One of the first steps in planning any management
activity on forested land is to conduct an inventory of the resources
on the property. This inventory, often called a cruise, when used to
estimate timber volume, can range from a quick walk-through
“eye-ball” estimate to the carefully planned and executed
timber inventory designs used for industrial and government
timberlands. Most landowners opt for something in between that’s
accurate enough to make informed decisions at a reasonable cost. This
leaflet will help you decide what type of inventory information might
be useful on your property and provides guidance on interpreting some
of the results.
The principal question that most timber
inventories answer is what is the volume and value of the timber
resource on the property. Timber inventories also answer questions
about the age and size structure and species composition of the
existing forest. Information about the structure of the forest helps
determine appropriate forest stand treatment. While other resources
and conditions can be inventoried, this is rarely done on smaller
ownerships because of the cost.
Why should you bother with the cost of an
inventory? That is an important question that is hard to answer
except generally. Depending on the experience of the forester and the
complexity of the forest, an “eye-ball” walk-through
estimate might be sufficient for some management decisions -
especially if the decision is to invest in a more formal inventory.
If you are considering a timber harvest, you will need an accurate
inventory for the Timber Harvest Plan (THP). Most management
decisions should be made with the best available information. It is
difficult to make an informed decision when you do not know what you
own.
The formal timber inventory helps guide forest management decisions from an objective point of view. After completion of an inventory, you have an estimate of the volume, value, and structure of the forest. Management objectives can be defined in terms of volume and stand structure. One of the themes of this series is for landowners to define measurable objectives to guide the process of forest management. By carefully specifying measurable objectives, landowners have a way to carry out their land management decisions.
While there is nothing really complex about
conducting a timber inventory, this is one task you should consider a
professional forester for, in most instances, because of the need for
credibility. Few people will believe numbers generated solely by a
landowner. Most Registered Professional Foresters (RPFs) will insist
on at least checking the inventory because their license depends on
providing accurate information. An inventory is useful for the
forester to get to know the property. The inventory process relies on
measuring a small sample and expanding it to reflect the entire
property. A very small bias, measuring the diameter an inch below
breast height, or slightly overestimating the heights of measured
trees, can have enormous impacts on the volume and value calculated
from the inventory.
The other major reason to use a forester would be
experience. Taking accurate inventory takes experience. Inventories
by inexperienced foresters are checked and rechecked until the
forester can be trusted to take accurate information every time. In
large inventories, check cruising – remeasuring a subsample of
the plots – is designed into the procedure. A landowner
conducting his or her own inventory could easily make a simple
mistake that would be unlikely to be discovered until important
decisions have been made with spurious information.
With that said, there is a leaflet in this series that describes how a landowner can conduct their own simple inventory. This will be useful if you just want to know what is on your land, or you would like to independently check others’ figures.
The most common timber inventory technique is a
stratified, systematic, random sample. So what does this mean and why
is it important? The basic technique is to group similar forest
stands (stratify), start at a randomly Determined place (random), and
measure small circular plots (sample), spaced equal distances apart
(systematic). Figure 1 shows a typical timber inventory overlaid on
an aerial photo.
Grouping trees of about the same age, stand
structure, species composition, and location improves the information
gathered by an inventory by minimizing the variability within a
strata (group) and maximizing the variability between the strata. In
Figure 1 we can see there are 4 vegetation types, Meadows (M), Mixed
Conifers in 2 density classes, Light and Dense (MCL and MCD
respectively), and Ponderosa Pine Light Density (PPL). The density
class refers to the amount of ground covered by tree crowns when
viewed from directly overhead. In the Dense class 80-100% of the
ground is covered. In the Light class 0-40% of the ground is covered
by tree crowns.
First, we need to take a sample of the trees to
obtain an understanding of what kind of forest we have to work with.
A sample is chosen at random from the entire forest so that we can
use the theories of probability and statistics to infer from a few
well-chosen samples, information about the forest as a whole.
Measuring trees is difficult and expensive. The goal is to reduce the
number of trees that you measure, but still have enough information
to make informed decisions.
Statistical theory limits some of the specific
ways in which we can take plots. In order to use cost-efficient
methods, we need to make sure that the individual sample points are
selected randomly. Otherwise, we might bias the sample by just
measuring the biggest and best parts of the stand. By selecting a
random starting point and systematically laying out the rest of the
sample plots, we satisfy the statisticians while still maintaining an
efficient sample. While there are many different methods for
selecting sample points, the systematic plots ensure that you sample
the entire property.
An example might help here. Looking at Figure 1
Plots 6, 7, 8, 9, 14, and 15 are in the Light Density Ponderosa Pine
Type, a 22-year-old Ponderosa pine plantation from an area harvested
after a fire. All of the trees are about the same age, diameter,
height, and species. Plots 5, 10, 13, 16, and 22 are in the Light
Density Mixed Conifer Type (MCL). This stand was damaged by the fire,
and some trees removed as salvage. It was not replanted and there is
little regeneration. Plots 1-4, 11, 12, 17-21 in the Dense Mixed
Conifer was not affected by the fire. It resulted from a timber
harvest in the 1930s using typical practices of the day. Essentially
every tree that had value was harvested. A few “seed trees”
of poor form and size were left, and the area regenerated from these.
The stand was initially taken over by brush, and the trees that we
see today had to grow through the brush. Table 1 outlines the
inventory data from these stands.
These stands are very different due to their
history. It does not make sense to treat them the same way, neither
does it make sense to combine their inventory information. The
Ponderosa pine plantation (PPL) is doing fine. It might might need a
thinning but otherwise should be left to grow. The dense conifer
stand (MCD) looks ready for a regeneration harvest. At this point, it
could transition to an uneven-aged stand, or an even-aged
regeneration treatment could be applied. The inventory helps you
decide on the treatment.
Averaging the inventory of these two stands distorts the on-the-ground information. Though it gives an average volume for the property, the average does not help with determining an appropriate management system. Most inventories are stratified, usually by timber type but sometimes by management unit.
The next choice to make is the type of plots to
measure. The two most commonly used plots are fixed radius or variable radius or
prism plots.
Fixed radius plot
are the easiest to picture. The forester locates the plot center
using a compass and pacing. Every tree within a certain distance of
the center of the plot is measured. Since you know the area of the
plot, you can expand the number of trees you measure to a per acre
figure. An example will help here.
A circle with a radius of 37.24 feet encloses 1/10
acre - or 4,356 square feet. Since we are measuring all of the trees
on 1/10 of an acre, we assume that the other 9/10 of an acre has a
similar distribution of trees. Over large enough areas, this works
out very well. Thus, every tree we visit on the plot represents 10
trees per acre. You can quickly see the cost savings by measuring at
most every 10th tree. You can also see a problem, unless the sample
is measured with great care, errors can multiply just as quickly.
Sampling also realizes cost savings because not every acre is
visited. Typically, only 1 – 10% of small properties are
actually measured.
Another technique is commonly used when timber
volume is the primary variable of interest. Variable radius or prism plots
use some clever geometry to further reduce the number of trees that
are actually measured, while still giving an accurate picture of
forest structure. Figure 2 demonstrates the theory behind variable
radius plots. From the plot center, a fixed angle is projected. Trees
that are larger in diameter than the fixed angle are measured, trees
smaller than the angle are rejected. One result is that every tree
diameter has its own plot size. Larger trees have larger plots and
are more likely to be selected for measurement. This is offset by the
fact that larger plots count for fewer trees per acre. Since larger
trees have more volume and value, you want very accurate information
on their size and distribution. Smaller trees are usually more
plentiful and have less volume and value. Using a prism cruise, you
can reduce the sampling intensity on these smaller trees and still
get an accurate idea of how many are present.
The trick in using prism plots is to choose a
fixed angle that will measure about 5 to 12 trees per plot. Using
this method has a benefit – with a bit of algebra, you can show
that every tree that is in it represents a fixed amount of basal area
per acre. Basal area is an expression of the density of a forest
stand and is described in more detail later. The number of square
feet per acre that each tree represents is called the Basal Area
Factor of the prism. Commonly used factors are 10 and 20.
The fixed angle is generated with a variety of
tools. The most commonly used is a prism that shifts the image of
part of the tree. Angle gages are simple and inexpensive. A real
“rule-of-thumb” approximates a 10 factor prism. Hold your
arm straight out at eye level with your thumb extended thumbnail
turned towards your eye. Slowly sweep in a circle keeping your thumb
over the center of the plot. Every tree that is wider than the fixed
angle formed by your eye and your thumb is “in” and should
be measured. If 12 trees are “in,” each represents 10
square feet of basal area per acre, so the stand has 120 square feet
of basal area per acre at that point. Note that everyone’s thumb
is a different size, and this is a very rough approximation and not
suitable for an inventory. It does give a very quick estimate of
basal area per acre.
There are some advantages in prism cruises over fixed plots in the amount of time it takes to measure a smaller number of trees, automatically calculating basal area per acre, and applying shortcuts to office calculations.
The purpose of the inventory determines exactly
what is measured. You should have an idea of why you are conducting
the inventory and what kind of information that you expect and need
to make management decisions.
Species is almost
always indicated. Sometimes species are grouped according to what can
be sold. Ponderosa pine and Jeffrey pine are often grouped as yellow
pines. Red fir and white fir as Fir. Generally, it is better to
record the exact species in the field and later group the
results.
Diameter at Breast Height (DBH) is usually measured for timber inventories. Because of the
root swell at the base of a tree, we measure diameter exactly 4 1/2
feet above the ground on the uphill side of the tree. Trees that fork
below 4 1/2 feet are measured as 2 trees. Trees with unusual
swellings at 4 1/2 feet are measured slightly above or below the
swelling to get a more accurate idea of the tree’s true
size.
Height is often but
not always measured. Accurately measuring height takes a great deal
of time per tree. Height is estimated by first carefully measuring a
certain horizontal distance from the tree. Then the angle between the
top and bottom of the tree is measured. Using trigonometry, you can
calculate the tree’s height. Because of the time it take to
measure heights, height is sometimes estimated by the number of logs
in the tree or by measuring a small sample of heights.
Site Quality is
usually indicated by site index or site class. Site quality expresses
the potential for the land to grow trees. In California, site quality
is often expressed by Site Class ranging from 1-5 with 1 being the
best. Site Class and quality are important in making management
decisions. Better sites are easier to manage. They are more forgiving
of mistakes, and trees just grow faster. Site Class 3 land, typical
of many smaller parcels, is at the lower limit of where you can
produce timber economically. There are, of course, other values that
can be found on lower site quality lands like wildlife habitat,
livestock grazing, and some specialty forest produces. Site quality
might be expressed by site index which predicts the height of the
biggest trees in a stand at various ages.
Age and Increment
can be a guide to treatments of all types, thinning, pruning, or
harvest. The age of a stand is determined from historical records,
counting rings from stumps, or counting rings using an increment
borer. The increment borer drills a small hole in the tree and
extracts a core. The tree's age can be estimated by counting its
rings. The increment borer also allows you to estimate recent growth.
The diameter growth over the last 5 or 10 years are measured to see
how fast the tree is growing.
Defect is often stated a percent of the total volume. Most individual trees have scars, rot, or crooked stems that reduce their value for lumber.
Trees Per Acre helps
describes the structure of a stand. When stands are young, they will
have many trees per acre, 1,500 or more. As they mature, some of them
die, and by the time they are harvested, there may be only 100 to 200
trees per acre.
Basal Area is the
area of an acre that has tree trunks on it. Usually basal area is
reported in square feet per acre. If you cut off every tree on an
acre cleanly at 4 1/2 feet above the ground – breast height
– and measure the area at the top of the stump, and add that
number up over the acre, you would get the basal area in square feet
per acre, an important indicator of density. It combines average
diameter and number of trees per acre into a single number that can
be used for many purposes. There is a very good relationship between
basal area and volume that is used for quick estimates. There are
guides that set target basal areas for certain stand management
options. Basal area is related to many wildlife habitat and aesthetic
concerns, giving an objective measurement to guide management
activities.
Volume represents
the bottom line of most inventories. How much timber is out there? In
California, volume is expressed in thousands of board feet (MBF). The
volume of standing trees is calculated to predict the number of
boards that it could produce. To account for the round edges and the
waste generated by sawing, many log rules have been developed. Tables
are available for most species using various conversion factor. In
California, the Scribner rule is the most common and is required for
Timber Yield tax when you have a harvest.
Volume is increasingly being reported in thousands of cubic feet
(MCF), especially on federal timberlands. This reflects the fact that
more of a tree is used than just flat boards. Most modern timber
inventory software reports both cubic and board foot volumes.
The Value of timber can change dramatically over short periods of time. Though a discussion of forest economics appears in another leaflet, a brief mention will be useful here. The value to the landowner is often reported as stumpage, that is the value of a log sitting “on the stump.” Stumpage is a great way to compare competitive bids for your timber. In a stumpage price, all costs of logging and marketing are
taken out of the equation, and the price given can
be multiplied by the volume present to get a bottom line return.
One source of average stumpage values is the Harvest Value Schedules
published twice a year by the State Board of Equalization, Timber Tax
Division. The Harvest Value Schedules are the Board’s best
estimate of return to the landowner given location, timber quality,
timber quantity, and harvesting method. The Harvest Value Schedules
will give you an independent ballpark estimate of the value of your
timber.
Table 1. Inventory Summary example
Per Acre
Total
Type
Species
Avg. DBH
Basal Area
Trees Per Acre
Cubic Foot Volume
Board Foot Volume
Acres
in.
sq. ft. / ac.
#
MCF
MBF
MBF
Meadow
5.25
MCL
Ponderosa Pine
9.5
26.7
48.4
0.9
4.5
Douglas Fir
9.0
24.8
44.6
0.7
3.5
Other Conifers
8.8
19.7
28.3
0.4
2.9
Hardwoods
12 .0
6.4
12.1
0.1
0
TOTAL
9.8
77.7
133.4
2.2
10.8
8.50
92 .0
MCD
Ponderosa Pine
7.2
84 .0
152 .0
2.9
14 .0
Douglas Fir
6.3
78 .0
140 .0
2.3
11 .0
Other Conifers
6.7
62 .0
89 .0
1.2
9 .0
Hardwoods
11 .0
20 .0
38 .0
0.4
TOTAL
7.8
244 .0
419 .0
6.8
34 .0
34.75
1,181.5
PPL
Ponderosa Pine
4.5
112 .0
675 .0
0.8
Douglas Fir
Other Conifers
Hardwoods
2 .0
22 .0
72 .0
0.1
TOTAL
3.3
134 .0
747 .0
0.9
12.25
Grand Total
7 .0
151.9
433.1
3.3
14.9
60.75
1,273.5
Fuels: Landowners
are rightly concerned with fire. Most often the person who does the
cruise will give a professional evaluation of fire risk based on
experience. There are procedures, typically piggybacked on a timber
inventory, that can objectively assess the amount and potential
flammability of fuels. Another leaflet in this series provides
specific information.
Wildlife Surveys are
more difficult to conduct than timber cruises. Mostly, because
animals move around, and are very good at hiding while trees tend to
stay on one place. You have to target specific wildlife species using
their behavior to get an accurate count.
Birds are a great example. Songbirds are inventoried using a line
plot method similar to timber inventories. Biologists walk along a
known path stopping at set distances. There, the biologist will
listen for a set period of time – identifying song birds by
their calls. This works fine for song birds in the day time but is
inadequate for birds that don’t sing or that are active at
night. The inventory has to take this biology into account. Spotted
owls are assessed by visiting plots and actively calling the birds by
hooting. The owls are territorial and will respond to the intruder
and come to investigate.
Fish are surveyed by walking stream segments with
a specially designed battery pack. The biologist temporarily stuns
the fish with an electric current, so they can be counted.
Wildlife habitat is the actual variable most often
inventoried. The types and arrangement of vegetation, information
gathered in a timber inventory, can tell you a great deal of what
kind of wildlife may be present. By manipulating habitat, a landowner
can encourage or discourage particular species. Knowing the specific
biology of the animal you are interested in is the key to wildlife
inventory.
Designing and conducting an inventory that objectively describes what you own is an crucial step in being able to effectively manage your property.
In accordance with applicable State and Federal laws and University policy, the University of California does not discriminate in any of its policies, procedures, or practices on the basis of race, religion, color, national origin, sex, marital status, sexual orientation, age, veteran status, medical condition, or handicap. Inquiries regarding this policy may be directed to the Affirmative Action Director, University of California, Agriculture and Natural Resources, 300 Lakeside Drive, 6th Floor, Oakland, CA 94612-3560. (510) 987-0096.
Publication of this series was in part funded by the
California Department of Forestry and Fire Protection
under Contract numbers 8CA96027 and 8CA96028