"Some data are worth going out on a limb for," Oldham jokes.
Water stress increases with height in a tree's canopygravity pulls down on the water column, which in turn decreases water pressure as you move up the tree. To compensate for this decreased water pressure with increasing height, trees make anatomical changes in leaves which can lead to a reduction in photosynthesis in the upper canopy. Of the variables the authors measuredleaf length, leaf width, and the amount of air space (or mesoporosity) in a leafall decreased with height (explaining decreasing photosynthesis with height) while leaf thickness and transfusion tissue (which improve water-stress tolerance) increased with height. In contrast, none of the 15 anatomical traits measured differed between the inner and outer crown positions, where light availability differed.
"In tall redwoods its not light that drives leaf anatomy and morphology, but rather a height-associated increase in water stress due to the force of gravity pulling down on the water column as it rises over 110 m from the roots to the tree top," Oldham explains. "This gravitational pressure, known as hydrostatic tension, decreases water availability with height and so directly reduces leaf expansion which in turn lowers photosynthetic capacity in the tree tops. At the same time, hydrostatic tension puts tall trees at increased risk during drought events thus driving investments in functional anatomical traits that may allow redwoods to reach such great heights, but are costly in terms of lost opportunity for growth."
Oldham notes that "For Earth's tallest trees the force of gravity may be the biggest obstacle to further increases in height." With increasing height, the increasing effects of gravity drive tissue investments toward water stress tolerance, resulting in tradeoffs with carbon gain per unit leaf mass in the upper crown. This could result in diminishing rates of height g
|Contact: Richard Hund|
American Journal of Botany