Re: Is there any absolute limit to how high a tree can transport water.

Complete question:

On a discussion forum someone asked about how tall a tree could get. I did 
a web search but couldn't find any info. I seem to remember in the past a 
study about the water transport system in trees - which (might) have 
concluded there was some limit to how high water could be lifted based on 
the physics involved? Have any conclusions been reached on the absolute 
limits for raising water in a living plant, or hasn't this issue been 
addressed? Thanks. 

Answer:

College plant physiology textbooks do discuss in detail the cohesion-tension 
theory of how water moves to the top of tall trees so you can make an attempt 
to answer your question using some of the figures that they give. The reference 
mentions that it requires about 3 MegaPascals (MPa) of water potential gradient 
to move water to the top of the tallest tree that may have ever lived at 150 
meters. The measured tensile strength of water has been as high as 10 to 30 
MPa. Thus, if tensile strength alone was the limiting factor, you could 
extrapolate that a plant could possibly raise water 3 to 10 times higher than 
150 meters. The water potential of the air which provides the gradient could 
provide that steep a gradient (30 MPa) if the relative humidity was low enough. 
The water potential of air at 50% relative humidity is -93.5 MPa. 

There are certainly lots of other water relations factors to consider such as 
the plant being about to construct xylem that can withstand the tensions 
without collapsing and generating the very negative water potentials in cells 
high in the tree needed to support the water potential gradients. Other non 
water-relation factors may come into play, such as the ability of such a tall 
tree to stand against the forces of wind and gravity. 

Reference

Salisbury, F.B. and Ross, C.W. 1985. Plant Physiology. Belmont, CA: Wadsworth.