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Questions Frequently Asked About Site Index
What is site index and what is it used for?
Site index (SI) is a measure of a stand's productive potential for a
particular tree species. SI in British Columbia is expressed as site tree
height at 50 years breast height age. SI provides standardized comparisons of
productive potential between sites, across a broad range of existing stand
conditions. As such, we use it as a silvicultural tool to prescribe treatments
and analyze investments. SI also serves as the main driver for many growth and
yield models, which predict future forest growth and timber yields.
What are the theories and assumptions behind SI?
Central to the concept are the theories that:
- 1) stand height is closely related to stand volume, and hence is a good proxy
for productivity in terms of timber volume; and,
- 2) relative to other growth measures (diameter, volume, basal area,
etc.), stand height is independent of stocking density, except possibly at
the extremes.
Theoretically, the SI concept applies only to single-species, even-aged
stands historically free from damage and competition. We require these
assumptions to avoid the confounding effects of competition, suppression,
repression, and top damage on the height-age relationship.
What happens when these assumptions are violated?
Luckily, in practice the concept and its assumptions are somewhat
forgiving. The more severe the violation(s), the more the resulting SI
estimate becomes confounded with individual stand history. Then SI's
usefulness as a comparative index declines. Operationally, the key is
recognizing the extent of the violation of the assumptions, and the bias in
the resulting SI estimate. This requires informed, professional judgement.
What determines SI?
SI reflects inherent physical site factors, such as climate (solar input,
precipitation, temperature, etc.) and soil (nutrients, moisture, etc.). SI
should be independent of stand history and management. Possible exceptions are
overstocking, genetic improvement, and fertilization.
How do we derive SI estimation functions?
We develop modern SI functions from historical height growth data derived
from extensive stem analysis (sectioning) of carefully selected trees. We
calibrate each species SI function across a defined age range determined by
the original data. Exercise caution when making extrapolations outside this
range.
What is breast-height age?
Breast-height (bh) age is the number of annual growth rings at breast height.
It is determined by increment boring or felling trees at breast height and counting
the annual rings. Breast height is 1.3 m on the uphill side of the tree.
What measure of height should I use with SI functions?
We use a measure of height that reflects the inherent productivity of the
site. The B. C. Forest Service has therefore adopted a definition for height
known as site height.
What is site height?
Site height is the height of the largest dbh tree of the target species in
a 0.01 ha plot. This tree is known as a site tree. Site trees must have other
characteristics such us being dominant or co-dominant, undamaged, unsuppressed,
vigorous, and healthy.
What are some common pitfalls in estimating SI?
In practice, we nearly always violate one or more of the SI assumptions to
some degree. Perhaps the least obvious and most common violations are the
assumptions regarding historical freedom from competition and damage. Clearly,
as a stand ages it becomes increasingly difficult to know its history, and
damage becomes the norm rather than the exception. It's easy to confound
measurements of SI in old-growth stands for this reason. This problem also
limits the availability of old-growth trees for the collection of stem
analysis data, used to construct SI functions. This is why SI functions rarely
contain data beyond 150 - 200 years.
How to select and measure site trees
Tree Selection
Select trees within a sample plot according to the following criteria:
- the largest diameter tree of the target species;
- are dominant or co-dominant;
- are not wolf, open-grown, or veteran trees;
- have a straight stem that is free of disease, damage, or breakage;
- free of suppression (above breast height) and repression; and
- are vigorous with full crowns.
Increment cores will assist in detecting suppression and abnormal growth
that may indicate insect or other damage. Other indicators of healthy,
unsuppressed trees are large branches or branch stubs close to the ground and
uniform internode growth. Abandon samples not meeting the above criteria.
Tree Measurements
Take the following measurements on each site tree:
- age at breast height (1.3 m).
Documentation
SiteTools comes complete with a comprehensive on-line reference manual, which
includes a context sensitive Help system.
Contacts
For additional information or specific help, please contact Gord
Nigh, Supervisor, Strategic Analysis Group at 250 387-3093.
For questions regarding the installation instructions, please contact Shelley
Grout, Software Applications Specialist at 250 387-6718.
Educational Materials and Training
The Southern Interior Growth
and Yield Co-operative (SIGY) web site lists training facilities and opportunities.
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