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.
- 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.
DBH Growth
Growth Index Ratio
Total Height Growth
Site Class
Crown Growth
Mortality
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
Sawlog Height
Merch Height
Total Height
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. Assisi implements many of the most popular taper models as well as integrates external models such as Flewelling.
Taper
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.
Site Index
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
Crown Width
Crown Competition
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) — calculates biomass as a function of tree volume (Heath et al. 2009; Woodall et al. 2011).
- National Scale Volume and Biomass (NSVB) — 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.