title: Elevational trends in hydraulic efficiency and safety of Pinus cembra roots.
abstract: In alpine regions, elevational gradients in environmental parameters are reflected by structural and functional changes in plant traits. Elevational changes in plant water relations have also been demonstrated, but comparable information on root hydraulics is generally lacking. We analyzed the hydraulic efficiency (specific hydraulic conductivity k s, entire root system conductance K R) and vulnerability to drought-induced embolism (water potential at 50 % loss of conductivity Ψ 50) of the roots of Pinus cembra trees growing along an elevational transect of 600 m. Hydraulic parameters of the roots were compared with those of the stem and related to anatomical traits {mean conduit diameter (d), wall reinforcement [(t/b)(2)]}. We hypothesized that temperature-related restrictions in root function would cause a progressive limitation of hydraulic efficiency and safety with increasing elevation. We found that both root k s and K R decreased from low (1600 m a.s.l.: k s 5.6 ± 0.7 kg m(-1) s(-1) MPa(-1), K R 0.049 ± 0.005 kg m(-2) s (-1) MPa(-1)) to high elevation (2100 m a.s.l.: k s 4.2 ± 0.6 kg m(-1) s(-1) MPa(-1), K R 0.035 ± 0.006 kg m(-2) s(-1) MPa(-1)), with small trees showing higher K R than large trees. k s was higher in roots than in stems (0.5 ± 0.05 kg m(-1)s(-1)MPa(-1)). Ψ 50 values were similar across elevations and overall less negative in roots (Ψ 50 -3.6 ± 0.1 MPa) than in stems (Ψ 50 -3.9 ± 0.1 MPa). In roots, large-diameter tracheids were lacking at high elevation and (t/b)(2) increased, while d did not change. The elevational decrease in root hydraulic efficiency reflects a limitation in timberline tree hydraulics. In contrast, hydraulic safety was similar across elevations, indicating that avoidance of hydraulic failure is important for timberline trees. As hydraulic patterns can only partly be explained by the anatomical parameters studied, limitations and/or adaptations at the pit level are likely.
authors.affiliation: Department of Botany, University of Innsbruck, Sternwartestr. 15, 6020, Innsbruck, Austria. adriano.losso@uibk.ac.at.
authors.forename: Adriano
authors.initials: A
authors.lastname: Losso
authors.affiliation: Dipartimento di Scienze della Vita, Università di Trieste, Via L. Giorgieri 10, 34127, Trieste, Italy.
authors.forename: Andrea
authors.initials: A
authors.lastname: Nardini
authors.affiliation: Department of Botany, University of Innsbruck, Sternwartestr. 15, 6020, Innsbruck, Austria.
authors.forename: Markus
authors.initials: M
authors.lastname: Nolf
authors.affiliation: Department of Botany, University of Innsbruck, Sternwartestr. 15, 6020, Innsbruck, Austria.
authors.forename: Stefan
authors.initials: S
authors.lastname: Mayr
mesh_terms: D000064:Acclimatization; D000222:Adaptation, Physiological; D000531:Altitude; D055864:Droughts; D017753:Ecosystem; D028223:Pinus / Q000033:anatomy & histology / Q000502:physiology*; D018517:Plant Roots / Q000033:anatomy & histology / Q000502:physiology*; D018547:Plant Stems / Q000502:physiology; D018526:Plant Transpiration; D013312:Stress, Physiological; D013696:Temperature; D014197:Trees / Q000502:physiology; D065929:Tundra; D014867:Water / Q000502:physiology*; D052584:Xylem / Q000033:anatomy & histology / Q000502:physiology
chemical_list: D014867:Water
keywords: Alpine timberline; Conifer; Hydraulic conductance; Root hydraulics; Xylem anatomy