Iowa’s Forests 

2008

Resource Bulletin

NRS-52

United States 

Department of Agriculture

Forest 

Service

Northern 

Research Station
Abstract

The second full annual inventory of Iowa’s forests (2004-2008) reports more than 3 million acres of 

forest land, almost all of which is timberland (98 percent), with an average volume of more than 1,000 

cubic feet of growing stock per acre. American elm and eastern hophornbeam are the most numerous 

tree species, but silver maple and bur oak predominate in terms of live tree volume. Iowa’s forest 

land is comprised of 65 percent sawtimber, 19 percent poletimber, and 16 percent sapling/seedling 

or nonstocked size classes. Average annual net growth of growing-stock trees on Iowa’s timberland 

increased during the past decade to the current estimate of nearly 105 million cubic feet. This report 

includes additional information on forest attributes, land use change, carbon, timber products, and 

forest health. A DVD included in this report includes 1) descriptive information on methods, statistics, 

and quality assurance of data collection, 2) a glossary of terms, 3) tables that summarize quality 

assurance, 4) a core set of tabular estimates for a variety of forest resources, and 5) a Microsoft 

Access database that represents an archive of data used in this report, with tools that allow users to 

produce customized estimates.

Acknowledgments

The authors would like to thank the many individuals who contributed both to the inventory and analysis 

of Iowa’s forest resources. Primary fi eld crew and QA staff over the 2004-2008 fi eld inventory cycle 

included John Benaszeski, James Blehm, Thomas Goff, Glenda Hefty, Peter Koehler, Benjamin Nurre, 

Eric Schneider, Earl Sheehan, Willard Smith, and Brian Wall. Data management personnel included Gary 

Brand, Mark Hatfi eld, Jay Solomakos, James Blehm, and Jeff Wazenegger. Report reviewers included 

Paul J. Tauke, Aron Flickinger, and Tivon Feeley of Iowa’s Department of Natural Resources. 

Published by: For additional copies:

U.S. FOREST SERVICE U.S. Forest Service

11 CAMPUS BLVD SUITE 200 Publications Distribution

NEWTOWN SQUARE, PA 19073-3294 359 Main Road

Delaware, OH 43015-8640

August 2011

Visit our homepage at: http://www.nrs.fs.fed.us

Cover: Forested bluffs and fl oodplain along Mississippi River, northeastern Iowa. Photograph by Linda Haugen, U.S. 

Forest Service.

Manuscript received for publication February 2011

The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of 

race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital or family 

status. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative means for 

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Room 326-W, Whitten Building, 14th and Independence Avenue, SW, Washington, DC 20250-9410, or call (202) 

720-5964 (voice or TDD). USDA is an equal opportunity provider and employer.
Mark D. Nelson, Matt Brewer, Christopher W. Woodall, Charles H. Perry, Grant M. Domke, 

Ronald J. Piva, Cassandra M. Kurtz, W. Keith Moser, Tonya W. Lister, Brett J. Butler, 

Dacia M. Meneguzzo, Patrick D. Miles, Charles J. Barnett, and Dale Gormanson

Contact Author: 

Mark D. Nelson, mdnelson@fs.fed.us

651-649-5104

Mark D. Nelson is a research 

forester with the Forest Inventory 

and Analysis Program, Northern 

Research Station, St. Paul, MN. 

Matt Brewer is the forest 

assessment leader for the Iowa 

Department of Natural Resources, 

Des Moines, IA.

Christopher W. Woodall is a 

research forester with the Forest 

Inventory and Analysis Program, 

Northern Research Station, 

St. Paul, MN.

Charles H. (Hobie) Perry is a 

research soil scientist with the 

Forest Inventory and Analysis 

Program, Northern Research 

Station, St. Paul, MN.

Grant M. Domke is a post-doctoral 

research forester with the Forest 

Inventory and Analysis Program, 

Northern Research Station, St. 

Paul, MN.

Ronald J. Piva is a forester with 

the Forest Inventory and Analysis 

Program, Northern Research 

Station, St. Paul, MN.

Cassandra M. Kurtz is a natural 

resource specialist with the 

Forest Inventory and Analysis 

Program, Northern Research 

Station, St. Paul, MN.

W. Keith Moser is a research 

forester with the Forest Inventory 

and Analysis Program, Northern 

Research Station, St. Paul, MN.

Tonya W. Lister is a forester with 

the Forest Inventory and Analysis 

Program, Northern Research 

Station, Newtown Square, PA.

Brett J. Butler is a research 

forester with the Forest Inventory 

and Analysis Program, Northern 

Research Station, Amherst, MA.

Dacia M. Meneguzzo is a 

research forester with the Forest 

Inventory and Analysis Program, 

Northern Research Station, 

St. Paul, MN.

Patrick D. Miles is a research 

forester with the Forest Inventory 

and Analysis Program, Northern 

Research Station, St. Paul, MN.

Charles J. Barnett is a forester 

with the Forest Inventory and 

Analysis program, Northern 

Research Station, Newtown 

Square, PA.

Dale Gormanson is a forester 

with the Forest Inventory and 

Analysis program, Northern 

Research Station, St. Paul, MN.

Iowa’s Forests 2008

About the Authors
Foreword

This report highlights the current status, ongoing trends, and future direction of Iowa’s forests. 

The Iowa forest inventory is conducted as a cooperative program between the Iowa Department 

of Natural Resources, Division of Forestry, and the Forest Inventory and Analysis program of 

the U.S. Department of Agriculture Forest Service. Periodic inventories of Iowa’s forests were 

conducted in 1954, 1974, and 1990. Starting in 1999, annual inventories were conducted 

wherein a portion of fi eld plots were inventoried each year and after 5 years a full inventory 

was completed. The fi rst Iowa annual inventory covers the period 1999-2003. This second 

annual inventory, which covers the period from 2004-2008, is completed with this report. With 

complete remeasurement of annual inventory plots, we produced better estimates of growth, 

mortality, and removals and detailed land-use change.

Results from this inventory show Iowa’s forests continue to grow more wood than is being 

harvested, providing an important and essential element to our state’s economy, the wood 

industry, and individual woodland owners. Iowa’s forests have expanded to more than 3 million 

acres with an average volume of over 1,000 cubic feet per acre of timberland, providing fi ne 

quality hardwood products, habitat for wildlife, clean water and air, soil protection from erosion, 

and additional ecosystem services. 

Although forest land area continues to expand, there are concerns about Iowa’s forests. The 

average size of forest tracts continues to decrease, and some of these holdings are changing 

ownership, both of which may affect current and future management opportunities. A large and 

growing number of nonnative invasive plants are competing with native vegetation for sunlight 

and nutrients. Maturing forests may lead to changes in tree species composition. These and other 

issues require ongoing monitoring to inform future management decisions. I invite you to read 

and interpret the latest results of Iowa’s forest inventory, to become more interested in our forest 

resources, and to participate in discussions about the future of forests and forestry in Iowa. 

Paul J. Tauke, State Forester, 

Iowa Department of Natural Resources
Contents

Highlights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Forest Features. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Forest Indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Forest Economics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Data Sources and Techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Literature Cited. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Statistics, Methods, and Quality Assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DVD
vi

Photograph courtesy of Tricia Knoot, Iowa State University.
1

On the Plus Side

Statewide forest land area continues to increase at a 

steady rate and now exceeds 3 million acres, 98 percent 

of which is classifi ed as timberland.

More than 96 percent of lands that were forested during 

1999-2003 persisted as forest land during 2004-2008, 

with reversions (gains) to new forest land exceeding 

diversions (losses) by fourfold.

Iowa’s forests contain more than 1 billion trees statewide, 

comprised of at least 61 species.

The quantity of live biomass on Iowa’s timberland has 

increased at a steady rate since 1990, now exceeding one 

hundred million oven-dry tons.

Total growing-stock volume on Iowa’s timberland has 

more than doubled since the 1954 and 1974 inventories, 

to a current statewide estimate of more than 3 billion 

cubic feet.

Forest growth exceeds volume losses from mortality, 

harvest, and land-use change combined.

Tree crowns across Iowa are on average healthy with 

minor levels of dieback.

The volume of sawtimber on Iowa’s timberland now 

exceeds 11 billion board feet (International 1⁄4-inch rule), 

with a decreasing fraction of low-grade trees.

Numbers of standing dead trees (snags) per acre have 

increased slightly between 1999-2003 and 2004-2008, to 

11.8 per acre of forest land, providing important habitat 

features for wildlife.

Iowa’s forests are growing on rich, productive soil, less 

affected by atmospheric pollution than other parts of the 

country.

Problem Areas

For “other eastern soft hardwoods” species group (mostly 

American elm), the total statewide average annual 

mortality is nearly 20 million cubic feet, a rate of 4 

percent per year.

Iowa’s forests are a maturing resource that may eventually 

experience density and age-related changes.

The forest land base of Iowa is highly fragmented due to 

agriculture and development.

Despite the diverse array of native plant species in the 

understory of Iowa’s forests, growing numbers of invasive 

plant species are competing with native vegetation.

The average parcel size of privately owned forest land in 

Iowa continues to decrease with indications that these 

holdings will be changing owners.

A relative scarcity of large coarse woody debris resources 

may reduce wildlife habitat quality for some species. 

The number of wood-processing mills has been steadily 

declining, making it harder for landowners to fi nd 

markets for the timber harvested from management 

activities on their forest land.

Issues to Watch

A majority of Iowa’s forest land lacks interior forest 

conditions and is subject to edge effects, like increasing 

nonnative invasive plants.

Increasing pressures to harvest Iowa’s forests for energy 

may compete with traditional forest product industries, 

and increased harvest utilization rates could possibly 

impact forest sustainability (decreased harvest residues) 

and habitat quality.

Highlights
2

Tree species composition of Iowa’s forests continues to 

change as oak species are slowly supplanted by other 

species.

The emerald ash borer, an exotic beetle, recently entered 

the state, and is expected to kill numerous ash trees in 

Iowa, both in forests and urban environments. Gypsy 

moth and bur oak blight also are expected to cause 

signifi cant damage to Iowa’s forests.

Given the gains in growing-stock volume in larger 

diameter classes for economically important tree species, 

the regeneration of these species should be monitored to 

ensure future sustainability.

The management of forests to maximize carbon 

sequestration may complicate timber management 

practices due to fl uctuating carbon credit markets and 

consideration of all carbon pools in forests (e.g., soil 

organic carbon, dead wood, and belowground stocks).
3

Forested bluffs and fl oodplain along Mississippi River, northeastern Iowa. Photograph by Linda Haugen, U.S. Forest Service.

Background
4

BACKGROUND

 An Overview of Forest 

Inventory

What is a tree?

Trees are perennial woody plants with central stems 

and distinct crowns. In general, the Forest Inventory 

and Analysis (FIA) program of the U.S. Department of 

Agriculture, Forest Service, defi nes a tree as any perennial 

woody plant species that can attain a height of 15 feet at 

maturity. An electronic record of every tree measured in 

this inventory can be found in the database on the DVD 

in the inside back cover of this bulletin.

What is a forest?

The FIA program defi nes forest land as land that is at 

least 10 percent stocked by trees of any size or formerly 

having had such tree cover and not currently developed 

for nonforest use. The treed area must be at least 1 acre 

in size and at least 120 feet wide to qualify as forest land. 

Trees in narrow windbreaks, urban boulevards, orchards, 

and other nonforest situations are very valuable too, but 

they are not described in this report. 

What is timberland, reserved forest 

land, and other forest land?

FIA defi nes three subcategories of forest land: timberland, 

reserved forest land, and other forest land. Reserved forest 

is land that has been withdrawn from timber utilization 

through legislation or administrative regulation. Other 

forest land is typical of poor soils where the forest is 

incapable of producing 20 cubic feet per acre per year at 

the culmination of mean annual increment. Timberland 

is forest land that is not reserved and meets minimum 

productivity requirements. 

During some periodic inventories conducted prior to 

1999, only trees on timberland plots were measured. 

We are therefore unable to report estimates for some 

attributes (e.g., volume) on non-timberland for those 

older inventories. Beginning with the implementation 

of FIA’s annual inventory system during 1999-2003, 

we were able to report forest attributes on all forest 

land—not just timberland. With the remeasurement of 

the same annual inventory plots during 2004-2008, we 

are now able to report growth, removals, and mortality 

on all forest land, whereas for prior inventories we 

could only report growth, removals, and mortality on 

timberland.

How do we estimate forest land area 

and number of trees?

Forest inventory plots have been established throughout 

Iowa, with one plot for every 6,000 acres, or so. Those 

plots located on forest land are measured ‘in the fi eld’, 

and they comprise a statistical sample of observations 

used for estimating various forest attributes. During the 

2004-2008 inventory, 12,547 trees that are at least 1 

inch in diameter at breast height (d.b.h., 4.5 feet above 

the ground) were measured on 659 forest plots in Iowa. 

Unless indicated otherwise, sampling errors reported 

in text and fi gures represent 1 standard error (SE). For 

information on sampling errors, see Statistics, Methods, 

and Quality Assurance on the DVD at the back of this 

bulletin.

How do we estimate a tree’s volume?

Forest inventories typically express volume in cubic feet 

(or cubic meters), but many Iowans are more familiar 

with cords (a stack of wood 8 feet long, 4 feet wide, 

and 4 feet high). A cord of wood, which is a typical 

unit for fi rewood, contains approximately 79 cubic 

feet of solid wood and 49 cubic feet of bark and air. 

Volume can be determined precisely by immersing a 

tree in a pool of water and measuring the amount of 

water displaced. A less precise, but much cheaper and 

easier method has been employed in forest inventories, 

whereby several hundred trees were cut, and detailed 

diameter measurements were taken along their lengths 

to accurately determine their volumes (Hahn 1984). 

Statistical tools were used to model this data by tree 

species group. Using these models, we can estimate 

tree volume based on species, diameter, and tree site 
5

BACKGROUND

index. This method was also used to calculate sawtimber 

volumes. FIA reports sawtimber volumes in International 

1/4-inch board foot scale as well as Doyle rule. To convert 

to the Scribner board foot scale, see Smith (1991).

How much does a tree weigh?

Building on previous work, the U.S. Forest Service’s 

Forest Products Laboratory and others have developed 

specifi c gravity estimates for many tree species (Miles 

and Smith 2009). These specifi c gravities were then 

applied to estimates of tree volume to derive estimates 

of merchantable tree biomass (the weight of the tree’s 

bole). All live tree biomass is estimated by including 

biomass estimates for the stump, limbs, and bark. We do 

not currently report the live biomass in roots or foliage. 

Forest inventory can report biomass as either green 

weight or oven-dry weight. Green weight is the weight of 

a freshly cut tree. Oven-dry weight is the weight of a tree 

with zero percent moisture content. On average 1 ton of 

oven-dry biomass is equal to 1.9 tons of green biomass

How do we estimate all the forest 

carbon pools?

FIA does not measure directly the carbon in standing 

trees; it estimates forest carbon pools by assuming that 

half the biomass in standing live/dead trees consists of 

carbon. Additional carbon pools (e.g., soil, understory 

vegetation, belowground biomass) are modeled based on 

stand/site characteristics (e.g., stand age and forest type).

How do we compare estimates from 

different inventories?

Estimates from new inventories are often compared 

with earlier inventories to determine trends in forest 

resources. This is certainly valid when comparing the 

Iowa 2003 inventory to the 2008 inventory, both of 

which are based on FIA’s ‘annual inventory’ system. 

Comparisons with older periodic inventories, however, 

are problematic because procedures for assigning stand 

characteristics such as forest type and stand size have 

changed as a result of FIA’s ongoing efforts to improve 

the effi ciency, reliability, and national consistency of the 

inventory. Several changes in procedures and defi nitions 

have occurred since the 1990 Iowa inventory. While 

these changes will have minimal impact on statewide 

estimates of forest area, timber volume, and tree biomass, 

they may have signifi cant impacts on plot classifi cation 

variables such as forest type and stand-size class. Some of 

these changes make it inappropriate to directly compare 

the 2008 and 2003 annual inventory tables with those 

published for the 1954, 1974, and 1990 periodic 

inventories. Note that estimates of timberland and forest 

land area reported in the 1954 Iowa report (Thornton 

and Morgan 1959) were subsequently adjusted in the 

1974 Iowa report (Spencer and Jakes 1980) to account 

for changes in defi nitions. Adjusted estimates for the 

1954 inventory are cited within this report. References 

to 1954, 1974, and 1990 periodic inventories each refer 

to that single year of inventory, but references to ‘2003’ 

and ‘2008’ annual inventories refer to the 5-year periods 

ending in those years, e.g., 1999-2003 and 2004-2008.

A word of caution on timberland 

suitability and availability

The FIA program does not attempt to identify which 

lands are actually suitable or available for timber 

harvesting. Land classifi ed by FIA as “timberland” is not 

necessarily suitable or available for timber production, 

but merely has the potential for such production. Actual 

suitability and availability are subject to changing laws, 

economic/market constraints, physical conditions, 

adjacency to human populations, ownership objectives, 

and other factors.

How do we produce maps?

Maps produced by FIA are for graphic display to 

meet general reporting requirements. A geographic 

information system (GIS) and various geospatial datasets 

were used to produce the maps portrayed within fi gures 

of this report. Unless indicated otherwise, forest resource 

data are from FIA and base map layers, e.g., state 

and county boundaries, were obtained from the GIS 
6

BACKGROUND

section of the Iowa Geological and Water Survey, Iowa 

Department of Natural Resources. Depicted FIA plot 

locations are approximate. Sources and intended uses of 

FIA data are available at: http://fi a.fs.fed.us/tools-data/. 

Sources of other geospatial datasets are cited within 

individual fi gures. Iowa maps are portrayed in Universal 

Transverse Mercator projection, Zone 15, North 

American Datum of 1983.
7

 Forest Features

Aerial photograph of forest/agriculture matrix in northeastern Iowa. Photograph by Linda Haugen, U.S. Forest Service.
8

FEATURES

Forest Area

Background

Characteristics of geology and climate affect forest 

distribution, composition, and structure. These 

characteristics are used to delineate ecological regions, 

which are shown in Figure 1 (Griffi th et al. 2008). A 

mixture of land uses occur in the ‘Central Irregular 

Plains’ in the south-central portion of the State, with 

wider forested strips occurring along streams. One-fi fth 

of the ‘Interior River Lowland’, in southeastern Iowa, 

is in forest, including bottomland deciduous forest, 

swamp forest, and mixed oak and oak-hickory forest. 

Flat-bottomed terraced valleys, forested valley slopes, and 

dissected glacial till plains characterize this ecoregion. In 

northeastern Iowa, the ‘Paleozoic Plateau’, also known as 

the ‘Driftless Area’, consists of hilly uplands and deeply 

dissected valleys, containing a signifi cant portion of Iowa’s 

forests, and the majority of its trout streams. Threefourths of the ‘Western Corn Belt Plains’ are used for 

cropland agriculture, with much of the remainder used for 

livestock forage. This region is characterized as nearly level 

to gently rolling, with warm, moist soils that support the 

most productive areas of corn and soybeans in the world. 

Spatial distribution and trends in forest area may indicate 

future forest resource trends resulting from changing 

land use or forest health conditions. Management and 

decisionmaking depend upon forest monitoring for 

providing essential information on forest trends.

What we found

Iowa forest land area is most concentrated in the 

northeastern and southern counties (Fig. 2). After 

declining from 2.3 million acres in 1954 to a low 

of 1.6 million acres in 1970s, area of forest land has 

increased at a steady rate, reaching a current estimate of 

approximately 3 million acres (Fig. 3). Almost all (98 

percent) of Iowa’s forest land is timberland; 1 percent is 

reserved forest land, and 1 percent is other forest land.

Figure 1.—Ecoregions of Iowa. Data sources: Ecoregions - GIS Section of the 

Iowa Geological and Water Survey, Iowa Department of Natural Resources; 

Inset – ESRI Data & Maps. 

0 100 Miles

Projection: UTM Zone 15N, NAD83

Timberland 

Forest land 

0 

500 

1,000 

1,500 

2,000 

2,500 

3,000 

3,500 

1950 1960 1970 1980 1990 2000 2010 

Area (thousand acres) . 

Year of Inventory 

Central Irregular Plains

Interior River Lowland

Paleozoic Plateau

Western Corn Belt Plains

Ecoregion

Figure 2.—County-level forest land area as a percent of total land area, Iowa, 

2008.

Figure 3.—Total forest land and timberland area, Iowa, 1954-2008. Error bars 

represent 1 standard error (68 percent confi dence interval) around estimate.

0 - 5

5.1 - 10

10.1 - 20

20.1 - 40

Percent Forest
9

FEATURES

What this means

At the state level, Iowa’s forest land and timberland 

areas have been gradually increasing since the 1970s. 

Because nearly all forest land in Iowa is timberland, 

subsequent sections of this report that refer to estimates 

on timberland likely are representative of all forest land 

within the State. Relatively large sampling errors are 

associated with the FIA inventory estimates at sub-state 

scales, so care should be given when monitoring forest 

land or timberland area changes at these scales.

Land-use Change

Background

Iowa’s land base is not static through time but is subject 

to ebbs and fl ows. Although total area of forest land has 

shown overall increases over time (Fig. 3), a larger “net” 

difference is occurring due to both reversions (gains) and 

diversions (losses) of forest land (Fig. 4). As some forest 

land is converted to uses such as suburban development, 

other areas of nonforest, such as former agricultural 

land, is reverting back to forest land. Monitoring 

the sustainability of Iowa’s forest resources requires 

information on trends in land use and resulting effects 

on forest resources.

What we found

Ninety-one percent of Iowa land was nonforest during 

both 2003 and 2008, with 7 percent of all land being 

forested during both time periods (Fig. 5). Very little 

land changed from forest to nonforest, but 2 percent of 

land changed from nonforest to forest during this period. 

Of lands that were forested in 2003, over 96 percent 

persisted as forest land in 2008. An estimated 120,000 

acres of forest land diverted to nonforest in 2008, but 

almost 495,000 acres of nonforest reverted to forest. 

While some fraction of this ‘change’ may be an artifact 

of changes in inventory methodology, the trend suggests 

that more land reverted to forest than was being diverted 

from forest. Agricultural land, which dominates Iowa’s 

landscape, is the predominant source of forest reversions 

and diversions (Fig. 6). Large diameter (sawtimber) 

stand-size class predominates forest land diversions, but 

large, medium (poletimber), and small (seedling/sapling) 

stand-size classes all contributed substantially to forest 

land reversions (Fig. 7). 

Figure 4.—Distribution of remeasured inventory plots showing plots that 

remained forest or nonforest and those that changed to and from forest 

between 2003 and 2008, Iowa. Depicted FIA plot locations are approximate.

Figure 5.—Land use status and change, Iowa, 2003 to 2008.

Forest in 2003 and 2008 

Nonforest to forest 

Forest to nonforest 

Nonforest in 2003 and 2008 

7% 2% 

0% 

91% 

Remained nonforest

Remained forest

Forest gain

Forest loss

FIA Remeasured Plots
10

FEATURES

What this means 

Patterns of Iowa land-use changes over the past 10 

years indicate that Iowa has a stable or increasing forest 

land base. For every acre of forest land lost to nonforest 

diversion, approximately 4 more new acres reverted to 

the forest land base. This consistent net increase in the 

Iowa’s total forest land area suggests that conservation of 

the State’s forest land area is continuing. 

Forest Ownership

Background

Ownership patterns are important because forest owners 

ultimately control the forest resources and decide how 

they will be managed. Family forest owners – families, 

individuals, and other unincorporated groups – comprise 

a dominant, diverse, and dynamic group of owners in 

Iowa. By understanding these owners, we can better help 

them meet their forest management needs, and in so 

doing, help conserve Iowa forests for generations to come. 

What we found

From the Missouri River bluffl ands in the west, through 

the small woodlots in the State’s agricultural heartland, 

to the deeply dissected Driftless Area near the Mississippi 

River to the east, forest ownership varies across Iowa 

(Figs. 8, 9). A total of 147,000 family forest owners 

control 85 percent of the forest land in Iowa. To better 

understand who these people are, why they own forest 

land, and how they use it, the Forest Service conducts 

the National Woodland Owner Survey (Butler 2008). 

According to the results of this survey, the average size 

of the forested holdings of these owners is 17 acres (Fig. 

10). The primary reasons for owning forest land are 

related to aesthetics, the land as part of their farm, and 

nature protection (Fig. 11). Although timber production 

is not an important ownership objective, 43 percent 

of the family forest land is owned by people who have 

0 100 200 300 400 500 600

Forest loss

Forest gain

Area (thousand acres)

Agriculture

Developed

Other nonforest

Figure 6.—Other land-use types contributing to gross forest loss (diversions) 

and forest gain (reversions), Iowa, 2003 to 2008.

0 

50 

100 

150 

200 

Large Medium Small Nonstocked 

Forest Land Area (thousand acres) 

Stand-size Class 

0 

50 

100 

150 

200 

Large Medium Small Nonstocked 

Forest Land Area (thousand acres) 

Stand-size Class 

Figure 7.—Forest stand-size classes contributing to gross forest loss (A; 

diversions) and forest gain (B; reversions), Iowa, 2008. 

A

B
11

FEATURES

commercially harvested trees. Eight percent of the land is 

owned by people who have a written management plan, 

and 27 percent of the land is owned by people who have 

received forest management advice. The owners’ major 

concerns are related to trespassing, keeping land intact 

for future generations, and threats from invasive plants. 

Although most family forest owners plan to do little or 

nothing with their land in the next 5 years, 1 in 3 acres is 

owned by someone who plans to harvest fi rewood, 1 in 5 

by someone who plans to harvest timber, and 1 in 10 by 

someone who plans to pass the land on to heirs or sell it. 

What this means

The average parcel size is decreasing and much land will 

soon be changing hands. Family legacy is an important 

ownership objective and also a major concern. What 

can be done to help the landowners and the land? It 

is clear that timber production is not on the forefront 

of landowners’ concerns, but it is also clear that many 

landowners are not adverse to harvesting and other 

activities in the woods. How can natural resource 

85%

3%

4%

6% 2%

Family

Other private

Federal

State

Local

Area

Owners

0

10

20

30

40

50

60

70

1-9 10-19 20-49 50-99 100-199 200-499 500+ 

Percentage 

Size of Forest Holdings (acres) 

Owners

Acres

0 10 20 30 40 50 60 70 

Timber production 

Firewood production 

Land investment 

Other recreation 

Part of home 

Hunting or fishing 

Privacy 

Family legacy 

Nature protection 

Part of farm 

Aesthetics 

Percent 

Forest: private

Forest: public

Nonforest: private

Nonforest: public

Water

Class

Figure 11.—Percentage by area and number of family forests by reason for owning 

forest land, Iowa, 2006. (Percent refers to portion of landowners who ranked each 

objective as very important or important on a seven-point Likert scale.)

Figure 8.—Private and public forest land in Iowa, 2001. Data source: 

ForestOwn_v1 (Nelson et al. 2010).

Figure 9.—Distribution of forest land by public and private ownership, 

Iowa, 2006.

Figure 10.—Area and number of family forests in Iowa by size of forest 

landholdings, Iowa, 2006.
12

FEATURES

professionals better communicate with family forest 

owners and help them better manage their woods? As 

Iowa’s forest is diverse, so too are the people who own 

it. It is important to provide information that meets the 

landowners’ needs. General statistics are good for a broad 

overview, but we need to better understand the different 

types of owners, their attitudes, and their behaviors, as 

well as effective and effi cient ways of communicating 

with them. One such attempt at learning more about 

landowners is through the “Sustaining Family Forests 

Initiative” (SFFI). (For more information, see www.

sustainingfamilyforests.org.)

Biomass

Background

Measures of total biomass and its allocation among stand 

components (e.g., small-diameter trees, limbs, stumps), 

help us understand the components of a forest stand and 

the resources available for different uses (e.g., biofuels, 

wildlife habitat, carbon sequestration). 

What we found

The quantity of live biomass on Iowa timberland has 

increased at a steady rate since 1990, exceeding 100 

million oven-dry tons by 2008 (Fig. 12). This increase 

corresponds to an increase in timberland area, with 

biomass per acre remaining nearly constant (Fig. 12). 

Most forest biomass in 2008 is in tree boles (72 percent), 

followed by tops and limbs, saplings, and stumps (Fig. 

13). The largest amounts of Iowa forest biomass are 

located in the northeast, southeast, and south-central 

portions of the State (Fig. 14). 

0 

5 

10 

15 

20 

25 

30 

35 

40 

45 

0.0 

20,000

40,000

60,000

80,000

100,000

120,000

1985 1990 1995 2000 2005 2010 

Biomass (oven-dry tons/acre) 

Biomass (thousand oven-dry tons) 

Inventory Year 

Total biomass 

Biomass per acre 

17%

7%

4%

72%

Tops and limbs

Saplings

Stumps

Bole

Figure 12.—Live-tree and sapling biomass (oven-dry tons) on timberland, 

Iowa, 1990, 2003, and 2008.

Figure 13.—Tree biomass on timberland by tree component, Iowa, 2008.

Figure 14.—Distribution of aboveground live-tree biomass on forest land, 

Iowa, 2001. Data source: Blackard et al. (2008).

< 35

35 - 55

> 55

Biomass (tons/acre)
13

FEATURES

What this means

The steady rates of increase in forest and timberland area 

and forest growth have resulted in a sustainable statewide 

resource of total forest biomass. Because timberland tree 

boles contain the most tree biomass, the management of 

private forest land strongly infl uences the future of not 

only the biomass resource but also the carbon cycles and 

future wood availability. Given the increasing desire to 

manage forest biomass components for both carbon and 

bioenergy use, the monitoring of Iowa’s forest biomass 

becomes even more critical.

Tree Species Composition

Background

Tree species composition within forests is constantly 

changing, infl uenced by the presence or absence of 

disturbances such as timber management, recreation, 

wildfi re, prescribed burning, extreme weather, and 

invasive species. As a result, the composition of species 

in a forest is an indicator of forest health, growth, 

succession, and need for stand improvement, or other 

management. Knowledge of the distribution of species 

allows for the measurement and prediction of change. 

What we found 

At least 61 tree species were observed on Iowa forest land 

in 2008. In terms of the total number of trees statewide, 

American elm and eastern hophornbeam were the two 

most numerous species, each estimated to have more 

than 100 million trees (Fig. 15). There were no oak 

or hickory species in the 12 most numerous species. 

However, several oak species are included among the 

top 12 in terms of the total statewide live-tree volume 

on forest land, with bur oak being the top oak in terms 

of volume, exceeded only slightly by silver maple, 

with each species containing nearly half a billion cubic 

feet of volume (Fig. 16). Due to large sampling errors 

associated with estimates for some individual tree species, 

comparisons across time are made for groups of related 

species. Most species groups had little or no change in 

total statewide live-tree volume between 2003 and 2008, 

but substantial increases in volume occurred for most 

species groups between 1990 and 2008 (Fig. 17).

0 50 100 150 200 

American elm

Eastern hophornbeam

Hackberry

Boxelder

Shagbark hickory

Red mulberry

Slippery elm

Silver maple

Black cherry

Black walnut

American basswood

Bitternut hickory

Number of Trees (million) 

Species 

0 200 400 600 

Silver maple

Bur oak

Eastern cottonwood

White oak

Black walnut

Northern red oak

American elm

American basswood

Hackberry

Shagbark hickory

Honeylocust

Boxelder

Total Live Volume (million ft3) 

Species

Figure 15.—Top 12 tree species in terms of number (million) of live trees 

on forest land, Iowa, 2008. Error bars represent 1 standard error (68 percent 

confi dence interval) around estimate.

Figure 16.—Top 12 tree species in terms of volume (million cubic feet) of 

live trees on forest land, Iowa, 2008. Error bars represent 1 standard error (68 

percent confi dence interval) around estimate.
14

FEATURES

What this means 

Iowa possesses a great diversity of tree species that will 

undoubtedly fi ll niches vacated by other species as forest 

ecosystems continue to evolve through the decades. As a 

group, select white oaks dominate the State in terms of 

total volume, even though no individual species of oak 

is represented among the top dozen species according to 

tree counts. This shows that many of the oak trees are 

large in size, indicating a maturing forest resource with 

sparse regeneration. In contrast, eastern hophornbeam, 

the second most numerous tree species, was not even 

listed among the top 12 species in terms of volume. 

Repeated inventories reveal that species composition of 

Iowa’s forests is changing, with some species increasing 

their dominance while others are decreasing. 

Tree Density, Size, and 

Stocking

Background

Tree density is the ratio of the number of trees to a 

specifi c area, e.g., trees per acre. Tree size typically is 

referred to in terms of height or diameter. Height is 

affected by a combination of tree species, age, and the 

productivity of a site. Tree diameter is also affected by 

species and age, with tree density or stocking being the 

other major determinant of diameter. Tree diameter is 

easily and accurately measured and is used to assign standsize class. Here, stand-size refers not to the area of a forest 

stand, but to the diameter of trees within a stand. Standsize classes are defi ned as large—where a plurality of 

stocking is in hardwoods 11 inches d.b.h. and larger and 

softwoods 9 inches d.b.h. and larger; medium—where a 

plurality of stocking is in softwood trees from 5 inches 

to 9 inches and hardwood trees from 5 to 11 inches; or 

small—where a plurality of stocking is in trees less than 

5 inches d.b.h. Tree diameter also can be used to 

compute tree basal area—the cross sectional area of a 

tree bole at breast height. 

Stocking provides information on the degree of occupancy 

of land by trees compared with a desired level for balanced 

health and growth. Stocking levels are calculated using a 

combination of number of trees, species, sizes, and spacing. 

Stocking is very important for determining and assessing 

forest management practices. A fully stocked stand 

indicates full utilization of the site. In stands of trees greater 

than 5 inches d.b.h., a fully stocked stand would typically 

have a basal area of greater than 80 square feet per acre. In a 

0 100 200 300 400 500 600 700 

Select white oaks

Other eastern

soft hardwoods

Cottonwood

and aspen

Soft maple

Black walnut

Hickory

Select red oaks

Other red oaks

Basswood

Other eastern

hard hardwoods

Ash

Hard maple

Other eastern

softwoods

Eastern white

and red pine

Other white oaks

Spruce/fir/

other yellow pines

Total Live Volume (million ft3

)

Species Group

2008

2003

1990

Figure 17.—Change in growing-stock volume on timberland between 1990, 

2003, and 2008 by tree species group, Iowa. Error bars represent 1 standard 

error (68 percent confi dence interval) around estimate.
15

FEATURES

seedling/sapling stand a fully stocked stand would indicate 

that the present number of trees is suffi cient to attain a 

basal area of 80 square feet per acre when the trees are more 

than 5 inches in diameter.

What we found

The density of Iowa’s timberland trees increased from 

fewer than 100 growing-stock trees per acre during the 

1950s to just over 300 in 1990, then declined slightly to 

a relatively stable density which now measures about 275 

trees per acre (Fig. 18). Basal area of growing-stock trees 

per acre of timberland increased from about 43 in 1954 

to 62 in 2003, then decreased slightly to the current 

measure of 57.

Nearly two-thirds (65 percent) of forest land in Iowa is 

in large diameter stand-size class, 19 percent is medium 

diameter, and the remaining 16 percent is in either 

small diameter or nonstocked classes. Just over onethird (36 percent) of Iowa’s forest land is fully stocked 

or overstocked, 42 percent is medium-stocked, and 22 

percent is poorly stocked or nonstocked. By combining 

these two classifi cations we see that large diameters 

comprise the majority of all stocking classes in Iowa, 

with the greatest percentage of large diameters in fully 

stocked stands (about 75 percent). Small diameters are 

most prevalent in overstocked stands, and the largest 

percentages of medium diameters are in medium and 

poorly stocked stands (Fig. 19).

What this means

The density and diameter of trees across Iowa provides 

information on the stages of forest stand development 

and stocking levels. Since 1954, both the number of trees 

per acre and basal area per acre increased substantially, 

refl ecting positive stewardship and conservation of Iowa’s 

forest resources. The dynamics between the number of 

trees per acre and average diameter or basal area per acre 

presents evidence of a maturing forest resource across 

Iowa. This trend should be expected to continue until 

stands reach a state of senescence or when affected by 

disturbances, either human-induced (e.g., removals) or 

naturally occurring (e.g., windstorm, fi re). However, 

nearly a quarter of forest land in Iowa is understocked 

or nonstocked, presenting an opportunity for additional 

management.

Growth

Background

A stand’s capacity for growth, i.e., for trees to increase in 

volume, is an indication of the overall condition of the 

forest and more specifi cally of tree vigor, forest health, 

TPA

BA

0 

10 

20 

30 

40 

50 

60 

70 

0 

50 

100 

150 

200 

250 

300 

350 

1950 1960 1970 1980 1990 2000 2010 

Basal Area (ft2) per Acre

Number of Trees (trees per acre) 

Inventory Year 

0

10

20

30

40

50

60

70

80

90

100

Proportion (%) 

Small diameter 

Overstocked Fully stocked Medium stocked Poorly stocked

Medium diameter 

Large diameter 

Figure 18.—Average number of growing-stock trees per acre and basal area 

per acre on timberland in Iowa, 1954-2008.

Figure 19.—Proportion of forest land area by stocking class and stand-size 

class Iowa, 2008.
16

FEATURES

and successional stage. Forest growth is reported as net 

growth, where net growth is equivalent to gross growth 

minus mortality. Average annual net growth represents 

an average for the annual change in volume between the 

two most recent inventories, 2003 and 2008. 

What we found 

Average annual net growth of growing-stock trees on 

Iowa’s timberland decreased from about 68 million 

cubic feet per year in 1954 to approximately 41 million 

cubic feet in both 1974 and 1990, followed by a large 

increase during the past decade to the current estimate 

of nearly 105 million cubic feet in 2008 (Fig. 20). The 

top fi ve species groups in terms of average annual net 

growth were select white oaks, soft maple, black walnut, 

cottonwood, and other eastern soft hardwoods, all of 

which exceeded 10 million cubic feet per year (Fig. 21). 

Other eastern hard hardwoods, ash, black walnut, and 

soft maple species groups all had ratios of annual net 

growth to volume exceeding 5 percent, with hard maple 

and cottonwood exceeding 3 percent, and values of less 

than 3 percent for oaks (Fig. 22).

0 

20,000 

40,000 

60,000 

80,000 

100,000 

120,000 

140,000 

1950 1960 1970 1980 1990 2000 2010 

Annual Growth (thousand ft3/year)

Inventory Year 

0 

4,000 8,000 

12,000 16,000 20,000 

Select white oaks

Soft maple

Black walnut

Cottonwood

and aspen

Other eastern

soft hardwoods

Other eastern

hard hardwoods

Ash

Select red oaks

Hickory

Other red oaks

Basswood

Hard maple

Volume Growth (thousand ft 3

/year) 

Species Group

0 1 2 3 4 5 6 

Other eastern

hard hardwoods

Ash

Black walnut

Soft maple

Hard maple

Cottonwood

and aspen

Basswood

Select red oaks

Select white oaks

Other red oaks

Other eastern

soft hardwoods

Hickory

Ratio of Growth to Volume (%)

Species Group

Figure 20.—Average annual net growth of growing stock on timberland acreage 

in Iowa, 1954-2008. Error bars represent 1 standard error (68 percent confi dence 

interval) around estimate, available only since 1990.

Figure 21.—Top 12 species groups in terms of average annual net growingstock volume growth on timberland in Iowa, 2008. Error bars represent 1 standard 

error (68 percent confi dence interval) around estimate.

Figure 22.—Top 12 species in terms of average annual net volume growth 

expressed as a percent of total growing-stock volume on timberland in Iowa, 

2008.
17

FEATURES

What this means 

The net growth of Iowa’s forests has increased in recent 

decades. Many of the economically desirable tree 

species, such as oaks, maples, and walnut, continue 

to accrue tremendous annual growth. However, 

other eastern hard hardwoods and ash show more 

growth relative to their total volume while oaks show 

comparatively less relative growth.

Mortality

Background 

Tree mortality infl uences a number of forest characteristics 

including health, vigor, and rate of accretion and 

depletion. Mortality can be caused by insects, disease, 

adverse weather, succession, competition, fi re, old age, or 

human or animal activity either alone or in combination. 

Although timber utilization and land clearing also result 

in tree mortality, those actions are accounted for separately 

as ‘harvest removals’ and ‘other removals’, respectively (see 

Removals, page 18). Growing-stock mortality estimates 

represent the average cubic foot volume of sound wood in 

growing-stock trees that died each year as an average for 

the years between inventories.

What we found 

The rate of average annual growing-stock volume 

mortality on Iowa’s timberland was fairly level during 

1954 and 1974 at about 0.6 percent, but has since 

increased to its current level of 1.2 percent (Fig. 23). 

In terms of total statewide average annual mortality, 

the “other eastern soft hardwoods” species group had 

almost 20 million cubic feet, the largest, by far, of any 

species group (Fig. 24). Select white oaks, cottonwood, 

and hickory followed with 3.3, 2.9, and 2.5 million 

cubic feet, respectively. Compared to total growing-stock 

volume on timberland for each species group, other 

eastern soft hardwoods had the largest percentage of 

mortality at 4 percent. All other species had much lower 

percentages of mortality, with hickory, other red oaks, 

select red oaks, basswood, and cottonwood all near 1 

percent (Fig. 25).

0 

0.2 

0.4 

0.6 

0.8 

1 

1.2 

1.4 

1950 1960 1970 1980 1990 2000 2010 

Average Annual Mortality (%/year) 

Inventory Year 

0 5,000 10,000 15,000 20,000 25,000 

Other eastern

soft hardwoods

Select white oaks

Cottonwood

and aspen

Hickory

Other red oaks

Select red oaks

Soft maple

Basswood

Ash

Other eastern

hard hardwoods

Black walnut

Other eastern

softwoods

Volume Mortality (thousand ft3

/year)

Species Group

Figure 23.—Average annual net growing-stock volume mortality by total 

statewide growing-stock volume on timberland in Iowa, 1954-2008.

Figure 24.—Top 12 species groups in terms of average annual net growingstock volume mortality on timberland in Iowa, 2008. Error bars represent 1 

standard error (68 percent confi dence interval) around estimate.
18

FEATURES

What this means 

The levels of tree mortality across Iowa are double the 

rate during the 1950s. Mortality is a natural process 

in forest stands as they develop and change over time. 

However, recent invasions by nonnative pests are 

increasing rates of mortality. Rate of mortality for ‘other 

eastern soft hardwoods’ species group is slightly higher 

than rate of growth. The most numerous tree species 

within this group is American elm; Dutch elm disease 

is a likely culprit affecting this high mortality rate. For 

all other species groups, however, rates of growth exceed 

rates of mortality, often by a substantial margin. Tree 

mortality is a crucial component of overall forest health 

and thus should be monitored into the future.

0 1 2 3 4 

Other eastern

soft hardwoods

Hickory

Other red oaks

Select red oaks

Basswood

Cottonwood

and aspen

Other eastern

softwoods

Select white oaks

Soft maple

Ash

Other eastern

hard hardwoods

Black walnut

Ratio of Mortality to Volume (%) 

Species Group Removals

Background 

Trees are removed from timberland to meet a variety of 

objectives. Changes in the quantity of growing stock 

removed help to identify trends in land-use change 

and forest management. Average annual removals of 

growing-stock volume on timberland can be classifi ed 

as either ‘harvest removals’ or ‘other removals’. As the 

name implies, the category ‘harvest removals’ refers to 

trees harvested for timber products or for other forest 

management purposes, but the long-term land use 

does not change. In contrast, ‘other removals’ refers to 

a change in either land use or protection status, either 

from actual diversion of forest land to a nonforest land 

use or from the reclassifi cation of forest land from 

timberland to ‘reserved’ or ‘unproductive’ forest land. 

Historical inventories were limited to timberland, and 

estimates of removals were limited to ‘annual cut’ of 

sawtimber or growing stock. Because removals generally 

occur on only a small number of plots during each 

inventory, the estimates for removals show greater 

variance than those for growth, mortality, or area which 

are recorded on nearly every plot. Like forest growth, the 

rate at which trees are removed represents the average 

annual growing-stock removals that occurred between 

2003 and 2008.

What we found

Average annual growing-stock removals as a percent 

of total statewide volumes was about 1.6 percent in 

1954, increasing to 4.8 percent in 1974, and returning 

to earlier levels in the current inventory, at 1.5 percent 

(Fig. 26). Most of the removals were focused on species 

groups of select white oaks, soft maple, other eastern soft 

hardwoods, select red oaks, black walnut, and other red 

oaks, all of which had removals in excess of 2 million 

cubic feet per year (Fig. 27). Average annual growingstock harvest removals as a percent of total statewide 

volumes were greatest for hard maple, at 2.3 percent, 

with select red oaks, other eastern hard hardwoods, other 

red oaks, and soft maple species groups all exceeding 

Figure 25.—Top 12 species groups in terms of average annual net growingstock volume mortality as a percent of total growing-stock volume on 

timberland in Iowa, 2008.
19

FEATURES

1 percent per year (Fig. 28). Average annual growingstock ‘other removals’ (associated with land-use change) 

was largest for soft maples, at 3.8 percent, with all other 

species groups below 1 percent (Fig. 29).

0 

0.5 

1 

1.5 

2 

2.5 

3 

3.5 

4 

4.5 

5 

1950 1960 1970 1980 1990 2000 2010 

Average Annual Removals (%/year) 

Inventory Year 

1,000 2,000 3,000 4,000 5,000 6,000 7,000 

0 

Select white oaks

Soft maple

Other eastern

soft hardwoods

Select red oaks

Black walnut

Other red oaks

Cottonwood

and aspen

Hard maple

Other eastern

hard hardwoods

Hickory

Basswood

Ash

Volume of Removals (thousand ft3

/year) 

Species Group

0 0.5 1 1.5 2 2.5 

Hard maple

Select red oaks

Other eastern

hard hardwoods

Other red oaks

Soft maple

Black walnut

Select white oaks

Other eastern

soft hardwoods

Cottonwood

and aspen

Hickory

Basswood

Ash

Ratio of Harvest Removals to Volume (%) 

Species Group

012 3 4

Soft maple

Cottonwood

and aspen

Select white oaks

Other eastern

soft hardwoods

Other red oaks

Ash

Hickory

Other eastern

hard hardwoods

Black walnut

Select red oaks

Hard maple

Basswood

Ratio of Other Removals to Volume (%) 

Species Group

Figure 26.—Average annual removals of growing stock as a percent of total 

statewide growing-stock volume on timberland in Iowa, 1954-2008.

Figure 27.—Top 12 species groups in terms of average annual net growingstock volume removals per acre of timberland in Iowa, 2008. Error bars 

represent 1 standard error (68 percent confi dence interval) around estimate.

Figure 28.—Top 12 species in terms of average annual net volume ‘harvest 

removals’ as a percent of total growing-stock volume on timberland in Iowa, 

2008.

Figure 29.—Top 12 species in terms of average annual net volume ‘other 

removals’ (land-use change) as a percent of total growing-stock volume on 

timberland in Iowa, 2008.
20

FEATURES

What this means 

Removal rates are indicative of both harvest and land-use 

change. Average annual rate of removals for all species 

groups combined (1.5 percent per year) is similar to that 

of mortality (1.2 percent per year). Net growth typically 

averages 3.4 percent, which exceeds the sum of removals 

and mortality. At these rates, it appears as though total 

volumes will continue to increase in Iowa. However, this 

may not apply to local scales or for specifi c species like 

silver maple, which predominates the soft maple species 

group and is the most prevalent species statewide. There 

is an ongoing need to monitor removals on a species-byspecies basis.

Forest Patterns

Background

The fragmentation of forest land areas continues to be 

a major ecological issue worldwide. Fragmentation is 

the process by which large, contiguous tracts of forest 

land are broken down into smaller, more isolated 

forest patches surrounded by nonforest land uses such 

as agriculture or urban development. Furthermore, 

fragmentation results in a loss of interior forest 

conditions and an increase in edge habitat, benefi tting 

some species, but having negative effects on others, 

including the loss of native species and increased 

populations of nonnative invasive species.

What we found

Land cover classifi cation data from the National Land 

Cover Database 2001 (NLCD 2001) (Homer et al. 

2007) was reclassifi ed to create a fi ve-class map of Iowa 

(Fig. 30). Using this map, forest pixels were characterized 

based on their proximity to developed edges, or edges 

due to urban development and agricultural land uses. 

Environmental differences along forest edges, also referred 

to as edge effects, can penetrate into a forest patch some 

distance (Collinge 1996). A commonly used threshold 

for edge effects is 100 to 300 ft, or approximately 30 

to 90 m, after which forest exhibit interior conditions 

(Riemann et al. 2008). Using the high end of 90 m, forest 

pixels were classifi ed as being within 90 m of a developed 

edge or greater than 90 m from a developed edge (Fig. 

31). According to this analysis, nearly three-fourths (72 

percent) of Iowa’s forest land is subject to edge effects and 

lacks interior forest conditions. 

Figure 31.—Forest edge status in Iowa, 2001. Data source: National Land 

Cover Database, 2001 (Homer et al. 2007).

Unit 1

Unit 1

Unit 2

Unit 2

Unit 3

Unit 3

Unit 4

Unit 4

Forest Edge Status

Forest within 300 ft of a 

developed edge

Forest greater than 

300 ft from a 

developed edge
21

FEATURES

What this means

Overall, forest makes up approximately 8 percent of 

Iowa’s land base. How this forest land is arranged across 

the landscape affects ecological processes. Based on the 

map pixel analysis (Fig. 30), the majority of forest was 

classifi ed as edge rather than interior forest, indicating 

that the forest land in Iowa is heavily fragmented. While 

much of the interior forest is found along stream and 

river corridors, the overall forest pattern is not consistent 

across the State. An assessment by FIA unit found that 

Units 1 and 2 contain the bulk of the forest land (84 

percent) and more interior forest, which is critical for 

maintaining biodiversity and healthy populations of 

native plants and wildlife. On the other hand, a higher 

percentage of the forest land in Units 3 and 4 was 

classifi ed as edge habitat and is, therefore, more heavily 

fragmented. This has more serious implications for these 

forest patches, such as higher susceptibility to invasion by 

nonnative invasive species and other negative edge effects.

Figure 30.—Iowa land cover, 2001. Data source: National Land Cover Database, 2001 (Homer et al. 2007).

Developed 

Forest 

Natural nonforest

Agriculture

Water

Land Cover
22

FEATURES
23

Linda Haugen, U.S. Forest Service, combats garlic mustard (Alliaria petiolata), a nonnative invasive plant, on a family forest in northeastern Iowa. Photograph 

by Dennis Haugen, U.S. Forest Service.

Forest Indicators
24

FOREST INDICATORS

Young Forest Habitat

Background 

Iowa forests provide habitat for numerous species of 

mammals, birds, reptiles, and amphibians, as well as 

for fi sh, invertebrates, and plants. Several indicators 

of wildlife habitat abundance can be derived from 

FIA data. Forest composition and structure affect the 

suitability of habitat for each species. Some species 

require interior forest, which is discussed previously 

in ‘Forest Patterns’. Other species depend upon early 

successional forests or the ecotone (edge) between 

different forest stages. Many species require multiple 

structural stages of forests to meet different phases of 

their life history needs. Abundance and trends in these 

structural and successional stages serve as indicators of 

population carrying capacity for wildlife species (Hunter 

et al. 2001). According to the Iowa Wildlife Action 

Plan (IWAP; Iowa Dept. of Nat. Res. n.d.), “[Iowa] 

habitats for birds that inhabit mature forests (warblers 

and thrushes) appear to be increasing at the expense of 

species requiring earlier successional stages (woodcock 

and ruffed grouse).” Both of these species are included 

among the IWAP’s species of greatest conservation need.

What we found 

Peak distribution of small-diameter stand-size class, 

which is an indicator of early successional stages, 

occurred during 1974 (19 percent), falling to a low of 

10 percent of all forest land during the 2003 inventory, 

and increasing moderately to a rate of 13 percent in 

2008 (Fig. 32). Concurrently, distribution of largediameter stand-size class increased from about 40 percent 

during 1954 to about 60 percent during the most recent 

inventories, with medium-diameter forest decreasing 

from about 30 percent during 1954 to about 20 percent 

in the current inventory (Fig. 32). The majority of Iowa 

forest land is in stand-age classes over 40 years. Small 

diameter stand-size class predominates young forests (0-

20 years), but there is more total area of small diameter 

stand-size class in forests older than 

20 years (Fig. 33).

What this means 

Increasing abundance of both small- and large-diameter 

stand-size classes is providing habitat for wildlife species 

at both ends of the successional-structural continuum, 

offset by declining abundance in medium-diameter 

class. Estimates of early successional forest area in Iowa 

provide indicators of habitat abundance for ruffed grouse 

(Bonasa umbellus), a permanent resident, and American 

woodcock (Scolopax minor), a migratory species. 

Iowa Department of Natural Resources has increased 

management of early successional habitat for these and 

other associated species, which appears to be refl ected by 

0

10

20

30

40

50

60

70

80

90

100

1954 1974 1990 2003 2008 

Proportion (%) 

Large diameter 

Medium diameter 

Small diameter 

Nonstocked 

Small diameter 

Medium diameter 

Large diameter 

0 

100 

200 

300 

400 

500 

600 

700 

800 

900 

0 - 20 21 - 40 41 - 60 61 - 80 81 - 100 101 - 150 151 - 200 

Foret Land Area (thousand acres) 

Stand Age (years) 

Figure 32.—Area (percent) of forest land by stand-size class, Iowa, 1954-2008.

Figure 33.—Area of forest land by age class and stand-size class, Iowa, 2008.
25

FOREST INDICATORS

0 4 8 12 16 20

Other eastern

soft hardwoods

Select white oaks

Other eastern

hard hardwoods

Other red oaks

Hickory

Soft maple

Ash

Select red oaks

Eastern noncommercial

hardwoods

Other eastern

softwoods

Cottonwood and aspen

Basswood

Black walnut

Hard maple

Other white oaks

Number of Standing Dead Trees (millions)

Species Group

Figure 34.—Number of standing dead trees by species group for the 12 most 

common standing dead tree species groups, Iowa, 2008.

the recent increase in fraction of small-diameter standsize class in the State. While both stand-size class and 

stand-age class provide indicators of early successional 

forests, the two attributes are not interchangeable and 

are best used in combination.

Standing Dead Trees

Background 

Specifi c features, like nesting cavities and standing dead 

trees (at least 5 inches d.b.h.) provide critical habitat 

components for many forest-associated wildlife species. 

Standing dead trees contain signifi cantly more cavities 

than occur in live trees (Fan et al. 2003). Standing dead 

trees that are large enough to meet habitat requirements 

for wildlife are referred to as ‘snags’. Standing dead trees 

serve as important indicators of wildlife habitat and 

past mortality events and also provide carbon storage. 

And, they serve as sources of down woody material 

(discussed elsewhere in this report), which also provides 

habitat features for wildlife. The number and density of 

standing dead trees, together with decay classes, species, 

and sizes, defi ne an important wildlife habitat feature 

across Iowa’s forests. 

What we found 

During the 2008 inventory, FIA collected data on 

standing dead trees of numerous species and sizes 

in varying stages of decay. According to the current 

inventory data, an estimated 36 million standing dead 

trees are present on Iowa forest land, at a density of 12.6 

per acre on public land, 11.6 per acre on private land, 

and an overall average of 11.8 per acre of forest land, 

compared to an overall average of 10 standing dead 

trees per acre in 2003. Half of these standing dead trees 

are in ‘other eastern soft hardwoods’ species group (Fig. 

34). Relative to the total number of live trees in each 

species group, select and other white oaks, other red 

oaks, and select red oaks topped the list with 8.1, 6.2, 

and 5.8 standing dead trees per 100 live trees (Fig. 35). 

The majority of standing dead trees were between 5 and 

6.9 inches d.b.h.—the smallest diameter class—with 

progressively fewer numbers in the larger diameter classes 

(Fig. 36). The greatest number of standing dead trees 

were estimated for the class of least decay, except for 

the larger diameter classes (15 inches d.b.h. or greater), 

where standing dead trees in intermediate decay classes 

predominated (Fig. 36).
26

FOREST INDICATORS

What this means 

Standing dead trees result from a variety of causes, 

including diseases and insects, weather damage, fi re, 

fl ooding, drought, and competition, and other factors. 

The dominance of standing dead trees in ‘other eastern 

soft hardwoods’ may be attributed to American elm, 

the most numerous species in the State, which has been 

severely impacted by Dutch elm disease. On a perlive-tree basis, however, standing dead trees are more 

dominant in the oak species groups, likely due their 

slower rate of decay and longer persistence. Standing 

dead trees provide areas for foraging, nesting, roosting, 

0 1 2 3 4 5 6 7 8 9

Select and

other white oaks

Other red oaks

Select red oaks

Other eastern

soft hardwoods

Soft maple

Cottonwood and aspen

Other eastern

hard hardwoods

Ash

Other eastern softwoods

Hickory

Basswood

Black walnut

Eastern noncommercial

hardwoods

Hard maple

Number of Standing Dead Trees 

per 100 Live Trees

Species Group

3 - Only limb stubs present 

4 - Few or no limb stubs remain 

5 - No evidence of branches remain 

1 - All limbs and branches present 

2 - Few limbs and no fine branches 

01 23 456

Standing Dead Trees (millions) 

Diameter Class (inches) 

5.0-6.9

23.0+ 

21.0-22.9 

19.0-20.9 

17.0-18.9 

15.0-16.9 

13.0-14.9 

11.0-12.9 

9.0-10.9 

7.0-8.9 

hunting perches, and cavity excavation for wildlife, from 

primary colonizers such as insects, bacteria, and fungi to 

birds, mammals, and reptiles. Most cavity nesting birds 

are insectivores that help to control insect populations. 

The state and federally endangered Indiana bat (Myotis 

sodalis), which has been recorded in 15 southern counties 

and one northeastern Iowa county, roosts primarily in 

the cavities and under exfoliating bark of snags of oak 

and hickory species groups. Providing a variety of forest 

structural stages and retaining specifi c features like snags 

on both private and public lands are ways that forest 

managers maintain the abundance and quality of habitat 

for forest-associated wildlife species in Iowa.

Figure 35.—Number of standing dead trees per 100 live trees for the 12 most 

common standing dead tree species groups, Iowa, 2008.

Figure 36.—Distribution of standing dead trees by decay and diameter 

classes for all dead trees in Iowa, 2008.
27

FOREST INDICATORS

Crown Conditions

Background 

Tree crown condition is one indicator of tree health. Full, 

dense crowns support vigorous tree growth, while sparse 

crowns suggest poor growth conditions. Factors affecting 

crown condition include disturbances such as disease, 

insect activity, and adverse weather; and unfavorable site 

conditions such as nutrient defi ciency, overcrowding, 

and moisture stress or excess. Three components of 

crown condition are monitored by FIA: crown density, 

crown dieback, and sapling crown vigor (Schomaker 

et al. 2007). Crown density is an estimate of crown 

fullness and represents the amount of foliage, branches, 

and reproductive structures that block light through 

the crown. Dieback is a measure of twig and branch 

mortality within the crown. Sapling crown vigor is an 

estimate of the crown condition and health of saplings. 

Crown vigor is based on estimates of crown ratio, 

dieback, and condition of foliage. 

What we found 

The frequency distribution of tree crown density across 

Iowa is normally distributed around a weighted mean of 

39 percent, indicating a slight increase from the previous 

inventory (32 percent; Fig. 37). Crown density of 30 

percent or more is an indicator of good tree vigor. The 

frequency distribution of crown dieback in Iowa’s tree 

crowns is dominated by the 0- to 5-percent class, with 

fewer than 12 percent of Iowa trees exhibiting more than 

10 percent dieback (Fig. 38). From 2003 to 2008, the 

frequency distribution of crown dieback was relatively 

stable with only slight shifts within dieback classes 

(Fig. 38). The vast majority of tree saplings have crown 

vigor of ‘good’ (75 percent) or ‘fair’ (23 percent), with 

an apparent slight decrease in vigor since the previous 

inventory (Fig. 39). 

0

5

10

15

20

25

30

0-10 11-20 21-30 31-40 41-50 51-60 61-70 71-80 81-90 91-100 

Frequency (%) 

Crown Density (%) 

2003

2008

2003

2008

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

Good Fair Poor 

Frequency (%) 

Sapling Crown Vigor (%)

Figure 37.—Crown density frequency distribution, Iowa, 2003 and 2008.

2003

2008

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

0-5 6-10 11-15 16-20 21-25 26-30 30+ 

Frequency (%) 

Crown Dieback (%)

Figure 38.—Crown dieback frequency distribution, Iowa, 2003 and 2008.

Figure 39.—Crown sapling vigor distribution, Iowa, 2003 and 2008.
28

FOREST INDICATORS

What this means 

The overall health of tree crowns across Iowa appears stable, 

as indicated by low levels of crown dieback and moderate 

levels of crown density. Because the condition of tree crowns 

is often the fi rst indicator of impending forest health issues, 

we might conclude that health of individual trees is good 

in Iowa. Given the relatively sparse tree crown sample size, 

specifi c conclusions on individual species or species groups 

cannot be made. The condition of Iowa’s tree crowns should 

continue to be monitored to assess effects of smaller scale 

forest health issues (e.g., recent invasion of emerald ash 

borer) on Iowa’s forest land area. 

Down Woody Materials

Background 

Down woody materials (DWM), in the form of fallen 

trees and branches, fulfi ll a critical ecological niche in 

Iowa’s forests. Down woody materials provide valuable 

wildlife habitat in the form of coarse woody debris, but also 

contribute towards forest fi re hazards via surface woody 

fuels. Components of DWM include fi ne and coarse woody 

debris, slash piles, duff, litter, and shrub/herbs cover and 

height, which are sampled using the line-intersect method, 

point sampling, and fi xed-radius sampling (Woodall and 

Monleon 2008).

What we found 

Fuel loadings of down woody materials (time-lag fuel classes) 

are not exceedingly high in Iowa (Fig. 40). When compared 

to the nearby states of Illinois and Missouri with similar 

forest ecosystems, Iowa’s fuel loadings of all time-lag fuel 

classes are not substantially different (Woodall and Monleon 

2008). The size-class distribution of coarse woody debris 

appears to be heavily skewed (77 percent) toward pieces less 

than 8 inches in diameter at point of intersection with plot 

sampling transects (Fig. 41A). With regards to decay-class 

distribution of coarse woody debris, there appears to be 

moderate stages of coarse woody decay across the State 

(decay classes 2, 3, and 4, totaling 91 percent; Fig. 41B). 

These decay classes are typifi ed by moderate to heavily 

decayed logs that are sometimes structurally sound but 

missing most/all of their bark with extensive sapwood decay. 

There is no strong trend in coarse woody debris volumes per 

acre among classes of live tree density (basal area per acre). 

Most of Iowa’s forests appear to have more than 200 cubic 

feet of coarse woody debris per acre (Fig. 42). 

0 

0.5 

1 

1.5 

2 

2.5 

3 

3.5 

4 

1-hr 10-hr 100-hr 1000+-hr 

 Fuel Loading (tons/acre) 

Fuel-hour classes (hours) 

Illinois

Iowa

Missouri

16%

6% 1%

77%

3.0 - 7.9

Coarse Woody

Diameter (in)

8.0 - 12.9

13.0 - 17.9

18+

25%

1% 8%

26%

40%

1

Decay Class

2

3

4

5

Figure 40.—Mean fuel loadings (tons/acre, time-lag fuel classes) on forest land 

in Iowa and two neighboring states, 2008. Error bars represent 1 standard error 

(68 percent confi dence interval) around estimate.

Figure 41.—Mean proportions of coarse woody debris total pieces per acre by 

transect diameter (inches - A) and decay classes (B) on forest land, Iowa, 2008. 

See Fig. 36 for defi nitions of decay classes.

A

B
29

FOREST INDICATORS

What this means 

The down woody fuel loadings in Iowa’s forests are 

not exceedingly different from those in nearby states. 

Only in times of extreme drought would these fuel 

loadings pose a hazard across the state. Of all down 

woody components, coarse woody debris (i.e., 1,000+-hr 

fuels) comprised the largest amounts. However, coarse 

woody debris volumes were still relatively low and were 

represented by small, moderately decayed pieces. The 

scarcity of large coarse woody debris resources may 

also indicate a lack of high quality wildlife habitat for 

some species. Overall, because fuel loadings are not 

exceedingly high across Iowa, possible fi re dangers are 

outweighed by the benefi ts of down woody material to 

wildlife habitat and carbon sinks.

Soils

Background

The soils that sustain forests are infl uenced by a number 

of factors, including: climate; the trees, shrubs, herbs, 

and animals living there; landscape position; elevation; 

and the passage of time. Climate-soil interactions are 

one signifi cant way that humans infl uence the character 

and quality of the soil and indirectly affect the forest. 

For example, industrial emissions of sulfur and nitrogen 

oxides lead to “acid rain.” The deposition of acids strips 

the soil of important nutrients, notably calcium (Ca) and 

magnesium (Mg). The loss of calcium and magnesium 

results in a shifting balance of soil elements toward 

aluminum (Al) and manganese (Mn). We can use the 

ratios of these elements as a measure of the impact of 

acid deposition on forest soils and tree health.

What we found

A specifi c example helps to illustrate the knowledge 

gained by linking trees to their underlying soil, even 

when these linkages only are sampled on a small number 

of Phase 3 (P3) plots (see documentation on the DVD 

at the back of this bulletin). The uncompacted live 

crown ratio is determined by dividing the live crown 

length by the actual tree length. Larger values are 

associated with healthier trees; low values of this ratio 

can be related to self pruning and shading from other 

tree crowns, but other reasons include defoliation due 

to dieback, and loss of branches due to breakage or 

mortality. (See also the previous section on Crown 

Conditions.) In Iowa, of many different variables 

available, tree diameter and height show the strongest 

relationship with live crown ratio (Fig. 43); smaller trees 

(in diameter and height) have larger live crown ratios. 

The next strongest predictors of live crown ratio are a 

series of soil parameters including the molar ratios of 

Mg:Mn, Ca:Al, and Mg:Al, and the total amount of Al 

and nitrogen (Fig. 43). These soil predictors are more 

important than traditional measures of competition 

and productivity such as basal area and stand-size code. 

Finally, it is important to put Iowa’s forests in the context 

of others around the country. While it is possible to 

identify linkages with soil parameters, and the indicators 

of soil acidifi cation have important predictive power for 

canopy health, these relationships are relatively weak 

in comparison with other states. For example, when 

states are ranked by median Ca:Al ratio, Iowa is near 

the bottom of the list. Iowa’s forests are growing on rich, 

productive soil, less affected by atmospheric pollution 

than other parts of the country.

0 

100 

200 

300 

500 

500 

600 

700 

0.0-30.0 30.1-60.0 60.1-90.0 90.1-120.0 120.1+ 

Coarse Woody Debris 

(cubic feet/acre) 

Live Tree Basal Area (ft2/acre) 

Figure 42.—Mean coarse woody debris volumes (cubic feet/acre) by live tree 

basal area (square feet/acre) class on forest land, Iowa, 2008.
30

FOREST INDICATORS

What this means

Tree species occupy different niches in the landscape. 

This provides a competitive advantage for colonization, 

growth, and reproduction. Atmospheric deposition of 

different compounds changes the soil substrate through 

additions and/or removals of nutrients and pollutants. 

These changes in the soil infl uence the ability of existing 

trees to thrive and reproduce in their current locations, 

as well as the ability of other trees to colonize new 

landscapes. It is important to document and understand 

natural and anthropogenic processes in the soil since 

they profoundly infl uence the current forest and success 

of future forest management plans. By studying the 

trees species found on P3 plots, it is possible to evaluate 

tree:soil interactions with some statistical rigor. The 

number of P3 plots in Iowa is small, so the power of this 

evaluation it limited. However, the data are consistent 

with observations in other states. The level of soil 

acidifi cation in Iowa is low, particularly in comparison to 

many eastern states. 

Understory Vegetation: 

Species Diversity and 

Nonnative Invasive Plants

Background

Trees dominate the overstory of Iowa’s forested ecosystems, 

but understory vegetation also plays a critical role in aiding 

the regulation of microclimate, helping to limit runoff, and 

providing wildlife habitat cover and forage. Collecting data 

on understory vegetation can assist managers in assessing 

soil nutrient and moisture availability, habitat quality, and 

stand structure. Furthermore, the data provide information 

on the presence of invasive or nonnative species. The 

plant community is invaluable for forest fauna, especially 

those wildlife species requiring the presence of a host plant 

species for survival. To further our understanding of plant 

communities, FIA assessed data from plant communities in 

Iowa from 2007 through 2008 on P3 plots (approximately 

6.25 percent of fi eld plots) and from 2007 through 2008 

on Phase 2 (P2) ‘Invasive’ plots (approximately 20 percent 

of fi eld plots).

What we found

P3 vegetation data were collected in 2007 through 2008 

from 15 P3 plots, where 301 plant species were observed. 

Classifi ed species were categorized based on the 

classifi cation system of the USDA Natural Resources 

Conservation Service (NRCS) PLANTS database 

(USDA NRCS 2011). The greatest number of classifi ed 

species were categorized as forbs/herbs (196), followed 

by graminoids (grass or grass-like plants; 33) (Table 1). 

Analyzing broader growth habit categories, there were 41 

trees, 22 shrubs, and 19 vines. Of the 301 plant species, 

220 (73.1 percent) are native to the United States and 

55 (18.3 percent) are introduced (Table 2). The most 

commonly observed plant species was common dandelion 

(13 plots) followed by common hackberry, American 

elm, and eastern prickly gooseberry, which were each 

observed on 12 plots (Table 3). Table 4 provides a list 

of common and scientifi c names for 43 invasive plant 

species inventoried on FIA P2 Invasive plots across the 

0 2 4 6 8 10 12 14 

Stand-size Code

Basal Area

Total N

Exchangeable Al

Mg:Al molar ratio

Ca:Al molar ratio

Mg:Mn molar ratio

Height

Diameter

Percent of Error Explained 

Variable 

Figure 43.—Percent of error in measurements of uncompacted live crown 

ratios explained by various predictors.
31

FOREST INDICATORS

Common dandelion (13) 1,724 310

Common hackberry (12) 1,643 364

American elm (12) 1,657 265

Eastern prickly gooseberry (12) 1,674 376

Sedge (11) 1,608 292

Riverbank grape (11) 1,724 369

Violet (11) 1,587 350

Virginia creeper (11) 1,792 397

Bristly greenbrier (10) 1,581 404

Stinging nettle (10) 1,766 375

Bitternut hickory (9) 1,452 347

Eastern redcedar (9) 1,968 438

Pennsylvania pellitory (9) 1,795 391

Slippery elm (9) 1,632 399

Common yellow oxalis (9) 1,398 342

White avens (9) 1,852 254

Great ragweed (8) 1,348 328

Littleleaf buttercup (8) 2,217 279

Kentucky bluegrass (8) 1,385 356

Blackseed plantain (8) 1,133 276

Canadian clearweed (8) 778 310

Stickywilly (8) 1,565 186

Green ash (8) 1,538 288

Canadian honewort (7) 1,585 169

American basswood (7) 1,759 345

Eastern waterleaf (7) 2,071 246

American lopseed (7) 2,007 364

Red mulberry (7) 1,510 460

Coralberry (7) 1,925 400

Black walnut (7) 1,681 459

Fragrant bedstraw (7) 1,889 236

Table 3.—The 33 most numerous plant species or undifferentiated genera 

found on Iowa P3 plots, the number of plots found on (in parentheses), and the 

mean number of tree seedlings and saplings per acre on those plots, 2007-

2008.

 Tree seedlings Tree saplings

Species per acre per acre

NRS region. Of the 33 most commonly observed species 

in Iowa, nine were trees and included none of the 43 

invasive species of interest (Table 4). Common dandelion 

was determined to be the most commonly occurring 

nonnative plant species (13 plots), followed by stinging 

nettle (10 plots) and Kentucky bluegrass (8 plots; Table 5). 

The majority of nonnative plant species fell in the forb/

herb category and only two of these species were of woody 

growth form. (Scientifi c names of invasive species are listed 

in Table 4.)

 Number of 

 Origin Species Percent

Table 2.—Number of species in Iowa on P3 plots by domestic or foreign origin 

(USDA NRCS 2011), 2007-2008.

 Cultivated or not in the U.S. 1 0.3

 Introduced to the U.S. 55 18.3 

 Native and Introduced to the U.S. 8 2.7

 Native to the U.S. 220 73.1

 Probably introduced to the U.S. 1 0.3

 Unclassifi ed 16 5.3

 

 Forb/herb 169 56.1

 Graminoid 33 11.0

 Shrub 9 3.0

 Shrub, subshrub, vine 3 1.0

 Subshrub, forb/herb 3 1.0

 Subshrub, shrub 3 1.0

 Subshrub, shrub, forb/herb 4 1.3

 Tree 21 7.0

 Tree, shrub 18 6.0

 Tree, shrub, subshrub 2 0.7

 Vine 2 0.7

 Vine, forb/herb 12 4.0

 Vine, shrub 1 0.3

 Vine, subshrub 2 0.7

 Vine, subshrub, forb/herb 2 0.7

 Unclassifi ed 17 5.6

 Total 301

Table 1.—Number of species on Iowa P3 plots by growth habit (USDA NRCS 

2011), 2007-2008.

Number of Species or

 Growth Habit Undifferentiated Genuses Percent
32

FOREST INDICATORS

Tree Species

Acer platanoides (Norway maple) 

Ailanthus altissima (tree-of-heaven) 

Albizia julibrissin (silktree) 

Elaeagnus angustifolia (Russian olive) 

Melaleuca quinquenervia (punktree) 

Melia azedarach (Chinaberry) 

Paulownia tomentosa (princesstree) 

Robinia pseudoacacia (black locust) 

Tamarix ramosissima (saltcedar) 

Triadica sebifera (tallow tree) 

Ulmus pumila (Siberian elm) 

Woody Species

Berberis thunbergii (Japanese barberry)

Berberis vulgaris (common barberry)

Elaeagnus umbellata (autumn olive)

Frangula alnus (glossy buckthorn)

Ligustrum vulgare (European privet)

Lonicera x. bella (showy fl y honeysuckle)

Lonicera maackii (Amur honeysuckle)

Lonicera morrowii (Morrow’s honeysuckle)

Lonicera tatarica (Tatarian honeysuckle)

Rhamnus cathartica (common buckthorn)

Rosa multifl ora (multifl ora rose)

Spiraea japonica (Japanese meadowsweet)

Viburnum opulus (European cranberrybush)

Vine Species

Celastrus orbiculatus (Asian bittersweet)

Hedera helix (English ivy)

Lonicera japonica (Japanese honeysuckle)

Herbaceous Species

Alliaria petiolata (garlic mustard)

Centaurea biebersteinii (spotted knapweed)

Cirsium arvense (Canada thistle)

Cirsium vulgare (bull thistle)

Cynanchum louiseae (Louise’s swallow-wort)

Cynanchum rossicum (European swallow-wort)

Euphorbia esula (leafy spurge)

Hesperis matronalis (dames rocket)

Lysimachia nummularia (creeping jenny)

Lythrum salicaria (purple loosestrife)

Polygonum cuspidatum (Japanese knotweed)

Polygonum x. bohemicum (P. cuspidatum/P. sachalinense hybrid)

Polygonum sachalinense (giant knotweed)

Grass Species

Microstegium vimineum (Nepalese browntop)

Phalaris arundinacea (reed canarygrass)

Phragmites australis (common reed)

Table 4.—Invasive plant species target list for NRS Forest Inventory and 

Analysis P2 Invasive plots, 2007 to present.

Common dandelion (13) 1,724 310

Stinging nettle (10) 1,766 375

Kentucky bluegrass (8) 1,385 356

Smooth brome (5) 2,686 459

Lambsquarters (5) 915 390

Garlic mustard (4) 1,267 341

Lesser burdock (4) 1,195 169

Curly dock (4) 1,136 567

Multifl ora rose (4) 1,897 352

Ground ivy (4) 413 76

Garden yellowrocket (3) 151 101

Common mullein (3) 1,389 705

White clover (3) 525 50

Red clover (3) 575 400

Common chickweed (3) 551 101

Common buckthorn (3) 1,593 225

Catnip (3) 1,489 405

Common motherwort (3) 1,181 446

Nodding plumeless thistle (3) 965 406

Common yarrow (3) 575 400

Common hop (3) 1,958 118

Table 5.—The 21 most commonly occurring nonnative plant species found on 

Iowa P3 plots, 2007-2008 the number of plots found on (in parentheses), and 

the mean number of tree seedlings and saplings per acre on those plots. Some 

plots may have multiple nonnative plant species and thus may be counted 

more than once.

 Tree seedlings Tree saplings

Species per acre per acre

For 2007 through 2008, FIA collected data from 39 

P2 “Invasive” plots in Iowa, on which multifl ora rose 

was the most commonly observed species (17 plots; 

Table 6; Fig. 44). Reed canarygrass was the second most 

common invasive species, occurring on 12 plots (30.8 

percent of P2 Invasive plots). Seven of the top 10 invasive 

species observed were of woody growth form. The high 

percentage of woody plants is likely infl uenced by the high 

proportion of the 43 target species (Table 4) that are of 

woody growth form. A large number of nonnative and 

invasive plants were detected on forested plots in the state 

of Iowa. Comparing Iowa with neighboring Missouri, 

18.3 percent of the species on P3 plots in Iowa were 

introduced (Table 2) versus 10.6 percent of the species in 

Missouri (Raeker et al., in press). The spatial distribution 

of multifl ora rose and 14 other species on P2 Invasive 

plots is portrayed in Figure 45. 
33

FOREST INDICATORS

Table 6.—Invasive species found on Iowa P2 Invasive plots, 2007-2008. 

Fifteen of the 43 invasive species FIA monitors were found in Iowa and are 

listed with the number of plots found on (in parentheses) and the mean number 

of tree seedlings and saplings per acre on those plots, 2007-2008.

Multifl ora rose (17) 368 2,386

Reed canarygrass (12) 286 1,236

Garlic mustard (7) 297 1,870

Amur honeysuckle (7) 385 2,578

Common buckthorn (5) 204 2,171

Autumn olive (4) 176 3,343

Morrow’s honeysuckle (4) 281 1,570

Japanese barberry (3) 613 1,459

Dames rocket (2) 37 187

Siberian elm (2) 66 3,156

Showy fl y honeysuckle (2) 173 3,039

Oriental bittersweet (1) 565 404

Bull thistle (1) 150 375

Tatarian honeysuckle (1) 132 4,738

Spotted knapweed (1) 0 1,574

 Tree seedlings Tree saplings

Species per acre per acre

Figure 44.—Multifl ora rose (Rosa multifl ora). Photograph by James H. Miller, 

U.S. Forest Service, Bugwood.org.

Plant Species

Multifl ora rose

Other

Forest Land

Figure 45.—Distribution of multifl ora rose (Rosa multifl ora) and other invasive 

plant species, Iowa, 2007-2008.

What this means

Iowa’s forests possess a diverse understory with plant species 

spread across fi ve growth habits (forb/herb, graminoid, 

shrub, tree, and vine). The presence of nonnative plant 

species in these forests causes concern as they are able 

to alter forested ecosystems by displacing native species 

and impacting the animal inhabitants that depend upon 

them. A contributing factor to the invasion of the forests 

is likely the high level of fragmentation (see Forest Patterns 

section, page 20) and anthropogenic infl uence. When 

invasive plant species outcompete native tree regeneration, 

they can negatively affect the carbon budget by reducing 

future tree cover. Furthermore, the establishment of and 

dominance by invasive plants can cause negative economic 

implications by reducing timber yields. Remeasurement of 

the P2 Invasive and P3 plots may discern infl uential site 

and regional trends. Understanding the importance of these 

characteristics will enable concerned individuals to better 

understand the forest understory, evaluate the threat of 

invasion, and determine future ecosystem response. Future 

remeasurement of these plots will help determine biotic and 

abiotic trends that infl uence the presence of the invasive 

plant species. This ongoing work will allow forest managers 

and concerned individuals to focus their prevention and 

control efforts on areas of increased invasion risk.
34

FOREST INDICATORS
35

Forest Economics

Maple timber harvest, northeastern Iowa. Photograph by Linda Haugen, U.S. Forest Service.
36

FOREST ECONOMICS

Growing-Stock Volume

Background

Growing-stock volume is the amount of sound wood in 

live, commercial tree species for trees of at least 5 inches 

d.b.h. and free of defect. This measure has long been 

used to assess wood volume available for commercial 

use, which is an important consideration in economic 

planning and evaluations of forest sustainability. 

What we found

Total growing-stock volume on Iowa’s timberland has 

more than doubled since the 1954 and 1974 inventories, 

to a current statewide estimate of over 3 billion cubic 

feet (Fig. 46). Almost all economically important 

species groups have shown signifi cant increases since 

1990, with the exception of select red oaks species 

group, which appears to be fairly stable (Fig. 47). There 

appear to be no signifi cant differences in growing-stock 

volumes between 2003 and 2008 inventories, with the 

possible exception of black walnut species group, which 

appears to have a slight increase. By viewing changes 

in growing-stock volume by d.b.h. classes, we can see 

more differences between selected species groups (Fig. 

48). In general, diameter distributions from the 2003 

and 2008 inventories are similar, with more substantial 

changes occurring between 1990 and the two most 

recent inventories. Increase in the smallest diameter class 

(5.0-10.9 inches) is most noticeable for ‘other eastern 

soft hardwoods’ species group, which is predominated 

by elms and hackberry species. Black walnut, soft maple, 

other eastern soft hardwoods, and hickory species groups 

all showed substantial increases within the 11.0-16.9 

inch diameter class. Cottonwood and aspen, and select 

white oaks exhibit the largest increase in the largest 

diameter class, 23.0+ inches (Fig. 48).

0 

500 

1,000 

1,500 

2,000 

2,500 

3,000 

3,500 

1950 1960 1970 1980 1990 2000 2010 2020 

Growing Stock Volume (million ft3

)

Inventory Year 

0 200 400 600 800 

Select white oaks 

Other eastern

soft hardwoods

Cottonwood

and aspen

Soft maple 

Black walnut 

Hickory 

Select red oaks 

Other red oaks 

Basswood 

Other eastern

hard hardwoods

Ash 

Hard maple 

Growing-stock Volume (million ft3

) 

Species Group 

1990

2003

2008

Figure 46.—Total growing-stocking volume (million cubic feet) on timberland, 

Iowa, 1954-2008. Error bars represent 1 standard error (68 percent confi dence 

interval) around estimate, available only since 1990.

Figure 47.—Total growing-stock volume on timberland by selected species 

groups, Iowa, 1990-2008. Error bars represent 1 standard error (68 percent 

confi dence interval) around estimate.
37

FOREST ECONOMICS

What this means

Growing-stock volume of economically critical species 

groups on timberland has increased substantially 

since Iowa’s earliest statewide inventories in 1954 and 

1974. None of the top 12 species groups in terms of 

volume shows signifi cant volume decreases. Volume 

increases vary both by species group and diameter class. 

Sustainability issues (e.g., regeneration) of mature forest 

stands containing economically vital tree species should 

continue to be monitored into the future. 

Sawtimber Volume

Background

Sawtimber trees are live trees of commercial species 

that contain either one 12-foot or two noncontiguous 

8-foot logs that are free of defect. Hardwoods must be 

at least 11 inches d.b.h. and softwoods must be 9 inches 

d.b.h. to qualify as sawtimber. Sawtimber volume is 

defi ned as the net volume of the saw log portion of live 

sawtimber, measured in board feet, from a 1-foot stump 

to minimum top diameter (9 inches for hardwoods and 

7 inches for softwoods). Estimates of sawtimber volume, 

expressed as board feet (International 1⁄4-inch rule), are 

used to determine the monetary value of wood volume 

and to identify the quantity of merchantable wood. 

What we found

Total statewide board-foot volume of sawtimber on 

timberland decreased slightly from 1954 to 1974, 

increased slightly in 1990, then increased substantially to 

the current inventory estimate of over 11 billion board 

feet (Fig. 49). Almost all of the nine most prevalent 

species groups in terms of total sawtimber volume 

demonstrated tremendous gains since 1990 (Fig. 50). 

Black walnut sawtimber increased most, followed by 

seven other species groups, all with increases exceeding 

0

40

80

120

160

200

5.0-10.9

11.0-16.9

17.0-22.9

23.0+

5.0-10.9

11.0-16.9

17.0-22.9

23.0+

1990 2003 2008

0 

50 

100 

150 

200 

250 

Select White Oaks

5.0-10.9

11.0-16.9

17.0-22.9

Growing Stock Volume (million ft

23.0+

3

)

5.0-10.9

11.0-16.9

17.0-22.9

23.0+

0 

50 

100 

150 

200 

250 

0

60

40

20

80

100

120

140

0

60

40

20

80

100

120

Cottonwood and Aspen

Black Walnut Hickory

Soft Maple

Other Eastern Soft Hardwoods

0

20

40

60

80

100

120

140

5.0-10.9

11.0-16.9

17.0-22.9

23.0+

Diameter Class (inches) 

5.0-10.9

11.0-16.9

17.0-22.9

23.0+

Diameter Class (inches) 

Figure 48.—Growing-stock volume by 6-inch d.b.h. classes on timberland, 

Iowa, 1990-2008, for top six species groups: select white oaks, other eastern 

soft hardwoods, cottonwood and aspen, soft maple, black walnut, and hickory.

0 

2 

4 

6 

8 

10 

12 

14 

1950 1960 1970 1980 1990 2000 2010 

Volume (billion board ft) 

Inventory Year 

Figure 49.—Volume of sawtimber on Iowa’s timberland, 1954 to 2008. 

Error bars represent 1 standard error (68 percent confi dence interval) around 

estimate, available only since 1990.
38

FOREST ECONOMICS

50 percent, and select red oaks showed the smallest 

increase. Relative to total sawtimber volume on 

timberland, net annual growth dropped from a high of 

4.5 percent in 1954 to a low of 2.3 percent during 1974, 

followed by a return to previous levels in subsequent 

inventories, with a current rate of 3.8 percent. Removal 

rates reveal an opposite pattern, with an increase in 1974 

followed by a substantial decrease to the current level of 

1.6 percent. During this same period, average annual 

mortality has increased gradually from a low of 0.5 percent 

in 1954 to the current rate of 1.1 percent (Fig. 51). 

What this means

Sawtimber volume resources on Iowa’s timberland have 

increased steadily since 1974, with annual rates of net 

growth exceeding mortality and removals combined, 

resulting in an overall annual rate of increase exceeding 1 

percent since 1990. This dynamic indicates that sawtimber 

on timberland is a sustainable component of the forest 

ecosystem. If current trends were to continue, one would 

expect increasing sawtimber volumes into the foreseeable 

future. However, this assumption may not hold locally, or 

following unexpected catastrophic forest disturbances (e.g., 

storms or future mortality from invasive insects or diseases).

Sawtimber Quality

Background

The economic value of Iowa’s sawtimber lies not only in its 

absolute amount (volume) but also in its quality. The high 

quality of hardwood sawtimber across the State indicates its 

value and supports Iowa’s forest products industries. 

What we found

Although grading techniques have changed since previous 

inventories, it appears that Iowa’s forests have had a major 

increase in total volume of high quality sawtimber in recent 

decades. 

The distribution of sawtimber appears to have increased 

within the highest tree grade (grade 1), from 13 percent 

in 1990 to 20 percent in the current inventory (Fig. 52). 

Other tree grades have shown more moderate fl uctuations. 

Comparisons of recent volumes by grade to inventories 

from 1990 and earlier are challenging because some 

sawtimber was not graded during those earlier inventories. 

Sawtimber in grades 1 and 2 combined totaled 4.7 billion 

board feet during both the 2003 and 2008 inventories, with 

stable or decreasing volumes for some species groups, but 

increasing volumes for others species groups, e.g., other red 

oaks and hickory (Fig. 53).

0 50 100 150 200 250 300 350 

Select red oaks

Basswood

Select white oaks

Hickory

Soft maple

Other eastern

soft hardwoods

Cottonwood and aspen

Other red oaks

Black walnut

Change 1990-2008 (% increase) 

Species Group

Figure 50.—Percent change in total sawtimber volume on timberland for nine 

most prevalent species groups in Iowa, 1990 to 2008.

Annual net growth 

Annual removals 

Annual mortality 

0.0 

0.5 

1.0 

1.5 

2.0 

2.5 

3.0 

3.5 

4.0 

4.5 

5.0 

1950 1960 1970 1980 1990 2000 2010 2020 

Annual Sawtimber Estimate / 

Total Volume Estimate (%) 

Inventory Year 

Figure 51.—Total average annual net growth, removals, and mortality as a 

percent of total sawtimber volume on Iowa’s timberland, 1954 to 2008.
39

FOREST ECONOMICS

What this means

Iowa’s sawtimber quality has been increasing for decades, 

with substantial increases in the amount of higher grade 

sawtimber. In recent inventories, changes in sawtimber 

grades have varied among species groups, with gains likely 

due to individual tree growth increasing the sawtimber 

volume and tree grades. Overall, it appears that the quality 

of Iowa’s sawtimber resource has increased from past 

decades and is currently stable.

Carbon stocks

Background

Collectively, forest ecosystems represent the largest 

terrestrial carbon sink on earth. The accumulation of 

carbon in forests (known as sequestration) helps to 

mitigate emissions of carbon dioxide to the atmosphere 

from sources such as forest fi res and burning of fossil fuels. 

The FIA program does not directly measure forest carbon 

stocks in Iowa. Instead, a combination of empirically 

derived carbon estimates (e.g., standing live trees) and 

models (e.g., soil organic carbon based on stand age and 

forest type) are used to estimate Iowa’s forest carbon 

stocks. Estimation procedures are detailed by Smith et al. 

(2006). Understory above- and belowground estimates 

include the carbon mass (tons per acre) of seedlings, 

shrubs, and bushes.

What we found

Carbon stocks have increased substantially in Iowa over 

the last several years from 152 million tons in 2003 to 

more than 170 million tons in 2008. Soil organic matter 

(SOM) represents the largest forest ecosystem carbon stock 

in the state at almost 76 million tons, followed by live tree 

and sapling carbon at 68 million tons (Fig. 54). Within 

the live tree and sapling pool, merchantable boles contain 

the bulk of the carbon (41 million tons) followed by roots 

(11 million tons) and tops and limbs (10 million tons). 

The majority of Iowa’s forest carbon stocks are found in 

13%

27%

38%

22%

26% 33%

22% 19%

1

2

3

4

Tree Grade

1

2

3

4

23%

36%

21% 20%

1

2

3

4

A 

B 

C 

Tree Grade

Tree Grade

-40 -20 0 20 40 60 80 100 

Soft maple

Hard maple

Select red oaks

Ash

Basswood

Other eastern soft hardwoods

Cottonwood and aspen

Black walnut

Select white oaks

Hickory

Other red oaks

Change in Sawtimber Volume (%) 

Species Group

Figure 52.—Distribution of sawtimber volume on timberland by tree grade 

(percent), Iowa, 1990 (A); 2003 (B); and 2008 (C).

Figure 53.—Percent change in high grade sawtimber volume (grades 1 and 

2) for selected hardwood species groups on timberland in Iowa between 2003 

and 2008.
40

FOREST ECONOMICS

relatively young stands aged 41-60 years (Fig. 55). Early 

in stand development, the majority of forest ecosystem 

carbon is in the SOM and belowground tree components. 

As forest stands mature, the ratio of above- to belowground 

carbon shifts and by age 61-80 years the aboveground 

components represent the majority of ecosystem carbon. 

This trend continues well into stand development as carbon 

accumulates in live and dead aboveground components. 

A look at carbon by forest-type group on a per-unit-area 

basis found that the elm/ash/cottonwood group contains 

the most carbon per acre of the types represented in Iowa 

(Fig. 56). Not surprisingly, the majority of carbon in each 

forest-type group was in the SOM followed by carbon in 

live biomass. 

What this means

Carbon stocks in Iowa’s forests have increased 

substantially over the last several decades. The majority 

of forest carbon in the State is found in relatively young 

stands dominated by moderately long-lived species. 

This suggests that Iowa’s forest carbon will continue 

to increase as stands mature and accumulate carbon in 

above- and belowground components. Given the ageclass structure and species composition of forests in Iowa, 

there are many opportunities to increase forest carbon 

stocks. That said, managing for carbon in combination 

with other land management objectives will require 

careful planning and creative silviculture beyond simply 

managing to maximize growth and yield.

1% 3% 4%

45%

6%

Live trees and saplings 

Soil organic matter

Total understory

Standing dead

Down dead

Litter

Saplings

Top and limbs

Merchantable boles

Stumps

Roots

7% 

1% 

24% 

6% 

2% 

Total Forest Carbon Stocks - 170 million short tons 

Aboveground 

Belowground 

77

59

51

46

41 41

23

41

49

54

59

59

0 

200,000 

400,000 

600,000 

800,000 

1,000,000 

1,200,000 

0 

10 

20 

30 

40 

50 

60 

70 

0-20 21-40 41-60 61-80 81-100 100+ 

Acres 

Short Tons (millions) 

Age Class (years) 

Acres 

0 10 20 30 40 50 60 70 80 

Elm/ash/cottonwood

675,294 acres

Maple/birch

62,430 acres

Oak/hickory

2,093,305 acres

Aspen/birch

6,420 acres

Nonstocked

76,652 acres

Oak/pine

81,407 acres

Other exotic hardwoods

23,548 acres

Exotic hardwoods

11,249 acres

Other hardwoods

 24,832 acres

C Stocks Per Acre (short tons) 

Forest-type Group

C in biomass

C in dead wood

C in litter

C in soil

Figure 54.—Estimated total carbon (C) stocks on forest land by forest 

ecosystem component, Iowa, 2008.

Figure 55.—Estimated percentage of above- and belowground carbon (C) 

stocks by stand-age class, Iowa, 2008.

Figure 56.—Estimated carbon (C) stocks on forest land by forest-type group 

and carbon pool per acre, Iowa, Estimated above- and belowground carbon 

stocks by stand age class, Iowa, 2008.
41

FOREST ECONOMICS

Timber Products Output

Background

The harvesting and processing of timber products 

produces a stream of income shared by timber owners, 

managers, marketers, loggers, truckers, and processors. 

In 2007, the wood products and paper manufacturing 

industries in Iowa employed 15,833 people, with an 

average annual payroll of $648 million (U.S. Census 

Bureau 2007). To better manage the State’s forests, it is 

important to know the species, amounts, and locations 

of timber being harvested.

What we found

Surveys of Iowa’s wood-processing mills are conducted 

periodically to estimate the amount of wood volume that 

is processed into products. This is supplemented with 

the most recent surveys conducted in surrounding states 

that processed wood harvested from Iowa. In 2005, 27 

active primary wood processing mills were surveyed to 

determine the species that were processed and where the 

wood material came from (Haugen and Michel 2010). 

These mills processed 118.5 million board feet of saw 

logs, veneer logs, and cooperage.

In 2005, 16.1 million cubic feet of industrial roundwood 

was harvested from Iowa’s forest land. Saw logs 

accounted more than 90 percent of the total industrial 

roundwood harvested (Fig. 57). Other products 

harvested were veneer logs, cooperage, and industrial 

fuelwood. White and red oaks accounted for almost 40 

percent of the total industrial roundwood harvest (Fig. 

58). Other important species groups harvested were 

maples, black walnut, and cottonwood.

In the process of harvesting industrial roundwood, 11.5 

million cubic feet of harvest residues were left on the 

ground (Fig. 59). Seventy percent of the logging residue 

came from nongrowing-stock sources such as crooked 

or rotten trees, nonforest trees, tops and limbs, and 

dead trees. The processing of industrial roundwood in 

the State’s primary wood-using mills generated another 

Saw logs

Veneer logs

Cooperage

Industrial fuel wood

(<1%)

7% 2% 

91% 

301,000 dry tons of wood and bark residues. More than 

80 percent of the mill residues generated were used 

for miscellaneous products such as mulch or animal 

bedding. Another 15 percent of the mill residues were 

used for industrial and residential fuelwood. Less than 

1 percent of the mill residues were not used for other 

products (Fig. 60).

0 1 2 3 4 

White oaks 

Red oaks 

Soft maple 

Black walnut 

Cottonwood 

Other hardwoods 

Hard maple 

Basswood 

Hickory 

Softwoods 

Industrial Roundwood Harvest (million ft3

) 

Species Group 

Figure 57.—Industrial roundwood and fuelwood production by product and 

softwoods and hardwood, Iowa, 2005.

Figure 58.—Industrial roundwood harvested by species group, Iowa, 2005.
42

FOREST ECONOMICS

What this means

The demand for wood products is likely to increase, 

placing a greater demand on the resource. An important 

consideration for the future of the primary woodproducts industry is its ability to retain industrial 

roundwood processing facilities. The number of woodprocessing mills has been steadily declining. The loss of 

processing facilities makes it harder for landowners to 

fi nd markets for the timber harvested from management 

activities on their forest land.

Another important issue is the volume of harvest residues 

that are generated in the State that go unused. More than 

30 percent of the harvest residue is from growing-stock 

sources (wood material that could be used to produce 

products). Iowa’s primary forest products industry is 

the processing of saw logs. This leaves a large volume of 

usable, small dimension wood material above the saw 

log top. Industrial fuelwood or pulpwood markets could 

lead to better utilization of merchantable trees. The use 

of logging slash for industrial fuelwood at cogeneration 

facilities and pellet mills could also result in better 

utilization of the forest resource.

Fiber products 

Fuelwood 

Miscellaneous products 

Not used 

0 

25 

50 

75 

100 

125 

Bark Residues Coarse Residues Fine Residues 

Mill Residues (1,000 dry tons)

0 

5 

10 

15 

Growing Stock Nongrowing Stock 

Logging Residue (million cubic feet) 

Used for product

Harvest residue

Figure 60.—Disposition of mill residues generated by primary wood-using 

mills, Iowa, 2005.

Figure 59.—Harvest residue generated by industrial roundwood harvesting 

by growing stock and nongrowing stock, and used for product and harvest 

residue, Iowa, 2005.
43

Data Sources and Techniques

Loess Hills, western Iowa. Photograph courtesy of Lisa Schulte-Moore, Iowa State University.
44

DATA SOURCES AND TECHNIQUES

Forest Inventory

Information on the condition and status of forests in 

Iowa was obtained from the Northern Research Station’s 

Forest Inventory and Analysis (NRS-FIA) program. 

Previous inventories of Iowa’s forest resources were 

completed in 1954 (Thornton and Morgan 1959), 1974 

(Spencer and Jakes 1980), 1990 (Leatherberry et al. 

1992), and 2003 (Leatherberry et al. 2006). Data from 

Iowa’s forest inventories can be accessed electronically on 

the DVD included with this report, or at: http://www.

nrs.fs.fed.us/fi a/. For detailed information on inventory 

methods, see section “Statistics, Methods, and Quality 

Assurance” on the DVD. 

National Woodland Owner Survey

Information about family forest owners is collected 

through the U.S. Forest Service’s National Woodland 

Owner Survey (NWOS). The NWOS was designed to 

increase our understanding of owner demographics and 

motivation. Individuals and private groups identifi ed 

as woodland owners by FIA are invited to participate 

in the NWOS. Data presented here are based on 

survey responses from randomly selected families and 

individuals who own forest land in Iowa. For additional 

information about the NWOS, visit: www.fi a.fs.fed.

us/nwos. 

Timber Product Output Inventory

This study was a cooperative effort of the Division of 

Forestry of the Iowa Department of Natural Resources 

and the U.S. Forest Service’s Northern Research Station 

(NRS). Using a questionnaire designed to determine 

the size and composition of Iowa’s forest products 

industry, its use of roundwood (round sections cut 

from trees), and its generation and disposition of wood 

residues, Iowa Division of Forestry personnel visited 

all “known” primary wood-using mills within the 

state. Completed questionnaires were sent to NRS for 

processing and analyses. As part of data processing and 

analyses, all industrial roundwood volumes reported 

on the questionnaires were converted to standard units 

of measure using regional conversion factors. Timber 

removals by source of material and harvest residues 

generated during logging were estimated from standard 

product volumes using factors developed from logging 

utilization studies previously conducted by NRS. 
45

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48

DVD Contents

Iowa’s Forests 2008 (PDF)

Iowa’s Forests 2008: Statistics, Methods, and Quality Assurance (PDF)

Iowa’s Inventory Database (CSV fi le)

Iowa’s Inventory Database (Microsoft Access fi le)

Field guides that describe inventory procedures (PDF)

Database User Guides (PDF)
Nelson, Mark D.; Brewer, Matt; Woodall, Christopher W.; Perry, Charles H.; Domke, Grant M.; Piva, Ronald J.; 

Kurtz, Cassandra M.; Moser, W. Keith; Lister, Tonya W.; Butler, Brett J.; Meneguzzo, Dacia, M.; Miles, Patrick D.; 

Barnett, Charles J.; Gormanson, Dale. 2011. Iowa’s Forests 2008. Resour. Bull. NRS-52. Newtown Square, 

PA: U.S. Department of Agriculture, Forest Service, Northern Research Station. 48 p. [DVD included].

The second full annual inventory of Iowa’s forests (2004-2008) reports more than 3 million acres of forest land, 

almost all of which is timberland (98 percent), with an average volume of more than 1,000 cubic feet of growing 

stock per acre. American elm and eastern hophornbeam are the most numerous tree species, but silver maple 

and bur oak predominate in terms of live tree volume. Iowa’s forest land is comprised of 65 percent sawtimber, 

19 percent poletimber, and 16 percent sapling/seedling or nonstocked size classes. Average annual net 

growth of growing-stock trees on Iowa’s timberland increased during the past decade to the current estimate of 

nearly 105 million cubic feet. This report includes additional information on forest attributes, land use change, 

carbon, timber products, and forest health. A DVD included in this report includes 1) descriptive information on 

methods, statistics, and quality assurance of data collection, 2) a glossary of terms, 3) tables that summarize 

quality assurance, 4) a core set of tabular estimates for a variety of forest resources, and 5) a Microsoft 

Access database that represents an archive of data used in this report, with tools that allow users to produce 

customized estimates.

KEY WORDS: inventory, forest statistics, forest health, FIA

 Iowa’s Forests 2008:

Statistics, Methods, and 

Quality Assurance

 United States 

Department of 

Agriculture

Forest Service

 Resource Bulletin 

NRS-52
http://www.nrs.fs.fed.us