Iowa Forests

2013

United States Department of Agriculture

Forest Service Northern 

Research Station

Resource Bulletin

NRS-102

Publication Date 

July 2016
Abstract

The third full annual inventory of Iowa’s forests (2009-2013) indicates that just under 3 million acres 

of forest land exists in the State, 81 percent of which is in family forest ownership. Almost all of 

Iowa’s forest land is timberland (96 percent), with an average volume of more than 1,000 cubic 

feet of growing stock per acre on timberland and more than 1,500 cubic feet of all live volume (for 

trees at least 5 inches diameter at breast height) per acre on timberland. American elm and eastern 

hophornbeam are the most numerous tree species, but bur oak and silver maple predominate in 

terms of live-tree volume. Iowa’s forest land is composed of 70 percent sawtimber, 17 percent 

poletimber, and 13 percent sapling/seedling or nonstocked size classes. Average annual net growth 

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

estimate of 71 million cubic feet. This report includes additional information on forest attributes, 

land-use change, carbon, timber products, wildlife habitat, forest health, and future projections. The 

following information is available online at http://dx.doi.org/10.2737/NRS-RB-102: 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 to both the inventory and 

analysis of Iowa’s forest resources. Primary field crew and quality assurance staff over the 2009-

2013 field inventory cycle were John Benaszeski, Brian Clennon, Dave Haugen, Dan Huberty, 

Peter Koehler, Dominic Lewer, Grant Nielsen, Benjamin Nurre, Willard Smith, and Brian Wall. Data 

management personnel were Chuck Barnett, James Blehm, Dale Gormanson, Mark Hatfield, Dan 

Kaisershot, Barb O’Connell, Jay Solomakos, Paul Sowers, and Jeff Wazenegger. Report reviewers 

were Paul Tauke, Tivon Feeley, and Dan Kaminski of the Iowa Department of Natural Resources.

Cover: Fall colors on a ridge trail in Waubonsie State Park, Fremont County, Iowa. Photo by Lindsey Barney, Iowa 

Department of Natural Resources, used with permission.

Manuscript received for publication May 2015.

July 2016

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Iowa Forests, 2013

Mark D. Nelson, Charles J. Barnett, Matt Brewer, Brett J. Butler, Susan J. Crocker, Grant M. Domke, 

Dale Gormanson, Cassandra M. Kurtz, Tonya W. Lister, Stephen Matthews, William H. McWilliams, 

Dacia M. Meneguzzo, Patrick D. Miles, Randall S. Morin, Ronald J. Piva, Rachel Riemann, James E. 

Smith, Brian F. Walters, James A. Westfall, and Christopher W. Woodall

Contact Author: 

Mark D. Nelson

mdnelson@fs.fed.us

651-649-5104

About the Authors

Mark D. Nelson, Susan J. Crocker, Grant M. 

Domke, Dacia M. Meneguzzo, Patrick D. Miles, 

and Christopher W. Woodall are research 

foresters with the Forest Inventory and Analysis 

(FIA) program, Northern Research Station, 

St. Paul, MN.

Charles J. Barnett is a biological scientist with 

the FIA program, Northern Research Station, 

Newtown Square, PA.

Matt Brewer is a forester with the Iowa 

Department of Natural Resources, 

Des Moines, IA.

Brett J. Butler is a research forester with 

the FIA program, Northern Research Station, 

Amherst, MA.

Dale Gormanson, Ronald J. Piva, and Brian 

F. Walters are foresters with the FIA program, 

Northern Research Station, St. Paul, MN.

Cassandra M. Kurtz is a natural resources 

specialist with the FIA program, Northern 

Research Station, St. Paul, MN.

Tonya W. Lister, William H. McWilliams, 

Randall S. Morin, and James A. Westfall 

are research foresters with the FIA program, 

Northern Research Station, 

Newtown Square, PA.

Stephen Matthews is an ecologist with the 

Sustaining Forests in a Changing Environment 

research unit, Northern Research Station, 

Delaware, OH.

Rachel Riemann is a research forester/

geographer with the FIA program, Northern 

Research Station, Troy, NY.

James E. Smith is a research plant physiologist 

with the FIA program, Northern Research 

Station, Durham, NH.
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, Forestry Bureau, 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 field plots were inventoried each 

year and after 5 years a full inventory was completed. Previous annual inventories were 

conducted for the periods 1999-2003 and 2004-2008. This inventory covers the period 

2009-2013. 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 also continue 

to provide fine-quality hardwood products, habitat for wildlife, clean water and air, 

protection of soil from erosion, and additional ecosystem services. 

Although Iowa’s woodlands continue to gain volume as trees become larger, there are 

some areas of serious concern. Acres of Iowa forest land have shrunk to 2.97 million 

acres with a net loss of 97,000 acres since the last reporting period. This is the first 

recorded net loss of forest acres in Iowa since the 1974 inventory. Other areas of concern 

include the continuing decrease in the average size of forest tracts, the increasing 

presence of nonnative exotic invasive woodland plant species, the increasing impact of 

insect and disease pests such as emerald ash borer and bur oak blight, browse impacts 

from white-tailed deer, and the loss of young oak and early successional forest stands 

from the Iowa landscape.

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Forest Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Forest Indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Forest Economics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Future Forests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

Data Sources and Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

Statistics, Methods, and Quality Assurance . . . . . . . . . . . . . . . . . . . . . . . . . 

Online at http://dx.doi.org/10.2737/NRS-RB-102

Summary Tables for Statistics, Methods, and Quality Assurance..........

Online at http://dx.doi.org/10.2737/NRS-RB-102

Additional Resources . . . . . Online at http://dx.doi.org/10.2737/NRS-RB-102
Black oak grandeur at Waubonsie State Park, Fremont County, Iowa. Photo by Lindsey Barney, Iowa 

Department of Natural Resources, used with permission.
 | 1

Highlights

On the Plus Side

• Iowa’s forests contain more than 1 billion trees of at least 60 species statewide.

• Iowa’s black walnut resource ranks first in number of trees per acre on timberland 

when compared with estimates for the surrounding states of Minnesota, Wisconsin, 

Illinois, Indiana, Missouri, Kansas, and South Dakota during 2008-2012 (the most 

recent year available for all states).

• Timberland makes up the vast majority (96 percent) of forest land.

• Total growing-stock volume on timberland has more than doubled since the 1954 

and 1974 inventories, to a current statewide estimate of more than 3 billion cubic 

feet.

• The volume of sawtimber on timberland exceeds 11 billion board feet (International 

1⁄4-inch rule).

• Average annual net growth is more than three times the average annual harvest 

removals of growing-stock trees on timberland.

• Iowa’s most important tree species are generally in good health, with low incidence 

of poor crowns and little dieback for most species.

• Number of standing dead trees (snags) per acre increased slightly between 2008 

and 2013, to the current density of 12.1 per acre of forest land, providing important 

habitat features for wildlife.

• Late successional forest of older age classes and larger diameter classes continues to 

increase in area.

• An estimated 95,000 family forest owners control 81 percent of the forest land in 

Iowa; the 5 most common reasons for owning family forest land are to protect 

wildlife habitat, to enjoy scenery, to protect nature, to protect water resources, and 

because it is part of their home.

• Downed dead wood biomass is dominated by coarse woody debris, providing 

wildlife habitat and carbon sequestration.

• In 2013, the wood products and related industries in Iowa employed almost 12,000 

people, with an average annual payroll of $648 million.
2 | 

• The number of employees working in the forest products industry increased from 

the previous year for the first time since 2005.

• More than 99 percent of mill residues were used for products.

• The quantity of live biomass on timberland has increased at a steady rate since 1990; 

total biomass now exceeds 100 million oven-dry tons.

• Overall forest carbon per acre increased by 4 percent relative to 5 years ago, and live 

tree carbon values increased by 9 percent.

• Nonforest trees in Iowa contribute an important resource beyond trees inventoried 

on forest land.

Areas of Concern

• After increasing for more than three decades, Iowa’s forest land area plateaued at 3 

million acres during the last inventory, and has since decreased.

• Although only 1 percent of Iowa’s land area changed land use between 2008 and 

2013, diversions (losses) from forest to nonforest exceeded reversions (gains) from 

nonforest to new forest land, leading to a 2 percent net loss of forest area.

• During the past decade, average annual growth has declined and average annual 

mortality has increased.

• The fraction of trees with low quality grades decreased between 1990 and 2003, but 

has since been increasing gradually. 

• For other eastern soft hardwoods species group (mostly American elm), the total 

statewide average annual mortality is nearly 19 million cubic feet, a rate of 4 percent 

of volume per year.

• Forest in the 0- to 20-year-old age class has decreased by nearly half since 2008, 

reducing habitat abundance for early successional species.

• The forest land base of Iowa is highly fragmented due to agriculture and 

development; a small minority of forest is “core” (having the highest spatial 

integrity) and a majority of forest is in unconnected fragments.

• Almost 95 percent of plots contain 1 or more of the 15 species of nonnative 

invasive plants recorded on Iowa plots inventoried by the U.S. Forest Service, 

Forest Inventory and Analysis (FIA) program. Multiflora rose is the most prevalent, 

occurring on more than 60 percent of plots.
 | 3

• Only 11 percent of family forest ownerships have a written management plan; only 

17 percent have received forest management advice.

• The last pulp mill in the State closed in 2005. Without this market, a large volume of 

small dimension wood material above the saw log top is underutilized.

• Emerald ash borer, an exotic wood-boring beetle that has become established in 

Iowa, threatens all North American ash tree species in both forests and urban 

environments; ash mortality was fairly stable from 1990 to 2008, but doubled 

between 2008 and 2013.

• Although most of the browse impact was found to be at a medium level, about onequarter of the samples had high or very high browsing impact.

Issues to Watch

• Rate of gross forest loss remained relatively stable between 2008 and 2013; the 

difference in net change between the two inventory periods can be attributed 

primarily to a drop in the amount of nonforest land that reverted to forest. If gross 

gains in forest no longer offset losses, net area of forest land will decrease in future 

years. 

• Increases in the human population and number of housing units are resulting in an 

increasing percentage of forest land in the wildland-urban interface.

• Future pressures to harvest Iowa’s forests for energy may compete with traditional 

forest products industries, and increased harvest utilization rates could diminish 

forest sustainability (via decreases in harvest residues) and habitat quality.

• Most tree species groups continued their slightly increasing trend in total statewide 

live-tree volume during the past 10 years, with the exceptions of cottonwood and 

aspen, soft maple, and other eastern softwoods species groups.

• Gypsy moth is a nonnative insect pest that recently entered Iowa; more than half 

of the live-tree volume in Iowa is in tree species preferred by gypsy moth. Bur oak 

blight also is expected to cause significant damage to Iowa’s forests.

• Future management of forests to maximize carbon sequestration could complicate 

timber management practices due to fluctuating carbon credit markets and 

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

and belowground stocks).
4 | 

• The average parcel size of privately owned forest land in Iowa continues to decrease 

with indications that these holdings will be changing owners; family legacy—

keeping land intact for future generations—is an important concern.

• Small, localized, industrial fuelwood or wood pellet manufacturers could increase 

future utilization of the forest resource.

• Oak/hickory forest-type group accounts for about two-thirds of Iowa’s forest and is 

critical for timber and for providing acorns for wildlife; young stands now account 

for less than 3 percent of the oak/hickory forest-type group, adding to concern 

about future abundance of this important species group.

• Volume of high quality sawtimber is increasing for some species groups and 

decreasing for others.

• With the improving economy and increasing demand for wood products, the forest 

products mills that were able to withstand the recession are beginning to increase 

their production, resulting in a potential increase in the number of employees in 

the forest products industry.

• Projections of future forest conditions through the year 2060 suggest that elm/

ash/cottonwood forest-type group will decrease in forest area and oak/hickory 

forest-type group will increase; these opposite trends are partially offsetting when 

mortality to individual ash and elm trees results in reclassification of forest-type 

groups.

• Future tree mortality resulting from emerald ash borer is projected to result in the 

loss of 16 million ash trees at least 5 inches in diameter at breast height from Iowa 

forest land by 2060.

• Changing climate patterns are projected to affect some bird species negatively and 

others positively.
 | 5

Background

Bur oak in Iowa. Photo by Iowa Department of Natural Resources, used with permission.

 | 5
6 | 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 defines a tree as any perennial 

woody plant species that can attain a height of 15 feet at maturity. A complete list of 

the tree species measured in Iowa during this inventory is included in the appendix. 

An electronic record of every tree measured in this inventory, as well as a glossary of 

additional terms, is available online at http://dx.doi.org/10.2737/NRS-RB-102. 

What is a forest?

The FIA program defines forest land as land that has at least 10 percent canopy 

cover of live tally tree species of any size or has had at least 10 percent canopy cover 

of live tally species in the past, based on the presence of stumps, snags, or other 

evidence, and not currently developed for nonforest use(s) that prevent normal tree 

regeneration and succession. 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 fully 

addressed in this report (see Nonforest Trees section on p. 61 for more information.)

What is the difference between timberland, reserved forest 

land, and other forest land?

FIA defines three subcategories of forest land: reserved forest, other forest land, and 

timberland. Reserved forest is land that has been withdrawn from timber utilization 

through legislation or administrative regulation. Other forest land typically has poor 

soils where the forest is incapable of producing 20 cubic feet per acre per year at the 

culmination of mean annual increment (growth). Timberland is forest land that is not 

reserved and meets minimum productivity requirements. 

During some periodic inventories conducted before 1999, only trees on timberland 

plots were measured. We are therefore unable to report estimates for some attributes 

(e.g., volume) on nontimberland 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, and again during 2009-2013, we 

can now report growth, removals, and mortality on all forest land, whereas for prior 

inventories we could report growth, removals, and mortality only on timberland.
 | 7

How do we estimate forest land area and number of trees?

Forest inventory plots have been established throughout Iowa, with 1 plot for 

approximately every 6,000 acres, including 5,426 nonforest plots and 641 forest plots. 

Only those plots located on forest land are measured “in the field.” Together these plots 

make up a statistical sample of observations used for estimating various forest attributes. 

During the 2009-2013 inventory, 13,259 trees that are at least 1 inch in diameter at 

breast height (d.b.h., 4.5 feet above the ground) were measured on forest plots in Iowa. 

Unless indicated otherwise, sampling errors reported in text and figures represent 1 

standard error (SE). For information on sampling errors, see the Statistics, Methods, 

and Quality Assurance section available at http://dx.doi.org/10.2737/NRS-RB-102.

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 firewood, contains about 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 used 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 these data by tree species group. 

Using these models, we can estimate tree volume based on species, diameter, and tree 

site 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 developed specific gravity estimates for many tree species (Miles and 

Smith 2009). These specific gravities are applied to estimates of tree volume to derive 

estimates of merchantable tree biomass (the weight of the tree’s merchantable bole). 

All live-tree biomass is estimated by including biomass estimates for the stump, top 

and 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 

no moisture content. On average 1 ton of oven-dry biomass is equal to 1.9 tons (2,000 

pounds/ton) of green biomass.
8 | BACKGROUND

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 and dead trees consists of carbon. 

Additional carbon pools (e.g., soil, understory vegetation, belowground biomass) are 

modeled based on stand and 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. References to 1954, 1974, and 1990 periodic 

inventories each refer to that single year of Iowa inventory, but references to 2003, 2008, 

and 2013 annual inventories refer to the 5-year periods ending in those years, i.e., 1999-

2003, 2004-2008, and 2009-2013, respectively. Comparisons are valid between 2003, 

2008, and 2013 inventories, all three 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 efficiency, reliability, and 

national consistency of the inventory. Several changes in procedures and definitions 

have occurred since the 1990 Iowa inventory. Although these changes will have minimal 

impact on statewide estimates of forest area, timber volume, and tree biomass, they may 

have significant impacts on plot classification variables such as forest type and standsize class (tree diameter class). Some of these changes make it inappropriate to directly 

compare the 2013, 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 definitions. Adjusted estimates for the 1954 inventory are cited within this 

report. Features for which no new data are available (e.g., soils, ozone, lichens) are not 

included in this report.

Reserved status—improved implementation 

FIA defines reserved forest land as forest land withdrawn by law(s) prohibiting the 

management of land for the production of wood products (not merely controlling 

or prohibiting wood-harvesting methods). All private forest lands, regardless of 

conservation easements that may restrict harvesting, are considered not reserved. 

Such lands are declared timberland if they meet minimum productivity requirements, 

and “other forest” if they do not. Timberland does not include reserved forest land.
 | 9

In an effort to increase consistency among states and across inventory years, a refined 

set of procedures determining reserved status has been implemented with version 6.0 

of the FIA field manual, which took effect with the 2013 inventory year (which began 

in October 2012). Furthermore, all previously collected annual inventory data (1999 

to present) have been updated by using the new standardized interpretation. 

Starting with this report, timberland estimates generated for earlier annual 

inventories will differ from previously published estimates. The 2012 inventory was 

the last inventory in which all data were available under the previous and improved 

implementations. Small changes are associated with timberland acreage, number of 

trees, volume, and biomass. The changes associated with the remaining timberland 

estimates are minor given the inherent variability in the associated estimates. The 

improved implementation of the reserved status definition increases the spatial and 

temporal precision of timberland estimates, allowing for higher quality trend analyses 

and potentially better forest management decisions.

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 classified 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 and 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 in this report. Unless indicated otherwise, 

forest resource data are from FIA, and base map layers, such as state and county 

boundaries, were obtained from the GIS section of the Iowa Geological and Water 

Survey (Iowa Department of Natural Resources, n.d.a). Depicted FIA plot locations 

are approximate. Sources and intended uses of FIA data are available at www.fia.fs.fed.

us/tools-data. Sources of other geospatial datasets are cited within individual figures. 

Iowa maps are portrayed in Universal Transverse Mercator (UTM) projection, Zone 

15, North American Datum (NAD) of 1983. 
10 | BACKGROUND
 | 11

Forest Features

Black oak in Militia Hollow, Waubonsie State Park, Fremont County, Iowa. Photo by Lindsey Barney, Iowa Department 

of Natural Resources, used with permission.

 | 11
12 | FOREST 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 (Griffith et al. 2008). A mixture of land uses occurs in the Central 

Irregular Plains in the south-central portion of the State, with wider forested strips 

occurring along streams. One-fifth 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 significant portion of Iowa’s forests and most of 

its trout streams. Three-fourths 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.

Ecoregion

Central Irregular Plains

Interior River Lowland

Paleozoic Plateau

Western Corn Belt Plains 0 100 Miles

N

0 100 Miles

Figure 1.—Level III Ecoregions of Iowa. Data source for ecoregions: Iowa Department of 

Natural Resources (n.d.b); for inset: ESRI Data & Maps.

Projection: UTM Zone 15N, NAD83

Ecoregion

Central Irregular Plains

Interior River Lowland

Paleozoic Plateau

Western Corn Belt Plains
 | 13

What we found

Iowa forest land area is most concentrated in the northeastern and southern counties, 

which are located in three of the four geographic inventory units (Fig. 2). After 

declining from 2.3 million acres in 1954 to a low of 1.6 million acres in the 1970s, area 

of forest land increased at a steady rate, reaching a peak of more than 3 million acres 

during the past several years, then declining slightly to the current estimate of 2.97 

million acres (Fig. 3). Almost all (96 percent) of Iowa’s forest land is timberland; 3 

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

Figure 3.—Area of forest land and timberland by inventory year, 

and associated sampling error, Iowa, 1954 to 2013. Error bars 

represent a 68 percent confidence interval around each estimate.

0

500

1,000

1,500

2,000

2,500

3,000

3,500

1950 1960 1970 1980 1990 2000 2010

Area (thousand acres)

Inventory Year

Forest land

Timberland

Figure 2.—County-level forest land area as a percentage of total land 

area, and location of the four FIA units in Iowa, 2013.

N

0 100 Miles

Projection: UTM Zone 15N, NAD83

4—Northwestern 1—Northeastern

3—Southwestern 2—Southeastern

Percent Forest

0-5

5.1-10

10.1-20

20.1-40

FIA Unit
14 | FOREST FEATURES

What this means

At the state level, Iowa’s forest land and timberland areas have been gradually 

increasing since the 1970s, peaking a few years ago. Because nearly all forest land 

in Iowa is timberland, subsequent sections of this report that refer to estimates on 

timberland most 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 changes in the area of forest land or 

timberland at these scales.

Land-Use Change 

Background

Although forests cover only 8 percent of the land area in Iowa, they are a critical 

resource and offer a wide range of benefits. Tree and vegetation cover limit the loss 

of Iowa’s fertile soils by protecting against erosion. Riparian forests serve as stream 

buffers, protecting and clarifying the State’s water resources. Forests provide habitat 

for forest-dwelling species and provide economic and other benefits for humans. 

Although the total area of forest land in Iowa remained fairly stable between 2008 

and 2013, some areas of the State experienced forest loss, whereas other areas saw 

increases in forest land. To better understand Iowa’s forest land dynamics, it is 

important to explore the underlying land-use changes occurring in the State.

FIA characterizes land area by using several broad land use categories: forest, 

rangeland, agriculture, water, developed, and other land. The conversion of forest land 

to other uses is referred to as gross forest loss (or diversion), and the conversion of 

nonforest land to forest is known as gross forest gain (or reversion). The magnitude 

of the difference between gross loss and gross gain is defined as net forest change. 

By comparing the land uses on current inventory plots with the land uses recorded 

for the same plots during the previous inventory, we can characterize forest landuse change dynamics. Understanding land-use change dynamics is essential for 

monitoring the sustainability of Iowa’s forest resources and helps land managers make 

informed policy decisions.
 | 15

What we found

The land area in Iowa is dominated by cropland and pasture. These agricultural land 

uses, along with urban and other nonforest land uses, cover 92 percent of the State’s 

surface area (Fig. 4). Commercial and residential development is limited and is 

primarily concentrated in the central part of the State surrounding Des Moines and 

Cedar Rapids. Between 2008 and 2013 most of the FIA plots in Iowa either remained 

forested (8 percent) or stayed in a nonforest land use (91 percent) (Fig. 4). 

On the 1 percent of FIA plots where land use changed between inventories, the 

amount of land that was diverted from forest to nonforest exceeded acreage of 

nonforest that reverted to new forest land, leading to a net loss of forest land of 2 

percent. According to FIA remeasurement data, Iowa lost 192,000 acres (6 percent) 

of forest land from 2008 to 2013, which was only partially offset by a gain of nearly 

95,000 acres (3 percent) during the same time period (Fig. 5). Seventy-one percent of 

the gross forest loss was due to diversion to agricultural land uses including cropland 

(31 percent), pasture (26 percent), and agricultural land grouped with idle farmland 

(14 percent). The other 29 percent of forest loss was forest land converted to water or 

marsh (15 percent), rangeland (6 percent), developed land (4 percent), and rightsof-way (4 percent) (Fig. 6). Sixty percent of forest gain in Iowa was from agricultural 

land, primarily cropland (40 percent) converting to forest. Other land use sources 

for new forest land included water and marsh (18 percent) and developed land (15 

percent) (Fig. 6). 

FIA data can be used to characterize the forest land that has been lost and gained to 

see if it differs from the characteristics of forest land in all of Iowa, the large majority 

of which is persisting. Iowa’s forests are dominated by stands in the large diameter 

size class (69 percent). However, new forest land that has been gained in the State is 

classified primarily as small diameter stands (53 percent). Similarly, the forest land 

that has been lost has a greater proportion of small diameter stands (41 percent) than 

in Iowa as a whole (12 percent). Figure 7 shows the distribution of remeasured plots 

across Iowa, highlighting plots where forest land has been lost and gained. Forest 

change plots are concentrated in the southern and eastern parts of Iowa, where forest 

cover is most prevalent.
16 | FOREST FEATURES

Figure 4.—Proportion of land with unchanged land use, gain in forest 

land from nonforest land, and loss of forest land to nonforest use, 

Iowa, 2008-2013.

8% 0%

1%

91%

Forest in 2008 and 2013

Nonforest to forest

Forest to nonforest

Nonforest in 2008 and 2013

Figure 5.—Change in forest land by land use category, and 

associated sampling error, Iowa, 2008-2013.

0 50 100 150 200 250

Loss of forest land to 

nonforest uses 

Gain in forest land from 

nonforest land

Area (thousand acres)

Components of Change Agriculture

Rangeland

Water 

Developed

Figure 6.—Forest gain by previous land use and forest loss by current land use, Iowa, 2008-2013.

18%

13%

40%

7%

15%

7%

Forest gain

15%

14%

31%

26%

6%

4% 4%

Forest loss

Water

Agricultural land and 

idle farmland 

Cropland

Pasture

Rangeland

Developed

Right-of-way
 | 17

What this means 

Agriculture is the dominant land use in Iowa and gains and losses in pasture and 

cropland appear to drive land-use change dynamics in the State. The forest land that 

was lost went mostly to agricultural uses and may be a result of increased commodity 

prices for corn during 2011-2013, increased demand for agriculture-based biofuels, or 

other possible causes. Iowa is one of the primary producers of ethanol in the United 

States. More than 50 percent of the national production of ethanol is attributed to 

the top-producing Corn Belt states of Iowa, Nebraska, and Illinois (Schnepf 2013). 

With increased interest in domestic fuel sources, there may be increased demand for 

suitable cropland. 

Gains in forest land may come from reverting agricultural land, especially land close 

to streams. The State’s public and private sectors have made a concerted effort to 

prioritize the reforestation of these riparian areas. Agroforestry efforts promote the 

maintenance of tree cover in the form of windbreaks and forest buffers, which help 

sustain a high agricultural output while conserving and protecting Iowa’s soil and 

water resources. These forested areas are also important to Iowa’s wildlife populations. 

Riparian forests often connect to form wildlife corridors, which may allow for greater 

species movement. 

Some of the diversion and reversion of forest land in Iowa are probably the result 

of marginal forest land moving into and out of the forest land base. This movement 

between forest and nonforest classifications may be a result of land meeting or not 

meeting FIA’s definition of forest land due to small changes in understory disturbance, 

Figure 7.—Distribution of remeasurement inventory plots showing forest gains and losses, Iowa, 2008-2013. 

(Depicted plot locations are approximate.)

Forest gain

Forest loss

Remained nonforest

Remained forest
18 | FOREST FEATURES

forest extent, or forest cover. These changes are generally not permanent and may be 

more prevalent in stands of small diameter trees (small stand-size class). The fact that 

much of the forest change in Iowa is occurring in stands of smaller diameter trees 

may support the idea that this type of nonpermanent land-use change is occurring in 

the State.

More forest land was lost to other land uses than was gained, which is a change from 

the results of the last inventory cycle from 2008 (Nelson et al. 2011). The difference in 

net change between the two inventory periods can be attributed primarily to a drop 

in the amount of nonforest land that reverted to forest; the amount of gross forest 

loss remained fairly stable between the two inventory cycles. The resulting net loss 

of forest land in this most recent inventory is relatively small and represents little 

appreciable net change in forest land area. The extent of forest cover may be starting 

to stabilize in Iowa after a period of growth since the 1970s.

Forest Ownership 

Background

Ownership patterns are important because forest owners ultimately control the forest 

resources and decide how they will be managed. FIA collects inventory data across 

all ownership categories, both public and private. Family forest owners—families, 

individuals, and other unincorporated groups—compose a dominant, diverse, and 

dynamic group of private owners in Iowa. By understanding these owners, we can 

better help them meet their forest management needs, and in so doing, help conserve 

Iowa’s forests for generations to come. FIA conducts the National Woodland Owner 

Survey (NWOS) in order to better understand who owns America’s forests, why 

they own it, what they have done with it in the past, and what they intend to do with 

it in the future. NWOS summaries reported here are based on the responses from 

randomly selected Iowa family forest owners, with 10 or more acres of ownership, 

surveyed between 2011 and 2013 (n=138) (Butler et al. 2016b).

What we found

From the Missouri River blufflands in the west, through the small woodlots in 

the State’s agricultural heartland, to the deeply dissected Driftless Area near the 
 | 19

Mississippi River to the east, forest ownership varies across Iowa (Fig. 8). An 

estimated 95,000 family forest owners control 81 percent of the forest land in Iowa, 

in 48,000 ownerships of at least 10 acres1 (Fig. 9). According to the results of NWOS, 

the average size of the family forest holdings is 47 acres (Fig. 10). There are many 

reasons for owning forest land, the top five of which are related to wildlife habitat 

protection, aesthetics, protection of nature, protection of water resources, and as part 

of the family’s home (Fig. 11). Timber production was ranked as important by only 

15 percent of ownerships and 23 percent of acreage. Eleven percent of the family 

forest ownerships have a written management plan, and 17 percent have received 

forest management advice. The following issues were of great concern or concern for 

at least 50 percent of ownerships: high property taxes, keeping land intact for future 

generations, trespassing or poaching, unwanted insects or diseases, and misuse of 

forest/woodland, such as dumping. The most frequently listed forestry activities (by 

percentage of ownerships) planned for the next 5 years are: cutting and removing 

trees for own use (47 percent), improving wildlife habitat (38 percent), eliminating 

or reducing invasive plants (36 percent), collecting nontimber forest products (29 

percent), and grazing livestock (29 percent). About 1 in 10 ownerships is owned by 

someone who plans to pass the land on to heirs or sell it within the next 5 years. 

1 The minimum size of a family forest ownership is 10 acres, a constraint not applied to the 147,000 family forest 

owners estimated in the 2008 report. Therefore, the 2008 and 2013 estimates are not directly comparable.
20 | FOREST FEATURES Figure 8.—Distribution of six forest ownership types, Iowa, 2009. Data source: Hewes et al. (2014).

Public

LocalStateFederal

Private

OtherCorporateFamily
 | 21

81%

4%

6% 3%

Family

Corporate

4%

Other private

2%

Federal

State Local

Figure 9.—Percentage of forest land and woodland by ownership 

category, Iowa, 2013.

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

landholdings, as a proportion of the total, and associated sampling 

error, Iowa, 2013. Error bars represent a 68 percent confidence 

interval around each estimate.

0 10 20 30 40 50 60 70 80 90

10-49

50-99

100-499

500+

Percent

Size of Forest Holdings (acres)

Acres

Ownerships
22 | FOREST FEATURES

What this means

Much land will soon be changing legal ownership. Family legacy—keeping land 

intact for future generations— is an important ownership objective and also a major 

concern to both landowners and natural resource managers. What can be done to 

help the landowners and the land? Timber production is not the primary goal of 

family forest owners, but it is clear that many landowners are harvesting timber 

and firewood on their ownerships. How can natural resource professionals better 

communicate with family forest owners and help them better manage their woods? 

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 efficient ways of communicating 

with them. One such attempt at learning more about landowners is through the 

Sustaining Family Forests Initiative (SFFI). For more information, visit 

www.sustainingfamilyforests.org.

0 10 20 30 40 50 60 70 80 90

Nontimber forest products

Cabin

Timber

Firewood

Family

Investment

Hunting

Recreation

Heirs

Farm

Privacy

Home

Water

Nature

Aesthetics

Wildlife

Percent

Reason

Acres

Ownerships

Figure 11.—Reasons for owning forest land by percentage of area 

owned and number of family forests, Iowa, 2013. These proportions 

refer to landowners who ranked each objective as very important or 

important on a 7-point Likert scale. Categories are not exclusive.
 | 23

Biomass

Background

Measures of total biomass and its allocation among stand components (e.g., 

large diameter and 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 aboveground live-tree biomass on Iowa timberland has increased 

at a steady rate since 1990, exceeding 107 million oven-dry tons by 2013 (Fig. 12). 

This increase corresponds to an increase in timberland area, with biomass per acre 

increasing at a slower rate (Fig. 12). Most forest biomass in 2013 is in tree boles (61 

percent), followed by belowground, 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). 

Figure 12.—Aboveground live-tree and sapling biomass on timberland, 

Iowa, 1990, 2003, 2008, and 2013.

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

45,000

0

20

40

60

80

100

120

140

1985 1990 1995 2000 2005 2010 2015

Biomass (oven-dry tons/acre)

Biomass (million oven-dry tons)

Inventory Year

Biomass

Biomass/acre
24 | FOREST FEATURES

Figure 13.—Aboveground live-tree and sapling biomass on timberland 

by component, Iowa, 2009-2013.

14%

5%

4%

61%

16% Tops and limbs

Saplings

Stumps

Bole

Belowground

biomass
 

| 25 Figure 14.—Distribution of aboveground live-tree biomass on forest land, Iowa, 2009. Data source for biomass: Unpublished data on file with B. Tyler Wilson, U.S. Forest Service, St. Paul, MN. Geospatial dataset produced by using the raster mapping methods described in Wilson et al. (2012).

Live Biomass on Forest Land (oven-drytons/acre)

<1010-20>20Nonforest
26 | FOREST FEATURES

What this means

Although total area of forest land and of timberland show a slight decrease, forest 

growth has 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 influences 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, influenced by 

the presence or absence of disturbances such as timber management, recreation, 

wildfire, 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 

About 1.08 billion trees were estimated on Iowa forest land in 2013, a slight decrease 

from the 2008 estimate of 1.16 billion trees. At least 60 tree species were observed 

on Iowa forest land in 2013. Of the total number of trees statewide, American elm, 

eastern hophornbeam, hackberry, and mulberry (including both red mulberry—a 

native species, and white mulberry—a nonnative species) were the four most 

numerous species, each estimated to have more than 50 million trees (Fig. 15). There 

were no oak species in the 12 most numerous species in 2013. However, several oak 

species are among the 12 species with the highest total statewide live-tree volume on 

forest land. Bur oak ranked the highest in terms of volume, containing nearly half 

a billion cubic feet of volume (Fig. 16). Black walnut is ranked seventh in terms of 

number of trees (Fig. 15), but compared with estimates for the surrounding states of 

Minnesota, Wisconsin, Illinois, Indiana, Missouri, Kansas, and South Dakota during 

2008-2012 (the most recent year available for all states), Iowa’s black walnut resource 
 | 27

ranks first in number of trees per acre on timberland: 12.9 trees per acre for all live 

trees of at least 1 inch d.b.h., and 5.6 trees per acre for growing-stock trees (Nelson 

and Brewer 2014).

Due to larger sampling errors associated with estimates for some individual tree 

species, comparisons across time are made for groups of related species. Most species 

groups had substantial increases in volume between 1990 and 2003, and a slightly 

increasing trend in total statewide live-tree volume during the past 10 years (Fig. 

17). Notable exceptions were cottonwood and aspen, soft maple, and other eastern 

softwoods species groups, which all show declining patterns.

0 20 40 60 80 100 120 140

American elm

Eastern hophornbeam

Hackberry

Mulberry (white and red)

Shagbark hickory

Boxelder

Black walnut

Slippery elm

Black cherry

American basswood

Bitternut hickory

Eastern redcedar

Number of Trees (millions)

Species

Figure 15.—Twelve most abundant tree species ranked by number 

of live trees on forest land, and associated sampling error, Iowa, 

2009-2013.

Figure 16.—Twelve tree species with the highest volume of live trees 

on forest land, and associated sampling error, Iowa, 2009-2013.

0 100 200 300 400 500 600

Bur oak

Silver maple

White oak

Eastern cottonwood

Black walnut

Northern red oak

Hackberry

American basswood

American elm

Shagbark hickory

Honeylocust

Boxelder

Total Live Volume (million ft3

)

Species
28 | FOREST FEATURES

What this means 

Iowa possesses a great diversity of tree species that will undoubtedly fill 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 dozen most 

populous 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. 

(See further discussion in Regeneration Status section on p. 64.) In contrast, eastern 

hophornbeam, the second most numerous tree species (92 million), was ranked 29th 

overall 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 in dominance. 

0

100

200

300

400

500

600

700

Total Live Volume (million ft3

)

1990

1999-2003

2004-2008

2009-2013

Select white oaks

Other eastern soft hardwoods

Select red oaks

Cottonwood and aspen

Soft maple

Hickory

Basswood

Other red oaks

Black walnut

Ash

Hard maple

Other eastern hard hardwoods

Other eastern softwoods

Other white oaks

Eastern white and red pine

Spruce/fir/other yellow pines

Figure 17.—Growing-stock volume on timberland by inventory year 

by tree species or species group, Iowa.
 | 29

Tree Density, Size, and Stocking

Background

Tree density is the ratio of the number of trees to a specific area, such as 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; the other major determinant of diameter is tree density or 

stocking. 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. Stand-size classes are defined 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 to 9 inches 

d.b.h. 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 (4.5 feet). 

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 by 

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 at least 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 seedling-sapling stand a fully stocked stand would indicate that the present 

number of trees is sufficient to attain a basal area of at least 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 60 growing-stock trees per 

acre during the 1950s to 78 in 2003, then declined slightly to a relatively stable 

density of about 68 trees per acre (Fig. 18). Basal area of growing-stock trees per 

acre of timberland increased from about 41 in 1954 to 53 in 2003, then decreased to 

the current estimate of 51. (Note that in the previous report, these results included 

growing-stock trees between 1 and 5 inches d.b.h.; here we include only growingstock trees of at least 5 inches d.b.h.)

More than two-thirds (69 percent) of forest land in Iowa is in the large diameter 

stand-size class, 18 percent is medium diameter, and the remaining 13 percent is in 
30 | FOREST FEATURES

either the small diameter or nonstocked class. Just over one-third (36 percent) of 

Iowa’s forest land is fully stocked or overstocked, 43 percent is medium stocked, and 

22 percent is poorly stocked or nonstocked. By combining these two classifications 

we see that large diameters compose 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).

Figure 18.—Average number of growing-stock trees per acre (TPA) 

and average basal area (BA) per acre for growing-stock trees of at 

least 5 inches d.b.h. on timberland, Iowa, 1954-2013.

0

10

20

30

40

50

60

0

10

20

30

40

50

60

70

80

90

1950 1960 1970 1980 1990 2000 2010

Basal Area (ft2) per Acre

Number of Trees (trees per acre)

Inventory Year

TPA

BA

Figure 19.—Proportion of forest land area by stocking class 

and stand-size class, Iowa, 2009-2013.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Overstocked Fully

stocked

Medium 

stocked

Poorly 

stocked

Proportion

Small diameter Medium diameter Large diameter
 | 31

What this means

The density and diameter of trees across Iowa provide 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 of growing-stock trees (of at least 5 inches d.b.h.) 

have increased, reflecting 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 present evidence of a maturing forest resource across Iowa. This 

trend may be expected to continue until stands reach a state of senescence or when 

they are affected by disturbances, either human induced (e.g., removals) or naturally 

occurring (e.g., windstorm, fire). However, nearly a quarter of forest land in Iowa is 

understocked or nonstocked, presenting an opportunity for additional management. 

(But see discussion of ownership objectives in Forest Ownership section on p. 18.) 

Tree Growth

Background

A stand’s capacity for growth, that is, for trees to increase in volume, is an indication 

of the overall condition of the forest and more specifically of tree vigor, forest health, 

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, 

2008 and 2013. 

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 about 41 million cubic feet in both 

1974 and 1990, followed by a large increase to nearly 118 million cubic feet in 2003 and 

a decrease during the past decade to 71 million cubic feet (Fig. 20). The five species 

groups with the greatest average annual net growth were black walnut, select white oaks, 

other eastern soft hardwoods, cottonwood, and select red oaks, all of which exceeded 5 

million cubic feet per year (Fig. 21). Black walnut, other eastern hard hardwoods, and 

ash species groups all had ratios of annual net growth to volume exceeding 3 percent, 

with seven other species groups each exceeding 2 percent (Fig. 22).
32 | FOREST FEATURES

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

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

associated sampling error, Iowa, 1954-2013. Error bars represent a 68 percent 

confidence interval around each estimate and are available only since 1990.

Figure 21.—Fourteen tree species or species groups ranked by average 

annual net growing-stock volume growth on timberland, and associated 

sampling error, Iowa, 2009-2013. 

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000 20,000

Black walnut

Select white oaks

Other eastern soft hardwoods

Cottonwood and aspen

Select red oaks

Other red oaks

Other eastern hard hardwoods

Basswood

Hickory

Ash

Hard maple

Soft maple

Other eastern softwoods

Other white oaks

Volume Growth (thousand ft3

/year)

Species/Species Group

Figure 22.—Fourteen tree species or species groups ranked by average 

annual net volume growth expressed as a percentage of total growing-stock 

volume on timberland, Iowa, 2009-2013. 

0 1 2 3 4 5 6

Black walnut

Other eastern hard hardwoods

Ash

Other red oaks

Basswood

Hard maple

Other white oaks

Other eastern softwoods

Select red oaks

Hickory

Cottonwood and aspen

Select white oaks

Other eastern soft hardwoods

Soft maple

Ratio of Growth to Volume (percent)

Species/Species Group
 | 33

What this means 

The net growth of Iowa’s forests increased during past decades, but has been 

decreasing during the last 10 years. Many of the economically desirable tree species, 

such as oaks and walnut, continue to accrue tremendous annual growth. Black walnut 

shows the highest growth rate—relative to total volume—followed by other eastern 

hard hardwoods and ash; oaks show comparatively less relative growth.

Tree Mortality

Background 

Tree mortality influences a number of forest characteristics such as health, vigor, and 

rate of accretion and depletion. Mortality can be caused by insects, disease, adverse 

weather, succession, competition, fire, 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” (see 

Tree Removals section on p. 35). 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 1.5 percent (Fig. 23). In terms of total statewide average annual 

mortality, the other eastern soft hardwoods species group had almost 19 million cubic 

feet of mortality, the largest, by far, of any species group (Fig. 24). Cottonwood and 

soft maple species groups followed with 0.8 and 0.75 million cubic feet, respectively. 

Compared to total growing-stock volume on timberland for each species group, 

eastern white and red pine had the largest percentage of mortality at 38 percent (but 

confidence in this estimate is low, due to large sampling error). Other eastern soft 

hardwoods had a mortality rate of 4 percent, and all other species groups had a lower 

rate (Fig. 25).
34 | FOREST FEATURES

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

1950 1960 1970 1980 1990 2000 2010

Average Annual Mortality 

(percent/year)

Inventory Year

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

total statewide growing-stock volume on timberland, Iowa, 1954-2013.

Figure 24.—Fifteen tree species or species groups ranked by average annual 

net growing-stock volume mortality on timberland, and associated sampling 

error, Iowa, 2009-2013. Error bars represent a 68 percent confidence interval 

around each estimate.

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

Other eastern soft hardwoods

Cottonwood and aspen

Soft maple

Hickory

Select white oaks

Other red oaks

Ash

Select red oaks

Black walnut

Basswood

Other eastern hard hardwoods

Eastern white and red pine

Hard maple

Other eastern softwoods

Other yellow pines

Species/Species Group

Volume Mortality (thousand ft3

/year)

Figure 25.—Fourteen tree species or species groups ranked by average 

annual net growing-stock volume mortality as a percentage of total 

growing-stock volume on timberland, Iowa, 2009-2013.

0 5 10 15 20 25 30 35 40

Eastern white and red pine

Other eastern soft hardwoods

Soft maple

Cottonwood and aspen

Ash

Hickory

Other red oaks

Basswood

Select red oaks

Black walnut

Other eastern softwoods

Select white oaks

Other eastern hard hardwoods

Hard maple

Other white oaks

Ratio of Mortality to Volume (percent)

Species/Species Group

0 5 10 15 20 25 30 35 40

Eastern white and red pine

Other eastern soft hardwoods

Soft maple

Cottonwood and aspen

Ash

Hickory

Other red oaks

Basswood

Select red oaks

Black walnut

Other eastern softwoods

Select white oaks

Other eastern hard hardwoods

Hard maple

Ratio of Mortality to Volume (percent)

Species/Species Group
 | 35

What this means 

The levels of tree mortality across Iowa are 2.5 times the rate during the 1950s, 

when removal rates were higher (see next section). 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. Total mortality for other eastern 

soft hardwoods species group is more than double that of any other species group. 

Dutch elm disease is a major contributor to this high mortality rate; American elm 

is the most numerous tree species within this group. Other than for eastern white 

and red pine species group, which shows a very high rate of mortality (but subject to 

large sampling error), 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.

Tree 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 classified 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. Also all trees cut or destroyed during 

changes in land use are considered “utilized” and are included in harvest removals. 

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 

reclassification 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 

occur on nearly every plot. Like forest growth, the rate at which trees are removed 

represents the average annual growing-stock removals that occurred between 2008 

and 2013 inventories.
36 | FOREST FEATURES

What we found

Average annual harvest removals of growing-stock trees on timberland as a 

percentage of total statewide volumes was about 1.6 percent in 1954, increasing to 

4.8 percent in 1974, then declining to 0.64 percent in the current inventory (Fig. 26). 

Most of the harvest removals were focused on species groups of soft maple, black 

walnut, other eastern soft hardwoods, cottonwood and aspen, other eastern hard 

hardwoods, select red oaks, and select white oaks, all of which had removals in excess 

of 1 million cubic feet per year (Fig. 27). Other removals were smaller than harvest 

removals for all species groups except cottonwood and aspen, where other removals 

accounted for 8.4 million cubic feet per year (Fig. 27). Average annual growing stock 

harvest removals as a percentage of total statewide volumes were greatest for soft 

maple, at 2.2 percent, with other eastern hard hardwoods and black walnut exceeding 

1.5 percent per year (Fig. 28). Average annual growing stock other removals was 

largest for cottonwood and aspen, at 2.6 percent, with all other species groups below 

1 percent (Fig. 29).

Figure 26.—Average annual harvest removals of growing stock 

as a percentage of total statewide growing-stock volume on 

timberland, Iowa, 1954-2013.

0

1

2

3

4

5

6

1950 1960 1970 1980 1990 2000 2010

Average Annual Harvest Removals

 (percent/year)

Inventory Year
 | 37

Figure 27.—Thirteen tree species or species groups ranked by average 

annual net growing-stock volume of harvest removals and of other 

removals per acre of timberland, Iowa, 2009-2013.

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000

Cottonwood and aspen

Black walnut

Soft maple

Other eastern soft hardwoods

Other eastern hard hardwoods

Select red oaks

Select white oaks

Ash

Other red oaks

Hickory

Hard maple

Basswood

Other eastern softwoods

Volume of Removals (thousand ft3

 per acre per year)

Species/Species Group

Harvest Removals

Other Removals

Figure 28.—Thirteen tree species or species groups ranked by average 

annual net volume harvest removals as a percentage of total growingstock volume on timberland, Iowa, 2009-2013.

0 0.5 1 1.5 2 2.5

Soft maple

Other eastern hard hardwoods

Black walnut

Cottonwood and aspen

Select red oaks

Other eastern soft hardwoods

Ash

Select white oaks

Other red oaks

Hard maple

Hickory

Basswood

Other eastern softwoods

Ratio of Harvest Removals to Volume (percent)

Species/Species Group
38 | FOREST FEATURES

What this means 

Removal rates are indicative of both harvest and land-use change. Average annual 

rate of removals (both harvest and other) for all species groups combined (1.2 percent 

per year) is slightly lower than that of mortality (1.5 percent per year) during 2009-

2013. Annual net growth (gross growth minus mortality) averaged 2.3 percent of total 

volume, which is almost double the rate of removals, indicating that Iowa removals 

are sustainable. During the previous 5 years (2004-2008), the average annual net 

growth of 3.4 percent per year exceeded the previous rate of removals (1.2 percent) 

by a larger amount, suggesting that the current ratio of growth to removals is lower 

than during the previous inventory. However, these rates may not apply to local scales 

or for specific species. For example, silver maple, which predominates in the soft 

maple species group and is the species with the largest volume of harvest removals 

(5.1 million cubic feet per year), has a rate of harvest removals (2.2 percent per year) 

exceeding rate of net growth (1.1 percent per year). In contrast, black walnut, which is 

one of the most valuable species in the State, has a growth rate (4.9 percent per year) 

that greatly exceeds the rate of harvest removals (1.7 percent per year). There is an 

ongoing need to monitor removals on a species-by-species basis. 

Figure 29.—Seven tree species or species groups ranked by 

average annual net volume of other removals (trees remaining 

after a land-use change) as a percentage of total growing-stock 

volume on timberland, Iowa, 2009-2013. 

0 0.5 1 1.5 2 2.5 3

Cottonwood and aspen

Black walnut

Other eastern hard hardwoods

Soft maple

Select red oaks

Select white oaks

Other eastern soft hardwoods

Ratio of Other Removals to Volume (percent)

Species/Species Group
 | 39

Forest Indicators

Ash tree infested with emerald ash borer, Lucas County, Iowa. Photo by Jeremy Cochran, Iowa Department of 

Natural Resources, used with permission.

 | 39
40 | FOREST INDICATORS 

Fragmentation and Urbanization of Forest Land

Background

In addition to direct loss of forest land due to conversion to other land uses, land 

development has a wider impact by increasing both the physical fragmentation and 

the urbanization of the remaining forested lands. Depending on the location of the 

development (or other activity), conversion from forest land to other uses can result 

in fragmenting remaining forest land. Changes in the size of remaining forest patches, 

in their level of connectivity to other large patches, in the amount of general forest 

cover surrounding each patch, and in the amount of forest–nonforest edge, all affect 

the quality of interior forest and the species and ecosystem functions that depend on 

interior conditions (Collinge 1996). These same factors also affect the ease with which 

exotic, invasive, or generalist species can gain a foothold, the ability of wildlife species 

to move across the landscape, and the ability of the forest to protect the quality and 

quantity of surface and ground water supplies. 

Spatial landscape pattern metrics help quantify these different characteristics of 

fragmentation. In the last 5-year report (Nelson et al. 2011), we examined the amount 

of forest in edge versus core situations with respect to the most widely used thresholds 

for interpreting likely impact. Values of metrics are sensitive to the grain (spatial 

resolution) of the land cover data source used (Moody and Woodcock 1995), similar 

to the way that animal species see the landscape very differently depending on the 

scale at which they operate. For example, the same patch that supplies interior forest 

conditions for one species is viewed as an unsuitable fragment by another. 

Important forest ecosystem processes operate at different scales, so in this report 

we examine current levels of fragmentation at two scales. We adapted a spatial 

integrity index (SII) developed by Kapos et al. (2000) for the Global Forest Resources 

Assessment that integrates three important facets of fragmentation affecting 

some aspect of forest ecosystem functioning—patch size, local forest density, and 

patch connectivity to core forest areas—to create a single resulting metric (SII) for 

comparison. Because even low misclassification rates in the source land cover data 

can be magnified into substantial errors in metric values (Langford et al. 2006, Shao 

and Wu 2008), we calculated spatial integrity at the two scales corresponding to two 

reliable and widely available source datasets: the 30 m (98.4 feet) scale of the 2011 

National Land Cover Dataset (Homer et al. 2015), and the 250 m (820 feet) scale of 

the 2009 FIA forest cover dataset (Wilson et al. 2012). Both scales fall within the 10 

to 1,000 km2

 (2,471 to 247,091 acres) scale at which linkages between patterns and 

processes are often of greatest management interest (Forman and Godron 1986). 
 | 41

Historically, portions of Iowa were home to oak savanna. This savanna was a matrix 

of prairie and trees, and thus not a completely closed tree canopy as is characteristic 

of forest land in other areas. Very little of this oak savanna is left, and only degraded 

remnants remain in the State (McGovern 2016). Depending on their tree canopy 

density, such forests may or may not be identified by the data sources and methods 

used in this analysis. Targeting those oak savanna forests would require data sources 

and methods more tuned to this intermix of forest and prairie.

In the SII calculation, core forest is defined by patch size and local forest density within 

a specified local neighborhood area. An unconnected forest fragment is defined by its 

patch size, local forest density, and distance to a core forest area, and the spatial integrity 

of all other forest lands is scaled between the forest fragment and the core forest area. 

Table 1 identifies the thresholds used to define both core forest and unconnected 

fragments, at the 250 m and 30 m scales, respectively.2 These thresholds are based on 

minimum core area reported by Kapos et al. (2000) (minimum of 100 km2 core area 

when using 1 km pixel resolution), rescaled to our pixel resolutions of 30 m and 250 m. 

These two scales capture a fairly broad range of definitions for core forest and spatial 

integrity that should encompass the scales appropriate for understanding impacts on a 

wide range of wildlife species and ecosystem processes affected by forest fragmentation 

in Iowa. At the 250 m scale where a patch must be 1,544 acres in size and have a local 

forest density of 90 percent within a 0.78-mile-radius circle, most of Iowa’s forest land 

does not qualify as core forest or forest with high spatial integrity. However, when 

examined at the 30 m scale, where a patch must be 22 acres in size and have 90 percent 

tree canopy cover within a 0.09-mile-radius circle, there are several counties with a 

substantial proportion of core or high-integrity forest land remaining. These areas of 

Table 1.—Spatial integrity index (SII) parameters used in calculations at each scale

Scale

Definition of Core 250 m (820 ft) 30 m (98 ft)

Patch size >1,544 acres >22 acres

Local forest density 90% 90%

 Neighborhood radius 0.78 mile 0.09 mile

Scale

Definition of Unconnected Fragment 250 m (820 ft) 30 m (98 ft)

Patch size <30 acres <2.5 acres

Local forest density ≤10% ≤10%

 Neighborhood radius 0.78 mile 0.09 mile

Distance to core >4.2 miles >0.5 miles

2 Unpublished data on file with Rachel Riemann, Troy, NY.
42 | FOREST INDICATORS 

forest land suggest an opportunity for planning that would retain their spatial integrity. 

In many areas, increasing the amount of tree cover in the surrounding landscape, and 

in particular creating additional connectivity of forest land between existing patches, 

would increase the spatial integrity of remaining forest land. 

What the SII identifies as core does not represent completely intact forest conditions 

because it is calculated from forest canopy alone and does not take into consideration 

underlying house densities or proximity to roads. Using Radeloff et al.’s (2005) 

definition of Wildland Urban Intermix areas (greater than 15.5 housing units per 

square mile [6.17 housing units per km2]), we identified how much forest, and 

particularly SII core or intact forest land, coincided with these intermix areas. The 

wildland-urban interface (WUI) is described as the zone where human development 

meets or intermingles with undeveloped wildland vegetation, and it is associated with 

a variety of human–environment conflicts. Radeloff et al. (2005) defined this area in 

terms of the density of houses (WUI “intermix” areas), the proximity to developed 

areas (WUI “interface” areas), and percentage of vegetation coverage present. We used 

maps of WUI intermix areas intersected with forest land in the 2011 National Land 

Cover Dataset (Fig. 30) to examine changes in the amount of forest land co-occurring 

with WUI house densities.

What we found

The human population of Iowa increased by 4.1 percent to 3.05 million between 2000 

and 2010. During that same time the number of housing units increased by 8.9 percent 

(U.S. Census Bureau 2011); that is, over those 10 years the increase in housing units 

occurred at a pace 2.5 times the rate of increase in population, a trend not limited 

to Iowa. In recent decades this housing growth has occurred not only in increasing 

suburban rings around urban areas but also in rural areas. Lepczyk et al. (2007), 

Theobald (2005), and Hammer et al. (2004) observed that the areas facing particularly 

rapid increases in housing density currently and into the future were amenity-rich rural 

areas around lakes and other recreation areas. The 27.6 percent increase in the number 

of reported second homes in Iowa from 2000 to 2010 (21,020 units) could be a partial 

reflection of this trend. This may put additional pressure on forested areas even above 

the general increases in population density and housing density. 

When we calculated SII classes for forest land in Iowa at the 250 m scale (in which 

core forest is defined as 1,544 acres and greater), 1 percent of the forest land is core 

forest, 6 percent has high spatial integrity, 6 percent has medium integrity, 1 percent 

has low spatial integrity, and 87 percent of the forest is in unconnected fragments 
 

| 43 Figure 30.—Iowa land cover, 2011. Data source: National Land Cover Database 2011 (Homer et al. 2015).

AgricultureDevelopedForest landNatural nonforestWater
44 | FOREST INDICATORS 

(total does not sum to 100 percent due to rounding error) (Table 2). At the 30 m scale, 

with 22 acres or greater considered core forest, 20 percent of the forest land in Iowa is 

core forest, 20 percent has high integrity, 12 percent has medium integrity, 1 percent 

has low integrity, and 47 percent of the forest is in unconnected fragments (Table 2). 

Forest connectivity is highest in the Northeastern and Southeastern Units and lowest 

in the Northwestern Unit (Fig. 31). Figure 32 compares the SII classes between the 

two scales for a four-county area in the northeastern corner of Iowa. It is important 

to note that the forest land datasets being used are depicting tree cover only and do 

not fully incorporate the presence of any development that might be associated with 

or underlying this tree cover. As we take this caveat into account in the following 

analysis, it is evident that some housing development incursions (e.g., WUI) into 

these areas are happening. 

Unlike many states, Iowa’s forest land does not appear to be immediately threatened 

by urban development. The amount of forest land with sufficient underlying housing 

density to qualify as WUI areas has been steadily increasing, but is still a small 

proportion of the total forest land in Iowa (Table 3). In 1990, 4 percent of Iowa’s forest 

land was in low- and medium-density WUI. In 2000 this share increased to 5 percent 

of the forest land, and in 2010 it was 6 percent of the forest land. Worth noting, 

however, is that when we integrate SII results at the 30 m scale with the WUI classes, a 

third of this 6 percent is affecting Iowa’s core forest, reducing it from 20 percent to 18 

percent of the forest land in Iowa at the 30 m scale. 

Proportion of forest land by 30 m 

(98 ft) spatial integrity class

 Proportion of forest land by 250 m 

 (820 ft) spatial integrity class

Unit

Forest 

fragment

Low 

SII

Medium 

SII

High 

SII

Core 

forest

Forest 

fragment

Low 

SII

Medium 

SII

High 

SII

Core 

forest

Northeastern 36% 2% 14% 23% 25% 80% 2% 9% 7% 2%

Southeastern 46% 1% 12% 21% 20% 91% 0% 3% 4% 1%

Southwestern 73% 1% 5% 11% 10% 85% 1% 5% 8% 1%

Northwestern 79% 1% 6% 9% 5% 100% 0% 0% 0% 0%

Iowa 47% 1% 12% 20% 20% 87% 1% 6% 6% 1%

Iowa after 

incorporating 

WUI areas

47% 2% 12% 21% 18% 87% 1% 6% 6% 1%

Table 2.—Percentage of forest cover by spatial integrity index (SII) class and FIA unit, at 30 m (2011) and 250 m spatial 

resolution, and adjusted for wildland-urban interface (WUI), Iowa
 | 45

Figure 31.—Proportion of forest land in core (spatial integrity index [SII] = 10) or high integrity (SII = 7 to 9) forest 

conditions at the 30 m scale, Iowa, 2011. Data source: National Land Cover Database 2011 (Homer et al. 2015).

0

0-0.25

0.26-0.5

0.51-0.75

0.76-1

Figure 32.—Forest land by spatial integrity index (SII) at the 250 m scale (A) and 30 m scale (B) in the northeastern four 

counties, Iowa, 2009 or 2011. Data source for 250 m scale: Unpublished data on file with B. Tyler Wilson, U.S. Forest 

Service, St. Paul, MN; for 30 m scale: National Land Cover Database 2011 (Homer et al. 2015).

A B

Forested Area

Unconnected

 Low integrity

 Medium integrity

 High integrity

Core

Table 3.—Proportion of forest land in the wildland-urban intermix, by FIA unit, Iowa, 1990-2010

 Proportion of forest land

Unit 1990 2000 2010

Northeastern 5% 7% 8%

Southeastern 3% 4% 5%

Southwestern 4% 5% 5%

Northwestern 2% 3% 3%

Iowa 4% 5% 6%
46 | FOREST INDICATORS 

What this means

Overall, forest makes up about 8 percent of Iowa’s land base. How this forest land 

is arranged across the landscape affects ecological processes. From its forest-prairie 

history, Iowa’s forest is currently primarily distributed within a matrix of agricultural 

land uses. Within that setting, we examined the spatial integrity of Iowa’s forest lands, 

and the extent of additional urban pressures. Based on the map pixel analysis, 1 percent 

of the area of Iowa’s forest is in core conditions forest when assessed at 250 m resolution, 

and 20 percent is core at 30 m resolution. These results indicate that the remaining 

forest land in Iowa is heavily fragmented at multiple scales of analysis. Overall forest 

pattern varies considerably across the State, as did historical forest cover. Northeastern 

and southeastern counties contain the bulk of the forest land in Iowa and also the least 

fragmented forest. However, even these counties contain substantial proportions of 

forest patches with no more than low spatial integrity (38 percent and 47 percent of 

forest land in the northeastern and southeastern counties, respectively) at our smallest 

unit of analysis. This degree of fragmentation has serious implications for these forest 

patches. Much of this forest is likely to be susceptible to negative edge effects, such as 

invasion by nonnative invasive species, a situation which makes it harder to maintain 

biodiversity and healthy populations of native plants and wildlife.

Tree Crown Health and Damage 

Background

The crown condition of trees is influenced by various biotic and abiotic stressors. 

Biotic stressors include native or introduced insects, diseases, invasive plant species, 

and animals. Abiotic stressors include drought, flooding, cold temperatures or freeze 

injury, nutrient deficiencies, soil physical properties affecting soil moisture and 

aeration, and toxic pollutants. 

Invasions by exotic diseases and insects are one of the most important threats to the 

productivity and stability of forest ecosystems around the world (Liebhold et al. 

1995b, Pimentel et al. 2000, Vitousek et al. 1996). Over the last century, Iowa’s 

forests have suffered the effects of native insect pests such as forest tent caterpillar 

(Malacosoma disstria) and the well-known exotic and invasive agent Dutch elm 

disease (Ophiostoma ulmi), among many others. More recently, invasions by the 
 | 47

emerald ash borer (Agrilus planipennis) and gypsy moth (Lymantria dispar) are 

threatening the health of trees. Additionally, although Asian longhorned beetle 

(Anoplophora glabripennis) and thousand cankers disease (Pityophthorus juglandis 

and its fungal associate Geosmithia morbida) have not yet been discovered in Iowa, 

they have been discovered in neighboring states.

Data on tree-level crown dieback are collected on P2+ plots (a subset of FIA Phase 

2 plots, on which additional measurements are taken). Crown dieback is defined as 

recent mortality of branches with fine twigs and reflects the severity of recent stresses 

on a tree. A crown was labeled “poor” if crown dieback was greater than 20 percent. 

This threshold is based on findings by Steinman (2000) that associated crown ratings 

with tree mortality. Additionally, crown dieback has been shown to be the best FIA 

crown variable to use for predicting tree survival (Morin et al. 2012).

Tree damage is assessed for trees at least 5.0 inches d.b.h. Up to three types of damage 

can be recorded. If more than three damage agents are observed, decisions about 

which three to record are based on the relative abundance of the damaging agents 

(U.S. Forest Service 2010).The types of damage that are recorded include defoliation, 

foliage disease, cankers, decay, rot, fire, animal damage, weather, and logging damage.

What we found

The incidence of poor crown condition is low across Iowa with no discernable 

spatial pattern (Fig. 33). The only species with more than 5 percent of live basal area 

containing poor crowns is American elm (Table 4). Mean dieback ranged from 1 

percent for shagbark hickory to 13 percent for American basswood (Table 5).

Figure 34 shows the proportion of remeasured trees that survived, died, or were cut in 

crown dieback classes based on the health of the crowns at the previous measurement. 

The proportion of the trees that died increased with increasing crown dieback. Nearly 

50 percent of trees with crown dieback above 20 percent during the 2008 inventory 

were dead when visited again during the 2013 inventory.

Damage was recorded on about 28 percent of the trees in Iowa, but there is 

considerable variation between species (Table 6). Because of Dutch elm disease, elm 

species have the highest mortality rate of all species in the State. The most frequent 

damage on all species was decay (17 percent of trees), but it ranged from 4 percent 

on American elm to 29 percent on silver maple. Damage from animals was noted on 

17 percent of silver maple trees. The occurrence of all other injury types was very low 

(Table 6).
48 

| FOREST INDICATORS 

.0.0

t

rest

Basal

Crowns

Figure 33.—Percentage of live basal area with poor crowns, Iowa, 2009-2013. Depicted FIA plot locations are approximate.Cartography: R.S. Morin, U.S. Forest Service, Jan. 2015.online at www.fia.fs.fed.us/tools-data/.Geographic base data are provided by the National Atlas of the USA. FIA data and tools are available Sources: U.S. Forest Service, Forest Inventory and Analysis program, 2013 and NLCD, 2006.Projection: Iowa State Plane North, North American Datum 83.

CrownsBasal Area with Poor Percentage of Live 

WaterNonforestForest5.1-9.00.1-5.000.0

0.1 - 5.05.1 - 9.0ForestNonforestWater

Percent of Live Basal

Area with Poor Crowns
 | 49

What this means

The trees of the most important species in Iowa’s forests are generally in good health, 

but substantial dieback was observed in American elm. Poor crown health in American 

elm is most likely due to the impacts of Dutch elm disease. The health of ash species, 

oak species, maple species, black walnut, and butternut should be monitored closely in 

the face of recent and likely future invasions by emerald ash borer, gypsy moth, Asian 

longhorned beetle, and thousand cankers disease. As in most eastern forests, decay is 

the most commonly observed damage in Iowa’s forests, an expected finding given that 

most of Iowa’s forests comprise mature trees. Additionally, silver maple is a relatively 

short-lived species that is likely to develop decay as it ages.

Table 4.—Percentage of live basal area with poor crowns, Iowa, 2008 and 2013

Proportion of Basal Area with Poor Crowns 

Species 2008 2013

American elm 7.1% 7.3%

American basswood 0.0% 4.7%

Silver maple 5.0% 2.5%

White oak 2.1% 1.8%

Bur oak 0.6% 0.6%

Cottonwood 33.6% 0.0%

Shagbark hickory 13.8% 0.0%

Black walnut 0.7% 0.0%

Hackberry 0.0% 0.0%

Northern red oak 0.0% 0.0%

Table 5.—Mean crown dieback and other statistics for live trees (at least 5 inches d.b.h.) on forest land by species, 

Iowa, 2013

Species Trees Mean SE Minimum Median Maximum 

number --------------------------------- percent --------------------------------

American basswood 36 12.5 4.6 0 5 95

American elm 56 8.6 2.3 0 5 99

White oak 22 6.1 2.8 0 5 65

Bur oak 55 6.1 1.8 0 5 99

Silver maple 64 6.0 0.7 0 5 40

Northern red oak 11 4.1 0.9 0 5 10

Cottonwood 14 3.9 0.9 0 5 10

Hackberry 45 3.3 0.4 0 5 5

Black walnut 25 2.0 0.5 0 0 5

Shagbark hickory 32 1.3 0.4 0 0 10
50 | FOREST INDICATORS 

Figure 34.—Crown dieback distribution by tree survivorship for 

remeasured trees, Iowa, 2004-2008 to 2009-2013.

0

10

20

30

40

50

60

70

80

90

100

0 1-10 11-20 >20

Frequency (percent)

Crown Dieback (percent)

Cut

Mortality

Survivor

Table 6.—Percentage of trees with damage, by species, Iowa, 2013

Damage 

type All

American 

basswood

American 

elm

Black 

walnut

Bur 

oak

CottonwoodHackberryNorthern 

red oak

Shagbark 

hickory

Silver 

maple

White 

oak

--------------------------------------------- percent -----------------------------------------------------

None 72 66 86 83 80 85 81 81 79 56 86

Animal 5 7 3 4 6 4 2 0 8 17 3

Cankers 2 0 0 1 0 0 1 0 0 3 0

Decay 17 24 4 11 11 9 11 14 9 29 6

Insect 

damage

2 0 0 0 0 0 2 0 7 2 0

Human 

activity (e.g., 

logging, 

pruning, 

vandalism) 

2 3 2 2 3 0 1 1 2 2 2

Other 2 2 4 1 1 0 1 2 1 4 2

Weather 5 4 4 4 2 3 5 4 2 2 3

Note that columns do not sum to 100 because multiple damages can be recorded on trees.
 | 51

Insects and Disease Pathogens

Background

Emerald ash borer (EAB), an exotic wood-boring beetle, was detected in the United 

States in 2002 near Detroit, Michigan. A pest of all North American ash (Fraxinus spp.) 

and white fringetree (Chionanthus virginicus), EAB has been present in Iowa since 2010 

(Cipollini 2015) (Fig. 35). 

Gypsy moth was captured across much of Iowa during the most recent inventory, with the 

largest concentrations along the northeast border, but it is not currently established in the 

State. Host tree species were split into preferred and nonpreferred suitability classes based 

on previously published field and laboratory tests (Liebhold et al. 1995a). Species from 

suitability class 1 were considered preferred and all others were considered nonpreferred. 

Bur oak blight (BOB) is a late-season leaf disease that is increasing in severity on bur 

oak trees in Iowa. It is caused by the newly described pathogen Tubakia iowensis. 

Symptoms include necrosis of leaf tissue along the veins, which eventually leads to 

death of the entire leaf starting around late July (Harrington et al. 2012). BOB was 

found on bur oaks throughout Iowa, but mature trees in upland forests appear to be 

most at risk of severe symptoms.

What we found

Emerald ash borer.—Iowa has 50.8 million ash trees of at least 1 inch d.b.h. on forest land. 

Of these, 16.1 million are at least 5 inches d.b.h., accounting for 4.7 percent of trees of all 

species. Highest densities of ash occur in the northeastern portion of the State (Fig. 35). 

Although ash growing-stock mortality on timberland was fairly stable from 1990 to 2008, it 

increased threefold in 2013, reaching 1.5 million cubic feet per year. Most of this mortality 

was concentrated in northeastern Iowa. 

Gypsy moth.—About 51 percent of the live-tree volume in Iowa included species preferred 

by gypsy moth. The most abundant preferred species in the State were the oaks and black 

willow. The highest density of host species preferred by gypsy moth occurred in the 

southeastern forests near the Des Moines River (Fig. 36). 

Bur oak blight.—Nearly 98 percent of Iowa’s 32 million bur oaks occurred on upland forest 

land. Sixty-two percent of upland forest bur oaks were in large, sawtimber-sized stands 

(Fig. 37). There were 10.5 million bur oaks in mature stands with an age of at least 76 years, 

with 84 percent of those in sawtimber-sized stands. Average annual mortality of bur oaks 

increased from 214,202 trees per year in the 2008 inventory to 235,855 trees per year in 

2013 (Fig. 38). Disease as the cause of death was not recorded in any bur oaks in the 2008 

inventory, but in the 2013 inventory the death of an average of 40,557 bur oaks per year was 

attributed to disease.
52 | FOREST INDICATORS 

E

A

B

iti

(

f

D

2

013)

A

s

h

B

a

s

al A

r

e

a

(ft

2

/a

c

re)

10

Figure 35.—Ash density on forest land and presence of emerald ash borer, Iowa, 2013.

Ash Basal Area (ft2 /acre)

EAB positive (as of January 2015)EAB positive (as of December 2013)Nonforest<33-9

>10
 | 53

0 to 4.95 to 24.925 to 49.950 to 74.975+NonforestWater

Percentage of Live Basal Area in

Species Preferred by Gypsy Moth

Figure 36.—Density of host tree species that are preferred by gypsy moth, Iowa, 2013.

Percentage of Live Basal Area in Species Preferred by Gypsy Moth

0-4.95-24.925-49.950-74.9≥75NonforestWater

Projection: Iowa State Plane North, North American Datum 83.Source: Wilson et al. (2012).Geographic base data are provided by the National Atlas of the USA. FIA data and tools are available online at www.fia.fs.fed.us/tools-data/.Cartography: R.S. Morin, U.S. Forest Service, Jan. 2015.

0 to 4.95 to 24.925 to 49.950 to 74.975+NonforestWater

Percentage of Live Basal Area in

Species Preferred by Gypsy Moth
54 | FOREST INDICATORS 

What this means

Emerald ash borer.—Because EAB is likely to be the largest contributor to ash 

mortality, the continued spread of EAB could have a considerable impact on ash 

health and the future composition of Iowa’s forests. 

Gypsy moth.—As gypsy moth continues to spread west across the United States, 

quantification of the amount of the forest resource that is preferred by this insect can 

help land managers prepare for future outbreaks. 

Bur oak blight.—The increase in bur oak mortality and better identification of disease 

as the cause of death may indicate a potential rise of BOB in Iowa. As climate change 

potentially creates conditions that may favor BOB, such as increased spring precipitation, 

Iowa’s valuable bur oak resource may be under further pressure from BOB. 

Figure 37.—Number of bur oak trees on uplands by standsize class and stand-age class, Iowa, 2013.

0

5

10

15

20

25

30

35

Total Sawtimber Poletimber Saplings

Number of Trees (millions)

Stand-size Class

126+ years

101 to 125 years

76 to 100 years

51 to 75 years

26 to 50 years

0 to 25 years

Figure 38.—Average annual mortality of bur oak trees by 

cause of death, Iowa, 2008 and 2013.

0

50

100

150

200

250

Number of Trees (thousands)

2008

2013

All Mortality

Disease

Animal

Weather

Vegetation

Unknown/Other
 | 55

Down Woody Materials

Background 

Down woody materials, in the various forms of fallen trees and shed branches, fill 

a critical ecological niche in Iowa’s forests. Down woody materials provide valuable 

wildlife habitat, stand structural diversity, and a store of carbon and biomass; they 

also contribute to forest fire hazards via surface woody fuels. 

What we found 

The total carbon stored in down woody materials (fine and coarse woody debris) on 

Iowa’s forest land exceeded 8 million tons (Fig. 39). Downed woody debris carbon was 

normally distributed by stand age class, meaning that moderate-aged stands (41-80 

years) had the highest total carbon (about 2.3 million tons), with less carbon in both 

younger and older ages classes (Fig. 39). More than 18.5 million tons of downed dead 

wood biomass occur within Iowa’s forests, dominated by coarse woody debris at 69 

percent; the remainder is fine woody debris (31 percent), with almost no dead wood 

biomass in residue piles. The total volume of coarse woody debris was highest in the 

private ownership category at roughly 1 billion cubic feet across Iowa’s forests (Fig. 

40). State and local forests had the second largest total of coarse woody debris volume 

(about 86 million cubic feet).

-500,000

0

500,000

1,000,000

1,500,000

2,000,000

2,500,000

3,000,000

3,500,000

4,000,000

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

Total Carbon (tons)

Stand Age (years)

Figure 39.—Total carbon in down woody materials (fine and 

coarse woody debris and piles) by stand-age class on forest 

land, and associated sampling error, Iowa, 2009-2013.
56 | FOREST INDICATORS 

What this means 

Given the relatively moist temperate forests across Iowa, only in times of drought 

would the biomass of down woody materials be considered a fire hazard. Although 

the carbon stocks of down woody materials are small compared to those of soils 

and standing live biomass across Iowa, down woody materials are still a critical 

component of the carbon cycle as a transitory stage between live biomass and other 

detrital pools such as the litter. Beyond transition of dead wood carbon to other 

pools, there is a potential for a reduction in these stocks due to increased rates of 

decay if future temperature and precipitation patterns change as projected (Russell et 

al. 2014a, 2014b). The loss of dead wood carbon stocks could indicate the reduction 

of other pools in the future. In contrast with southeastern states, where there is more 

widespread industrial management of forests (Woodall et al. 2013), no dead wood 

piles were sampled in this first down woody materials inventory of Iowa’s forests. 

Given that the vast majority of coarse woody debris volume was estimated to be in 

private ownership, it is the management of Iowa’s private forests that may affect future 

contributions from down woody materials to statewide forest carbon stocks and 

wildlife habitat (i.e., stand structure). Overall, because fuel loadings are estimated to 

not be exceedingly high across Iowa, the numerous ecosystem services provided by 

down woody materials may outweigh possible fire dangers.

Figure 40.—Total volume of coarse woody debris on forest 

land by ownership group, and associated sampling error, 

Iowa, 2009-2013.

0

200,000,000

400,000,000

600,000,000

800,000,000

1,000,000,000

1,200,000,000

1,400,000,000

National 

forest

Other 

Federal

State and 

local

Private

Volume (ft3

)

Ownership Group

0

200,000,000

400,000,000

600,000,000

800,000,000

1,000,000,000

1,200,000,000

1,400,000,000

Federal State and local Private

Volume (ft3

)

Ownership Group
 | 57

Nonnative Invasive Plants

Background

Invasive plant species (IPS) are both native and nonnative species that can cause 

negative ecological effects. These species can quickly invade forests, changing 

light, nutrient, and water availability. IPS can form dense monocultures which 

not only reduce regeneration but also affect wildlife habitat quantity and quality 

by altering forest structure and forage availability. Despite some beneficial uses 

of these invaders, such as culinary and medicinal purposes and soil contaminant 

extraction (reed canarygrass; Kurtz 2013), the negative effects are worrisome. 

Each year inspection, management, and mitigation of IPS cost billions of dollars. 

To aid in monitoring these species, FIA assessed the presence of 43 IPS and 

one undifferentiated genus (nonnative bush honeysuckles) on 107 P2 invasive 

plots in Iowa from 2009 through 2013. Because field crews code nonnative bush 

honeysuckles sometimes at the genus level and other times at the species level, this 

report lumps all of the nonnative bush honeysuckles into one category, reducing the 

number of species monitored to 40. 

What we found

Of the 40 invasive species monitored (Table 7), 15 were observed in Iowa (Table 8). 

Scientific names for nonnative invasive plants are listed in Table 7. Multiflora rose 

was the most commonly observed species (66 plots; 61.7 percent, Fig. 41). It was most 

common in the south-central and eastern regions of Iowa (Fig. 42). Reed canarygrass 

(48 plots), nonnative bush honeysuckles (39 plots), and garlic mustard (21 plots) were 

also found on many plots. The other 11 species were found on less than 9 percent of 

plots. One or more IPS were found on 94.4 percent of the monitored plots in Iowa. On 

plots with invasive plants, the percentage of a plot that was forested was lower than for 

those plots without invasive plants (71.4 

percent versus 91.3 percent, respectively).

Figure 41.—Multiflora rose. Photo by James H. Miller, U.S. 

Forest Service, Bugwood.org.
58 | FOREST INDICATORS 

Tree Species Vine Species

Ailanthus (Ailanthus altissima) English ivy (Hedera helix)

Black locust (Robinia pseudoacacia) Japanese honeysuckle (Lonicera japonica)

Chinaberry (Melia azedarach) Oriental bittersweet (Celastrus orbiculatus)

Chinese tallowtree (Triadica sebifera)

Norway maple (Acer platanoides) Herbaceous Species

Russian olive (Elaeagnus angustifolia) Black swallow-wort (Cynanchum louiseae)

Princesstree (Paulownia tomentosa) Bull thistle (Cirsium vulgare)

Punktree (Melaleuca quinquenervia) Canada thistle (Cirsium arvense)

Saltcedar (Tamarix ramosissima) Creeping jenny (Lysimachia nummularia)

Siberian elm (Ulmus pumila) Dames rocket (Hesperis matronalis)

Silktree (Albizia julibrissin) European swallow-wort (Cynanchum rossicum)

Garlic mustard (Alliaria petiolata)

Shrub Species Giant knotweed (Polygonum sachalinense)

Autumn olive (Elaeagnus umbellata) Japanese knotweed (Polygonum cuspidatum)

Common barberry (Berberis vulgaris) Leafy spurge (Euphorbia esula)

Common buckthorn (Rhamnus cathartica) Bohemian knotweed (Polygonum xbohemicum)

European cranberrybush (Viburnum opulus) Purple loosestrife (Lythrum salicaria)

European privet (Ligustrum vulgare) Spotted knapweed (Centaurea stoebe ssp. micranthos)

Glossy buckthorn (Frangula alnus)

Japanese barberry (Berberis thunbergii) Grass Species

Japanese meadowsweet (Spiraea japonica) Common reed (Phragmites australis)

Multiflora rose (Rosa multiflora) Nepalese browntop (Microstegium vimineum)

Nonnative bush honeysuckles (Lonicera spp.) Reed canarygrass (Phalaris arundinacea)

Table 7.—List of 39 invasive plant species and one undifferentiated genus monitored by the Northern Research Station 

on Forest Inventory and Analysis P2 invasive plots, 2007-2013

Table 8.—List of 14 invasive plant species and one undifferentiated genus detected on Iowa plots, 2009-2013

Name Observances Percentage of plots 

Multiflora rose 66 61.7

Reed canarygrass 48 44.9

Nonnative bush honeysuckles 39 36.4

Garlic mustard 21 19.6

Autumn olive 9 8.4

Bull thistle 8 7.5

Common buckthorn 8 7.5

Black locust 7 6.5

Siberian elm 6 5.6

Japanese barberry 4 3.7

Canada thistle 2 1.9

Creeping jenny 2 1.9

Dames rocket 1 0.9

Japanese knotweed 1 0.9

Oriental bittersweet 1 0.9
 

| 59 Figure 42.—Approximate location of P2 invasive plots showing the presence and absence of multiflora rose, the most commonly occurring invasive plant species of 

Projection: NAD83, UTM Zone 15N.Data source: U.S. Forest Service, FIA program, 2009-2013 Phase 2 invasive data.State and county layers source: ESRI Data and Maps 9.3.Forest/nonforest source: NLCD 2006. Depicted plot locations are approximate.Cartography: C.M. Kurtz, U.S. Forest Service, Jan. 2015.

those monitored in this region, 2009-2013.

Occurrence of Multiflora Rose

Present

Absent
60 | FOREST INDICATORS 

Changes in Iowa from 2007-2008 to 2009-2013

In the 2007-2008 survey 15 IPS were observed in Iowa (Nelson et al. 2011), the same 

number as in the 2009-2013 survey. However, there were actually more IPS observed 

in this survey because nonnative bush honeysuckles were combined into one group. 

Creeping jenny and Japanese knotweed are two IPS that were not observed last time 

but were found in this inventory. These two invasive plants are commonly used 

for landscaping. Japanese knotweed is of particular concern as it is very difficult to 

control, can penetrate pavement, and poses a fire hazard (Kurtz 2013). As in the 2007-

2008 survey, the two most commonly recorded species remained the same, multiflora 

rose and reed canarygrass, though both occurred on around 10 percent more plots 

in this inventory. This increase in presence was not found for all the invasive species 

monitored. Species such as autumn olive, common buckthorn, Japanese barberry, 

dames rocket, and Oriental bittersweet occurred on a lower percentage of plots in this 

inventory.

What this means

The presence of IPS within Iowa’s rich forests is a concern and it is important that 

these species are monitored over time to ensure that managers and the public are 

aware of their occurrence and spread. Further investigation of the inventory data may 

help to reveal influential site and regional trends. Invasive plants are good competitors 

and are able to alter forested ecosystems by displacing native species and affecting 

the fauna that depend upon them. Several characteristics contribute to their success, 

such as prolific seed production, ability to propagate vegetatively, rapid growth rate, 

and ability to survive in harsh conditions. Among the many factors that exacerbate 

invasion of forests by IPS are ungulates (e.g., cattle grazing), timber harvesting, 

development, and fragmentation (plots on a forest/nonforest edge were more likely 

to have IPS). When forests are invaded, IPS negatively affect the carbon budget by 

reducing future tree cover. Furthermore, these species can have costly economic 

implications through diminished timber yield and aesthetic beauty. 
 | 61

Nonforest Trees

Background

Areas of tree cover must be at least 1 acre in size and 120 feet wide to meet FIA’s 

definition of forest land. Some of the tree cover in Iowa, however, is configured in 

a way (e.g., narrow linear strips) that does not meet these requirements. Despite 

their small or narrow patch sizes, these groupings of trees are a critical resource 

and offer a wide range of benefits, such as preventing erosion, serving as riparian 

buffers, providing wildlife habitat, sequestering carbon, and protecting structures 

and livestock from harsh weather. Recently, natural resource agencies have noted a 

deficiency of available information on this important nonforest tree (NFT) resource. 

The lack of information is a concern, but obtaining such data is challenging. Remote 

sensing-based land cover geospatial datasets are often produced at spatial resolutions 

that are too coarse to identify narrow tree plantings or individual tree crowns. 

However, digital aerial imagery, such as that from the National Agriculture Imagery 

Program (NAIP) is acquired at a spatial resolution sufficient to identify such features. 

The Iowa Department of Natural Resources used multiple dates of NAIP imagery and 

elevation data derived from light detection and ranging (LiDAR) to create a 15-class 

high-resolution (i.e., 1 meter or 3.28 feet) land cover dataset (HRLCD) for the entire 

State with an effective date of 2009 (Fig. 43). We compared county- and state-level 

areas of tree cover from the HRLCD to county- and state-level estimates of forest 

land area reported by FIA in order to gain an understanding of the additional NFT 

resource in Iowa. 
62 | FOREST INDICATORS Figure 43.—High-resolution (1 meter) land cover data for Iowa, 2009.

Land Cover Class

Shadow/No dataRoads/ImperviousStructuresBarren/FallowSoybeansCornCut hayGrass 2Grass 1Deciduous tallDeciduous mediumDeciduous shortConiferous forestWetlandWater
 | 63

What we found

The high-resolution land cover data were available for all 99 counties in Iowa. The 

HRLCD identified about 4 million acres of tree cover whereas the FIA estimate of 

forest land for the State was just under 3 million acres. The HRLCD estimate of tree 

cover was larger for 83 counties compared to the FIA estimates. A county-by-county 

analysis found that the largest difference between the HRLCD and FIA estimates 

occurred in Dubuque County, where the HRLCD estimate was nearly 49,000 acres 

larger; the smallest difference in estimates was for Guthrie County with a mere 63 

acres of difference. On average, the HRLCD per-county estimate of tree cover was 

about 9,900 acres more than the FIA estimate of forest land area. Figure 44 shows the 

relationship between county-level estimates of tree cover obtained from the HRLCD 

versus estimates of forest land area from FIA. Even when the FIA estimate reported 

no forest land area for a county, the HRLCD identified some area of tree cover for that 

county. There were 16 counties where the FIA estimate of forest land was higher than 

the HRLCD estimate of tree cover. 

What this means

Iowa contains a large amount of treed lands that do not meet FIA’s definition of forest 

land but that are nonetheless a significant tree resource. It is important that these 

areas continue to be monitored and managed because they are vital for protecting soil, 

wildlife, domestic animals, homes, roads, water sources, and recreational areas. The 

results obtained from remote sensing-based procedures can provide useful information 

for the careful management of the NFT resource as well as the ecosystem services 

these trees provide. One important advantage of high-resolution datasets of tree cover 

is that not only do we know more about their total extent, we are also provided with 

Figure 44.—County-level estimates of Forest Inventory and Analysis 

(FIA) forest land area (2013) versus estimates of tree-covered areas 

obtained from high-resolution land cover data (HRLCD) for Iowa, 2009.

0

20

40

60

80

100

120

140

160

180

200

0 20 40 60 80 100 120 140 160 180 200

HRLCD Area of Tree Cover 

(thousand acres)

FIA Area of Forest Land (thousand acres)
64 | FOREST INDICATORS 

detailed locational information for more comprehensive monitoring of changes to the 

overall tree resources in the State. Changes in land use practices that affect nonforest 

windbreaks and other narrow linear tree features could go unnoticed if using traditional 

inventory definitions and methods. A remote sensing-based monitoring program 

provides enhanced monitoring of such resources. Note that FIA- and HRLCD-based 

estimates may also differ due to land use versus land cover definitions. Recently 

harvested forest stands in early stages of regeneration are still labeled as forest by FIA, 

but probably would show no tree cover in the HRLCD. Tree cover in a context of 

nonforest use would be labeled as nonforest in FIA, but would still be included within 

HRLCD. Forest loss (diversion) following 2009 would not be detected by HRLCD, but 

would be identified in FIA data. See additional discussion of land use and land cover in 

the sections on Land-Use Change and Fragmentation and Urbanization of Forest Land 

on pp. 14 and 40, respectively.

Regeneration Status

Background

The composition and abundance of tree seedlings (Fig. 45) drive the character of 

forest ecosystems in the early years of stand development. Benefits of forest in the 

early phases of development are described in the Forest Successional Stage section on 

p. 77. Young forests set the stage for future composition and structure, and hence, the 

viability and sustainability of timber and ecosystem services provided (e.g., riparian 

forest habitat, carbon sequestration, 

and scenic beauty). Nelson et al. 

(2011) recommended monitoring 

regeneration of Iowa’s forests because 

of the economic importance of 

larger-diameter species, which are 

aging, undergoing unprecedented 

mortality levels, and will eventually 

need replacement. These species 

are the cornerstone of Iowa’s wood 

products and manufacturing 

industry, which contributes nearly 

Figure 45.—Bur oak seedling. Photo by Carl Strang, courtesy of 

the Forest Preserve District of Du Page County, Illinois. 
 | 65

$4 billion in annual sales, $900 million in annual payroll, and more than $25 million 

in annual timber sales to private woodland owners (Iowa Department of Natural 

Resources 2009).

Forest systems of Iowa face numerous regeneration stressors, such as invasive plants, 

insects and diseases, herbivory, and climate change. As stands that make up these 

systems mature and undergo stand replacement disturbances, it is imperative to 

know the condition of the regeneration component. Although planting or seeding 

are options in some stands, the region is dominated by forest systems that regenerate 

naturally. In most situations, establishing desirable reproduction is the key to 

replacing stands in need with high-canopy species that meet managers’ objectives. 

Tending of young stands to control composition and stocking levels is also a very 

important consideration (Smith 1997). Regeneration data are critically important for 

understanding and projecting future forest character, which ultimately determines 

sustainability of the full suite of forest values available from Iowa’s forests. To address 

the need for more detailed information on regeneration, the FIA program of the 

Northern Research Station (NRS) has added measurement protocols collected on a 

subset of NRS-FIA sample plots measured during the growing season (McWilliams 

et al. 2015). The procedures measure all established tree seedlings less than 

1-inch d.b.h. by length class and include a browse impact assessment for the area 

surrounding the sample location. These Regeneration Indicator data improve NRSFIA’s ability to evaluate this important aspect of forest health and sustainability.

What we found

The results in this report are based on 26 sample plots measured in 2012 and 2013. 

Results of the browse impact assessment show that most of the samples (75 percent) 

had either medium or low levels of browse of understory plants (Fig. 46). The other 

one-quarter of the samples had high or very high browse impact. Examination of 

browse impact across the State does not reveal any distinct pattern other than small 

localized impacts (Fig. 47). It should be noted that there are relatively few samples to 

evaluate regional patterns.

The total number of seedlings was estimated at 20.1 billion, or an average of 7,700 

seedlings per acre of forest land. About 75 percent of the seedlings are less than 1 foot 

tall, 22 percent are 1.0 to 4.9 feet, and 3 percent are 5.0 feet and taller (Fig. 48). Nearly 

54 percent of all samples had 0-5,000 seedlings per acre, and over one-quarter had 

more than 10,000 seedlings per acre (Fig. 49). Overall seedling abundance exhibits no 

particular geographical pattern across Iowa (Fig. 50). Hackberry, silver maple, and white 
66 | FOREST INDICATORS 

ash make up two-thirds of the estimated total number of seedlings (Fig. 51A), and each 

of these species has more than 1,000 seedlings per acre (Fig. 51B), on average. Other 

species with at least 2 percent of the seedling sample are chokecherry (7 percent), an 

understory species; eastern hophornbeam (7 percent), slippery elm (3 percent), and 

mulberry (either red or white) (2 percent), typically mid-story species; and green ash (2 

percent) and bitternut hickory (2 percent), capable of reaching high canopy.

The Regeneration Indicator data collected so far for tree seedlings do not warrant 

detailed analysis by species because of the relatively small number of samples and 

high sampling errors. As an alternative, the status of saplings and adult trees provides 

insight into regeneration trends. Figure 52 depicts the number of live trees by 

diameter class for eight species of importance in Iowa. Both positive and negative 

findings are apparent. Species with decreasing numbers of saplings since the 2003 

inventory will eventually have less biomass and volume as this reduced pool of small 

trees reaches larger, more merchantable size classes. Bur oak had level numbers for 

the 2-inch diameter class and decreasing numbers for the 4- and 6-inch classes. 

Shagbark hickory had a decrease of almost one-third in the 2-inch class, but a 27 

percent decrease in the 4-inch class, and little change in larger diameter classes. Gains 

in regeneration are suggested by increases in the number of saplings for cottonwood, 

black walnut, and northern red oak. Findings were mixed and bear watching for 

silver maple, white oak, and black oak. Black ash had a 16 percent increase in 2-inch 

trees, but it also had decreases in the 4- to 10-inch classes. Eastern redcedar increased 

across all small and large diameters, thus continuing trends discussed by Schmidt and 

Leatherberry (1995).

0 10 20 30 40 50 60 70

Very high

High

Medium

Low

Proportion of Samples (percent)

Level of Browse

Figure 46.—Percentage of forested P2+ samples on forest land by 

browse impact, and associated sampling error, Iowa, 2012-2013. 
 | 67

Figure 47.—Distribution of forested P2+ samples on forest land by browse impact, Iowa, 2012-2013. (Depicted plot 

locations are approximate.)

Figure 48.—Average number of seedlings (A) and seedlings per 

acre (B) on forest land by height class, and associated sampling 

error, Iowa, 2012-2013. Error bars represent a 68 percent confidence 

interval around each estimate.

B

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

2.0 to 11.9 inches

1.0 to 4.9 feet

5.0 feet and taller

Number of Seedlings (millions)

Height Class

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

2.0 to 11.9 inches

1.0 to 4.9 feet

5.0 feet and taller

Seedlings per Acre

Height Class

A

Browse Impact

Nonforest

Low

Medium

High

Very high
68 | FOREST INDICATORS 

Figure 49.—Percentage of forested P2+ samples on forest land 

by class of seedlings per acre, and associated sampling error, 

Iowa, 2012-2013. 

0 10 20 30 40 50 60 70 80

10,001 to 20,000

5,001 to 10,000

0 to 5,000

Percentage of Samples

Number of 

Seedlings per Acre

Figure 50.—Distribution of forested P2+ samples on forest land by class of seedlings per acre, Iowa, 2012-2013. 

(Depicted plot locations are approximate.)

Seedlings per Acre

0-5,000

5,001-10,000

10,001-29,000
 | 69

Figure 51.—Number of seedlings (A) and average number of 

seedlings per acre (B) on forest land by species for species 

with at least 1 percent of the total number of seedlings, and 

associated sampling error, Iowa, 2012-2013. Mulberry includes 

both white and red species.

B

A

0 2,000 4,000

Eastern redcedar

Boxelder

Shagbark hickory

Black walnut

Black cherry

Shingle oak

Bur oak

Bitternut hickory

Green ash

Mulberry

Chokecherry

Slippery elm

Eastern hophornbeam

American elm

White ash

Silver maple

Hackberry

Seedlings per Acre

Species

0 5,000 10,000

Eastern redcedar

Boxelder

Shagbark hickory

Black walnut

Black cherry

Shingle oak

Bur oak

Bitternut hickory

Green ash

Mulberry

Chokecherry

Slippery elm

Eastern hophornbeam

American elm

White ash

Silver maple

Hackberry

Number of Seedlings (millions)

Species
70 | FOREST INDICATORS 

0

500,000

1,000,000

1,500,000

2,000,000

2,500,000

3,000,000

3,500,000

4,000,000

4,500,000

Number of Live Trees

Diameter Class (inches d .b .h .)

2003

2013

Bur Oak

Silver Maple

0

2,000,000

4,000,000

6,000,000

8,000,000

10,000,000

12,000,000

14,000,000

16,000,000

Number of Live Trees

White Oak

0

2,000,000

4,000,000

6,000,000

8,000,000

10,000,000

12,000,000

14,000,000

16,000,000

Number of Live Trees

Eastern Cottonwood

2500000

3,000,000

3,500,000

4,000,000

4,500,000

Live Trees 

2003

2013

2003

2013

2003

2013

15.0-16.917.0-18.919.0-20.921.0-28.9

29.0+

1.0-2.93.0-4.95.0-6.97.0-8.9

9.0-10.9

11.0-12.913.0-14.9

Diameter Class (inches d .b .h .)

15.0-16.917.0-18.919.0-20.921.0-28.9

29.0+

1.0-2.9

3.0-4.9

5.0-6.9

7.0-8.9

9.0-10.9

11.0-12.913.0-14.9

Diameter Class (inches d .b .h .)

15.0-16.917.0-18.919.0-20.921.0-28.9

29.0+

1.0-2.93.0-4.95.0-6.97.0-8.9

9.0-10.9

11.0-12.913.0-14.9

Figure 52.—Number of live trees on forest land, by diameter class, 

Iowa, 2003 and 2013, for eight species. (Continued on next page)
 | 71

Figure 52. (Continued)—Number of live trees on forest land, 

by diameter class, Iowa, 2003 and 2013, for eight species. 

(Continued on next page)

Eastern Cottonwood

0

500,000

1,000,000

1,500,000

2,000,000

2,500,000

3,000,000

3,500,000

4,000,000

4,500,000

Number of Live Trees Number of Live Trees Number of Live Trees 

ees 

2003

2013

Diameter Class (inches d .b .h .)

15.0-16.917.0-18.919.0-20.921.0-28.9

29.0+

1.0-2.93.0-4.95.0-6.97.0-8.9

9.0-10.9

11.0-12.913.0-14.9

Diameter Class (inches d .b .h .)

15.017.019.021.0

1357

9.0

11.013.0

Black Walnut

0

2,000,000

4,000,000

6,000,000

8,000,000

10,000,000

12,000,000

14,000,000

Northern Red Oak

0

500,000

1,000,000

1,500,000

2,000,000

2,500,000

3,000,000

3,500,000

4,000,000

4,500,000

Shagbark Hickory

25,000,000

30,000,000

35,000,000

2003

2013

Diameter Class (inches d .b .h .)

15.0-16.917.0-18.919.0-20.921.0-28.9

29.0+

1.0-2.93.0-4.95.0-6.97.0-8.9

9.0-10.9

11.0-12.913.0-14.9

2003

2013

Diameter Class (inches d .b .h .)

15.0-16.917.0-18.919.0-20.921.0-28.9

29.0+

1.0-2.93.0-4.95.0-6.97.0-8.9

9.0-10.9

11.0-12.913.0-14.9

2003

2013
72 | FOREST INDICATORS 

What this means

Early successional young forest habitat is critical for providing unique plant biota and 

landscape heterogeneity (Greenberg et al. 2011). The quality of Iowa’s young forest 

depends directly on the condition of the regeneration component. Regeneration is 

also the principal determinant of future benefits from mature forests, such as timber, 

carbon sequestration, and water quality. As Iowa’s forest stands continue to age, young 

forest is becoming increasingly rare. Older, more mature stands are dominating the 

forest landscape. Since 1990, the area of young forest (0 to 20 years old) decreased 

from 13 percent of the total forest area to only 5 percent. Young oak-hickory stands 

now make up less than 3 percent of the oak/hickory forest-type group. This is 

Figure 52. (Continued)—Number of live trees on forest land, by 

diameter class, Iowa, 2003 and 2013, for eight species.

Number of Live Trees Number of Live Trees

Shagbark Hickory

0

5,000,000

10,000,000

15,000,000

20,000,000

25,000,000

30,000,000

35,000,000

 Black Oak

0

200,000

400,000

600,000

800,000

1,000,000

1,200,000

1,400,000

1,600,000

1,800,000

2,000,000

Diameter Class (inches d .b .h .)

2003

2013

Diameter Class (inches d .b .h .)

15.0-16.917.0-18.919.0-20.921.0-28.9

29.0+

1.0-2.93.0-4.95.0-6.97.0-8.9

9.0-10.9

11.0-12.913.0-14.9

2003

2013

Diameter Class (inches d .b .h .)

15.0-16.917.0-18.919.0-20.921.0-28.9

29.0+

1.0-2.93.0-4.95.0-6.97.0-8.9

9.0-10.9

11.0-12.913.0-14.9
 | 73

important because the oak/hickory forest-type group accounts for about two-thirds of 

Iowa’s forest and is critical for timber and for providing hard mast for wildlife. 

The State’s maturing forest faces a wide variety of stress factors other than aging that 

work together to make regeneration a major issue for forest managers and policymakers. Conversion to nonforest land uses, forest fragmentation, and invasion by 

nonnative invasive plants, insects, and diseases are important stress factors (Reeder 

2012), particularly on private land, which makes up 9 out of 10 acres of the State’s 

forest land. Some of the most dominant and valuable species of older forests, such as 

the oaks, ashes, black walnut, and maples, are associated with the major risk factors 

described by the Iowa Department of Natural Resources (2013). The most significant 

emerging threats listed are gypsy moth, emerald ash borer, bur oak blight (Tubakia 

iowensis), thousand cankers disease, and Asian longhorned beetle. See Insects and 

Disease Pathogens section on p. 51 for additional discussion of these pathogens.

Browsing of tree seedlings by white-tailed deer (Odocoileus virginianus) is also a 

significant stressor. Although most of the browse impact was found to be at a medium 

level, about one-quarter of the samples had high or very high impact and browsing 

is a serious threat to regeneration in some local areas (Iowa Department of Natural 

Resources 2009). Eventually, most forest stands will undergo either anthropogenic 

or natural stand replacement events, such as harvest or mortality, and require 

regeneration to establish new young forest. Forest regeneration will be the key to 

successful establishment of new young forests. 

Information from the Regeneration Indicator offers insight into prospective trends for 

species of particular interest to Iowans. Iowa’s State Tree is the oak, of which 12 species 

are native to the State. Bur oak is found across Iowa and is important not only for 

timber, but also for wildlife. Bur oak regeneration bears watching because of decreased 

numbers of stems across the smaller diameter classes. Decreased number of smaller 

stems is also a concern for shagbark hickory. Both bur oak and shagbark hickory 

provide spring, summer, and fall roosting for Indiana bats (Myotis sodalis) when in 

proximity to hibernacula; limited regeneration could reduce the extent of future bat 

habitat (Menzel et al. 2001). See also Forest Wildlife Habitat section on p. 74.

The future of ash species is a concern due to the emerald ash borer and in the case 

of black ash, due to a lack of regeneration (see also Insects and Disease Pathogens 

section on p. 51). Decreases in smaller black ash stems are likely to have a negative 

impact on the availability of future source material, which is used by Native 

Americans for hand-woven baskets. Positive trends in cottonwood regeneration 

should offset some of the loss of ash species in riparian forests due to emerald ash 
74 | FOREST INDICATORS 

borer, although potential impacts to, or shifts in, wildlife species composition as a 

result of forest composition change are unknown.

Oak regeneration problems and difficulties are a well-known issue in the Midwest 

(Holt and Fischer 1979). Positive trends in the number of small northern red oak 

trees are encouraging; however, it appears that white oak regeneration needs to be 

watched due to a large decrease in 2-inch saplings. Technically, black walnut stands 

are included in the FIA definition of the oak/hickory forest-type group. Positive 

findings for walnut regeneration foretell increases in this valuable resource.

The results presented here reflect only two of the seven panels of measurements that 

will eventually constitute the first full baseline dataset for the Regeneration Indicator. 

Barring any extension of the inventory cycle length, the next 5-year inventory report 

for Iowa (for the year 2018) will coincide with the completion of the baseline dataset. 

This will fill in crucial data gaps and allow more detailed analyses by improving the 

level of statistical confidence in the estimates (in other words, narrower confidence 

intervals). The dataset will also facilitate research to evaluate plot-level regeneration 

adequacy for the major forest-type groups and a more complete understanding of 

future trends in composition, structure, and health of Iowa’s forests. Sound forest 

management, policy, and planning for regeneration will ensure a sustainable future 

for Iowa’s treasured forest resources for the rest of this century. 

Forest Wildlife Habitat

Forests, woodlands, and savannas provide habitats for many species of Iowa birds (154), 

mammals (42), and amphibians and reptiles (42) (NatureServe 2011). Different forest 

types at different structural stages provide natural communities (habitats) at a “coarse 

filter” scale of conservation (Fig. 53). Rare, imperiled, or wide-ranging wildlife species 

may not be fully served at this scale, so a “fine filter” approach is used to identify speciesspecific conservation needs. Representing an intermediate or “mesofilter” scale of 

conservation are specific habitat features (e.g., snags, riparian forest strips), which may 

serve particular habitat requirements for multiple species (Hunter 2005). This report 

characterizes habitats at the coarse-filter scale (forest age and size) and mesofilter scale 

(standing dead trees). We do not estimate habitat abundance for specific wildlife species.
 | 75

Like all states, Iowa has developed a state wildlife action plan (SWAP). The Iowa 

Wildlife Action Plan (IWAP) (Iowa Department of Natural Resources [2005]; 

revised in May 2006 and August 2012 [Reeder 2012]) identifies species of greatest 

conservation need (SGCN). Of the broad habitat categories in Iowa, wooded lands 

(forests, wet forest/woodlands, woodland, and shrubland habitats) have the greatest 

number of SGCN: 73 birds, 22 mammals, 23 reptiles and amphibians, 22 butterflies, 

8 land snails, and 1 dragonfly/damselfly. According to this plan, Iowa timber 

stands are maturing, providing habitat for wildlife species that depend upon late 

successional stages. Shade-tolerant tree species like sugar maple are overtaking some 

oak-dominated stands. Previously grazed pasture is reverting to early successional 

forest, but often because of invasive species like eastern redcedar. Degraded habitats 

and issues of landscape pattern—fragmentation and habitat block size—affect the 

suitability or productivity of habitats. One of the mesofilter conservation issues 

associated with forest habitats is the presence and abundance of snags and nest 

cavities. The IWAP also lists climate change as a high threat to forest-associated 

bird SGCN. We report on the condition and trends in forest attributes of forest 

age and size, and the quantity and distribution of standing dead trees. We project 

potential changes in tree species distributions that may influence future bird species 

distributions. Forest fragmentation, which is listed as a high threat to all taxa of Iowa’s 

SGCN, is addressed in the Fragmentation and Urbanization of Forest Land section 

on p. 40 of this report. Impacts of browsing (primarily by white-tailed deer) and 

regeneration are discussed in the Regeneration Status section on p. 64 of this report.
76 | FOREST INDICATORS Figure 53.—Distribution of forest-type groups on forest land, Iowa, 2009. Data source: Wilson et al. (2012). Oak/hickory Elm/ash/cottonwood Oak/pine Aspen/birch Maple/birch Nonforest
 | 77

Forest Successional Stage

Background 

Some species of wildlife depend upon early successional forests, which contain 

younger, smaller trees. Others require older, larger trees with complex canopy 

structure. Some species prefer interior forests; other species inhabit the ecotone 

(edge) between different forest stages. Many species require multiple structural stages 

of forests to meet different phases of their life history needs. According to IWAP, 

“Fire suppression and reduced cutting for firewood and wood products are changing 

the nature of Iowa’s forests. Habitats for birds that inhabit mature forests (warblers 

and thrushes) appear to be increasing at the expense of species requiring earlier 

successional stages.” (IWAP 2012 revision: 94). Examples of early successional species 

are several migratory songbird species; game species such as ruffed grouse (Bonasa 

umbellus) and American woodcock (Scolopax minor); cottontail rabbit (Sylvilagus 

floridanus); and a host of Lepidoptera (butterflies and moths), small mammals, and 

reptiles (Stambaugh 2014). Indiana bat and other bat species are also known to feed 

on insects common to young forests (Clark et. al. 1987, Loeb and O’Keefe 2006). 

Abundance and trends in structural and successional stages serve as indicators of 

population carrying capacity for wildlife species (Hunter et al. 2001). Historical 

trends in Iowa’s forest habitats are reported for timberland, which accounts for about 

96 percent of all forest land in the State. For current habitat conditions, estimates are 

reported for all forest land.

What we found 

Abundance of large diameter stand-size class has increased steadily in Iowa during 

the past several decades, making up 39 percent of timberland in 1954, and nearly 70 

percent today. Medium and small diameter stand-size classes have increased only 

slightly in area despite the substantial increase in total forest land area during recent 

decades (Fig. 54). Since 1990, timberland area less than 20 years of age has decreased 

by more than 40 percent. Area in other age classes changed moderately, increasing in 

some classes and decreasing in others (Fig. 55). Timberland older than 100 years has 

the smallest area of any age class, a pattern that has been consistent over the past two 

decades (Fig. 55).

In Iowa, all three stand-size classes contain forests of multiple ages (Fig. 56). In the 

medium diameter stand-size class, forest of 21-60 years of age predominates, with 

lower abundance of both young (0 to 20 years) and old (more than 60 years) forest. 
78 | FOREST INDICATORS 

0

500

1,000

1,500

2,000

2,500

3,000

3,500

1954 1974 1990 2003 2008 2013

Timberland Area (thousand acres)

Inventory Year

Nonstocked

Small diameter

Medium diameter

Large diameter

Figure 54.—Area of timberland by stand-size class, Iowa, 1954-2013.

Figure 55.—Area of timberland by stand-age class, Iowa, 1990-2013.

0

500

1,000

1,500

2,000

2,500

3,000

1990 2003 2008 2013

Timberland Area (thousand acres)

Inventory Year

100+ years

81 to 100 years

61 to 80 years

41 to 60 years

21 to 40 years

0 to 20 years

Figure 56.—Area of forest land by stand-age class and stand-size 

class, Iowa, 2009-2013.

0

100

200

300

400

500

600

700

800

Small diameter Medium diameter Large diameter

Forest Land Area (thousand acres)

Stand-size Class

 0 to 20 years

 21 to 40 years

 41 to 60 years 

 61 to 80 years

 81 to 100 years

 101 to 120 years

 121 to 140 years

 141 to 160 years

 161 to 180 years
 | 79

Somewhat surprisingly, the most abundant age classes in small diameter forest are 

also in the 21- to 60-year class, not the 0- to 20-year class (Fig. 56). In the large 

diameter stand-size class, forest of the 41- to 80-year class predominates; as expected, 

almost all forest older than 80 years is of large diameter size-class (Fig. 56). See also 

Tree Density, Size, and Stocking section on p. 29.

What this means 

Large diameter stand-size class has more than doubled in area over the past several 

decades. Timberland over 100 years of age amounts to only 5 percent of total area, but 

has nearly doubled in area during the past decade. Area of small diameter stand-size 

class has not kept pace with the increasing total area of timberland. Stand-size class 

and stand-age class are complementary indicators of forest structural/successional 

stage. Forests between 21 and 100 years of age contain at least some fraction of all 

three stand-size classes. As expected, there is very little small diameter forest in the 

oldest stand age classes and very little large diameter forest in the youngest standage class. The two age classes with the most size heterogeneity are 21- to 40-year 

and 41- to 60-year classes. Such mixtures of different-aged or -sized trees provide a 

vertical diversity of vegetation structure that can enhance habitat conditions for some 

wildlife species. There is a need to maintain forest conditions in both smaller and 

larger structural stages to maintain both early and late successional habitats for all 

forest-associated species. Managing forest composition and structure in a variety of 

conditions helps to conserve habitat and viable populations of many forest-associated 

wildlife species. See Regeneration Status section on p. 64 for more detail on seedling 

and sapling abundance.

Standing Dead Trees

Background 

Specific habitat features like nesting cavities and standing dead trees provide critical 

habitat components for many forest-associated wildlife species. Standing dead 

trees that are large enough to meet habitat requirements for wildlife are referred 

to as “snags.” According to one definition, “for wildlife habitat purposes, a snag is 

sometimes regarded as being at least 10 in (25.4 cm) in diameter at breast height and 
80 | FOREST INDICATORS 

at least 6 ft (1.8 m) tall” (Society of American Foresters 2008). Standing dead trees 

serve as important indicators not only of wildlife habitat, but also of past mortality 

events and carbon storage. They also serve as sources of down woody materials 

(discussed in the Down Woody Materials section on p. 55), which also provides 

habitat features for wildlife. The number and density of standing dead trees, together 

with decay classes, species, and sizes, define an important wildlife habitat feature 

across Iowa’s forests. 

What we found 

FIA plot data contain information on standing dead trees (at least 5 inches d.b.h. or 

diameter at root collar [d.r.c.]) of numerous species and sizes in varying stages of 

decay. According to current inventory data (2009-2013), almost 36 million standing 

dead trees are present on Iowa forest land. This equates to an overall density of 12.1 

standing dead trees per acre of forest land, with similar densities on public (12.8) and 

private (12.0) forest land. Nine species groups each contributed more than 1 million 

standing dead trees; the largest contributor, other eastern soft hardwoods group 

(more than three-fourths of which are elms) had nearly 20 million snags (Fig. 57). 

Relative to the total number of live trees in each species group (of at least 5 inches 

d.b.h. or d.r.c.), four species groups exceeded 10 standing dead trees per 100 live trees, 

with eastern white and red pine species group topping the list at 41 standing dead 

trees per 100 live trees (of at least 5 inches d.b.h. or d.r.c.) (Fig. 58). The great majority 

(70 percent) of standing dead trees were smaller than 11 inches d.b.h., with 34 percent 

between 5 and 6.9 inches d.b.h.; 6 percent were 17 inches or larger (Fig. 59). Only 14 

percent of standing dead trees were in the two classes of most decay (in which few 

or no limb stubs remain, or no evidence of branches remains); the other three decay 

classes were fairly evenly distributed, ranging from 24 to 35 percent of all standing 

dead trees in each class (Fig. 59).
 | 81

Figure 57.—Number of standing dead trees by species or 

species group for the 14 most common standing dead tree 

species groups, Iowa, 2009-2013.

0 5 10 15 20 25

Other eastern soft hardwoods

Select white oaks

Other eastern hard hardwoods

Hickory

Cottonwood and aspen

Soft maple

Ash

Other red oaks

Eastern noncommercial hardwoods

Select red oaks

Other eastern softwoods

Basswood

Black walnut

Hard maple

Number of Standing Dead Trees at least 

5 inches d .b .h . or d .r .c . (millions)

Species/Species Group

Figure 58.—Number of standing dead trees per 100 live trees for the 15 

most common standing dead tree species groups, Iowa, 2009-2013.

0 5 10 15 20 25 30 35 40 45

Eastern white and red pine

Other eastern soft hardwoods

Cottonwood and aspen

Select white oaks

Other red oaks

Ash

Soft maple

Other eastern hard hardwoods

Eastern noncommercial hardwoods

Select red oaks

Hickory

Other eastern softwoods

Basswood

Hard maple

Black walnut

Number of Standing Dead Trees at least 

5 inches d.b.h. or d.r.c. per 100 Live Trees at least 

5 inches d.b.h. or d.r.c.

Species/Species Group
82 | FOREST INDICATORS 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.0-

6.9

7.0-

8.9

9.0-

10.9

11.0-

12.9

13.0-

14.9

15.0-

16.9

17.0-

18.9

19.0-

20.9

21.0-

28.9

29.0+

Standing Dead Trees at least 5 inches 

d .b .h . or d .r .c . (millions)

Diameter Class (inches d .b .h .)

All limbs and branches present

Few limbs and no fine branches

Only limb stubs present

Few or no limb stubs remain

No evidence of branches remains

Figure 59.—Distribution of standing dead trees by decay and 

diameter classes for all dead trees in Iowa, 2009-2013.

What this means 

Snags and smaller standing dead trees result from a variety of potential causes, 

including diseases and insects, weather damage, fire, flooding, drought, and 

competition. Other eastern soft hardwoods species group contained the largest 

number of standing dead trees, but eastern red and white pine species group had the 

highest density of standing dead trees per 100 live trees. About 10 standing dead trees 

were present for every 100 live trees (of at least 5 inch d.b.h. or d.r.c.). But dead trees 

may contain significantly more cavities per tree than occur in live trees (Fan et al. 

2003), thereby providing a disproportionate number of habitat features for foraging, 

nesting, roosting, hunting perches, and cavity excavation for wildlife. Species that 

benefit from these features range from primary colonizers such as insects, bacteria, 

and fungi to birds, mammals, and reptiles. Most cavity nesting birds are insectivores 

which help to control insect populations. The availability of very large standing dead 

trees (snags) may be a limiting mesofilter habitat feature for some species of wildlife. 

Providing a variety of forest structural stages and retaining specific 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.
 | 83

Tree and Bird Species Projections

Background 

Climate change can alter species’ habitats, and these impacts may intensify if the 

climate continues to depart from current conditions. It is important that we begin to 

consider the impact of shifting habitats for species and management options in light 

of climate change. Species distribution models are important tools used to evaluate 

potential responses to changes. Across the eastern United States models of species’ 

current distributions and potential future habitat are considering the response of 147 

bird species’ habitats and 134 tree species’ habitats to climate change (Matthews et al. 

2011). These models provide insights into the important climate and edaphic variables 

that shape distributions and an understanding of how changes in conditions may 

affect species’ habitats. 

What we found

In Iowa, 71 tree species are projected to have habitat suitable by the end of the century 

(2100). Of these, 15 tree species, including white oak and shagbark hickory, have the 

potential to lose at least 20 percent of their current suitable habitat. Other important 

trees of the State, such as American elm, bur oak, and silver maple, are projected to 

remain fairly stable in habitat conditions. Nearly 35 tree species currently in the State 

are projected to increase in habitat, such as boxelder and pin oak. Others, such as 

yellow poplar, may find new habitat in the area.

For birds, the potential change in habitat dynamics under climate change scenarios 

also spans the domain of potential gains and losses of habitat; 117 bird species are 

projected to find current or potential future habitat in Iowa. Of these, 33 species are 

projected to have declines in habitat of at least 20 percent and 40 are projected to 

have increases in habitat. Some of these species reflect important forest associations 

in their distributions, and their future habitat dynamics most likely will be influenced 

by shifts in both tree species habitat and climate. Declines in habitat are projected 

for ruffed grouse, blue-winged warbler (Vermivora cyanoptera), and least flycatcher 

(Empidonax minimus). Habitat gainers may include yellow-billed cuckoo (Coccyzus 

americanus), whip-poor-will (Antrostomus vociferus), Acadian flycatcher (Empidonax 

traillii), prothonotary warbler (Protonotaria citrea), Kentucky warbler (Geothlypis 

formosa), and black-and-white warbler (Mniotilta varia). Habitat for wood thrush 

(Hylocichla mustelina) is expected to persist at current abundance.
84 | FOREST INDICATORS 

What this means

These models for trees and birds reflect how climate change has the potential to 

reshape the suitability of habitat within the State. Limited data, scenario-specific 

variability, and omission of specific habitat features all contribute to model 

uncertainty, so projections should be viewed as one possible outcome, not as definite 

predictions of the future. Planning and assessment of potential vulnerabilities of tree 

and bird species will be important if the climate continues to change.
Forest Economics

Red oak rising in Nodaway Valley Park, Page County, Iowa. Photo by Lindsey Barney, Iowa Department of Natural 

Resources, used with permission.

 | 85
86 | 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 reasonably free of form defect on the merchantable 

bole. 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. 60). 

All economically important species groups have shown significant increases since 1990 

(Fig. 61). There appear to be slightly increasing growing-stock volumes between 2008 

and 2013 inventories. By viewing changes in growing-stock volume by d.b.h. classes, we 

can see more differences between selected species groups (Fig. 62). In general, diameter 

distributions from the 2008 and 2013 inventories are similar, with more substantial 

changes occurring between 1990 and the three more recent inventories. Between the 

1990 and 2000s inventories, increases in the smallest diameter class (5.0 to 10.9 inches) 

were most noticeable for other eastern soft hardwoods species group, in which elms 

and hackberry species predominate. Black walnut, soft maple, and other eastern soft 

hardwoods groups all showed substantial increases within the 11.0- to 16.9-inch diameter 

class. Cottonwood and aspen, select white oaks, and select red oaks exhibited the largest 

increase in the largest diameter class, 23.0+ inches (Fig. 62). The most notable change in 

diameter distribution is for black walnut in the 17.0- to 22.9-inch diameter class, which 

more than doubled in volume during the past decade, and increased by nearly tenfold 

since 1990 (Fig. 62). 

What this means

Growing-stock volume of economically important species groups on timberland has 

increased substantially since Iowa’s earliest statewide inventories in 1954 and 1974, and 

continued to increase gradually during the past decade. None of the 12 species groups 

with the greatest volume shows significant volume decreases, although there are early 

indications that soft maple may be decreasing. 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. See Regeneration Status section on p. 64 for more detail.
 | 87

0

500

1,000

1,500

2,000

2,500

3,000

3,500

1950 1960 1970 1980 1990 2000 2010

Growing-stock Volume (million ft3

)

Inventory Year

Figure 60.—Total growing-stocking volume on timberland, and 

associated sampling error, Iowa, 1954-2013. Error bars represent 

a 68 percent confidence interval around each estimate and are 

available only since 1990.

0 100 200 300 400 500 600 700

Select white oaks

Other eastern soft hardwoods

Cottonwood and aspen

Black walnut

Select red oaks

Soft maple

Hickory

Basswood

Other red oaks

Hard maple

Other eastern hard hardwoods

Ash

Growing-stock Volume (million ft3

)

Species/Species Group

1990

2003

2008

2013

Figure 61.—Total growing-stock volume on timberland by selected 

species or species groups, and associated sampling error, Iowa, 

1990-2013. Error bars represent a 68 percent confidence interval 

around each estimate.
88 | FOREST ECONOMICS

Figure 62.—Growing-stock volume (trees at least 5 inches d.b.h.) 

on timberland, Iowa, 1990-2013, for six highest volume species or 

species groups. (Continued on next page)

Other Eastern

Soft Hardwoods

0

20

40

60

80

100

120

140

160

180

200

5.0-10.9 11.0-16.9 17.0-22.9 23.0+

Cottonwood

and Aspen

0

50

100

150

200

250

5.0-10.9 11.0-16.9 17.0-22.9 23.0+

0

50

100

150

200

250

5.0-10.9 11.0-16.9 17.0-22.9 23.0+

Growing-stock Volume (million ft3

)

Diameter Class (inches d .b .h .)

1990

2003

2008

2013

Growing-stock Volume (million ft3

)

Diameter Class (inches d .b .h .)

1990

2003

2008

2013

Growing-stock Volume (million ft3

)

Diameter Class (inches d .b .h .)

1990

2003

2008

2013

Select White Oaks
 | 89

Figure 62. (Continued)—Growing-stock volume (trees at least 

5 inches d.b.h.) on timberland, Iowa, 1990-2013, for six highest 

volume species or species groups. 

Select Red Oaks Growing-stock Volume (million ft3

)

Diameter Class (inches d .b .h .)

Soft Maple

0

20

40

60

80

100

120

140

5.0-10.9 11.0-16.9 17.0-22.9 23.0+

Black Walnut

0

20

40

60

80

100

120

140

5.0-10.9 11.0-16.9 17.0-22.9 23.0+

1990

2003

2008

2013

Growing-stock Volume (million ft3

)

Diameter Class (inches d .b .h .)

1990

2003

2008

2013

Growing-stock Volume (million ft3

)

Diameter Class (inches d .b .h .)

1990

2003

2008

2013

0

10

20

30

40

50

60

70

80

90

100

5.0-10.9 11.0-16.9 17.0-22.9 23.0+
90 | FOREST ECONOMICS

Sawtimber Volume

Background

Sawtimber trees are live trees of commercial species that contain either one 12-foot 

or two noncontiguous 8-foot logs. Hardwoods must be at least 11 inches d.b.h. and 

softwoods must be at least 9 inches d.b.h. to qualify as sawtimber. Sawtimber volume 

is defined 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 11.7 billion board feet (Fig. 63). Ten species groups demonstrated 

gains in sawtimber volume since 2008, and four species groups had decreases (Fig. 

64). Hard maple increased the most, at 57 percent. Eastern white and red pine species 

group decreased the most, corresponding to a high rate of mortality (but with high 

sampling uncertainty). Other declining species groups include other eastern softwoods, 

soft maple, and cottonwood and aspen (Fig. 64), all of which had a relatively high ratio 

of mortality to volume. 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, and a decline to the 

current rate of 2.7 percent. Removal rates reveal an opposite pattern, with an increase in 

1974 followed by a substantial decrease to the current level of 1.3 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 1.5 percent (Fig. 65). 
 | 91

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

1950 1960 1970 1980 1990 2000 2010

Volume (billion board feet)

Inventory Year

-80 -60 -40 -20 0 20 40 60 80

Eastern white and red pine

Other eastern softwoods

Soft maple

Cottonwood and aspen

Other eastern hard hardwoods

Select white oaks

Other red oaks

Other eastern soft hardwoods

Ash

Hickory

Basswood

Black walnut

Select red oaks

Hard maple

Percent Change, 2008-2013 

Species/Species Group

Figure 64.—Percent change in total sawtimber volume on 

timberland for the 14 highest volume species or species groups, 

Iowa, 2004-2008 to 2009-2013. 

Figure 63.—Volume of sawtimber on timberland, and 

associated sampling error, Iowa, 1954 to 2009-2013. Error 

bars represent a 68 percent confidence interval around each 

estimate and are available only since 1990.
92 | FOREST ECONOMICS

What this means

Sawtimber volume resources on Iowa’s timberland have increased steadily since 1974, 

with annual rates of net growth exceeding rates of removals. This dynamic indicates 

that sawtimber on timberland has been a sustainable component of the forest 

ecosystem. But if current trends were to continue, increasing sawtimber mortality and 

removals could eventually exceed growth in the future. However, this assumption may 

not hold locally.

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

Annual Sawtimber Estimate/ 

Total Volume Estimate (percent)

Inventory Year

Annual net growth

Annual removals

Annual mortality

Figure 65.—Total average annual net growth, removals, 

and mortality as a percentage of total sawtimber volume on 

timberland, Iowa, 1954 to 2009-2013.
 | 93

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 19 percent in the current 

inventory (Fig. 66). Other tree grades have shown more moderate fluctuations. 

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, and 4.6 billion board feet during the 2013 inventory; 

some species groups had stable or decreasing volumes, but others, such as hard maple, 

select red oaks, and ash, had increasing volumes (Fig. 67).

19%

20%

37%

20% 24%

23%

36%

21%

13%

22%

38%

27% 1

2

3

4

Tree Grade A B 20%

26%

33%

21%

C D

Figure 66.—Distribution of sawtimber volume on timberland by tree grade, Iowa, 1990 (A), 1999-2003 (B), 2004-

2008 (C), and 2009-2013 (D).
94 | FOREST ECONOMICS

What this means

Iowa’s sawtimber quality has been improving for decades, with substantial increases 

in the amount of higher grade sawtimber. In recent inventories, changes in sawtimber 

grades have varied among species groups. Gains in sawtimber volume and tree grades 

have probably come from individual tree growth. Overall, it appears that the quality 

of Iowa’s sawtimber resource has increased from past decades and is currently stable.

Carbon Stocks 

Background

Forest resource reporting in recent years has increasingly included estimates of carbon. 

This is primarily because forests tend to sequester carbon from the atmospheric 

greenhouse gas carbon dioxide, which is associated with global climate change. Among 

terrestrial ecosystems, forests contain the largest reserves of sequestered carbon. 

Regional and national greenhouse gas reporting forums include forest carbon stocks 

because increases in forest carbon stock represent quantifiable partial offsets to other 

greenhouse gas emissions. For example, carbon sequestration by U.S. forests provided 

an offset of more than 11 percent of total U.S. greenhouse gas emissions in 2013 (U.S. 

Figure 67.—Percent change in high grade sawtimber volume 

(grades 1 and 2) for selected hardwood species or species groups 

on timberland in Iowa between 2004-2008 and 2009-2013.

-60 -40 -20 0 20 40 60 80 100

Soft maple

Hickory

Cottonwood and aspen

Other red oaks

Select white oaks

Other eastern soft hardwoods

Basswood

Other eastern hard hardwoods

Black walnut

Ash

Select red oaks

Hard maple

Percent Change, 2008-2013

Species/Species Group
 | 95

Environmental Protection Agency [EPA] 2015), and the continuing increase in Iowa 

forest carbon stocks contributes to this effect. Total forest ecosystem carbon stocks in 

Iowa are estimated to be 169 million tons carbon, a 2 percent increase over 5 years ago. 

This section provides an overview of carbon in the State’s forest lands.

Carbon accumulates in growing trees via the photosynthetically driven production 

of structural and energy-containing organic (carbon) compounds that primarily 

accumulate in trees as wood. Over time, this stored carbon also accumulates in dead 

trees, woody debris, litter, and forest soils. For most forests, the understory grasses 

and forbs as well as nonvascular plants and animals represent minor pools of carbon 

stocks. Within soils, the larger woody roots are readily distinguished from the bulk of 

soil organic carbon, so the roots are generally reported as the belowground portion 

of trees and not included in the soils estimates. Carbon loss from a forest stand can 

include mechanisms such as respiration (including live trees and decomposers), 

combustion, runoff or leaching of dissolved or particulate organic compounds, or 

direct removal such as the harvest and utilization of wood. From the greenhouse gas 

reporting perspective, it is important to note that not all losses result in release of 

carbon dioxide to the atmosphere; some wood products represent continued longterm carbon sequestration.

The carbon pools discussed here include living plant biomass (live trees at least 1 inch 

d.b.h. and understory vegetation), dead wood and litter (standing dead trees, down 

dead wood, and forest floor litter; in other words, nonliving plant material), and soil 

organic matter exclusive of coarse roots and estimated to a depth of 1 meter (3.3 feet). 

The FIA program does not directly measure forest carbon stocks in Iowa. Carbon 

estimates, by ecosystem pool, are based on sampling and modeling; for additional 

information on current approaches to determining forest carbon stocks see U.S. EPA 

(2015), U.S. Forest Service, Forest Inventory and Analysis (2014), and O’Connell et al. 

(2014). The level of information available for making the carbon estimates varies 

among pools. For example, confidence is greatest in estimates of live tree carbon due 

to the level of sampling and availability of allometric relationships applied to the 

tree data. Lower confidence is associated with soil organic carbon estimates because 

of limited data and high variability. Interpretation of these estimates is therefore 

limited. Ongoing research is aimed at improving the estimates (U.S. EPA 2015). The 

carbon estimates provided here for Iowa’s forests are consistent with the data and 

methods used to develop the forest carbon reported in the U.S. EPA’s Inventory of U.S. 

Greenhouse Gas Emissions and Sinks: 1990-2013 (U.S. EPA 2015).
96 | FOREST ECONOMICS

What we found

The average relative proportions of total forest carbon within Iowa forest ecosystems 

are shown in Figure 68. Live trees and soil organic carbon account for 88 percent of 

forest carbon stocks, with more than a quarter of all carbon in the wood and bark of 

the bole of trees at least 5 inches d.b.h. Figure 69 illustrates average tons of carbon 

per acre on forest land by six stand-age classes for aboveground biomass (live tree 

and understory) versus aboveground nonliving (standing dead, down dead, and 

litter) pools. The average number of tons of carbon per acre on forest land increases 

with stand age, and greater net accumulation is within the biomass component. Total 

carbon stocks are the product of per-acre carbon and total acres of forest within each 

age class. More than 60 percent of total aboveground carbon stocks are represented by 

the middle two age classes (more than 30 percent each); in contrast, the youngest and 

oldest age classes together account for less than 10 percent of forest carbon stocks.

Species composition can affect carbon stocks. Figure 70 illustrates this with average 

tons carbon per acre on forest land by common Iowa forest-type groups. Carbon tons 

per acre on forest land is provided according to four classifications: biomass (live tree 

and understory), dead wood (standing dead trees and down dead wood), litter, and 

soil. Note that the sometimes considerable variability among forest-type groups is 

most closely associated with variability in biomass (which is essentially live tree; see 

Figure 68). Again, total stocks are the product of tons carbon per acre on forest land 

and acres of forest land, and in Iowa more than 90 percent of total carbon stocks are 

in the two most common forest-type groups: oak/hickory and elm/ash/cottonwood. 

The largest single pool is biomass within the oak/hickory forest-type group: 54 

million tons carbon or about 32 percent of all Iowa forest carbon stocks.

Actual stocks for a particular stand will depend on a combination of influences, 

such as site history, management, stand age, or component species. Consequently 

individual sites can differ from the summaries provided in Figures 68, 69, or 70. As 

an example, the statewide average tons of carbon per acre on forest land for live trees 

is 36 for stands that are identified as fully stocked, but the site-to-site variability is 

such that 50 percent of measured plots fall between 28 and 45 tons carbon per acre 

on forest land. Greater carbon tons per acre on forest land occur in 25 percent of 

measured plots, and lower levels occur in the remaining 25 percent.

The current carbon estimation methods and data were also applied to the 2008 

Iowa forest inventory (data not shown) to produce summaries consistent with 

those provided here for the 2013 inventory. Overall forest carbon tons per acre on 

forest land increased by 4 percent relative to 5 years ago, and live tree carbon values 
 | 97

5%

1%

Saplings, 3%

Top/branches, 7%

Bole, 26%

Stump, 1%

Coarse roots, 7%

Soil organic carbon

44%

Forest oor/litter

Down dead wood

4%

Understory

Standing dead trees

2%

Live tree 

carbon

44%

0

5

10

15

20

25

30

35

40

45

0-20 21-40 41-60 61-80 81-100 101+

Carbon (short tons per acre)

Age Class (years)

Aboveground deadwood and litter

Aboveground biomass

Figure 68.—Percentage of total forest carbon stocks within each of the listed 

classifications, Iowa, 2013. Note that live tree carbon is subdivided into live saplings 

(d.b.h. of 1-5 inches, 3 percent of total) and trees at least 5 inches d.b.h. (41 percent 

of total), which are further divided into four components. 

Figure 69.—Average per-acre carbon stocks on forest land according to six 

stand-age classes for aboveground biomass (live tree and understory) versus 

nonliving (standing dead, down dead, and litter) pools, Iowa, 2013.

increased by 9 percent. These increases in per-acre carbon meant that despite the 2 

percent decrease in forest area, total carbon stocks in 2013 were 2 percent greater than 

the equivalent values calculated for 2008.
98 | FOREST ECONOMICS

What this means

In general, forest carbon stocks or differences in stock broadly reflect other measures 

of forest resources such as stand age, volume, or stocking. However, these summaries 

offer a reference measure of carbon stocks for the State relative to published regional 

or national forest carbon reports, thereby providing a ready estimate of the role of 

Iowa’s forests. 

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 2013, the wood products and paper manufacturing industries in Iowa employed 

11,898 people, with an average annual payroll of $648 million (U.S. Department of 

Labor 2015). To better manage the State’s forests, it is important to know the species, 

amounts, and locations of timber being harvested. A survey of Iowa wood-processing 

mills was last conducted for 2005. For this report, the 2010 information is based on 

an adjustment of the 2005 survey and the harvesting trends of surrounding states 

that were receiving wood products from Iowa (Illinois 2010, Indiana 2008, Michigan 

0 10 20 30 40 50 60 70 80

All softwood types

Oak/pine

Oak/hickory

Elm/ash/cottonwood

Maple/birch

Additional hardwood types

Carbon (short tons per acre)

Forest-type Group C in biomass C in dead wood C in litter Soil organic C

Figure 70.—Average per-acre carbon stocks on forest land according 

to forest-type group by four classifications—biomass (live tree and 

understory), dead wood (standing dead trees and down dead wood), 

litter, and soil—Iowa, 2013. The less common forest-type groups are 

pooled as the softwood or additional hardwood types.
 | 99

2010, Minnesota 2010, Missouri 2009, Nebraska 2009, Virginia 2009, and Wisconsin 

2009). 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).

What we found

In 2010, 13.6 million cubic feet of industrial roundwood was harvested from Iowa’s 

forest land, a decrease of 16 percent from 2005 (Fig. 71). Saw logs accounted for 

more than 90 percent of the total industrial roundwood harvested. Other products 

harvested were veneer logs, cooperage, and industrial fuelwood. White and red oaks 

accounted for more than 35 percent of the total industrial roundwood harvest (Fig. 

72). Other important species groups harvested were the maples, black walnut, and 

cottonwood. In the process of harvesting industrial roundwood, 9.4 million cubic feet 

of harvest residues were left on the ground. The processing of industrial roundwood 

generated 250,000 dry tons of wood and bark residues. Eighty-three 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, and 2 percent was used for fiber products (Fig. 73). 

In 2013, the number of employees working in the forest products industry increased 

from the previous year for the first time since 2005 (Fig. 74). Although the number 

of employees decreased by 28 percent between 2007 and 2012, the total annual wages 

(actual dollars) decreased by only 17 percent (U.S. Census Bureau 2015). The total 

value of shipments decreased by 22 percent between 2007 and 2009, then increased by 

20 percent between 2009 and 2012 to reach the same level as they were in 2005. 

0

5

10

15

20

Saw logs Veneer Other 

products

All products

Industrial Roundwood Production

(million ft3

)

Products

1994

2000

2005

2010

Figure 71.—Industrial roundwood production by product and year, Iowa.
100 | FOREST ECONOMICS

Figure 72.—Industrial roundwood harvested by species or 

species group, Iowa, 2005 and 2010.

0

1

2

3

4

White

oaks

Soft

maple

Black

walnut

Red

oaks

CottonwoodHard

maple

Other

species

Industrial Roundwood Production

Species/ Species Group

(million ft3

)

2005

2010

Figure 73.—Disposition of mill residues generated by 

primary wood-using mills, Iowa, 2010.

Fiber products

2%

Industrial fuel

13%

Residential fuel

3%

Other uses

82%

Not used

<1%

Figure 74.—Number of employees, annual payroll, and total value 

of shipments for the forest products industry, Iowa, 2004-2013.

0

2

4

6

8

10

12

14

16

18

0

1,000

2,000

3,000

4,000

2004 2005 2006 2007 2008 2009 2010 2011 2012 2013

Number of Employees

(thousands)

Annual Payroll and Total Value

of Shipments (million dollars)

Year

Annual payroll Total value of shipments Number of employees
 | 101

What this means

The increase in total value of shipments between 2009 and 2012 reflects the increase 

in demand for higher value forest products such as white oak cooperage and black 

walnut veneer, much of which is being exported to other countries. As the economy 

improves, and the demand for wood products increases, the forest products mills 

that were able to withstand the recession are beginning to increase their production, 

resulting in an increase in the number of employees in the forest products industry.

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 growingstock sources (wood material that could be used to produce products). Iowa’s primary 

forest products industry is the processing of saw logs. The last pulp mill in the State 

closed in 2005. Without this market, a large volume of usable, small dimension wood 

material above the saw log top is left behind. Small, localized, industrial fuelwood or 

wood pellet manufacturers have the opportunity to better utilize the forest resource.
102 | FOREST ECONOMICS
 | 103

Future Forests

Quaking aspen fall color, Johnson County, Iowa. Photo by Mark Vitosh, Iowa Department of Natural Resources, used 

with permission.

 | 103
104 | FUTURE FORESTS

Forest Land Projections

This section focuses on anticipated changes to Iowa’s forests between 2010 and 2060. 

The Northern Forest Futures study (Shifley and Moser 2016) examined several 

alternative future scenarios that cover a range of different assumptions about the 

economy, population, climate, and other driving forces that will affect the future 

conditions of forests. The assumptions were incorporated into analytical models that 

estimated how northern forests are likely to change under each alternative scenario. 

The seven scenarios can be grouped by climate model (or “general circulation model”), 

storyline, and storyline variation. Two climate models, three storylines, and three 

variations were used to produce the seven scenarios listed in Table 9. Additional details 

on methods can be found in Shifley and Moser (2016).

A large component of future forest change will be the result of normal forest growth, 

aging, natural regeneration, and species succession. In addition, external forces are 

expected to drive forest change as follows: 

• Population increases will cause roughly 150,000 acres of Iowa’s forest land to be 

converted to urban land (Nowak and Walton 2005)

• Economic conditions will affect forest products consumption, production, and 

harvest rates

• Invasive species will spread and affect forest change

• Changes in population, the economy, energy consumption, and energy production 

will affect climate 

• Climate change will affect long-term patterns of forest growth and species succession.

Following are the three storylines, as developed by the Intergovernmental Panel on 

Climate Change (IPCC 2000), with a brief description of the characteristics of each:

Table 9.—Scenarios used to project future forest conditions for Iowa

General circulation modela IPCCb Storyline A1B IPCC Storyline A2 IPCC Storyline B2

CGCM3.1 Scenario A1B-C

Scenario A1B-BIO 

Scenario A2-C

Scenario A2-BIO

Scenario A2-EAB

CGCM2 Scenario B2-C

Scenario B2-BIO

a Source for these two coupled global climate models: Canadian Centre for Climate Modelling and 

Analysis (2012).

b IPCC is the Intergovernmental Panel on Climate Change.
 | 105

1.A1B—Rapid economic globalization. International mobility of people, ideas, and 

technology. Strong commitment to market-based solutions. Strong commitment to 

education. High rates of investment and innovation in education, technology, and 

institutions at the national and international levels. A balanced energy portfolio that 

includes fossil fuel-intensive sources and renewable energy sources.

2.A2—Consolidation into economic regions. Self-reliance in terms of resources 

and less emphasis on economic, social, and cultural interactions between regions. 

Technology diffuses more slowly than in the other scenarios. International disparities in 

productivity, and hence in per capita income, largely persist or rise in absolute terms. 

3.B2—A trend toward local self-reliance and stronger communities. Community-based 

solutions to social problems. Energy systems differ from region to region, depending on 

the availability of natural resources. The need to use energy and other resources more 

efficiently spurs the development of less carbon-intensive technology in some regions.

Descriptions of the three storyline variations are as follows:

1.C—Variations of the A1B, A2, and B2 storylines examine effects resulting from a 

continuation of the observed recent rates of forest removals due to timber harvesting 

and land use conversion from forest to another land use. These variations are referred 

to as scenarios A1B-C, A2-C, and B2-C.

2.BIO—Variations of the A1B, A2, and B2 storylines look at impacts of increased 

harvest and utilization of woody biomass for energy. They are referred to as scenarios 

A1B-BIO, A2-BIO, and B2-BIO. 

3.EAB—A variation of the A2 storyline examines the potential impact of the 

continued spread of the emerald ash borer with associated mortality of all ash trees in 

the affected areas. This is referred to as scenario A2-EAB. 

The anticipated declines in forest land amount to tens of thousands of acres (Fig. 

75). Specifically, over the next 50 years forest land area is projected to decline from 

an estimated 3.033 million acres in 2010 to 2.915 million acres (a loss of 4 percent) 

in 2060 under scenario A1B-C, to 2.953 million acres (a loss of 3 percent) under 

scenario A2-C, and to 2.977 million acres (a loss of 2 percent) under scenario B2-C. 

Only the storylines (developed around differing expectations of demographics and 

levels of economic activity) alter the projected area of forest land, scenarios with 

greater increases in population and economic activity project less future forest land. 

Because the choice of climate model and variations on the storylines do not affect 

projections of forest land area, only the three scenarios are represented in Figure 

75. The projected losses of forest land from 2013 to 2060 are small compared to the 

cumulative increase in forest area since the start of the 20th century. 
106 | FUTURE FORESTS

Emerald ash borer (EAB) was detected in Iowa in spring 2010. Under scenario A2-

EAB (Fig. 76) there is a sharp decrease in the area of elm/ash/cottonwood projected 

by 2060 as EAB is expected to virtually eliminate the ash tree resource. Ash species are 

less than 4 percent (146 million cubic feet) of the total live-tree volume in Iowa (Fig. 

77), so the impacts of EAB will not be as obvious as in other areas of the northeastern 

United States, where ash trees are more abundant. Nevertheless, 16 million ash trees 

at least 5 inches d.b.h. are likely to be lost from Iowa forest land (see also Insects and 

Disease Pathogens section on p. 51). By 2060 under all future scenarios, the oak/

hickory forest-type group is expected to increase in forest area, and the elm/ash/

cottonwood forest-type group is expected to decrease in forest area. 

Figure 75.—Past and projected forest land area by scenario, 

Iowa, 2010-2060.

1.0

1.5

2.0

2.5

3.0

3.5

1900 1920 1940 1960 1980 2000 2020 2040 2060

Acres (millions)

Year

Historical

A1B-C

A2-C

B2-C

Figure 76.—Forest land area by forest-type group, Iowa, 2010 and by 

scenario in 2060.

0.0

0.5

1.0

1.5

2.0

2.5

Oak/hickory

Elm/ash/cottonwood

Maple/birch

Aspen/birch

Other

Acres (millions) 

Forest-type Group

Inventory in 2010

A2-EAB in 2060

A1B-C in 2060

A2-C in 2060

B2-C in 2060
 | 107

The direct impacts of EAB-caused ash tree mortality are more pronounced when 

viewed in terms of forest volume rather than forest area (Fig. 78). For all scenarios 

live-tree volume is projected to peak in 2020 or 2030. After 2050 the A2-EAB scenario 

predicts a far more rapid decline in volume than the corresponding scenario modeled 

without projected losses to EAB (scenario A2-C). 

The scenarios (Fig. 78) with accelerated biomass removal for energy production 

(A1B-BIO, A2-BIO, and B2-BIO) show substantial declines in standing volume after 

2050, comparable to or greater than declines in the EAB scenario. This does not mean 

that increased utilization of biomass for energy is ill advised. Rather, forest managers 

0

1

2

3

4

5

6

A2-

EAB 

Other species

A2-

EAB 

Ash species

A2-C 

Other 

species

A2-C 

Ash 

species

Volume (billion ft3

)

2010

2020

2030

2040

2050

2060

Figure 77.—Past and projected live-tree volume for ash species 

and for all other species on forest land for scenarios A2-EAB and 

A2-C, Iowa, 2010-2060.

3.5

3.7

3.9

4.1

4.3

4.5

4.7

4.9

5.1

5.3

2010 2020 2030 2040 2050 2060

Volume (billion ft3

)

Year

A1B-C

A1B-BIO

A2-C

A2-BIO

B2-C

B2-BIO

A2-EAB

Figure 78.—Past and projected live-tree volume on forest land by 

scenario, Iowa, 2010-2060. 
108 | FUTURE FORESTS

will need to be prudent about how much biomass is utilized and how it is obtained 

and replenished. 

Under the scenarios with a continuation of current removals rates (A1B-C, A2-C, and 

B2-C), the live-tree volume is expected to increase until about 2030 (despite losses 

in forest land area). For the next 20 years all three scenarios have volumes just below 

the high levels set in 2030. Surprisingly, from 2050 to 2060 scenario B2-C and A1B-C 

achieve new highs whereas scenario A1B-C drops below 2030 levels. Although the 

total area of forest land is expected to decrease, the volume per acre in these three 

scenarios is expected to increase as forests continue to mature. In the past 50 years 

forest managers have had the luxury of rapidly increasing forest volume with growth 

greatly exceeding removals. If these projections hold true, that may not be the case for 

future generations of forest managers and wood-using industries. This trend results 

from the combined effects of gradually decreasing forest area and an aging forest 

resource with high volume but low net growth per acre. 
 | 109

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), 

2003 (Leatherberry et al. 2006), and 2008 (Nelson et al. 2011). Data from Iowa’s 

forest inventories are available online at http://www.nrs.fs.fed.us/fia/. For detailed 

information on inventory methods, see the Statistics, Methods, and Quality 

Assurance section at http://dx.doi.org/10.2737/NRS-RB-102. 

National Woodland Owner Survey

Information about family forest owners is collected annually 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 identified as woodland owners by FIA are invited to participate in the 

NWOS. Each year, questionnaires are mailed to 20 percent of private owners, with more 

detailed questionnaires sent out in years that end in 2 or 7 to coincide with national 

census, inventory, and assessment programs. 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.fia.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 by using regional 

conversion factors. Timber removals by source of material and harvest residues 

generated during logging were estimated from standard product volumes by using 

factors developed from logging utilization studies previously conducted by NRS. 
110 | LITERATURE CITED

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Appendix—List of tree species, greater than or equal to 1 inch in diameter, found on FIA inventory plots, Iowa, 2009-

2013

Common name Genus Species

Species 

code

Species 

group code

Ailanthus Ailanthus altissima 341 43

American basswood Tilia americana 951 38

American elm Ulmus americana 972 41

American hornbeam, musclewood Carpinus caroliniana 391 43

American plum Prunus americana 766 43

American sycamore Platanus occidentalis 731 41

Apple spp. Malus spp. 660 43

Bigtooth aspen Populus grandidentata 743 37

Bitternut hickory Carya cordiformis 402 29

Black ash Fraxinus nigra 543 36

Black cherry Prunus serotina 762 41

Black locust Robinia pseudoacacia 901 42

Black maple Acer nigrum 314 31

Black oak Quercus velutina 837 28

Black walnut Juglans nigra 602 40

Black willow Salix nigra 922 41

Boxelder Acer negundo 313 41

Bur oak Quercus macrocarpa 823 25

Butternut Juglans cinerea 601 41

Chinkapin oak Quercus muehlenbergii 826 25

Chokecherry Prunus virginiana 763 43

Eastern cottonwood Populus deltoides 742 37

Eastern hophornbeam Ostrya virginiana 701 43

Eastern redbud Cercis canadensis 471 43

Eastern redcedar Juniperus virginiana 68 9

Eastern white pine Pinus strobus 129 4

Green ash Fraxinus pennsylvanica 544 36

Hackberry Celtis occidentalis 462 41

Hawthorn spp. Crataegus spp. 500 43

Honeylocust Gleditsia triacanthos 552 42

Kentucky coffeetree Gymnocladus dioicus 571 42

Mockernut hickory Carya alba 409 29

Northern catalpa Catalpa speciosa 452 41

Northern pin oak Quercus ellipsoidalis 809 28

Northern red oak Quercus rubra 833 26

Oak spp. Quercus spp. 800 42

Ohio buckeye Aesculus glabra 331 41

Osage-orange Maclura pomifera 641 43

Paper birch Betula papyrifera 375 41

(Appendix continued on next page.)
(Appendix continued) 

Common name Genus Species

Species 

code

Species 

group code

Peachleaf willow Salix amygdaloides 921 43

Pignut hickory Carya glabra 403 29

Pin oak Quercus palustris 830 28

Ponderosa pine Pinus ponderosa 122 9

Post oak Quercus stellata 835 27

Quaking aspen Populus tremuloides 746 37

Red mulberry Morus rubra 682 42

Red pine Pinus resinosa 125 4

River birch Betula nigra 373 41

Russian-olive Elaeagnus angustifolia 997 43

Serviceberry spp. Amelanchier spp. 356 43

Shagbark hickory Carya ovata 407 29

Shingle oak Quercus imbricaria 817 28

Siberian elm Ulmus pumila 974 41

Silver maple Acer saccharinum 317 32

Slippery elm Ulmus rubra 975 41

Sugar maple Acer saccharum 318 31

Swamp white oak Quercus bicolor 804 25

White ash Fraxinus americana 541 36

White mulberry Morus alba 681 42

White oak Quercus alba 802 25
Nelson, Mark D.; Barnett, Charles J.; Brewer, Matt; Butler, Brett J.; Crocker, Susan J.; 

Domke, Grant M.; Gormanson, Dale; Kurtz, Cassandra M.; Lister, Tonya W.; Matthews, 

Stephen; McWilliams, William H.; Meneguzzo, Dacia M.; Miles, Patrick D.; Morin, Randall 

S.; Piva, Ronald J.; Riemann, Rachel; Smith, James E.; Walters, Brian F.; Westfall, James 

A.; Woodall, Christopher W. 2016. Iowa Forests, 2013. Resour. Bull. NRS-102. Newtown 

Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station. 

124 p.

The third full annual inventory of Iowa’s forests (2009-2013) indicates that just under 

3 million acres of forest land exists in the State, 81 percent of which is in family forest 

ownership. Almost all of Iowa’s forest land is timberland (96 percent), with an average 

volume of more than 1,000 cubic feet of growing stock per acre on timberland and more 

than 1,500 cubic feet of all live volume (for trees at least 5 inches diameter at breast 

height) per acre on timberland. American elm and eastern hophornbeam are the most 

numerous tree species, but bur oak and silver maple predominate in terms of live-tree 

volume. Iowa’s forest land is composed of 70 percent sawtimber, 17 percent poletimber, 

and 13 percent sapling/seedling or nonstocked size classes. Average annual net growth 

of growing-stock trees on Iowa’s timberland decreased during the past decade to the 

current estimate of 71 million cubic feet. This report includes additional information on 

forest attributes, land-use change, carbon, timber products, wildlife habitat, forest health, 

and future projections. The following information is available online at http://dx.doi.

org/10.2737/NRS-RB-102: 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, wildlife, carbon, FIA

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