Xeromorphy increases with height in needles of very tall Redwood trees

Redwoods (Sequoia sempervirens) are the world's tallest trees, exceeding heights of 115 m. These gymnosperms inhabit the fog belt of coastal northern California in stands characterized by strong vertical gradients of light and water stress, both of which increase with tree height. Lower foliage receives low light and little water stress, while upper foliage experiences high light and increased water stress.  Previous research has documented changes in needle physiology with height, including lower total water potential, reduced turgor, and an apparent increase in internal resistance to carbon dioxide diffusion. To determine whether needle allometry changes along a vertical gradient, and to relate any such changes to physiology, climbers collected foliage at 5 meter height intervals from the Stratosphere Giant, Mendocino, and Adventure trees in 2000, and in 2004 at 10 meter intervals from the Demetre, Federation, and Ballantine trees. We used confocal and epifluorescence microscopy with image analysis to measure the internal structure of 5-10 needles per height.  Dramatic and significant differences in needle structure occurred with the ascent of all six trees.  With increasing tree height, stomatal distribution shifted from hypostomatous to amphistomatous.  Needles became thicker with less lateral extension and had proportionately smaller photosynthetic mesophyll areas with greater vascular areas and proportionately more transfusion tracheids.  Outer branch needles were thicker with more upper stomata than inner branch needles. Needles exhibit increased xeromorphy with height, which correlates with physiological variations existing along the vertical climatic gradients associated with these very tall trees.