title: Linking carbon and water relations to drought-induced mortality in Pinus flexilis seedlings.
abstract: Survival of tree seedlings at high elevations has been shown to be limited by thermal constraints on carbon balance, but it is unknown if carbon relations also limit seedling survival at lower elevations, where water relations may be more important. We measured and modeled carbon fluxes and water relations in first-year Pinus flexilis seedlings in garden plots just beyond the warm edge of their natural range, and compared these with dry-mass gain and survival across two summers. We hypothesized that mortality in these seedlings would be associated with declines in water relations, more so than with carbon-balance limitations. Rather than gradual declines in survivorship across growing seasons, we observed sharp, large-scale mortality episodes that occurred once volumetric soil-moisture content dropped below 10%. By this point, seedling water potentials had decreased below -5 MPa, seedling hydraulic conductivity had decreased by 90% and seedling hydraulic resistance had increased by >900%. Additionally, non-structural carbohydrates accumulated in aboveground tissues at the end of both summers, suggesting impairments in phloem-transport from needles to roots. This resulted in low carbohydrate concentrations in roots, which likely impaired root growth and water uptake at the time of critically low soil moisture. While photosynthesis and respiration on a leaf area basis remained high until critical hydraulic thresholds were exceeded, modeled seedling gross primary productivity declined steadily throughout the summers. At the time of mortality, modeled productivity was insufficient to support seedling biomass-gain rates, metabolism and secondary costs. Thus the large-scale mortality events that we observed near the end of each summer were most directly linked with acute, episodic declines in plant hydraulic function that were linked with important changes in whole-seedling carbon relations.
authors.affiliation: Department of Biological Sciences, Idaho State University, Pocatello, ID 83209-8007, USA reinkeit@isu.edu.
authors.forename: Keith
authors.initials: K
authors.lastname: Reinhardt
authors.affiliation: United States Geological Survey, Forest and Rangeland Ecosystem Science Center, Boise, ID 83706, USA.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Germino
authors.affiliation: Sierra Nevada Research Institute, University of California, Merced, CA 95343, USA.
authors.forename: Lara M
authors.initials: LM
authors.lastname: Kueppers
authors.affiliation: Bordeaux Sciences Agro, INRA ISPA UMR 1391, 33175 Gradignan, France Nicholas School of the Environment, Duke University, Durham, NC 27708, USA.
authors.forename: Jean-Christophe
authors.initials: JC
authors.lastname: Domec
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Colorado-Boulder, Boulder, CO 80309, USA.
authors.forename: Jeffry
authors.initials: J
authors.lastname: Mitton
mesh_terms: D002244:Carbon / Q000378:metabolism*; D055864:Droughts; D052585:Phloem / Q000378:metabolism*; D028223:Pinus / Q000254:growth & development* / Q000378:metabolism; D014867:Water
chemical_list: D014867:Water; D002244:Carbon
keywords: carbon balance; hydraulic resistance; non-structural carbohydrates; productivity; respiration