Changes in Forest Structure and Biomass over Ten Years in a Lowland Amazonian Forest

The topographic effects on biomass and its increment were evaluated for a terra-firme forest near Manaus, Brazil. We established three plots (60 × 60 m) on a plateau and at the bottom of a valley (namely baixio), and measured stem diameter at breast height D (> 10 cm) for tree and palm species in 2002 and 2012. Small trees (10 cm > D > 5 cm) were measured in subplots (20 × 60 m) within each plot. We also estimated tree height with the D – tree height H relationship model based on a hundred samples of the tree species in each topography. The aboveground biomass AGB was estimated as 315.4 ± 33.2 (average ± SE) Mg ha1 on the plateau, which was higher than 224.2 ± 20.3 Mg ha1 in baixio though the difference was statistically marginal (p = 0.09). The difference in AGB was partly ascribed to the decrease of H for large trees and a lack of large trees whose D > 80 cm at the baixio site. The AGB of palm species in baixio was 2.7 ± 0.87 Mg ha1, which was higher than 0.24 ± 0.08 Mg ha1 on the plateau though the difference was statistically insignificant (p = 0.11). On the other hand, the increment of AGB did not differ significantly between the two sites (p > 0.05), and the variation of AGB increment among the plots was mainly ascribed to the variation of biomass loss caused by mortality. Discipline: Forestry and forest products Additional key words: biomass increment, topography, Amazon This study was conducted as a part of an international cooperative research project between Japan and Brazil entitled Carbon Dynamics of Amazonian Forests (2009-2015), and supported by the Science and Technology Research Partnership for Sustainable Development Program of the Japan Science and Technology Agency (JST) and the Japan International Cooperation Agency (JICA). Present address: 3 Forestry Department, Federal University of Mato Grosso (Sinop-MT 78550-000, Brazil) *Corresponding author: e-mail swrmp2005@yahoo.co.jp (R. Suwa) Received 31 August 2015; accepted 19 February 2016. Introduction An assessment of forest biomass in tropical forests is essential for studies on ecosystem productivity, which is crucial for a better understanding of global climate change (Fearnside 2003). There are large variations of biomass in the Amazon, such as the aboveground biomass (AGB) reportedly ranging from 155 to 492 Mg ha1 (Houghton et al. 2001). In a lowland Amazonian forest, namely the terrafirme forest, the topographic effects on the edaphic factors are distinct, and the plateau and bottom of the valley (hereinafter referred to as baixio) are generally characterized by clayish and sandy soils, respectively (Ferraz et al. 1998). The relatively infertile sandy soil in baixio as compared to the clayish soil on the plateau could result in higher biomass on the plateau than in baixio (Castilho et al. 2006). On the other hand, studies on the topographic effects on biomass increment are apparently scarce except for the short-term research (at two-year intervals) by Castilho et al. (2010). In the present study, we conducted repeated measurements of biomass for two types of topographic features (plateau and baixio) that included tree and palm species, in