Germination and soil seed bank traits of Podocarpus angustifolius (Podocarpaceae): an endemic tree species from Cuban rain forests
Pablo Ferrandis1, Marta Bonilla2 & Licet del Carmen Osorio3
1. Grupo de Biología de la Conservación de Plantas, Instituto Botánico de la Universidad de Castilla La Mancha, Jardín Botánico de Castilla-La Mancha, Campus Universitario s/n, Albacete 02071, España; pablo.ferrandis@uclm.es 2. Facultad de Forestal y Agronomía, Universidad de Pinar del Río, C/ Martí 270 Final, Pinar del Río, Cuba; mbon@af.upr.edu.cu 3. Estación Territorial de Protección de Plantas, Sancti Spiritus, Cuba.
Podocarpus angustifolius is an endangered recalcitrant-seeded small tree, endemic to mountain rain forests in the central and Pinar del Río regions in Cuba. In this study, the germination patterns of P. angustifolius seeds were ]]>
versus seeds extracted from soil samples) and pretreatment (i.e. seed water-immersion for 48h at room temperature). Germination was delayed for four weeks (≈30 days) in all cases, regardless of both factors analyzed. Moreover, nine additional days were necessary to achieve high germination values (in the case of fresh, pretreated seeds). These results overall may indicate the ]]>
P. angustifolius seeds. The water-immersion significantly enhanced seed germination, probably as a result of the hydration of recalcitrant seeds. Although germination of seeds extracted from soil samples was low, probably due to aging and pathogen effects throughout the time of burial, the study revealed the existence of a persistent soil seed bank (at least short-termed) of ≈42 viable seeds per m2 in the upper 10cm of ]]>
Key words: endangered plant ]]>
Resumen
Podocarpus ]]>
es un árbol endémico de los bosques lluviosos de la región de Pinar del Río y la parte central de Cuba, que se encuentra en peligro de extinción. En este estudio se evaluó la germinación de sus semillas y la naturaleza del banco de semillas del suelo. Específicamente, se analizó la respuesta germinativa de las semillas a dos factores: su procedencia (recolectadas directamente de los árboles versus extraídas de muestras de suelo) y ]]>
2 en los 10 primeros cm del suelo. Dicho hallazgo es destacable, ya que los casos descritos en la literatura sobre bancos persistentes en especies con semillas recalcitrantes son escasos. Además, se discute la importancia que tiene para esta especie amenazada la existencia de un banco de semillas persistente.
Palabras clave: planta en peligro de extinción, letargo morfofisiológico, banco de semillas del suelo persistente, tratamiento pregerminativo, semillas recalcitrantes. Seed germination and seedling ]]>
et al. (1994) stressed the importance of compiling information on the stages in the reproductive cycle of endemic, rare, and threatened plant species in order to enhance our understanding of their scarcity, and to design efficient conservation measures. So, basic information on dormancy and seed germination is very important for the development of conservation strategies of plant species (Ikeda ]]>
et al. 2005).
In the soil, water is the key resource triggering the physiological changes driving seed germination, but temperature and sometimes light/dark conditions also can be important (Nikolaeva 1977, Baskin & Baskin 1998). Many trees in tropical forests produce recalcitrant ]]>
et al. 1989), and if soil water conditions are appropriate, these seeds tend to germinate rapidly after being dispersed (Ng 1978). Typically, the radicle emerges in a few days and germination is completed in a few weeks. Indeed, recalcitrant seeds which fail to germinate usually lose viability promptly (Whitmore 1989, Choinski 1990). However, seeds of some species are able to tolerate a certain degree of drought in the soil (i.e. semi-recalcitrant or minimally recalcitrant seeds). If water content of tissues is reduced, seeds can enter a quiescent stage until humidity in the soil recovers ]]>
et al. 1997).
Innate physiological factors and environmental conditions in the habitat determine the length that seeds remain viable in the soil prior to germination, die by physiological processes, or suffer attack from predators or pathogens (Schafer & Chilcote 1969, Simpson
et al. 1989, Baskin & Baskin 1998). The soil seed bank is formed after viable seeds become buried in the soil and litter, or accumulate on the ground surface (Simpson et al. 1989). The number of seeds in the soil strongly depends on factors such as history, diversity, and the dynamics of the plant community covering the soil (Major & Pyott 1966, Brown & Oosterhuis 1981, Thompson 1992, Vázquez et al. 1997), and seeds are usually abundant in the soil under normal ]]>
et al. 1989).
Soil seed banks of individual species vary significantly in length from transient to persistent types. In a transient seed bank, all seeds disappear from the soil before the next seed dispersal episode occurs, so there will be no seed reserve in the soil for a period of time. In the case of persistent seed banks, a significant fraction of seeds ]]>
et al. 1997). Consequently, the ecological significance of soil seeds banks is transcendental since a persistent seed bank can contribute decisively to the resilience of plant populations (Baskin & Baskin 1978), even though no seeds are produced in a given year, and can even facilitate re-colonization after a local extinction (Milberg 1994). Obviously, such a point becomes particularly important in the case of threatened plant species.
The goals of the present study were to determine the nature of the soil seed bank and germination responses of the endangered Podocarpus angustifolius Griseb., a recalcitrantseeded tree endemic to mountain rain forests in the central and Pinar del Río regions in Cuba. ]]>
P. angustifolius, for which nearly no information is currently available, in order to contribute more effectively to its conservation.
Material and methods
Study area and species: The study was carried out in the Ecological Reserve of Alturas de Banao, located in the Guamuhaya mountain range (Sancti Spiritu municipality, central Cuba). The 6 159ha reserve is managed by the State Department for Protection of Flora and Fauna in Banao. The Reserve has a floral richness of over 1 200 species, of which a fifth is endemic or belongs to a rare, threatened or endangered ]]>
P. angustifolius is found. Mountain rain forests in Cuba range from 800 to 1 800m altitude, where the annual rainfall varies from 1 700 to 3 000mm (Borhidi 1996). The lowest temperatures in the reserve are reached in January and February, when values range from 20.7º to 21.3ºC. During July ]]>
Vegetation is ]]>
Magnolia cubensis Urb., Laplacea angustifolia Britt. et Wils., Laplacea wrightii Griseb, Cyrilla racemiflora L., Ocotea cuneata (Griseb.) Urb., ]]>
Ocotea floribunda (Sw.) Mez, Ocotea leucoxylon (Sw.) Mez and Myrsine coriacea (Sw.) R. Br. ex Roem. & Schult. In the lowest tree layer, tree species such as Clusia tetrastigma vesque, Gomidesia lindeniana Berg., Garrya fadyenii Hook, Miconia puntacta (Desv) D. ]]>
Alchornea latifolia Sw., Matayba domingensis (DC) Radlf., and an arborescent fern, Cyathea arborea Smith., are dominant. Hygrophilous orchids and Bromeliaceae plants mainly form the epiphyte community, and ferns are of secondary importance. In the understory, terrestrial orchids and Melastomataceae shrubs are abundant (Borhidi 1996).
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P. angustifolius belongs to the botanical family of Podocarpaceae (Coniferopsida). It is a small tree which reaches 8m in height and 30cm of normal trunk diameter. Leaves are lineal-lanceolate, 4-5cm wide. Microstrobili are arranged in clusters, whereas the ovules are isolated. The fruit has a red fleshy peduncle holding one 5-6mm diameter elliptic seed. Seeds are recalcitrant, a trait which is well represented in Podocarpaceae (Fountain
et al. 1989, Schaefer 1990). Fruit ripening occurs from January to March and seed dispersal occurs around the middle to end of April. P. angustifolius is endemic to mountains in the Escambray and Pinar del Río regions. Populations have undergone a recent decline due to the abusive use of P. angustifolius wood for the production of ornamental elements and hardwood devices. Recently, P. angustifolius has been classified as is ]]>
Plant nomenclature: Liogier (1962)
Soil seed bank: We selected a stand ]]>
P. angustifolius adult individuals with apparently healthy phenotype. Distance between P. angustifolius tree crowns ranged from 2m to 50cm, so branches from neighboring trees were interlaced in some cases. Eight 30cm x 30cm plots were placed at random in the stand. In early April 2007, just a few days before the beginning of
P. angustifolius seed dispersal, plots were excavated at 10cm depth with a small garden spade and collected individually in labeled polyethylene bags, forming each one a soil sample.
In the laboratory, healthy P. angustifolius seeds (intact seed coat, dark color similar to that when ripened, and resistance to slight pressure with tweezers) were extracted by washing soil samples on a ]]>
Checks for germination started five ]]>
Seeds directly collected from trees: During April 2007, freshly ripened seeds were collected directly from trees, using a ladder when ]]>
Pretreatments of seeds: One hundred seeds selected at random from both ]]>
To analyze the germination response of P. angustifolius seeds, a balanced factorial design including two factors was used. Factors and ]]>
The effects of both factors were evaluated on two dependent variables: (i) final cumulative germination and (ii) rate of germination. The ]]>
where VC is the coefficient of germination rate; SG is the number of seeds germinated; A is the number of germinating seeds between two consecutive times; and T is the time elapsed between the beginning of the germination and the interval determined as the mean.
Two-way ANOVAs were conducted with the statistical package SPSS v. 17 (2008). Prior to analysis, the value of germination (in percentage) was square-root arcsine transformed in order to improve the fit of data to the normal distribution. Significance of interactions was explored by contrasting confidence intervals.
Data of cumulative seed germination were fitted to logistic-model curves with the program Curve Expert 1.3.
Results
Germination response of P. angustifolius seeds: Germination began about 4 weeks after incubation, both in seeds extracted from soil samples and those collected from trees, regardless of the pretreatment (Fig. 1). Final cumulative germination was significantly affected by the two factors analyzed and their interaction (Table 1). Indeed, germination was higher in seeds collected from trees than in those from soil. Differences increased with the water-immersion seed pretreatment (Fig. 1). So, the lowest germination percentage was recorded in seeds extracted from the soil samples that were not pretreated (5%; Fig. 1A), whereas the highest value was achieved by pretreated freshly matured seeds collected from trees (70%; Fig. 1B). In addition, seeds coming from the soil, which were not exposed to water-immersion, suffered the attack of fungus Rhizopus stolonifer Ehrenb. (Ex Fr.) Lind, at the beginning of day 12 of incubation.
Germination rate was significantly affected by the pretreatment. Seed water-immersion accelerated germination. In contrast, seed source (soil ]]>
versus tree) was irrelevant for germination rate, as no significant interaction was detected (Table 1). Maximum cumulative germination (i.e., asymptote of the logisticmodel curve) without (Fig. 1A) and with (Fig. 1B) the germination pretreatment was achieved after 40 and 36-38 days, respectively.
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Soil seed bank: The mean number of P. angustifolius seeds with healthy appearance that were extracted from soil samples was 25.50 (SE=1.67), corresponding to a density (number of seeds per surface unit at a given depth) of c.a. 283 seeds/m2. If such a value is corrected by the maximum percentage of germination recorded in the incubation of seeds ]]>
Fig. 1B), then we can estimate a viable seed reserve of c.a. 42 seeds/m2 in the upper 10cm of soil.
Discussion
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Several findings in the present study strongly suggest that P. angustifolius seeds have non-deep, simple morphophysiological dormancy, according to the classification proposed by Baskin & Baskin (1998). Germination was delayed four weeks in all cases, regardless of seed source (soil seed bank versus direct seed collection from trees) and the pretreatment (water immersion versus non-immersion). Fors (1965) previously detected the same length of delay in seed germination of ]]>
P. angustifolius seeds (Osorio & Bonilla pers. observ.), a seed trait broadly found among members of the Podocarpaceae family (Baskin & Baskin 1998). This rudimentary embryo may need a period of 4 weeks to complete growth and reach the degree of maturation enabling it to germinate (Nikolaeva 1969, Baskin & Baskin 1998). Moreover, the fact that seeds need nine additional days to achieve a high value of germination after it is started may suggest the existence of an additional physiological dormancy mechanism ]]>
P. angustifolius (Baskin & Baskin 1998). Our findings are in accordance with those made by Ng (1978) and Ng & Asri (1979), who described the existence of morphological and morphophysiological dormancy in seeds of other Podocarpus species inhabiting tropical forests in Malaysia. ]]>
A noteworthy result of the study was the fact that seeds extracted from the soil immediately before the next seed dispersal event showed dormancy. Such a finding raises the following question: why did embryos of seeds at least one year old not complete their growth yet? The answer should be examined in the potential factor triggering the embryonic development and thus responsible for overcoming morphological dormancy. The only marked environmental difference to what seeds were ]]>
P. angustifolius seeds should be in an intensely shaded environment due to the dense canopy cover, whereas in the laboratory, seeds were directly exposed to day light during germination tests. In the literature, there are antecedents of direct exposure to light as a promoting factor of embryo growth and the ]]>
P. angustifolius under plant cover when it is highly dense (Bonilla pers. observ.). ]]>
Other notable results in the study were the highly significant promotion of final cumulative germination and the increase of germination rate when seeds were submitted to a water-immersion pretreatment for 48h, particularly in the case of seeds collected from trees. Fors (1965) found a similar germination response. In some recalcitrant seeds, it has been demonstrated that the embryo requires a high level of hydration to germinate, a level superior to that in ]]>
et al. 1973). It could be possible that the water-immersion of seeds prior to germination tests, conducted to quickly surpass the critical level of hydration, accelerated germination once dormancy was lost.
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In spite of the water-immersion pretreatment, seeds extracted from soil samples showed a much lower final cumulative germination than that achieved by freshly matured seeds collected from trees (15% versus 70%). Such a difference could be explained by the loss of viability of the former since they had been in the soil for at least one year. The factors involved in this decay of seed viability would be the attack of pathogens (indeed, a proliferation of fungus was detected during germination tests on seeds extracted from the soil and not washed by ]]>
2) in early April, just before the next seed yield. This finding implies that P. angustifolius is able to form persistent seed banks, at least over the short-term (i.e. a fraction of seeds remain viable in the soil between two consecutive seed dispersal episodes; sensu Thompson & Grime ]]>
P. angustifolius has the ability to form ]]>
Podocarpus henkelii Stapf ex Dallim. & A.B.Jacks. (Farrant et al. 1983); semi-recalcitrant conifer seeds which tolerate a certain degree of desiccation. Such a seed trait, along with the abundance of rainfall in the natural habitat and the preservation of soil humidity because of the dense canopy cover and the well-developed litter layer (Bonilla pers. observ.), may allow some seeds to be hydrated over the ]]>
Consequences of the formation of persistent soil seed banks on the plant population dynamics are important, particularly in the case of threatened species, such as P. angustifolius. A persistent seed bank can be crucial in ensuring the conservation of a species, since it ]]>
et al. (1997) suggested that short-term persistent soil seed banks play an essential role in the viability of populations, as Martínez-Duro et al. (2010) empirically demonstrated for a critically endangered perennial plant species.
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In conclusion, P. angustifolius seems to have seeds with morphophysiological dormancy, which may be overcome when they are directly exposed to light. This may be interpreted as an adaptive strategy to detect small gaps in the canopy where seedlings would have a higher chance of survival and establishment. The promotion of germination by water immersion of seeds induces us to consider a stimulating effect by embryo hydration. In spite of having recalcitrant ]]>
P. angustifolius is able to form persistent seed banks in the soil, probably short-termed but essential for population viability of this endangered plant species.
Acknowledgments
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The authors sincerely thank José M. Herranz and Alejandro Santiago for their helpful comments on a draft version of the manuscript, and three anonymous reviewers whose corrections/ suggestions substantially contributed to enhance the quality of the manuscript. This study has been done in the context of the research projects PEII10-0170-1830 (Consejería de Educación y Ciencia de la Junta de Comunidades de Castilla-La Mancha) and Montes-CSD2008-00040 (Consolider-Ingenio 2010). Kathryn Walsh checked ]]>
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Marta Bonilla. Facultad de Forestal y Agronomía, Universidad de Pinar del Río, C/ Martí 270 Final, Pinar del Río, Cuba; mbon@af.upr.edu.cu Licet del Carmen Osorio. Estación Territorial de Protección de Plantas, Sancti Spiritus, Cuba
Received 18-VIII-2010. Corrected 15-XII-2010. Accepted 20-I-2011.
