Approximately 2.5 × 10 ⁶ square kilometers of the Amazon forest are currently degraded by fire, edge effects, timber extraction, and/or extreme drought, representing 38% of all remaining forests in the region. Carbon emissions from this degradation total up to 0.2 petagrams of carbon per year (Pg C year ⁻¹ ), which is equivalent to, if not greater than, the emissions from Amazon deforestation (0.06 to 0.21 Pg C year ⁻¹ ). Amazon forest degradation can reduce dry-season evapotranspiration by up to 34% and cause as much biodiversity loss as deforestation in human-modified landscapes, generating uneven socioeconomic burdens, mainly to forest dwellers. Projections indicate that degradation will remain a dominant source of carbon emissions independent of deforestation rates. Policies to tackle degradation should be integrated with efforts to curb deforestation and complemented with innovative measures addressing the disturbances that degrade the Amazon forest.

The Brazilian Amazon is currently experiencing the world9s highest absolute rate of forest destruction and is likely to suffer even greater degradation in the future because of government plans to invest $40 billion from 2000 to 2007 in dozens of major new highways and infrastructure projects. We developed two computer models that integrate spatial data on deforestation, logging, mining, highways and roads, navigable rivers, vulnerability to wildfires, protected areas, and existing and planned infrastructure projects, in an effort to predict the condition of Brazilian Amazonian forests by the year 2020. Both models suggest that the region9s forests will be drastically altered by current development schemes and land-use trends over the next 20 years.

Soybeans represent a recent and powerful threat to tropical biodiversity in Brazil. Developing effective strategies to contain and minimize the environmental impact of soybean cultivation requires understanding of both the forces that drive the soybean advance and the many ways that soybeans and their associated infrastructure catalyse destructive processes. The present paper presents an up-to-date review of the advance of soybeans in Brazil, its environmental and social costs and implications for development policy. Soybeans are driven by global market forces, making them different from many of the land-use changes that have dominated the scene in Brazil so far, particularly in Amazonia. Soybeans are much more damaging than other crops because they justify massive transportation infrastructure projects that unleash a chain of events leading to destruction of natural habitats over wide areas in addition to what is directly cultivated for soybeans. The capacity of global markets to absorb additional production represents the most likely limit to the spread of soybeans, although Brazil may someday come to see the need for discouraging rather than subsidizing this crop because many of its effects are unfavourable to national interests, including severe concentration of land tenure and income, expulsion of population to Amazonian frontier, and gold-mining, as well as urban areas, and the opportunity cost of substantial drains on government resources. The multiple impacts of soybean expansion on biodiversity and other development considerations have several implications for policy: (1) protected areas need to be created in advance of soybean frontiers, (2) elimination of the many subsidies that speed soybean expansion beyond what would occur otherwise from market forces is to be encouraged, (3) studies to assess the costs of social and environmental impacts associated with soybean expansion are urgently required, and (4) the environmental-impact regulatory system requires strengthening, including mechanisms for commitments not to implant specific infrastructure projects that are judged to have excessive impacts.

Tropical tree height-diameter (H:D) relationships may vary by forest type and region making large-scale estimates of above-ground biomass subject to bias if they ignore these differences in stem allometry. We have therefore developed a new global tropical forest database consisting of 39 955 concurrent H and D measurements encompassing 283 sites in 22 tropical countries. Utilising this database, our objectives were: 1. to determine if H:D relationships differ by geographic region and forest type (wet to dry forests, including zones of tension where forest and savanna overlap). 2. to ascertain if the H:D relationship is modulated by climate and/or forest structural characteristics (e.g. stand-level basal area, A). 3. to develop H:D allometric equations and evaluate biases to reduce error in future local-to-global estimates of tropical forest biomass. Annual precipitation coefficient of variation (P_V), dry season length (S_D), and mean annual air temperature (T_A) emerged as key drivers of variation in H:D relationships at the pantropical and region scales. Vegetation structure also played a role with trees in forests of a high A being, on average, taller at any given D. After the effects of environment and forest structure are taken into account, two main regional groups can be identified. Forests in Asia, Africa and the Guyana Shield all have, on average, similar H:D relationships, but with trees in the forests of much of the Amazon Basin and tropical Australia typically being shorter at any given D than their counterparts elsewhere. The region-environment-structure model with the lowest Akaike's information criterion and lowest deviation estimated stand-level H across all plots to within amedian −2.7 to 0.9% of the true value. Some of the plot-to-plot variability in H:D relationships not accounted for by this model could be attributed to variations in soil physical conditions. Other things being equal, trees tend to be more slender in the absence of soil physical constraints, especially at smaller D. Pantropical and continental-level models provided less robust estimates of H, especially when the roles of climate and stand structure in modulating H:D allometry were not simultaneously taken into account.