Needle age and season influence photosynthetic temperature response and total annual carbon uptake in mature Picea mariana trees

• Background, Aims and Methods; 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. To assess the physiological significance of maintaining multiple foliar cohorts we measured photosynthetic capacity, foliar respiration (Rd), and leaf biochemistry and morphology of mature Picea mariana trees within an ombrotrophic bog ecosystem in Minnesota, USA. Results were applied to a simple model of canopy photosynthesis to simulate annual C uptake by cohort age under ambient and elevated temperature scenarios.Key Results; Temperature responses of key photosynthetic parameters (i.e., light-saturated rate of CO2 assimilation (Asat), rate of Rubisco carboxylation (Vcmax), electron transport rate (Jmax)) were dependent on season and generally less responsive in the developing current-year (Y0) needles compared to one-year-old (Y1) or two-year-old (Y2) foliage. Temperature optimums ranged from 18.7 - 23.7, 31.3 - 38.3 and 28.7 - 36.7°C for Asat, Vcmax and Jmax, respectively. Foliar cohorts differed in their morphology and photosynthetic capacity, which resulted in 64% of modeled annual stand C uptake from Y1&2 cohorts (LAI 0.67 m2 m-2) and just 36% from the Y0 cohorts (LAI 0.52 m2 m-2). Under warmer climate change scenarios, the contribution of Y0 cohorts was even less; e.g., 31% of annual C uptake for a modeled 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, these results illustrate the physiological and ecological significance of different foliar cohorts, and indicate the need for seasonal- and cohort-specific model parameterization when estimating C uptake capacity of boreal forest ecosystems under ambient or future temperature scenarios.

Ämnesord

NATURVETENSKAP  -- Biologi -- Botanik (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Botany (hsv//eng)

LANTBRUKSVETENSKAPER  -- Lantbruksvetenskap, skogsbruk och fiske -- Skogsvetenskap (hsv//swe)

AGRICULTURAL SCIENCES  -- Agriculture, Forestry and Fisheries -- Forest Science (hsv//eng)

Nyckelord

Black spruce; temperature adjustment; carbon assimilation; A/Ci curve; leaf age; Q10; evergreen; SPRUCE project; STELLA model; respiration

Forestry and Wood Technology

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