Document: GEO-3-32-40

Water potential, gas exchange, and carbon isotope discrimination in very tall coast redwoods, Sequoia sempervirens: Support for the hydraulic limitation hypothesis.

KOCH, G.W.* ¹ and S.C.SILLETT ²

Northern Arizona University, Flagstaff, AZ USA 86011 ¹
Humboldt State University, Arcata, CA, USA 95521 ²

Abstract:
It is unclear what slows the growth and limits the height of trees as they age. The hydraulic limitation hypothesis of Ryan and Yoder attributes this to the reduction in stomatal conductance (and consequent gas exchange) required to reduce the risk of cavitation in conducting tissues as hydraulic path length and resistance increase. To test the hydraulic limitation hypothesis, we examined patterns of water potential, gas exchange, and carbon isotope discrimination in foliage sampled along height gradients in very tall (90 110 m) and ancient (500 2000 yr) individuals of coast redwood, Sequoia sempervirens, at several sites in Mendocino and Humboldt County, California. We used rope techniques to access tree crowns with ecophysiological equipment. Predawn xylem pressure potential conformed closely to the theoretical static gravitational gradient, decreasing by an average of 0.011 0.001 MPa per meter in 8 trees over 90 m in height. Mid morning to midday gradients were steeper and more variable. Gas exchange and stable carbon isotope (¹³C) measurements indicated that foliage operates at lower internal carbon dioxide partial pressures as height in the canopy increases; ¹³C in sunlit foliage was strongly and positively correlated with height, increasing by 0.08 0.02 per mil per meter between 30 and 100 m. When the gravitational and pathlength constraints on water availability were bypassed, the uppermost foliage showed large and significant increases in conductance, internal carbon dioxide partial pressure, and photosynthesis. We believe gas exchange and growth in the upper crowns of S. sempervirens respond to seasonal and interannual variation in abiotic conditions in a dynamic manner that is consistent with the hydraulic limitation hypothesis. Our results have implications for understanding patterns of top die back in tall S. sempervirens and the response of this species to climate change.

Keywords: water relations, hydraulic limitation,carbon isotope,photosynthesis

This abstract is being presented at: 4:15 PM in session:
Oral Session #34: Water Relations in Trees.