The second largest South American biome (after the Amazon Forest), the Cerrado is the most biodiverse savannah environment in the world – and is disappearing rapidly.
Until the 1960s, the Cerrado remained largely preserved. Now, after the accelerated expansion of livestock and the agricultural frontier the biome has only 21% of its original vegetation intact, according to Conservation International.
The vegetation of the Cerrado is composed of grasses, shrubs and sparse trees. They are plants adapted to survive during the long periods of dry season that characterise the dry season. When the first rains arrive, however, everything changes and the Cerrado blooms. The seeds of the most diverse genera and plant families typical of the biome germinate at the same time, as if they were the instruments of a great orchestra playing in unison.
A study carried out at the State University of São Paulo (Unesp) reveals the different strategies of the different groups of plants of the Cerrado to fruit and disperse seeds that germinate with the arrival of the rains throughout the year.
In tropical regions with a seasonal climate, the availability of water in the soil is the main factor limiting the establishment and growth of seedlings. “In seasonal tropical ecosystems, the time the seeds germinate is regulated by the relationship between fruiting phenology and seed dormancy,” said Colombian biologist Diego Fernando Escobar , the first author of the article and a PhD student at the Institute of Biosciences of the State University Paulista (Unesp), in Rio Claro, with FAPESP scholarship .
In general, species that disperse seeds at the beginning of the rainy season have non-dormant seeds, which germinate rapidly if soil moisture content is adequate. Seeds scattered at the end of the rainy season and the beginning of the dry season – a period in which climatic conditions for planting (plant embryos) are inadequate – fall into a state of dormancy, preserving germination properties for the arrival of the next rainy season.
“The relationship between fruiting phenology and dormancy in the tropics has been tested at the community level for forest ecosystems, but studies on savannas are scarce, restricted to certain clades [phylogenetic tree branches], making it difficult to understand the general patterns of regeneration for this biodiversity hotspot,” said Escobar.
“In addition, such studies do not consider different classes of dormancy and dispersal syndromes. The relationship between dormancy classes and life history characteristics of species [such as different seasons of dispersal and seed characteristics] are not fully understood for the savannas,” said Escobar.
“We looked for to verify if the pattern of fruiting, dispersion and germination of seeds in the Cerrado corresponded to what happens in other seasonal tropical ecosystems,” he said.
Although seed dormancy is considered the main mechanism for controlling the timing of seed germination in seasonal ecosystems, some studies suggest that seed germination is controlled both by dormancy and the seed dispersal period. “That’s exactly what we’ve been able to verify,” Escobar said.
The study was conducted under the guidance of Professor Patricia Morellato , head of the Laboratory of Phenology, Department of Botany, Unesp Institute of Biosciences of Rio Claro, and collaboration of Professor Fernando Augusto de Oliveira and Silveira, Department of Botany, Universidade Federal de Minas General.
Escobar collected seeds of dispersed plants between March 2015 and March 2016, with regular intervals of 15 days between each collection. Seeds of 34 species belonging to 28 genera and 16 families were collected, including 31 woody and three herbaceous species.
Fruits of at least ten individuals of each species were collected, with the exception of Ground Clover (Qualea dichotoma), Red Ucuuba (Virola sebifera) and Pau Santo (Kielmeyera coriacea), where the team took fruits of only one individual per species.
The objective was to determine the proportion of species with dormancy in the Cerrado community and the climatic and natural history factors associated with dormancy.
They found the proportions of dormant and non-dormant Cerrado species were similar (47.1% and 52.9%, respectively). Once the germination data were tabulated, they proceeded to the second phase of the work: to identify the time when the various species fruit and disperse seeds.
“The fruiting patterns of the Cerrado are characterized by the production of ripe fruits throughout the year, but a large proportion of species fruit at the end of the dry season and beginning of the rainy season,” explained Escobar.
Among the species studied, 38.2% dispersed seeds during the rainy season, 14.7% in the transition between rain and drought, 20.6% in the dry season and 26.5% in the dry to rainy transition. This was only possible thanks to a database with information on the phenology of the fruiting of the Cerrado plants.
This phenology information has been collected since 2004 and was collected over 14 years of research supported by FAPESP in a private reserve in the municipality of Itirapina, in the State of São Paulo. Further, the dispersion method for which each species is adapted has been determined.
There are three types of seed dispersal among the Cerrado plants. Zoocoric seeds are dispersed by the action of animals and the anemocoric by the action of the winds. Autochoric (self-seeding) plants spread their seeds without the help of any external agent, that is, the seeds simply fall to the ground next to the parent plant.
Zoocoric species have fleshy fruits or fleshy structures around the seeds. The anemocoric species have seeds with structures adapted to take advantage of the force of the winds. The autochoric species, however, do not have fleshy structures nor do they show structures known to facilitate wind dispersion.
Analyzing the data, Escobar verified that the zoochory was the most common dispersion syndrome among the plants of Cerrado de Itirapina (64.7%), followed by anemochory (20.6%) and, finally, autochory (14.7% %).
In addition to determining which species of the Itirapina can lie dormancy and which do not, the germination tests in the laboratory allowed to determine the temperature conditions necessary to germinate the seeds of each of the 34 species.
For the germination experiments, the seeds were placed in Petri dishes with two layers of filter paper saturated with distilled water under white light 24 hours a day and up to five constant temperatures (15, 20, 25, 30 and 35 ° C). For each species, between 120 and 150 seeds were tested for each temperature, according to seed availability.
Germination was determined by the curvature of the radicle or protrusion of the aerial structures. The experiments were monitored three times a week for one month, after which the germination was monitored weekly for a maximum period of 12 months or until the germination curve was established.
The optimal germination temperature for each species was determined as the temperature or temperature matrix with the highest percentage of germination and germination rate. The optimum temperature for seed germination of most species was between 25° C and 30° C.
“Germination experiments indicated that the timing of seed germination in the Cerrado community is controlled by both the dispersal season (beginning of the rainy season) and dormancy, differing from other studies in seasonal ecosystems, including savannas, which recognise dormancy as the main mechanism to control germination, “said Escobar.
Most species germinated at the beginning of the rainy season, and both the dispersion season and seed dormancy controlled the germination time of the seeds.
The probability of a species being dormant depended on the interaction between season and type of dispersion, where species with limited dispersion (autochory) tended to be dormant – whereas species with dispersion of seeds at greater distances (anemochory and zoochory) tended to become dormant if there were dispersion during the transition from rain to drought.
“The dispersion during the transition between rain and drought favours the evolution of seed dormancy because the environmental conditions are favourable to germination, but not to the establishment of seedlings,” explained Morellato.
Avoiding germination during the dry season is an advantage to which all Cerrado plants have adapted. In the study, all species that dispersed seeds close to the beginning of the dry season produced dormant seeds independent of the taxonomy or the dispersion syndrome. On the other hand, dormancy in autochoric species may be related to decreased competence between sows, distributing the risk of mortality of the seedlings over time.
“We now know that phenology [the study of the relationships between biological processes and cycles and the climate] of fruiting and seed germination of plants in the Cerrado does not exactly follow the patterns found in other seasonal tropical ecosystems. This is the first comprehensive study addressing the ecology of seed dormancy in a community of Cerrado, aiming to evaluate the relationship between fruiting phenology and seed dormancy and how this relationship is modulated by classes of dormancy, dispersion syndromes and seed mass and humidity,” said Morellato.
According to the Unesp professor, besides showing patterns of fruiting and germination phenomena at the community level in the Cerrado, the results of the study clarify how the classes of dormancy are modulated by the interaction between season and dispersion syndrome, allowing a better understanding of the evolution of the seeds.
“One consequence of the study’s findings is that, in cases of ecological restoration attempts of the Cerrado with its native species, this needs to be done taking into account the time of year. If it is not so, it will not work. The seeds will either not germinate or the seedlings will die before they have time to extend roots and accumulate resources to survive in the dry season,” Morellato said.