm, the whip number, whip diameter, and flagella number increased and then decreased.Whip length increased and then decreased, and whip weight decreased and then increased and then decreased with the increase in enclosure years.The mean internode length varied considerably.Whip diameter increased by 9.7% after 41 years of enclosure compared with CK, and 41 years of enclosure was significantly higher than the other enclosure years (p < 0.05).The whip number, flagella number, whip length, and whip weight were significantly reduced after 16 and 41 years of enclosure compared to CK (p < 0.05).
From 40 to 60 cm in the soil layer, the whip and flagella numbers increased, then decreased, and then increased again.The whip length increased and then decreased with increasing enclosure age.The whip number increased significantly by 132.0% and 268.0% after 6 and 11 years of enclosure compared with CK, respectively (p < 0.05).The number of flagella and whip length increased significantly after 4, 6, 11, 16, and 41 years of enclosure with CK (p < 0.05).The changes in whip diameter, mean internode length, and whip weight varied substantially.With the increase in enclosure years, the Moso bamboo whip gradually developed into the shallow soil to adapt to the environmental changes.From 40 to 60 cm in the soil layer, the whip and flagella numbers increased, then decreased, and then increased again.The whip length increased and then decreased with increasing enclosure age.The whip number increased significantly by 132.0% and 268.0% after 6 and 11 years of enclosure compared with CK, respectively (p < 0.05).The number of flagella and whip length increased significantly after 4, 6, 11, 16, and 41 years of enclosure with CK (p < 0.05).The changes in whip diameter, mean internode length, and whip weight varied substantially.With the increase in enclosure years, the Moso bamboo whip gradually developed into the shallow soil to adapt to the environmental changes.As shown in Figure 2, from 0 to 20 cm in the soil layer, the total nitrogen, phosphorus, and potassium contents of bamboo whips decreased and then increased.Meanwhile, the organic carbon content increased and then decreased and then increased with the increase in enclosure years.The total nitrogen content was significantly reduced by 41.5%, 41.0%, 38.7%, 33.4%, and 16.3% after 4, 6, 11, 16, and 41 years of enclosure compared with that of CK (p < 0.05).The total phosphorus content was significantly reduced by 21.7%, 50.0%, 32.6%, and 19.6% after 4, 6, 11, and 16 years of enclosure compared with that of CK, respectively (p < 0.05).The total potassium content was significantly reduced by 24.8% after 6 years of enclosure compared with that of CK, and 6 years of enclosure was significantly lower than that of the other years of enclosure (p < 0.05).The organic carbon content was significantly increased by 14.8%, 12.7%, 7.2%, and 7.1% after 4, 6, 11, and 41 years of enclosure compared with that of CK, respectively.It significantly decreased by 8.01% after 16 years of enclosure compared with CK (p < 0.05).

Starch and Soluble Sugar Content of Bamboo Whips
As shown in Figure 3, the starch content of the bamboo whips increased, then decreased, and then increased.The soluble sugar content decreased, then increased, and then decreased with an increase in enclosure years.In the soil layer from 0 to 20 cm, the starch content increased significantly by 34.1%, 23.0%, and 62.7% after 6, 16, and 41 years of enclosure compared with that of CK, respectively.It was significantly higher than that of other enclosure years after 41 years of enclosure (p < 0.05).The soluble sugar content decreased significantly by 39.8%, 37.9%, 34.4%, 34.0%, and 34.0% after 4, 6, 11, and 41 years of enclosure compared with that of CK, respectively, (p < 0.05).From 20 to 40 cm in the soil layer, the total nitrogen content decreased and then increased.The total phosphorus and organic carbon content increased and then decreased and then increased.The total potassium content decreased and then increased and then decreased with an increase in enclosure years.The total nitrogen content was significantly reduced by 27.4% after 4 years of enclosure compared with CK (p < 0.05).The total phosphorus content was significantly reduced by 37.1% after 6 years of enclosure compared with CK.It increased significantly by 31.4% and 34.3% after 16 years and 41 years of enclosure compared with CK (p < 0.05).The total potassium content was significantly lower after 6 years of enclosure than in the other years of enclosure (p < 0.05).The organic carbon content was significantly increased by 14.5% and 16.2% after 4 and 6 years of enclosure compared with CK, respectively.It significantly decreased by 12.3% after 16 years of enclosure compared with CK, and 6 years of enclosure was significantly lower than other years of enclosure (p < 0.05).
From 40 to 60 cm in the soil layer, the total nitrogen and phosphorus contents decreased and then increased.The total potassium content increased and then decreased and then increased with an increase in enclosure years.The total nitrogen content decreased significantly by 27.2% and 23.9% after 4 years and 6 years of enclosure compared with that of CK, respectively (p < 0.05).The total phosphorus and total potassium content increased significantly by 29.0% and 15.1% after 41 years of enclosure compared with that of CK, and 41 years of enclosure was significantly higher than the other enclosure years (p < 0.05).The organic carbon content was significantly increased after 4 and 6 years of enclosure compared with CK, respectively, but there was no clear change seen in the organic carbon content.
Enclosure succession was favorable for the accumulation of organic carbon in the bamboo whips.The short-term enclosure was not favorable for the accumulation of total nitrogen, total phosphorus, and total potassium in bamboo whips.However, with the prolongation of the time of enclosure, the Moso bamboo increased the total nitrogen, total phosphorus, and total potassium content of the bamboo whips by adjusting the distribution pattern of the bamboo whips in the soil space to adapt to the environmental changes.

Starch and Soluble Sugar Content of Bamboo Whips
As shown in Figure 3, the starch content of the bamboo whips increased, then decreased, and then increased.The soluble sugar content decreased, then increased, and then decreased with an increase in enclosure years.In the soil layer from 0 to 20 cm, the starch content increased significantly by 34.1%, 23.0%, and 62.7% after 6, 16, and 41 years of enclosure compared with that of CK, respectively.It was significantly higher than that of other enclosure years after 41 years of enclosure (p < 0.05).The soluble sugar content decreased significantly by 39.8%, 37.9%, 34.4%, 34.0%, and 34.0% after 4, 6, 11, and 41 years of enclosure compared with that of CK, respectively, (p < 0.05).

Correlation between Morphological Characteristics and Nutrient Content of Moso Bamboo Whips
As shown in Table 3, whip number was significantly positively correlated with flagella number, whip length, whip heaviness, organic carbon, and starch (p < 0.01), and it was significantly negatively correlated with mean internodal length and soluble sugar (p < 0.01).The whip diameter was highly positively correlated with starch content (p < 0.01).In the soil layer from 20 to 40 cm, the starch content increased significantly by 40.0%, 29.2%, and 69.8% after 6, 16, and 41 years of enclosure compared with CK.It was significantly higher after 41 years of enclosure than the other years of enclosure (p < 0.05).The soluble sugar content decreased significantly by 36.1%, 33.4%, 26.6%, 29.8%, and 29.8% after 4, 6, 11, and 41 years of enclosure compared with CK, respectively (p < 0.05).
In the soil layer from 40 to 60 cm, the starch content increased significantly by 33.4%, 80.8%, 23.3%, and 63.7% after 4, 6, 16, and 41 years of enclosure compared with CK (p < 0.05).Meanwhile, the soluble sugar content decreased significantly by 34.5%, 33.2%, 32.6%, and 29.7% after 4, 6, 11, and 41 years of enclosure, respectively (p < 0.05).Enclosure succession is favorable for the accumulation of starch content, whereas soluble sugar content is closely related to the enclosure profile in Moso bamboo mixed forests.

Correlation between Morphological Characteristics and Nutrient Content of Moso Bamboo Whips
As shown in Table 3, whip number was significantly positively correlated with flagella number, whip length, whip heaviness, organic carbon, and starch (p < 0.01), and it was significantly negatively correlated with mean internodal length and soluble sugar (p < 0.01).The whip diameter was highly positively correlated with starch content (p < 0.01).The mean internode length was significantly positively correlated with the soluble sugar content (p < 0.01) and significantly negatively correlated with flagella number, whip length, whip weight, organic carbon, and starch (p < 0.01).The flagella number was significantly positively correlated with whip length, whip weight, organic carbon, and starch (p < 0.01), and significantly negatively correlated with soluble sugar (p < 0.01).The whip length was significantly positively correlated with whip heaviness and organic carbon (p < 0.01), and it was significantly negatively correlated with soluble sugar (p < 0.01).The whip weight content was significantly positively correlated with organic carbon and the soluble sugar content (p < 0.01).The total nitrogen and phosphorus contents were positively correlated (p < 0.01).The total phosphorus content was positively correlated (p < 0.01) with the total potassium content and negatively and significantly correlated with organic carbon (p < 0.05).The Organic carbon content was negatively and significantly correlated with the soluble sugar content (p < 0.05).

Redundancy Analysis between Quantitative Characteristics and Nutrient Content of Moso Bamboo Whips and Years of Closure
As shown in Figure 4, redundancy analysis was conducted with the quantitative characteristics of Moso bamboo whips and nutrient content as the response variables, and the years of enclosure as the explanatory variables.The results show that the first ordering axis explained 19.67% of the total spatial variance and the second ordering axis explained 47.95% of the total spatial variance, with a cumulative explanation of 67.62%.The enclosure years show a negative correlation with the mean internode length, organic carbon, and soluble sugar content, and there was a positive correlation with other indicators.
As shown in Figure 4, redundancy analysis was conducted with the quantitative characteristics of Moso bamboo whips and nutrient content as the response variables, and the years of enclosure as the explanatory variables.The results show that the first ordering axis explained 19.67% of the total spatial variance and the second ordering axis explained 47.95% of the total spatial variance, with a cumulative explanation of 67.62%.The enclosure years show a negative correlation with the mean internode length, organic carbon, and soluble sugar content, and there was a positive correlation with other indicators.

Principal Component Analysis and Evaluation of Quantitative Characteristics and Nutrient Content of Moso Bamboo Whips in Different Sequestration Years
As shown in Table 4, based on the principle of eigenvalue >1 and cumulative contribution rate ≥85%, three principal components were extracted in this study, with a cumulative contribution rate of 87.41%.Of these, the total variance contribution rate of the first principal component was 51.36%, the total variance contribution rate of the second principal component was 21.46%, and the total variance contribution rate of the third principal component was 14.59%.Therefore, these three principal components can explain most of the information and can be chosen to evaluate the growth potential of bamboo whips in Moso bamboo forests.As shown in Table 5, the growth status of bamboo whips in the six sample plots of the Moso bamboo forests was as follows: 41 years of enclosure > CK >11 years of enclosure >4 years of enclosure >16 years of enclosure >6 years of enclosure.

Principal Component Analysis and Evaluation of Quantitative Characteristics and Nutrient Content of Moso Bamboo Whips in Different Sequestration Years
As shown in Table 4, based on the principle of eigenvalue >1 and cumulative contribution rate ≥ 85%, three principal components were extracted in this study, with a cumulative contribution rate of 87.41%.Of these, the total variance contribution rate of the first principal component was 51.36%, the total variance contribution rate of the second principal component was 21.46%, and the total variance contribution rate of the third principal component was 14.59%.Therefore, these three principal components can explain most of the information and can be chosen to evaluate the growth potential of bamboo whips in Moso bamboo forests.As shown in Table 5, the growth status of bamboo whips in the six sample plots of the Moso bamboo forests was as follows: 41 years of enclosure > CK > 11 years of enclosure > 4 years of enclosure > 16 years of enclosure > 6 years of enclosure.

Discussion
In this study, the whip number, whip diameter, flagella number, whip length, and whip weight in the soil layer from to 0 to 20 cm increased significantly after 41 years of enclosure in Moso bamboo forests.This observed increase may be due to the clonal growth characteristics of Moso bamboo that give its root system higher morphological plasticity to obtain more resources, redistribute resources, and improve its adaptability to the growing environment [25,26].During enclosure succession, competition for space and nutrients between the root systems of Moso bamboo whips and those of other tree species allows Moso bamboo whips to adapt to the spatial and temporal variability of the growing environment and the unevenness of nutrient supply by modifying their own quantitative and structural characteristics [27].Therefore, exclusion measures can improve the plant root traits to a certain extent and, therefore, affect the morphological plasticity of the root system [19].The mean internode length was significantly reduced after 41 years of enclosure compared to CK.This phenomenon may be explained by the fact that, after a long period of enclosure management, the Moso bamboo whips fully occupied the underground space.Given the space limitations, the mean internode length was smaller than that at the beginning of the enclosure period [28].From 20 to 40 cm in the soil layer, the whip number, flagella number, whip length, and whip weight were significantly reduced compared with those of CK after 16 years and 41 years of enclosure.This phenomenon may be explained by the fact that the density of the mixed forests of Moso bamboo increased during the process of succession, and the competition for space and nutrients among the species intensified, such that Moso bamboo whips were restricted by the limitation of space and the scarcity of nutrients.This affected their growth and development and prompted the whips to tend to float upward to draw nutrients [29,30].In all the enclosure years, whip number, flagella number, whip length, and whip weight decreased with an increase in the soil layer.This phenomenon may be explained by the large number of stones, stumps, old bamboo whips, and old bamboo rattan retained in the soil layer of the forest floor, resulting in soil consolidation and a narrow subsurface space.This meant that the surface layer of the soil was more fertile and looser, which promoted the bamboo whips to move to the shallow layer [31,32].This suggests that Moso bamboo can respond to environmental changes by changing the number of bamboo whips and the vertical distribution pattern of bamboo whips in each soil layer.
With the increase in enclosure years, the contents of total nitrogen, phosphorus, and potassium of bamboo whips in the soil layer from 0 to 20 cm showed a tendency to decrease and then increase, but the organic carbon content showed a tendency to increase and then decrease and then increase.This phenomenon may be due to the fact that CK was a pure forest of artificially operated Moso bamboo and there was no inter-species competition within the forest, and the bamboo whips were able to obtain sufficient nutrients from the soil.However, in the early stages of enclosure succession, the competition for space and nutrients between the Moso bamboo whips and the roots of other tree species present reduced the total nitrogen, total phosphorus, and total potassium contents of the Moso bamboo whips [33][34][35].However, the organic carbon content increased, which may be related to an increase in apoptotic input and changes in soil texture and structure [36].With the progress of enclosure succession, the density of forest stands increased, and the competition of communities intensified, which affected the nutrient uptake of bamboo whips.In the late stage of succession, the bamboo whips adapted to the change in the growing environment through their strong morphologic plasticity, and the nutrient uptake efficiency of the whips increased from the change in the soil water content and the availability of apoplastic matter [37].This led to an increase in the nutrient content of the bamboo whips.The total nitrogen, phosphorus, and potassium contents of bamboo whips in the 40-60 cm soil layer were lower than those in other soil layers in all enclosure years, but the organic carbon content did not change significantly in all soil layers.The nutrient content of plant organs can reflect the soil nutrient level of the plant growth environment [38].The nitrogen, phosphorus, potassium, and carbon contents of the soil in Moso bamboo forests are mainly distributed in the soil layer from 0 to 40 cm [39][40][41].There was no significant change in the organic carbon contents of the bamboo whips in the soil layer.This may be the result of the combined effect of the number of years of closure, the climatic conditions, the composition of the species, and other factors.This may also be the result of a combination of factors, such as enclosure year, climatic conditions, and species composition [42][43][44], which need to be studied in depth.The starch content of Moso bamboo whips in the 0-20 cm and 20-40 cm soil layers was the highest and was significantly higher than that of other samples in 41 years of enclosure.The soluble sugar content was the highest in 16 years of enclosure and significantly higher than that of other enclosure years except CK.This phenomenon may be explained by the fact that the total photosynthetic area in the forest was increased by the succession of enclosure, which was sufficient to accumulate more photosynthetic products to maintain the growth of bamboo whips.Therefore, the starch and soluble sugar content increased to a certain limit and then remained stable in the late stage of the succession of enclosure [45,46].This pattern of change may be because of the mutual conversion and mutual supplementation between starch and soluble sugar content, and a similar phenomenon has been found in a previous study [47,48].This suggests that enclosure measures can promote an increase in nutrient content in Moso bamboo whips.
In this study, there was a significant correlation between the quantitative characteristics and nutrient content of Moso bamboo whips.Plants adjust the nutrient contents of different organs to adapt to the nutrient-limited conditions in the environment.Therefore, they work together for the acquisition of the maximum fitness of the plant and the reproduction of the progeny [49,50].This indicated that the quantitative characteristics of Moso bamboo whips and the nutrient contents of the bamboo whips showed coordinated roles in the process of Moso bamboo growth and worked together to maintain the growth of Moso bamboo.The results of redundancy analysis could explain 67.62% of the variation in the number of whips and nutrient content of Moso bamboo.This could more accurately assess the relationship between the number of whips and nutrient content of Moso bamboo and the length of the incubation period.A positive correlation was found between whip number, whip diameter, whip length, whip weight, total nitrogen, total phosphorus, total potassium, and starch content.It was concluded that enclosure measures were conducive to the optimization of the structure of Moso bamboo whips as well as the storage of more energy and nutrients required for growth [45].The quantitative characteristics and nutrient content of bamboo whips (whip diameter and starch content) were most affected by the number of years of conservation.Meanwhile, the least affected were whip length and organic carbon content.In this study, a detailed evaluation of the sample plots was conducted using principal component analysis.It was concluded that the growth of Moso bamboo whips was better at 41 years and 11 years of enclosure and under the conventional management mode, and the growth of Moso bamboo whips was slowest at 4, 16, and 6 years of enclosure, indicating that a longer enclosure could promote the growth and metabolism of the whips [51].However, the lowest scores in the sample plot were obtained at 6 years of the closed season, which may have been because of the higher density of Moso bamboo mixed forests after 6 years of the closed season [31] and the lower nutrient content of the soil, which restricted the growth of Moso bamboo.

Conclusions
Overall, our study has shown that with the progress in enclosure succession, the Moso bamboo whips gradually developed into shallow soil to adapt to environmental changes.The short-term enclosure was not conducive to the accumulation of total nitrogen, phosphorus, potassium, and soluble sugar content in the bamboo whip, but the long-term enclosure improved the accumulation of nutrients in the bamboo whip.The growth of the Moso bamboo whip was better in the 41 years enclosure and the bamboo whip was poorest in the 6 years enclosure.In summary, the use of enclosure measures for Moso bamboo forests in the Wuyi Mountain Nature Reserve promoted the growth of Moso bamboo whips and optimized their structure of bamboo whips.To better understand the impact of Moso bamboo succession on ecosystem function, it is necessary to conduct further research on the growth of Moso bamboo and changes present in the surrounding environment.

Figure 1 .
Figure 1.Morphological characteristics of bamboo whips with different enclosure years.Different lowercase letters indicate significant differences between enclosure years in the same soil layer (p < 0.05).

Figure 2 .
Figure 2. Changes in total nitrogen, total phosphorus, total potassium, and organic matter content in bamboo whips under different enclosure years (g•kg −1 ).Different lowercase letters indicate significant differences between enclosure years in the same soil layer (p < 0.05).

Figure 2 .
Figure 2. Changes in total nitrogen, total phosphorus, total potassium, and organic matter content in bamboo whips under different enclosure years (g•kg −1 ).Different lowercase letters indicate significant differences between enclosure years in the same soil layer (p < 0.05).

Figure 3 .
Figure 3. Changes in starch and soluble sugar content in bamboo whips under different enclosure years (g•kg −1 ).Different lowercase letters indicate significant differences between enclosure years in the same soil layer (p < 0.05).

Figure 3 .
Figure 3. Changes in starch and soluble sugar content in bamboo whips under different enclosure years (g•kg −1 ).Different lowercase letters indicate significant differences between enclosure years in the same soil layer (p < 0.05).

Figure 4 .
Figure 4. Redundancy analysis between morphological structure and nutrient content of bamboo whip and enclosure years.Note: EY represents enclosure years.

Figure 4 .
Figure 4. Redundancy analysis between morphological structure and nutrient content of bamboo whip and enclosure years.Note: EY represents enclosure years.

Table 1 .
Analysis of soil physicochemical properties in the experimental plot.

Table 2 .
General information about the sample plot.
Nutrient Content of Moso Bamboo Whip 3.2.1.Total Nitrogen, Total Phosphorus, Total Potassium and Organic Carbon Content of Bamboo Whips

Table 3 .
Correlation analysis of enclosure years and soil depth with bamboo whip quantity and nutrient content of Moso bamboo (Phyllostachys edulis).

Table 4 .
Initial factor loading matrix, eigenvector, and principal component contribution rate.

Table 5 .
The principal component value and comprehensive value of Moso bamboo forest with different enclosure years.