Quoted from: https://www.pnet.sr.unh.edu/index.html

PnET is a suite of three nested computer models which provide a modular approach to simulating the carbon, water and nitrogen dynamics of forest ecosystems. Though primarily a temperate forest canopy model, work is currently underway to generalize PnET and produce a simple, alternative model applicable to all terrestrial ecosystem types.

PnET-Day is the instantaneous canopy flux module, PnET-II adds nutrient allocation, a water balance and soil respiration to produce a monthly time-step carbon and water model which is driven by nitrogen availability. PnET-CN further extends the soil dynamics component and closes the N cycle by tracking nitrogen, along with carbon, throughout all compartments and fluxes.

The best source of information about PnET are the journal publications available for online viewing on the Publications page. The model itself is released as open source and users are encouraged to extend and enhance it for their own purposes.

PnET is available as both MATLAB and Visual Basic source. The distribution package contains example files which will allow the user to run each version of the model using data acquired at the Harvard Forest and Hubbard Brook experimental forests and a subset of the VEMAP continental U.S. data.

PnET was developed at the Complex Systems Research Center (a division of the Institute for the Study of Earth, Oceans and Space at the University of New Hampshire). Read more about the history of PnET at the CSRC.