Variation in nutrient distribution and potential nutrient losses by selective logging in a humid tropical forest of Rondonia, Brazil

Despite having one of the world's highest deforestation rates, the tropical forest of Rondonia state in the southwest Amazon is virtually unknown in t erms of nutrient dynamics. To fill this knowledge gap, the distribution of nutrients in a humid tropical forest located in the Ecological Reserve of S amuel, Rondonia state, was addressed. A total of 474 trees with diameter at breast height (DBH) > 10 cm were mapped at the Ecological Reserve of Samue l, a 20,000-ha area bordering the Samuel Hydroelectric Reservoir, which is located 50 km south of Porto Velho, Rondonia state, Amazon region, Brazil. These individuals belonged to ca 220 species distributed in 41 families. Fr om this total, leaves of 354 trees were collected for chemical compositiona l analysis; boles of 96 trees and fine branches of 75 trees also were colle cted. Soil samples were gathered for fertility analysis at two soil depth i ntervals: 0-50 and 50-100 cm. In general, soils were acidic and very poor i n terms of elemental contents. For plants, the highest concentrations of P, K, and Mg were found in leaves and fine branches, and the highest concentr ation of Ca was in the litter. The lowest nutrient concentration was observ ed in boles. The nutrient concentrations of leaves collected at Samuel were similar to that found in other terra firme forests of the Amazon region an d other tropical forests growing on poor soils. Comparisons of nutrient con centrations among families, genera, and species revealed that only N concen trations were distinct among botanical taxa; the variability found in conce ntrations of other nutrient were larger and did not reveal any significant differences. Like aboveground biomass, the total nutrient stocks of trees w as concentrated in a few large individuals. This fact reveals important imp lications about the impacts caused by selective logging. Although few trees are cut per hectare in this activity, the effects; on the ecosystem struct ure, due to the loss of nutrients, may be underestimated.