Models
Forest modeling is inherently diverse — for any given species and region, a dozen or more equations may be needed to describe taper, height, crown, growth, and beyond. Models are the foundation of any rigorous inventory effort, and the field continues to evolve. Assisi draws from a broad library: models maintained directly by their authors, equations published in peer-reviewed journals, and open-source implementations where available. Some models require a license from their authors; others are included with Assisi as standard. We actively collaborate with model developers to expand this library — if you've built models you'd like to see supported, we'd love to hear from you.
Growth Models
Growth models are coordinated sets of equations used together to project tree dimensions over time. There are four equations that most growth models implement: DBH, total height, crown height and mortality. Some growth models are implemented as external components and maintained by their authors. Others are implemented and maintained inside Assisi directly.
Growth Models
ORGANON - Integrated via the ORGANON DLL maintained by the OSU Center for Intensive Planted-forest Silviculture (CIPS).
Assisi FVS - An Assisi implementation of FVS growth models. Lake States and Southern Oregon regions are implemented as internal collections.Assisi Acadia - An Assisi implementation of the Univ. of Maine Cooperative Forestry Research Unit (CFRU) models.
Assisi OSU - An Assisi implementation of the Oregon State Univ. ORGANON models.
Assisi UCB - An Assisi implementation of the CAGYM models.
Assisi Northeast - An Assisi implementation of the NE TWIGS models.
Growth Equations
DBH Growth
Parse
ORGANON
Acadia
CAGYM
Assisi FVS
Kershaw
Tech & Hilt
Weiskittel
Wensel
Wykoff
Growth Index Ratio
Linear
5 Point Logistic
PMRC
Total Height Growth
Parse
Acadia
CAGYM
Assisi FVS
Schreuder
Sheppard
Weiskittel
Wensel
Linear
5 Point Logistic
Pienaar
Site Class
SI Difference
5 Point Logistic
Crown Growth
ORGANON
Acadia
CAGYM
Weiskittel
Mortality
Parse
Acadia
CAGYM
Assisi FVS
Biging
Pienaar
Weiskittel
Height Models
Height models estimate various heights of trees. Some of these equations are implemented as calls to external components, others as internally implemented equations. Most height model types support user-entered equations and can calibrate to field-measured heights, reducing measurement burden in the field.
Crown Height
Parse
Regressions
ORGANON
Acadia
Assisi FVS
Biging
FVS
Ritchie & Hann
Weiskittel
Sawlog Height
Parse
Regression
Taper
Yaussy
Merch Height
Parse
Regressions
Taper
Ek
Total Height
Parse
Regression
ORGANON
Acadia
CAGYM
Assisi FVS
Alexander
Barrett
Biging
Brickell
Cochran
Curtis
Dahms
Demars
Dolph
Ek
Harrington
Herman
Hegyi
Krumland
Larsen & Hann
Means
Powers
Rijal
Wiley
Wykoff
Tarif
Polynomial
Exponential
Iterate Merch
Iterate Clark Souter
Taper Models
Taper models estimate tree diameters at points along the main tree stem. Taper is essential for estimating tree volume when bucking stems into logs. Volume can also be calculated for a tree as a whole and by using volume tables. See Volume below for more.
Assisi implements many of the most popular taper models as well as integrates external models such as Flewelling.Taper Models
Behre
Clark & Souter
Simple Cone
Flewelling
Flewelling DLL
Hann
Kozac
Profile
Walters & Hann
Weiskittel
Wensel & Krumland
Site Index Models
Site Index is a critical component of growth and height models. Through site index, growth models become sensitive to local site productivity factors such as soil, topography and climate. There are a great many site index equations to describe all the species, region and site possibilities.
Parse
Conversion
Logistic
Alexander
Barrett
Brickell
Bruce
Carmean
Chapman-Richards
Cochran
Curtis
Dahms
Demars
Dolph
Dunning
Gregory & Haack
Hann & Scrivani
Harrington
Herman
King
King & Prodan
KuruczMeans
Powers
Schumacher
Wiley
Worthington
Crown Models
Crown is also critical in many growth models being the source of energy for growth itself. Crown is also an indicator of density competition.
Crown Height
ORGANON
Acadia
Assisi FVS
Biging
Ritchie & Hann
Weiskittel
Crown Width
Parse
Assisi FVS
Bechtold
Biging
Crookston
Donnelly
Krumland
Moeur
Paine & Hann
Ritchie & Hann
Russell & Weiskittel
Multi
Power
Polynomial
Total Height
Crown Competition
Multi
Polynomial
DBH Power
Crown width
Carbon Models
Carbon is a first-class output in Assisi. Carbon is estimated when compiling field data and when simulating management plans. Long term carbon projections are easy to see and track. There are three methods for calculating carbon in Assisi. The choice often depends on what a particular market requires.
Jenkins - The Jenkins biomass equations were published in 2003 (Jenkins, et. al., 2003) and updated in 2013 (Chojnacky et. al., 2013).
Component Ratio Method (CRM) - The Component Ratio Method calculates biomass as a function of tree volume (Heath et al. 2009; Woodall et al. 2011).
National Scale Volume and Biomass (NSVB) - The National Scale Volume and Biomass approach calculates carbon directly from DBH and total height (Westfall et. al., 2024).
Volume Models
Volume Models include Board, Cubic using bucking, volume equations or volume tables. Volume tables can be by sawlog, merch or total height with percent distributions by length.
Board Volume
Parse
Brereton
Bush
Doyle
Hahn
Int ¼ Inch
Scott
Scribner
Volume Tables
Volume Distributions
Cubic Volume
Parse
Bush
Hahn
Huber
Newton
Pillsbury & Kirkley
PMRC
Smalian
Scott
Walters, Hann & Clyde
Subneiloid
Two End Conic
Volume Tables
Volume Distributions
Stump Models
Walters & Hann
Hann
DIB Models
Bark
Linear
Power
Hann 2011
Bark Models
Parse
BTR
Linear
Power
PMRC
Clark
Density Models
Curtis
Reineke
Stage
Zeide
Max SDI
Form Point Models
Parse
Regression
Conversion
Clark Souter
Weight Models
Parse
PMRC
Clark
Fertilizer Models
Littke