Pervasive alteration of tree communities in undisturbed Amazonian forests

Laurance, W.F. et al. (2004) Pervasive alteration of tree communities
in undisturbed Amazonian forests. Nature 428, 171–175

Amazonian rainforests are some of the most species-rich tree communities on earth1. Here we show that, over the past two decades, forests in a central Amazonian landscape have experienced highly nonrandom changes in dynamics and composition. Our analyses are based on a network of 18 permanent plots unaffected by any detectable disturbance. Within these plots, rates of tree mortality, recruitment and growth have increased over time. Of 115 relatively abundant tree genera, 27 changed significantly in population density or basal area—a value nearly 14 times greater than that expected by chance. An independent, eight-year study in nearby forests corroborates these shifts in composition. Contrary to recent predictions2–5, we observed no increase in pioneer trees. However, genera of faster-growing trees, including many canopy and emergent species, are increasing in dominance or density, whereas genera of slower-growing trees, including many subcanopy species, are declining. Rising atmospheric CO2 concentrations6 may explain these changes, although the effects of this and other large-scale environmental alterations remain uncertain. These compositional changes could have important impacts on the carbon storage, dynamics and biota of Amazonian forests.

Cropping and fallowing sequences of small farms in the "terra firme" landscape of the Brazilian Amazon: a case study from Santarem, Para

Scatena, F. N.; Walker, R. T.; Homma, A. K. O.; de Conto, A. J.; Palheta Ferreira, C. A.; Carvalho, R. A.; Neves da Rocha, A. C. P.; Moreira dos Santos, A. I., and Mourao de Oliveira, P. 1996. Cropping and fallowing sequences of small farms in the "terra firme" landscape of the Brazilian Amazon: a case study from Santarem, Para.. Ecological Economics . 18(1) :29-40.

This paper analyzes field survey results and develops a conceptual model of the factors that influence cropping and fallowing practices on small farms in the terra firme landscape near Santarem, Brazil. A multi-fallow cultivation system that used rice, corn and bitter manioc in various relay-intercropping combinations was the most common cultivation practice observed. Five different types of fallow vegetation were identified and used by the farmers: (1) mature forest vegetation greater than 20 years old; (2) secondary forest vegetation 8 to 12 years old; (3) young secondary forest vegetation 3 to 6 years old; (4) brushy vegetation 2 to 4 years old; and (5) weed vegetation less than 2 years old. Distinct relay-intercropping sequences were associated with each of these fallows. We suggest that the selection of fallow length and cropping sequence is subject to the following general constraints: (1) the productivity of the landscape as determined by soil, water and climate; (2) ecological requirements and risks associated with particular crops; (3) land availability and the costs of site preparation, and cultural treatments; (4) the availability of hired labor; (5) the age structure of the families, their subsistence requirements and preferences for particular crops, leisure and non-farm-related production activities; and (6) local economic conditions including land values, access to credit and non-farm-related employment, and the conditions of commodity markets. To maximize agricultural production and general household utility given these constraints, the farmers have several options, including: (1) varying the length of fallows; (2) varying the types and sequences of crops that are planted following a given fallow; (3) modifying the clearing and cultivation practices; (4) improving subsequent yields by managing regeneration within a fallow; (5) developing diversified land use systems that contain combinations of pasture, perennials, semi-permanent annuals, areas of extractive reserves and true shifting cultivation; and (6) increasing production through the use of external inputs like fertilizer, irrigation and farm machinery. Most farmers in the study area have chosen to modify cropping sequences and vary the lengths of natural fallows rather than using expensive external inputs. Our data suggest that a major factor in selecting a fallow length is the cost of land clearance and preparation. Moreover, since clearing costs are dramatically reduced for young secondary vegetation, the reduction in site preparation costs over several short rotations compensates for the lost production caused by using short fallows instead of long fallows.
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