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Key decision-making criteria for dormancy-breaking and ability to form seed banks of Cerrado native tree species

ABSTRACT

Ecological restoration by direct seeding in the Cerrado biome still lacks information about native species germination, the need for dormancy overcoming and seed bank formation. This study aims to verify the effects of dormancy overcoming on germination of four tree species and the ability of 12 tree species to form seed banks for restoration use. Our results showed wide variation of species’ germination rates. Overcoming dormancy enhanced germination for Dimorphandra mollis, Hymenaea stigonocarpa, and Peltophorum dubium and decreased it for Copaifera langsdorffii, but was only cost-effective for H. stigonocarpa. Regarding the ability to form seed banks, only H. stigonocarpa and Cecropia pachystachya germinated and live seedlings of Terminalia corrugata were found after being buried for six months, thus forming a transient seed bank. Despite the fact that overcoming dormancy may optimize germination after direct seeding, maintaining dormancy mechanisms of species that can form seed banks could be essential for species establishment over time in restoration areas. Hence, our key decision criteria based on seed costs and seed and labor availability would be useful for the seeding actions of restoration practitioners.

Keywords:
direct sowing; ecophysiology; neotropical savanna; restoration ecology; seed photoblastism

Introduction

Major ecological restoration commitments around the world require information about the cost-benefit of techniques and species used for restoration (Aronson et al. 2011Aronson J, Durigan G, Brancalion PHS. 2011. Conceitos e definições correlatos à ciência e à prática da restauração ecológica. Instituto Florestal. Série Registros, São Paulo 44: 1-38. http://www.lerf.esalq.usp.br/divulgacao/recomendados/artigos/aronson2011.pdf
http://www.lerf.esalq.usp.br/divulgacao/...
; Bussato et al. 2015Bussato LC, Junior RC, Vieira J, Esperança AAF, Martins SV. 2015. Aspectos ecológicos na produção de sementes e mudas para restauração. In: Martins SV. (ed.) Restauração Ecológica de Ecossistemas Degradados. 2ed. edn. Viçosa, UFV. p. 102-168.). Compared to seedling planting, direct sowing of tree species has proven to be an efficient and inexpensive technique (Camargo et al. 2002Camargo JLC, Ferraz IDK, Imakawa AM. 2002. Rehabilitation of degraded areas of central Amazonia using direct sowing of forest tree seeds. Restoration Ecology 10: 636-644.; Woods & Elliott 2004Woods K, Elliot S. 2004. Direct seeding for forest restoration on abandoned agricultural land in northern Thailand. Journal of Tropical Forest Science 16: 248-259.; Sovu et al. 2010Sovu PS, Tigabu M, Odén PC. 2010. Restoration of former grazing lands in the highlands of Laos using direct seeding of four native tree species. Mountain Research and Development 30: 232-243. ; Guerin et al. 2015Guerin N, Isernhagen I, Vieira DLM, Campos RJB. 2015. Avanços e próximos desafios da semeadura direta para restauração ecológica. In: Martins SV. (ed.) Restauração ecológica de ecossistemas degradados. 2nd. edn. Viçosa, MG, Editora UFV. p. 331-370.; Ceccon et al. 2016Ceccon E, Gonzales EJ, Martorell C. 2016. Is direct seeding a biologically viable strategy for restoring forest ecosystems? Evidences from a meta-analysis. Land Degradation & Development 27: 511-520. ). However, to be viable, it is necessary to understand the characteristics of species used in direct seeding such as germination rates, seed dormancy, and seed bank formation (Engel & Parrotta 2001Engel VL, Parrota JA. 2001. An evaluation of direct seeding for reforestation of degraded lands central São Paulo state, Brazil. Forest Ecology and Management 152: 169-181. ). Characteristics such as timing of seed dispersal and dormancy seem to control germination timing, and recruitment of woody species in strongly seasonal ecosystems as Neotropical Savannas, also known as Cerrado in Brazil (Salazar et al. 2011Salazar A, Goldstein G, Franco AC, Miralles-Wilhelm F. 2011. Timing of seed dispersal and dormancy, rather than persistent soil seed-banks, control seedling recruitment of woody plants in Neotropical savannas. Seed Science Research 21: 103-116. ). In the Cerrado, a biome composed of a mosaic of vegetation varying from grasslands to forests, knowledge about dormancy, germination, and seed bank formation of native species is still scarce when compared to other Brazilian biomes (Lima et al. 2014Lima YBC, Durigan G, Souza FM. 2014. Germinação de 15 espécies vegetais do Cerrado sob diferentes condições de luz. Bioscience Journal 30: 1864-1872.; Palma & Laurance 2015Palma C, Laurance SGW. 2015. A review of the use of direct seeding and seedling plantings in restoration: what do we know and where should we go? Applied Vegetation Science 18: 561-568. ; Salazar et al. 2012aSalazar A, Goldstein G, Franco AC, Miralles-Wilhelm F. 2012a. Seed limitation of woody plants in Neotropical savannas. Plant Ecology 213: 273-287. ). Information about species biology, e.g., the need to overcome dormancy and the possibility of seed bank formation, is essential for restoration planning and success, as well as for reducing costs (Khurana & Singh 2001Khurana E, Singh JS. 2001. Ecology of seed and seedling growth for conservation and restoration of tropical dry forest: a review. Environmental Conservation 28: 39-52.; Salazar et al. 2012bSalazar A., Goldstein G, Franco AC, Miralles-Wilhelm F. 2012b. Differential seedling establishment of woody plants along a tree density gradient in Neotropical savannas. Journal of Ecology 100: 1411-1421.).

Seed viability loss could further reduce the seed supply of woody plants by preventing the formation of persistent seed banks (seeds living in the soil for over a year) from which seedlings can recruit (Salazar et al. 2012Salazar A, Goldstein G, Franco AC, Miralles-Wilhelm F. 2012a. Seed limitation of woody plants in Neotropical savannas. Plant Ecology 213: 273-287. a). Dormancy protects seeds from adverse conditions that do not favor species establishment (Finch‑Savage & Leubner‑Metzger 2006Finch-Savage WE, Leubner-Metzger G. 2006. Seed dormancy and the control of germination. New Phytologist 171: 501-523. ; Venable 2007Venable DL. 2007. Bet hedging in a guild of desert annuals. Ecology 88: 1086-1090.; Childs et al. 2010Childs DZ, Metcalf CJE, Rees M. 2010. Evolutionary bet‐hedging in the real world: empirical evidence and challenges revealed by plants. Proceedings of the Royal Society of London B: Biological Sciences 277: 3055-3064.; Footitt & Finch-Savage 2017 Footitt S, Finch-Savage WE. 2017. Dormancy and control of seed germination. Plant Physiology and Function. The Plant Sciences 6. doi: 10.1007/978-1-4614-7611-5_7-1
https://doi.org/10.1007/978-1-4614-7611-...
), and at the same time is responsible for seed bank formation (Garwood 1989Garwood NC. 1989. Tropical soil seed banks: a review. In: Leck MA, Parker VT, Simpson RL. (eds.) Ecology of soil seed banks. San Diego, Academic Press. p. 149-209.). The seed bank helps reestablish natural populations after disturbances and is an important component of species diversity conservation that is related to environmental resilience (Tres et al. 2007Tres DR, Sant’anna CS, Basso S, Langa R, Ribas UJ, Reis A. 2007. Banco e chuva de sementes como indicadores para a restauração ecológica de matas ciliares. Revista Brasileira de Biociências 5: 309-311.; Martins 2008Martins SV, Almeida DPA, Vaz Fernandes LV, Ribeiro TM. 2008. Banco de sementes como indicador de restauração de uma área degradada por mineração de caulim em Brás Pires, MG. Revista Árvore 32: 1081-1088.; Bussato et al. 2015Bussato LC, Junior RC, Vieira J, Esperança AAF, Martins SV. 2015. Aspectos ecológicos na produção de sementes e mudas para restauração. In: Martins SV. (ed.) Restauração Ecológica de Ecossistemas Degradados. 2ed. edn. Viçosa, UFV. p. 102-168.). Moreover, the seed bank could potentially be used for restoration purposes (Kiss et al. 2019Kiss R, Deák B, Török P, Tóthmérész B, Valkó O. 2019. Restoration potential of soil seed bank in a changing climate. International Network for Seed-based Restoration. https://ser-insr.org/news/2019/6/9/restoration-potential-of-soil-seed-bank-in-a-changing-climate. 11 Nov. 2019.
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) and seed dormancy is critical for that. Introducing species that can tolerate climatic changes through transient seed bank formation could support passive restoration, particularly in more degraded sites (Kiss et al. 2018Kiss R, Deák B, Török P, Tóthmérész B, Valkó O. 2018. Grassland seed bank and community resilience in a changing climate. Restoration Ecology 26: S141-S150. ).

In order to guarantee succession, ecological restoration must consider the characteristics of seed germination, the time in which seeds remain viable in soil banks, and the conditions that lead to dormancy breaking, if there are any (Khurana & Singh 2001Khurana E, Singh JS. 2001. Ecology of seed and seedling growth for conservation and restoration of tropical dry forest: a review. Environmental Conservation 28: 39-52.; Salazar et al. 2011Salazar A, Goldstein G, Franco AC, Miralles-Wilhelm F. 2011. Timing of seed dispersal and dormancy, rather than persistent soil seed-banks, control seedling recruitment of woody plants in Neotropical savannas. Seed Science Research 21: 103-116. ; 2012bSalazar A., Goldstein G, Franco AC, Miralles-Wilhelm F. 2012b. Differential seedling establishment of woody plants along a tree density gradient in Neotropical savannas. Journal of Ecology 100: 1411-1421.). Understanding seed germination traits helps determine which species are most likely to persist after a disturbance and important for vegetation management and conservation of genetic resources (Primack 1990Primack RB. 1990. Seed physiology, seed germination and seedling ecology - commentary. In: Bawa KS, Hadley M. (eds.) Reproductive ecology of tropical forest plants. Paris, UNESCO. p. 233-236.). In addition, it allows fast-growing species, along with species with potential seed bank formation that germinate over time, to contribute to in situ and ex situ restoration and conservation efforts (Merritt & Dixon 2011Merritt DJ, Dixon KW. 2011. Restoration seed banks-a matter of scale. Science 332: 424-425.). Therefore, restoration strategies in these environments can stimulate the formation of heterogeneous seed banks, enabling secondary recruitment. The target species used in restoration need to be tested to determine whether they remain viable in seed banks. Hence, our aim was to evaluate germination under darkness simulation in direct seeding conditions, the effect of seed dormancy breakage on germination and its cost-effectiveness, as well as the seed bank formation of some key species that could potentially be used in Cerrado restoration.

Materials and methods

Species

Considering that the Cerrado biome contains different types of native vegetation, we selected species that occur in both savannas and forests (i.e., generalists), as well as species that occur exclusively in savannas (i.e., savanna specialists) or forests (i.e., forest specialists) (Abreu et al. 2017Abreu RCR, Hoffman WA, Vasconcelos HL, Pilon NA, Rossatto DR, Durigan G. 2017. The biodiversity cost of carbon sequestration in tropical savanna. Science Advances 38: e1701284. doi: 10.1126 / sciadv.1701284
https://doi.org/10.1126 / sciadv.1701284...
) (Tab. 1). We attempted to select species that are commonly sold in seed markets. The seeds were purchased from the ‘Instituto Brasileiro de Florestas’ (IBF https://www.ibflorestas.org.br/) and the ‘Rede de Sementes do Xingu’ (https://www.sementesdoxingu.org.br/site/home/). All seeds purchased originated from the current annual harvest (<1 year of storage) and were processed manually/induced in the laboratory.

Table 1
List of native species, regional names, and habitat preference (Durigan et al. 2004Durigan G, Siqueira MF, Franco GADC, Contieri WA. 2004. A flora arbustivo-arbórea do Médio Paranapanema: base para a restauração dos ecossistemas naturais. Pesquisas em conservação e recuperação ambiental no Oeste Paulista: resultados da cooperação Brasil/Japão. São Paulo, Páginas & Letras. https://www.researchgate.net/profile/Giselda_Durigan/publication/332843226_A_Flora_Arbustivo-Arborea_do_Medio_Paranapanema_Base_para_a_Restauracao_dos_Ecossistemas_Naturais/links/5ccc432da6fdccc9dd89fc31/A-Flora-Arbustivo-Arborea-do-Medio-Paranapanema-Base-para-a-Restauracao-dos-Ecossistemas-Naturais.pdf.
https://www.researchgate.net/profile/Gis...
; 2012Durigan G, Ramos VS, Ivanauskas NM, Franco GADC. 2012. Espécies indicadoras de fitofisionomias na transição Cerrado-Mata Atlântica no estado de São Paulo. São Paulo, Governo do Estado de São Paulo, Secretaria do Meio Ambiente, Coordenadoria de Biodiversidade e Recursos Naturais.; Mendonça et al. 2008Mendonça RC, Felfili JM, Walter BMT, et al. 2008. Vascular flora of the Cerrado biome: checklist with 12,356 species. In: Sano SM, Almeida SP, Ribeiro JF. (eds.) Cerrado: ecology and flora. Vol. 2. Brasília, Embrapa Cerrados/Embrapa Informação Tecnológica. p. 421-1279.; Abreu et al. 2017Abreu RCR, Hoffman WA, Vasconcelos HL, Pilon NA, Rossatto DR, Durigan G. 2017. The biodiversity cost of carbon sequestration in tropical savanna. Science Advances 38: e1701284. doi: 10.1126 / sciadv.1701284
https://doi.org/10.1126 / sciadv.1701284...
) used for germination and seed bank experiments in APA Guariroba in Campo Grande, Mato Grosso do Sul, Brazil.

Germination

We conducted a germination test for 42 days in the laboratory, using 100 seeds from each species, distributed in four replications of 25 seeds each (Kildisheva et al. 2019Kildisheva OA, Erickson TE, Madsen MD, Dixon KW, Merritt DJ. 2019. Seed germination and dormancy traits of forbs and shrubs important for restoration of North American dryland ecosystems. Plant Biology 21: 458-469.). We germinated seeds in Petri dishes with filter paper moistened with 1 % nystatin solution (Garcia et al. 2006 Garcia LC, Barros FV, Lemos-Filho JP. 2006. Comportamento germinativo de duas espécies de canga ferrífera: Baccharis retusa DC. (Asteraceae) e Tibouchina multiflora Cogn. (Melastomataceae) . Acta Botanica Brasilica 20: 445-450. ). The plates were kept at a constant temperature of 25 °C. Since a thin layer of soil is usually used to cover seeds in direct seeding, we conducted germination tests in the dark to simulate dark soil conditions in the field. We counted germination on the plates everyday under a green safelight using root protrusion as a germination criterion.

Dormancy breakage and its cost-effectiveness

Of the species used in this study, we identified four with tegument impermeability dormancy - H. stigonocarpa (Pereira et al. 2011Pereira SR, Giraldelli GR, Laura VA, Souza ALT. 2011. Tamanho de frutos e de sementes e sua influência na germinação de jatobá-do-cerrado (Hymenaea stigonocarpa var. stigonocarpa Mart. ExHayne, Leguminosae - Caesalpinoideae). Revista Brasileira de Sementes 33: 141-148.), P. dubium (Oliveira et al. 2008Oliveira LM, Davide AC, Carvalho MLM. 2008. Teste de germinação de sementes de Peltophorum dubium (Sprengel) Taubert - Fabaceae. Floresta 38: 545-551.), C. langsdorffii (Pereira et al. 2013Pereira SP, Laura VA, Souza ALT. 2013. Superação de dormência de sementes como estratégia para restauração florestal de pastagem tropical. Pesquisa Agropecuária Brasileira 48: 148-156. ), and D. mollis (Salomão et al. 2003Salomão NA, Souza-Silva JC, Davide AC, et al. 2003. Germinação de sementes e produção de mudas de plantas do Cerrado. Brasília, Rede de Sementes do Cerrado.). Dormancy breakage was achieved through thermal shock, using boiling water at 60 ºC for five minutes and then an ice bath for one minute. Hence, for these species with hard seed coat structure, suggesting seed integumentary dormancy traits, after a thermal shock, we carried out a germination test again to compare it, under the same conditions, with results of the germination test without dormancy breakage conditions.

Based on our results, we compared cost scenarios between overcoming seed dormancy (100 % of seed lot - yes) or not (0 % - no), considering seed availability in the market; seed price per kilo found in the market; seed amount kg/ha based on Reis et al. (2019Reis LK, Guerra A, Colado MLZ, et al. 2019. Which spatial arrangement of green manure is able to reduce herbivory and invasion of exotic grasses in native species? Ecological Applications 28: e02000. doi: 10.1002/eap.2000
https://doi.org/10.1002/eap.2000...
); labor cost/ha to overcome seed dormancy; total cost/ha in both scenarios (with and without seed dormancy breaking); percentage of germination found in our study; and increasing or decreasing magnitude of germination after seed dormancy-breakage (i.e., dividing the percentage of germination without seed dormancy-breakage by the percentage germination with seed dormancy-breakage) (see an overview of the calculations in Tab. S1 in supplementary material).

Seed bank formation

To evaluate the seed bank formation, we used 200 seeds per species, which were arranged in fine-mesh nylon bags (20 per species) and buried about 5 cm deep (Fig. 1A) (Velten & Garcia 2007Velten SB, Garcia QS. 2007. Variation between three Eremanthus (Asteraceae) species in their ability to form a seed bank. Brazilian Journal of Botany 30: 713-719.) at the same site that Reis et al. (2019Reis LK, Guerra A, Colado MLZ, et al. 2019. Which spatial arrangement of green manure is able to reduce herbivory and invasion of exotic grasses in native species? Ecological Applications 28: e02000. doi: 10.1002/eap.2000
https://doi.org/10.1002/eap.2000...
) used for direct seeding experiments by (Fig. 1B). The nylon bags were taken to the field and randomly buried in previously selected areas on the Velho Saltinho (20º34’45. 41” S, 54º21’44” O) farm in the Mananciais do Córrego Guariroba Environmental Protection Area in Campo Grande, Mato Grosso do Sul, Brazil. All these seeds were not previously treated for dormancy breaking. After being buried for six months, we unearthed the bags (Fig. 1C, D) and performed the germination test in the laboratory again, under the same conditions as the previous test, to compare buried and non-buried seeds.

Figure 1
Seed bank formation test. (A) a pit where nylon bags were buried, (B) nylon bag with seeds before being buried, (C) nylon bag unearthed after 6 months, and (D) nylon bag unearthed after 6 months with predated/deteriorated seeds.

Data analysis

We calculated the percentage of species germination. We compared the species with hard seed coat structure that suggested seed integumentary dormancy traits to germination data after thermal shock. Moreover, we compared the germination percentage before and after seeds were buried for six months using the formula: G = Σni / N × 100, where ni is the number of seeds germinated and N is the number of seeds in each Petri dishes (Labouriau 1983Labouriau LG. 1983. A germinação das sementes. Washington, DC, Secretaria-Geral da Organização de Estados Americanos, Programa Regional de Desenvolvimento Científico e Tecnológico, Universidade do Texas. ). To determine the germination velocity of each species, we calculated average germination time (t-), which is an indication of germination process velocity; t- = Σ (ni × ti) / Σ, where ni is the number of germinated diaspores per day and ti is the day they germinated (Labouriau 1983Labouriau LG. 1983. A germinação das sementes. Washington, DC, Secretaria-Geral da Organização de Estados Americanos, Programa Regional de Desenvolvimento Científico e Tecnológico, Universidade do Texas. ). The seeds were classified into three categories: rapid twinning (< five days), intermediate germination (five <t - <10 days), and slow germination (> 10 days) (Ferreira et al. 2001Ferreira AG, Cassol B, Rosa SGT, Silveira TS, Stival AL, Silva AA. 2001. Germinação de sementes de Asteraceae nativas no Rio Grande do Sul, Brasil. Acta Botanica Brasilica 15: 231-242. ). After testing the data distribution and finding parametric distribution, the means were compared to the one-way analysis of variance (ANOVA) and Tukey post-hoc test (p <0.05) in the R program (R Development Core Team 2018R Development Core Team. 2018. R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna. https://www.R-project.org. 11 Nov. 2019.
https://www.R-project.org...
).

Results

Germination

The seeds that were not subjected to dormancy breakage and not buried for six months, 10 of the 12 species germinated in 40 days, ranging from 95 % for A. colubrina to 10 % for H. stignocarpa (Fig. 2). A. fraxinifolium, C. langsdorffii, and D. alata showed more than 50 % germination and P. dubium presented 46 % germination in the same period (Fig. 2). T. roseoalba, D. mollis, M. fistulifera, and C. pachystachya had lower germination rates, ranging from 10 to 29 % (Fig. 2). A. subincanum and T. corrugata did not germinate.

Figure 2
Percentage of germination in the laboratory (at 25 °C in the dark for 42 days) of native species commonly used to restore woody communities by direct seeding in the Cerrado biome.

For A. colubrina and A. fraxinifolium, germination started on the first day and reached maximum germination rates after ten days (Fig. 2). Peltophorum dubium and C. pachystachya took about 15 days to reach their maximum germination rates (Fig. 2). D. mollis started to germinate between 0-5 days and reached peak germination between 15 and 20 days (Fig. 2). Copaifera langsdorffii and D. alata took about 20 days to stabilize their germination rates (Fig. 2), while the other species took longer to germinate.

Dormancy breakage and its cost-effectiveness

For seeds that were subjected to dormancy breakage, D. mollis, H. stigonocarpa, and P. dubium showed an increase in germination rates after dormancy break (Fig. 3). However, the germination rate of C. langsdorffii decreased from 56 % to 32 % after seed dormancy breakage treatment, which is 1.8 times lower (Fig. 3). The total amount of money that could be saved is<~US$4-5/ha for some species (P. dubium and D. mollis, respectively) and ~US$50/ha for H. stigonocarpa (Tab. S1, and see values in “Reais” in supplementary material).

Figure 3
Comparison of germination percentage of native species with (treated seeds) and without dormancy overcoming treatment by thermal shock (untreated seeds).

Seed bank formation

After digging up the nylon bags that were buried for six months, we found seeds of D. mollis (one with live radicle and six with dead radicle), M. fistulifera (ten seeds with dead radicle), T. corrugata (ten seeds with dead radicle, one seed with live radicle, and seven live seedlings) and H. stigonocarpa (ten seeds with dead radicle and one live seedling) (Fig. 1). Additionally, we verified that only C. pachystachya, D. mollis, H. stigonocarpa, and M. fistulifera had enough seeds for the germination test. The other eight species had predated/deteriorated seeds (Fig. 1D).

Cecropia pachystachya doubled its germination percentage from 10 % to 30 % and H. stigonocarpa presented the largest difference, with a germination rate rising from 10 % to 50 % after being buried. Although M. fistulifera and D. mollis did not germinate in the experiment (<2 %) (Fig. 4), we found germinated seeds after digging up the nylon bags (Fig. 1).

Figure 4
Comparison of native species germination percentage without dormancy overcoming treatment (untreated seeds) and after being buried for six months.

Average germination time (t-)

On average, the germination period ranged from 5 to 25 days (Fig. 5). The t- of species that were neither treated nor buried in the seed bank (n=10 species) varied from species to species (Fig. 5A). Among these, T. rosealba (23), H. stigonocarpa (23), M. fistulifera (12), C. langsdorffii (15), and C. pachystachya (15) presented the highest average germination times (Fig. 5A). A. colubrina presented the lowest t- (Fig. 5A). The seeds that were treated for dormancy breakage were only different when compared to untreated H. stigonocarpa seeds (Fig. 5B). H. stigonocarpa and C. langsdorffii had a longer germination period compared to the other species (Fig. 5B), and the germination period for H. stigonocarpa was lower when there was dormancy breaking (eight days of difference). The opposite happened for seeds of this species that were buried, with seeds in the seed bank (after being buried for six months) presenting a higher average t- (three to seven days longer) than untreated seeds (without burying) (Fig. 5C), whereas burying D. mollis seeds increased its germination, while M. fistulifera did not germinate when buried (Fig. 5C).

Figure 5
Average time of seed germination (t-) in days of all evaluated species. (A) germination test of species without test of seed dormancy breaking and seed bank; (B) of species that we tested dormancy breaking; (C) of species that we tested seed bank after being buried for six months. *Species whose germination value was 0 was not added to the charts (Aspidosperma subincanum and Terminalia corrugata). Values followed by the same letter do not differ between the Tukey test at 5 % of significance.

Discussion

Germination and Average Germination Time

More than 50 % of A. colubrina, A. fraxinifolium, C. langsdorffii, and D. alata seeds germinated in less than 40 days (T50 ≤ 40 days). Among the studied species, A. colubrina and A. fraxinifolium were the fastest germinating species (T50 < 10 days). Serpa et al. (2010Serpa PRK, Passos MAA, Lima Torres JE, Machado IA, Coelho LCM, Cruz AFE. 2010. Análise germinativa em sementes de Anadenanthera colubrina (Vell.) Brenan Var. colubrina (Vell.) submetidas a diferentes tratamentos. Jornada de Ensino, Pesquisa e Extensão - JEPEX. Recife, Universidade Federal Rural de Pernambuco.) and Oliveira et al. (2012Oliveira KS, Oliveira KS, Aloufa MAI. 2012. Influência de substratos na germinação de sementes de Anadenanthera colubrina (Vell.) Brenan em condições de casa de vegetação. Revista Árvore 36: 1073-1078.) also observed the highest average germination rates and germination times for A. colubrine. Results suggest high reproductive vigor for these species due to accelerated and high germination in wide temperature ranges, with no seed dormancy (Maia 2004Maia GN. 2004. Caatinga: árvores e arbustos e suas utilidades. São Paulo, Leitura & Arte Editora.). This phenomenon differs from that observed for H. stignocarpa, P. dubium, C. langsdorffii, and D. mollis, which are species that presented low germination rates and longer times due to their integument dormancy (Salomão et al. 2003Salomão NA, Souza-Silva JC, Davide AC, et al. 2003. Germinação de sementes e produção de mudas de plantas do Cerrado. Brasília, Rede de Sementes do Cerrado.; Oliveira et al. 2008Oliveira LM, Davide AC, Carvalho MLM. 2008. Teste de germinação de sementes de Peltophorum dubium (Sprengel) Taubert - Fabaceae. Floresta 38: 545-551.; Pereira et al. 2011Pereira SR, Giraldelli GR, Laura VA, Souza ALT. 2011. Tamanho de frutos e de sementes e sua influência na germinação de jatobá-do-cerrado (Hymenaea stigonocarpa var. stigonocarpa Mart. ExHayne, Leguminosae - Caesalpinoideae). Revista Brasileira de Sementes 33: 141-148.; 2013). Improving germination performance may demand a species-by-species approach to re-establish the native plant community in restoration sites (Elzenga et al. 2019Elzenga JTM, Bekker RM, Pritchard HW. 2019. Maximizing the use of native seeds in restoration projects. Plant Biology (Stuttgart, Germany) 21: 377-379.).

Non-germination of A. subincanum and T. corrugata species in the laboratory may be related to the seed lot, type of photoblastism or low seed storage potential, which can reduce seed viability (e.g., A. subincanum, Oliveira et al. 2016Oliveira MC, Ogata RS, Andrade GA, et al. 2016. Manual de Viveiros e Produção de Mudas: espécies nativas do Cerrado. Rede de sementes do cerrado. 1st. edn. Brasília, Embrapa Cerrados-Outras publicações científicas (ALICE). https://www.alice.cnptia.embrapa.br/alice/bitstream/doc/1042301/1/ManualdeViveiroeproducaodemudas.pdf
https://www.alice.cnptia.embrapa.br/alic...
). Previous experiments suggest that A. subincanum has optimal germination at 25 ºC and a 12-hour photoperiod (Oliveira et al. 2016Oliveira MC, Ogata RS, Andrade GA, et al. 2016. Manual de Viveiros e Produção de Mudas: espécies nativas do Cerrado. Rede de sementes do cerrado. 1st. edn. Brasília, Embrapa Cerrados-Outras publicações científicas (ALICE). https://www.alice.cnptia.embrapa.br/alice/bitstream/doc/1042301/1/ManualdeViveiroeproducaodemudas.pdf
https://www.alice.cnptia.embrapa.br/alic...
), while 100 % of T. corrugata seeds can germinate in natural temperature and light conditions (Soares et al. 2006Soares JJ, Souza MHAO, Lima MIS. 2006. Twenty years of post-fire plant succession in a” cerrado”, São Carlos, SP, Brazil. Brazilian Journal of Biology 66: 587-602.; Farias et al. 2015Farias J, Sanchez M, Abreu MF, Pedroni F. 2015. Seed dispersal and predation ofTerminalia corrugataEichler (Combretaceae) in a Cerradosensu stricto, mid-west Brazil. Brazilian Journal of Biology 75: 88-96.). Therefore, regarding treatment homogeneity, the germination test simulating darkness of buried seeds and direct sowing conditions shows that direct seeding strategies must also be heterogeneous in the field. Hence, considering photoblastism, with some species germinating only in darkness, others in light and others indifferent, heterogeneous strategies for direct seeding (e.g., a thin layer of soil over seeds in some parts of the furrow and not in others) could be tested further.

Dormancy breakage

Germination of C. langsdorffii seeds decreased after thermal shock, therefore this technique cannot be recommended for germination of this species. Silva et al. (2016Silva CA, Teixeira AL, Gomes RLA, Marques FRO, Silva AAS, Silva ACA. 2016. Effects of pre-germination treatments on Copaifera langsdorffii seeds. African Journal of Agricultural Research 11: 4027-4030.) also came to such conclusion after finding that chemical scarification after immersion in sulfuric acid for 10 minutes was the best treatment. Another interesting option is partial seed burial and subsequent burning for 30 minutes, as fire could be a key factor for enhancing germination, with better results found for this species (Souza et al. 2015Souza ML, Silva DRP, Fantecelle LB, Lemos Filho JPD. 2015. Key factors affecting seed germination of Copaifera langsdorffii, a Neotropical tree. Acta Botanica Brasilica 29: 473-477.).

Conversely, the dormancy suppression of P. dubium, D. mollis, and H. stignocarpa seeds by thermal shock was successful, reflecting high germination percentage. This thermal shock can occur under natural conditions in the Cerrado because of recurring disturbance events during the dry and cold winter (Soares et al. 2006Soares JJ, Souza MHAO, Lima MIS. 2006. Twenty years of post-fire plant succession in a” cerrado”, São Carlos, SP, Brazil. Brazilian Journal of Biology 66: 587-602.). However, when correlating the germination percentage to the average germination period (days), seed treatment is not so feasible. The difference in germination period may not be significant in the long run, which can postpone the growth of the established individuals, as well as the canopy closure for forest vegetation types. In addition, the operational effort of dormancy-breaking may not be worthwhile for the species studied herein. Since those species have high potential for ecological restoration, an alternative that can compensate for dormancy-breaking is to increase the density of seeds at the time of sowing, but only if that strategy would reduce implantation costs. This decision should be based on the attributes of the species planted. In some cases, dormancy-breaking could be a disadvantage as it could increase the risk of seedling mortality from water stress during the dry season (Zaidan & Barbedo 2004Zaidan LBP, Barbedo CJ. 2004. Quebra de dormência em sementes. In: Ferreira AG, Borghetti F. (eds.) Germinação: do básico ao aplicado. Porto Alegre, Artmed. p. 135-146.) or an unexpectedly long drought in the wet season. Additionally, planting seeds without dormancy-breaking could increase seedling survival under microclimate oscillations (Mcivor & Howden 2000McIvor JG, Howden SM. 2000. Dormancy and germination characteristics of herbaceous species in the seasonally dry tropics of northern Australia. Austral Ecology 25: 213-222. ; Azania et al. 2003Azania AAPM, Azania CAM, Pavani MCMD, Cunha MCS. 2003. Dormancy breaking methods in Ipomoea and Merremiaseeds. Planta Daninha 21: 203-209. Tunes et al. 2009Tunes LM, Badinelli PG, Olivo F, Barros ADS. 2009. Treatments for overcoming dormancy of barley seeds. Scientia Agraria 10: 15-21.).

Considering the contrasting results, which depend on the species response, and for logistical purposes, we recommend overcoming dormancy to increase the seed germination rate or seed density depending on key decision criterion. Our study showed that dormancy-breaking enhanced germination in three of the four species tested, however, depending on labor costs of seed dormancy-breaking and seed availability, it may or may not be worthwhile to break seed dormancy (see Tab. S1 in supplementary material). Future studies could test part of the seed lot and take seed costs into consideration to evaluate this key decision criteria approach. The seed dormancy-breaking test is simple and inexpensive since it only uses a few parts of the seed lot (e.g., 200 seed = 100 for dormancy-breaking test and 100 as a control). Hence, based on seed cost, seed availability (species rarity or seed market availability), and labor availability (skilled labor or time to commit to dormancy-breaking), the practitioner could decide the best option for each species (e.g.Table S1 in supplementary material). Although the total amount of money saved is very low (<~US$4-5/ha) for some species, it is quite high and could be an interesting option for other species (e.g., ~US$50/ha for H. stigonocarpa, Tab. S1 in supplementary material). Hence, when comparing the four studied species, the thermal shock technique is recommended for large-scale direct seeding if the only intention is to enhance germination of H. stignocarpa. Even though we only tested germination in the laboratory, our results indicate that standardizing direct seeding in the field may not benefit all species equally because of the differences between species attributes and the distinct conditions required for successful emergence and establishment (Montalvo et al. 2002Montalvo AM, McMillan PA, Allen EB. 2002. The relative importance of seeding method, soil ripping, and soil variables on seeding success. Restoration Ecology 10: 52-67. ; Kiehl et al. 2010Kiehl K, Kirmer A, Donath TW, Rasran L, Hölzel N. 2010. Species introduction in restoration projects-Evaluation of different techniques for the establishment of semi-natural grasslands in Central and Northwestern Europe. Basic and Applied Ecology 11: 285-299. ).

Seed bank formation

Only two species were able to form a seed bank, which germinated after six months underground (C. pachystachya and H. stigonocarpa). Additionally, we also found live seedlings of T. corrugata and H. stigonocarpa inside buried nylon bags acting as transient seed bank builders, according to Thompson et al. (1993Thompson K, Band SR, Hodgson JG. 1993. Seed size and shape predict persistence in soil. Functional Ecology 7: 236-241.). This classification encompasses species whose seeds remain in the soil for up to one year. However, it is possible that C. pachystachya and H. stigonocarpa seeds remain in the soil longer, thus long-term investigations are necessary. Cecropia seeds are commonly found in abundance in seed banks, as their small size facilitates seed displacement in the litter, which is eventually incorporated into the soil (Chambers et al. 1991Chambers JC, Macmahon JA, Haefner JH. 1991. Seed entrapment in Alpine ecosystems: effects of soil particle size and diaspore morphology. Ecology 72: 1668-1677.; Guo et al. 2000Guo Q, Brown JH, Valone TJ, Kachman SD. 2000. Constraints of seed size on plant distribution and abundance. Ecology 81: 2149-2155. ; Souza 2003Souza PA. 2003. Efeito da sazonalidade da serrapilheira sobre o banco de sementes visando seu uso na recuperação de áreas degradadas. PhD Thesis, Universidade Federal de Viçosa, Viçosa.).

Our most interesting result is that H. stigonocarpa germination rates increased sharply (almost fourfold) after six months underground. The seed bank is extremely important in nature and in restoration processes for environmental resilience (Tres et al. 2007Tres DR, Sant’anna CS, Basso S, Langa R, Ribas UJ, Reis A. 2007. Banco e chuva de sementes como indicadores para a restauração ecológica de matas ciliares. Revista Brasileira de Biociências 5: 309-311.). The increased germination potential from 10 % to 45 % after soil incorporation for six months could be a strategy to initially reduce intraspecific competition for resources since there is a high rate of seed germination after direct sowing. The possibility of continuous germination over time, could enhance species survival even after stochastic events (Tres et al. 2007Tres DR, Sant’anna CS, Basso S, Langa R, Ribas UJ, Reis A. 2007. Banco e chuva de sementes como indicadores para a restauração ecológica de matas ciliares. Revista Brasileira de Biociências 5: 309-311.) such as droughts.

Our results show that A. colubrina, A. fraxinifolium, C. langsdorffii, and D. alata had the highest germination rates and are indicated for restoration projects that seek to quickly establish species. P. dubium and D. molis also had high germination rates after dormancy break and are also indicated for restoration projects. Only H. stigonocarpa and C. pachystachya showed seed bank formation potential, as well as signs that they remain active for over a year. When used in direct seeding with other species, these two species support the temporal dynamics of restoration. Considering the differences in germination responses of native species, we recommend applying dormancy-breaking in a portion of the seed lots when there is not enough information about a species, as it could provide heterogeneous results for direct seeding. In addition, we highlight that the species with low germination rates presented in this study should not be excluded from restoration activities and need to be further investigated. These species could be used to increase diversity, but to enhance their establishment the number of seeds need to be increased at the beginning of the project based on the goals of the restoration project, and available resources.

Acknowledgements

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance code 001. We are grateful to the “Programa Água Brasil,” WWF, Banco do Brasil, and Agência Nacional das Águas for financing this sowing experiment. We thanks Jessica Beatriz, Paula Isla, Adriano de Jesus, Adriano Melo, Luciene Lima, and Antonio Folleto (Velho Saltinho farm) for welcoming the experiment and helping establish it, and Mariana Abrahão and Larissa Reis for English review.

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Publication Dates

  • Publication in this collection
    22 Mar 2021
  • Date of issue
    Oct-Dec 2020

History

  • Received
    06 Feb 2020
  • Accepted
    09 Aug 2020
Sociedade Botânica do Brasil SCLN 307 - Bloco B - Sala 218 - Ed. Constrol Center Asa Norte CEP: 70746-520 Brasília/DF. - Alta Floresta - MT - Brazil
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