**3.4 Vertical distribution of seed densities of SSB in the different aged** *C. microphylla* **plantation**

The total level of germinated seeds in the SSB of different aged *C. microphylla* plantation followed the progression of: 25 years >7 years >16 years >37 years. The number of germinated seeds significantly differed among all plots, except for between the 7 and 16 years. As was observed for SSB species richness, the number of SSB germinating plants first increased in younger plantation, then declined. Seed densities were primarily concentrated in the 0–2 cm soils, followed by the 2–5 cm depth soils and then the 5–10 cm depth soils. The number of seeds in the 0–2 cm SSB of the semi-fixed dune was 227.50 seeds<sup>m</sup><sup>2</sup> , which accounted for 34.40% of the 0–10 cm seed abundance, and was significantly lower (*P* < 0.05) than in other plots. Thus, these analyses focused on the seeds below 2 cm in the semi-fixed dune plots, since there were considerably lower seed numbers in the 0–2 cm depth interval. The density of seeds in the 0–2 cm depths of the 7, 16, 25, and 37 years *C. microphylla* plantation were 9,665.00 seeds<sup>m</sup><sup>2</sup> , 8,216.25 seeds<sup>m</sup><sup>2</sup> , 12,981.25 seeds<sup>m</sup><sup>2</sup> and 4,986.25 seeds<sup>m</sup><sup>2</sup> , respectively, which accounted for 78.64%, 77.85%, 81.69%, and 79.65% of the total 0–10 cm SSB. Significant differences were observed for the seed densities within the 0–2 cm soils of each forest plot. These results thus indicate that planting *Caragana* had a positive enrichment effect on the seed density in the 0–2 cm surface layer of SSBs, and that the strength of this effect varied with *C. microphylla* plantation age wherein: seed density in the 25 years >16 years >7 years >37 years. Compared to the natural grassland plots, both the germination total seeds and densities in each vertical soil interval were significantly lower in the planted forest plots, and the degree of difference varied among plots (**Table 3**).

**Figure 2.** *Vertical distribution of species in the SSB of different aged* C. microphylla *plantation and control.*

*Evaluation of Soil Seed Banks in Different Aged* Caragana microphylla*… DOI: http://dx.doi.org/10.5772/intechopen.98712*


#### **Table 3.**

*Soil seed bank vertical distribution in different aged* C. microphylla *plantation and control.*

#### **3.5 Species diversity of SSB in different aged** *C. microphylla* **plantation**

The species richness of SSBs in *C. microphylla* plantation first increased with age, but then decreased (**Figure 3**). The number of species was highest in the 16 years *C. microphylla* plantation (22), and species richness was significantly different between each older plantation and the semi-fixed sand dunes. However, there was no significant difference between older plantation and the natural steppe (*P* > 0.05).

The Shannon-Wiener and Simpson indices trended similarly, with each value increasing as plantation age increased, but with different degrees of difference among each plantation age. The Pielou evenness index varied as plantation age increased. The number of species in the natural grassland (16.500 3.536) was lower than the number of species in each of *C. microphylla* plantation. Meanwhile,

#### **Figure 3.**

*Diversity of SSBs in different aged* C. microphylla *plantation and control. Note: Different letters indicate statistically significant differences among plots.*

the Shannon-Wiener, Simpson, and Pielou evenness indices were also lowest in the natural grassland, (0.146 0.032, 0.042 0.010, 0.052 0.007 respectively).
