Appendix 6 spacer Procedures for Obtaining Site Index for Silviculture Polygons
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BC Government logo Province of
British Columbia
Ministry of
Forests

File: 102-20/SISI

To: All District Managers

From: Janna Kumi, Assistant Deputy Minister, Operations

Re: Procedures for Obtaining Site Index for Silviculture Polygons in ISIS and MLSIS

Please distribute this memo to:

  • district staff in silviculture, inventory, and ISIS or MLSIS administration; and
  • all the major licensees operating in your district.

Silviculturists will soon be required to describe site productivity with a number (site index), rather than with a class (site class). Site index is required by MLSIS and will soon be required by ISIS and many standard silviculture forms, including the new Stand Management Prescription.

The attached procedures provide a method for obtaining site index values for silviculture polygons and strata. They will be updated as required. They have been approved by the ministry Growth and Yield Steering Committee.

Field procedures and data storage, as outlined in the attached procedures, will be consistent with the following principles:

  1. For each silviculture polygon in ISIS and MLSIS, an estimate of site index of accuracy sufficient for quality forest management will be obtained;
  2. Site index estimates will be stored corporately to avoid having to repeat site index data collection where an adequate site index estimate exists;
  3. Tools are provided to obtain accurate site index estimates for all important species and stand conditions; and
  4. Staff will receive the training required to select the appropriate tool to estimate site index and properly apply it under operational conditions.

If you have any questions on the recommended procedures, please contact:
Patrick Martin
Stand Development Officer
Silviculture Practices Branch
250-356-0305

Janna Kumi
Assistant Deputy Minister
Operations

Recommendations to Silviculturists on Obtaining Site Index

Silviculturists will soon be required to describe site productivity with a number (site index), rather than with a class (site class). Site index is required by MLSIS and will soon be required by ISIS and many standard silviculture forms, including the new Stand Management Prescription. This document provides recommendations to silviculturists on obtaining these site index values for silviculture polygons and strata. These recommendations will be updated as required.

  1. Definition of site index
    Site index is a measure of site productivity -- the capacity of an area of land to grow trees of a given species. The precise, technical definition of site index is "the top height of a given species at 50 years breast height age." A more informal definition is that site index is the average height of the largest diameter trees of a given species when they have achieved 50 years growth above breast height. Accurate estimates of site index are required to adequately describe site quality, formulate prescriptions, schedule treatments, allocate funds among competing projects, and predict stand growth and yield.
  1. Site productivity can be recorded as a number or a class
    Site productivity can be recorded as a number (e.g., site index is 18 m) or as a class (e.g., site class is medium). In MLSIS and ISIS, the productivity of silviculture polygons (strata) has been recorded as site class. These site class values are inadequate for many modern silviculture uses due to the wide range of productivity within an individual site class and the subjective or inaccurate methods that have been used to determine the class.
  1. There are several methods for estimating site index
    Several different methods can be used to obtain a site index estimate for a silviculture polygon (stratum). These different methods produce site index estimates that differ in accuracy and suitability for general silviculture use. Three methods that use ground collected data produce site index estimates best suited for silviculture use. These three are the biogeoclimatic, growth intercept, and site index curve methods. Several other methods produce site index estimates that are not well suited for general silviculture use. These methods include subjectively estimating site index, inputting inventory label height and age to a site index curve, and using the site index value from a nearby area.
  1. Changing the way site productivity is recorded and estimated
    For all silviculture polygons (strata), the existing site class values should be upgraded to more accurate site index estimates. To accomplish this, it is necessary to improve:
    1. The way site productivity is recorded by changing from site class to site index; and
    2. The methods used to estimate site productivity by using more accurate methods.
    Upgrading the existing site class values can be achieved by collecting the data required to accurately estimate site index during regularly scheduled silviculture surveys. Special surveys to obtain site index should not be required.
  1. Follow these steps to obtain site index for a silviculture polygon

    5.1. Polygon does not have existing site index estimate -- To obtain site index for a silviculture polygon (strata) which does not have an existing site index estimate, the silviculturist should select and apply an appropriate method to estimate site index. The three methods that produce site index estimates suitable for silviculture use are described in 5.4 Selecting and applying an appropriate site index method.

    5.2. Polygon has existing site index estimate -- To obtain site index for a silviculture polygon (strata) which has an existing site index estimate, the silviculturist should:
    1. Determine if the existing site index estimate is acceptable for general silviculture use; and
    2. If the existing estimate is acceptable, use it. If the existing site index estimate is not acceptable:
      1. Select and apply an appropriate method to estimate site index; or
      2. If an appropriate method cannot be used, due to operational limitations (time, money, manpower, etc.), carry forward the existing site index estimate. In many cases, this will involve converting an existing site class to a site index value (see 6 Converting site class to site index).


    5.3. Is the site index estimate acceptable for general silviculture use? -- For general silviculture use, a site index estimate is acceptable when site index was estimated using the biogeoclimatic, growth intercept or site index curve methods with ground collected data where an adequate number of samples were distributed over the strata to provide uniform and complete coverage. In general, an existing estimate needs to be updated to a more accurate site index estimate when it is:
    1. only available as a broad site class (G, M, P, L);
    2. based on a subjective estimate of site productivity;
    3. based on inventory label height and age input to a site index curve
    4. estimated from the site index value of a nearby area; or
    5. obtained by converting a site class to a site index.


    5.4. Selecting and applying an appropriate site index method -- Three methods are available to silviculturists to get a high quality site index estimate for a silviculture polygon (strata). The three methods are to:
    1. predict site index from the area's biogeoclimatic classification;
    2. predict site index from height and age measured on carefully selected sample trees and input to a growth intercept table; or
    3. predict site index from height and age measured on carefully selected sample trees and input to a site index curve.

    These methods are not available for all species in all regions under all stand conditions. In addition, operational considerations may further limit the use of any one of these methods in some surveys.

    In general, when surveying stands with:
    1. more than 30 years growth above breast height, attempt to estimate site index with the site index curve method. Exceptions to this rule include uneven-aged stands; stands that have received partial cutting; stands with extensive damage such as broken, forked, or dead tops; and stands where sample trees are very large or rotten at breast height. In these cases, it is usually best to use the biogeoclimatic method.
    2. 3-30 years growth above breast height, attempt to use the growth intercept method. Exceptions to this rule include uneven-aged stands and stands with extensive damage such as broken, forked, or dead tops. In these cases, it is usually best to use the biogeoclimatic method.
    3. less than 3 years growth above breast height, attempt to use the biogeoclimatic method.

    More detailed information on these three methods is available in Table 1.

Table 1. Characteristics of methods for estimating site index.

Characteristic of Method Biogeoclimatic Growth Intercept Site Index Curve
Description of method Predict site index from biogeoclimatic ecosystem classification Predict site index from height and age measured on carefully selected sample trees and input to growth intercept table Predict site index from height and age measured on carefully selected sample trees and input to a site index curve or table
The method can typically be combined with these silviculture surveys Surveys before free growing, some free growing surveys, and some post-free growing surveys Pre-stand tending surveys and some free growing surveys Some pre-stand tending surveys and some Silviculture Prescriptions
The stand must have these conditions to be suitable for the method Any. This method is not limited by stand condition Stand must be well stocked with suitable sample trees of the site index species having 3-30 years growth above breast height Stand must be well stocked with suitable sample trees of the site index species having more than 30 years growth above breast height
The method is currently available for these species in these regions Many commercial species on many sites in the Vancouver, Prince Rupert, Kamloops, and Nelson forest regions Interior Pl 11 interior species (At, Bl, Cw, Ep, Fd, Hw, Lw, Pl, Pw, Py, Sw)
6 coastal species (Ba, Cw, Dr, Fd, Hw, Ss)
Procedures for using this method are available from this source The relevant field guide for site identification and interpretation available from Crown Publications Inc., 546 Yates Street, Victoria, B.C. (Tel: 386-4636). Contact the Forest Sciences Officer in the relevant Ministry of Forests regional office for more information Patrick Martin, Silviculture Practices Branch, (Tel: 356-0305), in the document Growth Intercept Method Albert Nussbaum, Research Branch, (Tel: 387-6708), in Site Index Curves and Tables for British Columbia

Susan Penner, Resources Inventory Branch, (Tel: 356-9451), in Field Handbook - Classification and Sampling
Next developments of this method are scheduled for these dates In 1996, this method will be available for more species and sites in British Columbia By the summer of 1995 this method will be available for interior Sx and coastal Hw and Ss Updated Ss curves by summer 1995
Accuracy of the method Medium High High
  1. Converting site class to site index
    It will take several years to replace the existing site class values with improved site index estimates for all silviculture polygons (strata). During this transition period, it will be necessary to provide interim site index values for some silviculture polygons by converting the existing site class to a number. In time, these interim site index values should be replaced with more accurate site index estimates based on suitable, ground-sample methods. A site index value obtained by converting site class to site index is not sufficiently accurate for most silviculture decisions.

    Use Table 2 to generate an interim site index value by converting a site class to a number. Based on inventory label leading species, region, and site class, an interim site index value can be obtained. For example, if the inventory label leading species = Hw, region = coastal, and site class = M, then the interim site index value is 22 m.

Table 2. Interim site index values by inventory label leading species, region, and site class.

Inventory Label Leading Species Region Good Medium Poor Low
Ac Province 26 18 9 3
At Province 27 20 12 4
B, Ba, Bg Coastal 29 23 14 5
B, Ba, Bg Interior 18 15 11 5
Bl Province 18 15 11 5
Cw Coastal 29 23 15 6
Cw Interior 22 19 13 5
Dr Province 33 23 13 6
E, Ea, Ep Province 27 20 12 4
Fd Coastal 32 27 18 7
Fd Interior 20 17 12 5
H, Hm, Hw Coastal 28 22 14 5
H, Hm, Hw Interior 21 18 12 4
L, La, Lt, Lw Province 20 16 10 3
Mb Province 33 23 13 6
Pa, Pf, Pj, Pl Province 20 16 11 4
Pw Province 28 22 12 4
Py Province 17 14 10 4
S Coastal 28 21 11 4
S Interior 19 15 10 5
Ss Province 28 21 11 4
Sb, Se, Sw Province 19 15 10 5
Yc Coastal 29 23 15 6
Yc Interior 22 19 13 5

 

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