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Grazing Effects on the Mixed-Grass Prairie Seed Bank in the Coteau Region of North Dakota

Results



Seed Bank Species Composition

A total of 71 species were recorded by identifying the germinable seedlings: 13 grasses, 35 perennial forbs, 2 shrubs, 15 annual forbs, and 6 unknown species. The dominant species were Kentucky bluegrass, yellow wood sorrel (Oxalis stricta), narrow-leaved goosefoot (Chenopodium desiccatum), and western rock jasmine (Androsace occidentalis). For the different grazing intensities, collection/germination times, and range sites, the dominant species in the seed bank were slightly different. There were statistically significant differences in the number of species in soil seed banks between different times (F2, 12=23.77, P=0.000). The WS collection/germination time showed the highest species richness of the three times (Figure 1). There were, however, no statistically significant differences between grazing intensities, sites, two-way interactions (grazing intensity-time, grazing intensity-site, and time-site), and three-way interactions (grazing intensity-time-site).


Seed Bank Life Form Functional Group Size:


Grass germinable seed bank (GGSB): The most abundant group of species in the total germinable seed bank, the GGSB ranged from 40% to 84% of the total germinable seed bank at different times, different sites, and different grazing intensities (Figure 2). The most important component of the GGSB was Kentucky bluegrass. The GGSB showed significant differences in seeds per sample between grazing intensities, times, and sites. Light grazing showed significantly greater GGSB values than did extreme grazing in all times and sites. The WS time had the highest GGSB values of the three times. The silty site showed significantly greater GGSB values than did the overflow site. The GGSB showed significant differences in grazing intensity-time and grazing intensity-site two-way interactions; there is a significant difference in the three-way interaction (Table 1A).


Perennial forb germinable seed bank (PFGSB): PFGSB had the most species in the total germinable seed bank. The most important component of the PFGSB was the yellow wood sorrel. The overflow site showed significantly greater PFGSB values than did the silty site, and the WS germinate method showed the greatest PFGSB values of the three methods. The PFGSB values showed no significant differences between grazing intensities, but did show significant differences in grazing intensity-site two-way interaction (Figure 2 and Table 1B).


Shrub germinable seed bank (SGSB): The SGSB was not important not only in species component but also in abundance. The SGSB showed significant differences between grazing intensities, times, and their two-way interaction (Figure 2 and Table 1C). Germinating seeds of buckbrush (Symphoricarpos occidentalis) and prairie rose (Rosa arkansana) were not found even in the sites with higher shrub aboveground biomass or cover. Only occasionally did we find two species: fringed sagewort (Artemisia frigida) and wormwood (Artemisia absinthium).


Annual forb germinable seed bank (AFGSB): The AFGSB was the second most abundant in the total germinable seed bank. The most important components of the AFGSB were narrow-leaved goosefoot and western rock jasmine, although they represented only a minor percentage of the total aboveground biomass. The AFGSB showed significant differences between times and grazing intensity-site two-way interaction (Table 1D).


Total germinable seed bank (ToGSB): The ToGSB in the experimental sites ranged from 1637.6 germinating seeds /m2 to 5988.3 seeds /m2 in different grazing intensities, sites, and collection/germination times. The ToGSB values showed significant differences between grazing intensities and time. Light grazing showed significantly greater ToGSB values than did extreme grazing. The WS germination time had the highest ToGSB values of three germination times, but site showed no significant difference (Figure 2). The ToGSB values showed significant differences in the grazing intensity-time and grazing intensity-site two-way interactions and in the grazing intensity-time-site three-way interaction (Table 1E).


Table 1. Analysis of variance of cubic root transformed values of grass germinable seed bank (GGSB), perennial forb germinable seed bank (PFGSB), shrub germinable seed bank (SGSB), annual forb germinable seed bank (AFGSB) and total germinable seed bank (ToGSB).

 

Source of variation

DF

F values

P

A. Grass germinable seed bank (GGSB)

    Main effects

   Grazing intensity

1

163.44

0.000

   Time

2

27.72

0.000

   Site

1

29.92

0.000

   Two-way interactions

   Grazing intensity-Time

2

4.33

0.014

   Grazing intensity-Site

1

4.39

0.037

   Time- Site

2

0.35

0.702

    Three-way interaction

    Grazing intensity-Time-Site

2

5.67

0.004

B. Perennial forbs germinable seed bank (PFGSB)

   Main effects

   Grazing intensity

1

0.53

0.468

   Time

2

16.28

0.000

   Site

1

80.45

0.000

   Two-way interactions

   Grazing intensity-Time

2

0.75

0.472

   Grazing intensity-Site

1

20.27

0.000

   Time- Site

2

0.59

0.554

   Three-way interaction

   Grazing intensity-Time-Site

2

0.52

0.594

C. Shrub germinable seed bank (SGSB)

   Main effects

   Grazing intensity

1

46.75

0.000

   Time

2

8.90

0.000

    Site

1

0.03

0.853

   Two-way interactions

   Grazing intensity-Time

2

4.08

0.018

   Grazing intensity-Site

1

3.32

0.069

   Time- Site

2

2.20

0.112

   Three-way interaction

   Grazing intensity-Time-Site

2

0.51

0.603

D. Annual forbs germinable seed bank (AFGSB)

   Main effects

   Grazing intensity

1

2.31

0.129

   Time

2

17.50

0.000

   Site

1

0.05

0.831

   Two-way interactions

   Grazing intensity-Time

2

2.83

0.060

   Grazing intensity-Site

1

5.28

0.022

   Time- Site

2

0.64

0.529

   Three-way interaction

   Grazing intensity-Time-Site

2

0.99

0.374

E. Total germinable seed bank (ToGSB)

   Main effects

   Grazing intensity

1

112.61

0.000

   Time

2

40.10

0.000

   Site

1

1.16

0.282

   Two-way interactions

   Grazing intensity-Time

2

5.61

0.004

   Grazing intensity-Site

1

9.20

0.003

   Time- Site

2

0.84

0.434

   Three-way interaction

   Grazing intensity-Time-Site

2

5.77

0.003

Grazing intensity: light and extreme grazing; Site: silty and overflow; Time: WW, WS and SS (As described in Fig. 1 legend).


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