title: Needle age and season influence photosynthetic temperature response and total annual carbon uptake in mature Picea mariana trees.
abstract: OBJECTIVE
  The carbon (C) balance of boreal terrestrial ecosystems is sensitive to increasing temperature, but the direction and thresholds of responses are uncertain. Annual C uptake in Picea and other evergreen boreal conifers is dependent on seasonal- and cohort-specific photosynthetic and respiratory temperature response functions, so this study examined the physiological significance of maintaining multiple foliar cohorts for Picea mariana trees within an ombrotrophic bog ecosystem in Minnesota, USA.
  
  
  METHODS
  Measurements were taken on multiple cohorts of needles for photosynthetic capacity, foliar respiration (Rd) and leaf biochemistry and morphology of mature trees from April to October over 4 years. The results were applied to a simple model of canopy photosynthesis in order to simulate annual C uptake by cohort age under ambient and elevated temperature scenarios.
  
  
  RESULTS
  Temperature responses of key photosynthetic parameters [i.e. light-saturated rate of CO2 assimilation (Asat), rate of Rubisco carboxylation (Vcmax) and electron transport rate (Jmax)] were dependent on season and generally less responsive in the developing current-year (Y0) needles compared with 1-year-old (Y1) or 2-year-old (Y2) foliage. Temperature optimums ranged from 18·7 to 23·7, 31·3 to 38·3 and 28·7 to 36·7 °C for Asat, Vcmax and Jmax, respectively. Foliar cohorts differed in their morphology and photosynthetic capacity, which resulted in 64 % of modelled annual stand C uptake from Y1&2 cohorts (LAI 0·67 m(2 )m(-2)) and just 36 % from Y0 cohorts (LAI 0·52 m(2 )m(-2)). Under warmer climate change scenarios, the contribution of Y0 cohorts was even less; e.g. 31 % of annual C uptake for a modelled 9 °C rise in mean summer temperatures. Results suggest that net annual C uptake by P. mariana could increase under elevated temperature, and become more dependent on older foliar cohorts.
  
  
  CONCLUSIONS
  Collectively, this study illustrates the physiological and ecological significance of different foliar cohorts, and indicates the need for seasonal- and cohort-specific model parameterization when estimating C uptake capacity of boreal forest ecosystems under ambient or future temperature scenarios.
authors.affiliation: Climate Change Science Institute, Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6301, USA Anna.Jensen@lnu.se.
authors.forename: Anna M
authors.initials: AM
authors.lastname: Jensen
authors.affiliation: Climate Change Science Institute, Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6301, USA.
authors.forename: Jeffrey M
authors.initials: JM
authors.lastname: Warren
authors.affiliation: Climate Change Science Institute, Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6301, USA.
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Hanson
authors.affiliation: Climate Change Science Institute, Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6301, USA.
authors.forename: Joanne
authors.initials: J
authors.lastname: Childs
authors.affiliation: Climate Change Science Institute, Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6301, USA.
authors.forename: Stan D
authors.initials: SD
authors.lastname: Wullschleger
mesh_terms: D002244:Carbon / Q000378:metabolism*; D010788:Photosynthesis; D028222:Picea / Q000254:growth & development / Q000378:metabolism*; D018515:Plant Leaves / Q000254:growth & development / Q000378:metabolism; D012621:Seasons
chemical_list: D002244:Carbon
keywords: A/Ci curve; Black spruce; Picea mariana; Q10; SPRUCE project; STELLA model; carbon assimilation; climate change; evergreen; leaf age; photosynthesis; respiration; temperature adjustment