Generalization and evaluation of the process‐based forest ecosystem model PnET‐CN for other biomes

Alexandra M. Thorn,Jingfeng Xiao,Scott V. Ollinger

doi:10.1890/es14-00542.1

Abstract

Terrestrial ecosystems play an important role in carbon, water, and nitrogen cycling. Process‐based ecosystem models, including PnET‐CN, have been widely used to simulate ecosystem processes during the last two decades. PnET‐CN is a forest ecosystem model, originally designed to predict ecosystem carbon, water, and nitrogen dynamics of temperate forests under a variety of circumstances. Among terrestrial ecosystem models, PnET‐CN offers unique benefits, including simplicity and transparency of its structure, reliance on data‐driven parameterization rather than calibration, and use of generalizeable relationships that provide explicit linkages among carbon, water and nitrogen cycles. The objective of our study was to apply PnET‐CN to non‐forest biomes: grasslands, shrublands, and savannas. We determined parameter values for grasslands and shrublands using the literature and ecophysiological databases. To assess the usefulness of PnET‐CN in these ecosystems, we simulated carbon and water fluxes for six AmeriFlux sites: two grassland sites (Konza Prairie and Fermi Prairie), two open shrubland sites (Heritage Land Conservancy Pinyon Juniper Woodland and Sevilleta Desert Shrubland), and two woody savanna sites (Freeman Ranch and Tonzi Ranch). Grasslands and shrublands were simulated using the biome‐specific parameters, and savannas were simulated as mixtures of grasslands and forests. For each site, we used flux observations to evaluate modeled carbon and water fluxes: gross primary productivity (GPP), ecosystem respiration (ER), net ecosystem productivity (NEP), evapotranspiration (ET), and water yield. We also evaluated simulated water use efficiency (WUE). PnET‐CN generally captured the magnitude, seasonality, and interannual variability of carbon and water fluxes as well as WUE for grasslands, shrublands, and savannas. 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. Limitations in model performance included an overestimation of seasonal variability in GPP and ET for the two shrubland sites and overestimation of early season ER for the two shrubland sites and Freeman Ranch. Future modifications of PnET‐CN for non‐forest biomes 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.

Highlights

Terrestrial ecosystems play an important role in the carbon and water cycles
We reviewed the literature for estimates of model parameters for grasslands and shrublands
As in other terrestrial biogeochemistry models (Raich et al 1991), we model temperate savannas as two distinct biomes, independently simulating grassland and forest, and combining the outputs such that half of the land area is simulated as grassland, and half as forest

Summary

Introduction

Terrestrial ecosystems play an important role in the carbon and water cycles. Quantification of carbon and water fluxes at regional to continental scales requires modeling approaches that are applicable to all major biomes (Xiao et al 2014), while retaining enough simplicity that they can be parameterized and applied broadly. The most widely used ecosystem models include CENTURY (Parton et al 1987), CASA (Potter et al 1993), TEM (Raich et al 1991), Biome-BGC (Running and Hunt 1993), and the PnET family of models (Aber and Federer 1992), all of which were first developed in the late 1980s or early 1990s. All these models have been extensively tested and improved during the last two decades, often with the consequence of increasing model complexity. The PnET family of models, originally developed in 1992 to simulate temperate and boreal forests (Aber and Federer 1992), provide a unique simplified approach to modeling carbon and water exchange processes

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Conclusion