Reduced rainfall in the Amazon rainforest is increasing forest mortality. Fewer trees exacerbate regional periods of drought, leading to a self-amplified forest loss. A study conducted by an international team of researchers recommends supporting measures to safeguard diversity in the Amazon, as this maintains the resilience of this ecosystem.
It is one of the tipping point elements of the world’s climate system: the Amazon rainforest. If its surface area continues to shrink at the same rate as in previous decades, it could lead to drastic changes in Earth’s climate.
For a study published in ‘Nature Communications’, Professor Anja Rammig and Dr. Delphine Clara Zemp together with an international team examined how the Amazon is reacting to the more extreme periods of drought which are to be expected in the future due to global climate change. The situation is exacerbated by the human-caused land-use activities in the Amazon rainforest.
In the study, conducted at the Potsdam Institute for Climate Impact Research, the researchers demonstrate that on the one hand, the interplay between vegetation and the atmosphere sustains forest growth with the cascading effect of water transport. On the other hand, the loss of this effect can lead to a self-amplified forest loss. ‘Yet the consequences of this feedback between the plants on the ground and the atmosphere above them so far was not clear’, says Delphine Clara Zemp.
‘Our study provides new insight into this issue, highlighting the risk of self-amplifying forest loss which comes on top of the forest loss directly caused by the rainfall reduction.’ Such forest loss in the Amazon rainforest can affect contiguous areas while also having an effect on remote downwind regions, which may not seem to be directly connected to the rainforest.
The Amazon Reacts like an Organism
The explanation for the phenomenon analyzed by the researchers is that the trees of the Amazon together form a large functional network. The Amazon is essentially comparable to a single large organism: If one area is weakened, a ‘wound’ can open up in a completely different area. The researchers refer to this as non-linear forest decline, a phenomenon that can be explained using the analogy of an organism outlined above. ‘That makes it hard to make predictions on the effects of future droughts,’ states biologist Rammig.
But how does the rainfall cycle in the Amazon work? ‘The moisture is swept into the Brazilian Amazon region at the level of the equator, and it rains. The trees usually take up water through their roots and release it back into the atmosphere through their leaves. In this constant cycle, known as moisture recycling, water is transported across the Amazon from the Andes to Argentina,’ explains Rammig, demonstrating the process on a map of South America. ’The rainfall depends on how much moisture the trees release into the atmosphere, forest loss in the northern part of the Amazon can as a reaction lead to a additional forest loss further south or west.
‘As powerful as the cycle is, it is also surprisingly susceptible to environmental changes’, says co-author Henrique M.J. Barbosa from the Universidade de Săo Paulo - ‘and humankind is imposing massive perturbations on Amazonia by both cutting down the trees and heating up the air with greenhouse gases, which reduces large-scale moisture transport and precipitation, and end up affecting even the untouched patches of the forests.’
Data Comparison with the Last Ice Age
The vicious cycle is primarily triggered by extreme periods of drought, as the authors of the study conclude, which is a result from the German-Brazilian Research Training Group on Dynamical Phenomena in Complex Networks (IRTG1740) together with the Humboldt University in Berlin. They therefore used data from the last ice age, 20,000 years ago, for comparison. In the Amazon rainfall was reduced by 50 percent during this period and drought events were frequent, comparable to present-day weather phenomena.
Under a dry-season halving of rainfall , at least ten percent of the forest might be lost due to effects of self-amplification alone, adding to the substantial direct forest losses from reduced water availability.
A tenth might not sound like much. But if the forest ultimately recedes, there will be hardly any fertile soil left, because all the nutrients in the Amazon are stored in plants. It would also have a devastating effect on the ability to store carbon dioxide (CO2). We could as a result be heading for a tipping point, causing an irreversibly drastic change to the status quo.
Taking into account the puzzlements of the vegetation-atmosphere-feedback, self-amplified forest dieback could amount up to 38 percent of the Amazon basin. In combination with the direct effects of the droughts, in fact most of the Amazon forest might eventually be at risk.
Rainforest Resilience Overestimated
Current modelling studies assume a strong resilience of Amazon rainforest to drought because they assume that high amounts of atmospheric CO2 lead to increased forest growth. They also claim that an increased carbon dioxide concentration functions as a kind of buffer against water stress, causing the trees to give off less water so they can handle more severe droughts.
But there is a significant flaw in this theory according to Professor Rammig: ‘These relationships have never been measured in the tropics at the level of the entire ecosystem.’ If vital nutrients are missing in the soil, this assumption could already be false. The professor has no doubt that: ‘These models overestimate the resilience of the Amazon region.’
Diverse Forests Can Break the Downward Spiral
This scenario may sound gloomy, but there is hope for the Amazon organism: In order to break the vicious cycle of forest loss, its high species diversity needs to be maintained, as does the diversity of endogenous defense mechanisms so the forest can react to stress factors such as increased droughts.
The diversity of responses acts as a buffer capable of containing the cascading forest loss. This is a vital piece of information for the management of rainforest areas. The reason for this is that areas farmed as monocultures are not diverse, not resilient, and cannot replace the functions of the primary rainforest. ‘Since every species has a different way of reacting to stress, having a great variety of them can be a means for ecosystem resilience,’ says Marina Hirota from the Federal University of Santa Catarina, Brazil. ‘Preserving biodiversity may hence not just be about loving trees and weeds and birds and bugs; it may also be a tool to stabilize key elements of the Earth system.’
Delphine Clara Zemp, Carl-Friedrich Schleussner, Henrique M. J. Barbosa, Marina Hirota, Vincent Montade, Gilvan Sampaio, Arie Staal, Lan Wang-Erlandsson and Anja Rammig: Self-amplified Amazon forest loss due to vegetation-atmosphere feedbacks, Nature Communications 2/2017.