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Améglio, T., Cruiziat, P., and Bodet, C. (1994) A pressure chamber to reduce and control embolism in situ: consequence on the sap flow. Plant Sciences, Saint Malo (FR) Meeting of the SFPV, 1994/10/12-14. University of Rennes I, p. 281.
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Essay 2.3 How Water Climbs to the Top of a 112 Meter-Tall Tree
George Koch, Northern Arizona University; Stephen Sillett and Gregg Jennings, Humboldt State University; Stephen Davis, Pepperdine University
Plants grow tall where light competition is relatively strong and other essential resources–water and nutrients–are readily available and temperatures are not extreme. Thus, the tallest trees are found in temperate regions having high rainfall, moderate temperatures, and relatively fertile soils. The tallest trees on Earth today are coast redwoods, Sequoia sempervirens, of northern California. A total of 113 living redwoods are over 107 m tall, and the tallest individual is 112.5 m in height (Figure 1). Because 95% of the original redwood forests, including most prime habitat, has been logged, it is likely that individuals taller than today's record holders existed in the past. Anecdotal evidence indicates that individuals of Douglass fir, Pseudotsuga menziesii, from Washington and British Columbia may have exceeded 120 m, and mountain ash, Eucalyptus regnans, of Victoria, Australia may have been as tall as 114 m.
What limits the height growth of trees has long puzzled plant physiologists. The respiratory burden of large support structures, declining nutrient availability, and mechanical failure have all been proposed, but largely dismissed. Recently the "hydraulic limitation hypothesis" of Ryan and Yoder (1997) has sparked interest in whether constraints on the capacity of the hydraulic system to deliver water to upper crown foliage might become increasingly limiting to stomatal conductance and photosynthesis as trees grow taller, thereby reducing carbon availability for further height growth. As trees grow taller, a larger soil-to-leaf water potential gradient is required to overcome the effect of gravity and the increased hydraulic resistance of a longer flow pathway. Low leaf water potential, however, causes a reduction in stomatal aperture, which reduces transpiration and counteracts further reduction in water potential, but also reduces photosynthesis. Why should stomatal conductance be reduced as water potential declines? First, it would act to maintain cell turgor pressure, which is needed for cell expansion. Second, and perhaps more important, it acts to reduce the risk of xylem cavitation, which if unchecked can lead to the death of shoots, branches, or entire trees (Tyree and Sperry, 1988). The postulated mechanism underlying the hydraulic limitation hypothesis of Ryan and Yoder (1997) implies that trees should grow to a physiological "ceiling" height that reflects the tradeoff between turgor maintenance or cavitation avoidance on the one hand and photosynthetic carbon gain on the other, with the balance of these factors being determined by the regulation of stomatal conductance.
To test whether stomatal limitation of photosynthesis might underlie the limits to tree height, we examined patterns of water potential, gas exchange, carbon isotope composition, and xylem vulnerability to cavitation in coast redwood trees at sites along the northern California coast.
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