Site and Stocking

and Other Related Measurements

Height as a Measure of Site Quality

• Tree Height in relation to tree age has been found to be the most practical, consistent and useful indicator.

• What was discussed earlier as the limiting factor to fast growth?

Site Index• The average total height and age of dominant

and co-dominant trees in well-stocked, even-aged stands.

Crown Class• Dominants - Crowns receive full light from above, and partly

from the side. Trees extend above the general crown levels

• Co-dominants - Crowns form at the general level of the canopy. Crowns receive full light from above, but little from the sides.

• Intermediates - Crowns generally below the dominants and co-dominants, but extending up into the general canopy level. They receive little or no direct light from above, nor the sides.

• Overtopped or Suppressed - Crowns entirely below the general level of the canopy. They receive no direct light from above, nor the sides.

Crown Class

Crown Closure

Measured by what?

Crown Closure• Spherical Densiometer• Hemispherical photography• Aerial photography

• CLASSES• Very Sparse 1-9%• Sparse 10-29%• Low 30-49%• Medium 50-69%• Dense 70-84%• Very Dense 85-100%

Tree Age• Even-aged - The range of tree ages

generally does not exceed 20% of the rotation age assumed.

• Uneven-aged - Three or more distinct age classes, either intimately mixed, or in clumps.

Increment Borer

Site IndexTotal Heights and Ages of dominant and/or co-dominant trees are measured and curves are fitted.

Varies by species and region.

Site Index Limitations1. Exact Stand age is often difficult to determine

(diffuse porous wood, false rings and drought rings, skill with borer), and small errors can cause relatively large changes in the site-index estimate.

Alternate methods of determining stand age can include counting branch whorls (if applicable species) or historical information

Site Index Limitations - continued

2 Not well suited to uneven-aged stands, areas of mixed-species composition, or open lands.

3 Stand volume variables (DBH, stem form) are not taken into account.

4 Site-Index is not a constant, it changes periodically with climate and management.

5 Varies by species within the same site. So if you plan to change the site to a new species, it is difficult to predict.

Factors that Influence SI

SI Alternatives• Growth index

o Tree diametero Stand volume

• Output from a mechanistic growth modelo Indicator plantso NPP, evapotranspiration

• Leaf area indexo Can be assessed remotely

Site Index real world use

• Growth Intercept Models: These species-specific models are designed explicitly for young stands (5-50 years breast height age). The growth intercept technique estimates site index from the average annual height growth of site trees, which is determined either from the distance between annual branch whorls or from the height and breast height age of the tree.

• Site Index-Biogeoclimatic Ecosystem Classification (SIBEC) Models: This comprehensive tool correlates site index with site series within biogeoclimatic ecosystem classification (BEC) units and site series. The BEC system is designed specifically for British Columbia’s ecosystems. This model is best used for very young stands, very old stands, and stands not suitable for other methods. 

Indicator Plant Approach

• Curves fitted to measured data relating the presence of certain plants to the site quality for trees on a given site.

Site Index from Soil

Soil Survey• Forest lands• Site Index -  Measurements of site index are

usually extended to a number of like soils where data are unavailable.

• Erosion hazard• Equipment limitations• Seedling mortality• Windthrow hazard• Plant competition• Trees to plant

Site Productivity Measures

Tree GrowthPrinciples and their implications

General Growth Curve

Change in Tree Weight

Mean Annual Increment

• Increase in cubic-foot volume per acre per year

• Curves fit to known data – usually takes the equation form below

• MAI = Constant X Site Index +/- Intercept

• Attempts to predict future Annual volume growth

Annual Growth Increment

Generalized Chart for conifers

The concept of forest site productivity in terms of current annual volume increment vs age.

Skovsgaard J P , and Vanclay J K Forestry 2007;81:13-31

© Institute of Chartered Foresters, 2007. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Stocking

Stand DensityQuantitative measurement of the stand describing the degree of stem crowding within a stocked area.

Growth vs. Growing stock

The thinning response hypothesis.

Skovsgaard J P , and Vanclay J K Forestry 2007;81:13-31

© Institute of Chartered Foresters, 2007. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Assmanns theory of natural, optimal and critical basal area.

Skovsgaard J P , and Vanclay J K Forestry 2007;81:13-31

© Institute of Chartered Foresters, 2007. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Stocking• A description of the

number of trees, basal area, or volume per acre in a forest stand compared with a desired level for a management objective

• Central Hardwood Stocking Chart

Stocking Charts• Vary by species• Vary by site quality

• Compare current stand conditions to desired in order to make prescriptions