Revegetation in degraded areas with indigenous species

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The proposals for revegetation in degraded areas with indigenous tree species in Brazil has attempted to recuperate the high diversity of tropical forests and also to follow the natural regeneration processes of the species in these ecosystems. Thus, a discussion on how the individuals of the hundreds of tree species are distributed in a few hectares of a typical tropical forest seems to be essential, as well as the role of secondary succession in the renewal of these forests.

One of the main characteristics of tropical forests, which has direct implications with their diversities, refers to the high tree species richness of these ecosystems, and that also is correlated with the rarity (less than one individual per hectare) of the co-specific individuals in the forest. It is noteworthy that the high species diversity of tropical forests is given by the rare species and not by the common ones.

Analysing several botanical surveys on forests in the State of Sao Paulo, KAGEYAMA, NAMKOONG and ROBERDS (1992) found that while an average of 30% of the forests were shown to be rare, also about 30% of the individuals belonged to only three of the more common species.

On the other hand, pollinator foraging behaviour may be an evolutionary result of mutual adaptation between plants and pollinators which would be reflected in the tropical forest structure. KAGEYAMA (1989) have suggested that tree species pollinated by short-flying vectors may predominantly be clumped or common, while the species pollinated by animals with long distance flight may be highly dispersed or rare.

It is considered that the rarity or commonness of species results from their evolutionary history and, for that reason, it might be associated with other characteristics such as flight distance of pollinators and seed dispersers, regeneration strategies, herbivory patterns and associations with microorganisms. Thus, the spatial distribution patterns of species in the forest would, in a certain manner, be fixed, and should be followed in the planting schemes of the species in the field.

The concept of mixed reforestation with indigenous species involves criteria on how to associate the different species to be planted. Three criteria proposed for our conditions may be pointed out:

1. the one based on the randomness of the distribution of seedlings of the different species in the field (NOGUEIRA 1977);
2. the criterion based on the distribution indicated by phyto-sociological studies conducted in adjacent natural forests (JOLY 1987); and
3. the one based on the combination of groups of species characteristics of different stages of the secondary succession (KAGEYAMA and CASTRO 1989).

Secondary succession seems to be a concept that could be used to separate species into ecological groups with similar characteristics, especially with regard to the requirement or otherwise of small or large gaps in different development stages of the plants for their utilization in mixed plantings.

According to WHITMORE (1982), the gaps formed in the forest are reoccupied by different ecological groups of tree species adapted to regenerate in gaps of different sizes. Thus, the species could be separated based on their responses to gaps. Depending upon the criterion utilized, such groups are classified in different manners. The dichotomy between pioneer species (intolerant) and climax species (tolerant) is common in the literature. DENSLOW (1980) recognizes three groups: large gap species, small gap species and non gap species. WHITMORE (1975) identifies four groups, and he recognizes that each species is unique in its adaptive characteristics. In addition to those already cited, this author identifies the group of species that develop under closed forest, but benefit from gaps. BUDOWSKI (1965), using 21 characteristics of tropical forests which are modified through the serial stages, identifies four groups of species: pioneer, early secondary, late secondary and climax. For mixed plantings, it is necessary to design systems in which the trees of different tolerance classes have equal opportunities, each in its appropriate niche, resulting in a better utilization of the environment above and below the soil.

Therefore, mixed reforestation should be composed of species of different stages of succession, similar to natural forests, which are composed of a mosaic of successional stages. According to WHITMORE (1982), forests consist of a mosaic of structural phases which are constantly changing as one phase changes into the next, and its floristics composition depends on the frequency and size of gaps. This interpretation shows that the high diversity of tropical forests would be much more a function of the diversity between gaps than within gaps.

Thus, a mixed plantation may be considered as an assemblage of large gaps with different compositions of species providing the high diversity of species in the forest. The common and rare species would be placed in the field in proportion to their natural densities.

The Forest Science Department of ESALQ, University of São Paulo, is carrying out studies with the major objective of establishing models of association of tree species, using secondary succession as a reference. This research programme was consolidated as of 1988 as a cooperation effort between ESALQ and Companhia Energetica do Estado de Sao Paulo-CESP.

For this experimentation BUDOWSKl's (1965) terminology was used with a silvicultural interpretation for the different groups of species, according to the secondary succession. The proposed model considered that the pioneer species gave shade, whereas the climax species grew in the shade, and they complemented each other in the consortium. The early secondary species, in turn, would play the role of tutoring the late secondary. The pioneer and early secondary species would be full sunlight species (large gaps), the first short living (5 to 15 years), with shade giving crown (dense) and the latter longer living (30 to 50 years), with a narrow crown (monopodial growth). The late scondary and climax species would be apt to develop in the shade, where the first would benefit from partial light (small gaps) and the latter grow in the shade of the canopy.

In four conditions of the State of São Paulo 20 species were chosen from the four ecological groups, where all possible combinations of these species were tested. Tests included from pure plots of the species to combinations of the four groups simultaneously, amounting to 23 treatments, with 1,000 m' plots and a total experimental area of 36.8 ha.

The experimental results revealed that the association of tree species, separating them into ecological groups according to secondary succession, is an efficient manner of orientating mixed plantings of indigenous species. The main conclusions drawn from this research up to four years of age, with regard to the different ecological groups of species, were as follows (Table 1):

1. the pioneer and early secondary species grew more than the other two groups, showing similar behaviour in all of the combinations with the other species;
2. the late secondary species benefitted in regard to growth by partial shading of the early secondary species, and were impaired by total shading of the pioneer species;
3. the climax species also benefitted from partial shading of the early secondary species and showed a neutral behaviour when totally shaded by the pioneer species; however, they presented a healthier aspect under shade;
4. pioneer thinning after two years in the field, to simulate small natural gaps, has benefitted the growth of late secondary and climax species;
5. crown size and form of the trees of the different groups of species were deeply changed by the association of the species, especially for the groups of the more advanced stages of succession.

Table 1: Results of consortium of indigenous tree species according to the secondary succession in São Paulo, Brazil.

Pioneer: Croton floribundus. Early secondary: Lonchocarpus spp.. Late secondary: Paratecoma peroba. Climax: Myroxylun peruiferum. Source: Adapted from KAGEYAMA et al. 1992.