Improving the representation of Arctic photosynthesis in Earth system models

Abstract:

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. Although Arctic carbon fluxes are small - relative to global carbon fluxes - uncertainty is large. As part of a multidisciplinary project to improve the representation of the Arctic in ESMs (Next Generation Ecosystem Experiments - Arctic) we are examining the photosynthetic parameterization of the Arctic plant functional type (PFT) in ESMs. Photosynthetic CO₂ uptake is well described by the Farquhar, von Caemmerer and Berry (FvCB) model of photosynthesis. Most ESMs use a derivation of the FvCB model to calculate gross primary productivity. Two key parameters required by the FvCB model are an estimate of the maximum rate of carboxylation by the enzyme Rubisco (V_c,max) and the maximum rate of electron transport (J_max). In ESMs the parameter V_c,max is usually fixed for a given PFT. Only four ESMs currently have an explicit Arctic PFT and the data used to derive V_c,max for the Arctic PFT in these models relies on small data sets and unjustified assumptions. We examined the derivation of V_c,max  and J_max in current Arctic PFTs and estimated V_c,max  and J_max for 7 species representing both dominant vegetation and key Arctic PFTs growing on the Barrow Environmental Observatory, Barrow, AK. The values of V_c,max currently used to represent Arctic PFTs in ESMs are 70% lower than the values we measured in these species. Examination of the derivation of V_c,max in ESMs identified that the cause of the relatively low V_c,max value was the result of underestimating both the leaf N content and the investment of that N in Rubisco. Contemporary temperature response functions for V_c,max also appear to underestimate V_c,max at low temperature. ESMs typically use a single multiplier (JV_ratio) to convert V_c,max to J_max for all PFTs. We found that the JV_ratio of Arctic plants is higher than current estimates suggesting that the Arctic PFT will be more responsive to rising carbon dioxide than currently projected. Our data suggest that the Arctic tundra has a much greater capacity for CO₂ uptake, particularly at low temperature, and will be more CO₂ responsive than is currently represented in ESMs.