The limits to tree height.

Trees grow tall where resources are abundant, stresses are minor, and competition for light places a premium on height growth 1,2 . The height to which trees can grow and the biophysical determinants of maximum height are poorly understood. Some models predict heights of up to 120 m in the absence of mechanical damage According to the cohesion-tension theory, water transport in plants occurs along a gradient of negative pressure (tension) in the dead, tube-like cells of the xylem, with transpiration, water adhesion to cell walls, and surface tension providing the forces necessary to lift water against gravity 7 . Height growth may slow if the xylem tension and therefore leaf water potential (W) predicted for great heights, &22 MPa (ref. 7), reduces sufficiently the positive pressure (turgor) necessary for expansion of living cells or increases the risk of xylem cavitation-cavitation is the formation of embolisms that reduce hydraulic conductivity and can cause branch dieback and plant death Reduced water potential due to soil drought causes a decline in the turgor of living plant cells that is necessary for cell growth and leaf expansion 11 . To determine if this also occurs as water potential declines with height, we estimated turgor at dry-season water potentials from pressure-volume measurements. Turgor (in MPa) declined linearly with height, h, as turgor ¼ 2ð0:0074^0:0004Þh þð1:30^0:07Þ, n ¼ 4 trees, ranging from 0.93 MPa at 50 m to 0.48 MPa at 110 m. At night when xylem pressure increased, the turgor gradient was less steep, turgor ¼ 2ð0:0044^0:0023Þh þð1:39^0:19Þ, and turgor was 0.3-0.4 MPa higher than at midday. Given the role of turgor in leaf expansion, its reduction with height may underlie the distinct vertical gradient in leaf structure in redwood