Pinus tropicalis Morelet (Pino blanco; Pino hembra) is an endemic Cuban conifer, naturally distributed on sandy soils, with a preference for dry areas, in the province of Pinar del Río (Cubas western extremity) and the municipality of Isla de la Juventud (in southeastern Cuba). Trees grow to 20 m in height (Bisse, 1998). Ibáñez, Sosa and Manzanares (unpublished) maintain that the natural distribution of the species within Pinar del Río extends from San Diego de los Baños up to the eastern edge of the peninsula of Guanahacabibes and, in Isla de la Juventud, from its northern edge to the vicinity of the Ciènaga de Lanier in the south. Trees with straight, cylindrical boles may reach heights of 30 m and an aboveground diameter of 30 cm at breast height, with rough bark and deep fissures.
Data on ecologically related variations are reported in Del Risco (unpublished). The results of variations in growth, resin production, needle composition with reference to the use of Pinus needles as a feed supplement for battery chickens, and pulp and wood characteristics have been reported by Blanco et al. (1988); Alvarez et al. (1991); Mesa and Ramírez (1990); Leyva et al. (1990); Guyat et al. (1989); Irulegui and Ramírez (1986); Nacimiento (1979); Ibáñez, Sosa and Manzanares (unpublished) and Manzanares, Velázquez and Martínez (1989). Further data on pests, diseases and plant protection aspects are reported in Hochmut (1972); Echevarría, (1985) and Valdés, Mesa and Martínez (1985).
PHENOLOGY, FLOWERING AND SEED PRODUCTION
After five years of phenological studies of P. tropicalis in Viñales, Pinar del Río (the natural area of distribution), mature trees aged 10 to 20 years exhibited some needle fall, the needles sprouting between March and June. The trees flowered from January to May, with the peak period in late March-early April. Development continued throughout the remainder of the year, with maturity in July-August of the following year, at which time dehiscence and seed release took place. In Tope de Collantes, province of Sancti Spíritus (south-central Cuba), on very deep red ferralitic soils with abundant rocky outcroppings, at 400 m, in a plantation outside the natural area of the species and in mature trees aged 25 to 40 years, slight phenophases occurred. Needle fall and sprouting took place in April, May and August; but flowering only in specific trees from February to May (and not at all in two of the five years of observations). The fruits matured in August of the following year, at which time dehiscence and seed release took place (Hechavarría et al, 1990).
The characteristic seed production was 40 671 ± 272 seeds/kg, distributed as follows: 60.5 % ± 1.0 % healthy seeds, 7.2 % ± 0.4 % empty seeds and 2.3 % ± 1.0 % infected seeds. Seed germination rates were 14.1 % ± 0.7 %, representing a seed efficiency rate of 23.3 %, producing 3 512 ± 414 seedlings for each kg of seed used (Alvarez and Peña, 1980a).
An evaluation of the results of analyses of seed lots used by the forest companies for over a decade brought to light the existence of sharp variations in the above characteristics among the various provenances (Alvarez and Peña, 1980a). Immersing the seeds in water for 48 hours clearly reduced the efficiency of the germinative process, and the reported rate of infected seeds in the analysis was high. In any case, and despite 20 years of systematic research aimed at solving the germination problems of this species, the results achieved have been negative, and it is legitimate to assume that the level of phytopathological protection for these seeds is clearly insufficient (Alvarez and Peña, 1980b). The analysis of seed germination behaviour under different periods of cold storage (5° C ± 2° C) indicated the probable existence of a process of postmaturation, the process increasing steadily from the year of harvest for three more years, dropping off sharply thereafter (Alvarez and Peña, 1980a).
Pita and Bonilla (1999) have indicated a seed moisture content of 12 percent, and Bonilla (2001) maintained that the seed moisture content in four of his provenances varied from a minimum of 10 percent to a maximum of 13 percent. Therefore, in accordance with Navarrete (1980), the increase in respiratory intensity due to these high moisture contents can be considered a possible cause of the low germinative capacity reported for this species. Measuring the extent of seed deterioration due to ageing under environmental conditions and without altering the moisture content, using as an indicator the conductibility of 250 ml of deionized water in which 50 seeds were soaked for 24 hours, Bonilla (2001) reported that 30 days after harvesting the recorded conductibility was 163 m s.cm-1, which increased at nine months to 195 m s.cm-1 and at 24 months to 320 m s.cm-1, with an estimated potential maximum value of 425 m s.cm-1, after submitting the seeds to an autoclave. These results suggest that under these conditions, seeds of this species have already deteriorated by 38 percent one month after harvest. The figure goes up to 46 percent nine months after harvest and as much as 75 percent after 24 months. It is therefore reasonable to expect that post-harvest deterioration from ageing would be complete within 34 to 35 months, and that virtually no seeds would germinate.
Labrada (1973) pointed out that seedlings of this species are more susceptible to attacks of Fusarium spp. as compared to P. caribaea var. caribaea. Meanwhile, Duarte, Alonso and Pérez (1986) reported, in agreement with Alvarez and Peña (1980b), that the presence of saprophytic organisms of the genera Rhizopus Ehrenb (83.7 % affected), Aspergillus Michell ex Fries (2.65 % affected) and Penicilium Link ex Fries (0.08 % affected) on seed surfaces can be considered a factor in poor germination. This may be due to the fact that the external cover of 65 percent of all seeds presented fissures and rough spots, making it easier for spores to lodge on the seed surface, preventing fungicide penetration and allowing the spores to remain latent on the seed surface until such time as conditions favour spore germination. These authors also reported cases of exudation 24-48 hours after sowing, due to the possible presence of bacteria, but found no similar reports of bacteria as Pinus pathogens in the literature consulted.
These defective aspects of P. tropicalis seed germination and the ensuing problems in forest nurseries, as well as its characteristic pattern of herbaceous growth in the first three years after planting followed by an accelerated process of vertical growth sometimes associated with the appearance of fox tail phenotypes, have induced a tendency among forest companies in western Cuba to replace this species by P. caribaea var. caribaea after felling in its natural areas. This has justifiably aroused national and international concern over the risk of genetic erosion of this promising, fast-growing, tropical conifer. So much so that the Eighth Session of the FAO Panel of Experts on Forest Gene Resources included P. tropicalis on the list of species identified as having high global, regional and/or national priority, drawing attention to the need to obtain biological data on the species during the 1994-1999 period (natural distribution, taxonomy, genetics, phenology, etc), and for an ex situ evaluation of its genetic resources through progeny and provenance trials (FAO, 1994). Meanwhile, the International Union for the Conservation of Nature has classified it as an endangered species (IUCN, 1995)
GENETIC RESOURCE STATUS OF PINUS TROPICALIS
The first cycle of forest resource management in Pinar del Río ended in 1983 and was then repeated in 1986-1990. The results of the work showed that in late 1983 there were some 53 512 ha of natural P. tropicalis forest in Cuba, 50 790 ha in the province of Pinar del Río and 2,722 ha in Isla de la Juventud. By the end of 1990, natural national stocks of this species had dropped by 1 056 ha (1.97 percent) to a total of 52 456 ha, with 50 204 ha in Pinar del Río (1.15 per less than in 1983) and 2 252 ha in Isla de la Juventud (a 17.27 percent drop from 1983). As of December 1994, 6 595 ha of P. tropicalis plantations had been established in Cuba, with 6 567.6 ha in Pinar del Río (99.58 percent) and 27.4 ha in Isla de la Juventud (0.42 percent) (Dirección Forestal, 1996). In Pinar del Río in 1999, 121 ha of P. tropicalis were felled and only ten ha planted because very few seedlings were obtained. The data on forest dynamics received by the State Forest Service showed that in 2000 the province of Pinar del Río had 450 000 ha of forested areas, of which 67 773.8 ha were P. tropicalis (Bonilla, 2001), which would mean an increase of 33.4 percent over the 1983 data for this species in the province.
There are two selected seed stands totalling 150 ha in Pinar del Río producing improved P. tropicalis seed. Eleven experimental areas have been established under the management of the Instituto de Investigaciones Forestales for this species, including a seed nursery with first generation saplings (1982), four progeny trials (1972 - 1988), representing 10 geographical origins, and six freely pollinated progeny trials (1973 - 1988) including 70 families (Alvarez, 2000). The genetic improvement programme (recurrent selection) was launched in 1969 under the auspices of Project FAO-Cuba 3. By 1990, 225 plus trees had been selected, of which only five were subsequently lost. Of the 225, 91 were being evaluated and 134 remained for inclusion in the progeny trials. One technical aspect conspiring against progress in this programme so far has been the inability to achieve a satisfactory method of vegetative propagation for the species, even through the use of biotechnology (Alvarez, 1999).
A total of 11 provenances of P. tropicalis are protected, two through seed stands and nine through their representation in provenance trials, including the geographical origins San Simón, Guanito, Bartolo, El Burén, Galalón, Gramales, La Cañada, La Jagua, La Manaja, La Victoria and Mina Dora, all from Pinar del Río (Alvarez et al, unpublished). The genetic resources inherent in the Isla de la Juventud provenance remain for conservation. The provenance trials have indicated the probable existence of a process of adaptation of the geographical origins of this species in accordance with its natural distribution, in that the best provenances have generally proved to be the indigenous origins from each particular place (Fernández et al, 1985).
Concerning the progeny trials, as of five years from planting, sporadic differences have been observed among 25 families and the commercial controls used (seed stands). In any case, an evaluation on most of these trials by Pérez et al 19 years after planting indicated that out of a total of 49 families, twelve percent outperformed the controls by 16.5 percent in overall mean height(1.13 m) and by 17.8 percent in diameter (1.9 cm), an indication of realistic expectations for the improvement programme, which the authors summarize as follows:
From the data on P. tropicalis summarized here, we may conclude that it seems unlikely that either this species or most of its provenances are in any way endangered. Quite the reverse, in fact, as more than two decades of action have tended toward conservation of P. tropicalis genetic resources, on the whole. The only worrying exception to this situation is the natural population of Isla de la Juventud, where natural forest stocks have been systematically and significantly reduced, unaccompanied by a reforestation programme consonant with similar levels of felling. This is an important exception in that the existence of significant geographical barriers makes genetic exchange between this population and the remainder of the species unlikely. Therefore phenomena such as genetic drift, consanguinity or mutations may well have altered the gene frequencies of specific alleles, giving rise to specific differences among these provenances. Nonetheless, this has neither been verified nor specifically evaluated, because this sub-population is not represented in the provenance trials, there being no P. tropicalis seed stand in Isla de la Juventud. Moreover, no plus trees of this origin are among those already selected by the improvement programme, so the sub-population has not been included in the existing progeny trials either.
Most of the data in this paper conform to the recommendations of the FAO Panel of Experts on Forest Genetic Resources (FAO, 1994). The taxonomic classification of P. tropicalis was properly formulated in accordance its botanical identification, and later ratified on a number of occasions. Typological studies of the Pinar formation in Cuba including this species have recently concluded (Del Risco, unpublished). The basic facts on its distribution and phenology are given, together with the results of evaluations of progeny and provenance trials established over a prudent interval, as well as an analysis of its natural areas and the extent of conservation of its gene pool.
In any case, molecular studies of the genetic variability and structure are still needed, as are studies on crossing patterns using molecular markers. These would complement existing knowledge of the P. tropicalis phenotype and the long-term evolutionary history of this species (Eriksson, Namkoong and Roberds, 1995), perhaps helping to explain the marked differences between this and other Cuban pines (P. caribaea var. caribaea, P. cubensis and P. maestrensis), the similarities with Asian pines, why it is considered the most ancient species of the genus in Cuba, and perhaps to formulate a hypothesis on whence, how, and in what manner this species migrated to the island of Cuba.
Other unsolved problems are the vegetative propagation of P. tropicalis, the conservation of the Isla del la Juventud provenance, and providing a satisfactory response to the problem of plant protection and seed germination capacity.
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Further reference: CATIEs Forest Seed Project (PROFESOR) Pinus tropicalis, Morelet. Technical paper No. 124 on Forest Seed Management, Turrialba, Costa Rica., October 2000.