by
Paulo Kageyama1 & Ademir Reis2
2. Horto Botanico, Universidade Federal de Santa Catarina
Florianopolis, Brazil
SPECIES DIVERSITY AND SECONDARY FORESTS
The Itajai Açu River Valley, which was initially covered by the Atlantic Tropical Forest, was colonized by German and Italian immigrants and due, mostly, to this fact, that region is composed of small farms forming mosaics occupied by agriculture, pastures and secondary forests. The relicts of primary forests are scattered while a large part of the forest ecosystems is of recent formation and composed of secondary vegetation in different successional stages.
These secondary forests are, characteristically, formed by colonizing species which migrated from primary forest nuclei and other “atypical ecosystems”, and which found favourable conditions to establish themselves in areas cleared for agriculture and subsequently abandoned. The sources of such colonizing species may be identified on hill tops, seashores and in wetlands ("atypical ecosystems), or other, remnant forests.
This recently formed secondary vegetation has a constitution which is entirely different from that which existed before, with a mixture of only a few species characteristic of the original ecosystems. The occurrence of species is, to a large extent, the result of chance, reflecting the opportunity which each species had of arriving at a given location of this mosaic. It is noteworthy that the diversity of species in such secondary forests is considerably lower than that of the primary forests and their structure is much simpler, lacking the characteristic strata of these latter ones.
Reduction and Increase of Genetic Variation in Secondary Forests
The manner in which the species migrated from their natural ecosystems and occupied the secondary
sites in an anthropogenic environment may cause the genetic variability of the new populations to
decrease or increase, depending on the colonization process which occurred at each site.
The occupation of an area after deforestation, change in land use and abandonment of the land, may originate from a few individuals of a single species with the consequent narrowing of the genetic base at both inter and intraspecific levels. In this situation there will probably be a decrease in genetic variability as compared with the natural population, and there is a risk of future inbreeding associated with the small genetic population.
On the other hand, colonization of the area may occur through the arrival of a range of individuals from distant and possibly different populations, with subsequent interbreeding and genetic recombination in the new population, thus permitting the variability to be increased. In this situation, populations which were formerly isolated by distance, such as in the case of small populations of “atypical ecosystems” (hill tops, seashores, wetlands), may interbreed; and alleles formerly separated will recombine.
It is noteworthy that in such secondary vegetation, species with small populations, restricted in their natural condition, may become quite widespread due to the lack of competition in the new situation. Such is the case of extensive populations of Dodonaea spp, Rapanaea ferruginea and Miconia cinamomifolia in the Itajai Valley.
Also, rare species may become common in the disturbed environment, bringing about significant changes in their effective population size and, consequently, also in their genetic variation and the structure of their populations. Such is the case of Cedrela fissilis, Jacaratia spinosa, etc.
Alterations such as those described above may thus cause changes in the structure of genetic variation among and within populations. Attention should be paid to this aspect when considering secondary forests for management and/or genetic conservation purposes.
Strategies for Secondary Forest Management
Management of the secondary forests in the Itajai Valley region should guarantee the continuity of the
successional processes as well as an economic return of forest products to small farmers living in, or
close to, the forest. On the other hand, the characteristics of the new ecosystems and the new species
compositions, among other factors, must be clearly understood and considered if these secondary forests
are to be rationally and sustainably managed. Such management should consider genetic conservation
aspects while at the same time attempting to improve the characteristics of the existing forests, so that
the system may also provide goods and services for present-day use.
In this connection, various alternatives for the sustainable management of populations of economically important species already found in given areas, for the enrichment of low-density populations of desirably species, and for the introduction of potentially well-adapted species which are not presently found in the areas in question, will be discussed below.
Succession of forests into later stages is possible also in these anthropogenic ecosystems, provided propagules of species of more advanced successional stages find favourable conditions. This means that favourable conditions relating to soil, luminosity, moisture and interactions with e.g. pollinators and seed dispersers, among others, should occur. Species of advanced successional stages may also be favoured through enrichment planting, or re-introduced within the framework of an overall management plan for a forest area.
The secondary forest species of the Itajai Valley do not have a high economic potential, since the more valuable species of this type of forest occur in more advanced successional stages. However, these secondary forests at the age of some 20 years, usually contain at least one species with economic potential, such as Miconia cinamomifolia which is valued for its timber. This species is found in relatively dense populations, with a potential for selective exploitation and management of its natural regeneration. Other less valuable timber species, medicinal and aromatic plants, and native fruit trees, may also be found among the existing secondary forest species, and may also be the object of rational management.
However, the highest management potential in these ecosystems of secondary vegetation lies in enrichment planting using even more valuable species which are likely to have occurred in the area in earlier times, and subsequent intensive management for high production of wood, timber or other products. Among potential species for such use are Euterpe edulis, Ocotea catharinensis, Nectandra megapotamica, among others, which all show great economic potential.
A possible alternative strategy in the case of management of re-introduced high-value species, is their intensive management in “forest islands”. In this case, a system could be envisaged in which each local small farmer might plant a high density “forest island” inside the natural forest, consisting of one or a few species which may be different from those planted and managed by his neighbour. In this way a mosaic of small forest stands of a range of intensively managed species will be formed inside the larger, natural forest.
GENETIC CONSERVATION STRATEGIES OF SPECIES IN THE ATLANTIC SECONDARY FOREST
In the light of the patterns of genetic variability discussed above, the following alternative genetic conservation strategies may be envisaged for the secondary forests in the areas under review.
For very rare species as, for example, those occurring at a density of one tree each 50 or more hectares, relatively extensive areas are required to ensure the maintenance of a minimum viable populations size (at times 5 000 to 10 000 hectares). As these species are often known to be “nomadic” in character, with changes in their natural ranges occurring over centuries, these species will require a high number of populations to prevent chance extinction. This means that huge areas are required for their conservation, at times 50 000 to 100 000 hectares in extent.
For common species, i.e. those occurring in high densities and having a wide geographic distribution, it is likely that a fairly large number (≥ 20) of smaller reserves (≤ 500 ha) will capture most of the genetic variation of the species. The secondary vegetation could be used for genetic conservation of such species, while at the same time being managed for the provision of socio-economically important goods and services.
For colonizing pioneer species, possible increased genetic variation caused by their “opportunistic” behaviour may be conserved and managed in their new areas of occurrence. In the case of rare species which have increased their distribution range and population density through colonizing new areas, there may be a case for also paying attention to conservation in the secondary vegetation from which they originated.
Species endangered with extinction may be introduced and conserved in the “forest islands” described above. The activities could be complemented by action to enhance genetic variation in these species, e.g. through the planned and coordinated use of progenies from specified mother trees in adjacent, interbreeding “forest islands”.
Finally, the forest areas surrounding the conservation areas should be managed as “buffer conservation zones”, offering the possibility to increase the effective population size in the conservation areas themselves.
Forest Genetic Resources Information no. 21. FAO, Rome (1993)
Manuscript received May 1993
