Determining site index integrates the technician’s abilities to identify crown classes, measure tree height and estimate tree age. This is one of the few cases in which a biased sample of trees is chosen to measure. The trees as an indication of the site quality, so only trees that were relatively unimpeded by neighboring trees or disturbances are measured. The objective is very different than a typical sampling scheme to get an average measure of stand volume, size or growth rates. Therefore, the sampling method is different as well.
1. Select trees to measure. The number of trees to measure will depend upon how variable the stand is, and the degree of accuracy desired. The greater the variability in size and species on the site, the larger the sample size needed to get an accurate estimate of site quality for each species present. For species grouped into site classes, a relative ranking is the objective, so a large sample size is not required.
Criteria for selection:
a. Dominant or codominant crown class. Choose trees whose crowns are receiving full sunlight.
b. Check the outside—Free from past disturbance. Check all sides of a tree for signs of insect galls, conks, witches brooms, basal or trunk scars, breakage, etc. Check around the base of the tree for signs of root disease (e.g. Phaeolus spp. or Heterobasidion spp.). There may be instances in which a stand severely hit by an ice or windstorm will have very few suitable site index trees to choose from.
c. Check the inside—Free from past suppression. This may be difficult to assess on larger trees without looking at an increment core sample. Some people will core the tree first to make sure it is a usable site index tree before measuring the height to save time. On smaller trees, the distance between whorls can indicate general growth rate trends.
2. Extract a clean, intact core sample to estimate age.
a. Check core for evidence of rot, charcoal, past suppression or drought.
b. Make sure you can read the age – use a magnifying glass or hand lens on trees with tight rings. Look carefully at the regions indicating the center of the tree. Core samples more than a few of years from the pith are not reliable. Count the rings twice.
3. Measure total height. Obviously, this is a critical measurement. Measure your distance from the tree – do not pace. Make sure you can see the top. From a perspective that allows a clear view of the crown, look for evidence of breakage – flat tops, longer than expected side branches, etc. Also look for sucker limbs and forking in the crown.
4. Record your measurements. Record each tree as a pair of measurements – height and age. The two measurements are used together to obtain site index, so keep them as a pair. Always record breast height age in the field. This number can later be adjusted to total age by adding the number of years commonly needed for that species to reach breast height. This will vary by species and region. Typically, it is four to eight years for most low elevation conifers.