Why don't trees grow taller?

The main point of this article is about how they are testing an hypothesis about the hydraulic limitation of tree height.  The theory of Hydraulic limitation shows that tree height is limited by the difficulty of pulling water from tree roots to treetop.  They were also wondering if this was the case for just tall tree or if shorter trees can have this as well.  So what they did to test there hypothesis was the got pine needles from the top of a pine tree and they load the needles into a pressure bomb and it gives a number of how hard the needles at the top of the pine were sucking to get water. This test showed that water in the upper canopy needles was under more than enough suction to force the leaf openings to close.  Also they took needles from the top a pine all the way down to the bottom.  And the test showed that the treetops were 20% shorter than the bottom canapys.  That was just a brief summary of the tests that they did.  Overall I thought that this article was really interesting and it gave first hand facts about why trees aren't growing taller.