Disturbance as a factor in breaking dormancy and enhancing invasiveness of African grasses in a Neotropical Savanna

Abstract

The Cerrado is threatened by wildfires and invasive species. We aimed to evaluate in laboratory conditions whether temperature fluctuation at the soil surface, resulting from the absence of vegetation due to fire, can affect the germination of Urochloa decumbens and U. brizantha, two invasive African grasses. Seeds of both species were submitted to simulations: 1) temperature during fire at 1cm belowground (F); 2) temperature fluctuation at 1cm belowground without vegetation cover for a month (TF); 3) (F) + (TF); 4) control at 25ºC. After treatments, seeds were put to germinate at 25ºC for 40 days. We had four replicates per treatment and three temporal replicates. We compared germination percentage and the mean germination time among treatments using ANOVA. The treatments TF and F+TF had the highest germination values for both species. The results showed that fire per se could not stimulate seed germination, however, they suggest that a disturbance that produces a pattern of temperature fluctuation is able to break dormancy and enhance seed germination and, consequently, increase the invasiveness of the study species. Vegetation gaps resulting from disturbance may become new sites of invasion. This information is important for making management decisions regarding the control of these species.

Cerrado; fire; germination percentage; invasive species; physiological dormancy; seed dormancy; temperature fluctuation; Urochloa brizantha; Urochloa decumbens

Introduction

One of the major challenges in invasion ecology is whether it is possible to identify a set of traits that enables a species to be invasive. Reproductive traits are important determinants of the success of invasive species (Rejmánek 1996; Pyšeket al. 2015). The most relevant plant traits related to invasiveness are those associated with seed germination and seed bank longevity. These traits are related to germination capability as well as to the extended germination over time. Together, they enable fast establishment and allow the species to wait for better environmental conditions (Pyšek & Richardson 2007).

Disturbance is well recognized as a triggering factor in a number of cases of biological invasion (Hobbs & Huenneke 1992). Species whose seeds can germinate in both intact and disturbed environments are more likely to spread and occupy new habitats than those with restricted germination requirements. Thus, the ability to germinate in a wide range of environmental conditions should also be mentioned as a trait associated with plant invasiveness (Luo & Cardina 2012).

Events of disturbance create open spaces within the vegetation and lead to alterations in the microclimate (mainly through soil temperature and moisture), and usually increase the availability of soil nutrients or change in the soil microbiota (Davis et al. 2000;Carvalho et al. 2010). Disturbance may also alter a number of species interactions, and thus favor invasibility in the plant community (Lonsdale 1999; Davis et al. 2000). In addition, the type and intensity of a disturbance are critical in determining the success of exotics and their impact on the invaded community (Lockwood et al. 2007).

Fire is a natural disturbance in many ecosystems and it has been an important selective force in the evolution of seeds of fire-prone plant species (Keeley & Fortheringham 2000; Bond & Keeley 2005). High temperatures can break the physical dormancy of seeds resulting in higher germination (Hanley & Lamont 2000; Auld & Denham 2006; Keeley et al. 2012). Still, the physiological dormancy of seeds may be broken by chemical compounds released in the smoke (Ooi et al. 2014). High temperatures have also been reported to break physiological dormancy of grasses (Whiteman & Mendra 1982; Adkins et al. 2002) and other permeable seeds (Luo & Cardina 2012). On the other hand, some studies have shown decreased germination (Auld & O'Connel 1991; Ribeiro et al. 2013), or no relevant effect (e.g. Le Stradic et al. 2015; Fichino et al. 2016).

Besides fire direct effects on germination through high temperatures and smoke (Santana et al. 2012; Moreira & Pausas 2012), it also promotes indirect effects by changing the environmental conditions: gaps are open in the vegetation, that lead to higher solar radiation and light spectrum, as well as greater temperatures at the soil surface. In consequence, the range of daily soil temperatures is changed, as shown in different ecosystems (Coutinho 1990; Fidelis & Blanco 2014; Santana et al. 2012). The fluctuation of soil temperature can also break seed dormancy (Baeza & Roy 2008; Santana et al. 2012; Musso et al. 2015) and increase the germination of some "gap dependent" species (Ooi et al. 2014). Therefore, the establishment and persistence of invasive species in a community can be directed by fire through its influence on seed germination and seed bank longevity, and according to the burning regime and fire parameters.

In the Cerrado (Brazilian Tropical Savanna), fire has been acting as a selective force on plant communities since millions of years (Simon et al. 2009) and several species - especially in the herb layer - are believed to be fire-dependent (Coutinho 1982; 1990). Tolerance or enhanced germination at high temperatures have been reported for some Cerrado species (Coutinho 1982; Gottsberger & Silberbauer-Gottsberger 2006; Ribeiro & Borghetti 2014).

Therefore, the aim of this study was to evaluate in laboratory conditions whether the increase of soil temperature (direct effect of fire) and/or the daily fluctuation of soil temperature (indirect effect of fire) would affect the germination of two African grasses, Urochloabrizantha and U. decumbens, which show very aggressive invasive behavior in the Cerrado (Pivello et al 1999 a; b). The results of this study will help us to understand the germination strategies of invasive grasses in disturbed environments, giving support to management actions regarding the adequacy of prescribed fires in Cerrado invaded by these grasses.

Materials and Methods

Climate and fire in Cerrado

Cerrado has a tropical climate with wet summers and dry winters. The mean annual rainfall varies from 1200 to 1800 mm, and mean annual temperatures are 22-23ºC (Coutinho 2002). The maximum temperatures do not vary much throughout the year and can reach more than 40ºC, however, minimum temperatures may vary greatly and reach values close to zero in the coldest months (May-July), producing high temperature amplitudes in the coldest months (Coutinho 2002). The vegetation of Cerrado is characterized by a mosaic of different formations that include grasslands (Campo limpo), savanna-like vegetation (e.g.Campo sujo; Cerrado sensu stricto), and forest formations (Cerradão) (Coutinho 1982; Furley 1999). In aCampo sujo physiognomy in Central Brazil, Fideliset al. (unpubl. res.) registered temperatures of 50ºC at soil surface during the hottest hours of the day, which decreased to 12 - 17ºC at night during the dry season (September).

Fires in the Cerrado are 'surface fires', which run fast over the soil and rapidly consume the herbaceous biomass, causing no intense heating belowground. In Cerrado physiognomies the temperatures registered during a fire at 1 cm aboveground ranged from 85ºC to 840ºC (for more details, see Miranda et al. 1993) but belowground they increased only a few degrees, being the maximum values registered by Miranda et al. (1993; 2002) between 29ºC (3 cm) and 55ºC (1 cm).

Study species

We used two African species that are considered some of the most common and severe invasive species in the Cerrado: Urochloa decumbens(Stapf) R. D. Webster (signal grass) and Urochloa brizantha(Hochst. ex A. Rich.) R.D.Webster (palisade grass) (Pivello et al. 1999 a; b; Almeida-Netoet al. 2010; Lannes et al. 2012). They are C₄perennial grasses widely used in tropical pastures due to their high palatability, productivity and resistance to intensive grazing and trampling (Loch 1977; Kissmann 1997).Urochloa species were introduced in Brazil in the mid-1950s as cattle fodder (Kissmann 1997), and readily adapted to the low-fertile and aluminum-rich soils of Cerrado (Rao et al. 1996). The seeds of U. decumbensdisperse in December/January and of U. brizantha from April to June. However, some individuals of both species continue to disperse all over the year (personal observation). These invasive species have a strong competitive ability and they can rapidly dominate the Cerrado herbaceous community (Pivello et al. 1999 a; Almeida-Neto et al. 2010). In addition, the presence of Urochloaspp. in Cerrado can change fire behavior by affecting fire intensity, maximum temperature and flame height (Gorgone-Barbosaet al. 2015).

In the experiments of this study we used commercialized seeds of Urochloa decumbens and Urochloa brizantha, since they are widely used in planted pastures. The seeds were not submitted to any previous treatment, being stored at room temperatures of 25ºC.

Temperature treatments and simulated curves

We submitted the seeds of both species to four different treatments to simulate both the effect of belowground temperatures during the passage of fire and the daily temperature fluctuations after the removal of vegetation by fire. The seeds were placed in Petri dishes in an incubator (Binder KB E5.1) where all temperature treatments occurred. We applied the following treatments:Fire (F) = simulation of fire temperatures at 1 cm belowground: the initial temperature was 25ºC, rose up to 50°C in 20 minutes (based on previous data from Miranda et al. 1993) and then decreased continuously to 25ºC in the following 30 minutes;Temperature fluctuations (TF)= simulation of the daily temperature fluctuation at 1 cm belowground without vegetation cover: seeds were subjected to alternating temperature (10ºC to 40ºC, that represent the mean range of soil temperatures after fire during the dry season, measured in the field by A. Fidelis et al., (unpubl. res.) during thirty days;Fire + Temperature fluctuations (F + TF) = combination of the two treatments, to simulate both the passage of fire and daily temperature fluctuations resulting from vegetation removal; and Control(C): seeds were stored at a constant temperature of 25°C until put to germinate. According to the manual for the analyses of seeds of the Brazilian Ministry of Agriculture (Brasil 2009), temperatures recommended for germinating U. decumbens and U. brizantha must be between 15 and 35ºC. As the mean temperature in open cerrados is around 25ºC (Coutinho 2002) we chose it as the control temperature in our experiments, although it has been reported in the literature that germination and dormancy breaking ofUrochloa spp. was not influence by light or temperature (Adkins et al. 2002).

Seed germination experiments

Before starting the experiments we tested the seed viability through the Tetrazolium test 1%. The mean viability was 81% for U. decumbens and 87% for U. brizantha.

Urochloa decumbens and U. brizantha seeds were divided into four samples for each treatment (30 seeds/replication/treatment) and temporal replication. We replicated the experiment three times (temporal replication) to avoid pseudoreplication, since we had only one incubator (30 seeds x 4 samples x 4 treatments x 3 temporal replications = 1440 seeds/species). The entire duration of the experiment was 90 days (30 days for each replicate). Although a single species viability analysis was performed, the time elapsed between replicates was not considered to have influenced the results of the experiment, as it was depicted form previous tests. Seeds were placed in Petri dishes, covered with one layer of filter paper, moistened with distillated water, and put in germination chambers at 25ºC and dark conditions to simulate the buried seeds. The germination was recorded every other day, for 40 days. We considered germinated those seeds with 1 mm long radicle, and removed them from the Petri dishes after being counted. Seeds that did not germinate in 40 days were submitted to Tetrazolium test to check their viability.

Analyses

We calculated the percentage of germination and the mean germination time (MGT) in days, according to Ranal & Santana (2006).

To compare percentage of germination, MGT and viability between treatments, we used one-way analysis of variance (ANOVA, factor = treatment) applied to randomization tests (Euclidean distance between sampling units, 10000 interactions). All statistical analyses were performed using the software MULTIV (Pillar 2005).

Results

Seeds of both U. decumbens and U. brizantha showed percentage of germination which varied from 45% to 75%, independently of the treatment. Moreover, both species showed a similar pattern of response to the different treatments. A significant increase in the percentage of germination of both species was observed after TF and F+TF treatments when compared to the control (Fig.1).

Figure 1.
Percentage of germination (%) (mean, standard error and mean + 2* standard deviation) after the treatments: C = Control; F = fire; TF = temperature fluctuation; F+TF = fire followed by temperature fluctuation for (A) Urochloa decumbens and (B) Urochloa brizantha. Different letters represent significant differences, P≤ 0.05.

The percentages of germination of U. decumbens were 75±10% and 66±12% respectively for TF and F+TF, and not statistically different (P= 0.09), whereas the control (C, 48±10%) and F treatment (45±12%) values were lower (P< 0.0001, for all comparisons) and statistically similar to each other (P= 0.64) (Fig. 1A). The seeds of U. brizantha also showed the highest percentages of germination after TF and F+TF treatments (62±10% and 59±9%, respectively) and not statistically different between each other (P= 0.10), but significantly differed from the control (46±13%,P= 0.004 and P= 0.02, respectively, Fig. 1B) while F treatment resulted in percentage of germination (54±10%) not statistically different from those of the other treatments (P> 0.05, Fig. 1B). Considering all the viable seeds (germinated + not germinated, but still viable), the TF and F+TF treatments proportionally increased the percentage of germination for both species, since viability between the treatments did not differ (Tab. 1, U. decumbens,P= 0.31 and U. brizantha, P=0.80).

Table 1.
Percentage of total viable seeds (germinated + not germinated, mean±standard deviation) and dead seeds after the treatments: C = Control; F = fire simulation; TF = temperatures fluctuation; F+TF = fire simulation followed by temperature fluctuation for (A) Urochloa decumbens and (B) Urochloa brizantha. No statistical differences were found among treatments.

Treatments	Urochloa decumbens	Urochloa brizantha
	Viable	Dead	Viable	Dead
C	76.9±8.4	23.0±8.4	76.9±17.7	23.0±17.7
F + TF	79.1±12.9	20.8±12.9	81.6±9.3	18.3±9.3
TF	82.7±7.4	17.2±7.4	80.5±9.3	19.4±9.3
F	75.5±9.5	24.4±9.5	79.1±10.5	20.8±10.5

The mean germination time (MGT) did not differ between the treatments for both species (Fig. 2 - U. decumbens, P= 0.83 and U. brizantha,P= 0.63). The MGT of U. decumbens varied from 4.8±1.9 days in C to 3.7±1.1 days in F+TF.

Figure 2.
Mean germination time (MGT, days) (mean, standard error and mean + 2* standard deviation) in the treatments: C = Control; F = fire; TF = temperature fluctuation; F+TF = fire followed by temperature fluctuation for (A) Urochloa decumbens and (B) Urochloa brizantha.

Discussion

The ability to successfully germinate in a wide range of environments and to withstand disturbances are key reproductive traits that increase invasiveness of alien species (Pyšek & Richardson 2007; Luo & Cardina 2012). The two study species germinate more than 40%, which could be already advantageous to colonize and spread in invaded sites. Nevertheless, our results support the idea that part of both species' seeds (14-37%, considering the viable seeds of the treatments) are dormant, and such dormancy could be broken by daily fluctuation of temperature, generated for example, after disturbance events that remove the aboveground vegetation. Such temperature fluctuation enhanced germination, and consequently their invasiveness by increasing the probability of establishment of the species and therefore increasing the chance of a successful invasion.

Several grass species are known to have physiological dormancy (Baskin & Baskin 2004), as already reported for the genusUrochloa (Whiteman & Mendra 1982; Adkins et al. 2002). This strategy allows for long-term survival of populations by maintaining a persistent soil seed bank and delaying germination until the microclimatic conditions are optimal for seedling establishment (Baskin & Baskin 2004), being advantageous for the invasive species.

Temperature is the main environmental variable involved in the development, control, and the breaking of seed dormancy of species from different ecosystems (Moreira & Pausas 2012; Santana et al. 2012; Presotto et al. 2014). In our study we simulated the heating effect caused by Cerrado fires on seeds stored in the soil seed bank at 1 cm depth, and that mild temperature of 50ºC (F treatment) affected neither U. decumbens nor U. brizantha germination. A similar result was found by Martins & Silva (2001), where temperatures of 40ºC and 55ºC were not effective on stimulating the germination of U. brizantha. Recent studies showed that fire did not enhance the germination of grasses and forbs of open physiognomies of Cerrado but seeds were resistant to high temperatures (Le Stradic et al. 2015; Fichino et al. 2016). Also, heat-shock of 100 °C did not affect germination percentages of invasive species in the Chilean matorral (Gómez-González et al. 2011).

In this study, we found an indirect effect of fire, since both TF and F+TF were able to break seed dormancy and enhance germination. Thus, fire did not affect directly the germination of U. decumbens and U. brizanthaseeds from the soil seed bank, but the temperature fluctuation during one month was the effective triggering element. Musso et al. (2015) also found thatAristida setifolia and Echinolaena inflexa, two native Cerrado grasses, had their germination increased by temperature fluctuations (10-45ºC). However, Melinis minutiflora, another Cerrado invasive grass, was not affected by temperature fluctuations (Mussoet al. 2015).

Our results agree with the idea that disturbance events could facilitate the invasion process of U. decumbens and U. brizantha in Cerrado, not only by enhancing invasibility due to the increased resources and changes in competition, but also by enhancing the invasiveness of the species by breaking seed dormancy and facilitating germination. High percentage of germination directly affects the process of invasion by increasing recruitment and reproductive success of the invasive species, and as a consequence, its abundance, subsequent spread and colonization of new areas. Moreover, the pool of seeds in the soil may promote a considerable seed bank to maintain the invasive species in the community (as shown by Carmona 1995; Silva & Dias-Filho 2001). Therefore, the post-disturbance behavior of bothUrochloa species can be pointed out as an advantage over natives and probably other Cerrado invasive species, such as Melinis minutiflora.

It is important to highlight that we used commercial seeds with a high viability, and it is expected that seeds collected in the field would have lower viability. But even so, a percentage of these seeds would be stimulated by post-fire conditions. In fact, a great number of U. decumbens seedling was observed after fire in an invaded campo sujo in São Paulo state (E. Gorgone-Barbosa, unpubl. res.).

Our study revealed an important mechanism of the invasion process: increased germination and seedling recruitment after disturbance, which explains in part the invasion success of these species in the burned Cerrado. Local managers should thus consider the gaps created in the vegetation by any kind of disturbance as a potential new site of invasion, if seeds of invasive species are available in the area.

Conclusion

Mild temperatures (around 50ºC) caused by Cerrado fires in the soil seed bank at 1 cm depth do not affected the germination percentage of both U. decumbens and U. brizantha, aggressive invasives in that ecosystem. An important ecological mechanism that relates fire to the invasion of Urochloa species in disturbed cerrados is the post-fire fluctuation of daily temperatures at the soil level, which may facilitate their initial germination, enhance the percentage of germinated seeds, and probably increase the establishment success in recent burned areas.

Acknowledgements

We thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES/DGU 227/2010) for financial support. M.J.B. acknowledges the support from the programme FORESTERRA ERA-Net (Medwildfirelab, PCIN-2013-140-C04-03) and PROMETEO II (Desestres es/2014/038). We are also grateful to Faouzia Ayache for helping during lab experiments, and two anonymous reviewers who helped us to improve the manuscript.

References

• Adkins SW, Bellairs SM, Lock DS. 2002. Seed dormancy mechanisms in warm season grass species. Euphytica 126: 13-20.
• Almeida-Neto M, Prado PI, Kubota U, Bariani JM, Aguirre GH, Lewinsohn TM. 2010. Invasive grasses and native Asteraceae in the Brazilian Cerrado. Plant Ecology 209: 109-122.
• Auld TD, Denham AJ. 2006. How much seed remains in the soil after a fire? Plant Ecology 187: 15-24.
• Auld TD, O´Connell MA. 1991. Predicting patterns of post-fire germination in 35 eastern Australian Fabaceae. Australian Journal of Ecology 16: 53-70.
• Baeza MJ, Roy J. 2008. Germination of an obligate seeder (Ulex parviflorus) and consequences for wildfire management. Forest Ecology and Management 256: 685-693.
• Baskin CC, Baskin JM. 2004. A classification system for seed dormancy. Seed Science Research 14: 1-16.
• Bond WJ, Keeley JE. 2005. Fire as global 'herbivore': The ecology and evolution of flammable ecosystems. Trends in Ecology and Evolution 20: 387-394.
• Brasil - Ministerio da Agricultura, Pecuária e Abastecimento. 2009. Regras para Analises de Sementes. Brasilia, MAPA.
• Carmona R. 1995. Seed bank in the soil and the establishment of weeds in agro-ecosystems. Planta Daninha 13:3-9.
• Carvalho LM, Antunes PM, Martins-Loução AM, Klironomo LN. 2010. Disturbance influences the outcome of plant-soil biota interactions in the invasive Acacia longifolia and in native species. Oikos 119: 1172-1180.
• Coutinho LM. 1982. Ecological effects of fire in Brazilian Cerrado. In: Huntley BJ, Walker BH. (eds.) Ecology of tropical savannas. Berlin, Springer-Verlag. p. 273-291.
• Coutinho LM. 1990. Fire in the Ecology of Brazilian Cerrado. In: Goldammer JG. (ed.) Fire in the tropical biota: Ecological processes and global challenges. Berlin, Springer-Verlag. p. 82-105.
• Coutinho LM. 2002. O Bioma Cerrado. In: Klein AL. (ed.) Eugen Warming e o Cerrado brasileiro um século depois. São Paulo, Editora Unesp. p. 77-91.
• Davis MA, Grime PJ, Thompson K. 2000. Fluctuating resources in plant communities: a general theory of invasibility. Journal of Ecology 88: 528-534.
• Fichino B, Pivello VR, Fidelis A. 2016. Does fire trigger seed germination in the Neotropical Savannas? Experimental tests with six Cerrado species. Biotropica, (in press).
• Fidelis A, Blanco C. 2014. Does fire induce flowering in Brazilian subtropical grasslands? Applied Vegetation Science 17: 690-699.
• Furley PA. 1999. The nature and diversity of neotropical savanna vegetation with particular reference to the Brazilian cerrados. Global Ecology and Biogeography 8: 223-241.
• Gómez-González S, Torres-Díaz C, Valencia G, Torres-Morales P, Cavieres LA, Pausas JG. 2011. Anthropogenic fires increase alien and native annual species in the Chilean coastal matorral. Diversity and Distribution 17: 58-67.
• Gorgone-Barbosa E, Pivello VR, Bautista S, Zupo TM, Rissi MN, Fidelis A. 2015. How can an invasive grass affect fire behavior in a tropical savanna? A community and individual plant level approach. Biological Invasions 17: 423-431.
• Gottsberger G, Silberbauer-Gottsberger I. 2006. Life in the cerrado: a South American tropical seasonal ecosystem. Vol. 1. Ulm, Reta Verlag.
• Hanley ME, Lamont BB. 2000. Heat pre-treatment and the germination of soil- and canopy-stored seeds of south-western australian species. Acta Oecologica 21: 315-321.
• Hobbs RJ, Huenneke LF. 1992. Disturbance, Diversity, and Invasion: Implications for Conservation. Conservation Biology 6: 324-337.
• Keeley JE, Fotheringham CJ. 2000. Role of fire in regeneration from seed. In: Fenner M. (ed.) Seeds: The Ecology of Regeneration in Plant Communities. 2nd. edn. Oxon, CAB International. p. 311-330.
• Keeley JE, Bond WJ, Bradstock RA, Pausas JG, Rundel PW. 2012. Fire in Mediterranean climate ecosystems: ecology, evolution and management. Cambridge, Cambridge University Press.
• Kissmann KG. 1997. Plantas infestantes e nocivas. São Paulo, BASF.
• Lannes LS, Bustamante MMC, Edwards PJ, Venterink HO. 2012. Alien and endangered plants in the Brazilian Cerrado exhibit contrasting relationships with vegetation biomass and N : P stoichiometry. New Phytologist 196: 816-23.
• Le Stradic S, Silveira FAO, Buisson, E, Cazelles K, Carvalho V, Fernandes GW. 2015. Diversity of germination strategies and seed dormancy in herbaceous species of campo rupestre grasslands. Austral Ecology 40: 537-546.
• Loch DS. 1977. Brachiaria decumbens (Signal Grass) - A review with particular reference to Australia. Tropical Grasslands 11: 141-157.
• Lockwood JL, Hoopes MF, Marchetti MP. 2007. Invasion Ecology. Malden, Blackwell Publishing.
• Londsdale WM. 1999. Global patterns of plant invasions and the concept of invasibility. Ecology 80: 1522-1536.
• Luo J, Cardina J. 2012. Germination patterns and implications for invasiveness in three Taraxacum (Asteraceae) species. Weed Research 52: 112-121.
• Martins L, Silva WR. 2001. Comportamento da dormência em sementes de braquiária submetidas a tratamentos térmicos e químicos. Pesquisa Agropecuária Brasileira 36: 997-1003.
• Miranda AC, Miranda HS, Dias, Ide FO, Dias BFS. 1993. Soil and air temperatures during prescribed cerrado fires in Central Brazil. Journal of Trop, 9(3), 313-320.
• Miranda HS, Bustamante, MMC, Miranda AC. 2002. The Fire Factor. In: Olivera P, Marquis R. (eds.) The cerrados of Brazil: ecology and natural history of a neotropical savanna. New York, Columbia University Press. p. 51-68.
• Moreira B, Pausas JG. 2012. Tanned or Burned: The role of fire in shaping physical seed dormancy. Plos One 7: 1-8. doi:10.1371/journal.pone.0051523
  » https://doi.org/10.1371/journal.pone.0051523
• Musso C, Miranda HS, Aires SS, Bastos AC, Soares AMVM, Loureiro S. 2015. Simulated post-fire temperature affects germination of native and invasive grasses in cerrado (Brazilian savanna). Plant Ecology & Diversity 8: 219-227.
• Ooi MKJ, Denham AJ, Santana VM, Auld TD. 2014. Temperature thresholds of physically dormant seeds and plant functional response to fire: variation among species and relative impact of climate change. Ecology and evolution 4: 656-71.
• Pillar V. 2005. MULTIV: Multivariate exploratory analysis, randomization testing and bootstrap resampling. Porto Alegre, Universidade Federal do Rio Grande do Sul.
• Pivello VR, Carvalho V, Lopes P. 1999a. Abundance and distribution of native and alien grasses in a "Cerrado" (Brazilian Savanna) biological reserve. Biotropica 31: 71-82.
• Pivello VR, Shida CN, Meirelles ST. 1999b. Alien grasses in Brazilian Savannas : a threat to the biodiversity. Biodiversity and Conservation 8: 1281-1294.
• Presotto A, Povere M, Cantamutto M. 2014. Seed dormancy and hybridization effect of the invasive species, Helianthus annuus. Annals of Applied Biology 164: 373-383.
• Pyšek P, Manceur AM, Alba C, et al. 2015. Naturalization of central European plants in North America: species traits, habitats, propagule pressure, residence time. Ecology 96: 762-774.
• Pyšek P, Richardson, DM. 2007. Traits associated with invasiveness in alien plants: Where do we stand? In: Nentwig W. (ed.) Biological Invasions. Berlin, Springer. p. 97-125.
• Ranal MA, Santana DG. 2006. How and why to measure the germination process? Revista Brasileira de Botânica 29: 1-11.
• Rao IM, Kerridge PC, Macedo MCM. 1996. Nutritional requeriments of Brachiaria and adaptation to acid soils. In: Miles JW, Maass BL, Valle CB. (eds.) Brachiaria: biology, agronomy, and improvement. Brasília, CIAT/Embrapa. p. 53-71.
• Rejmánek M. 1996. A Theory of seed plant invasiveness: the first sketch. Biological Conservation 78: 171-181.
• Ribeiro LC, Borghetti F. 2014. Comparative effects of desiccation, heat shock and high temperatures on seed germination of savanna and forest tree species. Austral Ecology 39: 267-278.
• Ribeiro LC, Pedrosa M, Borghetti F. 2013. Heat shock effects on seed germination of five Brazilian savanna species. Plant biology (Stuttgart, Germany) 15: 152-157.
• Santana VM, Baeza MJ, Blanes MC. 2012. Clarifying the role of fire heat and daily temperature fluctuations as germination cues for Mediterranean Basin obligate seeders. Annals of Botany 111: 127-134.
• Silva DSM, Dias-Filho MB. 2001. Banco de sementes de plantas invasoras em solo cultivado com pastagens de Brachiaria brizantha e Brachiaria humidicola de diferentes idades. Planta Daninha 19: 179-185.
• Simon MF, Grether R, Queiroz LP, Skema C, Pennington RT, Hughes CE. 2009. Recent assembly of the Cerrado, a neotropical plant diversity hotspot, by in situ evolution of adaptations to fire. Proceedings of the National Academy of Sciences of the United States of America 106: 20359-20364.
• Whiteman PC, Mendra K. 1982. Effects of storage and seed treatments on germination of Brachiaria decumbens. Seed Science Technology 10: 233-242.

Publication Dates

• Publication in this collection
  Jan-Mar 2016

History

• Received
  15 Sept 2015
• Accepted
  17 Dec 2015