Line enrichment experiments using native species of commercial value were established in 1988-1990 in overexploited forests in Misiones, Argentina, on public and private lands. The overall goal of enrichment was to improve the forest composition in terms of both quantity and quality of commercially important species so as to allow harvests from short and medium cutting cycles (15-40 years). The species were planted in forests that had been recently harvested using the minimum diameter cutting system, the prevailing cutting method in the region. In the Misiones forests, generally about 40 m3 ha-1 of commercial trees is extracted using this cutting method, and the tree basal area of the residual forests ranges from 10-15 m2 ha-1. In the region, residual forests with <15 m2 ha-1 are considered overexploited, having relatively slow natural regeneration, and enrichment planting is recommended to accelerate their recovery.
Ten timber species were tested, as well as Euterpe edulis (palmito) which can be harvested after 10-12 years for its heart of palm. Enrichment was carried out in lines cut in the forest with east-west orientation to increase light availability for the planted seedlings. All lines were initially
2 m wide and were expanded to 4 or 6 m in the second or third year after planting to increase light incidence for each species, based on general information on the species' light requirements from other experiences in the region. Seedlings were 1 year old, 40-50 cm tall nursery specimens from pots. Seedlings that died were replaced up to the third year. The enrichment lines were weeded two to three times per year during the first
3 years and once or twice per year thereafter as needed.
Four to 7 years after planting, the timber species with greatest mean height and diameter at breast height (DBH) were Bastardiopsis densiflora, Enterolobium contortisiliquum, Nectandra lanceolata, Ocotea puberula, and Peltophorum dubium. Cordia trichotoma and Balfourodendron riedelianum, both highly valued timber species, could also be recommended for enrichment despite their relatively slow growth. Labor costs associated with establishment and care of enrichment plantings were similar to other reports of enrichment planting for the region. The incorporation of species with shorter harvest age and high economic value such as the palm E. edulis can accelerate and increase investment returns of enrichment plantings (Fig. 6.14).
Because of the relative management complexity of enrichment planting, some authors consider it economically viable only at a small or medium management scale (Ramos and del Amo 1992). The high cost of establishing and maintaining plantings in initial years has been cited as one disadvantage of enrichment techniques (Sips 1993). However, including species of medium and shorter harvest age could improve the economics of this technique, and enrichment planting could play an important role in the recovery of degraded forests (Weaver 1987; Montagnini et al. 1997).
It has also been said that it is difficult to provide optimal light conditions for each species planted within a transect (Weaver 1987) and that introduced species are more exposed to pests than in undisturbed forest (Lamprecht 1990). Most failures can be attributed to the use of species that are not well adapted to this method or to the use of canopy openings that are not conducive to good survival and growth of the species (Weaver 1987, 1993). However, it may be possible to adjust design and management of enrichment lines to avoid some of these problems. For example, light availability can be controlled by changing the width of the transect. Experimentation should include canopy openings of varying sizes, including full open conditions, in order to verify the performances of late-successional species and to develop more fully species selection criteria for enrichment planting. Different topographic positions (valley, ridge) can also influence the successful growth of planted seedlings (Ashton et al. 1997 a). Planting some species of the Meliaceae family, which contains many commercially important species, under a partial forest canopy has also resulted in reduced incidence of the shoot borer (Hypsipyla spp.) which commonly attacks and severely retards the growth of these species when planted in the open (Newton et al. 1993).
It is important to maintain and regulate the width of enrichment lines so that the requirements of each species are met. Control of surrounding vegetation is the most manageable factor in enrichment systems for secondary forests (Sips 1993). Appropriate line width and light incidence requirements of each species can be determined in controlled experiments. For example, in an enrichment trial on 10-year-old secondary forest in Veracruz, Mexico, the best growth for all species tested occurred at a 68% light transmission with respect to exterior conditions (Ramos and del Amo 1992). In the same experiment, survival was highest in open canopy plantings for the heliophilous species (Cordia alliodora and Swietenia macrophylla), while Brosimum alicastrum, a shade-tolerant, primary forest species, had the lowest survival rate (Ramos and del Amo 1992). In enrichment planting experiments with dipterocarp species in logged secondary forests in Indonesia, a 2-m-wide planting line gave the best results after 2 years (Adjers et al. 1995). Line plantings do not have to necessarily be in completely straight lines, but can detour around large trees that are in the way.
For mixed dipterocarp forest of middle elevations in Sri Lanka, Ashton et al. (2001) designed a comprehensive set of guidelines that are suited to the specific type of forest, and consider a set of studied understory and canopy species. The guidelines indicate silvicultural treatments that may be necessary in each case, mostly referring to the size of the canopy openings that are needed for each species, and mode of planting (isolated seedlings or in groups or patches), as well as the economic value of each species.
Management of enrichment plantings can be complemented by tending of natural regeneration within the lines. The weeding necessary for initial establishment and maintenance of the enrichment lines also tends to favor regeneration of native species (Montagnini et al. 1997). If regeneration in the enrichment lines is considered together with planted trees, enrichment planting may become a more economically attractive alternative. Therefore, silvicultur-al treatments should be designed to encourage the establishment and growth of line plantings and at the same time favor natural regeneration, especially of commercially important species (Sips 1993). Once the seedlings are established, the whole forest should be tended throughout, not just along the enrichment lines (Dawkins 1961).
The need for trained personnel and the costs associated with tending may limit the widespread applicability of this approach, especially on a large scale. However, in many regions enrichment planting is a low-risk investment in comparison to other alternatives such as plantation forestry. One way to compensate for the high labor costs of enrichment is to plant species that grow quickly and/or yield highly valued products. In Indonesia, enrichment of depleted dipterocarp forests has become an economically attractive alternative due to increasing lumber prices (Korpelainen et al. 1995). In another example, sensitivity analysis of enrichment plantings in Kalimantan, Indonesia, showed that enrichment of secondary forest with fruit trees, such as Dialium spp., Garcinia spp., and Willughbeia spp., was an economically and ecologically viable alternative (Schulze et al. 1994). In enrichment experiments in the forest of Veracruz, Mexico, Ricker et al. (2000) also showed initial good growth of seedlings of the local fruit trees Pouteria sapota and Diospyros di-gyna with optimal canopy openings of 60 and 55%, respectively.
Another option is to combine timber trees with species that produce an earlier profitable harvest, in order to accelerate returns on investment and to make this technique more economically attractive. In Thailand, tamarind (Ta-marindus indica), which yields a popular fruit, is sometimes interplanted in teak plantations (Jordan et al. 1992). Enrichment adds value to previously logged, low-volume forests by increasing the expected harvestable volume. This added value may prevent their conversion to plantations or other prevailing land uses.
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