Investigations of Mangrove Forest Dynamics in Amazonia, North Brazil

Abstract

The north coast of the Brazilian states of Para, and Maranhao, presents a large continuous belt of mangroves, covering an area of about 700 000 ha. Although mangroves in this region are relatively well preserved, expanding tourism, intensification of fisheries and of urban growth in the region may endanger this important coastal ecosystem. In spite of the immense area covered by mangrove forest, very few is known about species distribution and forest ecology in the region. The present study focuses on the growth dynamics of the mangrove species Rhizophora mangle L. under different conditions of inundation frequency, salinity and rainfall regime on the north coast of Brazil. The study concentrates on: 1) evidence of seasonality in primary production, reflected in phenology, litter fall and interactions with herbivores; 2) application of dendrochronological methods on R. mangle, including determination of tree age and influence of climatic and site-specific factors on tree growth; 3) applying these findings in an analysis of forest structure and dynamics on Ajuruteua Peninsula. Wood samples for dendrochronology were taken at three points along the coast of Para: Viseu, Sao Joao de Pirabas and Braganca. Detailed studies of phenology and litter fall, herbivory, forest structure and forest evolution were developed at different sites on Ajuruteua Peninsula, Braganca district.A study on litter fall and phenology demonstrated that rainfall seasonality is reflected in mangrove tree primary production. The description of a severe infestation by moth larvae in mangrove forests demonstrated that, besides climate and site-specific conditions, herbivory can be substantially influence the primary production.The wood of all studied specimens of R. mangle showed distinct rings. The rings were formed by a light and a dark layer. The light layer, formed at the end of the rainy season, is the result of a higher density of vessels; the dark layer, formed at the end of the dry season. The increase or decrease of vessels may be a reaction to seasonal salinity variation. Radiocarbon analysis showed that growth rings are annual. This allowed to the determination of tree age. The oldest tree was 111 years old.Growth curves revealed a linear growth (absence of trend-age). Growth rates varied strongly. The highest (however, not significantly different) rates were found at the sector of the Para coast with the highest tidal amplitudes, suggesting an influence of the tide on tree growth. The relationship between rainfall and growth was not easily interpretable, however, growth was positively correlated to rainfall in some areas. A presumed influence of rainfall may have been superimposed by the impact of other factors as salinity and input of fresh water.Growth rates on Ajuruteua Peninsula were highly variable. Trees were tentatively divided in three groups (fast, medium and slow growth). Although trees from each study sitecould occur in more than one group, there is an obvious trend that trees from frequently inundated areas dominate the group with the highest growth rate. Considering that all sites are submitted to similar climatic conditions, the absence of fast growing trees at certain sites may be due to the influence of local factors (e. g. salinity, inundation frequency or also neighbourhood competition among trees). It is assumed that neighbourhood competition is an important source of within-site variability of growth rates.The determination of the age of forest stands in combination with a classical study of forest structure allowed to propose a model of forest succession. This study demonstrated that the combination of forest structure surveys and dendrochronological methods provided informations concerning trees growth and forest development that were up to now not available. The application of these informations in mangrove forest management and preservation is suggested. In spite of the weak correlations between tree growth and climatic factors, dendrochronology may also help to understand changes of coastal vegetation within the past decades.