|
|
|
|
SILVICULTURAL SURVEY
NO. 1 Introduction In recent years, the Guyana Forestry Commission (GFC) has invested considerable resources to strengthen the regulatory system that applies within the forest sector. This work has been undertaken in response to a new National Forest Policy statement and several international agreements to which Guyana is a signatory. Two important planning guidelines have been published: the Code of Practice for Forest Operations (GFC, 1998) and the Forest Management Plan Guidelines (GFC, 1999). These two documents are intended to help guide the private sector towards implementing sustained yield timber harvesting within Guyana. At the same time, the GFC is strengthening its capability to monitor timber harvesting. Large-scale timber harvesting is regulated through the issue of Timbers Sales Agreements (TSAs) and Wood Cutting Leases (WCLs). At present, the 19 TSAs involved in logging cover an approximate area of 35,800 km2 of State Forests. WCLs, which apply to smaller operations, cover an additional approximate area of 3,900 km2. Although timber harvesting is restricted to a small portion of this area at any one time, the extensive nature of logging within Guyana's forests presents an obvious challenge to effective monitoring. Whilst monitoring inspections need to be carried out whilst logging is taking place, it is the state of the forest stand after logging has finished that defines its potential for further use. At present, this condition is very poorly known for most forested areas that have been logged. Heavy, comprehensive logging would require a long period of time for the stand to recover, whilst a light, selective cut could be repeated in a shorter time. In order to evaluate post-harvest conditions on a concession-by-concession basis a new method called the Silvicultural Survey has been developed (Bird, 2000a) and the first such survey is reported upon here. Objectives of the survey The purpose of the survey was to provide a clear indication of the quantity, quality and timing of the next two timber harvests in the area sampled and to compare these projections with the recently completed harvest. As such it was intended to provide a quantitative demonstration as to whether the logging operator had harvested the forest in a way that was consistent with the National Forest Policy. The three main objectives of the survey were:
Methods Study area Timber Sales Agreement 02/91, held by Demerara Timber Limited (DTL), covers an area of 221,940 ha on the Essequibo-Demerara watershed. The company has divided the licence area into 21 compartments. Four of these compartments accounted for over 60 per cent of timber production during the period 1996-1998, with logging occurring each year in three: Camoudi, Imbo and Wappu (DTL, 2000). The sampling frame for this study was limited to those felling blocks within these three compartments where timber harvesting had been completed over this period. This approach was adopted so that the results would reflect recent logging practice by the company. The compartment, and subsequently the felling block within the compartment, was randomly selected to safeguard against personal bias. This procedure led to the selection of Felling Block 2, an area of approximately 100 hectares, in the Wappu compartment (UTM co-ordinates of the NW corner of the block: 05 75 200 N, 03 12 800 W). Logging is believed to have been completed in this area in 1998. Block 2 occurs at an elevation of 75 to 150 m asl, and exhibits broken relief in the north-west, a flat, rocky plateau in the centre, and three discrete areas of swamp along the southern boundary. Annual rainfall over the general area is estimated at 2,700 mm. Lateritic soils occur throughout the block, with the localised exception of within the swamps. The forest cover over most of the block was classified by FAO (1970) as mixed forest on steep high hills occurring on reddish-brown gravelly clay soils (forest type 1h). This classification was validated by the survey. However, the central portion of the block had been misinterpreted as mixed forest on flat to gently undulating terrain (type 1e) occurring on brown sands. It more accurately corresponds with type 1k: low mixed forest found solely within type 1h, associated with shallow soils where sheet laterite is near or at the surface. Forest types 1h and 1k cover approximately 18 per cent of the TSA licence area (DTL, 2000). Field procedures The field procedures followed were similar to those of the pre-harvest inventory (or stock survey) already established by the GFC. However, certain adaptations were made to cater for the different stand conditions post-harvest and the specific objectives of this survey, as described in Bird (2000b). The field instructions for the survey are reproduced in Appendix 1. One important consideration was the choice of commercial tree species for assessment. A list (Appendix 2) was compiled based upon the company's current listing of sawmill and plywood species, to which were added three species (Burada, Hububalli and Tauroniro) considered to have commercial value. For the 42 species on this list all trees down to a minimum stem diameter of 30 cm dbh were assessed. This minimum size-class was chosen so that estimates of the next two timber harvests could be made. For the purpose of this survey, the sustained yield was therefore defined for each species as the maximum constant number of trees that could be cut over the next two felling cycles. All trees, regardless of species, that attained 60 cm dbh or more were also measured. These trees were assessed to see how large tree density varied at the micro-scale within the felling block and whether this variation could be attributed to the logging operation. Particular attention was paid to searching all tree fall gaps for tree stumps, which were measured and identified on the basis of their bark and stump shape. Site features recorded included all flowing streams, hill slopes considered too steep for ground extraction machinery (i.e. skidders and bulldozers), large rock outcrops and swamps. A 2 % validation check was carried out towards the end of the survey, with all trees within two 1 ha sample units re-measured. The results of this validation exercise appear in Appendix 3. Data analysis Each field sheet (see
Appendix 1) contained a tree location
map of the 1 ha sample unit. These maps were cut out and stuck together to
form a composite map of the entire felling block, which is now being
digitized. The tree data were entered into a spreadsheet for the compilation
of stand tables. Projection of the stand table to estimate future yield was
carried out using the potential crop trees only (i.e. those healthy trees of
good form). Three species groups were defined based upon expected growth
rates. This classification is necessarily imprecise due to the absence of
growth data, but is consistent with what little has been published (e.g. ter
Steege, 1990). One of the following growth rates was applied to each tree: 2,
4, and 6 mm/year (for listing of species growth estimates see Appendix 2).
Annual mortality for all tree species was assumed to be 1.5 % per year over
the projected time period. This estimate was taken from Alder (1995) and is a
general estimate applicable to all trees above 10 cm dbh in tropical moist
forest that has not been recently disturbed. The justification of using this
figure is based upon the observation that logging generally occurs in discrete
clumps and the next timber harvest can be expected to come from areas not
affected by the recently completed logging operation. Mortality was modelled
by removing the required number of trees from each 10-cm size-class per
species, starting with the smallest trees in each size-class. Future timber
harvesting was modelled by removing the largest trees per species until the
planned harvest level was achieved. Results Objective 1. To determine the level of the recently completed harvest Table 1 lists the total number of trees felled, showing the number of tree stumps by species and 10 cm size-class. One hundred and three (103) tree stumps were found within the 100 ha felling block. Wamara and Greenheart accounted for about half of the trees felled. The company's logging intensity was well below the maximum set in the GFC Code of Practice, which at present allows for up to 1,000 trees, including 300 Greenheart trees, to be felled per unit area of 100 ha. Felling was confined to trees above the statutory felling limits of 19 and 34 cm dbh (even allowing for taper between stump and beast height). The one small Wamara tree felled was clearly associated with the felling of an adjacent large Purpleheart tree. The felling of one Clump Wallaba tree is something of an anomaly as this is not a commercial tree species, but it may have been associated with the removal of a nearby Greenheart. Table 1. The number of tree stumps found in Felling Block
2 (100 ha)
The estimated bole volume (over bark) of the 103 trees felled is thought to lie between 500 and 600 m3. Volume estimation is hindered by the lack of locally derived volume equations. One available equation, which was developed for Greenheart on a nearby Brown Sand site (van der Hout, 1999), resulted in a volume estimate of 580 m3, whereas using an average form height factor of ten (Alder, 1999) gave a volume estimate of 480 m3. Clearly, volume estimation at present is imprecise. Objective 2. To characterize the logging operation and its overall impact on the forest Table 2 reviews the standard of the logging operation, in terms of the number of trees felled but left at stump. This is frequently mentioned as an inefficiency associated with poorly planned logging. Six (6) out of the 103 trees felled were found left lying at stump; of these trees only 2 two were found to be solid and hence could be attributed to poor extraction control. There as no sign of any tree left at the log landing and hence 94 % of all trees that were felled were removed from the felling block. Table 2. Trees felled, but left at stump
Table 3 lists for each felled species the number of remaining trees in three utilization categories: without defect & undamaged, without defect & damaged, and with defect. Logging damage was insignificant compared to the large reduction in potential crop trees caused by natural defects. A total of 72% (per cent) of the trees measured were found to be defective, either in terms of poor stem form or apparent decay. The percentage of defective trees was found to vary between species, being highest with Sarebebeballi (94%) and least with Locust (0%). Only 15 trees, which would otherwise have been considered potential crop trees, were found to have visible signs of logging damage. In 14 of these trees damage was confined to the stem, having been caused by contact with the skidder during extraction. Table 3. Residual trees: stem quality and logging damage
The impact of logging was further assessed using three simple measures: the number of 1 ha sample units where tree felling had taken place (and the intensity of logging within these units); the variation in large tree stocking across the felling block; and the length of logging road created. The results of all three measures suggest that the impact of the logging operation was not large, being restricted to less than half of the felling block area. A larger source of variation in tree stocking is caused by the natural variation in forest type, particularly as seen across the central portion of the block, an area of approximately 20 ha. The length of logging road was approximately 1,000 m, and passed through 14 ha of the block. Figures 1 and 2 show the felling intensity and the post-harvest stocking of large trees (>= 60 cm dbh) respectively. The maximum logging intensity was 12 trees/ha and this occurred once. Logging had been carried out at an intensity of 4 trees/ha or more over 9 ha and was recorded at a lower intensity over another 25 hectares. No felling had taken place throughout the remaining 66 hectares. Residual large tree stocking was highest in the broken terrain in the north-west of the block, where it generally exceeded 20 trees/ha and least within the low stature forest in the centre of the block, where it tended to be less than 10 trees/ha. The area corresponding to most logging activity in the eastern portion showed noticeable small-scale variation in stocking, reflecting the pattern of tree fall gaps. Figure 1. Felling intensity
Figure 2. Post-harvest stocking of large trees (>= 60 cm dbh)
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
