4 RARE AND COMMON TROPICAL TREE SPECIES

4.1. Species diversity and rarity

The most outstanding characteristic of a tropical forest is the large number of species and individuals that constitute the tree layer. One single hectare of forest in the Brazilian Amazon may contain 200–300 different tree species. In Mexico, Espinoza (1987) reports that in the late successional stages, up to 75 tree species per hectare are found in these forests. Jackson (1981), cited by Kageyama (1996), studied 227 different tree species in the Atlantic Forest of Brazil. He noted the presence of many taxonomic families, and at times a predominance of some of these. Generally, in the composition of the tropical forest, some species are represented by a large number of individuals, while others occur in very low densities per hectare.

For example, in the experimental site on Barro Colorado Island, Condit et al (1996) listed all the major woody, tree, shrub and vine species, from 10 millimetres in diameter to 1.30 meters in height, in three successive inventories carried out during 1982, 1985 and 1990 in a 50 hectare sample plot, which the Smithsonian Institute of Tropical Investigation operates in Panama. The authors report a total of 313 species with ranges of extreme presence which vary from one up to 40,511 individuals in the 50 hectare plot. The total population was composed of 244,228 individuals, representing an average of 4,885 plants per hectare. 36.74% of the species (115 individuals) are represented by 1 to 50 individuals, that is, from 0.02 to 1.0 individuals per hectare, on average. The data for some of the species is presented below, among which are some Meliaceae of the genera Cedrela, Guarea and Trichilia.

Table 3. RESULTS OF AN INVENTORY IN THE EXPERIMENTAL SITE OF BARRO COLORADO ISLAND, PANAMA. SMITHSONIAN INSTITUTE OF TROPICAL INVESTIGATION (1996)

Condit et al, 1996.

Kageyama, Namkoong and Roberds (1991) indicate that in the Atlantic Forest of Brazil, close to 30% of the tree species present are represented by one, or less than one, adult individual per hectare; these are considered “rare” species. About 30% of the individuals belong to the three most common tree species in the ecosystem.

Kageyama (1996) emphasizes that the rarity of a species can be considered a consequence of evolution in tropical forests, where the interaction between plants and animals has led to the development of a system of defence, rarity being one of the strategies for protection of plants against the predation by animals and attacks by micro-organisms (Janzen, 1970). Another evolutionary strategy is the production of secondary chemical compounds in some of these species (Kriecher, 1990).

Pollination and seed dispersal in rare species are usually done by long distance vectors, supporting the view that “rarity” is a consequence of evolution (Kageyama and Patiño, 1985; Bawa and Ashton, 1991). Gandara (1995) found, by analyzing iso-enzymes, that some rare alleles are found in individuals located at more than 950 meters apart in natural populations of Cedrela fissilis, in Brazil.

It is also interesting that Swietenia and Cedrela species are classified as monoecious (Gandara, 1995) (see also discussion in section 2.3 above). The pollinator of S. mahagony detected by Howard et al (1995) is a very small trips of the genus Thysanoptera. In Brazil, Cedrela fissilis was studied by Crestana (Kageyama, 1996) who found that only trips visited the flowers of this species; in Mexico, Parraguirre and Calix (1996) report the presence of trips visiting the flowers of S. macrophylla, confirming that these two genera seem to have the same, or closely related, pollen vectors.

4.2 Swietenia and Cedrela associated with rarity

According to existing evidence, the Meliaceae species do not follow the regeneration patterns of the majority of common tropical tree species, that is, those that are present at high density per hectare (Patiño and Marin, 1991; Kageyama, 1996). Many of the economically important species, such as Swietenia and Cedrela, are classified as “rare” Gandara (1995) found a density of 34 adult individuals per 270 hectares, or 1 tree every 8 hectares, in a natural primary forest in Brazil. For S. macrophylla, FUNTAC (1990) found 1 tree every 10 hectares (larger than 20 cm in diameter) in Acre, Brazil; Gullison and Hardner (1993) observed a density of 1 tree per 8 hectares in Bolivia; Verissimo et al (1995) notes a density of 1 tree per 4 hectares (larger than 30 cm in diameter) in the Brazilian Amazon; Miller (1940) found 1 tree per hectare in Belize; and Ciferri (1933) 4 trees per hectare in Santo Domingo.

The number of individuals of mahogany found in Mexico, in forest inventories carried out in the commons (Ejidos) in the south of Quintana Roo by the Forest Service, is shown in Table 4 below (Patiño, 1995).

In this regard it should be noted that trees with diameters of less than, or equal to, 15 centimetres were more numerous than those above. These can be incorporated into a class of trees available for future harvests, and indicates that a certain level of harvestable timber will be available also in the future.

In forest inventories carried out in the central and southern parts of the state of Campeche, Mexico, similar figures to those shown for Quintana Roo were obtained in regard to the frequency and abundance of tree species (Patiño, 1996). These results coincide, in general, with those of Patiño (1987).

Table 4. PRESENCE OF MAHOGANY (SWIETENIA MACROPHYLLA KING) IN THE PERMANENT SAMPLE PLOTS IN THE SOUTH OF QUINTANA ROO (DATA BASED ON FOREST INVENTORIES OF OCTOBER, 1990)

(Source: General Inventory Report, Ejidos/Forestry Producers Society of Quintana Roo, S.C.)

Figueroa (1994) reports that in Guatemala, with the exception of Petén, there have been no systematic inventories to determine the abundance of timber-yielding species and, on this subject, citing Lundell (1937), reports probable densities of 15 mahoganies per hectare for this region.

A study of this area in Guatemala (AHG-APESA, 1992; cited by Figueroa, 1994) which covered 3.67 million hectares (30% of the total ground surface of the country) with a 0.47% sampling intensity, indicates that the Department of Petén still has more than 2.6 million hectares of forests, which represents 70% of the total surface of that Department. Of this area, 75% has been only slightly modified, through the selective harvesting of Swietenia and Cedrela species.

Figueroa (1994) indicates that in the forests of the Petén on alluvial and limestone mother rock, the presence of Swietenia macrophylla was 2.25, 2.34 and 2.30 individuals per hectare. Mahogany was the most frequent species in only five of a total of 151 sample plots; the potential total volume estimated for this region of Guatemala is 4.0 million cubic meters of trees of commercial dimensions.

Cedrela odorata presents a lesser number of individuals per hectare than Swietenia macrophylla: in Quintana Roo, Mexico average is 0.65 trees per hectare when considering individuals of 15 cm and up to greater than 85 cm in diameter, while for trees between 45 and 65 cm in diameter, only 0.175 trees per hectare are found. See Table 5 below. This can be compared with numbers of Swietenia macrophylla with diameters of > 50 cm reported in Table 4 above (0.70 individuals per hectare).

Table 5. NUMBER OF INDIVIDUALS OF CEDRELA ODORATA PER HECTARE IN PERMANENT SAMPLE PLOTS IN THE SOUTH OF QUINTANA ROO, MEXICO (DATA FROM INVENTORIES IN OCTOBER 1990)

(Source: General Inventory Report, Ejidos/Forestry Producers Society of Quintana Roo, S.C.)

In secondary forests, species which are “rare” in primary forests present a high density; Gandara (1995) considers this as “abnormal”; and also notes that in many cases trees present bifurcations due to the high intensity of Hypsipyla attack in such forests. This could be considered as evidence of the fact that these species developed special characteristics which support their occurrence in low densities, that is, that “rarity” is the natural state of mahogany species.

This means that, for the utilization of the species in plantations, the special characteristics favouring “rarity” be given due consideration when defining management and plantation techniques. As stressed above the rarity of a species can be an evolutionary factor favouring escape from natural enemies, both insects and micro-organisms.

Kageyama (1996) comments that “rare” species, that is those with a naturally low density of individuals per ha, such as S. macrophylla, C. odorata and many others, generally have high mortality rates of plantlets when naturally regenerated. This is an inherent characteristic of “rare” species.

The populations of “rare” species frequently show an age-class distribution in which individuals in intermediate age-classes are lacking, with only a class of young seedlings and one class of adults (Flores-Negron and Lombardi, 1990). Small to young plants can only be found in rarely occurring openings in the canopy. This condition makes the sustainable management of populations of “rare” species quite difficult, and the prediction of regeneration following selective harvesting problematic (Negreros-Castillo and Mize, 1993).

Based on the above studies, species of the Meliaceae can serve as a model for predicting the behaviour of other “rare” tree species occurring in low density in natural tropical forests. The difficulty in regeneration makes these species difficult to manage, and it is difficult to manipulate age-classes in them. All of these difficulties, together with great pressure on the resources and often unmanaged harvesting, render the genetic resources of Swietenia and Cedrela species very vulnerable. This can be both a cause and a consequence of the lack of consistent information about the management of these species in natural forests. It strongly justifies a coordinated effort as suggested by FAO.