Size is a fundamental attribute of organisms, and studying the factors that limit size can improve understanding of the genetic and environmental constraints on organism function. The fossil record demonstrates that since plants began to colonize land some 400 million years ago they have steadily increased in size. Today the largest trees that can achieve heights in excess of 112 meters, or nearly 370 ft., (coast redwood, Sequoia sempervirens) and total volumetric mass that exceeds 1485 m3 (an estimated 365,000 kg for giant Sequoia; Sequoiadendron giganteum). The limits to tree height are not well understood, but may be determined by the problem of raising water from the soil to the uppermost leaves against the influence of gravity. Recent studies have demonstrated that a major index of plant water stress, the "water potential" of leaves, becomes progressively limiting to photosynthesis as trees grow taller. Although the tallest trees grow where soils remain moist throughout the year, the influence of gravity appears to increase water stress with increasing height, and this may ultimately limit how tall trees can grow. This reasoning predicts that if the uppermost branches of a very tall tree are provided with a "local" source of water - i.e., water that has not been raised under tension against gravity - then leaf water stress will be reduced and photosynthesis and height growth will be increased. To test this prediction, the proposed study will use several novel approaches to supplement water availability to the uppermost branches of redwoods over 110 m in height. A suite of physiological and growth measurements will reveal the extent to which the experimental manipulations improve water potential and release photosynthesis and height growth from the constraints due to gravity. The proposed research will develop new methodologies that may have broad implications for studying water stress in plants, including agriculturally important species. The research will increase understanding of water stress in tall trees, including an important timber species. Finally, it is expected that the results of this study will inform understanding of how climate change may affect the growth and productivity of forest trees. Funds will be used to engage undergraduate students in the research process.
