Seed production and procurement of indigenous species for revegetation programs

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The diversity of species within each ecological group and the density in which they occur in the natural forest should be a guide for the mixed protection plantations, and this has a direct implication for the seed production programmes of these species.

Even though the studies on a tropical forest community in a broader area scale (hundreds of hectares) are still quite insufficient, based on the existing data it may be inferred that the different ecological groups have different patterns of spatial distribution for their individuals, with tendencies to be more common or rare in the population

Among the most common species, those of the initial phase of succession, the pioneer species, as well as those of the final stage of succession, the climax (or tolerant) species, are predominant. It should be stressed that the pioneer species are common in large gaps, whereas the climax species are common in mature forest sites, considering the tropical forest as a mosaic of gaps in different stages of succession. On the other hand, the rare species in the forest include the secondary (or opportunist) species, which have been subject to few studies and little understanding, exactly because they are rare and difficult to be captured in the sampling (KAGEYAMA, NAMKOONG and ROBERDS 1992).

Thus, the pioneer and climax species are found in the forest in a lower number and high density and require a high amount of seed per species and per unit of the area to be planted; the secondary species are found in a high number and occur in a rare form in the forest, requiring a small amount of seed per species.

In this manner, following the patterns that characterize the natural occurence of tree species in tropical forests, it is possible to create mixed protection reforestation models, where the requirement of amount of seed per species and per hectare is minor, and differentiated by group species. For example, in an area of one hectare, 1,000 individuals of pioneer species, represented by a low number of species (I to 5), 300 individuals of secondary species of a higher number of species (20 to 60) and 300 individuals of climax species, belonging to a low number of species (5 to 15), may be planted together, as suggested by KAGEYAMA, SOUZA and VIANA (1992).

This seed production strategy, considering the species jointly, and separated by groups of species with common characteristics, assumes that the commercialization of the seeds as well as of the seedlings, should be effected in the form of a technological package, including the common species with a large amount per species as well as the rare species, with a small amount. The reason for this is that the production of rare species is much more difficult and complex than that of the common species, as discussed below.

Associating the ecological groups of tropical tree species with the technological characteristics of their seeds, KAGEYAMA and VIANA (1991) presented the research priorities for the indigenous species, which are complemented with additional information:

1. The priority for the pioneer tree species is to understand the factors that restrict the germination of their seed; storage is of minor importance due to the natural longevity of the seed; seed production holds the least priority because these species are prolific and bear fruit at an early age during a long period of the year.
2. The secondary tree species presented good storage perspectives of their seed under controlled conditions; seed germination does not present dormancy, they germinate with no difficulty; seed collection for the species comprising this group is quite difficult due to their rarity in natural forests, the height of their trees (many are emergent) and, in addition, flowering is not regular every year.
3. The free species of the climax group presented seed storage problems, especially the recalcitrant ones; many species presented germination problems associated with seed dormancy; seed production for these species is facilitated in view of the fact that they are common in natural forests.

Even though many generalizations were made, it may be inferred that, in general, the major seed production problems relate to the rare secondary species, which is the most numerous group in the tropical forest and, therefore, may also be extrapolated to mixed protection plantations. However, if we consider the need for seed per individual species, the rare secondary group is the one presenting the least amount of seeds.

Considering the example mentioned above for mixed planting in degraded areas, with the utilization of 1,000, 300 and 300 trees per hectare for the groups of pioneer, secondary and climax species, respectively, and an average of 3, 40, and 10 species for the three groups, respectively, one may require 334 seedlings per hectare for each pioneer species, only 8 seedlings per hectare per secondary species and 30 seedlings per hectare for each climax species (Table 2).

Table 2: Necessity of seeds and seedlings for planting of indigenous tree species following the succession.

Source Adapted from WAGED SOUZA and VIANA (1992).

To illustrate how this strategy of seed and seedling production and procurement of indigenous species, involving all of the species to be planted in a unit of area, has very drastic implications, mainly for the rare species, the following real example under our condition may be cited. The Chorisia speciosa, a very rare species in natural forests in the State of São Paulo (I tree in 5 ha) produces about 100 fruit per tree per year, with 150 seed per fruit, or a total of 15,000 seed per tree. Considering a seed utilization rate of 20 %, we would have a production of 3,000 seedlings from the production of one tree, which would be sufficient to plant only about 2 hectares in the form of the traditional pure planting, and about 380 hectares in the form of mixed planting, separating the species into groups according to succession. Obviously, this implies the commercialization of a group of 40 secondary species which would be planted in a certain area.