Do forests function like 'biotic pumps' for rainfall?

One of the more striking and controversial hypotheses to emerge in the last decade is the notion that intact tracts of forest, stretching from coastal to inland areas, may help to suck oceanic moisture far inland--functioning like a giant 'biotic pump'. 

This idea might sound slightly prosaic, but its potential implications are so profound and its putative mechanism so controversial that it has created fierce divides within the climatology and environmental-science communities.

No forest, no rain?

No forest, no rain?

The biotic-pump idea was first proposed in 2006 by a pair of Russian biophysicists, Anastasia Makareiva and Victor Gorshkov, in a highly theoretical analysis that even mathematicians found daunting.  The controversy has raged ever since then.

Everyone knows that forests (especially rainforests) emit a lot of water vapor, and Makareiva and Gorshkov argued that when this vapor condensed into rain it created a large suction.  That suction pulled cloudy, rain-bearing water from coastal areas inland, they argued, and thereby was crucial for maintaining rainfall in inland areas.  

The really striking implication of the Makareiva -Gorshkov hypothesis is this: If you break up the forest, you lose the suction, and the biotic pump stops working.  This, of course, would be an enormously important implication for forest conservation, if true.  Imagine millions of farmers clamoring for more forest in their region because their crops were drying up!

But the idea remains intensely controversial, because many physicists simply don't buy the idea of condensing water vapor creating a big suction.  For instance, just one website focused on this issue (and there are quite a number) had hundreds of individual comments, many very impassioned in nature.  This only happens in science if a debate is truly volcanic in nature.

This is a debate to keep your eye on--huge implications, and huge controversy about the biotic-pump idea itself.