Timing of seed dispersal and seed dormancy in Brazilian savanna: two solutions to face seasonality

Abstract

Background and aims: The relationship between fruiting phenology and seed dispersal syndrome is widely recognized; however, the interaction of dormancy classes and plant life-history traits in relation to fruiting phenology and seed dispersal is understudied. Here we examined the relationship between fruiting season and seed dormancy and how this relationship is modulated by dormancy classes, dispersal syndromes, seed mass and seed moisture content in a Brazilian savanna (cerrado).

Methods: Dormancy classes (non-dormancy and physical, morphological, morphophysiological, physiological and physiophysical dormancy) of 34 cerrado species were experimentally determined. Their seed dispersal syndrome (autochory, anemochory, zoochory), dispersal season (rainy, dry, rainy-to-dry and dry-to-rainy transitions), seed mass and moisture contents, and the estimated germination date were also determined. Log-linear models were used to evaluate how dormancy and dormancy classes are related to dispersal season and syndrome.

Key results: The proportions of dormant and non-dormant species were similar in cerrado. The community-estimated germination date was seasonal, occurring at the onset of rainy season. Overall, anemochorous non-dormant species released seeds during the dry-to-rainy transition; autochorous physically dormant species dispersed seeds during the dry season and rainy-to-dry transition; zoochorous species dispersed non-dormant seeds during the dry and rainy seasons, while species with morphological, morphophysiological or physiological dormancy dispersed seeds in the transitional seasons. Seed mass differed among dispersal seasons and dormancy classes, but seed moisture content did not vary with dispersal syndrome, season or dormancy class.

Conclusions: The beginning of the rainy season was the most favourable period for seed germination in cerrado, and the germination phenology was controlled by both the timing of seed dispersal and seed dormancy. Dormancy class was influenced by dispersal syndrome and season. Moreover, dormancy avoided seed germination during the rainy-to-dry transition, independently of dispersal syndrome. The variability of dormancy classes with dispersal syndrome allowed animal-dispersed species to fruit all year round, but seeds germinated only during the rainy season. Conversely, seasonally restricted wind-dispersal species dispersed and germinated their non-dormant seeds only in the rainy season.

Fig. 1.

Climate and fruiting phenology for the cerrado study site at Itirapina, south-eastern Brazil. (A) Average monthly temperatures and rainfall over a period of 30 years (1982–2012) (CRHEA–EESC/USP meteorological station). Maximum, mean and minimum temperatures (solids lines) and rainfall (bars). (B) Percentage of species fruiting per month according to the seed dispersal syndrome based on the total of 73 species monitored (6 autochorous, dashed line; 17 anemochorous, solid line; and 50 zoochorous, dotted line) during 11 years of phenological observations (Phenology Laboratory at UNESP, unpubl. data).

Fig. 2.

Dendrogram from UPGMA cluster analysis based on Euclidian distances showing the months clustered by season according to the climatic variables used: rainy season from November to March, dry season from May to August, rainy-to-dry transition in April and dry-to-rainy transition from September to October. The height scale represents within-dataset Euclidean distance. Data are the average for a period of 30 years (1982–2012) (CRHEA–EESC/USP meteorological station).

Fig. 3.

Number of species fruiting in each dispersal season according to (A) dispersal syndrome, (B) dormancy class and (C) number of species in each dispersal syndrome according to dormancy class in a cerrado community in south-eastern Brazil. ND, non-dormant; PY, physical; MPD, morphophysiological; MD, morphological; PD, physiological.

Fig. 4.

Circular histograms for the frequency of fruiting peak dates (A–D) and estimated germination dates (E–H) for the community and for each dispersal syndrome in a cerrado community in south-eastern Brazil. Black arrows indicate the mean angle (mean date) and arrow length corresponds to the mean vector (r) value, or the degree of seasonality. Absence of a black arrow in a histogram indicates that the mean angle was not significant and pattern is not significantly seasonal (for details see the Materials and methods section and Supplementary Data Tables S2 and S3).

Fig. 5.

Relationship between seed mass (A–C) and moisture content (D–F) according to dormancy class, seed dispersal season and dispersal syndrome for a cerrado community in south-eastern Brazil. Different capital letters indicate significant differences (Tukey test P ≤ 0.05); ns, not significant. ND, non-dormant; PY, physical; MPD, morphophysiological; MD, morphological; PD, physiological; Anemo, anemochory; Auto, autochory; Zoo, zoochory.