Foliar temperature acclimation reduces simulated carbon sensitivity to climate

نویسندگان

  • Nicholas G. Smith
  • Sergey L. Malyshev
  • Elena Shevliakova
  • Jens Kattge
  • S. Dukes
چکیده

Plant photosynthesis and respiration are the largest carbon fluxes between the terrestrial biosphere and the atmosphere1, and their parameterizations represent large sources of uncertainty in projections of land carbon uptake in Earth system models2,3 (ESMs). The incorporation of temperature acclimationof photosynthesis and foliar respiration, commonly observed processes, into ESMs has been proposed as a way to reduce this uncertainty2. Here we show that, across 15 flux tower sites spanning multiple biomes at various locations worldwide (10 S–67 N), acclimation parameterizations4,5 improve a model’s ability to reproduce observed net ecosystem exchange of CO2. This improvement is most notable in tropical biomes, where photosynthetic acclimation increased model performance by 36%. The consequences of acclimation for simulated terrestrial carbon uptake depend on the process, region and time period evaluated. Globally, including acclimation has a net e ect of increasing carbon assimilation and storage, an e ect that diminishes with time, but persists well into the future. Our results suggest that land models omitting foliar temperature acclimation are likely to overestimate the temperature sensitivity of terrestrial carbon exchange, thus biasing projections of future carbon storage and estimates of policy indicators such as the transient climate response to cumulative carbon emissions1. The terrestrial components of Earth system models (ESMs) simulate a variety of physiological and ecological processes to project carbon storage and terrestrial feedbacks to climate. These processes strongly affect simulated land–atmosphere interactions6, with consequences for historical and future climate projections that rival or exceed many atmospheric processes2,7. Although the temperature sensitivity of the terrestrial carbon cycle has been considered a fundamental characteristic of Earth’s carbon cycle, rigorously analysed in many studies and assessments (for example, ref. 1), some relevant, observed and well-characterized processes are still omitted from many ESMs (ref. 6), including those used in the Coupled Model Intercomparison Project, phase 5 (CMIP5; ref. 8). Examples of such processes are temperature acclimation of photosynthesis and leaf respiration, the gradual adjustment of the instantaneous temperature response of these two key processes of the terrestrial carbon cycle as a result of longer-term changes in growth temperature9–11. Previous studies have found that the uncertainty in ESMs’ projection of future carbon uptake is strongly related to the parametric uncertainty in the temperature response formulations for leaf photosynthesis and respiration2,3. For example, results of one set of simulations suggested that the primary parameter driving uncertainty in terrestrial carbon–climate feedbacks was the optimum temperature for photosynthesis (Topt), which was not allowed to change (that is, acclimate) in the study2. Consequently, it has been proposed that temperature acclimation might improve model functioning2 and, by decreasing the temperature sensitivity of acclimated processes, increase simulated carbon uptake on land12,13. These results, in part, have led to the inclusion of temperature acclimation of photosynthesis and/or leaf respiration in some global-scale land models14. Here, we explore the influence of state-of-the-art, empirically derived acclimation parameterizations of both photosynthesis and foliar respiration4,5 on model performance across multiple biomes and examine how these parameterizations will affect simulated leaf carbon exchange processes and simulated terrestrial carbon storage, from pre-industrial periods to 2100. We reasoned that, because temperature acclimation of photosynthesis and foliar respiration is widely observed (as reviewed in refs 11,14,15; see also citations in the SupplementaryMethods), acclimation parameterizations would enhance a model’s ability to reproduce observed carbon exchange rates. Additionally, we expected that acclimation would increase the net carbon assimilation of leaves by decreasing the sensitivity of leaf carbon exchange to temperature, resulting in high rates of carbon uptake across a wider range of temperatures, ultimately leading to increases in terrestrial carbon storage. Most terrestrial ecosystem models and CMIP5-class ESMs simulate photosynthesis as the minimum of two rate-limiting steps involved in the Calvin cycle: Rubisco carboxylation (Vc) and ribulose-1,5,-bisphosphate regeneration (J ), processes that are scaled by their maximum rates (Vcmax and Jmax, respectively)16. Vcmax and Jmax are commonly defined by a basal rate (that is, the rate at a standardized temperature) that is modified by a peaked Arrheniustype temperature function14. No mechanistic algorithm has been developed for leaf carbon release (that is, respiration)17. Instead, dark respiration (Rd) is commonly simulated as a function of a basal rate that is typically linked to the photosynthetic rate, and an exponential temperature function14. Temperature acclimation parameterizations have been developed for Vcmax, Jmax and Rd. At present, the most frequently used, robust, widely cited, and thus state-of-the-art, acclimation parameterizations are those defined by Kattge and Knorr4 for Vcmax and Jmax and Atkin et al.5 for Rd. The Kattge and Knorr4 formulation for photosynthetic acclimation was derived using empirical data from 36 different species and allows for the optimum temperature of the instantaneous temperature response of Jmax

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تاریخ انتشار 2016