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.

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