The Amazon Carbon Sink remains a mystery – feedbacks and landscape transformation

Recently, Brienen et al. 2015 published results from different long-term projects on Amazon forest dynamics, showing a generalized decline of the biomass carbon sink. While more than a decade ago, Betts et al. 2004 modelled the so-called die-back of the Amazonia, recent models have been showing an increase of the biomass carbon sink well into the 21st century due to CO2 fertilisation (for example Huntingford 2013). Brienen et al 2015 now argue a decline in the capacity of the carbon sink, caused by an accelerated tree life cycle, increasing the biomass mortality. The carbon uptake is simply getting oversaturated due to the high atmospheric CO2 concentrations. Will the new-generation models show a return of the die-back theory?  As yet, no effect of CO2 fertilisation or accelerated tree life cycles have been found in tree ring studies, such as done by Van der Sleen 2015.

Simultaneously, Haddad et al. 2015 published another study from long-term projects on world-wide forest dynamics. They show fragmentation has long-lasting impacts on biodiversity, nutrient cycling and ecosystem services. They furthermore state that currently more than 70% of the remaining forests are within a single kilometre from its edge and thus severely fragmented! The tropical forest around the Amazon is one of the two regions with a relatively contiguous area. Nevertheless, the fragmentation of the Amazon still results in a considerable carbon loss of approximately 10% of the total, according to Pütz et al. 2014.

The decline of the biomass carbon sink and the effects of encroaching human-transformed landscapes, including fragmentation, show dire consequences of declining ecosystem services potential and conservation value. The two processes might have a coupled effect, decreasing the total carbon storage of the tropical forests. Can this have consequences for the Payment for Ecosystem Services?

Fertilization from the driest to the wettest biosphere

A friend pointed me to a very interesting blog entry about how the African Sahara is fertilizing the Amazon basin. The post is written by Pierre Barthélémy on LeMonde.fr with the frensh title: L’Amazonie fertilisée par… la poussière du Sahara. You can find the scientific article by Yu et-al. (2015) here.

On the Geology of the Amazon River

Being a geologist from principle, I could not resist myself to have a look into literature on the paleo-Amazon. This information will help in the understanding of the Amazon vegetation structure nowadays. Perhaps even to understand future trends, although these future trends that we tend to predict have a very different time scales.

Initially the Amazon did not flow from west to east
The first indications of the Amazon River can be found back some 40 million years ago, during the Paleogene. The recently formed Andes and the existing high-topography shields of Guyana and Central Brazil formed a huge interior sea, in which the headwaters of the Amazon formed. This was the so-called Pebas basin (lake) and coincided with the current dense rainforest of the western Amazon region. In this basin, a network of smaller rivers was formed, which we might better categorize as being tributaries of the paleo-Orinoco. In those times, the Amazon did not flow to the east, but instead followed a route north draining into the present-day Gulf of Mexico. For a long time, the Pebas basin was a huge drainage pot for its surrounding mountains (Andes and Guyana Shield), and functioned like a kind of extension of the Gulf of Mexico. From the Gulf, sea level fluctuation, resulting climate change, induced the occasional flooding (incursions) and draining of the basin, forming the lacustrine (lake) sediments nowadays found in the western Amazon. The real Amazon was only a minor river draining on the west side of the Guyana shield.

A breakthrough!
During the Miocene things changed. We are jumping to approximately 20 million years ago. World-wide temperatures were decreasing from a very hot climate to a more temperate climate (similar to present-day). Meanwhile, the uplifting of the Andes continued rapidly and mountain ranges formed in the northern part of Latin America (Venezuela and Colombia). The Pebas basin was slowly cut off from its drainage point in the Gulf and the Amazon experienced more resistance to flow in that direction. Searching another route, the Amazon River cut itself loose from the Orinoco River and moved eastward. The Pebas basin developed into a large inland wetland, vegetated mainly with palm swamps and lowland riverine forest. Only at the end of the Miocene, some 10 million years ago, the river managed to breach through the so-called Purus Arch and connected to the much smaller eastern Amazon River. This break-through must have forced a violent burst and ecological disasters. In the fan delta of the current Amazon River,we find a sharp change in its sedimentary succession from this time onwards with significantly higher sedimentation rates. During the first couple of million years, the Amazon River is still not well developed and the Pebas wetland coexisted along with the river draining it.
Geo2Amazon

Fluctuations during ice ages
In the Pliocene (around 5 million years ago), ice ages started to appear. The era of ice ages meant a rapid fluctuation in temperature, rainfall and sea level. The waxing and waning of polar ice caps did not just happen once, but many times. During this phase the Amazon River developed to its present form, incising deeper and deeper into the gorge, which it had formed by connected east and west breaking through the Purus Arch. Swamps and lakes still persisted, and flooding events due to reformations of the river course were common. Only since the last ice age the current Amazon River channel along Manaus formed: 5,000 to 2,500 years ago major flooding events occurred establishing this flow channel. This was while the Romans conquered the old world.

 Thus, although the Amazon river nowadays seems to be a very stable river, it did not always exist in the way we know it and it shows to be a living and moving system.

Some papers on the Miocene river reversal: Hoorn, C.; Guerrero, J.; Sarmiento, G.A.; Lorente, M.A. (1995); Andean tectonics as a cause for changing drainage patterns in Miocene northern South America; Geology 23 (3); p. 237-240  &  Figueiredo, J.; Hoorn, C.; Ven, P. van der; Soares, E. (2009); Late Miocene onset of the Amazon River and the Amazon deep-sea fan: Evidence from the Foz do Amazonas Basin; Geology 37; p. 619-622
A paper on the Holocene Amazon: Fatima Rossetti, D. de; Toledo, P.M. De; Goes, A.M. (2005); New Geological framework for Western Amazonia (Brazil) and implications for biogeography and evolution; Quaternary Research 63; p. 78-89

A tree strategy to survive droughts: HD

After some time of literature and even more literature study on climate change, droughts, upcoming climatic disasters and the total dy-back of the Amazon rainforest, I was most amazed by the technology which trees develop in surviving these horrible futuristic scenarios.
Let me explain. The Amazon rainforest is a system, which keeps itself alive in different ways. One of them is the water recycling by ways of high evaporation rates, bringing moisture back in the atmosphere and pumping water-loaded air deeper into the forest.
The other is a mechanism called hydraulic distribution, which I want to address here. Despite the above mentioned water recycling, large parts of the Amazon rainforest experience an annual dry period of around three months. Nevertheless, the rainforest does not lose its leaves or reduces its biomass production (such as the trees in German winter!). Instead, the tree itself has the ability to govern its own water resources.

What happens exactly? The trees have two sets of root, namely the taproots and lateral roots. The taproot goes deep into the soil to reach deeper water resources in aquifers, while lateral roots spread directly from the tree to get water from the topsoil. Normally both the taproot and lateral roots will suck water from the soil into the trunk, which transports it to the canopy (middle picture b).
However, when rains fall down, this system changes overnight. Instead of tapping water from deep aquifers, the tree will only tap water from the topsoil and actually transports the water via the taproot down into the aquifer (right picture c). The tree is saving the rich harvest of fresh rain in the deeper soil layers! Now the tree will have some savings for the dry period.
However, trees seem to be very social and practical. During the dry period, it applies another process, which also happens overnight. While the taproot keeps on sucking soil water from deeper aquifers, the lateral roots use some of the water and push it into the topsoil, wetting it for its own use during the day and meanwhile providing water for fellow plants around (left picture a).

Hydraulic redistribution

The system is actually typical for trees in the Savanna and other dry regions, but seems to work well in the humid tropics as well. However, we do not really understand when the tree decides to change its fluid transport from storing to distributing it. Therefore, in physically based vegetation models it is often not modeled, but more or less lumped within some measure of resilience to drought. Does it remain too mystical or complicated to include the amazing ability of trees to redistribute soil water?

Want to read more? Oliveira and associates published a very interesting paper in Ecophysiology (March 2005), titled “Hydraulic redistribution of three Amazonian trees”. A bit later, Lee and associates published another paper in PNAS (October 2005), titled “Root functioning modifies seasonal climate”, that also gave an in-depth description, including the picture used in this post.