The short answer is, ultimately, the limits on the ability to transport water. According to the "cohesion-tension theory", water transportation in plants occurs along a gradient of negative pressure in the tube-like cells of the xylem, driven by a combination of transpiration (the "pull" of water vapor through stomata, pores in the leaves), water adhesion to the cell walls, and surface tension. All of these things are necessary to "lift" the water against gravity. However, there are simple mechanical limits to this; the "pull" of transpiration must increase with height in order to maintain sufficient water pressure in the plant cells to keep them rigid, and to provide sufficient water to power photosynthesis, but there is only so much pull to be had through this mechanism. Eventually the xylem becomes too narrow (in an attempt to maintain tension and pressure) to be effective. Several calculations have come up with the biomechanical limit of 130 meters height, absolute maximum, for the flow of water to be maintained to the top of the plant under ideal conditions with those mechanisms, and that does indeed seem to be the top height any tree can possibly achieve (most are under 120 m, even redwoods). Many trees have less-than-ideal xylem configurations, however, as the tissues of wood have different properties (grain width, %vascular tissue, etc.) which means the maximum height for effective water transport is much less. A key paper on this was published in 2004, by George W. Koch, Stephen C. Sillett, Gregory M. Jennings and Stephen D. Davis, "The limits to tree height" (Nature 428, 851-854, 22 April 2004), and further research in 2004-2005 has supported their understanding and interpretation.

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