PnET Carbon Model

PnET

Authors

John D. Aber, Morse Hall, University of New Hampshire, Durham, NH 03824-3525

Links

http://www.pnet.sr.unh.edu/

Description

The PnET models provide a nested set of modular approaches to simulating the carbon, water and nitrogen dynamics of forest ecosystems. The different versions of PnET are modular and build out from simplest to most complex. Algorithms such as photosynthesis are identical among model versions.

1. PnET-Day uses foliar mass, specific leaf weight, foliar N concentration, temperature and radiation flux to predict daily gross and net photosynthesis of whole forest canopies.
2. PnET-II adds carbon allocation and respiration terms, as well as a full water balance to predict NPP, transpiration and runoff. An empirical soil respiration terms allows prediction of total ecosystem carbon balance under ambient conditions. This version is used to predict the combined effects of climate change and increased atmospheric CO2 on these processes (Aber et al. 1995).
3. PnET-CN adds compartments for woody biomass and soil organic matter, as well as algorithms for biomass turnover and litter and soil decomposition to allow calculation of complete carbon and nitrogen cycles.

Model scale and resolution

The original PnET uses a monthly timestep. PnET-Day uses a daily timestep. The PnET models do not have an explicit spatial scale, but they are viewed as regional (i.e., in contrast with TEM, which is global in scale).

Precursors

The original PnET was a precursor to the other three versions.

Inputs

Climate variables

1. monthly average radiation
2. monthly average, minimum, and maximum temperature
3. monthly total precipitation
4. monthly average insolation

Site variables

1. latitude (degrees)
2. water holding capacity (cm)
3. canopy light attenuation constant
4. foliar percent nitrogen
5. foliar retention time (years)
6. leaf specific weight (mg cm^-2)
7. intercept of foliar N to max photosynthetic rate relationship
8. slope of the relationship
9. half saturation light level (J m^-2 s^-1)
10. constant effect of vapor pressure deficit on photosynthesis and transpiration
11. fraction of precipitation intercepted

Testing and validation

PnET-Day has been validated against daily summaries of eddy correlation carbon balance measurements from the Harvard Forest (Aber et al. 1996). PnET-II has been validated against annual NPP and monthly water yield data from the Harvard Forest and Hubbard Brook ecosystems. PnET-CN maintains the predictions for NPP and water balance used for validation in PnET-II. PnET-CN compares well with field data in predicting total annual, mean seasonal, and actual time series rates of nitrate loss in streams (Aber et al. 1997a, 1997b). PnET-II predictions of foliar production and NPP were validated against data from 10 additional forest types across North America (Aber and Federer 1992). Jenkins et al. (1999) found that PnET-II predictions were sensitive to soil water holding capacity, but not to temperature.

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., 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. and R. Freuder. 2000. Sensitivity of a forest production model to variation in solar radiation data sets for the Eastern U.S. Climate Research 15:33-43.

Jenkins, J.C., D.W. Kicklighter, S.V. Ollinger, J.D. Aber and J.M. Melillo. 1999. Sources of variability in NPP predictions at a regional scale: A comparison using PnET-II and TEM 4.0 in northeastern forests. Ecosystems 2:555-570

Ollinger, S.V., J.D. Aber and P.B. Reich. 1997. Simulating ozone effects on forest productivity: interactions between leaf,- canopy- and stand-level processes. Ecological Applications 7:1237-1251.