Prior to conducting any survey, the opening being surveyed should be divided into homogeneous units (strata). When the opening is stratified for a regular silviculture survey, usually no additional stratification is required to collect GI data.
The stratification criteria used for most silviculture surveys (i.e., site series, potential/past treatment, change in leading species, etc.) match the requirements of the GI method. The stratification process used for most silviculture surveys (i.e., use maps, air photos and opening file information to do the initial office stratification and confirm the boundaries with a walkthrough) provides enough information on the stratum to use the GI method.
To add GI data collection to a silviculture survey, there are two additional tasks during stratification:
To be suitable for GI data collection, a stratum must be well stocked with acceptable sample trees of the desired SI species (Figure 5.4).
It may be determined from the office information or the walkthrough that the age of the stand is not appropriate for the GI method or that the appropriate species tables are not available. In this case, the BEC or site index curve method will have to be considered. Use the decision key to decide which method is appropriate to obtain SI (see Lesson 10).
In some cases, the stratification required for a silviculture survey results in many small strata that may differ in history, forest health or stocking status, but do not differ in site productivity and forest cover attributes. In this case, it is possible to sample the many small strata as a single stratum for GI data collection.
During stratification it may be determined that the GI method is not suitable for a portion of the stratum or for the entire stratum. If the entire stratum is not suitable for the GI method, do not collect GI data on the plots falling in that stratum.
If a portion of the stratum is unsuitable for the GI method, either treat the entire stratum as unsuitable and use another method or divide the stratum into two smaller strata. It is not acceptable to put growth intercept plots in unsuitable ground. It is acceptable to use different methods to determine SI for the different strata within an opening.