Partilha de recurso e distribuição de riscos em espécies anuais da zona tropical semiárida

Abstract

Theoretical and empirical inferences about resources sharing and risk spreading based on seed germination and plants phenodynamics are concentrated in studies of annual desert species. These mechanisms should be standard for species that coexist in any environment where the risk of reproductive failure is high, as in Brazilian tropical semiarid. This study‟s intend is to investigate if annual species of the caatinga have mechanisms that demonstrate these strategies, such as germination asynchrony, differences in the germination speed, reproduction and life cycle. Germination and phenodynamic of annual plants were analyzed under continuous irrigation treatments and under three irrigation regimes, simulating rain events typical of the semiarid region (above average, intermediate and dry), whose standard was defined by analysis of the precipitation distribution in a temporal series for 30 years. To check if the responses of the germination in the soil seed bank were not influenced by the age structure of seeds, two experiments of continuous irrigation were conducted: 1) seeds from seed rain (same generation), and 2) seeds from the soil seed bank. The germination date of each individual was registered every two days and the reproductive phenodynamic and life cycle at weekly intervals. Differences in germination rate of species within and between treatments were observed using ANCOVA. This same analysis was performed to verify the relationship between the life cycle and reproduction time in each treatment. Changes in the initial time of flowering, reproductive duration period and life cycle of the species within and between treatments and their interaction was verified by two-way ANOVA. The results showed that the germination speed was different between the same species in different treatments for at least four species. Was the formation of three groups of species that differed in terms of speed, within the treatments. Four species of the seed bank had a higher germination speed among the soil seed bank and the rain of seeds. All species presented asynchronous germination independent of treatment and seeds generation. This indicates that the species have mechanisms that promote the interespecific sharing of resource and intraspecific dispersion of risk. Reproductive events and the life cycle of species also showed different interespecific responses and were affected within precipitation treatments. The covariance analysis showed that, the longer the supply of the resource, the greater the duration of reproductive period and life cycle of the species, with significant differences among all treatments. There were also significant differences between treatments between species and the interaction among these for flowering initial time (p <0.001), reproduction time (p <0.001) and life cycle duration of the species (p <0.001). These data suggest that herbaceous species of the caatinga present adaptations to unpredictability in the supply of water resources, and present a set of strategies that promote coexistence in the beginning and final stages of the populations‟ life cycle.