Rafael Gouma,2 Jean Marc Bouvet,3 Philippe Vigneron3 and Nicodème Kimbouma2
Forest genetic resource conservation can be carried out in situ (on site, within natural stands) or ex situ (off site). Ex situ stand conservation, i.e., outside a species' natural distribution range, can be contemplated if the introduced species or population plays a significant economic, social or cultural role in the country of introduction. The example of two eucalyptus species (Eucalyptus grandis and E. urophylla) introduced into the Congo illustrates this point neatly.
Industrial eucalyptus plantations represent a considerable economic asset in the Congo: close to 42 000 ha are planted and an estimated 450 000 tonnes of eucalyptus logs are exported every year for a turnover of 5 billion CFA francs. Besides, activities linked to wood harvesting provide around 4.000 jobs in a city of 500.000 inhabitants (Pointe-Noire) and thus have considerable social impact. In order to maintain and develop this industrial potential, Eucalyptus grandis and E. urophylla are utilized as parent species within the framework of an improvement strategy to produce hybrid clones E. urophylla x grandis. This article describes efforts made to prolong the survival of trial plots hosting the most promising species and provenances in order to maintain a broad genetic pool among base collections.
STATE OF AVAILABLE GENETIC RESOURCES
During the 1950s, trial plantations were established in the coastal savannah areas of the region with a view to supplying charcoal and fuelwood to the town of Pointe-Noire and energy for the Congo-Ocean railroad.
Despite unfavourable local conditions (annual average rainfall of 1200 mm but with strong annual fluctuations, and four months of marked dry season) and mineral-poor sandy soils, the eucalyptuses generally performed better than other local and introduced species. Therefore, considerable efforts were quickly made to introduce provenances and progenies of this genus (Brezard, 1982): in total, 62 eucalyptus species were introduced into the Congo. Most plant material was established on two different sites, Pointe-Noire and Loudima.
Few species became really acclimatized and the ecological environment of southern Congo is apparently a marginal zone for eucalyptus growth. On the whole, about thirty species still grow in the area on entire plots or relatively dense stands. This mosaic of tests covers a surface area of about 300 ha, more than two thirds in Pointe-Noire, and can be subdivided into two sub-groupings :
- the first sub-group (Figure 1), representing 95% of the total surface area of eucalyptus plots, groups together adapted species of known value.
- the second subset groups together species that are poorly or badly adapted and which survive on fragmented plots. These species currently offer little value but may constitute valuable genetic resources for future work.
UTILIZATION OF GENETIC RESOURCES
The pure species best adapted to Congo ecological conditions are relatively unproductive (7 to 10 m3/ha/year of maximum growth in volume). Selection in inter-specific hybrid progenies provided hybrids that were both suitable and vigorous. In this way, cross breeding of E. grandis (characterized by vigorous growth, reasonable shape, good adaptation to propagation by cuttings, good technological pulp quality but very badly adapted4 ) with E. urophylla (fairly well adapted, but badly suited to vegetative propagation, average pulp quality and a worse shape than E. grandis) produced E. urophylla x grandis hybrids that combined both vigour and adaptation, multiplication ability and good technological aptitude for pulp production, and which yielded 25 to 40 m3/ha/year in trial plantations. Figures 2 and 3 provide an idea of the growth of some pure species and inter-specific hybrids.
S. (in ha)
Figure 1: Preview of species adapted to the ecological conditions in the Congo and utilized as base material for eucalyptus improvement programmes
Key :Prov. : introduced or tested provenance; Prog. : introduced or tested progeny; S. : surface area. Adaptation and value: * * *: good; * *: means; *: mediocre; (*): provenances or progenies with local races.
SELECTING RESOURCES FOR CONSERVATION
Mainly established about 15 km from Pointe - Noire, the initial stands and tests represent genetic resources of immeasurable value for the continuity of eucalyptus improvement programmes in the Congo. They are particularly valuable since certain provenances can be threatened in their original site or the seed harvests made impossible (as on the island of Timor for E. urophylla). Ex situ conservation plots are thus vital for conserving the specie's genetic diversity and maintaining a vast genetic pool among core populations (Bouvet, 1998). In addition to the adaptation problems experienced among most first generation eucalyptuses, the old stands are subject to considerable man-made attack (illegal felling, ...), underlining the importance of these trees for local people
Technical initiatives have been launched to preserve this genetic heritage. Considerable financial resources are required, meaning that currently only certain plots are affected, including selected stands of E. grandis (planted between 1980 and 1983) and of E. urophylla (planted between 1973 and 1975; see Figure 4). These plots encompass most of the breeding populations participating in the system of recurrent, reciprocal selection (Vigneron, 1991). E. urophylla seeds were obtained partially thanks to cooperation between research institutes from Australia (CSIRO), Indonesia (General Forest Authority), and CTFT-Congo, under the aegis of FAO (Gouma, 1998).
Figure 3: Growth curves of certain hybrids in the Congo : commercial natural hybrid 1-41 (E. alba x unknown), artificial commercial hybrid 18-50 (E. urophylla x grandis), and hybrids under observation: 18-147 (E. urophylla x grandis), 43-28 (E. urophylla x pellita) and 59-10 (E. urophylla x brassiana). Plot R91-3, Kissoko Forest Station, Pointe - Noire (data from A.R Saya).
Figure 4: Information on the origins of E. urophylla and E. grandis introduced into plots 73-1 to 8, 75-3 to 7, 80-28 and 83-03 of Loandijili near Pointe-Noire in the Congo. Key : QLD: Queensland; NQLD: North Queensland; Zimb. : Zimbabwe; RSA: Republic of South Africa.
The conservation method outlined aims at improving the condition of standing volume and at prolonging stand survival. In order to do this, silvicultural treatment is conducted both to suppress competition from woody colonizers and to reinvigorate the stand by supplying fertilizer elements. Prominent signs and panels are installed to raise public awareness in these popular areas.
Clearly, prolonging stand survival will not be enough to conserve these genetic resources indefinitely. These operations are carried out in the Congo in order to keep the plant material alive as long as possible with a view to grafting and in vitro cultivation, and are complemented by seed harvesting.
Selected plots have been maintained differently according to initial plantation density and the type of regrowth present: (i) in the first instance, manual work was carried out in E. urophylla populations with tight spacing, where it is difficult for agricultural equipment to pass. To do this, smaller trees were thinned with a bush knife before larger trees were felled by chain saw, removing the remains in win rows and using chemical destruction with glyphosphate application; (ii) in a second example, it was possible to carry out mechanical maintenance in widely spaced plots (4,5 x m 4,5 m). These works cost an estimated 16 million CFA francs for a surface area of 15,75 ha (average of 1.000.000 F. CFA/ha). The cost has been entirely supported by the industrial plantation operator (Eucalyptus du Congo SA).
After thinning, N.P.K. fertilizer (13-13-21) was scattered in a circle around each of the trees (400 grams/tree) at the beginning of the rainy season to allow the trees to benefit from the growing season. For the time being, fertilizer has been applied to two E. grandis plots and will be extended to the remaining plots identified in line with available financial resources.
RESULTS AND CONCLUSION
Before the rehabilitation work, E. grandis trees in the selected plots were starting to rot and suffered from noticeable gummosis. Silvicultural treatment was beneficial and tangible signs of peak recovery can be observed. Similarly, although fertilisation did not take place, E. urophylla stands reacted well to the thinning work. The most remarkable feature is the activation of dormant buds that have spread numerous shoots along the trunk.
These voluntary initiatives aimed at conserving ex situ valuable genetic resources were undertaken using several straightforward silvicultural operations, which although costly, were motivated by a certain number of considerations. There were two kinds of motivation:
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