Generalizing a forest ecosystem model: Using PnET-CN to simulate carbon and water fluxes in grasslands and shrublands
Since terrestrial ecosystems play a key role in carbon cycling, there is strong interest in developing process-based terrestrial ecosystem models for these ecosystem processes. Such models enable forecasting of ecosystem dynamics under global climate change scenarios. Among terrestrial ecosystem models, PnET-CN offers unique benefits, including computational simplicity, linkage of photosynthesis rates to leaf nitrogen, and yearly assessment of the canopy biomass that produces maximum photosynthesis while accounting for shading. The model accurately predicts carbon exchange for temperate forests under various circumstances. The objective of our research was to generalize PnET-CN to other biomes: grasslands, shrublands, and savannas. To generalize the model, we determined parameter values for grasslands and shrublands using the literature and ecophysiological databases. We simulated carbon fluxes for six AmeriFlux sites: two grasslands (Konza Prairie and Fermi Prairie), two open shrublands (Heritage Land Conservancy Pinyon Juniper Woodland and Sevilleta Desert Shrubland), and two woody savannas (Freeman Ranch and Tonzi Ranch). Grasslands and shrublands were simulated using the biome-specific parameters, and savannas were simulated as a mixture of grasslands and forests. For each site, we used flux observations to evaluate modeled gross primary productivity (GPP), ecosystem respiration (ER), net ecosystem productivity (NEP), and evapotranspiration (ET).
Our generalized PnET-CN generally captured the magnitude, seasonality, and interannual variability of carbon and water fluxes as well as WUE for grasslands, shrublands, and savannas. Limitations in model performance included overestimation of seasonal variability GPP and ET for the two shrubland sites, early timing of peak GPP and ET for Freeman Ranch, and overestimation and early peak time of ER for the two shrubland sites and Freeman Ranch. Overall, our results show that PnET-CN is a promising tool for modeling ecosystem carbon and water fluxes for non-forest biomes (grasslands, shrublands, and savannas), and especially for modeling GPP in mature biomes. Future modifications of our generalized PnET-CN should focus on belowground processes, including water storage in dry shrubland soils, root growth and respiration in grasslands, and soil carbon fluxes for all biomes.