S&W Library  The Art of Timber Cruising: Part 2  FixedArea Plots
S&W Report / Spring / Summer 2001, Vol 23

In this second article on timber cruising, we will be looking at a simple method of preparing a basic description of the forest compartments in your woodlot. It is an easy method of collecting data that can be completed by most woodlot owners  this system is called fixedarea plot sampling.
The Concept of FixedArea Cruising
The concept of fixedarea cruising involves collecting information from a number of small sample plots in an area, which is then used to project statements about the volume, species composition, density, and condition of the trees in the total area.
Sampling a small portion of the woodlot and using this information to describe the entire forest compartment is a forestry standard practice. However, because many of your management decisions are based on the information collected from the cruise, it is important that the plots accurately represent the character of the forest area being sampled.
Fixedarea plot sampling offers the woodlot owner one method for sampling forests at all stages of development  from regeneration to mature timber.
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Planning Your Cruise
A part of the decisionmaking process, prior to undertaking a fixedarea plot cruise, is determining the plot size, shape, sample size and cruise layout.
Plot shape  the shape of the plot is generally a matter of preference of the forest manager. Sample plots can be square, rectangular or circular.
Circular plots are easy to lay out and can be established on the ground by one person holding the tape at zero over the plot centre, while the second person walks out the distance equal to the radius of the predetermined plot size. The plot radius should be measured a number of times along the circumference to determine the boundary of the plot. One disadvantage of circular plots is that they can be difficult to establish in dense undergrowth or in rough terrain.
Square or rectangular plots are best established by using a crew of two or more people. They usually take a little longer to establish; however, once established it is easier to determine the boundaries of the plot, which may provide for a more reliable tree tally.
Tables 1 and 2 provide a number of options for woodlot owners looking at using fixedarea plot sampling methods.

Table #1: FixedArea Sample Plots (Square) 
Area of Plot 
Dimensions 
Plot per Hectare Factor 
0.005 ha (50 m^{2}) 
7.1m x 7.1m 
200 
0.01 ha (100 m^{2}) 
10 m x 10 m 
100 
0.02 ha (200 m^{2}) 
14.15 m x 14.15 m 
50 
0.05 ha (500 m^{2}) 
22.4 m x 22.4 m 
20 
0.10 ha (1,000 m^{2}) 
31.6 m x 31.6 m 
10 
Source: Adapted from Avery  Natural resources Measurements. The plot per hectare factor is the number used to convert a plot total (trees tallied) to a per hectare basis. 

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Plot size Table #2 provides a number of recommendations in respect to plot size. The size of the plot is dependent on the type of vegetation being sampled. For example, it is recommended to use a smaller plot for young plantations (100 m^{2}) or a larger plot size (400 m^{2}) for natural stand conditions. 
Table #2: FixedArea Sample (Circular) Plot Sizes 
Area of Plot 
Radius of Plot 
Plot per Hectare Factor* 
When to Use 
.0001 (1 m^{2}) 
0.56 
10,000 
Ground plants, tree regeneration surveys. 
.002 ha (20 m^{2}) 
2.52 m 
500 
Shrubs and tree saplings under
2.5 cm diameter at breast height. 
.005 ha (50 m^{2}) 
3.99 m 
200 
Shrubs and tree saplings under
2.5 cm diameter at breast height. 
.01 ha (100 m^{2}) 
5.64 
100 
Young plantations, uniform stand conditions. 
.02 ha (200 m^{2}) 
7.98 m 
50 
Young plantations, uniform stand conditions. 
.03 ha (300 m^{2}) 
9.77 m 
33.3 
Natural stand conditions. Variable tree species and age distribution. 
.04 ha (11.28 m^{2}) 
11.28 m 
25 
Natural stand conditions. Variable tree species and age distribution. 
.05 ha (12.62 m^{2}) 
12.62 m 
20 
Natural stand conditions. Variable tree species and age distribution. 
The plot per hectare factor is the number to convert a plot total (trees tallied) to a per hectare basis. Source: Making Cents Out of Forest Inventories  A Guide for Small Woodlot Owners. 

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Sample size  the amount sampled will depend on the purpose of the inventory, the variability of the forest cover, the size of the forest compartment, and the how reliable the information needs to be.
As a rule:
 As the area to be sampled increases, the sample intensity decreases. Conversely, the smaller the area to be sampled, the sample intensity increases;
 The intensity of the sample should increase for operational decisions (e.g. harvesting). These types of management decisions require more precise information versus inventory requirements, which are used to make longerterm planning decisions.
 The sampling intensity needs to be increased when the forest compartment consists of a variety of growing conditions and/or when variable terrain is encountered; and
 Increased reliability equates to an increased sampling intensity.
For most small woodlot owners, a sampling intensity from 2% to 10% is common. However, if you are considering harvesting activities, you may want to consider a sampling intensity of 10% or higher. (Source  Making Cents Out of Forest Inventories)
Cruise layout  a series of transect lines (or cruise lines) are laid out parallel to each other to ensure that conditions throughout the forest compartment are sampled in an unbiased manner. The cruise lines should run against the topography (i.e. up and down the hills), and the sample plots are located at regular intervals along the line. The accuracy with which the sample plots represent the forest compartment depends on this unbiased selection of plots. See Figure 1.


Fig 1.
Cruise lines should be laid out in a grid fashion so that the lines are run at right angles to the topography.


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Instruments Required
In contrast to point sampling, where a special forestry tool called a wedge prism is required, collecting information from fixedarea plots requires no special tools. Most woodlot owners will have all the necessary tools in their workshop. These include: two tape measures (3m tape to measure tree diameters and a 25m tape to lay out the plot boundaries); wooden stakes; steel pins or flagging tape (to mark the centre or corners of the plot); a clipboard and tally sheets (to record your information).
Important note  if you are going to use a regular tape measure to measure the diameter of the tree you must convert this circumference measurement into diameter. The conversion factor is 3.1416. For example, a measurement of 38 cm using a regular tape would be recorded on your tally sheet as a
12cm diameter tree.
However, if you plan on undertaking a large cruising project, you should consider purchasing or borrowing forestry equipment specifically designed for measuring tree diameters, e.g. diameter tape, calipers, or a Biltmore stick. This equipment is available from most forest equipment retailers such as Canadian Forestry Equipment, Mississauga, ON, 18003874940.
Information Collected
The amount of information collected at each plot will depend on your objectives  timber production, wildlife management, or recreation. At a minimum, the tree species and the diameter of each tree in the plot should be collected, and an average age and height need to be determined for each forest compartment. Collecting this minimum amount of information will provide you with a general description of the forest compartment and will meet the minimum requirements for the Managed Forest Tax Incentive Program.
In most forest inventories tree diameters are measured at 1.3 m above the ground (referred to as dbh), and only those trees with a diameter of 10 cm or larger are tallied.


Fig 2.
A square sample plot measuring 22.4 metres by 22.4 metres is equal to an area of .05 hectares. Only those trees measuring greater than 10 cm in diameter at breast height within the plot are tallied. Note borderline trees are tallied only if more than onehalf of the trunk of the tree is within the plot. In this example, 20 trees are tallied representing 400 trees per hectare. Note  trees 1, 2, and 3 are outside the plot and are not tallied.


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Optional information that you may want to collect would include  tree quality (AGS or UGS); individual tree heights and ages; soil information; advanced regeneration; and flora information.

Preparing a Compartment Description
It is easy to calculate the species composition of your forest compartment by inserting the cruise information you have collected into the formula noted below.
Species Composition % = (species tally divided by total tree tally) x 100
For example, you have collected the following information in compartment 'A' of your woodlot:
Circular fixedarea sample plot = 9.77 m radius (0.03 ha)
Number of plots sampled = 3
Conversation factor = 33.3
Tree tally of all 3 plots = 42 (maple = 24, beech = 13, red oak = 5)
Trees per ha = (42 x 33.3) divided by 3 = 466
This calculation needs to be completed for each species tallied in your cruise.
Compartment 'A' has an average of 466 trees per hectare; of this total 266 were hard maple, 144 were beech, and the remaining 56 trees were red oak. Using the formula, hard maple would represent 57% of the composition of this compartment (i.e. 266/466 x 100 = 57); beech would represent 31% (i.e. 144/466 x 100 = 31) and red oak would make up the final 127% (i.e. 56/466 x 100 = 12). Therefore, the tree species and percent composition for this forest compartment would be hard maple 60%, beech 30% and red oak 10%.
Note  the tree species and percent composition of the forest compartment should only list species that make up at least 10%; percentages are normally rounded off to the closest multiple of 10; and the total of all species is not to exceed 100%.
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Expressing Density as Stems per Hectare Using a FixedArea Plot
Expressing the density of the woodlot based on the number of trees per hectare is a relatively straightforward measure that most woodlot owners can easily identify with. Expressing the density in this manner with the information collected in your fixedarea plots is simple.


Fig. 3
A circular plot with a radius of 9.77 metres is equal to an area of 300 square metres of .03 hectares. The trees tallied in the sample plot represent 466 trees per hectare
(14 x 33.3 conversion factor = 466). 

To calculate the stems per hectare you would total the trees tallied in the plot (or plots sampled) and multiply this number by the 'plot per hectare factor' as shown in Figure #3. Tables #1 and #2 provide you with the 'plot per hectare factor' for a variety of different sized sample plots.

Calculating the Compartment Basal Area
Although you are not measuring the compartments' basal area as you gather data from your fixedarea plots, the stem diameter information collected and the information contained in Table #3 can be used to calculate the basal area.
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Table 3: Basal Area Table 
Diameter
(cm) 
Basal Area
of Stem (m^{2}) 
10 
0.008 
12 
0.011 
14 
0.015 
16 
0.020 
18 
0.025 
20 
0.031 
22 
0.038 
24 
0.045 
26 
0.053 
28 
0.062 
30 
0.071 
32 
0.080 
34 
0.091 
36 
0.102 
38 
0.113 
40 
0.126 
42 
0.139 
44 
0.152 
46 
0.166 
48 
0.181 
50 
0.196 
Table 3  provides the basal area in square metres of individual trees measured at breast height (1.3 m from the ground). 

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For example, in compartment 'A' you have an average of 466 trees per hectare, with the diameter distribution as shown in Table #4. Using the information and the individual stem basal area information from Table #3, the basal area of this compartment is 20 m2 per hectare.

Table 4: Calculating Basal Area 
Diameter 
Stems 
Basal Area of stem (m^{2}) 
Total BA 
16cm 
333 
0.020 
6.7 
32cm 
100 
0.080 
8.0 
44cm 
21 
0.152 
3.2 
50cm 
12 
0.196 
2.4 
Total trees/ha 
466 
BA/ha (m^{2}) 
20.3 

Recommended Reading
For additional information on preparing an inventory for your woodlot, you may want to consider reading one of the following guides:
Making Cents Out of Forest Inventories  A Guide for Small Woodlot Owners, Ministry of Natural Resources  $10.00.
A True Picture  Taking Inventory of Your Woodlot, Eastern Ontario Model Forest  $10.00.
Copies of these two publications can be obtained from the LandOwner Resource Centre by calling (613) 6922390.

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