Population Structures and Dynamics of Rhododendron Communities with Different Stages of Succession in Northwest Guizhou, China

To explore the population structures and dynamics of Rhododendron shrub communities at different stages of succession in northwest Guizhou, China, this study examined the populations of Rhododendron annae and Rhododendron irroratum shrub with two different stages. A space-for-time substitution was employed to establish the diameter class/height structures, static life tables, and survival/mortality rate/disappearance rate curves of both Rhododendron populations with different orders of succession. Their structural and quantitative dynamics were analyzed, and their development trends were predicted. The results showed that, quantitatively, the populations of R. annae and R. irroratum in the two Rhododendron communities with different orders of succession were dominated by age classes one, two, and three as well as height classes i, ii, and iii. The number of Rhododendron plants at the three age classes and the three height classes accounted for 97.61–100% of the total. The quantitative dynamic indices of R. annae and R. irroratum were both greater than 0, with and without considering external interference. In terms of age class and height structures, both Rhododendron populations were expanding populations, presenting “inverted-J-shaped” and irregular pyramid patterns. There was a sufficient number of young individuals, but few or no old individuals. Both survival curves of the populations of R. annae and R. irroratum in the two Rhododendron communities with different orders of succession belonged to the Deevy-II type. In the late stage of succession, the mortality curves and disappearance curves of both Rhododendron populations in these communities presented a trend of increasing first and then decreasing with increasing age class. This result indicates that at each age class, R. annae and R. irroratum showed a trend of gradual increase after two, four, and six years. In brief, the populations of R. annae and R. irroratum have rich reserves of seedlings and saplings, but high mortality and disappearance rates. In this context, it is necessary to reduce human interference and implement targeted conservation measures to promote the natural renewal of Rhododendron populations.


Introduction
A population is the sum of individuals of the same species that occupy a certain space within a certain period.It reflects a link among individuals, communities, and ecosystems, and a fundamental component of communities [1].The biological and ecological characteristics of plant populations are the result of long-term adaptation to and selection of environmental conditions.The dynamic changes in the size or quantity of a population on Plants 2024, 13, 946 2 of 16 the spatiotemporal scale reflect the composition and development trends of the population; they also reflect the interactive relationship between the population and the environment and its position and role in the community [2,3].The dynamics of population structure and quantity, which are among the core contents of ecological research, cover diameter class, height class, age class, and other items.Changes in these items directly affect community characteristics and objectively characterize the development and evolution trends of communities [1,4].By examining the structure of a population using methods such as static life tables, survival curves, and survival analyses, it is possible to disclose the survival status of the population and the fitness and interactions of plants with the environment in the community [5][6][7].Succession-the process of one community being replaced by another-is the most important characteristic of the dynamics of plant communities, an important pathway for ecological restoration and reconstruction, and the foundation for maintaining community dynamics stability and sustainable development [8,9].In this sense, analyzing the structures and dynamics of plant populations is of great importance for elucidating the succession routes of plant communities.
Rhododendron (Ericaceae) is one of the largest genera of angiosperms, with important ornamental, cultural, scientific, economic, and medicinal values.There are over 1200 species of Rhododendron worldwide, over 900 of which are distributed in Asia and more than half of them in southwest China, which is the distribution and evolution center of Rhododendron plants in modern times [10][11][12][13][14]. Guizhou is located at the edge of the center and the transition zone of its eastward spread and is home to more than 110 naturally distributed Rhododendron species, second only to Yunnan, Sichuan, and Tibet [15,16].Northwest Guizhou is the most important region in the distribution of the Rhododendron taxa in Guizhou, with a total of six subgenera and more than 50 species, accounting for about half of the Rhododendron plants in Guizhou.In this regard, the Baili Azalea Nature Reserve, located at the junction of Qianxi County and Dafang County, is the most representative and important habitat [17,18].It is also a typical representative of the world's largest and continuously distributed natural wild Rhododendron communities in middle-to-lowaltitude mountainous areas [19].In this region, Rhododendron shrub-based communities with different orders of succession cover grass rangelands, rose shrubs, Rhododendron shrubs, oak shrubs, and oak evergreen forests.However, the position, competitiveness, and stability of Rhododendron shrubs in these communities remain unclear.
Many scholars have conducted preliminary research and analyses on the Rhododendron shrubs in northwest Guizhou, China, from the perspectives of germplasm resources, community characteristics, and interference effects of Rhododendron plants [20][21][22].However, these studies rarely touch upon the population structures or dynamics of Rhododendron in natural Rhododendron shrub communities at different stages of succession.This study takes the successional communities of Rhododendron annae and Rhododendron irroratum-two Rhododendron populations widely distributed throughout northwest Guizhou-as research objects.By analyzing the population structures and dynamics of R. annae and R. irroratum in two Rhododendron communities with different orders of succession, a theoretical basis is provided for the scientific protection, tourism development, and sustainable development of Rhododendron plants in northwest Guizhou.
The present study was conducted to achieve the following objectives: (1) to characterize the population structures or dynamics of Rhododendron in natural Rhododendron shrub communities at different stages of succession, (2) to explore the growth status of two Rhododendron species in communities at different successional stages, (3) to assess the self-renewal ability of the Rhododendron populations under natural conditions.R. annae (communities Ia, IIa, and IIIa) and R. irroratum (communities Ib, IIb, and IIIb) were both dominated by age classes one, two, and three.The total quantities of R. annae and R. irroratum for these three age classes accounted for 96.98% and 100.00% of the total quantity of Rhododendron plants, respectively.There were few Rhododendron plants in age class 4, 5, or 6, except for 12 R. annae plants.In terms of age structure, the two Rhododendron populations in each stage of succession were both expanding populations (Figure 2).The results of this survey found that a total of 765 Rhododendron plants were examined for the two orders of succession.To be specific, there were 430 R. annae plants (99, 186, and 145 in communities Ia, IIa, and IIIa, respectively) and 335 R. irroratum plants (73, 172, and 90 in communities Ib, IIb, and IIIb, respectively).The quantity of R. annae was 128.36% of that of R. irroratum (Figure 1).R. annae (communities Ia, IIa, and IIIa) and R. irroratum (communities Ib, IIb, and IIIb) were both dominated by age classes one, two, and three.The total quantities of R. annae and R. irroratum for these three age classes accounted for 96.98% and 100.00% of the total quantity of Rhododendron plants, respectively.There were few Rhododendron plants in age class 4, 5, or 6, except for 12 R. annae plants.In terms of age structure, the two Rhododendron populations in each stage of succession were both expanding populations (Figure 2).indices at other age classes (V 2 , V 3 , V 4 , and V 6 ) indicated a decrease in population size.In community II a , the quantitative dynamic indices of R. annae at all age classes (except age class 5; i.e., V 1 , V 2 , V 3 , V 4 , and V 6 ) were greater than 0. Notably, V 5 was equal to 0. In community III a , only the quantitative dynamic index of R. annae at age class 1 (V 1 ) was less than 0, whereas those at age classes two and three (i.e., V 2 and V 3 ) were both greater than 0. The quantitative dynamic indices of R. annae with and without considering external interference (V pi and V ′ pi ) were both greater than 0, with variation ranges of 39.57-48.15%and 2.67-7.90%,respectively.In communities I b , II b , and III b , the quantitative dynamic index of R. irroratum at age class one (V 1 ) was uniformly less than 0, whereas those at age classes two and three (i.e., V 2 and V 3 ) were both greater than 0. The quantitative dynamic indices of R. irroratum with and without considering external interference (V pi and V ′ pi ) were both greater than 0 as well, with variation ranges of 46.24-67.02%and 0.76-1.28%,respectively (Table 1).R. annae (communities I a , II a , and III a ) and R. irroratum (communities I b , II b , and III b ) were both dominated by height classes i, ii, and iii.The total quantities of R. annae and R. irroratum at these three height classes accounted for 99.53% and 97.61% of the total quantity of Rhododendron plants, respectively.There were few Rhododendron plants at height class iv, v, or vi, except for twelve R. annae plants and eight R. irroratum plants.The individual numbers of R. annae (communities I a , II a , and III a ) and R. irroratum (communities I b , II b , and III b ) both presented a trend of increasing first and then decreasing with increasing height.The plant numbers of R. annae at height classes i and ii presented a trend of increasing first and then decreasing with orders of succession.There were 19, 23, and 6 R. annae plants at height class i, as well as 74, 114, and 81 R. annae plants at height class ii, respectively.The plant number of R. annae at height class iii exhibited a gradually increasing trend with orders of succession.There were 5, 114, and 81 R. annae plants at height class iii, respectively.At height classes iv and vi, communities III a and I a each contained one R. annae plant.The height structures of R. annae in the three communities with different orders of succession both represented the expanding type (Figure 3).
In communities I b , II b , and III b , the plant numbers of R. irroratum at height class i presented a trend of decreasing first and then increasing with orders of succession.There were eight, one, and ten R. irroratum plants at height class i, respectively.The plant numbers of R. irroratum at height classes ii and iii presented a trend of increasing first and then decreasing with orders of succession.There were 54, 77, and 25 R. annae plants at height class ii, as well as 11, 94, and 47 R. annae plants at height class iii, respectively.At height class iv, there were only eight R. irroratum plants in the community I b .The height structures of R. irroratum in the three communities with different orders of succession were also both of the expanding type (Figure 3b).In communities Ib, IIb, and IIIb, the plant numbers of R. irroratum at height class i presented a trend of decreasing first and then increasing with orders of succession.There were eight, one, and ten R. irroratum plants at height class i, respectively.The plant numbers of R. irroratum at height classes ii and iii presented a trend of increasing first and then decreasing with orders of succession.There were 54, 77, and 25 R. annae plants at height class ii, as well as 11, 94, and 47 R. annae plants at height class iii, respectively.At height class iv, there were only eight R. irroratum plants in the community Ib.The height structures of R. irroratum in the three communities with different orders of succession were also both of the expanding type (Figure 3b).

Static Life Tables and Survival Rate Curves of Both Rhododendron Populations in Communities at Different Stages of Succession
There were many young individuals (age classes 1-2) of R. annae in communities Ia, IIa, and IIIa.The survival numbers (ax) of R. annae in these three communities presented a trend of increasing first and then decreasing with increasing age class and all peaked at age class two, reaching 46, 105, and 93, respectively.The life expectancies (ex) of R. annae in communities Ia and IIa showed a trend of decreasing first, then increasing, and finally decreasing again with increasing age class, and peaked at age class four (Ⅰa) and age class one (Ⅱa), reaching 2.50 and 1.60, respectively.The life expectancy (ex) of R. annae in community IIIa displayed a trend of gradual decrease with increasing age class, dropping from 1.35 at age class one to 0 at age classes four, five, and six.There were also many young individuals (age classes 1-2) of R. irroratum in communities Ib, IIb, and IIIb.With increasing age class, the survival numbers (ax) of R. irroratum in the three communities presented the same trend as those of R. annae, and also peaked at age class two, reaching 61, 135, and 57, respectively.The life expectancy (ex) of R. irroratum in community Ib showed a trend of decreasing first, then increasing, and finally decreasing again with increasing age class, and peaked at age class four, reaching 2.50.The life expectancies (ex) of R. irroratum in communities Ⅱb and Ⅲb displayed a trend of gradual decrease with increasing age class, dropping from 1.24 and 1.50 at age class one to 0 at age classes four, five, and six (Table 2).

Static Life Tables and Survival Rate Curves of Both Rhododendron Populations in Communities at Different Stages of Succession
There were many young individuals (age classes 1-2) of R. annae in communities I a , II a , and III a .The survival numbers (a x ) of R. annae in these three communities presented a trend of increasing first and then decreasing with increasing age class and all peaked at age class two, reaching 46, 105, and 93, respectively.The life expectancies (e x ) of R. annae in communities I a and II a showed a trend of decreasing first, then increasing, and finally decreasing again with increasing age class, and peaked at age class four (I a ) and age class one (II a ), reaching 2.50 and 1.60, respectively.The life expectancy (e x ) of R. annae in community III a displayed a trend of gradual decrease with increasing age class, dropping from 1.35 at age class one to 0 at age classes four, five, and six.There were also many young individuals (age classes 1-2) of R. irroratum in communities I b , II b , and III b .With increasing age class, the survival numbers (a x ) of R. irroratum in the three communities presented the same trend as those of R. annae, and also peaked at age class two, reaching 61, 135, and 57, respectively.The life expectancy (e x ) of R. irroratum in community I b showed a trend of decreasing first, then increasing, and finally decreasing again with increasing age class, and peaked at age class four, reaching 2.50.The life expectancies (e x ) of R. irroratum in communities II b and III b displayed a trend of gradual decrease with increasing age class, dropping from 1.24 and 1.50 at age class one to 0 at age classes four, five, and six (Table 2).
The survival, mortality, and disappearance curves of R. annae in communities with different orders of succession are shown in Figure 4.The logarithmic standardized survival numbers (lnlx) of R. annae in communities I a , II a , and III a gradually decreased with increasing age class, and its survival curves all fell between Deevey-II and Deevey-III.The survival curves of R. annae in communities I a , II a , and III a were tested using both exponential and power function models.As shown in Table 3, the R 2 of the exponential function model was uniformly greater than that of the power function model; therefore, the survival curves of R. annae in communities I a , II a , and III a tended to be closer to Deevey-II.The mortality rate and disappearance rate curves of R. annae in communities I a , II a , and III a presented a consistent trend with increasing age class (Figure 4b,c).Specifically, both the mortality rate and disappearance rate curves of R. annae in communities I a and II a showed a trend of increasing first, then decreasing, and finally increasing again with increasing age class, and peaked at age classes three and six.The mortality rate and disappearance rate curves of R. annae in community III a exhibited a trend of increasing first and then decreasing and peaked at age class three.The logarithmic standardized survival numbers (lnlx) of R. irroratum in communities I b , II b , and III b also gradually decreased with increasing age class, and all survival curves fell between Deevey-II and Deevey-III.As shown by the model tests in Table 3, the R 2 of the exponential function model for R. irroratum in communities I b , II b , and III b was uniformly greater than that of the power function model; therefore, the survival curves of R. irroratum in communities I b , II b , and III b tended to be closer to Deevey-II.The mortality rate and disappearance rate curves of R. irroratum in communities I b , II b , and III b both showed a trend of increasing first, then decreasing, and finally increasing again with increasing age class, and peaked at age classes three or four.

Time Sequence Analysis of the Two Rhododendron Populations in Communities at Different Stages of Succession
According to the population dynamics prediction of R. annae (Table 4), the population sizes of R. annae in communities I a , II a , and III a will decrease from their current values of 99, 186, and 145 plants to 15, 27, and 14 plants in six years.The quantities of R. annae in communities I a , II a , and III a all showed a trend of gradual increase after two, four, and six years at each age class, except for age class ii.The quantities at lower age classes always exceeded those at higher age classes.Specifically, the quantity of R. annae at age class two decreased by 39.13%, 40.00%, and 23.66%, respectively; that of R. annae at age class three increased by 17.65%, 44.64%, and 1500.00%,respectively.The quantities of R. annae in age classes four, five, and six also exhibited a gradually increasing trend.
According to the population dynamics prediction of R. irroratum (Table 4), the population sizes of R. irroratum in communities I b , II b , and III b will decrease from the current numbers of 126, 172, and 90 plants to 12, 21, and 10 plants in six years.Similarly, the quantities of R. irroratum in communities I b , II b , and III b all showed a trend of gradual increase after two, four, and six years at each age class, except for age class ii.Specifically, the quantity of R. irroratum at age class two decreased by 6.56%, 40.74%, and 31.58%,respectively, whereas that of R. irroratum at age class three increased by 227.27%, 469.23%, and 191.67%, respectively.The quantities of R. irroratum in age classes four, five, and six also exhibited a gradually increasing trend.Note: I a , R. annae + Lyonia ovalifolia shrubs; II a , R. annae shrubs; III a , R. annae + broad-leaved forest; I b , R. irroratum + Lyonia ovalifolia shrubs; II b , R. irroratum shrubs; III b , R. irroratum + broad-leaved forest.M 2 (1) , M 4 (1) , M 6 (1) is a prediction of the population after the time of age class 2, 4 and 6, respectively.

Population Structures and Types
Population structures and dynamic features provide an important foundation for a better understanding of the survival status and dynamic development laws of populations [23].The age class and height structures of a population can not only disclose the developmental stage of its individuals, but also elucidate the interactions between the biological characteristics of a species and its living environment [24].The findings of this study indicate that the individual numbers of both Rhododendron communities with different orders of succession presented a trend of increasing first and then decreasing with the direction of succession.That is, communities II a and II b had the highest number of individuals (186 and 172, respectively).This is due to differences in competitiveness and status of the two Rhododendron populations in communities with different orders of succession.Both populations had large individual numbers in age classes 1-2, but the number of individuals at age class one was far lower than that at age class two.This may be because Rhododendron plants mostly reproduce asexually through basal germination [25], but radial growth is slow.Additionally, the natural regeneration of seeds in Rhododendron plants requires extremely strict habitat conditions, and site conditions often limit their seed germination and seedling survival [26].The survival rate of seedlings at age class one is lower than that at age class two, resulting in far lower individual numbers at age class one than at age class two.
The two Rhododendron populations in communities with different orders of succession were both expanding populations; however, they differed in age and height structures.The age structures of R. annae in communities I a and II a presented an "inverted-J-shaped" pattern, whereas the age structure of R. annae in community III a and the age structures of R. irroratum in all three communities showed an irregular pyramid pattern.Similarly, the height structures of R. annae in the three communities and those of R. irroratum in communities I b and III b showed an irregular pyramid pattern, whereas the height structure of R. irroratum in community II b presented a "J-shaped" pattern.Because of differences in micro-environmental factors (such as light, nutrients, and survival space) between the two communities with different orders of succession, varying competition intensity within and between species further resulted in differences in population size.As a result, certain differences were found in age and height structures between the two Rhododendron populations.Both Rhododendron populations in communities with different orders of succession-characterized by large numbers of saplings, high survival rate at low age classes, and low survival rate at middle and high age classes-were classified as expanding populations.This classification was similar to those of Yang et al. [27].Perhaps because young individuals only require few environmental resources for growth and development and only face mild interspecific and intraspecific competition, the presence of a large number of young individuals can be maintained.With increasing tree age, the individual numbers of the two Rhododendron populations decrease rapidly, mainly because of resource limitations as well as self-thinning and allelopathic effects [26,28,29].Faced with intensified interspecific and intraspecific competition, the individual numbers of both populations experience a gradual decline.This observation explains why both Rhododendron populations had many young and middle-aged individuals but few old individuals.To sum up, the proportion of seedlings and young trees in two Rhododendron populations was large, and although the mortality rate of young individuals was relatively high, the period was still critical and sensitive.Therefore, further efforts should be made to protect Rhododendron shrubs and scientific measures should be taken to avoid human interference and prevent population reduction.

Population Dynamics and Development
The quantitative dynamics of a population reflect the interaction between its individual survivability and the environment, which is substantially influenced by the external environment.Time sequence analysis can predict the dynamics of plant populations to a certain extent [30].This study showed that the quantitative dynamic indices of both Rhododendron populations with different orders of succession (V pi and V ′ pi ) were both greater than 0. The V ′ pi of R. annae in community II a was much greater than that in communities I a and III a , and the V ′ pi of R. irroratum in communities II b and III b was much greater than that in community I b .This clarified that both Rhododendron populations held dominant positions and grew rapidly in the two Rhododendron communities with different orders of succession.The V ′ pi of R. annae was much greater than that of R. irroratum, and R. annae had few surviving individuals in both age classes five and six.This result suggests that although R. annae and R. irroratum are both expanding populations, R. irroratum grows slower and is more sensitive to the external environment.The survival, mortality rate, and disappearance rate curves of a population can be used to analyze its quantitative dynamics and gauge its development trends, thereby explaining the interactions between plant populations and the environment [6,7].As indicated by the static life tables of the populations of R. annae and R. irroratum, with increasing age class, the mortality and disappearance rates of both populations gradually increased.This may be because, as they age, Rhododendron populations experience increasing demands for resources.In particular, Rhododendron individuals entering the overstory face intensified competition in terms of nutrients, light, moisture, and other resources with increasing crown breadth.By contrast, old Rhododendron individuals are in the stage of physiological aging, which encompasses a sharp weakening of competitiveness and the occurrence of mass mortality.
The largest quantities of R. annae were observed at age class six in communities I a and II a and at age class three in community III a ; the largest quantities of R. irroratum were observed at age class six in community I b and at age class three in communities I b and II b .This result indicated that the mortality and disappearance rates of both populations gradually increased over their succession and peaked at age class three.Because the largest quantity of Rhododendron individuals was found at age class three, a self-thinning effect resulted under the combined action of density constraints and competition.At higher age classes, the quantity declined because of physiological decline.The life expectancies and survival rates of R. annae and R. irroratum were consistent with their mortality and disappearance rate trends.In the two orders of succession, the life expectancies of the Rhododendron populations at age classes one and two were significantly higher than those at other age classes.To reduce intraspecific competition among Rhododendron seedlings, relieve the self-thinning of seedlings and saplings caused by density constraints, and improve the efficiency of population supplementation from low to high age classes [31], the recommendation is to appropriately reduce the seedling numbers of both Rhododendron populations, thereby raising the early survival rate of Rhododendron individuals.Both quantities of R. annae and R. irroratum showed a trend of gradual increase after two, four, and six years at each age class, indicating that both Rhododendron populations were expanding populations.The survival curve test showed that both Rhododendron populations belonged to the Deevey-II type in communities with different orders of succession and were both stable populations.Although R. annae and R. irroratum had large population sizes in communities II a , III a , and II b , their individual numbers were small in communities I a and I b , especially at high age classes.In this context, if the survival rate of younger individuals cannot be improved, the old individuals in these communities will not be effectively supplemented, thus potentially causing the population to decline or even disappear [27].
The populations of R. annae and R. irroratum in northwest Guizhou have rich reserves of seedlings and saplings; therefore, they are expanding populations.Without human interference, Rhododendron populations can naturally reproduce and realize self-renewal and rejuvenation.However, considering the poor environmental adaptability and low survival rate of young Rhododendron individuals, protective measures should be implemented.The goal of these measures should be to increase both the quantity and survival rate of Rhododendron seedlings and saplings and promote the natural renewal of Rhododendron populations.At the same time, the on-site protection of existing Rhododendron populations should be strengthened to reduce human interference and protect plants from further damage.To effectively protect the germplasm resources of Rhododendron, efforts should also be made to conduct breeding, cultivation, and management of Rhododendron populations.Moreover, breeding and protection bases for them should be established, their quantities should be increased, and their spatial distribution should be expanded.

Overview of the Study Area
The study area is located in Xingxiu Township, Dafang County, Guizhou Province, China (27 • 23 ′ N, 105 • 51 ′ E), which is a transition zone from the highest plateau surface to the central plateau surface in northwest Guizhou.Its terrain is low in the north and south and high in the middle (Figure 5).It has a typical karst peak-cluster mid-slotted trough valley topography, with an altitude of 1730-1820 m, and a subtropical monsoon humid climate.The annual average temperature is 11.8 • C, and the average annual precipitation is 1150.4mm.The frost-free period is 257 d, and the annual sunshine duration is 1335.5 h.The zonal vegetation is a mountain evergreen broad-leaved forest, and the existing vegetation is dominated by Rhododendron shrubs, with important successional and transitional characteristics [32,33].The major dominant species include R. annae, R. irroratum, R. maculiferum, R. lilacinum, R. maculatum, and Lyonia ovalifolia.Most of the dominant species belong to the Rhododendron genus including shrubs or trees.Their leaves are evergreen or deciduous, semi deciduous, alternate, entire, rare and have inconspicuous small teeth.Their flower buds are mostly characterized by bud scales with varying shapes and sizes.The flowers are prominent, small to large in shape, and are usually arranged in umbrella-shaped or short racemes, with sparse single flowers, which are usually terminal and rarely axillary, corolla funnel shaped, bell shaped, tubular, or high-footed disc shaped, neat or slightly symmetrical, with lobes covered in tiles within the bud.The appearance of two types of Rhododendron is shown in Figure 6.The dominant soil type is yellow soil, with a pH value of 4.61-5.32[22].
varying shapes and sizes.The flowers are prominent, small to large in shape, and are usually arranged in umbrella-shaped or short racemes, with sparse single flowers, which are usually terminal and rarely axillary, corolla funnel shaped, bell shaped, tubular, or high-footed disc shaped, neat or slightly symmetrical, with lobes covered in tiles within the bud.The appearance of two types of Rhododendron is shown in Figure 6.The dominant soil type is yellow soil, with a pH value of 4.61-5.32[22].

Sample Plot Setting and Survey Methods
In March 2019, a survey was conducted on two orders of succession of the communities of R. annae and R. irroratum in the Baili Azalea Nature Reserve.To be specific, the orders of succession of R. annae included the three successional communities of R. annae + Lyonia ovalifolia shrubs (Ia), R. annae shrubs (IIa), and R. annae + evergreen forest (IIIa).The successional communities of R. irroratum also included three successional communities, namely, R. irroratum + Lyonia ovalifolia shrubs (Ib), R. irroratum shrubs (IIb), and R. irroratum + evergreen forest (IIIb) (Figure 5).Both orders of succession showed an alternation from southeast to northwest.The basic characteristics of all six communities are provided in Table 5.The constructive species of the R. annae succession changed from Lyonia ovalifolia to R. annae, and R. irroratum succession changed from Lyonia ovalifolia to R. annae.

Sample Plot Setting and Survey Methods
In March 2019, a survey was conducted on two orders of succession of the communities of R. annae and R. irroratum in the Baili Azalea Nature Reserve.To be specific, the orders of succession of R. annae included the three successional communities of R. annae + Lyonia ovalifolia shrubs (I a ), R. annae shrubs (II a ), and R. annae + evergreen forest (III a ).The successional communities of R. irroratum also included three successional communities, namely, R. irroratum + Lyonia ovalifolia shrubs (I b ), R. irroratum shrubs (II b ), and R. irroratum + evergreen forest (III b ) (Figure 5).Both orders of succession showed an alternation from southeast to northwest.The basic characteristics of all six communities are provided in Table 5.The constructive species of the R. annae succession changed from Lyonia ovalifolia to R. annae, and R. irroratum succession changed from Lyonia ovalifolia to R. annae.

Figure 1 .
Figure 1.Quantities of Rhododendron plants in the quadrats.Note: (a), the individual quantity of R.annae in different communities; (b), the individual quantity of R. irroratum in different communities; I a , R. annae + Lyonia ovalifolia shrubs; II a , R. annae shrubs; III a , R. annae + broad-leaved forest; I b , R. irroratum + Lyonia ovalifolia shrubs; II b , R. irroratum shrubs; III b , R. irroratum + broad-leaved forest.R. annae (communities I a , II a , and III a ) and R. irroratum (communities I b , II b , and III b ) were both dominated by age classes one, two, and three.The total quantities of R. annae and R. irroratum for these three age classes accounted for 96.98% and 100.00% of the total quantity of Rhododendron plants, respectively.There were few Rhododendron plants in age class 4, 5, or 6, except for 12 R. annae plants.In terms of age structure, the two Rhododendron populations in each stage of succession were both expanding populations (Figure2).

Figure 2 .
Figure 2. Age structures of R. annae and R. irroratum in different communities.Note: (a), age structures of R. annae in different communities; (b), age structures of R. irroratum in different communities.Note: I a , R. annae + Lyonia ovalifolia shrubs; II a , R. annae shrubs; III a , R. annae + broad-leaved forest; I b , R. irroratum + Lyonia ovalifolia shrubs; II b , R. irroratum shrubs; III b , R. irroratum + broad-leaved forest.The dynamic changes in quantity between different age groups of R. annae and R. irroratum populations between age classes at different stages of succession are presented.In community I a , the quantitative dynamic indices of R. annae at age classes one and five (V 1 and V 5 ) indicated an increase in population size, whereas the quantitative dynamic

Figure 4 .
Figure 4. Survival, mortality rate (q x ), and disappearance rate (K x ) curves of R. annae and R. irroratum in different communities.Note: (a), survival curves of R. annae in different communities; (b), mortality

Table 1 .
Dynamic indices for the age structures of Rhododendron plants.
Note: I a , R. annae + Lyonia ovalifolia shrubs; II a , R. annae shrubs; III a , R. annae + broad-leaved forest; I b , R. irroratum + Lyonia ovalifolia shrubs; II b , R. irroratum shrubs; III b , R. irroratum + broad-leaved forest.V n : quantitative dynamic index between age classes n and n + 1; V pi : quantitative dynamic index of the population without considering external interferences; V ′ pi : quantitative dynamic index of the population considering external interferences.

Table 2 .
Static life tables of R. annae and R. irroratum in different communities.

Table 2 .
Static life tables of R. annae and R. irroratum in different communities.
Note: I a , R. annae + Lyonia ovalifolia shrubs; II a , R. annae shrubs; III a , R. annae + broad-leaved forest; I b , R. irroratum + Lyonia ovalifolia shrubs; II b , R. irroratum shrubs; III b , R. irroratum + broad-leaved forest.ax : plant number within age class x; a ′ x : plant number within age class x after smoothing; l x : standardized survival number; lnl x : logarithmic standardized survival number; d x : death number; q x : mortality rate; L x : survival number within the interval from age class x to x + 1; T x : total survival number; e x : life expectancy; K x : disappearance rate.Plants 2024, 13, x FOR PEER REVIEW 8 of 17

Table 3 .
curves of R. annae in different communities; (c), disappearance rate curves of R. annae in different communities; (d), survival curves of R. irroratum in different communities; (e), mortality rate of R. irroratum in different communities; mortality rate of R. irroratum in different communities; (f), disappearance rate of R. irroratum in different communities; I a , R. annae + Lyonia ovalifolia shrubs; II a , R. annae shrubs; III a , R. annae + broad-leaved forest; I b , R. irroratum + Lyonia ovalifolia shrubs; II b , R. irroratum shrubs; III b , R. irroratum + broad-leaved forest.Function models for R. annae and R. irroratum in different communities.

Table 4 .
Time sequence analysis of population dynamics of R. annae and R. irroratum in different communities.

Table 5 .
Basic situation of different Rhododendron shrub communities.

Table 5 .
Basic situation of different Rhododendron shrub communities.