Atributos de historia de vida, estrutura e dinâmica populacional de árvore sob clima tropical sazonalmente seco

Abstract

The rainfall seasonality and stochasticity are the main regulators of structure and population dynamics in seasonally dry tropical vegetations. This stochasticity, however, can result in heterogeneous environments both in relation to water availability as in the availability of vertical and horizontal light. In deciduous thorny woodland vegetation of semiarid northeastern Brazil (caatinga), one of the most abundant woody species is Cordia oncocalyx, endemic tree and explored through extractivism due to its timber, energetic, foraging, medicinal, and scenic value. To understand how the caatinga trees respond to rainfall seasonality and stochasticity were analyzed, over two years, the structure and population dynamics of C. oncocalyx in Serra das Almas RPPN, Crateús-Ceará. Were determined: i) ontogenetic stages, architectural model and height-diameter relationship, ii) phenodynamics, iii) spatial pattern, iv) birth, mortality, recruitment and growth biometric rates. Parameters of structure and dynamics were correlated with rainfall, soil humidity, temperature and vertical and horizontal light availability. The structure of C. oncocalyx resembles that of shade-tolerant species of tropical rainforest to concentrate the highest density in the early stages, but differs because it forms the bank of infant rather than a seedling bank. The availability of vertical light, with only 4.7% reaching the ground, and Prévost architecture indicate light as a limiting factor in the early stages. Nevertheless, the allometric coefficient differ from geometric similarity, elastic similarity and constant stress models. The uninterrupted growth in diameter, even after individuals reach the maximum height, indicates the low density of trees and water restrictions as the main driving strenght of allometry, which may limit the growth in height, but not in diameter. Both the vegetative and reproductive phenophases respond similarly to variations in rain pulses, with adjustments in time, duration, and intensity, which were correlated with variations in rainfall and soil humidity, excluding photoperiod as a trigger. Lower synchrony, temporal separation of phenophases, and storage of fruits on the ground were risk-spreading strategies used by the population in the dry year. The change of aggregation in seeds and seedlings for randomness in the infant, and the decoupling of reproductive and seeds in relation to juvenile and immature stages, may indicate that the density-dependent mortality operates in the spatial structure of the population. However, the resumption of aggregation in juvenile, virginile and reproductive stages indicates that rainfall stochasticity creates the aggregate distribution of these stages. The dynamics of birth and recruitment occur in the rainy season, but recorded mortality only in seedling and infant occurs throughout the year with a peak in the dry months. The height and diameter growth showed decline or stagnation in the drought and increase in the rainfall. Juveniles showed greater growth in height in areas with more open canopy (rs = 0.24). The rate of population increase (ʎ) was 1.0336 and the highest sensitivity was in the transition from infant to juvenile. Local dominance and population growth tendency are explained by the formation of two banks, seeds and infants, which ensure a stock for recomposition even in drought years.