Impact of anomalous climates on carbon allocation to biomass production of leaves, woody components, and fine roots in a cool-temperate deciduous forest

We investigated carbon allocation in a cool-temperate forest in central Japan in years of contrasting climate anomalies: an early spring warming induced by the El Niño Southern Oscillation in 2002 and a low summer photosynthetic photon flux density (PPFD) induced by a stationary rain front in 2003. Observations of eddy flux, biometric variables, and chamber measurements from 1999 to 2006 and interannual variations in fine root net primary production (frNPP) were analyzed in conjunction with a terrestrial biosphere model simulation with multiple biometric constraints. Compared to the annual means (excluding 2002 and 2003), the low summer PPFD in 2003 reduced the annual gross primary productivity (GPP; −6%), soil respiration (SR; −11%), and ecosystem respiration (RE; −5%). Under the low summer PPFD, CO2 fluxes commonly decreased but components of the NPP were not affected. The low variation in NPP is explained by previous findings that NPP is more sensitive to climate conditions before or during the early stage of the growing season. The early spring warming in 2002 increased the GPP (8%) and woody tissue NPP (wNPP; 55%) and decreased the frNPP (−33%) and SR (−6%). Although early spring warming prolonged the growing season, the foliage NPP (fNPP) and litterfall were invariant. The increase in wNPP and the decrease in frNPP imply that the forest decreased frNPP in favor of wNPP under the high spring temperature. Although the frNPP was estimated by model-data integration, we argue that the decrease in frNPP is plausible because the decrease in SR cannot be explained without the contribution from fine root respiration. These results suggest that increasing or decreasing patterns of component fluxes cannot necessarily be inferred from the GPP. Factors such as the nature and duration of climate anomalies and allocation shift between components of the NPP may need to be considered when characterizing carbon allocation under anomalous climate events.