Stand Stability of Pure and Mixed- Eucalyptus Forests of Different Tree Species in a Typhoon-Prone Area

: Background and Objectives : The stable stand structure of mixed plantations is the basis of giving full play to forest ecological function and beneﬁt. However, the monocultural Eucalyptus plantations with large-scale and successive planting that caused ecological problems such as reduced species diversity and loss of soil nutrients have presented to be unstable and vulnerable, especially in typhoon-prone areas. The objective of this study was to evaluate the nonspatial structure difference and the stand stability of pure and mixed- Eucalyptus forests, to ﬁnd out the best mixed pattern of Eucalyptus forests with the most stability in typhoon-prone areas. Materials and Methods : In this study, we randomly investigated eight plots of 30 m × 30 m in pure and mixed- Eucalyptus ( Eucalyptus urophylla S. T. Blake × E. grandis W. Hill) plantations of different tree species ( Neolamarckia cadamba (Roxb.) Bosser, Acacia mangium Willd., and Pinus elliottii var. Elliottii Engelm. × P. caribaea Morelet) on growth status, characterized and compared the distribution of nonspatial structure of the monoculture and mixtures, and evaluated the stand quality and stability from eight indexes of the nonspatial structure, including preservation rate, stand density, height, diameter, stem form, degree of stem inclination, tree-species composition, and age structure. Results : Eucalyptus surviving in the mixed plantation of Eucalyptus and A. mangium (EA) and in the mixed plantation of Eucalyptus and P. elliottii × P. caribaea (EP) were 5.0% and 7.6% greater than those in pure Eucalyptus plantation (EE), respectively, while only the stand preservation rate of EA was greater (+2.9%) than that of the pure Eucalyptus plantation. The proportions of all mixtures in the height class greater than 7 m were fewer than that of EE. The proportions of EA and mixed plantation of Eucalyptus and N. cadamba (EN) in the diameter class greater than 7 m were 10.6% and 7.8%, respectively, more than that of EE. EN had the highest ratio of branching visibly (41.0%), EA had the highest ratio of inclined stems (8.1%), and EP had the most straight and complete stem form (68.7%). The stand stability of the mixed plantation of Eucalyptus and A. mangium presented to be optimal, as its subordinate function value (0.76) and state value ( ω = 0.61) of real stand were the largest. Conclusions : A. mangium is a superior tree species to mix with Eucalyptus for a more stable stand structure in the early growth stage to approach an evident and immense stability and resistance, which is of great signiﬁcance for the forest restoration of Eucalyptus in response to extreme climate and forest management.


Introduction
As a fast-growing tree species, Eucalyptus has been introducing and promoting genetic improvement in many regions to produce large amounts of wood for economic development [1]. However, large-area planting and continuous-planting rotation of commercial Eucalyptus plantations have caused various problems, such as reduced species diversity and loss of soil nutrients, which threaten ecological and timber security regionally and worldwide [1][2][3][4][5]. Constructing mixed forests can form a stratified stand structure, which helps to make full use of the forest land space and environmental resources, increase light energy utilization, regulate the climatic environment within and outside the forest, improve forest

Characteristics of the Experimental Plantations
These experimental plantations were constructed in February 2014 and included one pure plantation of Eucalyptus urophylla S. T. Blake × E. grandis W. Hill "DH32-29" (EE) as the control, and three mixed-Eucalyptus (DH32-29) plantations mixed with three kinds of tree species: (1) a mixed plantation of Eucalyptus and Neolamarckia cadamba (Roxb.) Bosser (EN) that was mixed by strip for four rows of Eucalyptus and four rows of N. cadamba; (2) a mixed plantation of Eucalyptus and Acacia mangium Willd. (EA) that was mixed by strip for four rows of Eucalyptus and four rows of A. mangium; and (3) a mixed plantation of Eucalyptus and Pinus elliottii var. Elliottii Engelm. × P. caribaea Morelet (EP) that was

Characteristics of the Experimental Plantations
These experimental plantations were constructed in February 2014 and included one pure plantation of Eucalyptus urophylla S. T. Blake × E. grandis W. Hill "DH32-29" (EE) as the control, and three mixed-Eucalyptus (DH32-29) plantations mixed with three kinds of tree species: (1) a mixed plantation of Eucalyptus and Neolamarckia cadamba (Roxb.) Bosser (EN) that was mixed by strip for four rows of Eucalyptus and four rows of N. cadamba; (2) a mixed plantation of Eucalyptus and Acacia mangium Willd. (EA) that was mixed by strip for four rows of Eucalyptus and four rows of A. mangium; and (3) a mixed plantation of Eucalyptus and Pinus elliottii var. Elliottii Engelm. × P. caribaea Morelet (EP) that was mixed by row for one row of Eucalyptus and one row of P. elliottii × P. caribaea. Since the biological characteristics of the different tree species are intrinsically different, Eucalyptus DH32-29 and P. elliottii × P. caribaea were planted with a spacing of 1.3 m × 3.0 m, N. cadamba was planted with a spacing of 3.9 m × 3.0 m, and A. mangium was planted with a spacing of 2.6 m × 3.0 m. They are all fast-growing excellent tree species with strong adaptabilities to grow in the Leizhou Peninsula. However, they were damaged annually by severe wind damages after planting (Table 1). Due to the catastrophic damage of the typhoon "Mujigae" (Super Typhoon, wind force ≥ 52 m/s, Beaufort scale ≥ 16) in October 2015, all Eucalyptus trees were blown down with stems broken; hence, all Eucalyptus were cut off from the basal stem to facilitate the germination of new branches and renew the whole stands. Recovery measures that erected leaning or fallen trees were taken for mixed species to restore stands. In June 2016, one strong branch was preserved and the others were removed. Other tending measures were consistent across the four stands.

Data Measurements
By the end of April 2017, two plots of 30 m × 30 m were set randomly in each stand type (including three mixed stands and one pure stand), adopting the random sampling method to measure the height, diameter, straight-fullness, and brunching status of stem form; the stem inclination of each tree; as well as to record the number of preserved and missing trees. The height (H, unit: m) was measured with a laser altimeter (Nikon Rangefinder Rieho 1000 AS) (precision: 0.1 m). The diameter at breast height (DBH, unit: cm) of each individual tree was measured with a tape (precision: 0.1 cm). Both the stem form and the degree of inclination of each tree were evaluated by the classification methods as described in Table 2   The tree has one single principal stem that is complete and straight (well-formed).

I (Assigning a score of 6)
The tree stem grows vertically without leaning to any side.

II (Assigning a score of 5)
The tree has one single principal stem that is complete and curved slightly.

II (Assigning a score of 5)
The tree stem has an inclination of 0 • to 30 • from the vertical axis.

III (Assigning a score of 4)
The tree has more than two principal stems that are branched below a third of its height.

III (Assigning a score of 4)
The tree stem has an inclination of 30 • to 60 • from the vertical axis.

IV (Assigning a score of 3)
The tree has more than two principal stems that are branched higher than one-third and lower than two-thirds of its height.

IV (Assigning a score of 3)
The tree stem has an inclination of 60 • to 90 • from the vertical axis. V (Assigning a score of 2) The tree has more than two principal stems that are branched higher than two-thirds of its height. V (Assigning a score of 2) The treetop was broken off. VI (Assigning a score of 1) The tree has one single stem that is bent badly. VI (Assigning a score of 1) The tree stem was broken off. The tree has more than two principal stems that are branched higher than twothirds of its height. V (Assigning a score of 2) The treetop was broken off. VI (Assigning a score of 1) The tree has one single stem that is bent badly. VI (Assigning a score of 1) The tree stem was broken off.

Data Processing and Statistical Analysis
Eight indicators of nonspatial structure were assigned: preservation rate, stand density, height, diameter, stem form, degree of stem inclination, tree-species composition, and age structure. The preservation rate means the percentage of tree survival after a period of afforestation, which indicates the degree to which trees are kept safe from damage of external disturbance. The calculation formula is as follows: The tree has more than two principal stems that are branched higher than twothirds of its height. V (Assigning a score of 2) The treetop was broken off. VI (Assigning a score of 1) The tree has one single stem that is bent badly. VI (Assigning a score of 1) The tree stem was broken off.

Data Processing and Statistical Analysis
Eight indicators of nonspatial structure were assigned: preservation rate, stand density, height, diameter, stem form, degree of stem inclination, tree-species composition, and age structure. The preservation rate means the percentage of tree survival after a period of afforestation, which indicates the degree to which trees are kept safe from damage of external disturbance. The calculation formula is as follows:

Data Processing and Statistical Analysis
Eight indicators of nonspatial structure were assigned: preservation rate, stand density, height, diameter, stem form, degree of stem inclination, tree-species composition, and age structure. The preservation rate means the percentage of tree survival after a period of afforestation, which indicates the degree to which trees are kept safe from damage of external disturbance. The calculation formula is as follows: where P is the preservation rate of each stand, n 0 is the number of retained plants, and N 0 is the number of planted plants.
The tree height distribution was calculated by using the tree height class integration method: every 2 m was integrated as one tree height class, and the median value of the group represented the tree height class and was involved in the calculation. The diameter distribution was calculated by using the diameter class integration method: every 2 cm was integrated as one diameter class, and the median value of the group represented the diameter class and was involved in the calculation. The stem form and inclination status of the stem were analyzed through their classification.
In the experimental field, two tree species and two ages exist in each mixed forest, one tree species and one age exist in the pure forest; hence, both the tree-species composition structure and age structure of the mixed forest were assigned a value of 2.00 when evaluating the stand stability of each forest, while that of the pure forest was assigned a value of 1.00. Other evaluation indexes were assigned as their average values. The values of the stability indexes were standardized first by adopting the method of subordinate function value of fuzzy mathematics [44][45][46] and forward to make it dimensionless between 0 and 1.00. The formula is as follows: where U ij is the subordinate function value of i index of j stand, U ij ∈ (0, 1.00); x ij is the measured value of i index of j stand; x imin is the minimum value of i index of j stand; and x imax is the maximum value of i index of j stand. The stability of the four Eucalyptus forests was assessed by the unit circle analysis method of π value rule of optimal stand state [47,48]. According to the method of π value rule of optimal stand state [48], when the values of all stand-state indicators are equal to one, the closed chart area formed is the largest and is equal to the unit circle area π, which can be regarded as the expected value of the optimal stand state. Therefore, the best stand state value (expected value) always equals the unit circle area π, namely the rule of π value of the best stand state. Hence, the stand state depends on the size of the closed chart area composed of real stand-state indicators, and the ratio of the real stand-state value (the size of closed chart area) to the optimal stand-state value (expected value) is the most appropriate measurement of the real stand quality [48]. The methods are as follows: Draw a circle with a radius of 1.00 and divide the circle of 360 • into eight sector areas, which represent eight nonspatial structure indexes of the stand, respectively. Starting from the center of the circle of the eight sector areas, draw corresponding index lines in the form of radiation and mark the index names. Sort the corresponding index value of each stand from large to small and mark them on the radiation with points. Connect adjacent points in turn to form a closed curve, which represents the stability state of the real stand.
When all indicators are equal to one, the maximum circular area of π can be regarded as the optimal stand state. The ratio of the state value of the real stand to the optimal stand is used to judge the stable degree of the state value of the real stand, and the formula is: where ω is the stable degree of the state value of the real stand; s 1 is sum of all sector areas in a closed figure; s 2 is sum of the areas of all triangles in a closed figure; n is the number of indicators (n ≥ 2); m is the number of indicators equal to 1; L 1 and L 2 are values of two adjacent indicators in the triangle part, respectively; and θ is the angle formed by two adjacent indicators. Therein, ω ∈ (0, 1.00). It indicates an excellent status when ω ≥ 0.70. A good status is indicated when ω ∈ (0.55, 0.70). A general status is indicated by ω ∈ (0.40, 0.55). It indicates a poor status when ω ∈ (0.25, 0.40). A terrible status is indicated when ω ≤ 0.25.
Microsoft Excel 2007 (Version 2007, Redmond, Washington D.C., USA) was used for data processing, R software (Version 3.5.1, Auckland, Auckland metropolitan area, New Zealand) was used for data analysis, and Python (Version 3.8, Amsterdam, Noord-Holland, the Kingdom of the Netherlands) was used for drawing the figure of stand-state unit circles.

Preservation of Eucalyptus Pure and Mixed Plantation
The preservation rates varied in tree species and stands, as shown in Table 3. Regarding the tree species in each experiment plantation, Eucalyptus surviving in EA and EP were 5.0% and 7.6% greater than those in the pure Eucalyptus forest (EE), respectively, while Eucalyptus surviving in the EN mixture was 18.8% fewer than those in EE. Therefore, both A. mangium and P. elliottii × P. caribaea had positive effects on the preservation of Eucalyptus. Among the mixed species, N. cadamba survived the most, at 88.9%, A. mangium survived at 59.3%, and P. elliottii × P. caribaea survived the least, at 50.2% (Table 3). As for the whole stand, the preservation rates of pure and mixed-Eucalyptus plantations demonstrated in Table 3 were all at a rather lower level (less than 70%) after experiencing severe wind damage four times, and only the preservation rate of EA was 2.9% greater than that of EE.

Tree Height Distribution
The distribution pattern of tree height structures of 18-month-old Eucalyptus was demonstrated to be similar in the pure and mixed-Eucalyptus forests ( Figure 4). The tree height class of 7 m of Eucalyptus received the largest percentage in four stands, followed by that of 9 m. As for 38-month-old mixed tree species, only the tree height distributions of N. cadamba and A. mangium were similar, since the number of trees with a height class of 7 m was the largest, followed by that of 5 m. The number of trees with a height class of 3 m was the largest in EP. All the mixed-Eucalyptus forests were demonstrated to be unevenly aged, multistoried structures ( Figure 4). Regarding the four forests of pure Eucalyptus and mixtures, the tree heights of EA were mainly distributed from 6.00 to 10.00 m (51.6% for the height class of 7 m and 22.2% for the height class of 9 m), which was identical with that of EE (Table 4). However, the tree heights of EN and EP were lower than that of EE in that tree heights of EN were mainly distributed from 4.00 to 8.00 m (21.6% for the height class of 5 m and 54.3% for the height class of 7 m), and tree heights of EP were mainly distributed from 2.00 to 4.00 m (31.3%) and 6.00 to 8.00 m (39.0%). In addition, even though the proportion of height class that was greater than 7 m of Eucalyptus was EA > EP > EE > EN from high to low in sequence, the proportion of height class that was greater than 7 m in EA was similar to EE (0.3% less), and that in EN and EP were 6.9% and 23.5%, respectively, less than that in EE (Table 4).    Regarding the four forests of pure Eucalyptus and mixtures, the tree heights of EA were mainly distributed from 6.00 to 10.00 m (51.6% for the height class of 7 m and 22.2% for the height class of 9 m), which was identical with that of EE (Table 4). However, the tree heights of EN and EP were lower than that of EE in that tree heights of EN were mainly distributed from 4.00 to 8.00 m (21.6% for the height class of 5 m and 54.3% for the height class of 7 m), and tree heights of EP were mainly distributed from 2.00 to 4.00 m (31.3%) and 6.00 to 8.00 m (39.0%). In addition, even though the proportion of height class that was greater than 7 m of Eucalyptus was EA > EP > EE > EN from high to low in sequence, the proportion of height class that was greater than 7 m in EA was similar to EE (0.3% less), and that in EN and EP were 6.9% and 23.5%, respectively, less than that in EE (Table 4).

Diameter Distribution
The diameter distribution of Eucalyptus in the different mixtures resembled that of EE: the diameter class of 7 cm accounted for the largest proportion, followed by the 9 cm diameter class, in spite of some subtle differences being demonstrated among the four stands ( Figure 5). With regard to mixed tree species, both the diameter distribution of N. cadamba and A. mangium were exhibited to be two-peak structures and ranged from 4.00 to 12.00 cm mostly, while most diameters of P. elliottii × P. caribaea ranged from 2.00 to 8.00 cm. The proportion of diameter class greater than 7 cm of Eucalyptus from high to low was EP > EA > EN > EE, and that of mixed species from high to low was A. mangium, N. cadamba, and P. elliottii × P. caribaea.
It was demonstrated ( Table 5) that the distribution of stand diameters of EN and EA were identical to EE for diameters that were mainly distributed in the 7 cm diameter class (ranged from 6.00 to 8.00 cm), followed by the 9 cm diameter class (ranged from 8.00 to 10.00 cm). The diameters of the mixed forest of Eucalyptus and P. elliottii × P. caribaea were mainly distributed in a diameter class of 7 cm (33.6% ranged from 6.00 to 8.00 cm), followed by a 5 cm diameter class (25.6% ranged from 4.00 to 6.00 cm). Generally, the proportion of diameter class that was greater than 7 cm of stands was EA > EN > EE > EP from high to low (Table 5). were identical to EE for diameters that were mainly distributed in the 7 cm diameter class (ranged from 6.00 to 8.00 cm), followed by the 9 cm diameter class (ranged from 8.00 to 10.00 cm). The diameters of the mixed forest of Eucalyptus and P. elliottii × P. caribaea were mainly distributed in a diameter class of 7 cm (33.6% ranged from 6.00 to 8.00 cm), followed by a 5 cm diameter class (25.6% ranged from 4.00 to 6.00 cm). Generally, the proportion of diameter class that was greater than 7 cm of stands was EA > EN > EE > EP from high to low (Table 5).

Structure of the Tree Stem Form
Apparent differences on the degree of straight-fullness and brunching are demonstrated in Figure 6. In different mixed patterns, only the proportion of straight-fullness of Eucalyptus stems in EA was larger than that in EE (1.9% larger in Grade I), and the proportion of branches was lower than that in EE (0.7% lower in Grade III, 3.8% lower in Grade IV, and 0.3% lower in Grade V). The proportion of Grade I Eucalyptus stems in EN was 26.2% lower than that in pure forests, and the proportion of Grade III, Grade IV, and Grade V Eucalyptus stems was 3.9%, 1.7%, and 6.2% greater, respectively, than that in EE. The stem straight-fullness proportion of Eucalyptus in EP was slightly lower than that of EE, and the branches were slightly more than that of EE.
Compared to other mixed species, the degree of straight-fullness and brunching of tree stems of N. cadamba in Grade I and Grade II accounted for the smallest proportions of 19.4% and 16.1%, respectively; and in Grade IV and Grade V, accounted for the largest proportions of 29.0% and 31.2%, respectively, indicating that branches of most N. cadamba sprouted from a position greater than a third of the height of the tree. Most of A. mangium

Structure of the Tree Stem Form
Apparent differences on the degree of straight-fullness and brunching are demonstrated in Figure 6. In different mixed patterns, only the proportion of straight-fullness of Eucalyptus stems in EA was larger than that in EE (1.9% larger in Grade I), and the proportion of branches was lower than that in EE (0.7% lower in Grade III, 3.8% lower in Grade IV, and 0.3% lower in Grade V). The proportion of Grade I Eucalyptus stems in EN was 26.2% lower than that in pure forests, and the proportion of Grade III, Grade IV, and Grade V Eucalyptus stems was 3.9%, 1.7%, and 6.2% greater, respectively, than that in EE. The stem straight-fullness proportion of Eucalyptus in EP was slightly lower than that of EE, and the branches were slightly more than that of EE.
Compared to other mixed species, the degree of straight-fullness and brunching of tree stems of N. cadamba in Grade I and Grade II accounted for the smallest proportions of 19.4% and 16.1%, respectively; and in Grade IV and Grade V, accounted for the largest proportions of 29.0% and 31.2%, respectively, indicating that branches of most N. cadamba sprouted from a position greater than a third of the height of the tree. Most of A. mangium mainly had a single slightly curved stem (46.5% for Grade II), and the proportion of branches of A. mangium was substantially greater than that of Eucalyptus, but lower than that of N. cadamba. The majority of P. elliottii × P. caribaea grew straight and full for Grade I, accounting for 81.3%. From the perspective of the whole stand, the stem structure of the trees in EN was mainly manifested as a single stem (including well-formed and slightly curved stems), and branched from the position that was higher than a third of the height of the tree ( Table  6). The stem structure of the trees of EA, EP, and EE was mainly manifested as a single stem (including well-formed and slightly curved stems), with the most well-formed stems (EP > EE > EA). In short, EN had the highest ratio of branching visibly (41.0%), and EP had the most straight and complete stem form (68.7%).

Degree of Stem Inclination
In the four experimental forests, more than 95% of the trees grew vertically without leaning to any side (Figure 7), except the A. mangium (no more than 85%), as its inclination proportion is particularly evident, up to 15.1%. The degree of inclination of Eucalyptus in mixed plantations was more obvious than that in the pure forest, among which EN > EA > EP. There was no inclined stem, but broken-off tree stems of N. cadamba at 2.2% in EN. There was a slight inclination (1.8% for Grade III, 0.9% for Grade IV) of the stems of P. elliottii × P. caribaea and 0.5% broken-off stems of Eucalyptus in the mixed plantation of Eucalyptus and P. elliottii × P. caribaea. As a whole, the degree of stem inclination of EA was identified to be greatest, followed by EN, EP, and EE (Table 7). From the perspective of the whole stand, the stem structure of the trees in EN was mainly manifested as a single stem (including well-formed and slightly curved stems), and branched from the position that was higher than a third of the height of the tree ( Table 6). The stem structure of the trees of EA, EP, and EE was mainly manifested as a single stem (including well-formed and slightly curved stems), with the most well-formed stems (EP > EE > EA). In short, EN had the highest ratio of branching visibly (41.0%), and EP had the most straight and complete stem form (68.7%).

Degree of Stem Inclination
In the four experimental forests, more than 95% of the trees grew vertically without leaning to any side (Figure 7), except the A. mangium (no more than 85%), as its inclination proportion is particularly evident, up to 15.1%. The degree of inclination of Eucalyptus in mixed plantations was more obvious than that in the pure forest, among which EN > EA > EP. There was no inclined stem, but broken-off tree stems of N. cadamba at 2.2% in EN. There was a slight inclination (1.8% for Grade III, 0.9% for Grade IV) of the stems of P. elliottii × P. caribaea and 0.5% broken-off stems of Eucalyptus in the mixed plantation of Eucalyptus and P. elliottii × P. caribaea. As a whole, the degree of stem inclination of EA was identified to be greatest, followed by EN, EP, and EE (Table 7).

Evaluation of Stand Stability
The values of eight indicators of nonspatial structure were characterized and presented in Table 8. The stability of the four stands was assessed by combining the subordinate function value of the fuzzy comprehensive and the π value rule of the optimal stand state (Table 9, Figure 8). The subordinate function value of EA was 0.17 greater than the pure forest, and its state value of real stand was 0.20 greater than the pure forest. The subordinate function value of EP was 0.09 greater than the pure forest, and its state value of real stand was 0.09 greater than the pure forest. The subordinate function value of EN was 0.06 less than the pure forest, and its state value of real stand was 0.05 greater than the pure forest. Hence, the stand stability of the mixed forest of Eucalyptus and A. mangium presented as optimal for its subordinate function value of 0.76, and the state value (ω2 = 0.61) of real stand was the largest, followed by the mixed forest of Eucalyptus and P. elliottii × P. caribaea (ω3 = 0.50), pure Eucalyptus forest (ω4 = 0.41), and mixed forest of Eucalyptus and N. cadamba (ω1 = 0.36).

Evaluation of Stand Stability
The values of eight indicators of nonspatial structure were characterized and presented in Table 8. The stability of the four stands was assessed by combining the subordinate function value of the fuzzy comprehensive and the π value rule of the optimal stand state (Table 9, Figure 8). The subordinate function value of EA was 0.17 greater than the pure forest, and its state value of real stand was 0.20 greater than the pure forest. The subordinate function value of EP was 0.09 greater than the pure forest, and its state value of real stand was 0.09 greater than the pure forest. The subordinate function value of EN was 0.06 less than the pure forest, and its state value of real stand was 0.05 greater than the pure forest. Hence, the stand stability of the mixed forest of Eucalyptus and A. mangium presented as optimal for its subordinate function value of 0.76, and the state value (ω 2 = 0.61) of real stand was the largest, followed by the mixed forest of Eucalyptus and P. elliottii × P. caribaea (ω 3 = 0.50), pure Eucalyptus forest (ω 4 = 0.41), and mixed forest of Eucalyptus and N. cadamba (ω 1 = 0.36).  Note: The values are mean ± standard error for height, diameter, stem form, and degree of stem inclination.

Discussion
The characteristics of stand structure can express and reflect the inter-or intraspecific competition state and the stability of stands, and have great impacts on the maintenance

Discussion
The characteristics of stand structure can express and reflect the inter-or intraspecific competition state and the stability of stands, and have great impacts on the maintenance of biodiversity, the production of target tree species, and stand qualities [35,[49][50][51]. Stability of the forest represents the comprehensive ability of forest ecosystem to resist external environment disturbance and restore the initial state after disturbance [52,53], and ecological restoration is the fundamental measure of ecological security to maintain the relative stability of the plant ecosystem [54]. Assessing stability from the perspective of nonspatial structure can reveal its stand resistance, resilience, and durability in response to extreme climate or other natural disturbances. Building a fast-growing, high-yield, and high-stability Eucalyptus mixed plantation in a windy area is a way of guaranteeing protection to ensure economic benefits and improve the woodland ecological environment. Prodigious changes in nonspatial structure and stand stability of three kinds of mixed patterns of Eucalyptus and other tree species were demonstrated in detail, and were in line with expectations, but not all mixed patterns could improve stand stability.

Preservation Rate Changes with Mixed Tree Species
The preservation rate is not only an important indicator of the overall stability and resistance of stands, but also an important indicator of stand quality and forest health. The preservation rate reflects the internal living ability of the forest and the adaptability to the environment of the forestland, which is the quantity basis for maintaining the reasonable structure of the stand [55]. Under the condition of the same site, the stand preservation rate of each experimental forest was mainly affected by external disturbance factors or internal competition and interaction among different tree individuals in the stand. External disturbance factors mainly included human disturbance and natural disturbance. The management and tending measures of the experimental forests were all consistent except for the mixed pattern, and all stands located at the same site grew under identical climate and weather conditions, so they suffered from the same natural disturbances, especially the wind damage in the Leizhou Peninsula. Therefore, the difference in stand preservation rate was mainly affected by the mixed pattern, and the competition and interaction among different individual trees.
In our study, both A. mangium and P. elliottii × P. caribaea had a positive effect on the preservation of Eucalyptus, but only when Eucalyptus were mixed with A. mangium could the overall stand preservation rate of the mixed forest be promoted (Table 3). This is because the self-preservation rate of P. elliottii × P. caribaea was not high under the influence of Eucalyptus and the natural environment, so the stand preservation rate of EP was lower than that of pure Eucalyptus. Other studies in mixed forests of Eucalyptus and other tree species obtained comparable results, even though they were conducted on different mixed ratios and sites. Zheng and He demonstrated that the stand preservation rate of the mixed forest of Eucalyptus "Leizhou No.1" and A. auriculiformis mixed by row (1 row of Eucalyptus and 1 row of A. auriculiformis A. Cunn. ex Benth.) was greater than the pure forest, while that mixed by two tree individuals was lower than the pure forest [56]. Yang et al. showed that only when Eucalyptus "U6" and A. crassicarpa Benth. were mixed by ratios of 3:1 (3 rows of Eucalyptus and 1 row of A. crassicarpa) or 3:2 (3 rows of Eucalyptus and 2 rows of A. crassicarpa), their stand preservation rates were lower than the pure forest, but other mixed ratios were greater than that of the pure forest [37]. Chen indicated that the preservation rate of E. wetarensis Pryor × E. camaldulensis Dehnh. "Wc3" had increased by 1.79% after mixing with Cunninghamia lanceolata (Lamb.) Hook. (mixed ratio was 1 to 3) and by 1.2% after mixing with P. massoniana Lamb. (mixed ratio was 1 to 6) as compared with pure Eucalyptus forest. Meanwhile, the preservation rate of C. lanceolata increased, and that of P. massoniana decreased in the mixtures [57]. Deng also demonstrated that the stand preservation rates of the mixed forest of E. dunnii Maiden and C. lanceolata were different when they were mixed at different ratios [58]. Therefore, different mixed tree species and mixed ratios can lead to different stand preservation rates of mixed-Eucalyptus plantations. In addition, the preservation rate of N. cadamba was the highest, up to 88.9%, even though it had a negative effect on the survival of Eucalyptus (Table 3), which indicated that N. cadamba had stronger resistance than Eucalyptus, and N. cadamba may become the dominant tree species in the future. Therefore, the mixed pattern, competitive ability, and interaction among tree species under the specific sites were the main factors from which the effects will determine the future succession tendency of the community.

Mixed Pattern Affects Structure Distribution of Tree Height and Diameter
Diameter distribution and tree height distribution can reflect the degree of tree differentiation and the competition among trees in the stand, which are important indexes of the structural stability of the stand, and also important variables to measure the quality of the stand and formulate the adjustment and optimization scheme when performing management [59,60]. Diameter class structure describes the distribution characteristics of tree diameter, which can reflect the sustainability of the community to a certain extent [61]. The effect of different mixed patterns tended to be different on the diameter structure distributions in view of other scholars' studies. Yang et al. demonstrated that Eucalyptus "U6" of eight mixed patterns (six-year-old Eucalyptus mixed with A. crassicarpa by individual ratios of 1:1, 2:1, 3:1, and 3:2, and by row ratios of 1:1, 2:1, 3:1, and 3:2) had a larger proportion of diameter class above 15 cm than pure Eucalyptus forest, and the maximum over 30.0% only when Eucalyptus was mixed with A. crassicarpa by individual (1 individual of Eucalyptus and 1 individual of A. crassicarpa) and by row (1 row of Eucalyptus and 1 row of A. crassicarpa) [37]. Zhao's study showed that whether E. urophylla × E. grandis or Styrax tonkinensis (Pierre) Craib ex Hartw. was used for mixed forest, the proportion of wood of larger than 12 cm diameter was more than their respective pure stands [62]. Yang showed that the proportion of trees whose DBH was less than the average DBH in the mixed forest of E. urophylla × E. grandis and Castanopsis fissa (Champion ex Bentham) Rehder et E. H. Wilson was less than that in the pure forest, which is quite beneficial to the cultivation of large-diameter Eucalyptus [60]. In our study, Eucalyptus showed evident advantages of fast growth on height by comparing the growth rate of mixed tree species so that all the mixed plantations had a lower proportions of trees with tree height class greater than 7 m than the pure forest (Table 4, Figure 4), which showed that the competition for resources among trees in pure Eucalyptus forests was greater than that in mixed forests. Generally, broad-leaved forest stands with complex structure and high species diversity have more advantages in resisting disturbance, self-regulation, maintaining stability, and exerting ecological benefits than pure forests of the same age [63]. Therefore, the results of our study that the proportion of trees with a height class more than 7 m was EE > EA > EN > EP and the proportion of trees with diameter class above 7 cm was EA > EN > EE > EP indicated that the stand resistance and resilience of the mixed forest of Eucalyptus and A. mangium (EA) were the strongest in response to external disturbances, which is consistent with the assessment of stability (Tables 4 and 5, Figure 8).

Stem Form and Status of Stem Affects Stand Quality
The shape of stem growth is generally called stem form. An excellent stem form can not only improve the growth of trees, but also the wood quality [64,65]. The stem form and morphology of stem are often included in the selection index when people carry out the process of superior tree selection [66,67], and the structure and status of stem are often the most important observable indicators in the study of the resistance of stand response to natural disturbance factors such as wind, ice, and rainfall [42,68]. Therefore, the structure of the stem form of a stand can assess the overall stand resistance and present stand quality effectively. Windstorms constantly occur and land in southern China [69][70][71], and extreme windstorms manifest almost every year on the Leizhou Peninsula, Zhanjiang City [39], which leads to a general concern and numerous studies on wind resistance. The results of the research on the mixed forest of Eucalyptus grandis × E. urophylla and Casuarina equisetifolia Forst. were a little different from ours, in that they concluded that not only Eucalyptus in two ratios of mixtures (3:2 and 4:1), but also the whole mixtures, suffered less wind damage than Eucalyptus pure forest after being hit by typhoon "Vicente" (STY, wind force ≥ 45 m/s), during which wind fall was the main damage to Eucalyptus in mixtures, and branches or stems broken were mostly in pure forest [72]. Hence, the wind resistance varied enormously with different Eucalyptus clones [42,73] with mixed ratio and mixed tree species. The distribution structure on the degree of straight-fullness and brunching and stem inclination clearly demonstrated that most of the tree individuals presented a single stem, vertical growth, and no inclination of the stem regardless of whether it was a mixed forest or a pure forest (Tables 6 and 7), which is basically identical to their status under natural growth condition [74][75][76][77]. However, the integral stand-stem form of the mixed forest of Eucalyptus and P. elliottii × P. caribaea (EP) was more straight and complete under the same site quality, since P. elliottii × P. caribaea had little impact on Eucalyptus trees due to its slow growth, and suffered less wind resistance compared to the other two broad-leaf species. The mixed forest of Eucalyptus and A. mangium had a higher ratio of inclined stems, and the mixed forest of Eucalyptus and N. cadamba had a higher ratio of tree branching stems. According to previous studies, the wind resistance was proved to be P. elliottii × P. caribaea > A. mangium > Eucalyptus urophylla × E. grandis from high to low in sequence after suffering the typhoon "Mujigae" [78], and the wind resistance of N. cadamba was better than A. mangium [79]. Therefore, there may be some mechanisms of interaction existing in tree individuals. Generally, the tree individuals may secrete some chemical substances to interfere with each other in the process of growth to compete for more resources in a limited space or resist external disturbance [80]. Such allelopathic mechanisms of interaction and growth mechanisms adapting to the environment are still worth further study.

Limitations
The row spacing was treated differently when planting these experimental forests, in view of the differences in canopy morphology of each tree species and the demand for space resources. Nevertheless, the size of row spacing and the stand density can influence the growth of stand, so stand density was also considered an important indicator when assessing the stand quality and stability. However, whether there is a better density and row spacing to make the stand structure more stable remains to be further studied. In addition, the structural characteristics of the shape of crown, the distribution of branches and roots, species diversity, and distribution in the forest may affect the stability and stress resistance of the stand, which is also worth further study.

Conclusions
Assessing the stand stability of pure and mixed-Eucalyptus forests in typhoon-prone areas is critical for choosing appropriate mixed patterns and species. In this study, we found that the overall stand preservation rate of the mixed forest can be promoted when Eucalyptus were mixed with A. mangium. The height of EN and EP, as well as the diameter of EP, distributed at a lower level than that of the pure Eucalyptus plantation, and the height of EP appeared to have a two-peak distribution. The stem of EN was testified to be more curving and brunching than that of the pure Eucalyptus plantation, the stem of EA was testified to be more curving and inclined than that of the pure Eucalyptus plantation, and the stem of EP was testified to grow straightly. Only A. mangium can markedly advance the stand stability of Eucalyptus plantations; P. elliottii × P. caribaea can improve the stand stability of Eucalyptus plantations mildly, and N. cadamba is not conducive to the stand stability of Eucalyptus plantations. To sum up, A. mangium is a superior tree species to mix with Eucalyptus for a more stable stand structure in windy areas to approach an evident and immense stability and resistance in response to extreme climate, followed by P. elliottii × P. caribaea and N. cadamba, which is of great significance to renovate inefficient Eucalyptus plantations and restore forest ecology.

Data Availability Statement:
The data presented in this study are available on request from the corresponding authors. The data are not publicly available due to privacy.