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Criterion 3 Indicator 15
Rationale
Area and Percent of Forest Affected by Processes or Agents Beyond the Range of Historic Variation
 
People have either changed or have the ability to change many of the natural processes that have shaped Oregon’s forests. Fire suppression, timber harvesting, development, agriculture, and grazing can all cause changes in the processes that historically shaped the environment. Disturbance processes are key factors affecting species diversity and the abundance of individual species across the landscape. Therefore, disturbances have direct effects on the ecological composition and integrity of the forest. One indication of overall forest health is how closely ecological processes are maintained within the range of historic variation.

Can This Indicator Be Quantified
It is very difficult to determine what range of historic variation to use as a reference point. Major ecological disturbances include fire and windstorms. These events vary dramatically over time and across the landscape. Years may go by without a major fire or windstorm, or a severe event may significantly change the forest in one area in a short time. In most cases direct measurement is impossible. A lack of field plot data about current conditions also makes it difficult to quantify this indicator. For this indicator, the discussions will concentrate on the changes in three ecological processes that have been affected by human interactions: disturbance cycles, insect and disease outbreaks, and exotic or invasive species. Landslides are another important disturbance and are discussed in Indicator 18.
 
Disturbance cycles — Historically fire was probably the natural process that caused the greatest amount of change in the forest. Across all Douglas-fir forests of the Pacific Northwest, fire return intervals are thought to have averaged around 230 years. In the Oregon Coast Range, fires tended to be less frequent but larger. High severity, stand replacement fires occurred at 200 to 400 year intervals in the Sitka spruce forests of the coastal fog belt. In the central Cascades, fire cycles are thought to have ranged from 95 to 145 years. Farther south where conditions are warmer and drier, fire return intervals were more frequent and have been estimated at 9 to 42 years in the area around Crater Lake National Park. In the Pacific silver fir forests of the high Cascades, fire return intervals have been calculated by stand age analysis, and range from 300 to 600 years at high elevations, and 100 to 300 years at lower elevations. Fire return intervals averaged only 18 to 20 years over one-hectare areas in the eastern Siskiyou Mountains in southwest Oregon. (The information above is summarized from Agee 1993.)
 
In western Oregon, the large stand replacement fires of the past are now controlled with fire suppression tactics. Fire has been replaced by timber harvesting as the dominant disturbance regime. Maps 15-1 and 15-2 illustrate the reduction in disturbance size. Although this is an "apples and oranges" type of comparison because different methods were used to create the two maps, it is still worthwhile to examine the changes. During historic periods, average fire sizes ranged from 2,500 acres to 46,000 acres, with the largest fires in these periods from 11,000 to 400,000 acres (Map 15-1). In contrast, the average sizes of harvest units range from 30 to 45 acres, depending on the period. The largest accumulations of harvest units (for example, units adjacent to each other and harvested within a few years of each other) ranged from 400 to 1,300 acres, still much smaller than the large fires from historical periods (Map 15-2).
 
 
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In the transitional forest zone of the eastern Oregon Cascades, dominated by true firs, low to high severity fire intervals ranged from 33 to 100 years. Farther east, in the ponderosa pine types, pre-settlement fires were frequent and widespread due to lots of perennial herbaceous plants in the understory. Fire return intervals ranged from 4 to 36 years, depending on the area and ignition source (Agee, 1993). In many places the frequent fires cleared the vegetation on the forest floor and created open stands of ponderosa pine. The current fire regime has significantly changed from these historical patterns in eastern Oregon. There has been a decrease in the number of small fires that do not kill most trees in the stand, but an increase in the number of higher intensity, stand replacement or damaging fires (USDA Forest Service, 1997).
 
The number of fires declined in the 1960s in eastern Oregon, due to the advent of improved technology for fire detection, prevention, and suppression. However, with fewer fires, more fuel accumulated in forests, and the number of wildfires has increased steadily since then. Currently in eastern Oregon, fire frequency and intensity are approaching or exceeding the levels of the early 1900s, a time when many wildfires occurred. The average cost of wildfire suppression, fatalities of firefighters, and the number and size of high intensity fires were all doubled in the period from 1970 to 1995, in comparison to levels from 1910 to 1970 (USDA Forest Service, 1996).
 
Insect and disease cycles — Aerial surveys of insect and disease damage are conducted annually by the U.S. Forest Service and Oregon Department of Forestry. The survey plane flies at low elevation and observers sketch damaged areas onto a map. In recent surveys in western Oregon, 171,670 acres of coastal Douglas-fir were detected with discoloration from Swiss needle cast (Map 15-3). Insects have heavily damaged the forests of eastern Oregon in recent years (Maps 15-4 through 15-7). In 1991 bark beetles or spruce budworm infested more than 4.7 million acres. (Note: Mapped areas do not indicate that all trees are affected or dead. The intensity of damage is highly variable).
 
 
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Exotic/invasive species — There is no comprehensive database that quantifies the abundance of different species of concern like Scotch broom, blackberries, cheat grass, and English ivy.

Trends
   
Disturbance cycles — Timber harvest units are usually much smaller in size than historic fires, and the 50 to 60 year rotation lengths on private lands are shorter than most of the stand replacement fire cycles. It is unknown what the consequences are of this change in disturbance regimes, and more study is needed. However, some of the changes include the creation of more forest edge habitat and less interior habitat (see Indicator #5), and a reduction in the number of large trees and amounts of large woody debris present on the landscape (see Indicator #17).
 
Very little timber harvest is planned on the federal forest lands in the future, and over time these forests will tend to become dominated by older dense stands with large amounts of biomass on the forest floor. The effects of these policies on disturbance cycles have not been quantified, but the intensity of wildfires is likely to increase.
 
Insect and disease cycles — Fire suppression and selective timber harvest have combined to change the species composition and density of many eastern Oregon forests. Lehmkuhl, et. al., (1994) found significant changes in both the overstory and understory composition of forests in eastern Oregon and Washington. The researchers found fewer early successional trees (the first species of trees to grow in open areas after a disturbance), but large increases in the number of shade-tolerant species in the overstory. They also found open grass/forb and shrub understories being replaced by dense layers of ponderosa pine, or shade-tolerant Douglas-fir and grand fir. In some areas like the Deschutes River basin, they also found grassland being replaced by forest cover.
 
Lehmkuhl, et. al., (1994) compared aerial photos from the 1930s to 1950s with photos from the 1990s, to determine changes in insect and disease hazard. They rated hazards into 12 categories according to the "presence of susceptible host vegetation in susceptible arrangements." In the majority of watersheds, the change in hazard rating was within 10 percent, with some ratings increasing and others decreasing. However, the researchers found large increases in susceptibility to insects and disease in some watersheds.
 
Exotic/invasive species — People have introduced several exotic species into Oregon that have spread rapidly and are capable of dominating sites. Of particular concern are Scotch broom, blackberries, cheat grass, and English ivy. These species have replaced native species on a large portion of the forest land base, although no precise data exists on how many acres.
 
Juniper woodlands occur on sites that are too dry to support other tree species, but juniper is very susceptible to fire and its historic range was limited. Grazing and fire suppression have contributed to a large expansion of juniper range since the 1800s. It is very difficult to determine the exact extent of the change, because fire usually kills most junipers and there is little evidence left for researchers to decipher the fire history.

Data Source and Availability
   
 
Insect and disease information from the U.S. Forest Service is available at: http://svinet2.fs.fed.us/r6/nr/fid/wid.htm.

Reliability of Data
   
Swiss needle cast data — The disease was mapped during aerial surveys of stands within approximately 20 miles of the coast line during April and May, in 1996, 1997, and 1998. Areas of discoloration were sketch mapped on 1:100,000 USGS quad maps, and then digitized into a geographic information system (GIS). The GIS maps show polygons that correspond to the areas of discolored forest, primarily Douglas-fir forests. These maps are suitable for regional applications only (1:100,000 scale or smaller). Locations of some polygons may not be accurate due to errors in the aerial sketch maps, the compilation process, digitizing, or a combination of these factors. The Oregon Department of Forestry does not guarantee the validity or accuracy of these data.
 
Insect information — Aerial surveys may cover a single drainage or larger planning units. The surveys are done every year in mid-July, or as soon as defoliator damage is most visible. Defoliation is sketch-mapped at various scales, depending on management objectives (minimum scale 1:125 000). All high hazard forest types are surveyed. The observers look for a reddish tinge, which indicates defoliated trees or stands. When defoliation is severe or has occurred over many years, stands may appear gray. Defoliation is classified into one of three categories: light, moderate, or severe. Ground checks are done to verify the defoliation.

Scale
Varies.

Recommended Action for Data Collection
   
Basic research needs to be done to describe the size, intensity, and pattern of historic disturbances such as fire. Further analysis needs to be done to compare historic patterns with the current disturbance patterns caused by fire and timber harvesting.
Common protocols need to be developed to rank forest susceptibility to insects and diseases. The federal permanent plot inventories may be able to provide the data needed to track susceptibility over time. Coordination should be developed among inventories for insect and disease damage data.
 
Currently, there are no adequate inventories of exotic and invasive species. This information could be taken on the permanent plots measured by the federal agencies if there is enough interest in the indicator.

Definitions
   
Fire cycle — The average time between fires in a given area.
 
Fire regime — The characteristic frequency, predictability, intensity, seasonality, and extent of fires in an ecosystem.
 
Lethal fire — A wildland fire that kills the overstory vegetation on a site.
 
Non-lethal fire — Rangeland fires in which vegetation structure and composition, three years following the fire, are similar to pre-burn conditions.

Selected References
   
Agee, James K. 1993. Fire ecology of Pacific Northwest forests. Island Press. 493 pp.
 
Lehmkuhl, John, and Paul Hessburg, Richard Everett, Mark Huff, Roger Ottmar. 1994. Historical and current forest landscapes of eastern Oregon and Washington. USDA Forest Service Pacific Northwest Research Station, Portland, OR. PNW-GTR-328. Part 1: Vegetation pattern and insect and disease hazards.
 
USDA Forest Service. 1997. An assessment of ecosystem compo
nents in the interior Columbia Basin and portions of the Klamath and Great Basins. USDA Forest Service Pacific Northwest Research Station, Portland, OR. General Technical Report PNW-GTR-405. Volume 2, June 1997.
 
USDA Forest Service. 1996. Status of the interior Columbia Basin: summary of scientific findings. (Part of the "ICBEMP" project: interior Columbia Basin ecosystem management project.) USDA Forest Service Pacific Northwest Research Station, Portland, OR. General Technical Report PNW-GTR-385.