Characteristic on the Stability of Haloxylon Ammodendron Plantation in the Southern Fringe of Gurbantunggut Desert, Northwest China

Using chronosequence theory and method, the characteristics of vegetation-soil coupling and structure stability of Haloxylon ammodendron plantations in the southern fringe of Gurbantunggut Desert were analyzed. The results showed, the canopy storey of H. ammodendron plantation experienced three stages, rapid growth (the age of 7 to 20), then slow growth (the age of 20 to 28) and last decline (over the age of 28). The best natural regeneration started from 17-yr-old plantation. Vegetation-soil system coupling degree (C) and coupling coordinative degree (D) of plantations with different age were not one-to-one correspondence. The system of H. ammodendron plantations always stayed in disorder recession, vegetation and soil were prone to loss type during the process of sand-fixation. Five principal components evaluated that the first rank was 42-yr-old plantation. It was inferred that the trend of the vegetation and soil system was from senescence to harmonious development. So the trend of coordinated development between vegetation and soil would be promoted, if the artificial tending and management measures strengthened.


Introduction
Structure and function are two vital aspects to determine and measure the stability of forests community [1]. Population structure, which reflects the distribution of individuals within a population, may provide insights into past and present regeneration [2,3]. Life table and survival curve are the main methods when analyzing population structure. By using life table analysis, it was discovered that H. ammodendron seedling had two rapid drop stages of survival rate and high mortality rate in natural forest [4]. Meanwhile, by using survival curve analysis, H. ammodendron population was suggested closing to progressive or stable status in some suitable habitats [5,6]. This, however, has been unknown in the plantation which had similar inhabits but different growth stage.
Plantation has been applied in ecological restoration worldwide [7]. A key issue for plantation is that population regeneration is often challenged by seedling establishment in many plantation ecosystems, which is critical to the stability of plantation species as well as to the plantation sustainability [8]. For H. ammodendron, failure in regeneration resulted in the population degradation occurred in old stand in the Hexi Corridor Desert [9,10], suggesting that the regeneration barrier possibly occurs in H. ammodendron plantation. This, however, has not been assessed in the biggest H. ammodendron plantation developed in the Gurbantunggut Desert edges.
The wind prevention and sand fixation forest ecosystem, which is a kind of relatively heterogeneous open system, and flow of the material, energy and information constantly happens among the subsystems, thereinto, the two major subsystems, vegetation and soil formed an interdependent and mutually restrictive relationship between them [11]. Exploring the relationship between vegetation and soil has significant meaning in understanding the process and effect of sand fixation vegetation, and further putting forward corresponding optimal management measures [12]. In the ecological process of settlement, growth and succession of artificial H. ammodendron population, the accumulation and distribution of soil moisture and nutrient will change. In turn, that will affect the characteristics of vegetation. Therefore, such mutual restriction relationship between vegetation and soil will definitely change according to the spatial and temporal scale [13].
The Gurbantunggut Desert in northwestern China, the third largest desert in the world, is sensitive to climate change and human activities [14]. The desert vegetation is dominated by the H. ammodendron, which is greatly tolerant to drought, wind erosion, sand burial and other stress factors [15][16][17][18]. Therefore, H. ammodendron has been widely used in vegetation restoration and sand-fixation engineering [19]. In the edge of the Gurbantunggut Desert, i.e., the desert-oasis ecotone, H. ammodendron forest play an important role in preventing dune movement toward oasis. However, large area of H. ammodendron forest was suffered deforestation and overgrazing during the 1970s and 1980s, which leads to severe desertification in this area [20]. Since then H. ammodendron plantation has been extensively developed along the edge of the Gurbantunggut Desert. In recent decades, these ammodendron plantations have gradually developed into the mature stage, but, some of them tend to decline, which directly threatened the stability of the artificial population system and the normal exertion of ecological functions in this area.
This chapter, the change characteristics of the canopy storey and the regeneration storey structures of H. ammodendron plantations at different chronosequence stages, as well as the growth and death dynamic development law of populations ware studied. Meanwhile, the feedback relationship between vegetation and soil were analyzed. Further, the current situation of growth and development of H. ammodendron population in the edge of the Gurbantunggut Desert was evaluated accurately and comprehensively. Then, we explained why the H. ammodendron population is stable, developing or declining, and whether H. ammodendron plantation has the ability of sustainable natural regeneration. On this basis, we put forward the scientific suggestion for the vegetation construction and management of the degraded artificial forest ecosystem in this region. We hope this research could provide basic data and scientific evidence for further research on the vegetation restoration and reconstruction of desert ecosystem in arid region.
which are constructed on moving sandy land within an area of about 57.61 km 2 . Thus soil and climate is similar among these plantations. All plantations were established by transplanting 1-yr-old H. ammodendron seedlings in regular spacing ( Table 1). Thus H. ammodendron individuals could be classified into two main storeys, i.e. transplanted individuals (canopy storey) and seedlings including those naturally regenerated between transplanted ones (regeneration storey). None of the plantations were subjected to irrigation, pruning, thinning or any other silvicultural practices after transplantation. In addition, no incident of insect, rodents and fire was recorded since 1981. Except H. ammodendron seedlings, understory plants were dominated by Ceratocarpus arenarius and Salsola ruthenica, with a low total cover less than 8%. (Table 1, Figure 1).
In August, when soil water content of the study site was relatively stable and close to the average level in a year [21], vegetation survey was carried out in nine H. ammodendron plantations. In each plantation, three 20 m Â 30 m sample plots were established, with each at least 100 m away from the others. Within each sample plots, the numbers of survived and dead individuals for canopy storey was recorded (Table 1). Meanwhile, height, basal stem diameter and crown width (north-south and east-west) of survived individuals were measured one by one. Then the crown projected area, above-ground biomass and canopy density of major storey were calculated according to Phillips and Macmahon's ellipse formula [22] and Song's Eq. [23], respectively.

Characteristics of canopy storey in six H. ammodendron plantations at different Chronosequence stage
Vegetation survey was carried out in six H. ammodendron plantations, which having capacity of normal natural regeneration. According to Rundel's method [24], combining with the characteristic of slow growth and false ring of H. ammodendron [25], the structure of canopy storey was analyzed by quantity and size dynamic of tree height and basal stem diameter. Tree height class was divided into 0 $ 0.  moved to a higher section gradually, and the structure of size class showed a trend of continuous differentiation first and then concentration. It indicated that the intraspecific competition among individuals was intensifying, and the population structure tended to be more and more complex, which was conducive to the rational use of spatial resources. The tree height and basal stem diameter class of canopy storey showed obvious differentiation in the plantations with the age of 17 and 20. It indicated that, after the competition and self-thinning, some individuals having competitive advantage had entered in the rapid growth stage, and most individuals could grow to the canopy storey (HT 2.5-3.0 m, BSD 8-10 cm). After the age of 28, H. ammodendron plantation would start to decline, when branch aging and withering appeared, and the natural death rate of H. ammodendron in canopy storey would reach 5% $ 8%.

Characteristics of seedling with different grade
Following the method of the Blackman [26] and Forest resources planning survey [27], seedlings in regeneration storey were divided into three grades, namely grade I   seedling was HT ≤30 cm, grade II seedling was 31 ≤ HT <50 cm, and grade III seedling was HT ≥50 cm. respectively. The survival numbers of each grade seedlings were recorded respectively according to the following criteria. The grade I seedling status was considered 'Good', 'Medium' and 'Bad', if their number was >5000, 3000 $ 4999 and < 2999. Grade II seedling status was considered 'Good', 'Medium' and 'Bad', if their number was >3000, 1000 $ 2999 and < 999. Grade III seedling status was considered 'Good', 'Medium' and 'Bad', if their number was ≥5000, 500 $ 4990 and < 500.
It can be seen from Table 2 and 7-yr-old plantation already had the weaker ability of regeneration. In the plantation age of 7, 12, 17, 28, and 33, grade I seedlings all accounted greater proportion. Especially in 28-yr-old plantation, 67.3% of the total regenerated individuals (36,383 No.hm À2 ) was grade I seedling, and grade III only accounted for 1.9%. Grade I seedlings were presented "bad" in 20-yr-old plantation. Grade II seedlings were "good" in the plantations with the age of 17 to 28. Grade III seedlings were "good" only in 17 and 33 yrs. old plantation. In contrast, there were not only abundant grade I seedlings but also larger proportion of grade III individuals (29.3%) in 17-yr-old plantation. For 33-yr-old plantation, although seedling density was only 9,433 No.hm À2 , the ratio of grade III reached 40.3%. The growth quality of grade III seedling was better than others apparently in 12-yr-old and 17-yr-old plantations. Especially in the 17-yr-old one, the average height and basal stem diameter of grade III seedling even reached 1.2 m and 1.9 cm, respectively ( Table 2).
The quantity and growth quality of older seedlings is more important for the sustainable development of population [7]. As shown in Figure 4, the best growth status of grade III seedlings was in 17-yr-old plantation, average height and basal stem diameter reached 1.10 m and 1.91 cm respectively, and the maximum were 2.19 m and 3.89 cm, respectively. Height of grade III seedlings was more evenly in 28-yr-old plantation, which was mainly distributed between 0.51 $ 1.02 m. For 33-yr-old plantation, the density and growth of the grade III seedlings were similar with the 28 yrs. old one.

Analysis on influencing factors of natural regeneration in H. ammodendron plantation
Results of the relational degree (Rd) between natural regeneration and its influencing factors shown in Table 3. Among six vegetation factors, above-ground biomass of the H. ammodendron individuals in canopy storey was closely related with density of seedlings in the regeneration storey (Rd = 0.77). But the density of grade III seedlings was mainly influenced by the density (Rd = 0.71) and age (Rd = 0.70) of the individuals in the canopy storey. Furthermore, density of seedlings had greatest Rd. with SHN (Rd = 0.87), as followed by SEP (Rd = 0.84) and SWC (Rd = 0.79). While the density, height and basal stem diameter of grade III  seedlings were strongly influenced by SOM, Rd. was 0.78, 0.87 and 0.94 respectively, which explained that influence of SOM become more important than SHN for grade III seedlings. The soil pH of 8.1 to 8.6 had less influence on natural regeneration. In Minqin Desert, when soil pH was equal to or higher than 8.6, natural regeneration of H. ammodendron would be limited [28]. On the whole, soil environmental factors generally had greater influence on natural regeneration than that of vegetation factors of the canopy storey.

Population development characteristics of H. ammodendron plantation
Static Life   storey, the mutual relationship of parameters were as follow: l x = a x /a 0 Â 1000, d x = l x -l x + 1 , q x = d x /l x Â 100%, L x = (l x + l x + 1 )/2, T x = P ∞ x Lx, e x = T x /l x . Where a x is the existing number of age x, l x was standardized number, d x was number of death interval, q x was average survival rate per age stage, L x was life between the interval, T x was total life, e x was mean expectation of life. When H. ammodendron individuals in canopy storey grew up to the age of 17, q x rose obviously at age class above V, and the highest q x appeared at age class below IV. In 7-yr-old plantation, the largest e x was at age class I. And in the plantation with the age of 12, 17, 20, 28 and 33, the largest e x presented at age class IV, II, VI, III and IV, respectively. It inferred that survival ability of H. ammodendron could be declined gradually with its growth ( Table 4).
Variation tendency of survival and death curves were similar in the same plantation, and discrepancy still existed among six H. ammodendron plantations. The survival curve of 7-yr-old plantation was a skewed normal curve, and the survival rate and death rate were higher for I-age-class seedlings than others, and they were the highest for II-age-class seedlings. The survival curve of the 12-yr-old plantation showed Deevey-III type, that is, the early death rate was higher, the selection intensity of the environmental seize was greater, less than 10% of the seedlings could pass through this seize and enter the II-age-class. The transition from the seedling stage to the vegetative development stage was relatively balanced. Along with the increase of age, the environmental seize intensity at the vegetative development stage weakened, and the survival and death amount tended to level off ( Figure 5).

The change characteristics of the vegetation and soil of H. ammodendron plantation
The growth of H. ammodendron was rapid during 7 to 12 years after planting, and it began to decline after age 20, when the height, basal stem diameter, and crown width reached the maximum. During the age of 7 to 42, the tissue water content (TWC) and Proline mass fraction in the assimilated branches of H. ammodendron showed three curvilinear changes trend, i.e., increased from age 7 to 15, and then decrease slowly from age 30 to 35, after that increased again. Along with the increase of H. ammodendron age, the cell damage rate of H. ammodendron decreased inversely, the nitrogen mass fraction increased linearly and chlorophyll mass fraction increased exponentially. The critical change point of phosphorus mass fraction occurred at about the age of 25, showing a typical parabolic change characteristic. Meanwhile, the change trend of potassium mass fraction was irregular. For seven soil factors, only SHN (Y) had a power function regression relationship with plantation age (X) (Y = 0.219X 1.289 , F = 10.997, R 2 = 0.611, P = 0.013), while SEP and SAP had no linear regression relationship with the plantation age. The EC increased along with plantation age, which might be caused by the accumulation and deposition of soluble salts in litters ( Table 5).

Classification system and evaluation criteria of vegetation and soil coupling type of H. ammodendron plantation
According to the coupling coordination degree (D), combined with the comprehensive evaluation function of vegetation f(x) and soil environment g(y) calculated based on the classification method of Peng [29], the vegetation-soil coupling  coordination type and evaluation criteria of H. ammodendron plantation were proposed, result shown in Table 6. If the ratio of f(x) to g(y) was bigger than 1, it indicated that the growth and development speed of vegetation was faster than that of soil. If the ratio was smaller than 1, it indicated that the growth and development speed of vegetation was slower than that of soil, and soil fertility resources were not fully utilized by the vegetation. If the ratio was closer to 1, it indicated that the succession state between them was more synchronous and coordinated. According to the D and the ratio between f(x) and g(y), the vegetation and soil coordinated development could be divided into 5 categories and 15 sub-categories. The coupling intensity and coupling coordination were not in one-to-one correspondence relationship, and both of them often appeared alternately, which was in line with the fluctuating development law between vegetation and soil subsystems.

Coupling coordination analysis of vegetation and soil system of H. ammodendron plantation
According to the data of Table 7, vegetation-soil coupling degree (C) of plantation was arranged on the order of 33 > 23 > 42 > 20 > 17 > 28 > 12 > 15 > 7. When H. ammodendron grew up to the age of 17, the interaction between vegetation and soil increased obviously and tended to be stable. The C of the 33-yr-old plantation was 73.85% increased than 7-yr-old one. The D of H. ammodendron plantations was arranged on the following order: 28 > 42 > 20 > 17 > 33 > 12 > 23 > 15 > 7. The D of 28-yr-old plantation was 96.63% increased than 7-yr-old one.
On the whole, the C and D of nine plantations showed a fluctuating and increasing trend along with the age increasing. H. ammodendron populations always kept at the level of disorder and declining, and vegetation and soil were in the state    of profit and loss. However, the degree of disorder on profit and loss showed the trend from serious to slight. (Table 7).

Comprehensive evaluation of vegetation and soil system of H. ammodendron plantation
Through principal component analysis, the overall information of the original 18 variables were replaced by the first 5 principal components, contribution rate was 90.91% (Table 8). Among them, the Eigen Value of the first principal component was 7.531, which accounted for 41.84% of the total variation and formed a correlation with above-ground biomass (0.900), crown projected area (0.874), basal stem diameter (0.831), and tree height (0.830) etc.. Therefore this principal component could represent the growth characteristic index of vegetation. The Eigen Value of the second principal component was 3.486, which accounted for 19.37% of the total variables. The chlorophyll mass fraction (0.918) had a greater positive effect on it, and the cell damage rate (À0.858) had a strong negative correlation, so this    Table 9, it could be seen that the 42-yr-old one ranked the first, followed by plantation with the age order of 28, 20, 23, 12, 15, 17, 33 and 7. That is to say, during the process of sand fixation, the comprehensive score calculated based on the five principal component scores increased gradually, thus the comprehensive ranking was gradually higher than before. (Table 9).

Conclusions and discussion
The constructive species could regenerate successfully, which is the basic premise of sustainable development of artificial forest. The 7-yr-old H. ammodendron plantation, in the southern fringe of Gurbantunggut Desert, had weak selfgeneration ability and it could maintain such ability until the age of 33. During the process of individuals growth in canopy storey, there were abundant regenerated individuals of the grade I seedlings under canopy, which indicated that the H. abundance of grade I seedling (HT < 30 cm). The number of grade I seedlings accounted for the largest proportion in regeneration storey, and rich seedling bank could provide a strong guarantee for the regeneration and succession of the population [30]. However, the large seedling bank could not guarantee proper regeneration and ensure sufficient number of higher grade seedlings, and grade III seedlings (HT ≥ 50 cm) were the key for transferring from the regeneration storey to the canopy storey, and thus realizing natural regeneration and sustainable development of the H. ammodendron population. Therefore, moderate thinning and tending measures should be taken for H. ammodendron plantation to form a more stable structure. The SHN was the main influencing factor of seedling regeneration in H. ammodendron plantation. Therefore, avoiding destroy the shrub and grass with strong nitrogen fixation ability, and properly retaining withered litters on soil surface would increase the SHN content in soil, and at last would promote the growth of seedlings. The H. ammodendron plantation system, at the south edge of Gurbantunggut Desert, had weak self-optimization ability, the interdependence and mutual restriction relationship between vegetation and soil. Along with the increase of the plantation age, the ratio of f(x)/g(y) gradually increased too. It showed that H. ammodendron population restoration measures had a strong ability to adapt to the environment, and the growth and development speed of population exceeded the development speed of soil, and this may indicate that such vegetation was able to adapt to the local environment, but had a limited ability to reconstruct the soil. When the plantation developed to the age of 42, the coupling degree (C) and coupling coordination degree (D) between vegetation and soil system, f(x)/g(y) were relatively larger, it could be inferred that the vegetation-soil system of the H. ammodendron plantation tended to transform from a declining type to a coordinated development type. It is concluded that the competition of vegetation growth would be more intense in the coming years. Therefore, artificial management measures, such as spring water collection, regular fertilization and tree pruning are needed to promote the healthy growth of H. ammodendron and the natural regeneration of seedlings, so as to improve the diversity and coverage of vegetation, in order to enhance the coordination degree of vegetation and soil, and finally realize the transformation to a high-level coupling state.