Ecofisiologia evolutiva de sementes de Melastomataceae de campos rupestres

Abstract

Seed biology plays a central role in plant ecology and evolution. The multiple benefits provided by seeds to seed plants were crucial in their rise and dominance throughout theglobe. Seeds are of primary significance for reproductive success and seed traits are understrong selective pressure because without successful establishment following germination,no postgermination phenotype can ever be expressed. Seed traits are associated withfecundity, range size, habitat segregation, adult plant abundance, seed persistence in thesoil, dispersal ability and seedling survival. In this study, we implemented phylogenetic comparative methods to investigate the evolution of seed size and germination traits inNeotropical montane savannas (rupestrian fields). We take advantage of microenvironmentalspatial heterogeneity and contrasting geographic distribution patterns inplant communities of rupestrian fields to explore the association of seed traits and plantdistribution in both global and local scales. We focused on Melastomataceae because thispioneer-dominated family comprises one of the most functionally distinctive, ecologicallydiverse and species-rich clade of Angiosperms. In the first chapter, we reviewed thefunctional ecology of life cycle transitions in Melastomataceae in two Neotropical hotspots,namely the Brazilian Cerrado and the Atlantic Rain Forest. The life-history traits of Melastomataceae are very variable in terms of fruiting phenology and seedling establishment, but not in terms of dispersal ecology and seed germination. All examined18species produce small-sized, photoblastic seeds, dominant across Neotropical seed banks.With regards to frugivory, a taxonomic wide range of frugivores (mainly birds) consumethe water- and sugar-rich berries. Although it is frequently argued that melastomes areexpected to benefit from human-induced changes, decreases in germination andestablishment caused by high temperatures and enhanced fire frequency are likely tooverride possible increases in plant density in forest edges. We discuss how seed- andseedling-based restoration techniques can help assist ecosystem recovery in restoration ecology. In the second chapter, we examined whether gut passage affects seed germinationin ten melastome species. We compared the dispersal quality of seven bird species bycontrasting seed germination of hand-extracted, gut-passed seeds and seeds within intactfruits. Seed cleaning is required prior to germination for all species, because germination within intact fruits was < 4% across species. Most mechanical/chemical action of guts onthe seeds was non-significant and a few responses were species-specific. Our results suggest that the variable outcomes resulting from complex fruit-frugivore interactionspotentially affect the recruitment of Cerrado melastomes. In the third chapter, we addressedthe role of seeds in determining the geographic range and ecological breadth. We used amultitude of statistical approached in order to find whether seed germination requirementsare associated with plant distribution in global (endemics vs. widespreads) and local scales(xeric vs. mesic sites). Germinability and temperature range greatly varied among species,but variation was not related to plant endemism. Therefore, we argued that factorscontrolling seedling establishment, rather than seed germination, constrain the geographicexpansion of endemic species. Temperature ranges were higher for plants in mesicmicrohabitats compared to plants associated with xeric microhabitats, though hygrophilousexperience lower daily soil temperature fluctuations. Decreased germination under hightemperatures suggests major negative impacts during regeneration from seed as global airand soil temperature increases. In the fourth chapter, we provide the framework for theevolution of seed dormancy in tropical Melastomataceae. Anatomical analysis andgermination experiments with seeds collected in different years and sites indicated theoccurrence of physiological dormancy in four out of 50 species. Phylogeneticreconstruction methods allowed us to detect multiple evolution of physiological dormancyin species and populations dispersing seeds in xeric microhabitats during unfavourable19times for establishment. Therefore, drought-induced mortality may have been a strongselective pressure favouring the evolution of dormancy in Melastomataceae. In chapter 5,we have investigated the evolution of seed size and germination traits in 50 species inNeotropical montane savannas. Trait-trait correlations along the phylogenetic tree of Melastomataceae were significant for germinability and germination synchrony, but not forseed size or light requirement, since these two traits were phylogenetically conserved. Wefound a consistent pattern of divergences in germination traits between Merianieae and Miconieae and within the Miconieae whereas coordinated patterns of convergences ingermination traits were found between Melastomeae and Microlicieae and within Microlicieae. These contrasting patterns indicate different selective pressures driving theevolution of seed strategies in different groups and coordinated evolution of dispersal modeand growth-form. In the last chapter, we have evaluated seed behaviour of 18 melastomespecies artificially buried in soils associated with rocky outcrops and grassy fields forregular intervals of six months up to 18 months. Secondary dormancy and annualemergence cycles are documented for the first time for the family. Buried seeds are longlivedbut emergence was independent of microhabitats or favourable conditions forestablishment, suggesting that ecological filtering and niche conservatism both operate inmaintaining the patchy distribution in complex habitats. Overall, our data contribute to theunderstanding of the evolution of seed dormancy and germination in the Neotropical areaand also shed light into the roles of seeds in determining plant distribution in complexhabitats.