Relationship between Tree Richness and Temporary Stability of Plant Communities: A Case Study of a Forest in Northeast China

Abstract

The relationship between diversity and stability is a classic issue in ecology, but no general consensus has been achieved. To address this relationship, a field survey of a forest in Northeast China was conducted. The temporary stability was defined from the perspective of community characteristics. The results showed that communities with the highest temporary stability value were characterized by a single dominant species. A significant linear relationship with a low R² value was observed between temporary stability and tree richness. When dominant and non-dominant tree species were studied, no significant linear relationship was obtained between temporary stability and non-dominant tree richness. However, the relationship between temporary stability and dominant tree richness was significant with a high R² value, and the temporary stability decreased with increasing dominant tree richness. This study demonstrates that dominant tree richness is closely related to temporary stability, and temporary stability can serve as a stability indicator. The results provide a new perspective for understanding stability and additional information for revealing the relationship between diversity and stability in forest ecosystems.

1. Introduction

The diversity–stability relationship is a classic issue in ecology [1]. Although several studies have been conducted [2,3], no general consensus has been achieved; one potential explanation is that multiple definitions of diversity and stability have been used [4,5]. Various definitions can enrich their respective connotations; however, multiple definitions can also result in different relationships and further obfuscate discussions that are needed to clarify these relationships.

Many types of diversity–stability relationships have been investigated in forest ecosystems [6,7,8,9,10] to determine whether one general relationship exists, which would require a unified terminology and limited scope. Among different relationships, the relationship between species diversity and plant community stability is an important topic with a clear scope. If the concept of diversity and stability in this relationship can be clearly defined, and easy-to-understand and implement indicators are used, it will help to reveal the diversity–stability relationship.

Richness is the most simple and suitable indicator of diversity [11]. However, many indicators of stability are quantitative, such as biomass [12,13], number [14,15], and age structure [16,17], and are rarely based on community characteristics. Communities are dynamic such as seasonal and interannual changes [18], but variations in the quantity do not necessarily lead to changes in characteristics. Community characteristics are more stable and suitable than quantitative characteristics for defining the concept of stability.

To address the relationship between diversity and stability, a field survey of a forest in Northeast China was conducted. The objectives were to (1) define the temporary stability from the aspect of community characteristics, (2) calculate the temporary stability of the plant community, and (3) clarify the relationship between tree richness and temporary stability. The aim was to provide a new perspective for understanding community stability and additional information for revealing the relationship between diversity and stability in forest ecosystems. Considering the effect of dominant species on plant communities [18,19], the hypothesis was that the diversity–stability relationship is closely related to the effect of dominant trees.

2. Materials and Methods

2.1. Field Survey

Surveys to collect data on the forest plant community were conducted in three counties of Northeast China in July 2014: Fusong county, Antu county, and Changbai county in Jilin Province (Figure 1). The three counties are rich in forest resources; the forest coverage of these three counties exceeds 80%. Many important tree species are growing in this area, including Picea jezoensis var. microsperma, Larix olgensis, Betula costata, Populus ussuriensis, Tilia amurensis, Quercus mongolica, Abies nephrolepis, Pinus koraiensis, Betula platyphylla, and Fraxinus mandshurica. Many important wild animals are living in this area, such as the sika deer (Cervus nippon), black bear (Ursus thibetanus), badger (Meles meles), wild pig (Sus scrofa), and leopard cat (Prionailurus bengalensis).

A total of 46 natural forest plots were surveyed in the field (Figure 1). Each plot was 600 m² in size, and the distance between any two plots was not less than 4 km. In each plot, species of all trees with DBH ≥ 5 cm and the diameter at breast height (1.3 m) (DBH) were recorded and measured.

2.2. Definition of Temporary Stability

Dominant species have a proportionally larger effect on the forest plant community [19]; consequently, if the most dominant species change, the characteristics of the communities change [20]. Since the plant community is dynamic, the dominant species may change over time. In this study, the temporary stability was defined as the possibility of the most dominant species in the community to remain unchanged in a short period (e.g., a tree species always has the largest basal area in the community within a certain time), reflecting the ability of the community to resist change over time.

The most likely type of species change is that the most dominant species is being replaced by other species in the community, especially the second-most dominant species because they are the closest in terms of dominance. The greater the difference in the basal area between the first two most dominant species, the less likely it is that the most dominant species will be replaced, and the more stable the community characteristics are. Thus, the dominant relationship between the first two most dominant species was used to characterize the temporary stability.

Based on the above description, the dominance ratio between the second-most dominant species and the most dominant species was calculated by comparing their dominance:

$$y=\frac{d_2}{d_1}$$

(1)

where y denotes the dominance ratio between the second-most dominant species and the most dominant species; d₂ is the dominance of the second-most dominant species; d₁ is the dominance of the most dominant species.

The basal area was used to reflect the dominance of the species. Thus, in Equation (1), d₂ represents the total basal area of the species with the second-largest basal area; d₁ denotes the total basal area of the species with the largest basal area.

According to Equation (1), a larger y value indicates that the closer the dominance between the first two most dominant species, the more uncertain it is that the most dominant species will continue to dominate the community, and the lower the stability of the community is. The equation was modified as follows to permit larger values to indicate higher stability:

$$\mathrm{TS}=1-y=1-\frac{d_2}{d_1}$$

(2)

where TS is the temporary stability of the forest plant community; y, d₂, and d₁ are the same as in Equation (1). Larger TS values indicate higher stability.

2.3. Calculation of Tree Richness

Species richness was used to represent diversity. Since different species in forest plant communities have different roles [19], all trees surveyed in the plots were divided into two categories—dominant and non-dominant tree species. To determine dominant and non-dominant tree species, the relative basal area (RBA) [21] of all tree species in each plot was calculated. Dominant and non-dominant tree species in each plot were distinguished according to their RBA. Dominant tree species were defined as species with RBA values ≥ 0.10 in a plot [20], and the other species were defined as non-dominant tree species.

In this study, three richness variables were used to represent diversity: tree richness (TR), dominant tree richness (DR), and non-dominant tree richness (NR). They were defined as the numbers of tree species (dominant and non-dominant tree species), dominant tree species, and non-dominant tree species in a plot, respectively [19].

2.4. Data Analysis

To describe the temporary stability characteristics of forest plant community, the TS values of the plots were calculated, and the first two dominant species and their RBA values were listed out. Then, all plots were ranked according to their TS. Last, the dominant species composition and RBA values of different plots with the changes of TS were analyzed.

To characterize the relationship between tree richness and temporary stability, the values of TS, TR, DR, and NR of each plot were calculated. Next, the correlations between the TS and the three richness variables were calculated using Pearson’s correlation coefficient. Lastly, different linear regression models were established to describe the relationship between the TS (dependent variable) and different richness variables (independent variables). In the process of modeling, a scatter diagram was used to visualize the trend. Analysis of variance (ANOVA) was used to test the linear relationship, and a t-test was used to test the contribution of the independent variable. The determination coefficient (R²) was used to determine the goodness of fit of the model, and residual analysis was used to determine whether the assumptions of the regression model were satisfied [22,23,24]. The significance level was set to p < 0.05.

3. Results

3.1. Temporary Stability of the Plant Community

The TS values differed in different plots. The plots can be generally divided into four groups based on the TS values: ≤0.20, 0.20–0.60, 0.60–0.80, and >0.80. The TS values of plots 1 to 10 were less than 0.20, and the RBA values of the first two dominant tree species were close. The TS values of plots 11 to 29 were between 0.20 and 0.60; the first dominant tree species such as PJ, BC, and PU had certain dominance in these plots. The TS values of plot 30 to 39 were between 0.60 and 0.80; the first dominant tree species such as LO and PJ had obvious dominance in these plots. The TS values of plots 40 to 46 were greater than 0.80; the first dominant tree species had absolute dominance in these plots. PJ, BE, and BC were the single dominant species in plots 44, 45, and 46, respectively (

Table 1. Temporary stability (TS), the first two dominant tree species, and the relative basal area (RBA) value in different plots.

Plot Number	TS	First Dominant Tree Species and RBA	Second Dominant Tree Species and RBA
1	0.04	PS (0.29)	LO (0.28)
2	0.07	TA (0.29)	PU (0.27)
3	0.08	LO (0.28)	PK (0.26)
4	0.09	BC (0.21)	UL (0.19)
5	0.09	LO (0.37)	AN (0.33)
6	0.09	AN (0.34)	PJ (0.31)
7	0.12	PU (0.32)	TA (0.28)
8	0.14	BC (0.32)	PJ (0.28)
9	0.16	LO (0.30)	BP (0.25)
10	0.16	BC (0.33)	AU (0.27)
11	0.22	TA (0.36)	PK (0.28)
12	0.23	BC (0.51)	PJ (0.39)
13	0.23	PU (0.38)	TA (0.30)
14	0.24	PJ (0.40)	AN (0.30)
15	0.27	PK (0.30)	TA (0.22)
16	0.34	BC (0.43)	AN (0.29)
17	0.34	PJ (0.52)	BC (0.34)
18	0.39	BC (0.37)	PJ (0.23)
19	0.41	PJ (0.40)	AN (0.23)
20	0.42	PU (0.41)	PJ (0.24)
21	0.43	BP (0.38)	QM (0.22)
22	0.44	QM (0.42)	AN (0.24)
23	0.46	AN (0.47)	TA (0.25)
24	0.48	PJ (0.56)	BC (0.29)
25	0.50	QM (0.50)	FM (0.25)
26	0.52	PJ (0.46)	PK (0.22)
27	0.52	PU (0.47)	BP (0.23)
28	0.54	TA (0.52)	AM (0.24)
29	0.55	PJ (0.66)	BC (0.30)
30	0.62	PJ (0.60)	AN (0.23)
31	0.63	LO (0.44)	TA (0.16)
32	0.65	TA (0.53)	AN (0.18)
33	0.66	LO (0.58)	BP (0.19)
34	0.67	PK (0.44)	PJ (0.14)
35	0.69	QM (0.62)	AN (0.20)
36	0.71	PU (0.51)	PK (0.15)
37	0.72	PJ (0.70)	BC (0.20)
38	0.74	PJ (0.62)	PU (0.16)
39	0.77	LO (0.61)	AN (0.14)
40	0.84	LO (0.86)	BP (0.14)
41	0.85	LO (0.76)	BP (0.11)
42	0.86	LO (0.84)	PJ (0.12)
43	0.89	LO (0.90)	AN (0.10)
44	0.98	PJ (0.97)	\
45	1.00	BE (1.00)	\
46	1.00	BC (1.00)	\

AM: Acer mono; AN: Abies nephrolepis; AU: Acer ukurunduense; BC: Betula costata; BE: Betula ermanii; BP: Betula platyphylla; FM: Fraxinus mandshurica; LO: Larix olgensis; PJ: Picea jezoensis var. microsperma; PK: Pinus koraiensis; PS: Pinus sylvestris var. sylvestriformis; PU: Populus ussuriensis; QM: Quercus mongolica; TA: Tilia amurensis; UL: Ulmus laciniata.

).

3.2. Relationship between Tree Richness and Temporary Stability

TS had a significant negative correlation relationship with TR (r = −0.39, N = 46, p < 0.01). For the linear regression model of TS built by TR, the R² value was 0.15, the p-value of ANOVA was <0.01 (F = 7.98), and TR was significant in the model (p < 0.01;

Table 2. The linear regression model of the temporary stability (TS, dependent variable) built by tree richness (TR, independent variable).

Model	Unstandardized Coefficient	Standard Error	t Value	Significance Level	95% Confidence Interval of Unstandardized Coefficient
TR	−0.04	0.01	−2.83	0.007	[−0.06, −0.01]
Constant	0.72	0.10	7.53	0.000	[0.53, 0.92]

). The standardized residual value ranged from −1.59 to 1.54. There was a significant linear relationship between TS and TR (Figure 2).

3.3. Relationship between Dominant Tree Richness and Temporary Stability

TS had a significant negative correlation relationship with DR (r = −0.84, N = 46, p < 0.001). For the linear regression model of TS built by DR, the R² value was 0.70, the p-value of ANOVA was <0.001 (F = 103.09), and DR was significant in the model (p < 0.001;

Table 3. The linear regression model of the temporary stability (TS, dependent variable) built by dominant tree richness (DR, independent variable).

Model	Unstandardized Coefficient	Standard Error	t Value	Significance Level	95% Confidence Interval of Unstandardized Coefficient
DR	−0.25	0.03	−10.15	0.000	[−0.30, −0.20]
Constant	1.27	0.08	15.56	0.000	[1.11, 1.44]

). The standardized residual value ranged from −1.78 to 1.70. There was a significant linear relationship between TS and DR (Figure 3).

3.4. Relationship between Non-Dominant Tree Richness and Temporary Stability

TS had no significant correlation relationship with NR (r = −0.16, N = 46, p > 0.05). For the linear regression model of TS built by NR, the R² value was 0.02, the p-value of ANOVA was >0.05 (F = 1.09). There was no significant linear relationship between TS and NR (Figure 4).

4. Discussion

4.1. Temporary Stability Indicator

The TS was defined in this study based on the role of the dominant species in the plant community and the dominance relationship between the first two most dominant species. In the plots with the highest TS values, a single tree species was dominant and was difficult to be replaced, even after disturbance to a certain extent. Therefore, these plots were resistant to change over short periods, and plant community stability was high. However, in the plots with the lowest TS values, the dominance of the first two dominant tree species was close; the most dominant species were likely to be replaced, especially after disturbance. Consequently, it was uncertain if the most dominant species could continue to dominate the community, and plant community stability was low. The results were consistent with common sense and indicated that the TS could be used as a stability indicator.

Compared with other indicators and methods used to describe stability in previous studies [15,25,26,27], the TS proposed in this study is simple and can generate quantitative results quickly and efficiently, even when little is known of the life history of each species in the community and long-term continuous observational data are lacking. Several previous studies focused on temporal stability [28,29,30,31], a widely used concept and indicator. More studies using the TS proposed in this study are needed to test its applicability.

Furthermore, trees are usually more stable and live longer than small shrubs and herbs in the forest. Plant communities also provide habitats for many wild animals in this area, such as the black bear and wild pig. Trees are less affected than shrubs or herbs by animal activity, including competition, foraging, and resting. Thus, tree richness provides a superior approach for framing the concept of diversity.

4.2. Dominant Tree Richness Is Correlated with Temporary Stability

A significant linear relationship (with a low R² value) was observed between TS and TR. Furthermore, TS decreased with increasing TR, indicating that TR can contribute to explaining the TS. However, when the relationships between TS, DR, and NR were studied further, no significant linear relationship was obtained between TS and NR. Nevertheless, the relationship between TS and DR was significant with a high R² value, and TS decreased with increasing DR, indicating that DR had a close relationship with TS and had high explanatory power.

The decrease in TS with the increasing DR may be attributed to competition and the ecological niche concept [32]. Breaking the dominance of the most dominant species in communities with a single dominant species is difficult; thus, the community is stable and its characteristics are difficult to change. However, in communities with two or more dominant species, the co-dominant species not only occupies a large area of community space but is also expected to expand their living space by growing and competing for unoccupied space with the most dominant species thereby challenging its dominant status. The presence of co-dominant species and the competition between them increase the uncertainty in the community characteristics and potentially decrease the community stability.

Many studies have examined the relationship between diversity and stability, providing various findings and explanations [1]. This study focused on the relationship between diversity and stability of a forest in Northeast China. Specifically, a new definition of TS was used to characterize this relationship; thus, the results are not directly comparable to previous studies. Some previous studies have hypothesized the role that dominant species play in community stability [33,34,35]. In this study, TS and DR had a significant linear relationship, and TS decreased as DR increased. The forest types in this study mainly included PJ forest, LO forest, BC forest, and PU forest (Table 1). More studies in other forest types are needed to confirm this relationship.

4.3. Limitations and Suggestions

Species richness and composition are two primary components of species diversity [36]. The richness was mainly considered in this study; however, the role of species composition and relationship was less considered. Both components should be considered in future research. Furthermore, the age of the forest stand, the presence of old trees in plant communities, and the amount of deadwood in the environment were not considered in this study; they may be very important for stability and deserve exploratory research.

Stability is not a one-dimensional concept [37]. This study was based on the plot scale, but more studies at different spatial scales are needed. With respect to the temporal scale, this study provided an estimate of temporary stability; however, long-term fixed monitoring would increase the understanding of regular changes in community stability.

Furthermore, external factors influencing the plant community were not considered in this study for convenience. However, many factors can affect diversity and stability, such as anthropogenic activities [38], pests [39], and climate extremes [40], which should be considered in future research.

5. Conclusions

The diversity and stability relationship of a forest in Northeast China was analyzed by providing a new definition of stability. The TS was defined based on plant community characteristics, providing a new perspective of stability. More studies using TS are needed to test its applicability. Based on the temporary stability characteristics of the plant community, the TS can serve as a stability indicator. A significant linear relationship was observed between TS and DR; TS decreased with increasing DR. More studies are needed to confirm this relationship, including different forest types in different areas, as well as long-term observations. Furthermore, future studies should narrow the scope of diversity and stability and use unified definitions to clarify the relationship between diversity and stability.