Growth Intercept Method for Silviculture Surveys

Growth Intercept Method for Silviculture Surveys AUGUST 1995 Canada CANADA-BRITISH COLUMBIA PARTNERSHIP AGREEMENT ON FOREST RESOURCE DEVELOPMENT: FRDA II BC

Growth Intercept Method for Silviculture Surveys AUGUST 1995 Prepared by B.C. Ministry of Forests Silviculture Practices Branch Canada CANADA-BRITISH COLUMBIA PARTNERSHIP AGREEMENT ON FOREST RESOURCE DEVELOPMENT: FRDA II BC

ii Growth Intercept Method for Silviculture Surveys Funding for this publication was provided by the Canada-British Columbia Partnership Agreement on Forest Resource Development: FRDA II a five year (1991 96) $200 million program cost-shared equally by the federal and provincial governments. Canadian Cataloguing in Publication Data British Columbia. Silviculture Practices Branch. Growth intercept method for silviculture surveys Canada-British Columbia Partnership Agreement on Forest Resource Development: FRDA II. Includes bibliographical references: p. ISBN 0-7726-2627-8 1. Trees British Columbia Growth. 2. Trees Yields British Columbia. 3. Site index (Forestry) British Columbia. 4. Forest management British Columbia. I. British Columbia. Ministry of Forests. II. Canada British Columbia Partnership Agreement on Forest Resource Development: FRDA II. III. Title. SD396.B74 1995 634.9'285'09711 C95-960350-6 For information regarding additional copies contact: B.C. Ministry of Forests Silviculture Practices Branch Stand Development Forester 3rd Floor, 31 Bastion Square Victoria, B.C. V8W 3E7

Growth Intercept Method for Silviculture Surveys iii Preface This document is designed to help you, the silviculture survey coordinator, use the growth intercept method to estimate site index in conjunction with free growing or pre-stand tending silviculture surveys. Site index measures the capacity of an area of land to grow trees of a given species. It is a measurement equal to the average height of the 100 suitable trees per hectare, with the largest DBH, of one species at 50 years breast height age. 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. Many branch specialists (Research Branch, Resources Inventory Branch, and Silviculture Practices Branch), regional stand tending foresters, district silviculture staff and silviculture contractors helped develop the growth intercept method for silviculture surveys. Particular thanks are due to Frank Barber, Silviculture Practices Branch, Ministry of Forests; Gord Nigh, Research Branch, Ministry of Forests; and Jim Thrower, J.S. Thrower and Associates Ltd. If you have any comments on the growth intercept method or this document, please send them to: Patrick Martin Stand Development Forester Silviculture Practices Branch Ministry of Forests 31 Bastion Square Victoria, B.C. V8W 3E7 Tel: 387-1191 FAX: 387-1467 Ovid: PJMARTIN

iv Growth Intercept Method for Silviculture Surveys Contents Preface... iii Introduction... 1 1 When to Use the Growth Intercept Method... 2 2 How to Use the Growth Intercept Method... 4 2.1 Pre-stratify the Opening... 4 2.2 Select the Site Index Species... 5 2.3 Collect the Growth Intercept Data... 7 2.3.1 Sample Lines... 7 2.3.2 Selecting a Sample Tree... 8 2.3.3 Sample Tree Measurements... 9 2.3.4 Plot Does Not Contain Suitable Growth Intercept Sample Tree... 13 2.3.5 Survey Timing... 13 2.3.6 Sample Intensity... 13 2.4 Compute the Average Site Index for Each Stratum... 14 3 Advanced Options for Using the Growth Intercept Method... 17 3.1 Measuring More than One Sample Tree per Plot... 17 3.2 Determining Age by Counting Branch Whorls... 17 3.3 Estimating Site Index from Partial Growth... 19 3.4 Assessing the Reliability of the Site Index Average... 22 3.5 Site Index Species Conversion... 22 4 Recommended Contract Specifications... 24 4.1 Pre-work Conference... 24 4.2 Marking Growth Intercept Sample Trees... 24 4.3 Measurement Standards... 24 5 Example to Illustrate the Use of the Growth Intercept Method... 25 6 Where to Get More Information... 30 7 Tables to Estimate Site Index from Growth Intercept... 31

Growth Intercept Method for Silviculture Surveys v Figures 1. The best method for estimating site index depends, in part, on stand age... 3 2. Pre-stratifying the opening, assessing if the stratum is suitable for the growth intercept method, and selecting the site index species... 7 3. Good growth intercept sample plot location achieves complete and uniform coverage of the entire stratum area... 8 4. Good growth intercept sample tree selection contributes to an accurate site index estimate... 9 5. Method for collecting growth intercept data on a sample tree... 11 6. Two options for recording growth intercept data on field cards... 12 7. Compute the average site index for each stratum... 15 8. Example of submission of a site index estimate with an MLSIS forest cover update... 16 9. Determining age by counting branch whorls... 18 10. Collecting growth intercept data on a sample tree to estimate site index from partial growth... 21 11. Example of the questions used to assess the suitability of the strata for growth intercept data collection and select site index species... 25 12. Example of a field sheet used to collect growth intercept data... 27 13. Plot location map... 28 Tables 1. Acceptable plot sizes and number of sample trees... 17 2. Site index species with distinct annual branch whorls... 18 3. Number of plots required to achieve a reliable site index average for the stratum... 22 4. Site index species conversions... 23

Growth Intercept Method for Silviculture Surveys 1 Introduction The growth intercept method predicts site index from height and age measurements taken on carefully selected sample trees. This method of calculating site index is being calibrated for many tree species in B.C. As of August 1995, the method is available for interior lodgepole pine, interior spruce, and coastal western hemlock. By August 1996, growth intercept tables will be available for coastal Douglas-fir, interior Douglas-fir, and interior western hemlock. These procedures replace those described in the memo Using the preliminary growth intercept method to estimate site index in young lodgepole pine stands. 1 The old preliminary method required the surveyor to identify the first five annual branch whorls immediately above breast height and measure this amount of height growth. The new method, described in this document, requires the surveyor to measure total tree height and age at breast height. Compared to the old method, the new method has the following benefits: Can be applied to younger stands; Is suitable for species with and without regular annual branch whorls; Produces a more accurate site index estimate under many circumstances; and Integrates better with existing and proposed silviculture survey methods. 1 Memo dated October 14, 1994, from Henry Benskin, Director, Silviculture Branch, to all regional silviculture officers and district silviculture resource officers.

2 Growth Intercept Method for Silviculture Surveys 1 When to Use the Growth Intercept Method As a survey coordinator, one new task that you face in planning free growing or pre-stand tending surveys is to select, for each silviculture polygon (stratum) in an opening, the best method to obtain site index. In many cases, more than one method will be available to you. In each survey, you will have to choose the most appropriate method for each stratum. The different methods produce site index estimates that differ in accuracy. Three methods, which use ground collected data, produce site index estimates that are accurate enough for most silviculture purposes: 1. Biogeoclimatic method where site index is predicted from the strata s biogeoclimatic ecosystem classification (usually subzone and site series); 2. Growth intercept method where site index is predicted from height and age of sample trees that are 3 30 years old; and 3. Site index curve method where site index is predicted from height and age of sample trees that are over 30 years old. For a stratum, the method to choose to obtain site index depends on: 1. the tools that are available to estimate site index; 2. the stand age and condition; and 3. the requirements and restrictions unique to the survey. In those cases where all three site index tools are available, stand condition is suitable, and there are no additional survey restrictions, the best method to use is determined by stand age (Figure 1).

Growth Intercept Method for Silviculture Surveys 3 Biogeoclimatic method Growth intercept method Site index curve method Less than three-years growth above breast height, use the biogeoclimatic method. Stand age Three- to thirty-years growth above breast height, use the growth intercept method. More than thirty-years growth above breast height, use the site index curve method. Figure 1. The best method for estimating site index depends, in part, on stand age. In general, to obtain site index for a stratum, use the growth intercept method when: 1. Growth intercept tables are available. As of August 1995, the growth intercept method is available for the following species: Coastal western hemlock; Interior spruce; and Interior lodgepole pine. 2. You are willing to collect height and age on sample trees. 3. You require a very precise site index estimate. 4. The stand and sample tree condition are suitable for the growth intercept method (see 2.1, Pre-stratify the Opening and 2.2, Select the Site Index Species).

4 Growth Intercept Method for Silviculture Surveys 2 How to Use the Growth Intercept Method There are four steps to the growth intercept method of estimating site index in a silviculture survey: 1. Pre-stratify the opening; 2. For each stratum, select the site index species; 3. In each stratum, collect growth intercept data on the site index species; and 4. Compute the average site index for each stratum. Each of these four steps are described in more detail below. Advanced options for using the growth intercept method are described in 3, Advanced Options for Using the Growth Intercept Method. 2.1 Pre-stratify the Opening Before collecting growth intercept data during a standard silviculture survey, it is helpful to pre-stratify the opening. As the survey coordinator, you must follow the steps described below. 1. Divide the opening into the strata required for the silviculture survey. Use maps, photos, the opening file, and a walk-through to divide the opening into homogeneous strata. 2. Assess the suitability of each stratum for growth intercept data collection. To be suitable for growth intercept data collection, each stratum must be well stocked with acceptable sample trees of the desired site index species (see 2.2, Select the Site Index Species and Figure 1). 3. Re-stratify or change the method of site index estimation if the strata are not suitable for collection of growth intercept data. The following situations may arise: If the entire stratum is unsuitable, do not collect growth intercept data on plots that fall in the stratum. An accurate site index estimate cannot be obtained for this stratum using the growth intercept method. If a portion of the stratum is unsuitable, either treat the entire stratum as unsuitable or divide the stratum into two smaller strata suitable and unsuitable. Do not collect

Growth Intercept Method for Silviculture Surveys 5 growth intercept data on plots that fall in the unsuitable stratum. An accurate site index estimate cannot be obtained for the unsuitable stratum with the growth intercept method. If the stratum contains large areas that differ in site productivity, consider dividing the stratum into smaller strata of uniform productivity. In some cases, the stratification required for a silviculture survey results in many small strata that differ in history, forest health, or stocking status, but do not differ in site productivity and forest cover attributes. In these cases, it may be feasible to sample the many small strata as a single large stratum. When this occurs, distribute the 10 growth intercept plots over the small strata and compute one average site index that can be applied to each of the small strata. After completing the pre-stratification, there may be one or more strata in the opening that cannot be assigned a site index estimate by the growth intercept method. To obtain a site index estimate for these strata, you must use some other method (e.g., the biogeoclimatic or the site index curve). 2.2 Select the Site Index Species For each stratum, you must select the site index species. The site index species is the tree species on which growth intercept data are collected. To properly use site index, it is necessary to know the site index value and the species to which this value applies. The site index species must be specified because different tree species often have different site index values on the same site. Whenever reasonable, select as the site index species the leading species in the inventory component. This will ensure that site index data collected in the field are compatible with the requirements of the ISIS, MLSIS, forest cover map labels, and the inventory database. The inventory component leading species is not a reasonable choice for site index species when it is a silviculturally unacceptable species, has low vigour, is suppressed, or is scheduled to be eliminated by treatment (e.g., spacing or spraying). In these cases, choose the silviculture component leading species as the site index species and apply a site index species conversion when determining site index for the inventory label (see 3.5, Site Index Species Conversions).

6 Growth Intercept Method for Silviculture Surveys For a stratum, a suitable site index species is a tree species to which the following apply: The growth intercept table is available; More than 500 stems/ha (total trees) evenly distributed over the entire stratum; Dominant and co-dominant trees have completed between 3 and 30 years of growth above breast height; and Sample tree height growth reflects site productivity. Height growth above breast height may not reflect long-term site productivity if the stand has experienced wide-spread forest health damage, thinning shock, fertilization, or brush competition. A stand is not suitable for the growth intercept method if these factors have altered the normal height growth of sample trees above breast height. If a suitable site index species cannot be selected for a stratum, do not collect growth intercept data on plots that fall in the stratum. A useful site index estimate cannot be obtained with the growth intercept method. Obtain the site index estimate with either the biogeoclimatic method or the site index curve method. During pre-stratification in some mixed species stands, it is difficult to guess which species will end up as the leading species after the survey data are collected and compiled. In these cases, you may end up collecting growth intercept data on a species that is not the inventory component leading species. When this happens, you will need to adjust your site index in order to supply ISIS or MLSIS with a site index appropriate for the inventory component leading species. Tables to assist you with this conversion are presented in 3.5, Site Index Species Conversion. Careful pre-stratification will minimize the need for site index species conversion.

Growth Intercept Method for Silviculture Surveys 7 Stratum must have more than 500 total trees of the site index species per hectare uniformly covering the entire stratum area. Sample trees must have 3- to 30-years growth above breast height. A B C D Stratum B is unsuitable for the growth intercept method due to inadequate density of sample trees. Stratum A is suitable. To use the growth intercept method, the growth intercept tables must be available for the site index species. Stratum Inventory Label C Pl8Sx2-4-0.4-5718/0+(91)(7,0) D Pl8Sx2-12-3.5-3940/0+(91)(7,0) Stratum C is unsuitable for the growth intercept method due to inadequate years of growth above breast height on sample trees. Stratum D is suitable. Sample tree height growth must reflect long-term site productivity and not the temporary effects of damage, treatment, or growth suppression. E F At10 Pl10 Stratum E is unsuitable for the growth intercept method since growth intercept tables have not been developed for the site index species (At). Stratum F is suitable. The stratum is unsuitable for the growth intercept method since irregular height growth is common on sample trees. Figure 2. Pre-stratifying the opening, assessing if the stratum is suitable for the growth intercept method, and selecting the site index species. 2.3 Collect the Growth Intercept Data After pre-stratification and selection of the site index species, you must collect the growth intercept data in each stratum. 2.3.1 Sample Lines Within each stratum, survey sample lines should be laid out to achieve uniform coverage of the entire stratum area (Figure 3).

8 Growth Intercept Method for Silviculture Surveys 1818 NSR-G 289 NSR-M 21 77 km to Prince George PlF 110-G1 POC 1 2 S/L 3 1 2 3 4 S/L 2 2 2 3 1 3 S/L 1 1 TP TP 288 Pl 110-M1 285 Pl(F) 832-G6 1834 NSR-M 1835 Pl 833-M6 334 Pl 632-G6 22 PI(SA) 110-G2 23 Pl 110-G1 Figure 3. Good growth intercept sample plot location achieves complete and uniform coverage of the entire stratum area. 2.3.2 Selecting a Sample Tree At plot centre, select one growth intercept sample tree from a 5.64 m radius plot. For the growth intercept sample tree, select the plot tree with the following characteristics (Figure 4): Species Age DBH Stem condition Crown position Ring width site index species 3 to 30 years growth above breast height largest DBH undamaged stem vigorous, uniform annual height growth above breast height dominant or co-dominant crown class not overtopped by trees or brush vigorous, uniform ring widths from pith to bark at breast height Generally, advanced regeneration, veterans, and residuals are not acceptable sample trees.

Growth Intercept Method for Silviculture Surveys 9 Sample tree must have vigorous, uniform ring widths from pith to bark at breast height. Sample tree must have an undamaged stem with vigorous, uniform annual height growth above breast height. Poor sample tree Good sample tree Poor sample tree Poor sample tree Good sample tree Sample tree must be a dominant or co-dominant tree, not overtopped by trees or brush, with largest DBH of the site index species. Sample tree must have 3- to 30-years growth above breast height. 2 years growth above breast height 5 years growth above breast height 1.3 m 1.3 m Poor sample tree Good sample tree Poor sample tree Good sample tree Figure 4. Good growth intercept sample tree selection contributes to an accurate site index estimate. 2.3.3 Sample Tree Measurements Follow four steps to collect growth intercept data: 1. Locate breast height: On the sample tree, accurately locate breast height at 1.3 m above ground level (Figure 5). 2. Measure total height: Tree height from ground to the top of the leader (total height) can be measured with a suunto and distance tape, a height pole, or a carpenter s measuring tape. For species with a droopy tip

10 Growth Intercept Method for Silviculture Surveys (e.g., western hemlock), total height is height with the tip straightened up. 3. Determine breast height age: Breast height age is equal to the number of rings at breast height. On small diameter sample trees, this must be determined by felling the tree at breast height and counting rings. On larger diameter sample trees, felling can be avoided by using an increment borer. Re-drill the tree if the increment core does not contain the pith. If the site index species produces branch whorls that indicate annual height growth, it is possible to: determine breast height age by counting annual branch whorls above breast height (see 3.2, Determining Age by Counting Branch Whorls); and determine site index from a portion of the total height of the sample tree (see 3.3, Estimating Site Index from Partial Growth). 4. For the sample tree, record the growth intercept measurements on the field cards. On the field cards, record: Tree species; Total height; and Breast height age. The current field cards, including the FS 658 and the FS 748, were not designed to record growth intercept measurements. If you choose to use these cards to record growth intercept data, you must either use the comments column or a separate line below the standard survey data collected on each plot. Alternatively, you could choose to record your growth intercept data on a separate field card (Figure 6).

Growth Intercept Method for Silviculture Surveys 11 Step 1: Locate breast height Step 2: Measure total height 3.1 m 1.3 m 1.3 m Step 3: 1. fell tree and count rings Determine breast height age by one of two methods 2. count rings on increment core Step 4: Plot no. Record species, height, and age on field card Species Total height (m) Breast height age (yr) 1 Pl 3.1 4 4 years old 4 years old Figure 5. Method for collecting growth intercept data on a sample tree.

12 Growth Intercept Method for Silviculture Surveys Option 1: Record your growth intercept data in the comments column or on a blank line below the data for each plot on the standard silviculture survey cards. REPORT NO. Province of British Columbia SURVEY TYPE OPENING NO. BEARING (AZIMUTH) PLOT NO. EA./ LAYER Ministry of Forests TOTAL TREES TOTAL CONIFERS STRATUM POINT OF COMMENCEMENT SILVICULTURE SURVEY PLOT FG 93B 024 018 A Road junction on east side of block W E L L - S P A C E D PREFERRED AND ACCEPTABLE SPECIES TOTAL TOTAL W FG LEADER HEIGHT (cm) TOTAL HEIGHT (cm) AGE (year) PLANT- ABLE PRE- PAR- ABLE COMPETING VEGETATION SPECIES % COVER HEIGHT (cm) PAGE sof LINE COMMENTS Pl Lw Fd Sx 270 1 1 16 9 6 1 M 290 12 Pl8 Lw Fd 12 3 m 1 1 Growth Intercept Data Spp: Pl Height: 3.1 Age: 4 Growth Intercept Data Spp: Pl Height: 3.2 Age: 5 2 1 6 5 5 5 3 1 11 7 3 2 5 Growth Intercept Data Spp: Pl Height: 3.7 Age: 5 4 1 16 11 6 1 1 M Growth Intercept Data Spp: Pl Height: 3.1 Age: 4 5 1 8 4 2 1 1 4 270 12 Pl7 Fd Lw 12 2.7 m 2 1 Growth Intercept Data Spp: Pl Height: 3.5 Age: 5 COMMENTS: (prescriptions, snags, watercourses, wildlife, stand damage, etc.) DATA ON THE LEADER HEIGHT, TOTAL HEIGHT AND AGE OF WELL-SPACED TREES, MAIN COMPETING VEGETATION AND INVENTORY COMPONENT SHOULD BE RECORDED ON THE FIRST PLOT ESTABLISHED IN THE STRATUM, AND EVERY FOURTH PLOT THEREAFTER. PEST TYPE, INCIDENCE AND SEVERITY SHOULD BE NOTED IN THE COMMENTS SECTION, WHEN OBSERVED. FS 658 SIL 90/4 Figure 6. Two options for recording growth intercept data on field cards (continued).

Growth Intercept Method for Silviculture Surveys 13 Option 2: Record your growth intercept data on a separate field card. Growth Intercept Plot Card Opening number 93B 024 018 Stratum: A Page 1 of 1 Date: October 12, 1994 Growth intercept plot radius (m): 5.64 POC: Road junction on east side of block Bearing Total Breast and Plot height height Site index distance number Species (m) age (yr) Comments (m) 270 deg. 1 Pl 3.1 4 50 m 270 deg. 2 Pl 3.2 5 100 m 270 deg. 3 Pl 3.7 5 100 m Figure 6. Two options for recording growth intercept data on field cards (concluded). 2.3.4 Plot Does Not Contain Suitable Growth Intercept Sample Tree If the plot does not contain a suitable sample tree, collect the growth intercept data on a suitable sample tree on the way to the next plot. 2.3.5 Survey Timing Growth intercept data should not be collected in the spring or early summer while the tree is still growing. For sample tree species that form annual branch whorls, this restriction can be eliminated with the methods described in 3.3, Determining Site Index from Partial Growth. 2.3.6 Sample Intensity As a general rule, collect growth intercept data on one plot per hectare (one sample tree/plot) to a maximum of 10 plots per stratum.

14 Growth Intercept Method for Silviculture Surveys 2.4 Compute the Average Site Index for Each Stratum After the growth intercept data are collected, complete the following steps: 1. Finalize the strata boundaries (Figure 7); 2. Use the appropriate table in 7, Tables to Estimate Site Index from Growth Intercept to obtain site index from measured height and age for each growth intercept sample tree; and 3. Compute the average site index from all plots in the stratum.

Growth Intercept Method for Silviculture Surveys 15 A B Step 1: Finalize strata boundaries. A B 6 7 5 4 16 17 18 15 14 13 12 11 8 9 10 3 2 1 Strata boundaries before data collection. Strata boundaries after data collection. Step 2: Use growth intercept table to assign a site index to each plot based on measured height and age. Plot no. 1 2 3 Species Pl Pl Pl Total height (m) 3.1 3.2 3.7 Breast height age (yr) 4 5 5 Site index (m) 21 19 Breast height age Growth Intercept Table Species: Lodgepole pine (Pl) Site Index (m) (yr) 15 16 17 18 19 20 21 Tree total height (m) 3 4 3.1 5 3.2 6 Step 3: Compute the average site index from all plots in the stratum. Growth Intercept Plot Card Opening number 93B 024 018 Stratum: A Page 1 of 1 Date: October 12, 1994 Growth intercept plot radius (m): 5.64 POC: Road junction on east side of block Bearing Total Breast and Plot height height Site index distance number Species (m) age (yr) Comments (m) 270 deg. 1 Pl 3.1 4 21.0 50 m 270 deg. 2 Pl 3.2 5 19.0 100 m 270 deg. 3 Pl 3.7 5 21.5 100 m 270 deg. 4 Pl 3.1 4 21.0 100 m 270 deg. 5 Pl 3.5 5 20.5 100 m 0 deg. 6 Pl 4.1 6 21.0 100 m 90 deg. 7 Pl 3.3 5 19.5 100 m 90 deg. 8 Pl 3.4 5 20.0 100 m 90 deg. 9 Pl 3.8 6 20.0 100 m Total = 183.5 Average = 183.5/9 = 20.4 Figure 7. Compute the average site index for each stratum.

16 Growth Intercept Method for Silviculture Surveys Last, if you have obtained site index values for forest cover strata in an opening, update the site index values in the following locations: 1. Opening file; 2. ISIS or MLSIS forest cover attributes (e.g., FS 708C); and 3. Forest cover map labels and the inventory database (e.g., FS 810A). Province of British Columbia Ministry of Forests MAJOR LICENCE SILVICULTURE INFORMATION SYSTEM FOREST COVER DEFINITION - FORM C SECTION 35 A. TIMBER MARK OBLIGATION BLOCK OPENING NO. PREPARED BY TELEPHONE NO. DATE Y M D Site Index B. STRATA LAYER RANK AREA (ha) STOCKING STATUS TYPE SPECIES COMPOSITION CROWN SITE AGE HEIGHT REF. CLOSURE (yrs) (0.1) M YEAR SPECIES % SPECIES % SPECIES % SPECIES % SPECIES % (%) CL. INDEX A 3 0 0 I M M N A T P L 8 0 S x 20 1 2 3 6 4 0 9 4 2 0 4 4 7 1 A S 3 0 0 I M M U A T PL 9 0 Sx1 0 12 3 8 9 4 2 0 1 2 0 0 1 1 0 0 AGE PLT DENSITY (stems / ha) WELL-SPACED (stems / ha) FREE- BASAL PEST INFESTATION GROWING AREA (stems / ha) (m 2/ ha) CODE % CODE % RES. TYPE C. ADMINISTRATION D. RPF SIGNATURE AND SEAL REVIEWED BY DATE REGION DISTRICT Y M D RPF SIGNATURE DATE RPF NO. FS 708C HSP 95/1 Figure 8. Example of submission of a site index estimate with an MLSIS forest cover update.

Growth Intercept Method for Silviculture Surveys 17 3 Advanced Options for Using the Growth Intercept Method The basic application of the growth intercept method is described in 2, How to Use the Growth Intercept Method. Once you have mastered the basics of the growth intercept method, the following advanced options may be of interest to you. 3.1 Measuring More than One Sample Tree per Plot To reduce between-plot variation, you may choose to increase plot size and measure more than one growth intercept sample tree per plot (Table 1). To compile data when more than one sample tree per plot is measured, site index is estimated for each tree and the site index values are averaged. Plot average values are used in all subsequent calculations. Table 1. Acceptable plot sizes and number of sample trees Plot radius Plot area Number of growth (m) (ha) intercept sample trees 5.64 0.01 1 7.99 0.02 2 9.74 0.03 3 3.2 Determining Age by Counting Branch Whorls If the site index species produces annual branch whorls (Table 2), it is possible to determine breast height age by counting whorls above breast height. However, mid-year branch clusters and other variations in branch growth may cause silviculture surveyors to make errors in age determination with this method. Therefore, counting rings, not counting whorls, is the recommended way to determine breast height age. On sample trees with seven or fewer years growth above breast height, a count of whorls above breast height is recommended to confirm the age determined by a ring count.

18 Growth Intercept Method for Silviculture Surveys Table 2. Site index species with distinct annual branch whorls Site index species Pl, Sx Hw Distinct annual branch whorls Yes No Breast height age can be determined by counting the number of annual branch whorls above breast height plus one (for the annual whorl within the bud at the tip of the leader). The shape of individual branches that is created by annual branch growth is used to help distinguish an annual whorl from a mid-year cluster of branches (Figure 9). 6 5 4 Stem cross section at breast height 3 2 1 Breast height age is equal to the number of annual branch whorls above breast height plus one. 1.3 m This tree has a breast height age of 6 years. Figure 9. Determining age by counting branch whorls.

Growth Intercept Method for Silviculture Surveys 19 3.3 Estimating Site Index from Partial Growth If the site index species has distinct annual branch whorls, it is possible to estimate site index from a portion of the total growth above breast height. Estimating site index from partial growth has several advantages: 1. Allows the growth intercept method to be used on trees during the growing season; 2. Allows a growth intercept estimate of site index to be obtained from a sample tree that has stem defect high up the stem; and 3. Speeds the process of measuring height and age. The important disadvantage is that it is less accurate than the method based on obtaining a ring count and total height. To estimate site index from partial growth, follow these five steps to collect growth intercept data (Figure 10): 1. Locate breast height: On the sample tree, locate breast height at 1.3 m above ground level. 2. Select the upper measurement point: The upper measurement point is an annual branch whorl below the leader. Look up the stem above breast height and select the upper measurement point. The accuracy of the site index estimate improves as the number of years of measured height growth increases. Therefore, make the upper measurement point as high up the stem as possible subject to the following restrictions: Reduce the height of the upper measurement point to get below a stem defect observed up the stem; and If the survey is conducted in the spring or summer while the tree is still growing, choose as the upper measurement point the annual whorl below the currently growing leader. 3. Measure the tree height from ground to the upper measurement point: Tree height from ground to the upper measurement point can be measured with a suunto and distance tape, a height pole, or a carpenter s measuring tape. 4. Determine the number of years growth between breast height and the upper measurement point: The number of years of growth between breast height and the upper measurement point is equal to the number of annual

20 Growth Intercept Method for Silviculture Surveys branch whorls above breast height up to and including the whorl at the upper measurement point (see 3.2, Determining Age by Counting Branch Whorls). 5. For the sample tree, record: Tree species; Height from ground to upper measurement point; Number of years growth between breast height and the upper measurement point; and A comment, partial growth, indicating that the recorded height is not total height and the recorded age is not breast height age.

Growth Intercept Method for Silviculture Surveys 21 Step 1: Locate breast height. Step 4: Determine the number of years growth between breast height and the upper measurement point. Years growth equals the number of annual branch whorls above breast height up to and including the node at the upper measurement point. 3 4 Upper measurement point Step 2: 1.3 m Select the upper measurement point. Breast height 2 1 years growth = 4 Upper measurement point selected at node below currently growing leader Step 5: 1.3 m Record species, height, age, and partial growth comment on a field card. 1.3 m Breast Total height Plot height age no. Species (m) (yr) Comments 1 Pl 3.0 4 Partial growth Step 3: Measure height from ground to the upper measurement point. 3.0 m 1.3 m Figure 10. Collecting growth intercept data on a sample tree to estimate site index from partial growth.

22 Growth Intercept Method for Silviculture Surveys 3.4 Assessing the Reliability of the Site Index Average If desired, confidence limits on average site index can be computed as shown in the FS 1138 Calculation Card for Silviculture Survey Confidence Limits. To do this, input site index values in the formulas instead of the number of well-spaced trees and do not multiply your results by the plot size multiplier (p). The reliability of the site index average for the stratum can also be assessed by calculating the standard deviation of site index values of all plots in the stratum. As the standard deviation of plot site index increases, more plots are required to achieve a reliable site index average for the stratum (Table 3). Table 3. Number of plots required to achieve a reliable site index average for the stratum Standard deviation of site index (m) Number of plots required to achieve a reliable site index average for the stratum 1 7 2 18 3 35 3.5 Site Index Species Conversion Occasionally, you may need to estimate site index for one species from the site index measured on a second species on the same site. This may occur when you collect growth intercept data on a species that is not the inventory component leading species. For example, if you use the Pl growth intercept to estimate site index in a FdPl stand, you must convert the Pl site index in order to obtain a site index appropriate for Fd on that site. Table 4 will assist you with these conversions. The accuracy of the site index value is reduced when site index species is converted. When an accurate site index is required, field measurements must be taken on the species of interest. Do not rely on converting site index species.

Growth Intercept Method for Silviculture Surveys 23 Table 4. Site index species conversions Region Species pair Average site index (m) relationship for species pair Interior Pl 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 Bl 9.7 11.5 13.4 15.2 17.0 18.9 20.7 22.6 24.4 26.2 28.1 Interior Pl 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 Fd 10.1 11.9 13.8 15.7 17.5 19.4 21.3 23.1 25.0 26.9 28.8 Interior Pl 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 Lw 11.5 13.4 15.4 17.3 19.2 21.1 23.0 25.0 26.9 28.8 30.7 Interior Pl 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 Sx 8.8 10.9 13.1 15.3 17.5 19.7 21.8 24.0 26.2 28.4 30.6 Interior Sx 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 Bl 10.3 12.0 13.7 15.4 17.2 18.9 20.6 22.3 24.0 25.8 27.5 Interior Sx 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 Fd 12.1 13.6 15.1 16.5 18.0 19.5 21.0 22.4 23.9 25.4 26.9 Interior Sx 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 Pl 11.2 13.0 14.9 16.7 18.5 20.4 22.2 24.1 25.9 27.7 29.6 Coast Hw 15.0 18.0 21.0 24.0 27.0 30.0 33.0 36.0 39.0 42.0 Fd 17.1 20.5 23.8 27.1 30.5 33.8 37.1 40.4 43.8 47.1 Source: Nigh, G.D. 1994. Site index conversion equations for mixed species stands. B.C. Min. For., Research Br., Victoria, B.C. Res. Rep. 01.

24 Growth Intercept Method for Silviculture Surveys 4 Recommended Contract Specifications 4.1 Pre-work Conference Consider adding to the pre-work conference a field review of the growth intercept data collection procedures. 4.2 Marking Growth Intercept Sample Trees To facilitate checking, consider requiring that all growth intercept sample trees be marked with a specified colour of flagging tape. 4.3 Measurement Standards Height Total height should be measured to the nearest 10 cm. An error of 5% is the maximum acceptable. Unless the survey is conducted during the growing season, site index should be determined from total height, not from partial growth. Age Breast height age should be determined to the exact year. Because a small error in age results in a large error in site index, especially with young sample trees, no error is acceptable in the determination of breast height age. In a contract silviculture survey, breast height age should be determined from ring counting, not from counting branch whorls above breast height. Species No error is acceptable in the determination of growth intercept sample tree species.

Growth Intercept Method for Silviculture Surveys 25 5 Example to Illustrate the Use of the Growth Intercept Method A pre-stand tending survey was scheduled for an opening to provide the information required to prepare a Stand Management Prescription. Site index was required for each Standards Unit in the opening. Steps 1 and 2: Pre-stratify the opening and select the site index species Maps, photos, the opening file, and a walk-through were used to divide the opening into Standards Units. Two Standards Units were delineated: pine leading fir (PlFd stratum) on the upper hill, and pure fir (Fd stratum) on the toe of the slope. By asking a series of questions, the survey coordinator assessed the suitability of each stratum for growth intercept data collection and selected a site index species (Figure 11). Questions Stratum 1 (PlFd) Stratum 2 (Fd) Can one site index species be Yes. Suitable Pl sample Yes. Suitable Fd sample selected for the entire area? trees are present over the trees are present over the entire stratum. entire stratum. Is the growth intercept Yes. The growth intercept No. The growth intercept table available for the table is available for table is not yet available selected site index species? interior Pl. for interior Fd. Does the site index species Yes. The stratum is Yes. The stratum is have more than 500 stems/ha adequately stocked with Pl adequately stocked with Fd (total trees)? growth intercept sample trees. growth intercept sample trees. Have dominant trees of the Yes. The growth intercept Yes. The growth intercept site index species between sample trees have completed sample trees have completed between 3 and 30 years of enough growth to provide enough growth to provide growth above breast height? good estimates of site index. good estimates of site index. Does sample tree height Yes. The height growth above Yes. The height growth above growth reflect site breast height on the growth breast height on the growth productivity? intercept sample tree has not intercept sample trees has not been reduced by brush been reduced by brush competition, pest attack, etc. competition, pest attack, etc. Figure 11. Example of the questions used to assess the suitability of the strata for growth intercept data collection and select site index species.

26 Growth Intercept Method for Silviculture Surveys Based on this assessment, the survey coordinator decided that the PlFd stratum was suitable, but the Fd stratum was not suitable, for growth intercept data collection. Pl was selected as the site index species in the PlFd stratum. Growth intercept data were not collected on plots in the Fd stratum. The survey coordinator chose to obtain the site index estimate for the Fd stratum with the biogeoclimatic method. Since the site index species was the same as the leading species in the inventory label, the survey results were compatible with the requirements of the Stand Management Prescription form (and ISIS, MLSIS, forest cover map labels, and the inventory database). Step 3: Collect the growth intercept data Survey sample lines were laid out on a grid so that the entire PlFd stratum was thoroughly sampled. In each 5.64 m radius plot, one suitable Pl growth intercept sample tree was selected. In the PlFd stratum, growth intercept data were collected on 10 sample plots and recorded on field sheets specially designed for this purpose (Figure 12).

Growth Intercept Method for Silviculture Surveys 27 Growth Intercept Plot Card Opening number 93B 024 018 Stratum: 1 (PlFd) Page 1 of 1 Date: October 12, 1994 Growth intercept plot radius (m): 5.64 POC: Road junction on east side of block Bearing Total Breast and Plot height height Site index distance number Species (m) age (yr) Comments (m) 270 deg. 1 Pl 3.1 4 21.0 50 m 270 deg. 2 Pl 3.2 5 19.0 100 m 270 deg. 3 Pl 3.7 5 21.5 100 m 270 deg. 4 Pl 3.1 4 Partial GI 21.0 100 m 270 deg. 5 Pl 3.5 5 20.5 100 m 0 deg. 6 Pl 4.1 6 21.0 100 m 90 deg. 7 Pl 3.3 5 Partial GI 19.5 100 m 90 deg. 8 Pl 3.4 5 Plot 8 lacked 20.0 150 m suitable sample tree. Sample tree selected halfway to plot 9 90 deg. 9 Pl 3.8 6 20.0 100 m 90 deg. 10 Pl 2.8 4 19.5 100 m Figure 12. Example of a field sheet used to collect growth intercept data.

28 Growth Intercept Method for Silviculture Surveys Step 4: Compute average site index for each stratum For each plot, the site index estimated from height and age was identified from the appropriate table (see 7, Tables to Estimate Site Index from Growth Intercept). The survey coordinator finalized the strata boundaries based on plot data. As a result, plot 10 was grouped in the Fd stratum (Figure 13). The average site index for the PlFd stratum was computed by averaging the 9 remaining site index values in the stratum. The average site index for the PlFd stratum was: = sum of site index values/number of plots = (21.0+19.0+21.5+...+20.0)/9 = 20.4 m 16 17 18 15 14 13 12 11 Fd stratum 6 7 8 9 5 4 3 2 10 1 PlFd stratum Figure 13. Plot location map. Optional: Check precision of estimate To check if sufficient plots were established to achieve a reliable estimate of site index, the survey coordinator computed the standard deviation of plot site index values. The standard deviation (STDDEV) was computed in three steps: 1. The sum of plot site index values was computed: χ= 21. 0 + 19. 0 + 21. 5 + χ= 183. 5 2. The sum of squared values was computed: χ 2 = ( 21. 0 21. 0) + ( 19. 0 19. 0) + ( 21. 5 21. 5) + χ 2 = 3746. 75

Growth Intercept Method for Silviculture Surveys 29 3. These values were entered into the formula for the standard deviation with n equal to the number of plots (n=9): STDDEV = χ 2 2 χ n 1 ( ) 2 3746. 75 ( 183. 5) 9 = 8 = 08. m n By comparing the number of plots and the standard deviation of site index to Table 3, the survey coordinator concluded that site index had been estimated on enough plots to produce a reliable stratum average site index. Last, site index was written into the Stand Management Prescription for each Standards Unit. In addition, the new site index values were passed to the opening file, ISIS or MLSIS, the forest cover map, and the inventory database.

30 Growth Intercept Method for Silviculture Surveys 6 Where to Get More Information Additional information on the growth intercept method of estimating site index can be obtained by contacting Silviculture Practices Branch or Research Branch specialists: Patrick Martin Gord Nigh Stand Development Forester Biometrician Silviculture Practices Branch Research Branch Ministry of Forests Ministry of Forests 31 Bastion Square 506-1175 Douglas Street Victoria, B.C. Victoria, B.C. V8W 3E7 V8W 3E7 TEL: 387-1191 TEL: 387-3093 FAX: 387-1467 FAX: 387-8197 OVid: PJMARTIN OVid: GNIGH For more information on the development of the growth intercept equations, see the following publications: Nigh, G.D. 1995a. Variable growth intercept models for lodgepole pine in the Sub-Boreal Spruce biogeoclimatic zone, British Columbia. B.C. Min. For., Research Branch, Victoria, B.C. Res. Rep. 02. 22p. Nigh, G.D. 1995b. Variable growth intercept models for spruce in the Sub-Boreal Spruce and Engelmann Spruce-Subalpine Fir biogeoclimatic zones of British Columbia. B.C. Min. For., Research Branch, Victoria, B.C. [In review]. Nigh, G.D. 1995c. Growth intercept models for species without distinct annual branch whorls: western hemlock. B.C. Min. For., Research Branch, Victoria, B.C. [In review]. For more information on tools to estimate site index, see the following publications: British Columbia Ministry of Forests. 1995. Procedures for silviculturists to obtain site index. Silviculture Practices Branch. Victoria, B.C. 6p. Thrower, J.S. and A.F. Nussbaum. 1991. Site index curves and tables for British Columbia: coastal species. B.C. Min. For., Victoria, B.C. Land Manage. Hdbk. Field Guide Insert 3. Thrower, J.S., A.F. Nussbaum, C.M. DiLucca. 1991. Site index curves and tables for British Columbia: interior species. B.C. Min. For., Victoria, B.C. Land Manage. Hdbk. Field Guide Insert 6. Nelson, Prince Rupert, and Vancouver Forest Regions. Various dates. Field Guide For Site Identification and Interpretation.

Growth Intercept Method for Silviculture Surveys 31 7 Tables to Estimate Site Index from Growth Intercept GROWTH INTERCEPT TABLE Species: Lodgepole pine (Pl) Region: Interior B.C. Source: G. Nigh, 1995a Breast height Site index (m) age (yr) 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Tree total height (m) 3 1.4 1.4 1.4 1.5 1.5 1.6 1.6 1.7 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 4 1.4 1.4 1.5 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.6 2.7 2.9 5 1.4 1.5 1.6 1.6 1.7 1.8 1.9 2.0 2.2 2.3 2.5 2.6 2.8 3.0 3.2 3.4 6 1.5 1.5 1.6 1.7 1.8 1.9 2.1 2.2 2.4 2.5 2.7 2.9 3.1 3.3 3.6 3.8 7 1.5 1.6 1.7 1.8 1.9 2.1 2.2 2.4 2.6 2.8 3.0 3.2 3.5 3.7 4.0 4.3 8 1.5 1.7 1.8 1.9 2.1 2.2 2.4 2.6 2.8 3.1 3.3 3.6 3.9 4.2 4.5 4.8 9 1.6 1.7 1.8 2.0 2.2 2.4 2.6 2.8 3.1 3.3 3.6 3.9 4.2 4.6 4.9 5.3 10 1.6 1.8 1.9 2.1 2.3 2.5 2.8 3.0 3.3 3.6 3.9 4.3 4.6 5.0 5.4 5.8 11 1.7 1.8 2.0 2.2 2.5 2.7 3.0 3.2 3.6 3.9 4.2 4.6 5.0 5.4 5.8 6.3 12 1.8 1.9 2.1 2.3 2.6 2.9 3.2 3.5 3.8 4.2 4.5 4.9 5.4 5.8 6.3 6.7 13 1.8 2.0 2.2 2.5 2.8 3.0 3.4 3.7 4.1 4.5 4.9 5.3 5.7 6.2 6.7 7.2 14 1.9 2.1 2.3 2.6 2.9 3.2 3.6 3.9 4.3 4.7 5.2 5.6 6.1 6.6 7.1 7.7 15 2.0 2.2 2.5 2.7 3.1 3.4 3.8 4.2 4.6 5.0 5.5 6.0 6.5 7.0 7.6 8.1 16 2.0 2.2 2.5 2.8 3.2 3.5 3.9 4.4 4.8 5.3 5.8 6.3 6.8 7.4 8.0 8.6 17 2.1 2.3 2.7 3.0 3.4 3.7 4.2 4.6 5.1 5.6 6.1 6.6 7.2 7.8 8.4 9.0 18 2.2 2.5 2.8 3.2 3.5 4.0 4.4 4.9 5.4 5.9 6.4 7.0 7.6 8.2 8.8 9.5 19 2.3 2.6 2.9 3.3 3.7 4.2 4.6 5.1 5.6 6.2 6.8 7.3 8.0 8.6 9.2 9.9 20 2.4 2.7 3.1 3.5 3.9 4.4 4.9 5.4 5.9 6.5 7.1 7.7 8.3 9.0 9.6 10.3 21 2.5 2.8 3.2 3.7 4.1 4.6 5.1 5.7 6.2 6.8 7.4 8.0 8.7 9.3 10.0 10.7 22 2.6 2.9 3.4 3.8 4.3 4.8 5.3 5.9 6.5 7.1 7.7 8.4 9.0 9.7 10.4 11.1 23 2.7 3.1 3.5 4.0 4.5 5.0 5.6 6.2 6.8 7.4 8.0 8.7 9.4 10.1 10.8 11.5 24 2.7 3.2 3.7 4.1 4.7 5.2 5.8 6.4 7.0 7.7 8.3 9.0 9.7 10.4 11.1 11.9 25 2.9 3.3 3.8 4.3 4.9 5.4 6.0 6.6 7.3 7.9 8.6 9.3 10.0 10.8 11.5 12.3 26 3.0 3.4 3.9 4.5 5.0 5.6 6.2 6.9 7.5 8.2 8.9 9.6 10.4 11.1 11.9 12.7 27 3.1 3.6 4.1 4.7 5.2 5.9 6.5 7.1 7.8 8.5 9.2 9.9 10.7 11.5 12.2 13.0 28 3.2 3.7 4.3 4.8 5.4 6.1 6.7 7.4 8.1 8.8 9.5 10.2 11.0 11.8 12.6 13.4 29 3.3 3.8 4.4 5.0 5.6 6.3 6.9 7.6 8.3 9.1 9.8 10.6 11.3 12.1 12.9 13.7 30 3.4 4.0 4.6 5.2 5.8 6.5 7.2 7.9 8.6 9.3 10.1 10.9 11.6 12.4 13.3 14.1 How to use the table to get a site index estimate from measured height and age. Step 1: Look down the breast height age column to find the row corresponding to the breast height age of your sample tree. Step 2: Look across the row to find the total height that is closest to the total height of the sample tree. Step 3: Look up the column to find the site index estimated by sample tree height and age.

32 Growth Intercept Method for Silviculture Surveys GROWTH INTERCEPT TABLE Species: Lodgepole pine (Pl) Region: Interior B.C. Source: G. Nigh, 1995a Breast height Site index (m) age (yr) 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 Tree total height (m) 3 2.6 2.7 2.8 3.0 3.1 3.3 3.5 3.7 3.8 4.0 4.2 4.4 4.7 4.9 5.1 4 3.1 3.2 3.4 3.6 3.8 4.0 4.2 4.5 4.7 4.9 5.2 5.5 5.7 6.0 6.3 5 3.6 3.8 4.0 4.2 4.5 4.7 5.0 5.3 5.6 5.9 6.2 6.5 6.8 7.2 7.5 6 4.1 4.4 4.6 4.9 5.3 5.6 5.9 6.3 6.6 7.0 7.4 7.8 8.2 8.6 9.1 7 4.6 4.9 5.2 5.6 5.9 6.3 6.7 7.1 7.5 7.9 8.4 8.8 9.3 9.8 10.2 8 5.1 5.5 5.9 6.3 6.7 7.1 7.5 8.0 8.4 8.9 9.4 9.9 10.4 10.9 11.5 9 5.7 6.1 6.5 7.0 7.4 7.9 8.4 8.9 9.4 9.9 10.5 11.0 11.6 12.2 12.8 10 6.2 6.7 7.1 7.6 8.1 8.6 9.2 9.7 10.3 10.9 11.5 12.1 12.7 13.4 14.1 11 6.7 7.2 7.7 8.2 8.8 9.3 9.9 10.5 11.1 11.8 12.4 13.1 13.8 14.5 15.2 12 7.2 7.8 8.3 8.9 9.4 10.0 10.6 11.3 11.9 12.6 13.3 14.0 14.7 15.4 16.2 13 7.7 8.3 8.9 9.4 10.1 10.7 11.3 12.0 12.7 13.4 14.1 14.8 15.6 16.3 17.1 14 8.2 8.8 9.4 10.0 10.6 11.3 12.0 12.7 13.4 14.1 14.8 15.6 16.4 17.2 18.0 15 8.7 9.3 9.9 10.6 11.2 11.9 12.6 13.3 14.1 14.8 15.6 16.4 17.2 18.0 18.9 16 9.2 9.9 10.6 11.3 12.0 12.7 13.5 14.2 15.0 15.9 16.7 17.6 18.4 19.3 20.2 17 9.7 10.4 11.1 11.8 12.5 13.3 14.0 14.8 15.6 16.5 17.3 18.2 19.1 20.0 20.9 18 10.1 10.8 11.5 12.3 13.0 13.8 14.6 15.4 16.2 17.1 17.9 18.8 19.7 20.6 21.5 19 10.6 11.3 12.0 12.8 13.5 14.3 15.1 16.0 16.8 17.6 18.5 19.4 20.3 21.2 22.2 20 11.0 11.8 12.5 13.3 14.0 14.8 15.7 16.5 17.3 18.2 19.1 20.0 20.9 21.8 22.7 21 11.5 12.2 13.0 13.7 14.5 15.3 16.2 17.0 17.9 18.7 19.6 20.5 21.4 22.4 23.3 22 11.9 12.6 13.4 14.2 15.0 15.8 16.6 17.5 18.4 19.2 20.1 21.0 22.0 22.9 23.9 23 12.3 13.0 13.8 14.6 15.4 16.3 17.1 18.0 18.9 19.8 20.7 21.6 22.5 23.4 24.4 24 12.7 13.5 14.3 15.1 15.9 16.7 17.6 18.5 19.4 20.3 21.2 22.1 23.1 24.0 25.0 25 13.1 13.9 14.7 15.5 16.3 17.2 18.1 19.0 19.9 20.8 21.7 22.6 23.6 24.5 25.5 26 13.4 14.3 15.1 15.9 16.8 17.6 18.5 19.4 20.3 21.2 22.2 23.1 24.1 25.0 26.0 27 13.8 14.6 15.5 16.3 17.2 18.1 18.9 19.8 20.7 21.7 22.6 23.5 24.5 25.5 26.4 28 14.2 15.0 15.9 16.7 17.6 18.5 19.4 20.3 21.2 22.1 23.0 24.0 24.9 25.9 26.9 29 14.6 15.4 16.3 17.1 18.0 18.9 19.8 20.7 21.6 22.6 23.5 24.4 25.4 26.4 27.4 30 14.9 15.8 16.6 17.5 18.4 19.3 20.2 21.1 22.0 23.0 23.9 24.9 25.8 26.8 27.8 How to use the table to get a site index estimate from measured height and age. Step 1: Look down the breast height age column to find the row corresponding to the breast height age of your sample tree. Step 2: Look across the row to find the total height that is closest to the total height of the sample tree. Step 3: Look up the column to find the site index estimated by sample tree height and age.