Distribuição e estrutura da população de quatro espécies madeireiras em uma floresta sazonalmente alagável na reserva de desenvolvimento sustentável Mamirauá, Amazônia central

Abstract

km2 and are influenced by periodical inundations of sediment-rich white-water rivers originating from the Andes and the Andean foothills. Besides intense timber exploitation, várzea forests are endangered through an increasing conversion of forests into agricultural areas and pastures. The aim of the present study was to describe the spatial distribution and the population structure of four high-várzea timber species in dependence of abiotic variables within the Mamirauá Sustainable Development Reserve (MSDR), in order to contribute to the developpment of sustainble management plans. The study was conducted near the area of the community Jarauá, municipality of Alvarães, MSDR. All individuals ≥ 1,0m height of the species Hura crepitans, Guarea guidonia, Sterculia elata and Ocotea cymbarum were inventoried on an area totalling 7.5 ha, and measured in dbh (diameter at breast height) and height. The basal area, volume, and aboveground wood biomass was estimated for each population. The height and period of inundation on all individuals was recorded during the highest water level 2006, and calculated for previous years basing on the daily water-level records of the Negro River at the harbour of Manaus since 1903. The relative photosynthetically active radiation (rPAR) was measured on all individuals ≤10 cm dbh. In total, 344 individuals were inventoried. Ocotea cymbarum showed the highest number of individuals (n=128), followed by Hura crepitans (n=91), Guarea guidonia (n=83) and Sterculia elata (n=42). The major proportion of the individuals of Hura crepitans and Sterculia elata were characterized by diameters ≤ 10 cm dbh (51.6%, and 62.7%, respectively), indicating high establishment rates in these species. Hura crepitans was the species with the highest basal area, volume, and aboveground wood biomass. The populations of Guarea guidonia, Hura crepitans, Ocotea cymbarum, and Sterculia elata were subjected to mean inundation heights of 0.10±0.08 m, 0.31±0.23 m, 0.36±0.22 m, and 0.16±0.15 m, corresponding to a mean period of inundation of 10.9±7.4 days year-1, 27.4±18.0 days year-1, 31.5±17.0 days year-1 and 15.9±13.3 days year-1, respectively. Average rPAR amounted to 3.2±1.8 % in Hura crepitans; 2.5±1.4 % in Sterculia elata; 1.9±0.7 % in Ocotea cymbarum and 1.5±0.6 % in Guarea guidonia. The topographically variations in the populations of Ocotea cymbarum and Hura crepitans indicate wider amplitudes of distribution than that recorded in the populations of Sterculia elata and Guarea guidonia. Despite the low rPAR values and variations recorded within the study site, the different light requirements in each species suggest different ecological niches. The results about the population structure indicate that all species present individuals > 50 cm dbh, which is the minimum logging diameter in the normative instructions established by the IBAMA. However, individual densities in these diameter classes were very low. If managed as suggested by the normative instructions, the populations of the investigated tree species maybe endangered with risk of extinction at the study site. Due to its topographically position, high-várzea forests are subjected to low impacts of the hydro-geomorphologic dynamic. However, the micro-topographical variations directly influence on the distribution of the studied tree species.