Tag Archives: secondary forests Amazon

Deforestation Tolerance: Amazon

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Amazon generates approximately half of its own rainfall by recycling moisture 5 to 6 times as airmasses move from the Atlantic across the basin to the west.  From the start, the demonstration of the hydrological cycle of the Amazon raised the question of how much deforestation would be required to cause this hydrolological cycle to degrade to the point of being unable to support rain forest ecosystems.

High levels of evaporation and transpiration that forests produce throughout the year contribute to a wetter atmospheric boundary layer than would be the case with non-forest.This surface-atmosphere coupling is more important where large-scale factors for rainfall formation are weaker, such as in central and eastern Amazonia. Near the Andes, the impact of at least modest deforestation is less dramatic because the general ascending motion of airmasses in this area induces high levels of rainfall in addition to that expected from local evaporation and transpiration.

Where might the tipping point be for deforestation-generated degradation of the hydrological cycle? The very first model to examine this question  showed that at about 40% deforestation, central, southern and eastern Amazonia would experience diminished rainfall and a lengthier dry season, predicting a shift to savanna vegetation to the east.

Moisture from the Amazon is important to rainfall and human wellbeing because it contributes to winter rainfall for parts of the La Plata basin, especially southern Paraguay, southern Brazil, Uruguay and central-eastern Argentina; in other regions, the moisture passes over the area, but does not precipitate out. Although the amount contributing to rainfall in southeastern Brazil is smaller than in other areas, even small amounts can be a welcome addition to urban reservoirs…

In recent decades, new forcing factors have impinged on the hydrological cycle: climate change and widespread use of fire to eliminate felled trees and clear weedy vegetation. Many studies show that in the absence of other contributing factors, 4° Celsius of global warming would be the tipping point to degraded savannas in most of the central, southern, and eastern Amazon. Widespread use of fire leads to drying of surrounding forest and greater vulnerability to fire in the subsequent year.

We believe that negative synergies between deforestation, climate change, and widespread use of fire indicate a tipping point for the Amazon system to flip to non-forest ecosystems in eastern, southern and central Amazonia at 20-25% deforestation.

We believe that the sensible course is not only to strictly curb further deforestation, but also to build back a margin of safety against the Amazon tipping point, by reducing the deforested area to less than 20%, for the commonsense reason that there is no point in discovering the precise tipping point by tipping it. At the 2015 Paris Conference of the Parties, Brazil committed to 12 million ha of reforestation by 2030. Much or most of this reforestation should be in southern and eastern Amazonia.

Excerpts from Amazon Tipping Point  by Thomas E. Lovejoy and Carlos Nobre, Sciences Advances,  Feb. 21, 2018

Survival of Tropical Forests: bird predators

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[T]he Amazon rainforest contains more than 1,500 bird species. Around a quarter of them are found nowhere else on Earth. Many of these birds have evolved to fill a specific role – whether that means eating particular types of insects, or scattering a certain size of seed….A new paper published in Proceedings of the Royal Society explores  the link between deforestation in the Amazon and local bird diversity…[B]ird data was collected in 330 different sites in the Brazilian state of Pará, including arable and pastoral farmland and both primary and secondary forests. Primary forests are the original native vegetation, now increasingly degraded by logging and wild fires. Secondary forests are those which grow back in areas, often farmland, which have been abandoned by people…

The study focused on seed dispersal and insect predation, two ecosystem processes where birds play important roles. Fruit-eating (or frugivorous) birds spread the seeds of forest trees. Insect-eating (insectivorous) birds ensure that any germinating saplings have a fighting chance at survival. ..[S]witching from primary tropical forests to farmland dramatically reduced the “services” birds were able to provide.

This may seem fairly intuitive so far, given that there is a world of difference between a forest and cattle pasture. However, more significantly it was found that the traits were only partially restored in regenerating secondary forests. These areas have been branded as the “forests of the future” but we found them coming up short. These “forests of the future” cannot conserve all the biological interactions realised in primary forests, undisturbed or otherwise, which are essential for biodiversity conservation.  Once large seed-dispersing birds such as guans or cotingas are lost in an area, trees species with large seeds find it harder to recover. Regeneration becomes unlikely or impossible. Research from Brazil’s coastal Atlantic Forest has shown that the loss of such key species is driving the evolution of palm trees with smaller seeds. Some of these links may have been lost before we even knew them….The sorts of generalist insect-eaters that come to dominate farmland aren’t generally able to capture the well-disguised insects found in adjacent patches of forest.

Excerpt from Without birds, tropical forests won’t bounce back from deforestation, the conversation.com, Nov. 8, 2016