Lesson 2 spacer Introduction to Site Index
Lesson 1
Lesson 2

Lesson Objectives

What is Site Index

Where is SI Used?

Site Index in Forest Cover Labels

Problems with Incorrect Site Index Determinations

When/Where is SI Recorded?

FPC and SI

SI Source Codes

Biased Site Index and Stands Growing Below Potential

Site Index and Yield

Tree Growth Response to Improved Site Quality

Lesson 3
Lesson 4
Lesson 5
Lesson 6
Lesson 7
Lesson 8
Lesson 9
Lesson 10
Lesson 11
Lesson 12
Appendix 1
Appendix 2
Appendix 3
Appendix 4
Appendix 5
Appendix 6
Appendix 7
Appendix 8
Appendix 9
Course Homepage


Site Index and Yield

The actual timber yield achieved by a stand depends on many factors. While the SI of the growing site is one important factor, other important factors include species composition, stocking, treatment, and losses to pests, disease, and damage. High SI does not guarantee high yield. For example, high site land with no trees on it, produces no timber volume. Figure 2.11 compares the yield expected from an understocked Pl stand on SI = 20 m with the yield that can be expected with full stocking.

A site may have high growth potential (SI) for conifers, but if deciduous trees establish a thick canopy above the young conifers, the site will produce little conifer volume. A high site may have the potential to grow trees rapidly, but if the trees are killed or repeatedly damaged by pests and disease, little timber volume will accumulate. Figure 2.12 provides a hypothetical example of the different yields that can be achieved by Pl stands on SI = 20 m. Scenario A includes delayed regeneration, inadequate stocking, and heavy brush competition and losses to pests. Scenario B includes patchy but adequate stocking, with some brush competition and moderate losses to pests. Scenario C includes prompt regeneration, full stocking, and no brush competition or losses to pests. To produce an accurate estimate of yield, a stand growth model requires an accurate description of SI, stocking, species composition, and expected losses to pests and disease. The model user must not focus solely on SI.

Figure 2.11. Yield predictions for understocked and well-stocked Pl stands on SI = 20 m. (Source: TIPSY 2.1.)

Figure 2.12. Yields of hypothetical Pl stands on SI = 20 under three management scenarios.

Tree Growth Response to Improved Site Quality

Compared to a poor site, a site that is “better” for a given tree species provides the required resources (e.g., light, soil water, soil nutrients, CO2) and environmental conditions (e.g., temperature, soil aeration) at levels that are closer to optimum for more days of the year. Under these favourable conditions, trees achieve more growth; top height growth is greater and stands attain a greater top height at bh age 50 years. Thus, the site has a greater site index. The increased height growth of trees on better sites can be understood as a result of four factors (Figure 2.13):

  1. Resource availability—a greater quantity of resources are available per day and there are more days in the growing season.

  2. Resource capture—trees can support more foliage thus capturing more of the available light.

  3. Efficiency of resource use—the resources that have been captured are used more efficiently. More photosynthate is produced per unit of resources captured. The efficiency of resource use is greater as:

    1. there are fewer days in the growing season where soil moisture deficit, frosts, air moisture vapour pressure deficits, and other environmental conditions limit photosynthesis; and

    2. improved nutrient levels in the leaves improves the yield of photosynthate per unit of captured light.

  4. Allocation to roots—underground resources (e.g., soil moisture and nutrients) are acquired more easily. Thus, the tree needs to allocate less of its photosynthate to the roots, leaving a greater proportion to be allocated to stem growth. The increased top height growth on better sites is the result of the combined effect of these tree responses to improved site quality.

Good Site Poor Site
More site resources Less site resources
More foliage, more site
resources captured
Less foliage, less site
resources captured
Increased photosynthetic efficiency Decreased photosynthetic efficiency
Less allocation to roots More allocation to roots

Figure 2.13. Tree height growth is greater on better sites due to greater resource availability, resource capture, efficiency of resource use, and allocation to stem growth.


PreviousTop of Pagenext