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RESULTS

In this chapter results are presented for the study inventory (6.1), the harvesting performance study (6.2), and the harvesting impact assessment (6.3). The study methodology is described in Chapter 5.

Study inventory

The harvesting impact assessment is based on a comparison of the unharvested forest with the harvested forest. A study inventory was carried out prior to harvesting in order to obtain information on the natural forest. Tree species and breast height diameter were recorded. From the literature only very little information is currently available on the forests of the study region and their growth.

The inventory layout consisted of 10 strips 20 x 1,000 m each. Only 9 of the 10 strips were actually forested. All trees greater than or equal to 10 cm dbh were measured and recorded. The inventoried area is 11.8% of the total study area. An attempt was made to classify all trees into categories of commercial, potentially commercial, and others. The identification of commercial species other than Okoumé was not possible with the desired degree of certainty, especially for younger trees. Thus only two classes were used: Okoumé and other species.

Table 6-1 and Table 6-2 give the inventory results by strip. The mean diameter of all inventoried trees was 23.8 cm; the maximum diameter found was 133.7 cm.

Table 6-1: Number of trees by inventory strip for all trees 10 cm dbh

Inventory Strip

 

A

B

C

D

E

G

H

I

K

Total/
Average

Trees 10 cm

 

907

927

989

809

669

1011

1049

1024

677

8,062

Area

ha

1.93

1.96

1.82

2.06

1.87

2.02

2.00

2.08

1.99

17.72

Trees 10 cm/ha

trees/ha

471

472

544

394

358

500

525

493

339

455

Table 6-2: Mean, median and maximum dbh
for all trees 10 cm dbh

Inventory Strip

 

A

B

C

D

E

G

H

I

K

All strips

Mean

cm

23.8

25.1

23.8

23.8

23.3

23.3

23.1

23.7

24.7

23.8

Median

cm

19.1

20.7

18.8

19.1

18.5

18.5

19.7

19.7

20.7

19.4

Maximum

cm

133.7

95.5

114.6

109.8

117.8

117.1

121.0

104.4

111.4

133.7

Standard dev.

cm

15.6

15.5

15.4

14.2

15.2

14.3

12.4

13.6

14.0

14.5

Table 6-3 shows results by diameter class for the study area. The total stocked area of the ten inventory strips was 17.7 ha; the total number of inventoried trees was 8,062. The average density of all trees greater than or equal to 10 cm diameter was 455 per hectare, 15.3 or 3.3% Okoumé. The basal area for all trees 10+ cm was 28.6 m2 per hectare, for Okoumé it was 2.7 m2. The inventory found 46.9 trees per hectare between 40 and 80 cm diameter breast height, 5.1 Okoumé trees/ha (10.8 %). The total number of trees above 80 cm dbh was 3.9 per hectare, 1.2 Okoumé trees/ha (30.1%). The proportion of Okoumé increases considerably in diameter classes above 50 cm.

Table 6-3: Number of trees 10 cm dbh per hectare

Diam. class
(cm)

Okoumé

Others

Total

Okoumé
%

_ Okoumé

_ All Species

10.0-19.9

3.6

233.3

237.0

2

15.3

455.0

20.0-29.9

2.7

113.4

116.1

2

11.7

218.0

30.0-39.9

2.8

48.3

51.1

5

9.0

101.9

40.0-49.9

1.6

20.1

21.7

7

6.3

50.8

50.0-59.9

1.6

11.9

13.5

12

4.7

29.1

60.0-69.9

1.0

6.0

7.0

14

3.0

15.6

70.0-79.9

0.9

3.8

4.7

18

2.1

8.6

80.0-89.9

0.3

1.5

1.8

19

1.2

3.9

90.0-99.9

0.6

0.7

1.3

48

0.9

2.1

100.0-109.9

0.0

0.2

0.2

0

0.3

0.8

110.0-119.9

0.2

0.2

0.4

57

0.3

0.6

120

0.1

0.1

0.2

33

0.1

0.2

Total

15.3

439.6

455.0

     

%

3.3

96.7

100.0

     

Figure 6-1 and Figure 6-2 provide a graphical presentation for the diameter distribution of all species and Okoumé.

Figure 6-3 shows the occurrence of Okoumé in each diameter class. Its proportion increases with increasing diameters. Since the database for this graph is very limited, the results, especially the shares of 48% and 57% in the 90 and 110 cm classes should be taken as indicative only. During harvesting operations almost all Okoumé trees above 80 cm dbh are removed; future harvestable trees are those with diameters between 40 and 80 cm dbh. The share of Okoumé in these classes is considerably lower than in higher diameter classes. If future crop trees become damaged during harvesting operations, the future harvestable volume and thus the commercial value of the forest would be further reduced.

Figure 6-1: Diameter distribution for all trees 10 cm dbh

Figure 6-2: Diameter distribution for Okoumé 10 cm dbh

Figure 6-3: Portion of Okoumé in each diameter class

Volume

Volume equations for forests in South-Congo are not available. Only a rough estimate of the volume of Okoumé trees could be attempted using an equation that was developed for Okoumé in Gabon (Chapter 5, Methodology). Results are presented in Table 6-4.

Table 6-4: Estimated volume for Okoumé

Diam. class

(cm)

Okoumé

Assumed mean diameter
(cm)

Estimated
stem volume
(m3/ha)



(m3/ha)

10.0-19.9

3.6

15.0

0.81

37.53

20.0-29.9

2.7

25.0

1.69

36.72

30.0-39.9

2.8

35.0

3.43

35.03

40.0-49.9

1.6

45.0

5.67

31.60

50.0-59.9

1.6

55.0

4.84

25.93

60.0-69.9

1.0

65.0

4.23

21.09

70.0-79.9

0.9

75.0

5.06

16.86

80.0-89.9

0.3

85.0

2.17

11.80

90.0-99.9

0.6

95.0

5.42

9.63

100.0-109.9

0.0

105.0

0

4.21

110.0-119.9

0.2

115.0

2.65

4.21

120

0.1

125.0

1.56

1.56

TOTAL

15.3

 

37.53

 

The equation used for table Table 6-4 is:

Based on the formula, the total standing volume of Okoumé trees equal or greater than 10 cm dbh would be 37.5 m3 per hectare. The harvestable volume ( 80 cm dbh) based on this equation would be 11.8 m3 per hectare. This volume is gross including defect stems, harvesting losses and stem cut-offs.

Figure 6-4 presents graphically the results of the computation based on the volume equation. The same source reports an average annual diameter increment of 0.8 cm for adult and dominant Okoumé trees. Young Okoumé can show a diameter increment up to 1.5 cm per year.

Figure 6-4: Standing Okoumé volume; estimate based on volume equation

Harvestable trees

All harvestable trees in the study area (111 Okoumé, 5 others) were located and marked on a sketch map. The total number of harvestable trees was 116 with an average of 0.8 trees per hectare. This is less than the expected density of harvestable Okoumé trees derived from the average density in the study inventory, which was 1.2 harvestable Okoumé per hectare. The reason for the difference between the study inventory and the commercial harvest survey is mainly due to the assessment of the stem quality of trees above 80 cm dbh. Not all trees recorded in the inventory are harvestable because of decay and poor stem shape. Some of the difference might be caused by the fact that during the commercial inventory some trees with diameters close to the minimum diameter may have been overlooked or left out in order to ensure that the required minimum diameter limit was met.

Harvesting Performance

Wood production

A detailed wood production record was created for Zones A, B, and C of the study area (Figure 5-2). The total area of Zones A-C amounts to 59.5 ha and 58 trees were harvested in this area. The average harvesting density was 1.0 trees per hectare or 5.8 m3. The total wood volume extracted from Zones A-C was 345.6 m3. All volumes are net log volumes after skidding and crosscutting. Table 6-5 provides summarised results of the wood production survey.

Table 6-5: Wood production in Zones A-C of the study area

Subject

 

Zone A

Zone B

Zone C

Total/
Average

Total area

ha

22.3

21.0

16.2

59.5

Trees felled

 

20

30

8

58

Trees/ha

trees/ha

0.9

1.4

0.5

1.0

Volume harvested*

m3

132.2

170.4

43.0

345.6

Volume harvested/ha*

m3/ha

5.9

8.1

2.7

5.8

* net volume after crosscutting

         

Wood recovery

The wood volume of 96 Okoumé logs from 93 trees harvested in the study area was computed in order to compare it with losses during felling and crosscutting. Felling losses are due to improper felling and topping techniques and to the removal of second quality stem wood. Further losses occur during crosscutting at the landing site. The net volume (under bark after crosscutting) of all 96 logs was 560.1 m3. This is an average of 5.8 m3 per log. The maximum log volume was 11.10 m3; the minimum was 2.63 m3 (Table 6-6).

Table 6-6: Net wood volume under bark for 96 Okoumé veneer logs

Mean

Standard
deviation

Variation
coefficient

Median
(50%/50%)

Minimum

Maximum

m3

m3

%

m3

m3

m3

5.83

1.63

28

5.57

2.63

11.10

The diameter at breast height (dbh) and the log taper for the 96 logs were also recorded. The mean breast height diameter was 91 cm, with a standard deviation of 9.5 cm. The mean log taper was 1.5 cm/m. The dbh was taken at the landing 20 cm above the buttress; the log taper was obtained from crosscut logs and therefore does not include buttresses. Table 6-7 provides results for the average breast height diameter and the log taper.

Table 6-7: Diameter at breast height (dbh) and log taper for 96 Okoumé logs

   

Mean

Standard
deviation

Median
(50%/50%)

DBH

cm

91

9.5

89

Log taper

cm/m

1.53

0.44

1.51

Wood recovery was computed for the felled trees and the crosscut logs (Chapter 4, Methodology and Figure 5-4). The stem volume of the standing tree, including the stump and up to the first branch, is taken as 100%. For comparison of the results with recoveries obtained in other studies, it should be born in mind that the assumptions for the 100% volume differ widely. Figure 6-9 shows results for the volume recovery computations. The average recovery is 86% after felling and 70% after crosscutting, which is done after skidding to the landing. Both recoveries refer to the same stem volume for the standing tree.

The extremely low minimum recovery of 39% in one case (Table 6-8) is due to decay in the centre of the stem. However, the relatively low standard deviation of all measurements indicates a relatively good estimate of the mean volume. This is confirmed by a small difference between mean and median. In the evaluation of the results it should be recognised that neither uprooted nor damaged trees are included in the losses. Only the felled tree volume and losses directly related to topping and crosscutting were computed. The total wood loss is thus considerably higher.

Table 6-8: Recovery after felling (r1 ) and after crosscutting (r2 )

Operation level

 

Mean

%

Standard deviation
%

Median

%

Minimum

%

Maximum

%

Felling

r1

86

9

89

51

95

Crosscutting

r2

70

9

72

39

82

Felling time

Table 6-9 shows the results of the time analysis for felling 60 Okoumé trees. The dbh for all felled trees was equal to or greater than 80 cm.

Usually the cutting time depends on the dbh of the tree and the tree volume. Often other factors, such as saw chain condition or the occurrence of climbers, affect cutting time. The relatively low variance coefficient for this element could be an indicator of the consistency of the chainsaw operator. Time for topping is much more influenced by site difficulties (steep terrain, swamps, brush) and blocked chainsaws, occurring when cutting difficult "hangers" or "bridges".

Table 6-9: Mean felling time for 60 Okoumé trees

   

Recon-naissance

Prepa-ration

Cutting (felling)

Topping (crosscutting)

Mainte-nance

Complete Observations

No of observations

 

55

41

60

58

21

40

Mean time

Min.

4.9

3.1

5.5

4.9

2.2

20.6

% of total time

%

24

15

26

24

11

100

Variation coefficient

%

81

70

35

70

204

36

Figure 6-5 illustrates the average time required for each work element during the felling cycle, expressed in percentage of the total mean felling time. Fifty percent of the felling cycle time was used for maintenance, reconnaissance, and preparation of trees, the other fifty percent for cutting (felling and crosscutting). The time required for the reconnaissance could probably be reduced by using harvesting maps during all operation phases.

Figure 6-5: Time distribution for felling 60 Okoumé trees

Harvesting impacts

The harvesting impact assessment carried out in the case study focuses on impacts that can be measured or estimated using simple instruments and methods. The harvesting impacts surveyed during the field work are: road length and area, felling and skidding damages, and soil disturbance.

Skidtrails, roads and landings

Forestry operations require access to the site, which is usually provided by roads and skidtrails. These elements are the most troublesome features of timber harvesting operations especially with regard to soil erosion and landscape protection. Nevertheless, with traditional ground skidding systems as used in most countries, roads and skidtrails are essential for timber extraction and forest management.

The road and skidtrail survey carried out in the case study concentrates on three aspects of roads: the total length of the road system, the surface area covered by roads, and the degree of soil disturbance from these elements. Quantitative estimates were made for all three of these characteristics.

Skidtrails

Figure 6-6, Figure 6-7, and Figure 6-8 show the skidtrail layout for Zones A-C. Zone A (Figure 6-6) is characterised by a medium harvestable timber density of 5.9 m3 per hectare and relatively easy terrain conditions. The average skidtrail length per cubic metre was only 8.5 m. Three trees were felled in the swampy area. The maximum skidtrail gradient was 30%. All trees were skidded to a temporary landing at the main road. The average skidding distance was 331 m.

Figure 6-6: Skidtrail layout Zone A

Zone B is characterised by the highest timber density in the study area (8.1 m3 per hectare) and easy terrain conditions. Thirty trees were felled in this zone. The measured skidtrail length was 11.1 m per cubic metre. The maximum skidtrail gradient was 30%. The skidtrail layout follows the location of harvestable trees except for one unnecessary connection (Figure 6-7). Logs were skidded directly to the main road. The average skidding distance was 393 m.

Figure 6-7: Skidtrail layout Zone B

Zone C is characterised by the lowest timber density and the most difficult terrain conditions of all three zones. The average harvested volume per hectare was only 2.7 m3. Eight trees were felled in this zone. The skidtrail length was 24.4 m per cubic metre. The maximum skidtrail gradient was 35%. Relatively long sections of the skidtrails were in steep terrain. The logs were skidded directly to the main road. The average skidding distance was 486 m per log.

Figure 6-8: Skidtrail layout Zone C

Table 6-10 displays the skidtrail lengths and the area covered by skidtrails. The average width of skidtrails in the study area was 4 m. Results of the soil disturbance survey are presented in Table 6-13.

Table 6-10: Results of the skidtrail survey in Zones A - C

Subject

 

Zone A

Zone B

Zone C

Total/
Average

Length

m

1,130

1,891

1,049

4,070

Area

ha

0.45

0.76

0.42

1.63

Proportion of area

%

2.0

3.6

2.6

2.7

Density

m/ha

50.7

90.0

64.8

68.4

 

m/tree

56.5

63.0

131.1

70.2

 

m/m3 (*)

8.5

11.1

24.4

11.8

Maximum gradient

%

30

30

35

 

* net volume after crosscutting

The average skidtrail density for Zones A-C was 68.4 m/ha, 70.2 m/tree, and 11.8 m/m3. The maximum skidtrail gradient was 35%. With the difficult terrain conditions, this maximum gradient seems reasonable. The terrain slopes are up to 50%. Assuming an average skidtrail width of 4 m, the area covered by skidtrails is approximately 2.7% of the total area of Zones A-C. Table 6-11 displays skidtrail gradient classes obtained from surveying Zones A-C. All skidding was uphill except for one short skidtrail segment in Zone C.

Table 6-11: Skidtrail gradients in Zones A, B, C

Gradient class

Proportion of
total length
%

0-9%

74

10-19%

21

20-29%

2

30-39%

3

Table 6-12 presents results of the skidding distance survey for each Zone. The average skidding distance is 403 m, with a maximum and minimum of 705 m and 20 m respectively. The extremely low minimum of 20 m is because the main road crosses the study area. The highest average skidding distance occurs in Zone C, which has the lowest timber density. Skidding distance is highly influenced by the location of the harvesting site in relation to the roads and landings. The results of the skidding distance survey are indicative but represent only a small part of the concession.

Table 6-12: Skidding distances in Zones A, B, C

Subject

 

Zone A

Zone B

Zone C

Total/
Average

Average

m

331

393

486

403

Maximum

m

705

625

555

705

Minimum

m

30

20

326

20

Roads

The area actually cleared for roads is presented in Table 6-13. Results from the skidtrail survey and landing estimates are included in the table in order to provide a complete picture of the road network. Since road construction had not been finished in VMA95 by July 1995, planned road information had to be used for the evaluation of primary and secondary roads.

Landings

The total number and size of landings were estimated using the road planning map. For VMA95 (15,550 ha), a total of 34 temporary landings were planned. The average size of a landing is estimated at 80x100 m. The total estimated surface covered by landings is thus 27 ha. This results in an average of 17.5 m2 of landing area per hectare of forest area (Table 6-13) for VMA95.

Table 6-13: Evaluation of the road and landing network for VMA95

 

Total
length

km

Density

m/ha

Average clearing width

m

Area

m2/ha

Proportion of total area

%

Primary road

28

1.8

40

72

0.7

Secondary road

61

3.9

25

98

1.0

Skidtrail

1,063

68.4

4

274

2.7

Landing

   

80x100

17.5

0.2

TOTAL

       

4.6

The road and skidtrail network, including landings, covers approximately 4.6% of the total surface of VMA95 (15,550 ha), approximately 715 ha.

Damages

Felling damages

Felling damages can be crown damage, bark damage, and uprooted or broken trees. Felling damages were recorded on 30 felling sites. The total number of damaged trees on the 30 felling sites was 531; an average of 17.7 damaged trees per felled tree. The range is from 6 to 31. The average computed size of a felling site was 389 m2 and a range from 132 m2 to 703 m2. For this calculation the average tree density derived from the inventory was used.

Twenty of the 531 damaged trees were Okoumé trees. This equals 3.8% and closely represents the average proportion of Okoumé trees, 3.3%. The proportion of damaged Okoumé trees increases in larger diameter classes. The mean diameter of all damaged trees was 26.5 cm while the mean for damaged Okoumé trees is 46.2 cm. The diameter range of damaged trees was from 10.2 to 103.1 cm. Table 6-14 provides summarised results for the felling damage assessment.

Table 6-14: Felling damage on 30 felling sites

Diameter
class
(cm)

All
species

Okoumé

(%)

Uprooted/
broken
(%)

Crown
damage
(%)

Bark
damage
(%)

10.0-19.9

229

0.4

51.1

47.2

6.6

20.0-29.9

151

2.6

44.3

55.0

7.3

30.0-39.9

75

5.3

52.0

46.7

2.7

40.0-49.9

30

10.0

40.0

60.0

6.7

50.0-59.9

19

21.1

31.6

68.4

5.3

60.0-69.9

15

6.7

26.7

73.3

0.0

70.0-79.9

8

12.5

0.0

87.5

12.5

>/= 80.0

4

50.0

0.0

100.0

25.0

TOTAL

531

3.8

46.1

52.5

6.2

The sum of uprooted/broken trees, crown damage, and bark damage in Table 6-14 is slightly more than 100%. This is because trees were sometimes counted twice since they had both crown and bark damage. However, uprooted or broken trees were counted only once. These uprooted or broken trees have been considered as unproductive and without any future commercial value.

Figure 6-9: Diameter distribution for 531 felling damages

The average number of felling damages per felled tree is used to compute the average felling damages per hectare of harvested area. A total of 58 trees were felled in Zones A, B, and C. The average number of felling damages per felled tree was 17.7. Thus the total computed number of felling damages in Zones A, B, and C amounts to 1026. This equals 17.3 felling damages per hectare. The number of felling damages per cubic metre of harvested volume (net after crosscutting) is 3.0.

Skidding damages

Damages to the residual stand along skidtrails occur during skidtrail construction and log skidding. The total length of skidtrail surveyed in Zones A, B, and C was 3,214 m. The total number of damaged trees was 683 (19 Okoumé, 2.8 %). This proportion corresponds reasonably well with the total percentage of Okoumé, which is 3.3%. The average number of skidding damages was 11.5 trees per hectare, which equals an average rate of 2.5% of all trees. The number of trees damaged per kilometre of skidtrail was 212 (5.9 Okoumé). Forty-six per cent of all damaged trees were fully or partly uprooted; the others were bark damaged. The share of damaged Okoumé trees increases significantly in larger diameter classes. The mean diameter of all skidding damaged trees is 22 cm, with a range from 10.2 cm to 89.1 cm. Uprooted trees occur almost exclusively in diameter classes below 40 cm dbh. Usually the skidder operator attempts to avoid big trees in order to speed-up construction work. Table 6-15 shows summarised results of the skidding damage assessment.

Table 6-15: Skidding damage along 3,214 m skidtrails

Diameter
class
(cm)

Total
damaged
trees

Okoumé

(%)

Bark damage
(%)

Uprooted

(%)

10.0-19.9

417

0.7

36.7

63.3

20.0-29.9

152

0.7

72.4

27.6

30.0-39.9

49

8.2

89.8

10.2

40.0-49.9

36

5.6

100.0

0.0

50.0-59.9

15

20.0

93.4

6.6

60.0-69.9

7

28.6

100.0

0.0

70.0-79.9

4

25.0

100.0

0.0

_ 80.0

3

100.0

100.0

0.0

TOTAL

683

2.8

54.3

45.7

Figure 6-10 presents the diameter distribution for skidding damages. This distribution is commensurate with the diameter distribution for all species obtained in the harvesting inventory.

Figure 6-10: Diameter distribution for 683 skidding damages

Summary of skidding and felling damages

The following summaries compare the felling and skidding results. Table 6-16 presents totals for the felling and the skidding damage assessment. The results are based on a relatively small sample area of 58 hectares located in Zones A, B, and C, whereas the total area for harvesting in 1995 was approximately 15,550 hectares. However, the average harvested volume in Zones A, B, and C is 5.8 m3/ha after crosscutting, which is about the average volume per hectare harvested by the enterprise during the past 30 years.

Table 6-16: Summary of felling and skidding damage assessment

Number of damaged trees...

 

Felling

Skidding

Total

...per hectare harvested area

/ha

17.3

11.5

28.8

...per felled tree

/tree

17.7

11.8

29.5

...per m3 harvested volume*

/m3

3.0

2.0

5.0

* net volume after crosscutting

       

Damages to Okoumé trees are considered separately since Okoumé is currently the most important commercial species. During skidding, 19 Okoumé trees, distributed on 59.5 ha, were damaged. Of these Okoumé trees, eight were under 40 cm dbh, eight were from 40 to 80 cm dbh, and three were above 80 cm. The felling survey revealed a total of 20 damaged Okoumé trees on 30 felling sites. Of these Okoumé trees, nine were under 40 cm dbh, nine were in the class 40-80 cm dbh, and two were above 80 cm.

The most important group from a commercial point of view is the 40-80 cm class, since most of these trees will become harvestable in the next 30-40 years. Table 6-17 gives summarised results of Okoumé tree damage. For easier reference, data are presented on a per-hectare basis and as a percentage of the total average occurrence of Okoumé trees.

Table 6-17: Damage frequency for Okoumé

Diameter class

Average Okoumé trees per hectare

Average damaged Okoumé trees per hectare

% Damaged Okoumé trees

cm

 

Felling

Skidding

Total

%

10.0-39.9

9.1

0.29

0.17

0.46

5

40.0-79.9

5.1

0.29

0.17

0.46

9

80.0

1.2

0.13

0.06

0.19

16

TOTAL

15.3

0.71

0.40

1.11

7.3

Figure 6-11: Portion of damaged Okoumé trees in three diameter groups

Approximately 9% of the future Okoumé trees in the diameter class 40-80 cm were damaged during harvesting operations. Although this percentage seems relatively low, it is not only a damage to the biological potential but also to the future commercial value of the forest. The overall damage frequency to Okoumé in all diameter classes is 7.3%; the damage frequency for all species is 6.3%.

Soil disturbance

Regarding the three zones under investigation, the total soil disturbance was: Zone A 5.5%, Zone B 9.2%, and Zone C 4.5%. The highest disturbance value occurs in Zone B, where harvesting intensity is much higher compared to Zone A and Zone C.

In order to estimate the total disturbed surface in VMA95, the areas covered by roads, skidtrails, felling sites, and landings were summed. Table 6-18 shows that approximately 8.4% or 1,306 ha of the total forest area (15,550 ha) was disturbed by these elements. Facilities such as the worker camps, workshops, and the private airport are not included in this estimate. These facilities are used for the exploitation of several annual coupes. Their contribution to the `surface consumption' is therefore relatively low.

Table 6-18: Total disturbed area, in % of the total VMA95 area (15,550 ha)

Disturbance Type

Disturbed area
%

Primary road

0.7

Secondary road

1.0

Skidtrail

2.7

Felling site

3.8

Landing

0.2

TOTAL

8.4

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