Skip to main content

Tree-Growth-Site-Quality

How to select trees for site index measurements?​

Vigor: Dominant and co-dominant

  • Normal growth

Obsolete: Predominant height

  • Based on tallest 100 trees / ha
    • Used for site index

Select trees based on top height

  • Average height of the 100 trees / ha with the largest DBH

Tree growth as an indicator of site quality: Calculating site index with the site curve method:

  • Top Heigght: 52 m
  • Average DBH-age: 75
  • Site Index = 41

Limitations of site index:

  • Most useful in even0aged, single species conifer stands
    • Crown shape affects height measurements
    • more than 30 years of DBH-age needed

Note of caution: Site index usually does not include a measure of uncertainty (confidence interval) but there are many sources of variability:

  • Selection of trees and variability within the stand
  • Measurement errors
  • Uncertainty of growth curves
  • Low sample size

Site quality assessment with indicator plants​

  • Predict site index with understory vegetation
  • In BC, supported through the BEC system (SIBEC)
  • Measure of uncertainty provided

Growth intercept method:​

  • For stands between 3 years and 30 years
    • greater than or equals 3 years of growth
    • Not overtopped
    • Undamaged stems

Site quality assessment with soil analysis​

Predicting site index with soil properties

  • Multiple regression equations based on independent site variables
  • Labor-intensive
  • Useful when no other methods are possible (e.g. in reforestation or plantation forestry)
  • Variables that affect water availability most important
  • Variables that describe nutrient status are less important

We look for:

  • topography -- water shedding / standing water
  • Soil texture -- coarse fragments, fine fraction
  • Organic matter