Results and Discussion

Fig. 1 - Proportion of the study area in multi-story
large and very large tree structure classes

Multistory large and very large tree forests declined over the 200-year simulation period under scenario one due to a combination of wildfire and insect outbreaks (Figure 1). This suggests, based on our modeling assumptions, that passive management on Federal lands in the study area might produce no more than about the current abundance of large and very large tree forests in the study area and that those forest conditions might decline on a long-term basis.

Single-story large and very large tree forests remained relatively constant at about 2 percent of the landscape area under scenario 1, on average, over 200 years (Figure 2). The relatively low levels of single-story large and very large tree forests that did occur resulted from an uncommon coincidence of slow regeneration of small trees and random low- or moderate-severity wildfire.

Fig. 2 - Proportion of the study area in single-story
very large and large tree structure classes

High-severity wildfires burned more landscape area under scenario 1 than scenarios 2 and 3 (Figure 3). High-severity wildfire was proportionately greatest in WUI areas dominated by grass, forb, and shrub communities. Although these communities are highly susceptible to wildfires that kill most of the aboveground vegetation, wildfires in such vegetation are much more easily controlled than those burning in dense forests. The other ownership/allocation categories were largely forested throughout our simulations. High-severity wildfire affected Federal general forests somewhat more than other ownership/allocation classes because Federal general forests were mostly in lower-elevation, drier environments subject to higher fire probabilities.

Mechanical fuel treatments and stand thinnings only occurred on private lands under scenario 1 and remained below 1000 ha treated per decade (Figure 4). Likewise, prescribed fire treatments only occurred on Federal lands and were absent under scenario 1 (Figure 5). Treatments that might produce at least some commercial timber products averaged less than 10,000 ha per decade under scenario 1 and slowly declined to about 5,000 ha per decade in the last ten decades (Figure 6).

Fig. 3 - Area affected by high severity wildfire

Scenario 2 produced moderate amounts of multistory large and very large tree forests (Figure 1). Contrary to our design objectives, scenario 2 did not increase multistory large tree forest by much compared to current conditions. In fact, scenario 2 produced lower amounts of multistory large and very large tree forest than scenario 1 for the first 100 years. Both scenarios 1 and 2 simulations produced, on average, about half the current amount of multistory large tree and very large tree forest at the end of 200 years. Perhaps alternative approaches to protecting and developing multistory large tree forests on Federal general forests could be formulated and might be more successful than our scenario 2. This also suggests, at least given assumptions in our models, that current levels of multistory large and very large tree forests in the study area are perhaps an artifact of fire suppression and other factors and may not be sustainable in the study area over the long run.

Single-story large and very large tree forests substantially increased across the entire landscape under scenario 2 (Figure 2). Much of the increase occurred in Federal general forest, and WUI were due to thinning to produce large trees quickly in scenario 2 and very active fuel treatments that produced open stands in scenario 3. Single-story large and very large tree forests remained at very low levels in reserves and in the wilderness.

Fig. 4 - Area treated with mechanical
fuel treatments and thinnings

Scenario 2 produced lower levels of high-severity wildfire compared to scenario 1 (Figure 3). Even though fuel treatments were not extensive in scenario 2, some did occur on Federal general forests and in reserves to foster early development of large trees. As a result, the area burned in high-severity wildfires was, on average, about 20 percent to 30 percent less than in scenario one, especially after the first two decades. As in scenario one, the highest proportion of high-severity wildfire in scenario 2 was in open forests dominated by grass/forb/shrub communities in WUI. High-severity wildfires are relatively easy to control in open grass/forb/shrub communities compared to high-severity wildfire in dense forest. From a wildfire protection perspective, active fuel treatment changed potential fire behavior rather than eliminating wildfire. Federal general forests experienced considerably lower amounts of high-severity wildfire compared to scenario one, due to fuel treatment effects, but amounts in reserves and wilderness were similar to those under scenario one.

Management activity levels were higher in scenario two than in scenario one due to thinnings to promote large tree development in Federal general forests. Scenario 2 produced about 25,000 ha of commercial treatment activities per decade over twenty decades (Figure 6). Mechanical fuel treatment rates declined slightly after the first decade, then varied over the remaining nineteen decades. Mechanical fuels and thinning were highest in the first decade as the initial round of mechanical fuel treatments peaked (Figure 4). Prescribed fire rose to about 10,000 ha per decade as fire replaced mechanical fuel treatment for fuel reduction, then remained at relatively stable levels (Figure 5).

Fig. 5 - Area treated with prescribed fire

Scenario three produced the lowest overall abundance of multistory large and very large tree forests (Figure 1). Multistory large and very large tree forests declined from about 13 percent of the study area to a minimum of about 4 percent at the end of the simulation. Much of the decline occurred in the first 100 years as dense forests burned or were killed by insect outbreaks. Initial declines on Federal general forests were due to thinnings designed to quickly move dense forests to more open conditions followed by thinnings and fuel treatments at maintenance levels.

Conversely, scenario 3 produced abundant single-story large and very large tree forests (Figure 2). Single-story large and very large tree forests initially occupied less than 5 percent of the study area, but increased fourfold to over 20 percent by the end of the simulation. Increases were nearly greatest on Federal general forest lands due to active thinning and fuel treatment. Smaller increases occurred in WUI and reserves that were treated at lower rates. Though the trend was flattening after 200 years, single-story large and very large tree forests were still increasing across the landscape as a whole. Landscape levels (about 20 percent) at 200 years were lower than those estimated by Hann and others (1997) for historical conditions in the southern Cascades area in Oregon and Washington (about 57 percent).

Fig. 6 - Area treated with management activities
that may produce commercial timber products

Scenario 3 also produced the lowest overall rates of high-severity wildfire (Figure 3). After the first five decades, the proportion of area burned in high-severity wildfires was generally 30 percent or more below that in scenario 1 and 5 percent to 10 percent lower than that in scenario 2. WUI areas experienced the highest proportion of high-severity wildfire, again in grass-/forb-/shrub- dominated open forests where wildfire is most easily controlled. Of the other three ownership/allocation classes, wilderness areas were most highly impacted by high-severity wildfires, in contrast to scenarios 1 and 2. This resulted from fuel treatments that reduced wildfire outside wilderness. Federal general forests, on the other hand, experienced lower levels of high-severity wildfire compared to both scenarios 1 and 2 due to fuel treatment effects.

Scenario 3 produced about 35,000 ha of commercial timber harvest per decade (Figure 6). Mechanical fuel treatments and thinnings occurred on about 9,000 ha in the first decade, then varied between 6,000 and 7,000 ha per decade after that (Figure 4). Not surprisingly, given the emphasis on reducing fire risks and generating open forests on Federal general forests, scenario 3 produced the high levels of prescribed fire (Figure 5). The initial ramp-up in prescribed fire took place over the first four decades after initial mechanical fuel treatments reduced fuel levels so that prescribed fire could be used for subsequent fuel treatments.