1

What is the indicator and 

why is it important?

The forests of the United States continue to face a variety 

of biotic threats to long-term health and sustainability. 

Indicator 15 quantifies observed activity and effects from 

key biotic agents, such as insects, disease, and nonnative 

species. Current analysis of these agents, systematically 

measured at regular intervals and contrasted with 

reference conditions, provides evidence of changing 

ecological conditions and can be used to support practical 

forest health planning and management. 

Primarily, the indicator is based on Insect and Disease 

Survey (IDS) data compiled by U.S. Department of 

Agriculture (USDA), Forest Service, Forest Health 

Protection. These geospatial data are collected mainly 

through aerial detection surveys, supplemented by 

ground surveys and remote sensing with modeling and 

analysis techniques. The IDS methodology does not provide 

complete geographic coverage of the Nation’s forests. In 

particular, Hawaii, Puerto Rico, and other U.S. territories 

have not been surveyed consistently. Nevertheless, it does 

cover a substantial portion of the United States—more 

than 500 million acres annually—via a targeted approach. 

Data generation and attribution methods (i.e., demarcation 

of damaged areas and identification of causal agents) 

have become increasingly standardized and consistent 

since 1997. Notably, the IDS data are not limited to insects 

and diseases: they document a variety of biotic and abiotic 

disturbances, including drought and other weather-related 

phenomena. The two principal types of forest damage 

recorded in the IDS data are mortality and defoliation; only 

data documenting forest mortality caused by biotic agents 

are reported here. Additionally, plot data from USDA Forest 

Service, Forest Inventory and Analysis (FIA) were used in 

a separate examination of the distributions of nonnative 

plant species in U.S. forests.

What does the indicator 

show?

Figure 15-1 shows, for the United States and five regions, 

total forest acres with mortality caused by biotic agents 

annually from 1997 to 2017. Nationally, the annual totals 

exceeded 5 million acres, or 0.5 percent of the U.S. forest 

area, every year from 2002 onward. The peak mortality 

year for the United States was 2009, when nearly 12 million 

forest acres experienced mortality from biotic agents. This 

coincided with the peak year in the Rocky Mountain Region, 

which saw mortality on 8.4 million forest acres, 

representing 5.4 percent of the region’s forest area. 

Likewise, elevated levels of biotic mortality observed 

nationally from 2015 to 2017 are explained by a substantial 

increase in mortality in the Pacific Coast Region, 

particularly in 2016, when 5.7 million forest acres (5.8 

percent of the region’s forest area) experienced mortality 

caused by biotic agents. In sharp contrast, Alaska and the 

Indicator 3.15

Area and percent of forests affected by biotic 

processes and agents beyond reference 

conditions, 2017

Frank H. Koch and Kevin M. Potter

January 2024 | FS-1217-Indicator-3.15
2

South Region saw little mortality from biotic agents, 

regardless of year. 

Figure 15-1 also indicates the proportion of the annual 

mortality acreage caused by nonnative biotic agents 

versus agents of native or unknown origin. Only the North 

Region showed a significant impact from nonnative biotic 

agents. In fact, most of the biotic mortality observed 

in the North Region throughout the 21-year data period 

was due to nonnative agents. For example, in 2005—the 

peak mortality year in the region—nonnative agents were 

responsible for 99.4 percent of the 5.1 million forest acres 

with mortality. Mortality in the North Region due to agents 

of native or unknown origin exceeded mortality from 

nonnative agents only in 2008 and 2009, when the former 

accounted for 697,000 out of 977,000 forest acres (71.3 

percent) and 608,000 out of 825,000 forest acres (73.6 

percent), respectively.

Maps provide added context regarding some of the trends 

shown in figure 15-1. Figure 15-2 depicts the combined 

extents of forest mortality caused by biotic agents in the 

conterminous United States during the 2008–2012 and 

2013–2017 periods. As noted from figure 15-1, most of the 

mortality reported for the United States during the 2008–

2012 period occurred in the Rocky Mountain Region, but 

the map for this period (fig. 15-2, top) illustrates further 

that the mortality was pervasive in the forests of the 

Central and Northern Rockies, particularly in Colorado, 

Idaho, and Montana. Similarly, the map for the 2013–2017 

period (fig. 15-2, bottom), when much of the mortality from 

biotic agents occurred in the Pacific Coast Region, reveals 

that the mortality was mainly concentrated in California’s 

Sierra Nevada Mountains, especially in the southern portion 

of the range. 

Figure 15-1—Forest acres with mortality caused by biotic agents, 

summarized annually from 1997 to 2017 by agent origin (nonnative vs. 

native or unknown). Hawaii is included in the totals for the Pacific Coast 

Region for years when data were reported (2013 and 2015–2017). Forest 

area defined using National Land Cover Database 2011 percent tree 

canopy data (conterminous United States and Hawaii) and classified 

land cover data (Alaska). Agent data source: Forest Health Protection, 

National Insect and Disease Survey Database.

Figure 15-2—Maps of the conterminous United States showing the 

combined geographic extent of all areas with forest mortality caused by 

biotic agents during the 5-year periods 2008–2012 (top) and 2013–2017 

(bottom). Forest area masks developed from National Land Cover 

Database 2011 percent tree canopy data. Data source: Forest Health 

Protection, National Insect and Disease Survey Database.
3

What has changed since 

2010?

On average, the annual forest acreage of the United States 

with mortality from biotic agents has been lower since 

2010 (average = 6.5 million acres) than it was during the 

previous decade (average = 8.6 million acres). While total 

U.S. mortality exceeded 9 million acres six times between 

2001 and 2010, it has not surpassed this threshold since, 

even with the uptick in mortality that began in 2015. 

Nevertheless, it would be inappropriate to interpret 

this as a meaningful decrease in biotic mortality at the 

national scale. The annual mortality totals from the last 

three years of available data (2015–2017) may indicate 

the start of an increasing trend. They are also three to 

four times greater than the national mortality totals in 

the years prior to 2001. Fundamentally, forest mortality 

is a spatially and temporally variable phenomenon. This is 

markedly true for mortality caused by biotic agents: the 

mortality observed nationally in any given year is mostly 

determined by the activities of a few key agents operating 

at the regional level. For example, the mountain pine beetle 

(Dendroctonus ponderosae) was responsible for most of 

the mortality in the Rocky Mountain Region in 2009; by 

extension, this pest accounted for fully 75 percent of total 

mortality at the national scale (fig. 15-1). Thus, regionalscale patterns are more relevant for evaluating the status 

of this indicator. 

To this end, figures 15-3 through 15-7 show the most 

important biotic mortality agents through time in each of 

five regions. Recently, Alaska (fig. 15-3) has seen a 

resurgence of the spruce beetle (Dendroctonus rufipennis) 

after almost two decades of low mortality. In fact, 2017 

saw more spruce beetle mortality (nearly 400,000 forest 

acres) than any year since 1997. Spruce beetle outbreaks 

have tended to be cyclical in Alaska, but changing climatic 

conditions (e.g., milder winters and warm summers) could 

allow the outbreaks to persist longer and become more 

widespread. Still, other biotic agents exhibited much less 

impact during the data period, and as noted earlier, the 

percentage of Alaska’s forest area that experienced 

mortality from the spruce beetle or any other agent 

remained low (<0.3 percent) throughout the period.

Forest mortality in the Pacific Coast Region (fig. 15-4) has 

been caused by an assortment of biotic agents, including 

bears (Ursus spp.). From 2005 to 2014, the mountain pine 

beetle was responsible for the largest share of the 

mortality, but it was not particularly dominant, only 

exceeding 40 percent of the region’s total mortality 

acreage in 2007 and 2008. Furthermore, its relative 

importance as an agent has declined during the region’s 

recent surge in mortality. Instead, the fir engraver 

(Scolytus ventralis) and the western pine beetle 

(Dendroctonus brevicomis) have become more prominent. 

Persistent and intense drought conditions in the Pacific 

Coast Region, probably amplified by climate change, have 

facilitated outbreaks of these agents. Since 2015, mortality 

caused by the fir engraver and western pine beetle has far 

surpassed the levels observed earlier in the data period. 

Figure 15-3—Total forest acres with mortality caused by biotic agents, 

1997–2017, in Alaska. The three agents with the most extensive impacts 

are depicted separately: spruce beetle (Dendroctonus rufipennis), 

northern spruce engraver (Ips perturbatus), and eastern larch beetle (D. 

simplex). Forest area defined using National Land Cover Database 2011 

classified land cover data. Agent data source: Forest Health Protection, 

National Insect and Disease Survey Database.

Figure 15-4—Total forest acres with mortality caused by biotic agents, 

1997–2017, in the Pacific Coast Region. Hawaii is included in the regional 

totals for years when data were reported (2013 and 2015–2017). The 

five agents with the most extensive impacts are depicted separately: 

mountain pine beetle (Dendroctonus ponderosae), fir engraver (Scolytus 

ventralis), bears (Ursus spp.), unidentified bark beetle species, and 

western pine beetle (D. brevicomis). Forest area defined using National 

Land Cover Database 2011 percent tree canopy data. Agent data source: 

Forest Health Protection, National Insect and Disease Survey Database.
4

Historically, the mountain pine beetle has been the most 

destructive biotic agent in the Rocky Mountain Region (fig. 

15-5). In 2008 through 2010, when mountain pine beetle 

populations were at epidemic levels, the insect accounted 

for more than 85 percent of all biotic mortality in the 

region. Since 2010, its impact has decreased dramatically: 

in 2017, the mountain pine beetle caused mortality on just 

102,000 forest acres (<0.1 percent of the forest area) 

regionwide. This decrease has occurred as susceptible 

pines have been depleted from the region’s forests by the 

insect. Spruce beetle mortality has become more 

prevalent as mountain pine beetle has declined, but in 2017, 

total biotic mortality in the Rocky Mountain Region dropped 

to a level almost as low as in years prior to 2001.

The emerald ash borer (Agrilus planipennis), a nonnative 

beetle from Asia, has emerged as the most consequential 

biotic agent in the North Region (fig. 15-6). The insect was 

responsible for two-thirds of all forest mortality due to 

biotic agents since 1997. Most of that mortality occurred in 

the 5 years after the insect’s initial discovery (2002–2006) 

or in 2016 and 2017, when the data suggest that its range 

expanded considerably. Two other nonnative species—the 

hemlock woolly adelgid (Adelges tsugae) and the beech 

scale (Cryptococcus fagisuga), the insect vector for beech 

bark disease—also ranked among the region’s important 

biotic agents, but mortality by agents besides the emerald 

ash borer, nonnative or otherwise, totaled less than 1 

million forest acres (<0.5 percent of the forest area) 

annually. 

In the South Region (fig. 15-7), most of the biotic mortality 

during the data period was caused by the southern pine 

beetle (Dendroctonus frontalis). From 1998 to 2002, the 

insect accounted for 99.9 percent of all mortality from 

biotic agents. However, its regional impact decreased 

sharply after 2002; in 2010 and 2011, mortality caused by 

the southern pine beetle occurred on less than 800 forest 

acres. This may be explained by better management and 

improved conditions (e.g., less overstocking) in the region’s 

pine forests. Conversely, the increase in southern pine 

beetle mortality in 2016 and 2017 can be traced to a 

Figure 15-5—Total forest acres with mortality caused by biotic agents, 

1997–2017, in the Rocky Mountain Region. The five agents with the 

most extensive impacts are depicted separately: mountain pine beetle 

(Dendroctonus ponderosae), Douglas-fir beetle (D. pseudotsugae), 

spruce beetle (D. rufipennis), fir engraver (Scolytus ventralis), and 

pinyon ips (Ips confusus). Forest area defined using National Land Cover 

Database 2011 percent tree canopy data. Agent data source: Forest 

Health Protection, National Insect and Disease Survey Database.

Figure 15-6—Total forest acres with mortality caused by biotic agents, 

1997–2017, in the North Region. The five agents with the most extensive 

impacts are depicted separately: emerald ash borer (Agrilus planipennis), 

beech bark disease complex, bronze birch borer (A. anxius), hemlock 

woolly adelgid (Adelges tsugae), and eastern larch beetle (Dendroctonus 

simplex). Forest area defined using National Land Cover Database 2011 

percent tree canopy data. Agent data source: Forest Health Protection, 

National Insect and Disease Survey Database.

Figure 15-7—Total forest acres with mortality caused by biotic agents, 

1997–2017, in the South Region. The five agents with the most extensive 

impacts are depicted separately: southern pine beetle (Dendroctonus 

frontalis), baldcypress leafroller (Archips goyerana), forest tent 

caterpillar (Malacosoma disstria), hemlock woolly adelgid (Adelges 

tsugae), and ips engraver beetles (Ips spp.). Forest area defined using 

National Land Cover Database 2011 percent tree canopy data. Agent data 

source: Forest Health Protection, National Insect and Disease Survey 

Database.
5

localized resurgence of the insect on overstocked stands 

in Alabama and Mississippi. Regardless, total biotic 

mortality stayed below 0.3 percent of the region’s forest 

area, even in the peak years of southern pine beetle 

activity. Notably, the nonnative hemlock woolly adelgid 

spread into the South Region and began causing mortality 

in the mid-2000s. The IDS data suggest that its impact has 

been limited to date, but this is at least partially an artifact 

of the restricted distribution of hemlocks in the region.

Are there important 

regional differences?

In all regions but the North Region, the most consequential 

mortality-causing agents are native bark beetle species. 

Each of these species has a documented history of 

periodic outbreaks in forests of its native region (or 

regions). These outbreaks can be widespread regionally, 

depending on the condition and distribution of host trees, 

as well as predisposing factors such as persistent drought. 

Analysis and monitoring of these predisposing factors 

will become increasingly critical as changing climatic 

conditions lead to altered dynamics of drought and other 

forest disturbances, both within and between regions.

As highlighted earlier (fig. 15-1), a distinctive characteristic 

of the North Region is the relative significance of nonnative 

biotic agents. This pattern is consistent across taxa. For 

example, figure 15-8 depicts the average number (i.e., 

richness) of nonnative plant species found on surveyed FIA 

plots in counties of the conterminous United States. Most 

of the counties with the highest richness (>4.0) are in the 

North Region. More generally, nonnative plant species 

richness is considerably higher in the eastern portion of 

the country than in the West, which can be explained by its 

longer history of trade and human settlement, as well as 

denser transportation networks. This disparity between 

regions will probably diminish through time as invasion 

pathways continue to proliferate.

Figure 15-8—Map of the conterminous United States showing mean richness of nonnative plant species on surveyed Forest Inventory and Analysis (FIA) 

plots in each county. Counties where an insufficient number of FIA plots were surveyed for nonnative plants are displayed in grey. Data source: USDA 

Forest Service, Forest Inventory and Analysis.
6

U.S. Forest Sustainability Indicators:

 https://www.fs.usda.gov/research/inventory/sustainability

In accordance with Federal civil rights law and U.S. Department of Agriculture (USDA) civil rights 

regulations and policies, the USDA, its Agencies, offices, and employees, and institutions participating 

in or administering USDA programs are prohibited from discriminating based on race, color, national 

origin, religion, sex, gender identity (including gender expression), sexual orientation, disability, age, 

marital status, family/parental status, income derived from a public assistance program, political 

beliefs, or reprisal or retaliation for prior civil rights activity, in any program or activity conducted or 

funded by USDA (not all bases apply to all programs). Remedies and complaint filing deadlines vary 

by program or incident.

Persons with disabilities who require alternative means of communication for program information 

(e.g., Braille, large print, audiotape, American Sign Language, etc.) should contact the responsible 

Agency or USDA’s TARGET Center at (202) 720-2600 (voice and TTY) or contact USDA through the 

Federal Relay Service at (800) 877-8339. Additionally, program information may be made available in 

languages other than English.

To file a program discrimination complaint, complete the USDA Program Discrimination Complaint 

Form, AD-3027, found online at How to File a Program Discrimination Complaint and at any USDA 

office or write a letter addressed to USDA and provide in the letter all of the information requested in 

the form. To request a copy of the complaint form, call (866) 632-9992. Submit your completed form 

or letter to USDA by: (1) mail: U.S. Department of Agriculture, Office of the Assistant Secretary for 

Civil Rights, 1400 Independence Avenue, SW, Washington, D.C. 20250-9410; (2) fax: (202) 690-7442; 

or (3) email: program.intake@usda.gov.

USDA is an equal opportunity provider, employer, and lender.

Koch, Frank H.; Potter, Kevin M. 2024. Indicator 3.15: area and percent 

of forests affected by biotic processes and agents beyond reference 

conditions, 2017. FS-1217-Indicator-3.15. Washington, DC: U.S. Department of 

Agriculture, Forest Service, Washington Office. 6 p. https://doi.org/10.2737/

FS-1217-Indicator-3.15.