**2.3 Characteristics of regeneration layer in six** *H. ammodendron* **populations at different chronosequence stage**

#### *2.3.1 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

*Characteristic on the Stability of* Haloxylon ammodendron *Plantation… DOI: http://dx.doi.org/10.5772/intechopen.99050*

**Figure 2.** *Dynamitic distribution of* H. ammodendron *height (HT) in canopy storey at different chronosequence stage.*

**Figure 3.**

*Dynamitic distribution of* H. ammodendron *basal stem diameter (BSD) in canopy layer at different chronosequence stage.*

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<sup>2</sup> ) 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<sup>2</sup> , 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.

### *2.3.2 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


*Grade I: HT < 30 cm, Grade II: 31* ≤ *HT < 50 cm, Grade III: HT* ≥ *50 cm. The unite of density is No.hm<sup>2</sup> . 'G, M and B' in the table stand for 'good, medium and bad' respectively.*

#### **Table 2.**

*Quality and quantity of seedlings with three grades in* H. ammodendron *plantation at different chronosequence stage.*

*Characteristic on the Stability of* Haloxylon ammodendron *Plantation… DOI: http://dx.doi.org/10.5772/intechopen.99050*

**Figure 4.**

*Height and Basal Stem Diameter of grade III seedling (HT* ≥ *50 cm) in* H. ammodendron *plantation at chronosequence stage.*


*X1i: density of seedling (Nohm<sup>2</sup> ), X2i: density of grade III seedling (Nohm<sup>2</sup> ), X3i: height of grade III seedling (m), X4i: basal stem diameter of grade III seedling (cm), X1j: plantation age (yr), X2j: planting density (Nohm<sup>2</sup> ), X3j: density of canopy storey (%), X4j: height of regeneration storey (m), X5j: crown projected area of canopy storey (m2tree<sup>1</sup> ), X6j: Above-ground biomass of canopy storey (kgtree<sup>1</sup> ), X7j: SWC(%), X8j: pH, X9j: EC (mscm<sup>1</sup> ), X10j: SOM (gkg<sup>1</sup> ), X11j: SHN (mgkg<sup>1</sup> ), X12j: SEP (mgkg<sup>1</sup> ), X13j: SAP (mgkg<sup>1</sup> ).*

#### **Table 3.**

*The relational degree (Rd) between characteristic of seedling and its related influencing factors.*

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.

#### **2.4 Population development characteristics of** *H. ammodendron* **plantation**

Static Life Table was made according to relevant parameters calculated by actual measurment data of survival individual (ax) in canopy storey and regeneration

storey, the mutual relationship of parameters were as follow: lx = ax/a0 1000, dx = lx-lx+1, qx = dx/lx 100%, Lx = (lx + lx+1)/2, Tx= P<sup>∞</sup> <sup>x</sup> Lx, ex = Tx/lx. Where ax is the existing number of age x, lx was standardized number, dx was number of death interval, qx was average survival rate per age stage, Lx was life between the interval, Tx was total life, ex was mean expectation of life. When *H. ammodendron* individuals in canopy storey grew up to the age of 17, qx rose obviously at age class above V, and the highest qx appeared at age class below IV. In 7-yr-old plantation, the largest ex was at age class I. And in the plantation with the age of 12, 17, 20, 28 and 33, the largest ex 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**).
