Brazilian savanna’s germination patterns are unique
July 04, 2018
By Peter Moon | Agência FAPESP – The Cerrado is the world’s most biodiverse savanna environment and the largest biome in South America apart from the Amazon Rainforest. More importantly, it is fast disappearing.
Until the 1960s, the Cerrado was relatively well preserved, but since then, it has been increasingly overrun by cattle ranching and the expanding agricultural frontier, among other factors. Only 21% of its original vegetation is now intact, according to Conservation International.
The vegetation of the Cerrado consists of grasses, bushes and sparse trees. These plants with twisted trunks and branches have adapted to the arid environment during the long dry season. When the first rains come, the Cerrado bursts into flower. The seeds of a wide variety of plant genera and families typical of the biome germinate at the same time like the instruments of a symphony orchestra playing in unison.
A study performed by researchers at São Paulo State University (UNESP) in Rio Claro, Brazil, reveals the different strategies used during the year by the various groups of plants in the Cerrado to bear fruit and disperse seeds that often lie dormant and germinate when the rains arrive. The results of the study have recently been published in Annals of Botany.
In tropical regions with a seasonal climate, the availability of groundwater is the key limiting factor for the establishment and growth of seedlings. “In seasonal tropical ecosystems, the timing of seed germination is governed by the relationship between fruiting phenology and seed dormancy,” said Colombian biologist Diego Fernando Escobar, first author of the article and a PhD researcher at UNESP’s Rio Claro Bioscience Institute (IBRC) with a scholarship from FAPESP.
Generally, species that disperse seeds at the start of the rainy season have nondormant seeds that germinate quickly if the soil moisture is appropriate. Seeds dispersed in the late rainy season and early dry season, a period when the climate is unsuitable for the establishment of seedlings, retreat into a state of dormancy during which they preserve their germinative properties for the arrival of the next wet season.
“The relationship between fruiting phenology and dormancy in the tropics has been tested at the community level for forest ecosystems, but studies of savanna ecosystems are few and far between and are confined to certain clades [branches of the phylogenetic tree], hindering an understanding of the general patterns of regeneration for this biodiversity hotspot, ” Escobar said.
“Moreover, the studies available don’t consider different dormancy classes and dispersal syndromes. The relationship between dormancy classes and the life-history characteristics of species [such as different dispersal seasons and seed traits] are not fully understood for savannas.
“We set out to investigate whether the fruiting, dispersal and germination patterns in the Cerrado corresponded to those described for other seasonal tropical ecosystems.”
While seed dormancy is considered the main mechanism that controls the timing of seed germination in seasonal ecosystems, some studies suggest that seed germination is controlled by both seed dormancy and the seed dispersal period. “That’s exactly what we succeeded in confirming,” Escobar said.
The principal investigator for the study was Professor Patricia Morellato, head of the Botany Department’s Phenology Laboratory at IBRC-UNESP. Fernando Augusto de Oliveira e Silveira, a professor in the Botany Department of the Federal University of Minas Gerais (UFMG), collaborated.
Escobar collected seeds dispersed between March 2015 and March 2016, with regular 15-day intervals between collections. Seeds from 34 species belonging to 28 genera and 16 families were sampled, including 31 woody species and three herbaceous species.
Fruits were collected from at least ten individuals per species except for Qualea dichotoma, Virola sebifera (red ucuuba) and Kielmeyera coriacea, for which fruits were collected from only one individual per species.
The goal was to assess the proportion of dormant species in the Cerrado community and the climate and natural history factors associated with dormancy.
The proportions of dormant and nondormant species in the Cerrado were found to be similar (47.1 and 52.9%, respectively). The germination data were tabulated, and the second stage of the study focused on fruiting and seed dispersal timing for the various species.
“Fruiting patterns in the Cerrado are characterized by the production of ripe fruit throughout the year, but a large proportion of species bear fruit late in the dry season and early in the wet season,” Escobar said.
Among the species studied, 38.2% dispersed seeds during the rainy season, 14.7% in the transition between the rainy and dry seasons, 20.6% during the dry season, and 26.5% in the transition from the dry season to the rainy season.
These results were made possible by a database on Cerrado plant fruiting phenology built using data collected since 2004. This research, conducted for 14 years, has been supported by FAPESP and has focused on a private reserve in Itirapina, São Paulo State.
The study also analyzed the dispersal method for which each species is adapted. Three types of seed dispersal are found in the Cerrado: zoochory (dispersal by animals), anemochory (dispersal by wind) and autochory (dispersal by the plant itself without help from external agents, with the seeds falling near the mother plant or being ejected by it).
Zoochorous species have fleshy fruits or structures that partially or totally encase the seeds. Anemochorous species have winged or flat seeds adapted to leverage wind power. Autochorous species do not have fleshy fruits or structures known to facilitate wind dispersal.
Escobar’s analysis of the data showed that zoochory was the most common dispersal syndrome among Cerrado plants in Itirapina (64.7%), followed by anemochory (20.6%) and autochory (14.7%).
Laboratory germination tests showed which species from the Itirapina reserve displayed dormancy and which did not, as well as gauging the temperature required for the germination of seeds from each of the 34 species.
For the germination experiments, seeds were placed in Petri dishes with two layers of filter paper saturated with distilled water and left under white light for 24 hours per day at five constant temperatures (15, 20, 25, 30 and 35 °C). For each species, between 120 and 150 seeds were tested at each temperature, according to seed availability.
Germination was defined in terms of radicle curvature or the protrusion of aerial structures. The experiments were monitored three times a week for a month, after which germination was monitored weekly for a maximum of 12 months or until the germination curve stabilized.
The optimal germination temperature for each species was defined as the temperature or temperature range with the highest germination percentage and germination rate. The optimal germination temperature range for seeds of most species was found to be 25 °C-30 °C.
“Our germination experiments showed that the timing of seed germination in the Cerrado community is controlled both by the dispersal season, meaning the onset of the rainy season, and by dormancy. In this respect, our findings differ from those of other studies in seasonal ecosystems, including savannas, which identify dormancy as the main mechanism controlling germination,” Escobar said.
Most species germinated at the start of the wet season, and both the dispersal period and seed dormancy controlled the timing of seed germination.
The probability of dormancy in any species depended on the interaction between season and dispersal type. Species with limited dispersal (autochory) tended to be dormant, while those with long-distance dispersal (anemochory and zoochory) became dormant when the seeds were dispersed during the transition from wet to dry season.
“Dispersal during the wet-to-dry transition favors the evolution of seed dormancy because environmental conditions are conducive to germination but not to the establishment of seedlings,” Morellato explained.
Avoiding germination during the dry season is an advantage to which all Cerrado plants have adapted. In this study, all species that dispersed seeds close to the onset of the dry season had dormant seeds regardless of taxonomy or dispersal syndrome. On the other hand, dormancy in autochorous species may relate to a reduction in competition between sibling plants, distributing the risk of seedling mortality over time.
“We know now that the phenology [seasonal rhythms and recurring biological processes or cycles, especially as influenced by climate] of fruiting and seed germination in Cerrado plants doesn’t exactly match the patterns found in other seasonal tropical ecosystems. This is the first comprehensive study to address the ecology of seed dormancy in a Cerrado community, focusing on the link between fruiting phenology and seed dormancy and on how this link is modulated by dormancy classes, dispersal syndromes, seed mass, and seed moisture content,” Morellato said.
In addition to fruiting phenology and germination patterns at the community level in the Cerrado, she added, the results of the study elucidate how dormancy classes are modulated by the interaction between season and dispersal method, permitting a better understanding of seed evolution.
“One consequence of the findings is that attempts to restore Cerrado ecology with native species must take the time of year into account. Otherwise they won’t work,” she stressed. “The seeds won’t germinate or the seedlings will die before they have time to develop roots and accumulate resources for survival during the dry season.”
The article “Timing of seed dispersal and seed dormancy in Brazilian savanna: two solutions to face seasonality” by Diego F. E. Escobar, Fernando A. O. Silveira and Leonor Patricia C. Morellato can be read at: academic.oup.com/aob/article-abstract/121/6/1197/4841709?redirectedFrom=fulltext.
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