PnET Model

Dr. Ge Sun (ge_sun@ncsu.edu)

11/21/2003

 

The PnET model consists of a series of variations the PnET family model that can simulate the forest ecosystem processes (C and Water cycles) at daily (PnET-Day) (Aber et al, 1996) and month time step (PnET-II)  (Aber and Feder, 1992) and nitrogen cycle at monthly time step (PnET-CN) (Aber et al 1997). Codes of the model were written in Visual Basic with a good user interface and are open to the public. Users can download the codes from the official PnET web site: http://www.pnet.sr.unh.edu

A recent 13-model, including PnET-II, validation and comparison study using hardwoods data multiple-year eddy covariance measurements from the Oak Ridge National Lab concluded that PnET-II did well for predicting total annual evapotranspiration but not for leaf NPP and NEE. Other variations of the PnET model has not been widely validated in the Southeastern US.  The PnET-CN model was validated and applied for the northeastern region and the Chesapeake Bay basins (Pan et al. 2003a, 2003b). The submodels used in the above ground processes in the PnET model have been integrated with the DNDC model to develop a more integrated biogeochemical cycling model (PnET-N-DNDC) (Li e al. 2000; Stange 2000). This integrated model has been further modified for forested wetland dominated by anaerobic conditions (Zhang et a., 2002; Li et al., 2003). A physically based hydrologic model MIKE SHE (DHI, 2003) is being integrated with this wetland ecosystem model to examine the spatial dynamics of C, N and water cycles at the watershed and regional scale. Below is a brief description of the PnET-II model that has been well validated across the eastern US.

 

What’s the PnET-II Model?  A slide presentation can be viewed at: http://www.sgcp.ncsu.edu/projects/gsun/pnet/pnetdemo.ppt

 

PnET-II was originally developed for studying forest ecosystem processes in northern forests (Aber and Federer, 1992). It is a lumped-parameter, monthly-time-step, and stand-level model that describes carbon and water dynamics in mature forests. It simulates both carbon and water cycles in a forest ecosystem using simplified algorithms that describe key biological and hydrologic processes. This model has been well validated with field data for deciduous upland hardwoods (Aber et al. 1995; Aber et al. 1996) and southern pines (McNulty et al. 1996; Sun et al. 2000), and it has been applied at a regional scale to study the potential effects of climate change (U.S.  Global Change Program 2000).

             Input parameters for vegetation, soil and site locations, and climate may be derived from the literature or measured from a local study site. Stand level vegetation parameters include those regulating the physiological and physical processes such as photosynthesis, light attenuation, foliar nitrogen concentration, plant and soil respiration, and rainfall interception. Only one soil parameter, soil water holding capacity (field capacity in percentage ´ rooting depth), is required. Climate input variables include minimum and maximum monthly air temperature, total monthly photosynthetic active radiation (PAR), and total monthly precipitation.

            The model simulates the carbon cycle by tracking absorbed carbon during photosynthesis, allocation to foliage, wood, and root, and respiration from leaf, stem and roots.  PnET calculates the maximum amount of leaf-area which can be supported on a site based on the soil, the climate and parameters specified for the vegetative type.  The model assumes that leaf area is equal to the maximum amount of foliage that could be supported due to soil water holding capacity, species, and climate limitations.  Predicted NPP equals total gross photosynthesis minus growth and maintenance respiration for leaf, wood, and root compartments.  PnET calculates respiration as a function of the current month's minimum and maximum air temperature.  Changes in water availability and plant water demand also place limitations on leaf area produced, so total leaf area decreased as vapor pressure deficit and air temperature increased above optimal levels.  Reduced leaf area decreased total carbon fixation and altered ecosystem hydrology.

The hydrologic cycle is simulated by the water balance equation. The input component of soil water storage is represented by net precipitation (i.e., precipitation - canopy interception), and outputs consist of canopy interception, plant transpiration, fast or macro-pore flow representing water not available for extraction by plant roots, and lateral and deep drainage. Soil evaporation is neglected in fully stocked forest ecosystems. Evapotranspiration is defined as the sum of plant transpiration and canopy interception. The model assumes that water that is not subjected to evapotranspiration eventually flows to streams as runoff. Transpiration is directly linked to forest photosynthesis and forest carbon gain processes by modeling transpiration as a function of water use efficiency and vapor pressure deficit. Therefore, PnET-II closely integrates forest hydrology with the biological processes. 

 

References:

Aber, J. D. and C. A. Federer, 1992. A Generalized, Lumped-Parameter Model of Photosynthesis, Evapotranspiration, and Net Primary Production in Temperate and Boreal Forest Ecosystems. Oecologia 92:463-474.

 

Aber, J. D., S. V. Ollinger, C.A. Feder, P.B. Reich, M.L. Goulden, D.W. Kicklighter, J. M. Mello and R.G. Lathrop, Jr., 1995.  Predicting the Effects of Climate Change on Water Yield and Forest Production in Northeastern U.S. Climate Research. 5:207-222.

 

Aber, J. D., P.  B. Reich, and M. L. Goulden, 1996. Extrapolating leaf CO2 Exchange to the Canopy: A Generalized Model of Forest Photosynthesis Validated by Eddy Correlation. Oecologia. 106:257-265.

 

Aber, J.D., S.V. Ollinger, and C.T. Driscoll. 1997. Modeling nitrogen saturation in forest ecosystems in response to land use and atmospheric deposition. Ecological Modeling. 101:61-78.

 

Danish Hydraulic Institute (DHI). 2003. MIKE SHE User Guide.  174 p.

 

Li, C., J. Aber, F. Stange, K. Butterbach-Bahl, and H. Papen. 2000. A process-oriented model of N2O and NO emissions from forest soils: 1. Model development. . J. Geophysical Research. 105:4369-4384.

 

Li, C., J. Cui, G. Sun, and C. Trettin. 2003. Modeling Impacts of Management on Carbon Sequestration and Trace Gas Emissions in Forested Wetland Ecosystems. Environmental Management. (In Press).

 

Stange, F., K. Butterbach-Bahl, and H. Papen, S. Zechmeister-Boltenstern, Li, C., and J. Aber.  2000. A process-oriented model of N2O and NO emissions from forest soils: 2. Sensitivity analysis and validation. J. Geophysical Research. 105:4385-4398.

 

Pan, Y, Hom J, Birdsey R, McCullough K. 2003a Impacts of rising nitrogen deposition on N  exports from forests to surface waters in the Chesapeake Bay Watershed. Environmental Management (in press).

 

Pan Y, Hom J, Jenkins J, Birdsey R. 2003b. Importance of foliar nitrogen concentration to predict forest in the Mid-Atlantic region. Forest Science (in press).

 

Sun, G.; Amatya, D.M.; McNulty, S.G.; Skaggs, R.W.; Hughes, J.H. 2000. Climate change impacts on the hydrology and productivity of a pine plantation. Journal of the American Water Resources Association. 36(2): 367-374.

 

McNulty, S. G, J. M. Vose, and W. T. Swank, 1996. Loblolly Pine Hydrology and Productivity across the Southern United States. Forest Ecology and Management, 86:241-251.

U.S. Global Change Program. 2000. Climate Change Impacts on the United States: The Potential Consequences of Climate Variability and Change. Overview. A Report to the National Assessment Synthesis Team. Cambridge University Press. 154 p.

 

Zhang, Y., C. Li, Carl C. Trettin, H.Li, and G.Sun. 2002. An integrated model of soil, hydrology and vegetation for carbon dynamics in wetland ecosystems. Global Biogeochemical Cycles 16(4)1061, doi:10.1029/2001GB001838.