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Disentangling plant establishment in sandy coastal systems: biotic and abiotic factors that determine Allagoptera arenaria (Arecaceae) germination

ABSTRACT

Germination rate and establishment success of plants in harsh environments depend on the ability of seeds to withstand unfavorable environmental conditions and avoid predators. Brazilian coastal plains, known as restinga, are subject to environmental factors that seriously limit plant establishment and survival (e.g. salinity, desiccation, oligotrophy, flooding, high temperature and radiation levels). We tested, both in field and laboratory experiments, conditions for germination and establishment of Allagoptera arenaria, a palm tree often found in restinga ecosystems of southeastern Brazil, and which have a principal role in plant community dynamics. Our results showed that the absence of mesocarp, high radiation exposure, and temperature were the main drivers of seed germination. In the field, the highest germination rate was linked to nude seeds buried in open areas. High temperatures and/or predation damaged seeds that remained on the soil surface, especially if they were close to the mother plant and alongside dung piles made by dispersers. Under controlled conditions, seeds exhibited optimum germination at 35 ºC. Therefore, the germination and establishment of A. arenaria depend as much on environmental conditions as on a network of interactions including vertebrates and invertebrates, which allow this species to colonize harsh, open areas in restinga ecosystems.

Keywords
Arecaceae; facilitation; resting; secondary dispersion; seedling

Introduction

The structure of plant populations is intimately linked to recruitment, and seed germination and seedling establishment are periods of high risk in the plant life cycle, which must occur at the right time and under appropriate conditions (Silvertown & Charlesworth 2001Silvertown J, Charlesworth D. 2001. Introduction to plant population biology, 4th. edn. Oxford, Blackwell.; Tobe et al. 2005Tobe K, Zhang L, Omasa K. 2005. Seed germination and seedling emergence of three annual growing on Desert Sand Dunes in China. Annals of Botany 95: 649-659.). Many post-dispersal factors can have profound influence on recruitment (Roberts & Heithaus 1986Roberts JT, Heithaus ER. 1986. Ants rearrange the vertebrate-generated seed shadow of a neotropical fig tree. Ecology 67: 1046-1051.; Schupp 1990Schupp EW. 1990. Annual variation in seed fall, post dispersal predation, and recruitment of a Neo- tropical tree. Ecology 71: 504-515.; Howe 1993Howe HF. 1993. Aspects of variation in a neotropical seed dispersal system. Vegetatio 107/108: 149-162.; Shepherd & Chapman 1998Shepherd VE, Chapman CA. 1998. Dung beetles as secondary seed dispersers: impact on seed predation and germination. Journal of Tropical Ecology 14: 199-215.) including environmental conditions such as temperature, radiation, and water availability that greatly influence the development of the embryonic axis during germination (Carvalho & Nakagawa 2000Carvalho NM, Nakagawa J. 2000. Sementes: ciência, tecnologia e produção. 4th. edn. Jaboticabal, FUNEP.; Vandelook et al. 2008Vandelook F, Moer V, Assche JA. 2008. Environmental signals for seed germination reflect habitat adaptations in four temperate Caryophyllaceae. Functional Ecology 22: 470-478. ; Baskin & Baskin 2014Baskin CC, Baskin JM. 2014. Seeds: ecology, biogeography, and evolution of dormancy and germination. San Diego, San Diego Academic Press.), and seed predation (Janzen 1971Janzen DH. 1971. Seed predation by animals. Annual Review of Ecology, Evolution and Systematics 2: 465- 492. ; Howe & Smallwood 1982Howe HF, Smallwood J. 1982. Ecology of seed dispersal. Annual Review of Ecology and Systematics 13: 201-228.; Schupp 1990Schupp EW. 1990. Annual variation in seed fall, post dispersal predation, and recruitment of a Neo- tropical tree. Ecology 71: 504-515.). Dealing with unfavorable environmental conditions (Hölzel & Otte 2004Hölzel N, Otte A. 2004. Assessing soil seed bank persistence in flood-meadows: the search for reliable traits. Journal of Vegetation Science 15: 93-100.) and escaping predation often involve mutualistic interactions, such as long seed dispersal away from the mother plant (Janzen 1970Janzen DH. 1970. Herbivores and the number of tree species in tropical forests. American Naturalist 104: 501-528.; Wicklow et al. 1984Wicklow DT, Kumvar R, Lloyd JE. 1984. Germination of blue grain seeds buried by dung beetles (Coleoptera: Scarabacidae). Environmental Entomology 13: 878-881.; Estrada & Coates-Estrada 1991Estrada A, Coates-Estrada R. 1991. Howler monkeys (Alouatta palliata), dung beetles (Scarabaeidae) and seed dispersal: ecological interactions in the tropical rain forest of Los Tuxtlas, Mexico. Journal of Tropical Ecology 7: 459-474.; Comita et al 2014Comita LS, Queenborough SA, Murphy SJ, et al. 2014. Testing predictions of the Janzen-Connell hypothesis: a meta-analysis of experimental evidence for distance- and density-dependent seed and seedling survival. Journal of Ecology 102: 845-856. ). This is the case in restinga communities, which occur in sandy plains along the Brazilian coast and show great spatial and temporal fluctuations in water availability (Martin et al. 1993Martin L, Suguio K, Flexor JM. 1993. As flutuações de nível do mar durante o quaternário superior e a evolução geológica de "deltas" brasileiras. Boletim do Instituto de Geologia-USP 15 Publicação Especial:1186). Even during the rainy season, periods of water shortage can occur due to dry spells and the low water retention capacity of sandy soil (Cavalin & de Mattos 2007Cavalin PO, Mattos EA. 2007. Spatio-temporal variation of photosynthetic pigments in CAM tree Clusia hilariana Schlechtendal associated with dry spells during rainy season in south-eastern Brazil. Trees 21: 671-675.; Braz & Mattos 2010Braz MIG, Mattos EA. 2010. Seed dispersal phenology and germination characteristics of a drought-prone vegetation in south-eastern Brazil. Biotropica 42: 327-335.).

The ability to germinate under environmental constraints gives a species the chance to become established in a community (Bolfrey-Arku et al. 2011Bolfrey-Arku GEK, Chauhan BS, Johnson DE. 2011. Seed germination ecology of itchgrass (Rottboellia cochinchinensis). Weed Science 59: 182-187.). In contrast to species unable to stand high temperatures and low water availability (Fialho & Furtado 1993Fialho RF, Furtado ALS. 1993. Germination of Erythroxylum ovalifolium (Erythroxylaceae) seeds within the terrestrial bromeliad Neoregelia cruenta. Biotropica 25: 359-362.; Pinheiro & Borghetti 2003Pinheiro F, Borghetti F. 2003. Light and temperature requirements for germination of seeds of Aechmea nudicaulis (L.) Griesebachand and Streptocalyx floribundus (Martius ex Schultes F.) Mez (Bromeliaceae). Acta Botanica Brasilica 17: 27-35.; Mantovani & Iglesias 2008Mantovani A, Iglesias RR. 2008. Factors limiting seed germination of terrestrial bromeliads in the sandy coastal plains (restinga) of Maricá, Rio de Janeiro, Brazil. Rodriguésia 59: 135-150. ), some restinga species establish in open, sandy sites (Zaluar & Scarano 2000Zaluar HLT, Scarano FR. 2000. Facilitação em restingas de moitas: um século de busca por espécies focais. In: Esteves FA, Lacerda LD. (eds.) Ecologia de restingas e lagoas costeiras. Rio de Janeiro, Nupem-UFRJ. p 3-23.; Scarano 2002Scarano FR. 2002. Structure, function and floristic relationships of plant communities in stressful habitats marginal to the Brazilian Atlantic rain forest. Annals of Botany 90: 517-524.) and often act as facilitators of other plant species (Scarano 2002Scarano FR. 2002. Structure, function and floristic relationships of plant communities in stressful habitats marginal to the Brazilian Atlantic rain forest. Annals of Botany 90: 517-524.; Scarano et al. 2004Scarano FR, Cirne P, Nascimento MT, et al. 2004. Ecologia vegetal: integrando ecossistema, comunidades, populações e organismos. In: Rocha CFD, Esteves FA, Scarano FR. (eds.) Pesquisas de longa duração na Restinga de Jurubatiba: ecologia, história natural e conservação. São Carlos, Editora Rima. p. 77-97.).

The palm tree, Allagoptera arenaria grows in restinga areas in southeastern Brazil (Menezes & Araujo 2000Menezes LFT, Araujo DSD. 2000. Variação da biomassa aérea de Allagoptera arenaria (Gomes) O. Kuntze numa comunidade arbustiva de Palmae na Restinga da Marambaia, RJ. Brazilia Journal of Biology 60: 47-157.), showing high recruitment rates on bare soil, and acting as nucleus of vegetation patches (Zaluar & Scarano 2000Zaluar HLT, Scarano FR. 2000. Facilitação em restingas de moitas: um século de busca por espécies focais. In: Esteves FA, Lacerda LD. (eds.) Ecologia de restingas e lagoas costeiras. Rio de Janeiro, Nupem-UFRJ. p 3-23.). Mature palms modify the environment under their canopy through litter accumulation, increased soil nutrient content, shade, and decreased wind velocity (Menezes & Araujo 1999Menezes LFT, Araujo DSD. 1999. Estrutura de duas formações vegetais do cordão externo da restinga de Marambaia-RJ. Acta Botanica Brasilica 13: 223-235.). Although we have a good understanding of A. arenaria life history and dynamics in restinga environments, there is a lack of knowledge regarding recruitment conditions in exposed restinga soils.

Assuming that seed desiccation and predation are the main post-dispersal causes limiting plant establishment, and that long-distance dispersion from mother plants reduce predation rates (Janzen-Connell effect), we explored mechanisms by which A. arenaria thrives in restinga habitats. Our hypotheses were that establishment does occur through processes that include mitigation of adverse conditions, especially by burial, and protection against predators through a network of positive interactions involving several species of vertebrates and invertebrates.

Materials and methods

Field site and species

Our field site was at the Restinga de Marambaia (23(02’S 43(35’W, sea level), Rio de Janeiro State, where vegetation patches are scattered over a sand matrix with a sparse herbaceous layer (Menezes & Araujo 2005Menezes LFT, Araujo DSD. 2005. Formações vegetais da restinga de Marambaia, Rio de Janeiro. In: Menezes LFT, Peixoto AL, Araujo DSD. (eds.) Historia Natural da Marambaia. Seropédica, EDUR. p. 67-12.). Climate is tropical rainy (Aw in Köeppen classification) with a mean annual temperature of 23.7 ºC, being February the warmest month (mean of 26.8 ºC) and August the coldest (20.9 ºC). Mean annual precipitation is 1,240 mm, 37 % of which falls in summer. Winter months are significantly drier, with only about 15 % of the annual precipitation falling in approximately 21 days (Mattos 2005Mattos CLV. 2005. Caracterização Climática da Restinga da Marambaia. In Menezes LFT, Peixoto AL, Araujo DSD. (eds.) História Natural da Marambaia. Seropédica, Editora da Universidade Federal Rural do Rio de Janeiro. p. 55-66.). Restinga sites show significant spatial variation in environmental conditions, which are buffered in vegetation patches; surface temperature of bare sand can reach 70 oC on a typical summer day (Dias et al. 2005Dias ATC, Zaluar HT, Ganade G, Scarano FR. 2005. Canopy composition influencing plant patch dynamics in Brazilian sandy coastal plain. Journal of Tropical Ecology 21: 343-347.), while maximum soil temperatures beneath vegetation patches remain near 25 oC (Mantovani & Iglesias 2008Mantovani A, Iglesias RR. 2008. Factors limiting seed germination of terrestrial bromeliads in the sandy coastal plains (restinga) of Maricá, Rio de Janeiro, Brazil. Rodriguésia 59: 135-150. ).

Allagoptera arenaria is a palm species about 2 m high which forms dense populations in certain restinga sites (Menezes & Araujo 1999Menezes LFT, Araujo DSD. 1999. Estrutura de duas formações vegetais do cordão externo da restinga de Marambaia-RJ. Acta Botanica Brasilica 13: 223-235.) and has a small distribution range between southern Bahia State and the northern coast of São Paulo State (Lorenzi et al. 2004Lorenzi H, Sousa HM, Costa JTM, Cerqueira LSC, Ferreira E. 2004. Palmeiras Brasileiras: nativas e exóticas cultivadas. Nova Odessa, Instituto Plantarum.). Infructescences contain five to 130 orange to yellowish one-seeded fruits, 12-20 mm long and 10-13 mm in diameter with a sweet fibrous mesocarp and a stony endocarp. The corolla is persistent, covering the fruit. The micropile is lateral and the three germ pores are located slightly basal or lateral. The endosperm is homogeneous and highly rich in lipids (Henderson et al. 1995Henderson A, Galeano G, Bernal R. 1995. Field guide to the palms of the Americas. New Jersey, Princeton University Press.; Moraes 1996Moraes M. 1996. Allagoptera (Palmae). Flora Neotropica 73:1-34.; Henderson 2006Henderson F. 2006. Morphology and anatomy of palm seedlings. Botanical Review 72: 273-329.; Grenha et al. 2010Grenha V, Macedo MV, Pires AS, Monteiro RF. 2010. The role of Cerradomys subflavus (Rodentia, Cricetidae) as seed predator and disperser of the palm Allagoptera arenaria. Mastozoologia Neotropical 17: 61-68.). Fruits are mainly dispersed by the crab-eating fox, Cerdocyon thous, and the raccoon, Procyon cancrivorus (Gatti et al. 2006Gatti A, Bianchi R, Rosa CRX, Mendes SL. 2006. Diet of two sympatric carnivores, Cerdocyon thous and Procyon cancrivorus, in a restinga area of Espirito Santo State, Brazil. Journal of Tropical Ecology 22: 227-230.; Andreazzi et al. 2009Andreazzi CS, Pires AS, Fernandes FAS. 2009. Mamíferos e palmeiras neotropicais: interações em paisagens fragmentadas. Oecologia Brasiliensis 13: 554-574.). As other palm species, A. arenaria fruits are highly predated by Bruchinae beetles (Grenha et al. 2008Grenha V, Macedo MV, Monteiro RF. 2008. Seed predation onAllagoptera arenaria(Gomes) O'Kuntze (Arecaceae) byPachymerus nucleorum Fabricius (Coleoptera, Chrysomelidae, Bruchinae). Revista Basileira de Entomologia 52: 50-56.) and exit holes of Bruchinae species evidencing fruit predation, are easy to recognize at naked eye. The type of germination in A. arenaria seeds is the remote tubular. After germination, the seedling develops a plagiotropous form and becomes upright after a short or long period of growth (Moraes 1996Moraes M. 1996. Allagoptera (Palmae). Flora Neotropica 73:1-34.).

The A. arenaria seeds used in the experiments were collected at the Marambaia restinga in November 2004. Twenty-five thousand fruits were collected, 4 fruits per individual, to guarantee maximum genetic variability in the sample of the tested seeds. The ripe fruits were collected directly from the inflorescences selecting only the yellow staining ones. After harvesting, the fruits were taken to the laboratory and the epicarp and mesocarp were mechanically removed with a knife. A manual selection was performed to discard wilted and damaged seeds, and then the samples were homogenized. Those that did not reach 1.2 cm in width were discarded, since this was the limit found by Leite (1990Leite CO. 1990. Biologia de reprodução de Allagoptera arenaria (Gomes) O. Kuntze (Diplothemium maritimum MART) - Palmae, Rio de Janeiro. MSc. Thesis, Universidade Federal do Rio de Janeiro, Rio de Janeiro.) for aborted and unviable seeds.

Field experiments

Germination rate of A. arenaria was determined under four conditions: either seeds with or without mesocarp were placed on the soil surface or buried 5 cm deep. This burial depth was selected, as temperature 5 cm below the soil surface does not exceed 30 ºC at midday (Scarano 2002Scarano FR. 2002. Structure, function and floristic relationships of plant communities in stressful habitats marginal to the Brazilian Atlantic rain forest. Annals of Botany 90: 517-524.). We used 8 replicates of 25 seeds each, totaling 1600 seeds. Experimental plots were protected by wire mesh to avoid predation and seeds were not surface-sterilized. Buried seeds were extracted every two weeks along a 150-day period to determine germination status; non-germinating seeds were buried again. The germination criterion used was the emergence of the 1 mm-long cotyledonary petiole. At the end of the experiment, seeds that did not germinate were sectioned to look for the presence of Bruchinae larvae.

To test for the Janzen-Connell effect we collected five fresh fruits with mesocarp from under 20 A. arenaria individuals and five seeds from 20 dung piles of dispersers animals selected 50 m far from the nearest A. arenaria individual. Predation criteria were either the presence of Bruchinae larvae or exit holes.

Lab experiments

Four thousand A. arenaria seeds were collected in November 2004 from approximately 800 individuals. Mesocarps were removed manually using a knife, and old seeds and those damaged by insects were discarded; seeds less than 1.2 cm wide were also excluded as non-viable (Leite 1990Leite CO. 1990. Biologia de reprodução de Allagoptera arenaria (Gomes) O. Kuntze (Diplothemium maritimum MART) - Palmae, Rio de Janeiro. MSc. Thesis, Universidade Federal do Rio de Janeiro, Rio de Janeiro.).

To examine the combined effects of temperature and light on germination, seeds were exposed to five treatments at steady temperature (15, 20, 25, 30 and 35 ºC) under constant white light (fluorescent light at 30 µmol m-2 s-1) and dark conditions, using five replicates of 16 seeds each (800 seeds in total). Before the experiment, seeds were surface-sterilized by soaking them three times for 3 min in 5 % sodium hypochlorite solution and then rinsed three times for 3 min with deionized water. Seeds were subsequently sown onto sterilized sand in germination boxes and periodically watered with distilled water. Seeds were checked for germination on a weekly basis for 150 days.

Data analyses

We used t-test and Analysis of Variance (ANOVA), followed by Tukey test to compare mean number of germinated and infested A. arenaria seeds when data conformed to parametric premises either before or after transformation. If data did not conform to parametric premises, we compared mean number of germinated and infested A. arenaria seeds with the randomization test suggested by Pillar & Orlóci (1996Pillar VP, Orlóci L. 1996. On randomization testing in vegetation science: multifactor comparisons of relevé groups. Journal of Vegetation Science 7: 585-592.). Post-hoc tests to evaluate significance of differences between means compared with the randomization procedure were done using contrasts between levels of factors (Pillar & Orlóci 1996Pillar VP, Orlóci L. 1996. On randomization testing in vegetation science: multifactor comparisons of relevé groups. Journal of Vegetation Science 7: 585-592.). Randomization and post-hoc (contrasts) tests were conducted with the MULTIV software package (Pillar 1997Pillar VP. 1997. Multivariate exploratory analysis and randomization testing using Multiv. Colnoses 12: 145-148.). Data are presented as mean ± sd throughout the manuscript. Parametric analyses were performed using the basic interface of the software R v3.0.1 (R Development Core Team 2013R Development Core Team. 2013. R: A language and environment for statistical computing, R Foundation for Statistical Computing, Vienna, Austria. http://www.R-project.org.
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Results

Seed germination in the field

Seed germination of Allagoptera arenaria in the field depended on the presence of mesocarp (Q = 165.8; P = 0.001, Fig. 1) being the germination rate greater when mesocarp was absent than when it was present (3.7 ± 5.5 vs. 0.5 ± 1.0, respectively). Position was also a determinant for A. arenaria germination (Q = 276.4; P = 0.001), being higher when seeds were buried than when they were on the soil surface (4.2 ± 5.2 vs. 0, Fig. 1). Finally, light condition also influenced germination (Q = 118.3; P = 0.001), and seeds fully exposed to sunlight germinated more than those in shade (3.4 ± 5.6 vs. 0.7 ± 1.2). Interaction between factors was significant (Q = 83.3; P = 0.006) and showed A. arenaria germination rate was affected by the interaction between the three factors (presence of mesocarp, light condition and seed position [buried or not]). Optimal conditions for A. arenaria germination would then be buried seeds without mesocarp in areas fully exposed to sunlight (Fig. 1).

Figure 1
Germination rate of Allagoptera arenaria according to habitat, presence of mesocarp and environmental factors. Bars with the same letters are not significantly different (P > 0.05).

Seed predation by bruchinae beetles

The frequency of seeds with Bruchinae larvae depended on the presence of mesocarp (Q =19.1; P = 0.01) since the number of seeds with beetles was greater when mesocarp was present than when it was absent (2.4 ± 2.9 vs 1.3 ± 1.8). When seeds were laying on the soil surface, the presence of Bruchinae larvae were higher (Q = 206.64; P = 0.001) than when seeds were buried (3.7 ± 2.4 vs. 0.1 ± 0.3), and under shade (Q = 15.01; P = 0.007) than in fully exposed sunlight (2.4 ± 2.7 vs. 1.4 ± 2.2). There were significant interactions between seed position (sowing conditions) and light condition (Q = 9.8; P = 0.03) and between sowing conditions and presence of mesocarp (Q = 13.1; P = 0.01). Allagoptera arenaria seeds were more exposed to Bruchinae larvae when laying on the soil surface under shade and with mesocarp (Fig. 2A-B). There were no interactions between seed light condition and presence/absence of mesocarp (Q = 1.3; P = 0.44 - Fig. 2C) nor between all three factors (Q = 0.14; P = 0.87 - Fig. 2D). Additionally, we found that A. arenaria seeds laying on the soil surface next to the mother plant were more predated by beetles than seeds laying in dung piles 50 m apart from a mother plant (t = 13.125, sd= 37.998, p-value < 0.0001).

Figure 2
Mean incidence of Allagoptera arenaria seed predation by Bruchinae beetles under different conditions. First and second order interactions between environment, mesocarp and sowing condition are shown: A. between microhabitat and environmental condition, B. between sowing and mesocarp presence; C. between environment and mesocarp; D. between environment, mesocarp and sowing microhabitat. Bars with the same letters are not significantly different (P> 0.05).

Laboratory seed germination

We found strong effects of mesocarp, light and temperature on A. arenaria germination rate in the lab. Seeds without mesocarp had greater germination rate than seeds with mesocarp (t = -4.7; gl = 8; p-value = 0.001, Fig. 3A); seeds germinated more in light than in the dark (Q = 4.8; P = 0.005, Fig. 3B), and germination rate increased linearly with temperature (Q = 74.4; P = 0.001, Fig. 3C). The interaction between light and temperature was not significant (Q = 2.2; P = 0.398).

Figure 3
Mean number of seeds germinated in lab conditions A. with and without mesocarp, B. in conditions of light and dark; and C. in five temperature conditions. Black dots represent the dispersion of raw data for each treatment of temperature. Bars with the same letters are not significantly different (P > 0.05).

Discussion

As expected, seeds of Allagoptera arenaria strongly depended for germination and establishment on environmental conditions and interactions with dispersers. In both, field and laboratory experiments, the absence of mesocarp, exposure to light, and temperature were critical for seed germination. Field experiments showed that A. arenaria seeds placed directly onto the soil surface had poor germination rates, both in the open and under the canopy. In addition, the presence of mesocarp likewise limited germination. Temperature oscillations may trigger the germination process by breaking the testa in some seeds and in palms could interfere with the process of clearing the germ pore, weakening the pore plate or the operculum (Henderson 2006Henderson F. 2006. Morphology and anatomy of palm seedlings. Botanical Review 72: 273-329.; Baskin & Baskin 2014Baskin CC, Baskin JM. 2014. Seeds: ecology, biogeography, and evolution of dormancy and germination. San Diego, San Diego Academic Press.). Thus, and even though there is greater water availability beneath tree canopies (Pugnaire et al. 1996Pugnaire FI, Haase P, Puigdefábregas J. 1996. Facilitation between higher plant species in a semiarid environment. Ecology 77: 1420-1426.; Zaluar & Scarano 2000Zaluar HLT, Scarano FR. 2000. Facilitação em restingas de moitas: um século de busca por espécies focais. In: Esteves FA, Lacerda LD. (eds.) Ecologia de restingas e lagoas costeiras. Rio de Janeiro, Nupem-UFRJ. p 3-23.) the buffering of temperatures did not favor seed germination. This pattern is typical of pioneer species (sensuSwaine & Whitmore 1988Swaine MD, Whitmore TC. 1988. On the definition of ecological species groups in tropical rain. Plant Ecology 75: 81-86.) such as A. arenaria, which prefer open areas even though seeds would be more exposed to predators (Grenha et al. 2008Grenha V, Macedo MV, Monteiro RF. 2008. Seed predation onAllagoptera arenaria(Gomes) O'Kuntze (Arecaceae) byPachymerus nucleorum Fabricius (Coleoptera, Chrysomelidae, Bruchinae). Revista Basileira de Entomologia 52: 50-56.).

Allagoptera anrenaria, is often a critical component of the restinga community as it contributes to the survival of other species acting as facilitator (Menezes & Araujo 2000Menezes LFT, Araujo DSD. 2000. Variação da biomassa aérea de Allagoptera arenaria (Gomes) O. Kuntze numa comunidade arbustiva de Palmae na Restinga da Marambaia, RJ. Brazilia Journal of Biology 60: 47-157.; Zaluar & Scarano 2000Zaluar HLT, Scarano FR. 2000. Facilitação em restingas de moitas: um século de busca por espécies focais. In: Esteves FA, Lacerda LD. (eds.) Ecologia de restingas e lagoas costeiras. Rio de Janeiro, Nupem-UFRJ. p 3-23.; Scarano 2002Scarano FR. 2002. Structure, function and floristic relationships of plant communities in stressful habitats marginal to the Brazilian Atlantic rain forest. Annals of Botany 90: 517-524.; Scarano et al. 2004Scarano FR, Cirne P, Nascimento MT, et al. 2004. Ecologia vegetal: integrando ecossistema, comunidades, populações e organismos. In: Rocha CFD, Esteves FA, Scarano FR. (eds.) Pesquisas de longa duração na Restinga de Jurubatiba: ecologia, história natural e conservação. São Carlos, Editora Rima. p. 77-97.). Facilitators are positively associated to other species and are instrumental in increasing vegetation complexity over time (Armas & Pugnaire 2005Armas C, Pugnaire FI. 2005. Plant interactions govern population dynamics in a semiarid plant community. Journal of Ecology 93: 978-989.). This palm tree shows high recruitment rates on bare soil, becoming dominant and occupying a central position in vegetation patches that thrive around them, showing a positive spatial association with seedlings of other species (Zaluar & Scarano 2000Zaluar HLT, Scarano FR. 2000. Facilitação em restingas de moitas: um século de busca por espécies focais. In: Esteves FA, Lacerda LD. (eds.) Ecologia de restingas e lagoas costeiras. Rio de Janeiro, Nupem-UFRJ. p 3-23.). Mature individuals act as facilitator species by modifying the environment under their canopy through accumulation of litter, increased soil nutrient content, lowering soil surface and air temperature as well as decreasing wind velocity (Menezes & Araujo 1999Menezes LFT, Araujo DSD. 1999. Estrutura de duas formações vegetais do cordão externo da restinga de Marambaia-RJ. Acta Botanica Brasilica 13: 223-235.).

Our results suggest that a complex sequence of interactions is required for germination success of this restinga species. For instance, the high germination rate of naked A. arenaria seeds buried in open areas is consistent with dispersal by the Scarabaeidae Ateuchus squalidus (Leite 1990Leite CO. 1990. Biologia de reprodução de Allagoptera arenaria (Gomes) O. Kuntze (Diplothemium maritimum MART) - Palmae, Rio de Janeiro. MSc. Thesis, Universidade Federal do Rio de Janeiro, Rio de Janeiro.). These Scarabaeidae use mammalian dung and fruits as food and/or as oviposition sites (Cambefort & Walter 1991Cambefort Y, Walter P. 1991. Dung beetles in tropical forests in Africa. In: Hanski I, Cambefort Y. (ed.) Dung beetle ecology. Princeton, Princeton University Press. p. 198-210.; Hanski 1991Hanski I. 1991. The dung insect community. In: Hanski I, Cambefort Y. (eds.) Dung beetle ecology. New Jersey, Princeton University Press. p. 5-21.; Halffter & Matthews 1966Halffter G, Matthews EG. 1966. The natural history of dung beetles of the subfamily Scarabaeinae (Coleoptera, Scarabaeidae). Folia Entomologica Mexicana 38: 29-107). In A. arenaria seeds this beetle is essential for germination. Beetles remove fruits and bury them ca. 5 cm deep, where they feed on the pulp (Leite 1990Leite CO. 1990. Biologia de reprodução de Allagoptera arenaria (Gomes) O. Kuntze (Diplothemium maritimum MART) - Palmae, Rio de Janeiro. MSc. Thesis, Universidade Federal do Rio de Janeiro, Rio de Janeiro.). Just slightly below the soil surface, temperatures do not exceed 30 ºC, close to the optimal germination range for this species. By contrast, seeds remaining on the soil surface in the open would rapidly be damaged by desiccation and high temperatures, often reaching 70 ºC (Scarano 2002Scarano FR. 2002. Structure, function and floristic relationships of plant communities in stressful habitats marginal to the Brazilian Atlantic rain forest. Annals of Botany 90: 517-524.). Therefore, reproductive success of A. arenaria strongly depends on the feeding behaviour of A. squalidus, an interaction that could be termed as mutualistic. While mutualistic relationships are often difficult to recognize, they are very important in terms of plant population and community structure (Price 1997Price PW. 1997. Insect Ecology. 3rd. edn. NewYork, John Willey & Sons. ).

Our results show that A. arenaria seeds in dung piles away (50 m) were less predated than those near the parent tree, evidencing the Janzen-Connell effect. Species of the Pachymerini beetle tribe (Bruchinae) feed almost exclusively on palm seeds (Johnson et al.1995Johnson CD, Zona S, Nilsson JA. 1995. Bruchid beetles and palm seeds: recorded relationships. Principes 39: 25-35.), being fruit exposure the main cause of bruchinae predation (Silvius & Fragoso 2002Silvius KM, Fragoso JMV. 2002. Pulp handling by vertebrate seed dispersers increases palm seed predation by bruchid beetles in the northern Amazon. Journal of Ecology 90: 1024-1032.). We recorded that mesocarp presence increased seed predation rate. There is a misconception that palm fruits only become predated by Bruchiane beetles after their consumption by frugivores (Wright 1983Wright SJ. 1983. The dispersion of eggs by a bruchid beetle among Scheelea palm seeds and the effect of distance to the parent palm. Ecology 64: 1116-1021.). However, in our case intact mesocarps significantly contributed to greater Bruchinae predation rates.

Predation by invertebrates is density-dependent, so that the greater accumulation of seeds the greater number of beetles (Wilson & Janzen 1972Wilson DE, Janzen DH. 1972. Predation on Scheelea palm seeds by bruchid beetles: seed density and distance from the parent palm. Ecology 53: 954-959.; Shepherd & Chapman 1998Shepherd VE, Chapman CA. 1998. Dung beetles as secondary seed dispersers: impact on seed predation and germination. Journal of Tropical Ecology 14: 199-215.; Galetti et al. 2001Galetti M, Keuroghlian A, Hanada L, Morato MI. 2001. Frugivory and seed dispersal by the lowland tapir (Tapirus terrestris) in southeast Brazil. Biotropica 33: 723-726.). However, Bruchinae beetles generally have low dispersal capacity and move short distances between plants, so that individuals are often restricted to the understory of a palm tree for their whole life (Janzen 1971Janzen DH. 1971. Seed predation by animals. Annual Review of Ecology, Evolution and Systematics 2: 465- 492. ). Frugivores, by contrast, take fruits away from the mother plant, preventing the accumulation of fruits and thus reducing Bruchinae predation. Therefore, dung piles deposited by dispersers somehow are a way of seed protection against Bruchinae while helping dispersal of seeds to significant distances from the mother plant (Fragoso 1997Fragoso JMV. 1997. Tapir-generated seed shadows: Scale- dependent patchiness in the Amazon rain forest. Journal of Ecology 85: 519-529.; Quiroga-Castro & Roldán 2001Quiroga-Castro VD, Roldán IA. 2001. The fate of Attalea phalerata (Palmae) seeds dispersed to a tapir latrine. Biotropica 33: 472-477.; Fragoso et al. 2003Fragoso JMV, Silvius KM, Correa JA. 2003. Long-distance seed dispersal by tapirs increases seed survival and aggregates tropical trees. Ecology 84: 1998-2006.) avoiding competition for resources (Janzen 1970Janzen DH. 1970. Herbivores and the number of tree species in tropical forests. American Naturalist 104: 501-528.; Pimentel & Tabarelli 2004Pimentel DS, Tabarelli M. 2004. Seed dispersal of the palm Attalea oleifera in a remnant of the Brazilian Atlantic Forest. Biotropica 36: 74-84; Rios & Pacheco 2006Rios RS, Pacheco LF. 2006. The effect of dung and dispersal on post dispersal seed predation of Attalea phalerata (Arecaceae) by bruchid beetles. Biotropica 38: 778-781.). Bruchinae predation can reach up to 100 % of exposed seeds in some palm species (Wenny 2000Wenny DG. 2000. Seed Dispersal, Seed Predation, and Seedling Recruitment of a Neotropical Montane Tree. Ecological Monographs 70: 331-351.; Russo & Augspurger 2004Russo SE, Augspurger CK. 2004. Aggregated seed dispersal by spider monkeys limits recruitment to clumped patterns in Virola calophylla. Ecology Letters 7: 1058-1067.) in contrast to low predation rates in dung piles (Quiroga-Castro & Roldán 2001Quiroga-Castro VD, Roldán IA. 2001. The fate of Attalea phalerata (Palmae) seeds dispersed to a tapir latrine. Biotropica 33: 472-477.; Rios & Pacheco 2006Rios RS, Pacheco LF. 2006. The effect of dung and dispersal on post dispersal seed predation of Attalea phalerata (Arecaceae) by bruchid beetles. Biotropica 38: 778-781.). For instance, only 2 % of Attalea maripa seeds in dung piles were predated by Bruchinae beetles as compared to 77 % of seeds lying under parent palm trees (Fragoso 1997Fragoso JMV. 1997. Tapir-generated seed shadows: Scale- dependent patchiness in the Amazon rain forest. Journal of Ecology 85: 519-529.); and only 6 % of Attalea phalerata seeds in mammalian dung piles were predated, against 61 % of seeds under parent trees (Quiroga-Castro & Roldán 2001Quiroga-Castro VD, Roldán IA. 2001. The fate of Attalea phalerata (Palmae) seeds dispersed to a tapir latrine. Biotropica 33: 472-477.).

Long-distance dispersal of A. arenaria seeds in restinga environments depends on two vertebrates, Cerdocyon thous and Procyon cancrivorus, in which more than 80 % of scats contain palm seeds (Gatti et al. 2006Gatti A, Bianchi R, Rosa CRX, Mendes SL. 2006. Diet of two sympatric carnivores, Cerdocyon thous and Procyon cancrivorus, in a restinga area of Espirito Santo State, Brazil. Journal of Tropical Ecology 22: 227-230.; Andreazzi et al. 2009Andreazzi CS, Pires AS, Fernandes FAS. 2009. Mamíferos e palmeiras neotropicais: interações em paisagens fragmentadas. Oecologia Brasiliensis 13: 554-574.). They are also responsible for the clumped distribution of A. arenaria. Dung beetles use both fruits and mammalian dung as fodder and oviposition sites (Cambefort & Walter 1991Cambefort Y, Walter P. 1991. Dung beetles in tropical forests in Africa. In: Hanski I, Cambefort Y. (ed.) Dung beetle ecology. Princeton, Princeton University Press. p. 198-210.; Hanski 1991Hanski I. 1991. The dung insect community. In: Hanski I, Cambefort Y. (eds.) Dung beetle ecology. New Jersey, Princeton University Press. p. 5-21.; Halffter & Matthews 1966Halffter G, Matthews EG. 1966. The natural history of dung beetles of the subfamily Scarabaeinae (Coleoptera, Scarabaeidae). Folia Entomologica Mexicana 38: 29-107), acting as secondary dispersers and positively affecting seed survival by preventing predation (Wicklow et al. 1984Wicklow DT, Kumvar R, Lloyd JE. 1984. Germination of blue grain seeds buried by dung beetles (Coleoptera: Scarabacidae). Environmental Entomology 13: 878-881.; Estrada & Coates-Estrada 1991Estrada A, Coates-Estrada R. 1991. Howler monkeys (Alouatta palliata), dung beetles (Scarabaeidae) and seed dispersal: ecological interactions in the tropical rain forest of Los Tuxtlas, Mexico. Journal of Tropical Ecology 7: 459-474.; 1996Estrada A, Coates-Estrada R. 1996. Tropical rain forest fragmentation and wild populations of primates at Los Tuxtlas, Mexico. International Journal of Primatology 17: 759-784.). The minimum temperature for A. arenaria germination was 20 ºC (2.7 %), with the greatest germination rate at 35 ºC (40 %), unusually high compared to other tropical species (Mello & Barbedo 2007Mello JIO, Barbedo CJ. 2007. Temperatura, luz e substrato para a germinação de sementes de pau-brasil Caesalpinia echinata Lam., Leguminosae-Caesalpiniodeae. Revista Árvore 31: 645-655.; Cardoso & Pereira 2009Cardoso VJM, Pereira FJM. 2009. Dependência térmica da germinação de sementes de Drymaria cordata (L.) Willd. ex Roem. & Schult. (Cariophyllaceae). Acta Botanica Brasilica 23: 305-312.; Pires et al. 2009Pires LA, Cardoso VJM, Joly VA, Rodrigues RR. 2009. Germinação de Ternstroemia brasiliensis Cambess. (Pentaphylacaceae) de Floresta de Restinga. Acta Botanica Brasilica 23: 57-66. 2009.; Pimenta et al. 2010Pimenta RS, Luz PB, Pivetta KL, Castro A, Pizetta PUC. 2010. Efeito da maturação e temperatura na germinação de sementes de Phoenix canariensis Hort. ex Chabaud - Arecaceae. Revista Árvore 34: 31-38. ). Optimum temperature for germination corresponds to temperatures usually experienced at the time of seedling emerge (Baskin & Baskin 2014Baskin CC, Baskin JM. 2014. Seeds: ecology, biogeography, and evolution of dormancy and germination. San Diego, San Diego Academic Press.; Bell et al. 1993Bell DT, Plummer JA, Taylor SK. 1993. Seed germination ecology in southwesternWestern Australia. Botanical Review 59: 286-305.) which in A. arenaria is the summer period, when water availability is greatest. Temperature control of germination could determine the geographical range of many plant species (Thompson 1973Thompson PA. 1973. Seed germination in relation to ecological and geographical distribution. In: Heywood VH. (ed.) Taxonomy and ecology. London, Academic Press. p. 93-119.; Probert 1992Probert RJ. 1992. The role of temperature in germination ecophysiology. In: Fenner M. (ed.) Seeds: The Ecology of regeneration in plant communities. Wallingford, CAB International. p. 285-325.) and for A. arenaria may represent a limiting factor as temperatures decrease further south.

Conclusion

Our results show that A. arenaria colonizes open areas in restinga systems mostly through mutualistic interactions that help escape both, unfavorable conditions and predation, thus confirming our hypotheses. On the one hand, vertebrates contribute to long-distance dispersal and the protection of seeds within dung piles, crucial against Bruchinae beetles, the main predators of A. arenaria seeds. On the other hand, secondary dispersal by the beetle Ateuchus squalidus contributes to germination success. Then, establishment of A. arenaria in open areas of restinga is the outcome of a complex interaction network between dispersers and predators, which place seeds in the best possible microhabitat for establishment. This interactions network is essential to maintain restinga community dynamics.

Acknowledgements

We are grateful to Fátima Piña Rodrigues for advice, assistance and technical help; Fabio Rubio Scarano for comments on an earlier draft of this manuscript. The Centro de Avaliações do Exército (Brazilian Army) for research permission and access to the study area. This work was supported by the Fundação Coordenação de Aperfeiçoamento de Pessoal de Nível Superior under Grant CAPES - Proc. 2543/13-3, and Fundação o Boticário de Proteção à Natureza under Grant Proc. 0512-20012.

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Publication Dates

  • Publication in this collection
    19 Oct 2017
  • Date of issue
    Jan-Mar 2018

History

  • Received
    19 Apr 2017
  • Accepted
    15 Aug 2017
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