Forest Service

North Central

Research Station

Research Note NC-379

2002

ABSTRACT.—The North Central Research Station’s Forest

Inventory and Analysis program began fieldwork for the sixth

forest inventory of Michigan in 2000. This initiates a new

annual inventory system. This Research Note contains

estimates of Michigan’s forest resources derived from data

gathered during the first year of the inventory.

KEY WORDS: Annual inventory, forest land, forest type,

growing-stock volume, Michigan.

BACKGROUND

The North Central Research Station’s Forest Inventory and

Analysis (NCFIA) program began fieldwork for the sixth

forest inventory of Michigan in 2000, in partnership with the

Michigan Department of Natural Resources. This inventory

initiates a new annual inventory system in the State. Onefifth of the field plots in the State are measured each year

under this system. As a result, the current inventory of

Michigan’s forest resources will not be fully implemented

until 2004. However, because each year’s sample is a

systematic sample of the State’s forest and because timely

information is needed about Michigan’s forest resources,

estimates have been prepared from data gathered during the

first year of the inventory. Due to the limited number of

field plots measured, future estimates using data from

this report are subject to change when ensuing annual

inventories are completed and data complied. The results

presented are estimates based on sampling techniques. As

additional inventories are completed, the precision of the

estimates will increase and additional data will be released.

Reports of previous inventories of Michigan are dated 1935,

1955, 1966, 1980, and 1993. Data from new inventories are

often compared with data from earlier inventories to

determine trends in forest resources. However, for the

comparison to be valid, the procedures used in the two

inventories must be similar. As a result of our ongoing

efforts to improve the efficiency and reliability of the

inventory, several changes in procedures and definitions

have occurred since the last inventory of Michigan in 1993

(Leatherberry and Spencer 1996). Some of these changes

make it inappropriate to directly compare portions of the

2000 data with data published for 1993.

RESULTS

Before Euro-Americans settled the region that is Michigan,

forests occupied an estimated 33.1 million acres, or slightly

more than 90 percent of the land area (Smith et al. 2001).

From the early 1800s to the 1935 inventory, forest land area

in Michigan declined to 19.1 million acres, about half of the

State’s total land area. During the 1935 inventory, approximately one-fifth of the forest land area (3.6 million acres) in

Michigan was considered “deforested” (Cunningham and

Moser 1938). Deforested lands were lands that had once

been forest but that were now in grass, brush and marsh, or

lightly wooded pasture. Deforested lands were viewed as

acreage needing planting (Cunningham and Moser 1938).

The conventional wisdom was that with proper management

deforested lands could be reforested.

Between 1935 and 1966, timberland1

 area in Michigan

hovered around 19 million acres (fig. 1). The general

stability in timberland area during the middle third of the

20th century can be attributed to forest management

activities dedicated to reforestation. For example, between

EARL C. LEATHERBERRY is a Resource Analyst with the

North Central Research Station, St. Paul, MN.

Michigan’s Forest Resources in 2000

Earl C. Leatherberry

1

 Timberland is forest land that is capable of growing trees at

a minimum level (20 cubic feet per acre per year) and that is

not restricted from harvest.
1933 and the start of World War II, the Civilian Conservation Corps (CCC) planted about 500 million trees in

Michigan. Also, the significant amount of public ownership

of forest in Michigan helped maintain a stable base of

timberland area. However, between 1966 and 1980, the area

of timberland in Michigan declined to a historical low of

17.5 million acres. Much of the decline in timberland area

was related to increased tillage and the pasturing of livestock

on marginal timberland. During the 1980s and early 1990s,

the area of timberland rebounded to 18.6 million acres. The

increase came predominantly from abandoned cropland and

pasture and marginal forest land that were productive

enough to be reclassified as timberland (Schmidt et al.

1997). Between 1993 and 2000, Michigan’s timberland area

increased by an estimated 442 thousand acres to an

estimated 19.1 million acres. The increase in timberland

area between 1993 and 2000 should be viewed with the

caveat that the 2000 estimate of timberland area is based on

a partial inventory and therefore has a higher sampling error

than inventories since 1955 (fig. 1). The 2000 estimate

indicates that timberland area is at least holding steady. A

stable timberland base is noteworthy considering that

suburban and second-home developments continue to

expand into rural areas. Also, resort communities or

enclaves, including golf courses, continue to expand into

timberland areas. Timberland that converted to other land

uses is apparently supplanted by land, some of it agricultural

land, that reverted back to forest. However, land that

converted to timberland may be different from timberland

that was converted to other land uses. Differences may be

associated with species composition, productivity, and

ownership. Subsequent inventory data will allow for a more

in-depth analysis of regional changes in Michigan’s timberland area.

Private ownership controls 62 percent of the timberland area

in Michigan. These include individual, as well as partnership, corporate, and other group ownerships (Leatherberry

et al.1998). Michigan’s timberland owners provide an array

of goods and services. For example, Michigan’s timberland

owners provide wildlife habitat, watershed protection,

timber and non-timber products, recreation opportunities,

and environmental benefits such as biodiversity. Further,

some private owners have had their forest formally certified

by an independent authority as ecologically sustainable.

Managers of public timberland, such as national and state

forests, have more of an explicit responsibility to foster

societal benefits through forestry. The conservation and

2

Figure 1.—Area of timberland, Michigan, 1935-2000 (Note:

the sample error associated with each inventory is

represented by the vetical line at the top of each bar).

0

2

4

6

8

10

12

14

16

18

20

1935 1955 1966 1980 1993 2000

Year

Million acres

restoration of the pine resource in Michigan is an illustration

of how public ownership provides societal benefits. Before

the period of extensive logging in the late 1800s and early

1900s, pines were an important component of the State’s

forests. Logging largely decimated the pine resource.

However, some pockets where pines survived logging were

placed in public ownership. Places such as Hartwick Pines

State Park and Estivant Pines are remnant old-growth pine

forest. Extensive pine restoration programs were carried out

on public land. About one-fourth of the timberland area

now in the softwood type groups was planted, and nearly 60

percent of that was planted on public land.

Hardwood forest types occupy about three-fourths of the

State’s timberland area. The maple-beech-birch forest type

group is the most extensive forest type in Michigan,

occupying an estimated 36 percent of the State’s timberland

area (fig. 2). The aspen-birch type occurs on about 19

percent of timberland area. The oak-hickory group, and the

elm-ash-cottonwood type group, when combined occupy

about 20 percent of Michigan timberland area. The white

pine-red pine-jack pine group and the spruce-fir type group

cover about one-fourth of the timberland area, accounting

for virtually all the area in the softwood forest type groups.

As Michigan’s forests mature and are affected by natural and

human-caused events, they take on certain stand-size

characteristics. Stand-size class is a measure of the average

diameter of the dominant trees in a stand. There are three

stand-size classes: sawtimber—large trees, softwoods at least
trees greater than 5 inches d.b.h., from 1 foot above the

ground to a minimum 4-inch top diameter. Growing-stock

volume increased between 1993 and 2000, from 26.6 to

27.3 billion cubic feet. Total growing-stock volume has

increased in every inventory since 1955. However, between

1993 and 2000 hardwood growing stock declined from 19.1

to 18.3 billion cubic feet (fig. 4). Fully two-thirds of total

growing-stock volume is now in hardwood species. The

apparent decrease in hardwood growing-stock volume is

probably associated with the decline of area in sawtimbersize stands. Some of the sawtimber-size stands may have

been harvested or otherwise removed, resulting in declining

hardwood volume. In addition, the higher sampling error

associated with the 2000 estimate may distort actual change

(Hansen et al. 2000).

3

Figure 2.—Area of forest land by forest type, Michigan, 2000.

9 inches in diameter at breast height (d.b.h.) and hardwoods at least 11 inches d.b.h.; poletimber—medium trees,

trees 5 inches in d.b.h. to sawtimber size; and sapling/

seedling—small trees, trees 1 to 5 inches in d.b.h. Between

1993 and 2000, the area of larger trees—sawtimber-size

trees—declined while the area in poletimber-size stands

increased. The proportion of area in sapling-seedling size

stands stayed about the same (fig. 3). Ensuing annual

inventories of the sixth Michigan inventory will provide

information for more definitive conclusions about forces

driving the changes in stand-size class.

Michigan’s growing-stock volume totals 27.3 billion cubic

feet. Growing-stock volume is the amount of solid wood in

Figure 3.—Stand-size class as a percentage of total timberland area, Michigan, 1993 and 2000.

Other

softwoods

1%

Elm/ash/

cottonwood

9%

Oak/

hickory

10%

Aspen/

birch

19% Maple/beech/

birch 36%

Spruce-fir

14%

White/red/

jack pine

11%

0

10

20

30

40

50

Sawtimber Poletimber Seedling-sapling

Stand-size class

Percent

1993 2000
Softwood growing-stock volume has increased during every

inventory since 1955 (fig. 5). Between 1993 and 2000, the

volume of softwood growing stock increased from 7.5

billion cubic feet to 9 billion cubic feet.

Maples account for the largest part of growing-stock volume

at 28 percent of total growing-stock volume. Other species

with significant growing-stock volume are eastern white and

red pines (13 percent), followed by aspen and cottonwood

(11 percent), oaks (10 percent), and other eastern softwoods

(10 percent).

In summary, data from the 2000 inventory of Michigan

forest resources indicate the directions of change in the

State’s forest resources. Timberland area increased slightly

between 1993 and 2000. Although total growing-stock

volume increased, hardwood growing-stock volume appears

to have decreased during the same time. These findings are

presented with the caveat that data are not yet sufficient to

make definitive statements about how Michigan’s forest

resources have changed since the 1993 inventory. As

additional data become available under the annual inventory system, a clearer picture of the direction of Michigan’s

forests will emerge.

4

Figure 4.—Hardwood growing-stock volume, Michigan, 1935-2000 (Note: the sampling error associated with each inventory is represented by a vertical line at the top of each bar.)

Figure 5.—Softwood growing-stock volume, Michigan, 1935-2000 (Note: the sampling error associated with each inventory is represented by a vertical line at the top of each bar.)

Softwoods

0

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4

6

8

10

1935 1955 1966 1980 1993 2000

Year

Billion cubic feet

Hardwoods

0

5

10

15

20

25

1935 1955 1966 1980 1993 2000

Year

Billion cubic feet
INVENTORY METHODS

Changes Between Inventories

Since the 1993 inventory of Michigan, several changes have

been made in the NCFIA inventory methods to improve the

quality of the inventory as well as meet the increasing

demands for timely forest resource information. The most

significant change between the inventories has been the

change from periodic inventories to annual inventories.

Historically, the NCFIA inventoried each State on a cycle

that averaged about 15 years. However, the need for timely

and consistent data across large geographical regions,

combined with national legislative mandates, resulted in

NCFIA’s implementation of an annual inventory system. The

annual inventory system began in Michigan in 2000.

With an annual inventory system, approximately one-fifth of

all field plots are measured in any single year. After 5 years,

the entire inventory will be completed. After the initial 5-

year period, NCFIA will report and analyze results as a

moving 5-year average. For example, NCFIA will be able to

generate inventory results for 2000 through 2004 or for

2001 through 2005. While there are great advantages for an

annual inventory, one difficulty is reporting on results in the

first 4 years. With the 2000 inventory, only 20 percent of all

field plots have been measured. Sampling error estimates for

the 2000 inventory are 0.89 percent for timberland area and

1.76 percent for growing-stock volume. Thus, caution

should be used when drawing conclusions based on this

limited data set. As ensuing measurements are completed,

we will have additional confidence in our results due to the

increased number of field plots measured. As each measurement year is completed, the quantity and quality of the

results will expand.

Other significant changes between inventories include the

implementation of new remote sensing technology, implementation of a new field plot design, development of new

volume equations, and gathering of additional remotely

sensed and field data. The use of new remote sensing

technology since the previous inventory has allowed NCFIA

to use computer-assisted classifications of Multi-Resolution

Land Characterization (MRLC) data and other available

remote sensing products to stratify the total area of the State

and to improve estimates. Previous inventories used manual

interpretation of aerial photographs to stratify the sample.

The new volume equations, developed by USDA Forest

Service research scientists and cooperative researchers, more

accurately estimate the true growing-stock and sawtimber

volumes. As additional annual inventories are implemented

and comparisons between the current inventory and

previous inventory become possible, FIA will update the

1993 inventory using the new volume equations.

New algorithms were used in 2000 to assign forest type and

stand-size class to each condition observed on a plot. These

algorithms are being used nationwide by FIA to provide

consistency from State to State and will be used to reassign

the forest type and stand-size class of every plot in the 1993

inventory when it is updated. This will be done so that

changes in forest type and stand-size class will reflect actual

changes in the forest and not changes due to algorithms.

The list of recognized forest types, groupings of these forest

types for reporting purposes, equations used to assign

stocking values to individual trees, definition of nonstocked

(stands with a stocking value of less than 10 percent for all

live trees), and names given to the forest types changed with

the new algorithms.

Another change with the current inventory is the determination of the exact plot location of every ground plot in the

new inventory. For plots that are visited in the field, this is

done using a global positioning system (GPS) device at plot

center. For plots not visited in the field, the plot location is

determined by transferring the old plot location from aerial

photography to an unclassified, geo-corrected remotely

sensed image. Both procedures provide an accurate location

that is used to link the ground plots to the classified

remotely sensed data used for stratification.

PROCEDURES

The 2000 Michigan survey used a two-phase sample for

stratification that included remeasuring inventory plots from

the 1993 inventory and measuring new field plots. Twophase sampling, also called double sampling, consists of a

phase 1 sample used to estimate area by strata and a phase 2

sample used to estimate the average value of parameters of

interest within the strata. The estimated population total is

the sum across all strata of each stratum’s estimated area

multiplied by its estimated mean per unit area. The only

land that could not be sampled was private land where field

5
personnel could not obtain permission to measure a phase 2

plot. These denied access plots were rare in Michigan (about

2 percent of the total plots statewide), and the methods used

in the preparation of this report made the necessary

adjustments to account for sites where access was denied.

Phase 1

Phase 1 and phase 2 plots were placed systematically across

the entire State without regard to specific land characteristics. All lands have the same probability of being sampled

under this inventory system. The 2000 inventory used a

computer-assisted classification of satellite imagery for

classification. FIA used the imagery to form two initial

strata—forest and nonforest. Pixels within 60 m (2 pixel

widths) of a forest/nonforest edge formed two additional

strata—forest/nonforest and nonforest/forest. Forest pixels

within 60 m of the boundary on the forest side were

classified as forest/nonforest. Pixels within 60 m of the

boundary on the nonforest side were classified into the

nonforest/forest strata. An overlay of national forest land

ownership was used to identify all lands owned by the

Ottawa, Hiawatha, and Huron-Manistee National Forests.

These national forest lands where treated separately but

were also stratified into one of the above four strata.

Stratification and estimation were conducted at the State

level for national forest lands and at the FIA Inventory Unit

level for other lands. Final estimation of area by stratum was

based on these five strata—national forest, forest, forest/

nonforest, nonforest/forest, and nonforest for all lands.

In the 1993 inventory, aerial photographs were assembled

into township mosaics and a systematic grid of 121 one-acre

photo plots (each plot representing approximately 190.4

acres on the ground) was overlaid on each township mosaic.

Each of these photo plots was stereoscopically examined by

aerial photo interpretation specialists and classified based on

land use, forest type, and stand-size density. From these

photo plots, a systematic sample of plots (without regard to

their aerial photo classification) was selected as ground plots

and further examined by survey crews to verify the classification and to take further measurements. These 1993

ground plots formed the basis for the remeasured ground

plots in the 2000 inventory. Additional information related

to the procedures for the 1993 inventory can be found in

Leatherberry and Spencer (1996).

The move to satellite imagery changed NCFIA’s phase 1

sample from being based on one photo plot for every 190.4

acres to a sample based on a classified pixel every 0.22 acres.

The increased intensity of the phase 1 sample greatly

improved estimates of the area within each stratum,

particularly at the county level. Also, because the classification was conducted using a computer-assisted algorithm

across the entire State, biases in the photo plot sampling

method that resulted from differences in photo quality, age

of photography, and experience of the photo interpreter were

eliminated and classification was consistent across the entire

State.

Phase 2

Phase 2 of the inventory consisted of the measurement of an

annual sample of field plots in Michigan. Current FIA

precision standards for annual inventories require a sampling intensity of one plot for every 5,937 acres. To satisfy

this requirement, the geographical hexagons established for

the Forest Health Monitoring (FHM) program were divided

into 27 smaller NCFIA hexagons, each of which contained

5,937 acres (McRoberts 1999). A grid of field plots was

established by selecting one plot from each of the smaller

hexagons based on the following rules: (1) if an FHM plot

fell within a hexagon, it was selected as the grid plot; (2) if

no FHM plot fell within the hexagon, the existing FIA plot

nearest the hexagon center was selected as the grid plot; and

(3) if neither FHM nor existing NCFIA plots fell within the

hexagon, a new NCFIA plot was established in the hexagon

(McRoberts 1999). This grid of plots is designated the

Federal base sample and is considered an equal probability

sample; its measurement in Michigan is funded by the

Federal government. The State of Michigan supplemented

the Federal base sample and tripled the number of sample

plots across the State.

The total Federal base sample of hexagonal grid plots was

systematically divided into five interpenetrating, nonoverlapping subsamples or panels. Each year the plots in a

single panel are measured with panels selected on a 5-year,

rotating basis (McRoberts 1999). For estimation purposes,

the measurement of each panel of plots may be considered

an independent random sample of all lands in a State. Field

crews measured vegetation on plots in the forested and

straddler (nonforest/forest and forest/nonforest) categories;

plots classified as non-forested were checked to ensure

correct classification.

6
NCFIA has two categories of field measurements—phase 3

(formally FHM plots) and phase 2 field plots to optimize our

ability to collect data when available for measurement. It is

imperative that each type of plot be uniformly distributed

both geographically and temporally. Phase 3 plots are

measured with the full array of vegetative and health

variables collected (Mangold 1998) as well as the full suite

of measures associated with phase 2 plots. Phase 3 plots

must be measured between June 1 and August 30 to

accommodate measurement of non-woody understory

vegetation, ground cover, and other variables. We anticipate

that in Michigan the complete 5-year annual inventory will

involve about 400 phase 3 plots. On the remaining plots,

only variables that can be measured throughout the entire

year are collected. In Michigan, the complete 5-year annual

inventory is expected to involve about 3,500 phase 2

forested plots and about 1,380 phase 2 straddler plots. With

intensification, the number of field plots will be tripled.

The new national FIA 4-point cluster plot design was used

for data collection (fig. 6) in 2000 and will be used in

subsequent years. For all remeasured field plots in the

Federal base sample, the new 4-point cluster plot was

established and measured at the old plot (1993) location. In

addition, the first five subplots of the old 10-point (subplot)

cluster were remeasured in 2000 to estimate change. All

trees previously measured on the five subplots were

remeasured or otherwise accounted for. These measurements form the basis for change estimates between the 1993

7

and 2000 inventories for characteristics such as average

annual net growth, mortality, and removals. Thus, until a

complete cycle of annual inventories for Michigan has been

accomplished, both the new 4-point cluster plots and part

of the old 10-point cluster plots will be measured. If the

anticipated 20 percent of the State is sampled each year, by

the sixth year of annual inventories in Michigan, the new 4-

point cluster plots will begin to be remeasured and the

former plot design will be abandoned. The national plot

design also requires mapping forest conditions on each plot.

Due to the small sample size (20 percent) each year, the

precision associated with change factors such as mortality

will be relatively low. Consequently, change estimates will

not be reported until at least three annual inventories have

been completed, and even then we anticipate that estimates

of change will be limited in detail. When the complete

annual inventory has been implemented in 2005, the full

range of change variables will be available.

The overall plot layout for the new design consists of four

subplots spaced 120 feet apart in a triangular arrangement.

Subplots 2, 3, and 4 are spaced 120 degrees apart. The

center of the new plot is located at the same point as the

center of the previous plot if a previous plot existed within

the sample unit. All trees less than 5.0 inches in diameter at

breast height (d.b.h., or 4.5 feet above ground level) are

measured on a 6.8-foot-radius (1/300 acre) circular

microplot located 12.0 feet due east of the center of each of

the four subplots. Trees with diameters 5 inches and larger

are measured on a 24-foot-radius (1/24 acre) circular

subplot. The forest condition of each subplot is recorded.

Factors that can determine a change in forest condition

from subplot 1 are changes in forest type, stand-size class,

land use, ownership, and density. Each condition that

occurs anywhere on one of the subplots is identified,

described, and mapped if the condition in total meets or

exceeds 1 acre in size (the 1-acre minimum size for a

condition to be identified could include land off the

subplot). Each condition is assigned a condition number,

and condition information is recorded.

Field plot measurements are combined with phase 1

estimates in the compilation process. As additional annual

inventories are completed, tables will be generated for

Figure 6.—Current NCFIA field plot design. publication. In year 5, all statewide inventory summary

1

2

4 3

Plot

center

MAGNETIC

N
Printed on recyclable paper.

tables will be available in both printed and electronic

formats. For additional information, contact:

Program Manager

Forest Inventory and Analysis

North Central Research Station

1992 Folwell Ave.

St. Paul, MN 55108

or

State Forester

Michigan Department of Natural Resources

Forest Management Division

Box 30452

Lansing, MI 48909-7952

LITERATURE CITED

Cunningham, R.N.; Moser, H.C. 1938. Forest area and

timber volumes in the Lake States. Econ. Notes 10. St.

Paul, MN: U.S. Department of Agriculture, Forest

Service, Lake States Experiment Station. 46 p.

Hansen, M.H.; Brand, G.J.; Wendt, D.G.; McRoberts, R.E.

2001. Estimation based on the first cycle of the annual

forest inventory system: methods, preliminary results,

and observations. In: Reams, G.A.; McRoberts, R.E.; Van

Deusen, P.C., eds. Proceedings, 2d annual Forest

Inventory and Analysis symposium; 2000 October 17-

18; Salt Lake City, UT. Gen. Tech. Rep. SRS-47.

Asheville, NC: U. S. Department of Agriculture, Forest

Service, Southern Research Station: 109-113.

Leatherberry, E.C.; Spencer, J.S. 1996. Michigan forest

statistics, 1993. Resour. Bull. NC-170. St. Paul, MN: U.S.

Department of Agriculture, Forest Service, North Central

Forest Experiment Station. 144 p.

Leatherberry, E.C.; Kingsley, N.P.; Birch, T.W. 1998. Private

timberland owners of Michigan, 1994. Resour. Bull. NC191. St. Paul, MN: U.S. Department of Agriculture,

Forest Service, North Central Forest Experiment Station.

84 p.

Mangold, R.D. 1998. Forest health monitoring field

methods guide (National 1998). Research Triangle Park,

NC: U.S. Department of Agriculture, Forest Service,

National Forest Health Monitoring Programs. 429 p.

(Revision 0, April 1998).

McRoberts, R.E. 1999. Joint annual forest inventory and

monitoring system, the North Central perspective.

Journal of Forestry. 97(12): 27-31.

Schmidt, T.S.; Spencer, J.S.; Bertsch, Robin. 1997.

Michigan’s forests 1993: an analysis. Resour. Bull. NC179. St. Paul, MN: U.S. Department of Agriculture,

Forest Service, North Central Forest Experiment Station.

96 p.

Smith, W.B.; Vissage, J.S.; Darr, D.R.; Sheffield, R.M. 2001.

Forest resources of the United States, 1997. Gen. Tech.

Rep. NC-219. St. Paul, MN. U.S. Department of

Agriculture, Forest Service, North Central Research

Station. 190 p.

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