**2.5 Comprehensive evaluation of vegetation and soil system of** *H. ammodendron* **plantation**

#### *2.5.1 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 = 0.219X1.289, F = 10.997, R2 = 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**).
