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An Introduction to Forest Measurements

from a field forester's perspective

as applied in the appalachian hardwoods of Eastern Kentucky

by Duane Bristow

1. A description of a timber cruising computer program
2. Forest Mensuration info from Australia
3. A sample timber cruise report.

Meanings of values in terms of average stands

It is necessary to be able to look at a stand of timber and know what typical values to expect or to look at values in a report and visualize the stand from which they came.

• Timber type - typical stands include:
  • cove hardwoods - yellow poplar, hard maple, beech, hemlock
  • mid slope oak-hickory - red oak, white oak, hickory, hard maple
  • upper slope and ridge hardwoods - scarlet oak, hickory, black gum, chestnut oak, pine
  • old field - virginia pine
  • old field - yellow poplar
• Stand structure - typical structures include:
  • All aged
  • Even aged - sapling, pole, small sawtimber, large sawtimber
  • Two story - usually sawtimber overstory with sapling or pole understory.
  • High graded - sapling and pole understory with large scattered cull wolf tree overstory
• Stand density - measured by basal area and crown closure (basal area in sq. ft. per acre)
  • Understocked - less than 40 BA - abundant sunlight
  • Scattered - 40 to 70 BA - open with patches of sunlight
  • Well stocked - 70 to 90 BA - not much sunlight
  • Overstocked - greater than 90 BA - little light
• Site quality - indicated by growth rate, site index, plant indicators, etc. Trees on better sites will have longer smoother bodies with fewer knots, sweep and defect.
  • Good to excellent - deep moist soil in coves and lower north and east facing slopes as well as benches and some broad flat ridges - diameter may be increasing 2 inches or over in ten years - expect trillium, ferns, yellow trout lily, false solomon's seal, iris, jack-in-the-pulpit, bloodroot, wild ginger, and little brown jug or heartleaf.
  • Medium - Usually mid slopes and upper north and east facing slopes as well as some broad ridges - diameter growth of 1 1/2 inches or more every ten years. Expect bloodroot, wild ginger, and little brown jug or heartleaf.
  • Poor - shallow, dry, rocky soils, steep south and west facing slopes and narrow ridge tops. Diameter growth 1 inch or less every ten years. Lots of mountain laurel thickets, huckleberries, scrubby pines.
• Stand condition
  • Scars and hollow lower trunks due to fire damage particularly on uphill side of tree.
  • Broken and small tops and short bodies due to wind and ice storm damage.
  • Scars on lower trunks and broken tops due to logging damage.
  • Dead and dying trees or dying limbs due to insect and/or disease damage.
  • Large spreading tops and dying limbs and down trees due to overmaturity.
  • Slow growth due to overstocking
  • Limby short bodied trees due to understocking.
  • Large numbers of trees forked below breast height due to coppice reproduction from stumps rather than from sprouting from roots or root collars. Forking above breast height is usually caused by a top broken by logging or wind or ice damage and, in some cases, by insect damage to terminal buds.

The Numbers

Below is a discussion of the meaning of the numbers obtained by some common forest measurement methods. This applies to typical mid slope oak-hickory stands in Eastern Kentucky. Timber volumes are commonly expressed in terms of board feet (bf) or thousand board feet (mbf) on the doyle log scale. Dbh means diameter breast high or 4 1/2 feet above the ground.

• Age and dbh - Economic maturity often occurs at 18 to 22 inches dbh and at an average tree age of 60 to 90 years. Harvesting usually occurs long before economic maturity is reached. Stands are typically harvested every 25 to 30 years. Although a light selective harvest on a cutting cycle of this length might be very desirable, stands are usually overcut. Loggers often tell landowners that they will not overcut the timber because they will not cut anything below a 12" diameter stump. They also tell landowners that they will get full value from their timber because they will cut all trees above a 12" diameter stump. As a general rule of thumb trees below 20 to 22 inches at the stump or below 18 inches dbh should not be cut unless they are defective or the stand is overstocked or for some other silvicultural purpose. In the case of yellow poplar and some other fast growing trees on good sites this diameter should be 22 inches dbh.
• Volume per acre - typical timber harvests average 2,000 to 3,500 board feet per acre with all or most sawtimber size trees being harvested and cull or defective trees left in the woods but harvests from well managed stands should yield 5,000 to 10,000 board feet per acre harvesting only selected mature and defective trees. Typical harvest volumes are usually on a 30 to 40 year cutting cycle while the ideal volumes mentioned are on a 50 to 60 year cutting cycle.
• Volume per tree - typically averages about 150 board feet at harvest. It should average 200 to 250 board feet.
• Average annual growth - 50 to 100 board feet per acre per year. Average potential annual growth with good management is probably around 200 board feet.
• Sawtimber trees per acre - In Kentucky the average tree is about a 13 inch tree with a 12 foot grade 3 butt log. There are the equivalent of about 47 of these per acre.
  We should have an average tree 16 inches in diameter 2, 16' logs in merchantable height with a butt log grade 2. With 46 of these per acre our average volume per acre would be increased to 6,000 - 8,000 board feet and our average value of timber per acre would go from about $150 to between $900 and $1,600 on the stump.
• Total trees per acre - Seedlings from natural reproduction within two years after a clearcut may number as many as 10,000 per acre. In planting seedlings a goal of 600 to 800 per acre surviving is reasonable. Throughout the life of the stand the number of trees per acre will decrease to a number of perhaps 500 trees of which only 50 may be sawtimber size and 350 to 400 in the sapling size class. This assumes an all aged stand. In an even aged plantation perhaps 80 trees will reach sawtimber size.

Timber Cruising

Timber cruising is done to determine the condition of a stand, its timber volume, and its dollar value in order to plan harvests, make recommendations for future management or for purposes of financial analysis such as obtaining loans or evaluation for land sales or for testimony in court cases involving timber trespass.

• Planning
  • Be sure to have landowner info and authorization.
  • Get map and determine boundaries, topography and acreage of tract.
  • Plan number of plots needed and plot location scheme.
  • Draw plot grid and locate plots on map.
  • Plan data collection specifications.
  • Plan timing and personnel to be used in cruise. Usually on steep ground plan on 20 to 25 plots per day. In extremely difficult conditions 10 to 15 plots. In easy terrain 30 to 50 plots.
  • Obtain information on markets, access to markets, and logging difficulty.
• Principles
  • Number of plots needed is based on expected stand variation, acreage, and purpose of cruise.
  • Tracts less than about 25 acres require 100% cruise rather than sampling.
  • Most tracts require at least 40 to 60 plots.
  • Need for stratification of tract and accuracy of data on sub strata or sub tracts will determine total number of plots needed.
  • Plots need not be precisely located in the field but locations must be random, not biased, and must be uniformly located throughout the tract.
  • Since the most expensive part of the cruise is plot location rather than data collection, all data that can possibly be of use should be collected.

  

  

• Field Procedures
  • Sample tree selection
    • Point Sampling, Basal Area Factor, Plot Radius Factor
    • Effect of slope
    • Procedures for handling borderline trees
  • Stand status - strata - silvicultural and statistical significance.
  • Species and species value groups
  • Dbh - methods of measurement and precision
  • Log and total height - effect of defects
  • Tree grade
  • Tree status - leave, harvest, cull
  • Crown Class - D, CD, I, S
  • Age and growth - uses of, methods of measurement, significance
• Office Procedures
  • Calculations - volume, value, stand & stock
  • Statistics - variability and precision and accuracy
  • Computers - use of my analysis program.
  • Reports - essential elements.
    • Cover letter
      • Purpose of cruise
      • Owner
      • Location
      • Methods
      • Summary of results, recommendations and discussion
    • Report - full
      • Owner, date, location, personnel
      • Description of area, topography, soils, and access
      • Assumptions of cruise
      • Overall timber and area descriptions and sizes
      • Timber volumes and values by species groups and grade
      • Discussion and recommendations
      • Additional data on stocking and sizes such as
        • Number of trees per acre
        • Basal Area per acre
        • Average volume per tree
        • Average volume per acre
        • Discussion of stocking, volumes and values by strata or acres in various classes of stocking and volumes.
        • Stand and stock tables.
    • Report - brief - Same as full but without value information or recommendations.

Forms

Below are some sample forms for use in field collection of data.

Instructions for Data Collection

Field Equipment needed:

Prism or angle gauge, D-tape, 100 foot tape, Cruiser stick, Compass, Increment borer, clinometer, topo map, covered metal clipboard, pencils, field data sheets, sounding ax

Office Equipment needed:

Computer with proper software or Volume factor tables, tree factor tables, and calculator.

Field procedures:

Each crew is to take data on at least ten plots.

• Tally all trees within plot 5 inches dbh and up by species.
• Measure dbh to the nearest inch (or 1/10 inch) of each tree.
• Estimate merchantable height in 16 foot logs to the nearest 1/2 log of all trees 12 inches dbh and above.
• Estimate merchantable height in feet to nearest 4 feet of all trees less than 12 inches dbh.
• Record tree Grade 1, 2, or 3 (4 for cull, 0 for not applicable) for each tree.
• Record Leave, Harvest, or Cull for each tree.
• Record crown class - D, CD, I, S for each tree.
• Record slope position and stand characteristics for each plot.
  • Slope - 1 bottoms and coves, 2 mid slopes, 3 upper slopes and ridges, 4 broad ridges
  • Stand by size class and type

Office procedures:

In the office calculate:

• Trees per acre 5" dbh and up
• Total board foot volume per acre doyle scale
• Harvest volume per acre
• Average basal area per acre
• Average board foot volume per harvest tree
• Average harvest value per acre
• Number of trees and basal area per acre by size class
• Pulpwood volume per acre in trees less than 12" dbh and in sawtimber topwood.
• Statistical values
  • Mean volume per acre (M)
  • Standard deviation (S)
  • Standard error of the mean (SM)
  • Half confidence Interval (1/2 CI at 95% confidence.)
  • Allowable error (AE)
  • Coefficient of variation (CV)

FORMULAS

• Trees per acre - (Sum of Ft)/n
• Volume per acre - (Sum of Fv)/n
• Fv=V*Ft
• BA=(BAF*tree count)/n
• Av. dbh=(sum of (Ft*dbh))/(sum of Ft)
• Mean(X)=(Sum of X)/n
• S=sqr(((sum of X)^2)-(sum of X^2)/n)/(n-1)
• SM=S/(sqr(n))
• 1/2 CI=t*SM
• For 95% confidence t=1.960
• For 90% confidence t=1.645
• For 80% confidence t=1.282
• AE=1/2 CI/Mean
• CV=S/Mean
• n=((t^2)*(CV)^2))/(AE)^2
• Rule of thumb CV=Range/(4.5*Mean)
• Another very rough rule of thumb - doyle vol. per acre for BAF 10 plot = 400 * (number of logs tallied on plot)

Expansion Factors

dbh      Ft      dbh*Ft
------  -------  -------
  5      73.34    367
  6      50.93    306
  7      37.42    262
  8      28.65    229
  9      22.64    204
 10      18.34    183
 11      15.15    167
 12      12.74    153
 13      10.85    141
 14       9.35    131
 15       8.15    122
 16       7.16    115
 17       6.34    108
 18       5.66    102
 19       5.08     97
 20       4.59     92
 21       4.16     87
 22       3.79     83
 23       3.47     80
 24       3.18     76
 25       2.93     73
 26       2.71     70
 27       2.52     68
 28       2.34     66
 29       2.18     63
 30       2.04     61
 31       1.91     59
 32       1.79     57
 33       1.68     55
 34       1.586    54
 35       1.496    52
 36       1.414    51
 37       1.339    50
 38       1.269    48
 39       1.205    47
 40       1.145    46

Fv FC 78 doyle scale

dbh      1/2      1     1 1/2   2       2 1/2   3       3 1/2
---------------------------------------------------------------
10       128      257   312     367     385     403
11       167      333   409     485     530     576
12       185      369   459     548     612     675     688
13       206      412   521     640     716     792     825
14       224      449   580     701     785     870     916
15       244      489   636     782     880     986    1043
16       258      516   673     831     945    1067    1146
17       273      545   716     888    1021    1154    1243
18       283      566   747     928    1075    1217    1313
19       300      599   792     986    1143    1300    1402
20       309      618   824    1030    1195    1360    1475
21       320      641   861    1082    1256    1431    1556
22       330      659   887    1118    1304    1486    1618
23       338      677   916    1152    1346    1541    1676
24       343      687   932    1177    1377    1577    1714
25       353      706   961    1213    1424    1635    1784
26       360      721   981    1244    1461    1677    1837
27       368      736  1003    1273    1497    1724    1887
28       371      742  1016    1289    1521    1755    1919
29       377      754  1036    1317    1557    1796    1966
30       384      767  1055    1342    1587    1832    2007
  

Woodland Preliminary Examination Field Data

Plot Data

 
 
 
 
 
 
 
Tree Data:
------------------------------------------------------------------------
Tree  Species   dbh  Merc. Ht.  Grade  Vigor  CC  LHC  Trees/   Vol/
No.                   (logs)                           Acre     Acre
------------------------------------------------------------------------
1.   ________  ____   ________   ___    ___  ____ ___ ________  ________
2.   ________  ____   ________   ___    ___  ____ ___ ________  ________
3.   ________  ____   ________   ___    ___  ____ ___ ________  ________
4.   ________  ____   ________   ___    ___  ____ ___ ________  ________
5.   ________  ____   ________   ___    ___  ____ ___ ________  ________
6.   ________  ____   ________   ___    ___  ____ ___ ________  ________
7.   ________  ____   ________   ___    ___  ____ ___ ________  ________
8.   ________  ____   ________   ___    ___  ____ ___ ________  ________
9.   ________  ____   ________   ___    ___  ____ ___ ________  ________
10.  ________  ____   ________   ___    ___  ____ ___ ________  ________
11.  ________  ____   ________   ___    ___  ____ ___ ________  ________
12.  ________  ____   ________   ___    ___  ____ ___ ________  ________
13.  ________  ____   ________   ___    ___  ____ ___ ________  ________
14.  ________  ____   ________   ___    ___  ____ ___ ________  ________
15.  ________  ____   ________   ___    ___  ____ ___ ________  ________

                                 Totals               ________  ________

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Last revised February 17, 1997.

URL: http://webcom.com/~duane1/wood/measure.html

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All contents copyright (C) 1997, Duane Bristow. All rights reserved.