Kirk Wythers

ABSTRACT [1] Evidence suggests that respiration acclimation (RA) to temperature in plants can have a substantial influence on ecosystem carbon balance. To assess the influence of RA on ecosystem response variables in the presence of global change drivers, we incorporated a temperature-sensitive Q10 of respiration and foliar basal RA into the ecosystem model PnET-CN. We examined the new algorithms' effects on modeled net primary production (NPP), total canopy foliage mass, foliar nitrogen concentration, net ecosystem exchange (NEE), and ecosystem respiration/gross primary production ratios. This latter ratio more closely matched eddy covariance long-term data when RA was incorporated in the model than when not. Averaged across four boreal ecotone sites and three forest types at year 2100, the enhancement of NPP in response to the combination of rising [CO2] and warming was 9% greater when RA algorithms were used, relative to responses using fixed respiration parameters. The enhancement of NPP response to global change was associated with concomitant changes in foliar nitrogen and foliage mass. In addition, impacts of RA algorithms on modeled responses of NEE closely paralleled impacts on NPP. These results underscore the importance of incorporating temperature-sensitive Q10 and basal RA algorithms into ecosystem models. Given the current evidence that atmospheric [CO2] and surface temperature will continue to rise, and that ecosystem responses to those changes appear to be modified by RA, which is a common phenotypic adjustment, the potential for misleading results increases if models fail to incorporate RA into their carbon balance calculations.

Background/Question/Methods While large changes in atmospheric CO2, temperature and precipitation are predicted by 2100, the long-term consequences for carbon cycling in forests are poorly understood. We used the PnET-CN ecosystem model to evaluate the effects of changing climate and atmospheric CO2 on productivity in forests of the North American Great Lakes region at 1 km spatial resolution. We examined two statistically downscaled and contrasting climate projections (PCM B1 and GFDL A1FI, with the latter predicting more warming and reduced precipitation) to represent a range in potential future climate. Each climate projection was simulated under two CO2 scenarios (constant and increasing atmospheric CO2 concentration) to separate the effects of rising CO2 from warming and precipitation changes. Results/Conclusions Increased productivity under future climate projections and higher CO2 was largely driven by CO2 fertilization effects on reduced stomatal conductance and water stress...