Estabelecimento e distribuição de espécies arbóreas em florestas de várzea na Amazônia Central

Abstract

The seasonal inundation is the main factor influencing establishment, growth and survival of Amazonian floodplain tree species. However, hydrological seasonality is influenced by climatic variability such as El Niño and La Niña, which either causes an extension (La Niño) or reduction (La Niña) of the terrestrial phase and thus the vegetation period. The present study investigated the influence of inundation and climatic variability on establishment, growth, distribution, and structure of different tree species populations of Central Amazonian várzea forests. The study was performed in the Mamirauá Sustainable Development Reserve, near the confluence of the Japurá and Solimões Rivers, Alvarães, Amazon State, Brazil. Two plot types were established along the flood-level gradient: I) Inventories of sapling tree individuals (tree height 1.0 m, diameter at breast height-DBH < 10 cm) and adult tree individuals (DBH 10.0 cm) were realized in 50 plots, each of the size of 2500 m2 (50 x 50 m), totaling an inventoried area of 12.5 ha, and II) Inventories of tree seedlings (tree height between 15 cm - 100 cm) were performed in 50 rectangular plots, each of the size of 50 m2 (1 x 50 m), totaling an inventoried area of 0.25 ha. According to their abundance along the flood-level gradient, tree species were classified as I) high-várzea species (HVS) (mean inundation height: 1.2±0.8 m), low-várzea species (LVS) (mean inundation height: 5.2±1.2 m), and transitional species (TS) (mean inundation height: 3.1±1.7 m). Inundation height and diameters (DBH in adult and sapling individuals, diameter near the soil surface in seedling individuals) of all individuals was recorded. To determine tree ages and diameter increment rates, wood samples with a diameter of 5 mm were extracted from adult tree populations (20 samples per species, 204 in total), and stem disks obtained from regenerating tree populations (min.: 10, max.: 20 per species, 160 in total). Tree ages and diameter increment rates were correlated to the flood-level gradient, using historical daily water-level data of the Negro/Amazon River at the harbor of Manaus since 1903, and to ENSO activity by using data from 1886 until 2010. A total number of 2058 individuals were sampled, which distributed to 1086 adult individuals, 495 seedlings and 477 saplings. Highest tree densities were recorded in LVS, followed by and TS and HVS. In all species populations, diameters were positively correlated to tree ages. In general, adult trees demonstrated higher diameter increment rates than sapling trees. Mean flood duration per year differed significantly I) between El Niño and La Niña years, II) between the investigated species populations, and III) between different age classes within the same species. No impact of La Niña events on tree species establishment and growth could be detected. However, tree regeneration clearly reacted more sensitive to climatic variability than the adult populations. The majority of investigated individuals (52.3%) established during El Niño years, indicating that the extension of the vegetation period is crucial for tree species establishment. Moreover, during El Niño years, tree regeneration demonstrated elevated diameter increment rates compared to other years. Although the seasonal inundations are the most important factor for tree establishment in floodplains, other factors like seed dispersal, competition and diverging recruitment strategies seem importantly to contribute to the distribution patterns of Amazonian várzea tree species.