Fenologia da dispersão e estratégias de germinação de sementes de plantas lenhosas e herbáceas no Cerrado
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Data
2019-08-05
Autores
Escobar, Diego Fernando Escobar [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
A germinação de sementes e estabelecimento de plântulas são as etapas iniciais e cruciais do ciclo de vida das plantas, sendo importantes determinantes dos padrões de distribuição, diversidade e dinâmica das comunidades vegetais. Nos ecossistemas sazonais, a disponibilidade de água no solo e a heterogeneidade do ambiente são os principais fatores limitantes da germinação e estabelecimento das plântulas. Nestes ambientes a fenologia da dispersão das sementes, o nicho de germinação e a dormência devem sincronizar a germinação com o início da estação chuvosa, o que maximiza o período favorável ao estabelecimento, incrementando as chances de sobrevivência das plântulas à subsequente estação seca. Por outro lado, as plantas respondem à heterogeneidade ambiental distribuindo a progênie através do espaço ou do tempo. Distribuir as sementes ao longo de diferentes áreas aumenta a chance de que pelo menos uma parte delas chegue a lugares adequados para a germinação e estabelecimento das plântulas. Por outro lado, a dormência incrementa a variação do tempo de germinação distribuindo a germinação em vários eventos, o que diminui a probabilidade de que todas as plântulas morram devido a eventos imprevisíveis. Os estudos dos mecanismos que sincronizam a germinação com o início da estação chuvosa nos ambientes tropicais sazonais não levam em consideração o nicho de germinação nem a história evolutiva das espécies, e estão focados majoritariamente nos ecossistemas florestais. Portanto, não temos uma clara compreensão da importância relativa e da interação dos diferentes mecanismos que controlam o tempo de germinação nos ambientes sazonais tropicais, nem de como são afetados por outras características da história natural e evolutiva das espécies. Além disso, o papel da dormência (i.e., dispersão temporal) como mecanismo para enfrentar a heterogeneidade ambiental tem sido pouco explorado nos ambientes sazonais. Desta forma, nesta tese avaliei os mecanismos que as plantas do cerrado utilizam para sincronizar a germinação com o início da estação chuvosa, e para enfrentar a heterogeneidade ambiental, e como variam tais mecanismos em relação a características funcionais e evolutivas das espécies. Para tanto, caracterizei o nicho térmico de germinação e a dormência mediante experimentos de germinação, e a síndrome de dispersão, a fenologia da dispersão, o peso das sementes e a forma de crescimento de 82 espécies de cerrado de 26 famílias, que incluem as principais linhagens de angiospermas e uma ampla gama de formas de crescimento. Os resultados mostram que a dormência é uma característica adaptativa no cerrado, já que incrementa as chances de estabelecimento das plântulas ao sincronizar a germinação com o início da estação chuvosa e distribuir o risco de recrutamento das espécies com capacidade limitada de dispersão espacial (i.e., autocóricas). A dormência está correlacionada evolutivamente com a dispersão espacial, mas não com o peso das sementes ou a fenologia da dispersão, indicando um trade-off entre a dispersão espacial e temporal. Além disso, a dormência e o nicho de germinação são mecanismos alternativos para enfrentar a sazonalidade climática e a importância relativa destes mecanismos varia com a estação de dispersão. Desta forma, a dormência é chave para entender a grande diversidade vegetal do cerrado, não só porque incrementa as chances de estabelecimento das plântulas, mas também porque é crucial para o recrutamento de espécies autocóricas em locais que são desfavoráveis para o estabelecimento das plântulas de espécies zoocóricas e anemocóricas. Finalmente, nossos resultados mostram que a dormência e o comportamento germinativo não diferirem entre fisionomias de cerrado, de lenhosas a herbáceas, o que indica que a sazonalidade climática é um filtro forte para a germinação e estabelecimento das plântulas ao longo das vegetações do cerrado, e respondem de forma similar à sazonalidade climática independentemente do habitat.
Seed germination and seedling establishment are the initial and crucial stages of a plant’s life cycle and are important in determining plant community dynamics and plant distribution and diversity patterns. In seasonal environments, water availability in the soil and environmental heterogeneity are the main factors that limit seed germination and seedling establishment. In these environments, seed dispersal season, germination niche, and dormancy synchronize germination with the onset of the rainy season, which maximizes the favorable period for seedling establishment and increases the chances of seedlings surviving the subsequent dry season. Plants respond to environmental heterogeneity by distributing germination through space and time. Distributing seeds throughout different areas (spatial dispersal) increases the chances that at least some will arrive in sites adequate for germination and establishment. On the other hand, dormancy increases the variation in germination time, distributing germination into multiple reproductive events (temporal dispersal), thus decreasing the likelihood that all seedlings die due to unpredictable events. However, most studies addressing the mechanisms that synchronize germination with the onset of the rainy season in seasonal tropical environments do not consider germination niche or species evolutionary history and are mainly focused on forest ecosystems. Therefore, a clear understanding on the relative importance of the mechanisms controlling germination time, on the interaction between these mechanisms, and on how they might be affected by life history traits and evolution is still lacking in seasonal tropical environments. Moreover, the role dormancy plays as a mechanism to face environmental heterogeneity has not been thoroughly explored in seasonal environments. Thus, in this thesis, I evaluated the mechanisms that cerrado plants use to synchronize germination with the onset of the rainy season and to face environmental heterogeneity. I also analyzed how such mechanisms vary in relation to plants functional and evolutionary characteristics. For such, dormancy and the thermal germination niche of 82 cerrado plant species, belonging to 26 families and including several growth forms, were determined from germination experiments. Additionally, dispersal syndrome, dispersal season, and seed mass were also determined for each species. Our results show that dormancy is an adaptative characteristic in cerrado as it increases the chances of seedling establishment by synchronized germination with the onset of the rainy season and by distributing the risk of recruitment of species with limited spatial dispersal (i.e., authochorous). Our results also demonstrate that dormancy is evolutionarily correlated with spatial dispersal, but not with seed mass or dispersal season, indicating there is a trade-off between spatial and temporal dispersal. Furthermore, dormancy and germination niche are alternative mechanisms used to face seasonality, and the relative importance of these mechanisms varies with dispersal season. Hence, dormancy is key to understanding the great plant diversity found in cerrado, not only because it increases the chances for seedling establishment, but also because it is crucial for the successful recruitment of autochorous species in sites that are unfavorable for the establishment of zoochorous and anemochorous species. Finally, our results show that plant species from closed to open cerrado physiognomies show similar germination traits and proportion of dormancy, suggesting that seasonality is a strong filter for both germination and seedling establishment throughout the cerrado plant communities and other seasonally dry ecosystems.
Seed germination and seedling establishment are the initial and crucial stages of a plant’s life cycle and are important in determining plant community dynamics and plant distribution and diversity patterns. In seasonal environments, water availability in the soil and environmental heterogeneity are the main factors that limit seed germination and seedling establishment. In these environments, seed dispersal season, germination niche, and dormancy synchronize germination with the onset of the rainy season, which maximizes the favorable period for seedling establishment and increases the chances of seedlings surviving the subsequent dry season. Plants respond to environmental heterogeneity by distributing germination through space and time. Distributing seeds throughout different areas (spatial dispersal) increases the chances that at least some will arrive in sites adequate for germination and establishment. On the other hand, dormancy increases the variation in germination time, distributing germination into multiple reproductive events (temporal dispersal), thus decreasing the likelihood that all seedlings die due to unpredictable events. However, most studies addressing the mechanisms that synchronize germination with the onset of the rainy season in seasonal tropical environments do not consider germination niche or species evolutionary history and are mainly focused on forest ecosystems. Therefore, a clear understanding on the relative importance of the mechanisms controlling germination time, on the interaction between these mechanisms, and on how they might be affected by life history traits and evolution is still lacking in seasonal tropical environments. Moreover, the role dormancy plays as a mechanism to face environmental heterogeneity has not been thoroughly explored in seasonal environments. Thus, in this thesis, I evaluated the mechanisms that cerrado plants use to synchronize germination with the onset of the rainy season and to face environmental heterogeneity. I also analyzed how such mechanisms vary in relation to plants functional and evolutionary characteristics. For such, dormancy and the thermal germination niche of 82 cerrado plant species, belonging to 26 families and including several growth forms, were determined from germination experiments. Additionally, dispersal syndrome, dispersal season, and seed mass were also determined for each species. Our results show that dormancy is an adaptative characteristic in cerrado as it increases the chances of seedling establishment by synchronized germination with the onset of the rainy season and by distributing the risk of recruitment of species with limited spatial dispersal (i.e., authochorous). Our results also demonstrate that dormancy is evolutionarily correlated with spatial dispersal, but not with seed mass or dispersal season, indicating there is a trade-off between spatial and temporal dispersal. Furthermore, dormancy and germination niche are alternative mechanisms used to face seasonality, and the relative importance of these mechanisms varies with dispersal season. Hence, dormancy is key to understanding the great plant diversity found in cerrado, not only because it increases the chances for seedling establishment, but also because it is crucial for the successful recruitment of autochorous species in sites that are unfavorable for the establishment of zoochorous and anemochorous species. Finally, our results show that plant species from closed to open cerrado physiognomies show similar germination traits and proportion of dormancy, suggesting that seasonality is a strong filter for both germination and seedling establishment throughout the cerrado plant communities and other seasonally dry ecosystems.
Descrição
Palavras-chave
Dormência, Nicho térmico de germinação, Sazonalidade, Heterogeneidade ambiental, Savana neotropical, Mecanismos de redução de risco, Dormancy, Thermal germination niche, Seasonality, Environmental heterogeneity, Neotropical savanna, Risk reduction mechanisms