Photosynthetic Characterization of Plant Functional Types from Coastal Tundra to Improve Representation of the Arctic in Earth System Models

The primary goal of Earth System Models (ESMs) is to improve understanding and projection of future global change. In order to do this they must accurately represent the carbon fluxes associated with the terrestrial carbon cycle. Photosynthetic CO₂ uptake is well described by the Farquhar, von Caemmerer and Berry model of photosynthesis, and most ESMs use a derivation of this model. One of the key parameters required by the Farquhar, von Caemmerer and Berry model is an estimate of the maximum rate of carboxylation by the enzyme Rubisco (V_c,max). In ESMs the parameter V_c,max is usually fixed for a given plant functional type (PFT) and often estimated from the empirical relationship between leaf N content and V_c,max. However, uncertainty in the estimation of V_c,max has been shown to account for significant variation in model estimation of gross primary production, particularly in the Arctic. As part of a new multidisciplinary project to improve the representation of the Arctic in ESMs (Next Generation Ecosystem Experiments - Arctic) we have begun to characterize photosynthetic parameters and N acquisition in the key Arctic PFTs. We measured the response of photosynthesis (A) to internal CO₂ concentration (c_i) in situ in two sedges (Carex aquatilis, Eriophorum angustifolium), a grass (Dupontia fisheri) and a forb (Petasites frigidus) growing on the Barrow Environmental Observatory, Barrow, AK. The values of V_c,max (normalized to 25^oC) currently used to represent Arctic PFTs in ESMs are approximately half of the values we measured in these species in July, 2012, on the coastal tundra in Barrow. We hypothesize that these plants have a greater fraction of leaf N invested in Rubisco (F_LNR) than is assumed by the models. The parameter V_c,max is used directly as a driver for respiration in some ESMs, and in other ESMs V_c,max is linked to leaf N content and N acquisition through F_LNR. Therefore, these results have implications for ESMs beyond photosynthesis, and suggest that physiological characterization of a more extensive list of PFTs would be a useful resource for ESMs.