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Logging techniques in the mountains of Jamaica

John Sessions

John Sessions wrote this article at the conclusion of a year's assignment as an FAO logging expert in Jamaica.

The author analyses costs for thinning and clear felling in a subtropical, mountainous Carib pine plantation and compares the use of two pieces of light equipment, one truck-mounted, the other on skids, for skyline logging.

1. UNIMOG/URUS SETUP FOR UPHILL EXTRACTION

Although many developing countries have adopted plantation forestry programmes in recent years the relatively higher and more rapid economic returns of food production, as well as the needs of expanding populations, have often resulted in forestry plantations being relegated to less desirable areas, poor land, steep slopes and mountainsides. Additional reasons why a country may decide to utilize mountainous regions for plantations include, of course, erosion control and watershed protection. It may also be that a country simply consists mostly of mountains: an example is the West Indian island nation of Jamaica, which is 80 percent mountainous.

Forestry in mountains is inherently more costly than on flat lands, especially in terms of stand treatment, and it calls for special logging techniques and equipment. This article attempts to present production data and costs for thinning and clear felling of Carib pine (Pinus caribaea) in mountains as part of the Forestry Development and Watershed Management Project of FAO and the Forest Department of the Government of Jamaica, financed by the United Nations Development Programme. The work described was carried out in good part under the supervision of the author during 1973.

The mountainous areas in Jamaica are typically over 25 degrees in slope, and some areas of existing plantations are on slopes of 40 degrees. In addition, the geology is complicated by clay, shale, disintegrated granite, various conglomerates and rock, often found intermingled over short distances. The terrain is exceedingly irregular, and long, uniform slopes are rare. Topographic conditions, with frequent deep gullies and sharp lateral ridges, generally favour ridge-top road systems. Rainfall in plantation areas varies from 80 to 160 inches per year, with some very heavy rains of short duration.

The silvicultural prescription for Carib pine plantations in Jamaica recommends a thinning during the first 6 to 10 years of age and a clear felling at 18 to 20 years of age. Thinnings would require removal of 200 to 309 stems per acre, with volume removals of 500 to 1000 cubic feet per acre of material 5 to 9 inches diameter at breast height. Final harvest would require clear felling of 200 to 250 stems per acre, with volume removal of 4 000 to 5 000 cubic feet per acre of material averaging 12 to 13 inches in diameter at breast height.

Two cable extraction machines, the Unimog/Urus and the thinnings unit, were introduced to Jamaica by the project for the testing, evaluation in thinning, and final harvest of pine plantations on steep terrain.

The Unimog/Urus is a cable crane, consisting of an Austrian-made Urus winch set and tower mounted on a four-wheel drive Mercedes-Benz Unimog carrier. The Urus winch set has three drums: skyline, mainline and haulback. Various line sizes can be used on the drums; those chosen for Jamaica were 1 500 feet of 9/16-inch line for the skyline and 3/8-inch line for the mainline and haulback. Each line had an independent wire rope core. Skidding distances could be increased to 2 000 feet by adding an extension to the skyline, using quick-coupling wire rope attachments which allow passage of the carriage. Further increases in skidding capability could not be achieved without sacrificing line size on the mainline drum.

Maximum payload for this model of the Unimog/Urus is approximately 1 ton if logs are suspended completely free of the ground, and 1.5 tons if logs are extracted with only one end of the log lifted from the ground. The Unimog/Urus is capable of both uphill and downhill extraction, and the carriage can pass over intermediate supports. The crew usually consisted of six men: a working foreman, an equipment operator, a chaser to unhook logs at the landing, and three choker setters in the field

2. RIGGING DIAGRAM FOR THINNINGS UNIT

The daily cost of the operation, including depreciation allowances for the US$33 000 Unimog/Urus, was approximately $120, with a labour/equipment cost ratio of approximately 40:60.

Rigging time-when no intermediate supports were required-averaged six hours. Using intermediate supports tended to increase the rigging time by approximately 4.5 hours each. For downhill extraction additional rigging was required, and the time averaged an additional 1.1 hours per 100 feet of skyline.

Communication between the Unimog/Urus and the field crew was by radio-telephone.

The thinnings unit was the smaller cable extraction machine. It consisted of a small, 17 hp. two-cylinder two-cycle engine with two-speed transmission and a cable winch mounted on an iron frame with skids. Weight of the unit was about 300 pounds. When moving over short distances, the machine winched itself by mainline. This type of thinnings unit (Figure 2) is limited to uphill extraction, since the carriage must return to the field by the force of gravity.

The line capacity of the thinnings unit was approximately 600 feet of ¼-inch line, with the skyline stored on a separate anchor reel and wound by hand through a double reduction gear for skyline tensioning. The reel had a capacity of 550 feet of 3/8-inch line, and the weight of the skyline reel with cable was 260 pounds.

The carriage was of simple design, relying on a movable block positioned along the skyline to stop it at the desired location; like the Unimog/Urus, it was capable of passing intermediate supports.

3. RIGGING TIME FOR THINNINGS UNIT

Crew size was three to four men: an equipment operator, a chaser, and one or two choker setters.

Cost per day for the operation of the thinnings unit, including depreciation of the $5 000 unit, was estimated at $40, with a labour/equipment cost ratio of approximately 70:30.

Rigging time varied from six to eight hours depending on the length of the skyline. Figure 3 shows rigging time as a function of skyline length.

Communication between the thinnings unit and the crew in the field was by hand signal.

Both the thinnings unit and the Unimog/Urus were used to thin stands from 8 to 11 years old.

Volume production for the thinnings unit varied from 275 cubic feet per day for very short distances to 165 cubic feet per day for skidding distances Of 450 feet. Volume production for the thinnings unit is shown in Figure 4.

All trees were felled by chainsaw, in a herringbone pattern leading to the skyline. Trees were bucked into 7- to 14-foot lengths for extraction. Residual stand damage was minimal.

4. DAILY PRODUCTION FOR THINNINGS UNIT

Extraction costs for the thinnings unit were obtained by adding the rigging costs and skidding costs. The unit costs for rigging and skidding are shown in Figure 5. As an example of costing, the average extraction costs for the thinnings unit on a 400-foot skyline with a removal of 660 cubic feet per acre are estimated thus:

5. RIGGING AND SKIDDING COSTS FOR EXTRACTION BY THINNINGS UNIT

Figure 1

Figure 2

Figure 5 indicates a rigging cost of $0.06 per cubic foot for an average 200-foot skid. Summing costs, the average unit extraction cost for the operation would be $0.25 per cubic foot.

In general, extraction costs for the thinnings unit decreased as span length increased until spans approached 300 feet; after this point extraction costs became relatively constant.

Lateral skidding distances were held to 60 feet. Attempts to skid logs to the skyline from greater distances resulted in increased cycle time, owing to hangups which more than offset the advantages of reduced rigging time.

6. FENCE-POST PRODUCTION BY UNIMOG/URUS

In utilizing the Unimog/Urus two methods were employed for extracting thinnings. In eight-year-old stands of mainly fence-post material and little saw timber, the thinnings were bucked into 6- to 8-foot lengths and manually bunched together along the skyline. When the bunching was completed, the Unimog/Urus was set up and the pieces bundled together for transport up the skyline to the landing. In later thinnings, where most of the material was of sawtimber size (greater than 6.5 inches over bark at the small end), the pieces were skidded to the skyline using conventional cable-crane techniques.

Volume production for manual bunching of fence-post material averaged 90 pieces per man-day for an average skid of 50 feet. The technique used was to place men along the upper side of the skyline at 15-to-25-foot intervals and then to proceed down toward the skyline, pitching logs end over and down the slope. Fence posts were never manually extracted uphill. Felling of trees for manual bunching was always carried out downhill to accumulate as much material as possible near the skyline. Top damage to recoverable products in the eight-year-old stand was low. For Jamaica, the cost of manually bunching fence posts could be estimated at one-tenth of a cent per cubic foot, per foot of average skid.

7. RIGGING TIME FOR UNIMOG/URUS

Volume production by the Unimog/Urus for the forwarding of fence posts along the skyline depended on the allowable payload for the particular cableway. For cableways with allowable payloads fully suspended of 1 500 pounds or more, fence posts were forwarded in bundles of 20 posts. For lower allowable payloads, bundles of 10 or 15 posts were used. Fence-post production is shown in Figure 6. The horizontal curve segments reflect idle machine time at short skidding distances, when the carriage returned to the field before the next load had been completely bundled

Rigging times are given in Figure 7, assuming the rigging of an intermediate support every 400 feet.

8. RIGGING, BUNCHING AND SKIDDING COSTS FOR FENCE-POST EXTRACTION BY UNIMOG/URUS

Rigging and skidding costs are shown in Figure 8 for bundled fence-post material. The rigging costs depend on the amount of material expected to be bunched beneath the skyline, and the greater the bunching distance, the lower the unit rigging cost.

9. VOLUME PRODUCTION FOR UPHILL EXTRACTION BY UNIMOG/URUS

As an example of the costing of fence-post extraction, given a 1 200-foot skyline with an allowable payload of 20 fence posts per load and a maximum bunching distance of 200 feet, the rigging, skidding and bunching costs are determined as follows: Figure 8 indicates a rigging cost of $0.11 and a skidding and bunching cost of $0.24 for an average skid of 600 feet. The total extraction cost would thus be $0.35 per cubic foot.

Residual stand damage was low because lateral skidding took place outside the skyline corridor

Bunching was attempted only in uphill extraction, but downhill extraction experience with the Unimog/Urus indicates that costs will be approximately 25 to 50 percent higher than for uphill extraction.

Volume production for later thinnings in 10- and 11-year old stands are shown in Figures 9 and 10 for uphill and downhill extraction. Loads with the Unimog/Urus in thinnings and clear fellings varied from 10 to over 40 cubic feet, while in THINNINGS the average load volume was about 15 cubic feet. There were two main reasons for this: it was often difficult to choke more than 15 cubic feet of logs in any one location because of the small piece size, and volumes in excess of 15 cubic feet tended to lead to an increase in the frequency of hangups during lateral skidding. Hangups increased cycle time, and this fact often more than offset the gains in volume per cycle. Lateral skidding distances in thinnings averaged 75 feet.

Occasional barking of the lower stems of residual trees occurred during the lateral skidding process. In addition, some trees were pulled over, or the tops broken off, during lateral excursions of the skyline, as logs were pulled to the skyline corridor.

10. VOLUME PRODUCTION FOR DOWN-HILL EXTRACTION BY UNIMOG/URUS

The Unimog was used in the final clear-felling harvest of pine plantations in uphill and downhill extractions. Volume production for clear fellings is shown in Figures 9 and 10. As in thinnings, clear-cutting production is dependent on skidding distance and load size. Load size for final harvests was considerably higher than that for thinnings, owing to the larger piece size; it was limited primarily by the allowable payload, a function of the particular clearances along an individual skyline profile. The average load efficiency was about 75 percent of the allowable payload.

11. RIGGING AND SKIDDING COSTS FOR UPHILL EXTRACTION BY UNIMOG/URUS

Figure 1

Figure 2

Log hangups in clear cutting were less frequent during lateral skidding than in the thinnings operation. Studies conducted indicated that the sum of rigging and skidding costs over an entire unit tended to remain constant for lateral skidding distances of 50 to 100 feet. If lateral skidding distances were limited to less than 50 feet, sharp increases in the total extraction cost occurred.

Rigging and skidding costs are shown in Figures 11 and 12 for uphill and downhill extraction, and these graphs should be interpreted in the same way as those discussed previously. The collection of cost data is an important function of logging studies since it enables a comparison to be made of alternative logging plans.

12. RIGGING AND SKIDDING COSTS FOR DOWNHILL EXTRACTION BY UNIMOG/URUS

Consider the use of cost studies in the following example:

Given a skyline of 1200 feet with a planned final harvest of 3 000 cubic feet per acre and an allowable payload of 1 500 pounds or 25 cubic feet, determine whether the logs should be extracted uphill or downhill. If the logs are extracted downhill, the logging may be done from an existing road at the lower end of the unit. If extraction is to be uphill a spur road must be constructed to the top of the unit, at an estimated cost of $5 000. The total volume to be removed is 100 000 cubic feet, and truck haul costs are assumed to be unaffected by the choice of alternatives.

Evaluation:

Using a 75 percent load efficiency, the average load becomes (0.75) (25 cubic feet) = 19 cubic feet.

For uphill extraction (from Fig. 11):

Rigging cost = 2 cents per cubic foot
Skidding cost¹ = 20 cents per cubic foot
Average unit cost = rigging cost + skidding cost = 22 cents per cubic foot

For downhill extraction (from Fig. 12):

Rigging cost = 4 cents per cubic foot
Skidding cost¹ = 29 cents per cubic foot
Average unit cost = rigging cost + skidding cost = 33 cents per cubic foot

Adding the roading costs

Downhill extraction alternative:

Total logging cost = roading cost + extraction cost = 0 + (100 000)
($0.33) = $33 000

Uphill extraction alternative:

Total logging cost = roading cost + extraction cost = 5 000 + (100 000)
($0.22) = $27 000

¹Average skid 600 feet.

The uphill extraction alternative is more efficient, all other things being equal. Not only are benefits accrued on the initial entry, but on future entries the roading cost will only involve reopening a spur road, rather than new road construction.

Production volumes and some idea of costs have been presented for thinnings and clear fellings under conditions in Jamaica. Similar conditions exist in subtropical plantations on steep terrain in other developing countries, for which this production data may be a guide to equipment selection and management practices.

In thinnings uphill extraction costs in Jamaica were minimized by using the thinnings unit for distances less than 500 feet and the Unimog/Urus for skyline lengths of 500 to 1 500 feet. For early thinnings of small-dimension material by Unimog/Urus, extraction was made more economically by manually bunching the logs prior to forwarding them along the skyline in bundles.

In final harvests, it is suggested that if a significantly large harvesting programme exists (at least 200 acres per year), lower extraction costs could be obtained by equipment specialization. Although the 1-ton model of the Unimog/Urus demonstrated the physical capability of logging pine in final harvests, larger machines of greater load-carrying ability would lower extraction costs.

Finally, the value of logging planning should be stressed. Decisions such as whether to log uphill or downhill should be made only after careful consideration of the costs and benefits.

Additional particulars of the extraction equipment can be obtained from the author, or from the manufacturer, R. Hinteregger, Villach, Austria.

Watershed management in Jamaica where forestry hydrology and farming meet

Hillside farming on steep slopes and erosion from forest and mountain roads constitute major watershed problems in Jamaica. Since 1969, an FAO/UNDP project in the upland regions there has established two demonstration watersheds, conducted five intensive training courses, maintained an experiment in soil loss and runoff, and drawn up a development plan for a 2 600-acre government property.

In the project's demonstration watersheds, the following major conservation treatments for cultivated slopes were carried out:

- Bench terraces, which are a reverse type of terracing particularly suited to humid tropical countries for draining excess runoff. A special type of stepped waterway was introduced where mechanized agriculture was involved.

- Hillside ditches, which are narrow bench ditches. They are easy to maintain and can be used as roads.

- Individual basins, useful where bananas and fruit trees are grown.

- Miniconvertible terraces, well suited for mixed cultivation and may be adjusted to changing future needs.

- Hexagons, a road and terrace system, useful where there is full mechanization of orchards on slopes up to 20°.

By applying these conservation methods, intensive cultivation on slopes up to sometimes 30° was possible. Specification tables for these techniques, including vertical intervals, volumes to be cut and filled, and percent in flat strips, were worked out.

The project introduced a "wattling and staking" method on the fill slopes of mountain and forest roads. Hydroseeding on cut slopes is being tried experimentally

The project also introduced a classification scheme for land capability. For classifying hilly lands, particularly in developing countries where professional staff are scarce, this system has the advantages of being simple, easily understood and practical. The major factors considered in the scheme are slopes and soil depth; stoniness, wetness, gully dissection and floodings are also taken into consideration. The scheme includes soil conservation measures necessary in cultivating sloping lands. If a piece of land can be physically treated by conservation techniques, it can be classified safe for cultivation, otherwise it should be classified as forest land or pasture.

Further details on the above and other activities may be obtained from T. Sheng, Watershed Management Officer, UNDP, P.O. Box 280, Kingston, Jamaica.


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