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Growth-and-Yield-Modelling

Important for silviculture because there is terminology we need to use, and tools allow us to predict and understand what our treatments will do.

Growth and yield modelling aims to answer:​

  • What yield can be expected from a particular species or mixture on a particular site?
  • What kind of treatments should be used to achieve particular objectives?
  • What has gone wrong? What yield should we see on a particular site?
  • How should tese treatments be designed?

How is this done?​

  • Simulation of alternatives to pick the "best" option

What is needed for reliable models?​

  • A lot of data

G&Y is very important for silvicultural prescriptions/plans

Growth and yield terminology​

Quadratic mean diameter​

  • Measure of central tendency
  • If every tree is treated the same in the average, then the measurement is not as representative of total volume per hectare
  • Weighted towards larger trees
    • Better proxy for tree volume

Volume Yield​

Quanityt of wood or biomass at a particular point in time

  • For example:
    • Volume yield per ha - m3/ha or m3 ha-1
    • Biomass in kg per ha - kg/ha or kg ha-1

Growth/increment​

  • Change in stand quantity over time
    • For example:
      • Diameter growth per year

Mean Annual Increment (MAI)​

  • DBH divided by age
  • A tree aged 20 years with a DBH of 30 cm has a MAI of:
    • 30 cm / 20 years = 1.5 cm/yr
  • Can also be used with volume

Period Annual Increment (PAI)​

  • Average growth over a defined period
    • Usually last over the 5 or 10 last years
      • E.g. diameter growth
      • E.g. stand volume growth

Current Annual Increment (CAI)​

  • Growth in the most recent year
    • E.g. Basal Area increment in 2020 (cm2)
    • E.g. Diameter increment in 2020 (cm)

Basal Area per Tree (BA, m2)​

Growth and yield terminology​

Development over time

  • Volume yield
  • Mean annual increment (MAI)
  • Periodic annual increment (PAI)
  • Intersection between MAI and PAI indicates point at which stand can be harvested

Stem density = number of trees per HA

  • More measures of density available

Scale of modelling​

Model classification

  • Whole stand models

    • Yield tables

  • Size class models

    • Based on distribution of diameter classes
      • The number of trees in each class is based on empirical data
      • Each diameter class is based on an average tree
    • Growth and volume is calculated by diameter class

  • Individual tree models

    • Single tree models / tree list models
    • Distance dependent / Distance independent

  • Whole stand models