Now there’s a mouthful.  A relatively unheralded study released nearly two years ago by scientists at UC Berkeley explains the significance of this phenomenon on forests and climate.  In a report on the UC Berkeley news website entitled “Deep-rooted plants have much greater impact on climate than experts thought,” hydraulic redistribution is defined as the internal process whereby trees will use their tap roots to deposit water deep underground during the rainy season, then through “hydraulic redistribution” they will move this water up to the shallow soil surrounding their surface roots to use during the dry season.

The implications of this are consistent with what we’ve been trying to say all along - forests, tropical forests in particular - are probably more significant drivers in climate change than burning fossil fuels.  As the report states: 

“During the wet season, these plants can store as much as 10 percent of the annual precipitation as deep as 13 meters (43 feet) underground, to be tapped during the dry months.” 

The report also stated: 

” ‘Global climate models don’t do a very good job of capturing plant effects on how climate might behave,’ Lee (one of the study’s authors) said.   Lee accounted both for daily and seasonal dryness in the Amazon, and showed that the two together have a large impact on the climate over the region. The increased moisture in the soil created by hydraulic redistribution during the dry season allows the plant to carry on photosynthesis at a higher rate, leading to greater carbon uptake. This also leads to greater evaporation from the leaves of water, which takes heat with it. Thus, the summer dry-season temperatures are cooler than would be expected.”

We learned of this study after recently being linked to by an excellent blog “Transect Points,” authored by soil scientist Philip Small, that offers further expert analysis of the implications of this study, particularly on soil nutrition, in a post entitled “Tap Roots;” this is good reading.

So what do we know?

(1)  Tropical rainforests have shrunk from 8 million square miles to 3 million square miles in the last 150 years, most of that in the last 50 years, and they are currently being further decimated to grow biofuel, ironically because environmentalists think biofuel is less likely than petroleum to cause catastrophic climate change.

(2)  Rainforest trees, through hydraulic lift (energy provided by evaporation of water out of the leaves) “transpirate” sufficient volumes of water into the atmosphere to increase and moderate precipitation, which impacts the climate globally.  Transpiration from rainforests add moisture to clouds blowing in from the ocean, giving them critical mass to release evaporation from the ocean as rainfall, adding to the reserves of land based fresh water and reducing incidence of droughts.

(3)  The cloud cover that forms over rainforests is reflective, unlike the open land of biofuel plantations that are replacing them, which collectively has a thermal impact that impacts the climate globally.

(4)  “Hydraulic redistribution” is part of a phenomenon where trees absorb at least 10% of precipitation in the rainy season in the earth around tap roots, and then lift this water into the shallower soil near their surface roots during the dry season - this, along with trees stablizing soil and therefore harvesting runoff that otherwise would run back into the ocean, means trees raise the water tables - something of interest to anyone operating an irrigation pump to (hopefully) grow food.

Will the recognition that rainforests are the key to moderating extreme weather, averting drought, and lowering global temperatures, arrive in time to avert destroying the last of them to grow biofuel?