Criterion 3 measures forest disturbance processes and contains only two indicators. The first (Indicator 3.15) addresses biological processes, such as insect infestations and the influx of invasive species, that can affect forest health, and the second (Indicator 3.16) addresses physical processes, such as fire and storms, that likewise affect forests. The relatively small number of indicators, however, is no indication of the relative importance of this criterion. The processes described here have a crucial effect on the health, character, and extent of forest ecosystems and are, thus, closely linked to all the other indicators contained in this report.
In many cases, forest disturbances—both biological and physical— can be seen as leading indicators foreshadowing changes in the distribution of forest ecosystem types across the landscape. Disturbances also affect the ability to provide an array of valuable goods and services, whether traditional commodity outputs like timber or livestock forage, ecosystem services such as water purification and streamflow regulation, or more intangible values such as aesthetic character or species habitat. The indicators in this criterion may register major changes that are not yet apparent in the other indicators describing the biophysical characteristics of forests and their associated values and outputs. Moreover, to the extent that climate change will affect our forests, these effects will likely first be clearly apparent within Criterion 3.
A certain level of disturbance is natural in healthy ecosystems. The real question is not the absolute level of disturbance, but whether it represents a significant departure from the background, or “natural,” level of disturbance for a given ecosystem. For this reason, the MP C&I definitions for both Indicators 3.15 and 3.16 stipulate “reference conditions” against which current levels of disturbance are to be measured. Of course, determining valid reference conditions can be a difficult and controversial undertaking. The strategy used in this report is to identify the average measures for the 1997-to-2002 time period as the reference and analyze current measures accordingly. This approach is not without its problems, since the 1997-to-2002 reference period may not represent a natural or sustainable level of disturbance. Because of fire suppression activities throughout much of the past century, for example, fire incidence in many of our forests is less than occurred before suppression, and, as a result of accumulated fuels, fire intensity is higher today in many fires that do burn. So the reference conditions should be taken merely as benchmarks for comparison and not as targets representing an ideal situation.
The findings for the indicators in this criterion point to a substantial increase in the levels of biotic disturbance and an increase in fire extent and intensity relative to the 1997-to-2002 reference period. In the lower 48 States, cumulative total forested area with notable mortality due to biotic agents has risen to 37 million acres, compared to the reference condition of 12 million acres. Bark beetle, engraver beetle, and gypsy moth are the leading contributors to this increase, along with increasing mortality in the pinon-juniper forest type. When defoliation is taken into account alongside mortality, the number of acres affected since 2003 rises to 50 million, or 8 percent of forest area in the lower 48 States.
The growing incidence of nonpathogenic invasive plants and animals likewise threaten forest health, although here the effects are not registered in terms of forest mortality so much as changing species distributions. Aside from radically altering forest character and displacing native species, these invasive species can predispose forest stands to other types of disturbance such as insect infestation and fire.
Drought and the increasing density of forest stands, because of tree growth and fire suppression have been cited as important factors undermining forest health and thereby the ability of trees to resist insects and disease. Another factor may be the increased senescence of shorter lived species, such as lodgepole pine (Pinus contorta), which are now reaching older ages in the absence of traditional disturbance agents such as fire. In the future, climate change may further complicate the picture, as water availability, precipitation patterns, and the ranges of certain insects and pathogens are expected to change. The causes and possible effects to forest ecosystems are complex, and many of the processes themselves can be considered natural, even if they are in response to anthropogenic changes such as fire suppression or climate change. Therefore, the implications of these changes for sustainability are difficult to determine both in both a conceptual and a practical sense. What is clear, however, is that the findings for Indicator 3.15 point to a major increase in biotic forest disturbance with the potential for broadscale impacts, many of which society will likely find undesirable.
For most forest ecosystems, fire is the most important abiotic (nonbiological) disturbance category in Indicator 3.16. Other disturbance factors considered in the indicator include weather damage, damage from airborne pollutants, and impacts from human development. Climate change is also identified as a potential abiotic disturbance factor, but there are numerous specific pathways through which it can affect forests, including biotic disturbance agents alongside more direct paths such as drought and fire. This fact brings up an important point: disturbance factors are often linked through various biophysical processes, and evidence of one type of disturbance may indicate the presence, or probable future occurrence, of other types of disturbance. Catastrophic fire following insect induced mortality is a common example of this.
Fire. The findings for Indicator 3.16 point to an increase in fire extent and intensity relative to the 1997-to-2003 time period. Current fire levels are significantly less than those witnessed before the advent of broadscale fire suppression efforts in the first half of the last century, but the fires that do burn are likely more intense, and, without significant forest management efforts, the number and extent of fires are likely to continue to increase in the future. Increases in biotic disturbance and mortality documented in Indicator 3.15 support this conclusion.
Weather. Weather-related damage has also increased significantly relative to the reference period, rising from approximately 800 thousand acres to nearly 1.8 million acres during the past decade. Most of this is related to a roughly 10-fold increase in the forest area affected by drought, and this, in turn, may foreshadow increases in other disturbances, such as fire and disease, to which drought-stressed trees are more susceptible. Storm damage is another aspect of weather disturbance that is locally significant though not all that visible in national level statistics.
Pollution. Little direct evidence exists linking airborne pollution to widespread forest mortality or decline at the regional scale, but this does not necessarily mean pollution is not a problem; it is just hard to identify and may be more clearly seen in other indicators such as Indicator 4.19, which addresses soil degradation.
Development. Human development impacts a growing area of forest land. In 2000, the past year for which consistent data were available for this report, our development footprint (meaning affected area) accounted for more than 13.3 percent of total land area in the United States, up from 10.1 percent in 1980. This expansion significantly exceeds population growth, and it has no doubt continued since 2000.
Climate Change. Climate change will potentially affect forests in numerous and complex ways. Some of these are identified in the analysis of Indicator 3.16. But as yet little data exists documenting these effects or providing direct evidence that climate change is the proximate cause.