title: Growth maximization trumps maintenance of leaf conductance in the tallest angiosperm.
abstract: Structural and physiological changes that occur as trees grow taller are associated with increased hydraulic constraints on leaf gas exchange, yet it is unclear if leaf-level constraints influence whole-tree growth as trees approach their maximum size. We examined variation in leaf physiology, leaf area to sapwood area ratio (L/S), and annual aboveground growth across a range of tree heights in Eucalyptus regnans. Leaf photosynthetic capacity did not differ among upper crown leaves of individuals 61.1-92.4 m tall. Maximum daily and integrated diurnal stomatal conductance (g s) averaged 36 and 34% higher, respectively, in upper crown leaves of ~60-m-tall, 80-year-old trees than in ~90-m-tall, 300-year-old trees, with larger differences observed on days with a high vapor pressure deficit (VPD). Greater stomatal regulation in taller trees resulted in similar minimum daily leaf water potentials (Ψ L) in shorter and taller trees over a broad range of VPDs. The long-term stomatal limitation on photosynthesis, as inferred from leaf δ (13)C composition, was also greater in taller trees. The δ (13)C of wood indicated that the bulk of photosynthesis used to fuel wood production in the main trunk and branches occurred in the upper crown. L/S increased with tree height, especially after accounting for size-independent variation in crown structure across 27 trees up to 99.8 m tall. Despite greater stomatal limitation of leaf photosynthesis in taller trees, total L explained 95% of the variation in annual aboveground biomass growth among 15 trees measured for annual biomass growth increment in 2006. Our results support a theoretical model proposing that, in the face of increasing hydraulic constraints with height, whole-tree growth is maximized by a resource trade-off that increases L to maximize light capture rather than by reducing L/S to sustain g s.
authors.affiliation: Center for Ecosystem Science and Society, Northern Arizona University, Flagstaff, AZ, 86011, USA, george.koch@nau.edu.
authors.forename: George W
authors.initials: GW
authors.lastname: Koch
authors.forename: Stephen C
authors.initials: SC
authors.lastname: Sillett
authors.forename: Marie E
authors.initials: ME
authors.lastname: Antoine
authors.forename: Cameron B
authors.initials: CB
authors.lastname: Williams
mesh_terms: D018533:Biomass; D005052:Eucalyptus / Q000254:growth & development / Q000502:physiology*; D008027:Light; D010788:Photosynthesis / Q000502:physiology*; D018515:Plant Leaves / Q000502:physiology*; D054046:Plant Stomata / Q000502:physiology; D018526:Plant Transpiration / Q000502:physiology*; D014197:Trees / Q000254:growth & development / Q000502:physiology*; D055548:Vapor Pressure; D014867:Water / Q000502:physiology*; D014934:Wood / Q000254:growth & development*
chemical_list: D014867:Water