NORTHERN COLORADO FRONT RANGE: MONTANE ZONE

Central Questions: Are disturbance patterns and forest conditions within or outside of their Historic Range of Variability? Have insect outbreaks and/or mistletoe infestations increased in the 20^th century due to fire suppression or other management actions? How do modern forest compositions and structures compare with those of the pre-20th century? Has 20^th century fire suppression resulted in an unnatural accumulation of fuels so that recent and future fires are unnaturally severe and widespread?

The key to addressing all these questions is to consider the spatial heterogeneity within the montane zone so that we can reach conclusions for particular habitats rather than generalizing for the entire montane zone of ponderosa pine and Douglas fir forests.

I. OVERVIEW OF DISTURBANCE HISTORY

-- Fire–

A. Evidence: studies of fire-scar dates in Rocky Mtn. N.P., Poudre Canyon, Boulder County generally.

B. Common trends:

1. Native American Period (pre-1859)-- moderately high fire frequencies, especially at lowest elevations; lightning and human-set fires.

2. Euro-American Settlement Period (1859-1916)-- sharp increase in fire frequency; fires set by prospectors and ranchers; increased climatic variation was also favorable to increased fire during mid- and late-1800s. Humans probably increased the frequency of ignition and the frequency of small fires, but during the second half of the 19^th century the increase in years of widespread fire is primarily explained by climatic variation and changes in broad-scale climate drivers (i.e. sea surface temperatures in the Pacific and Atlantic Oceans).

3. Modern Fire Suppression Period (post-1917)-- very low fire frequencies.

-- Logging in the Montane Zone of the Front Range--

A. Native American Period-- very minor logging.

B. Euro-American Settlement Period-- extensive logging for railroads, mines, and towns.

C. Modern Period (post-1917)-- minor logging for fuel and control of insect outbreaks.

-- Major 20^th century insect outbreaks in the Montane zone:

A. Mountain pine beetle: 1920s, 1930s, 1950s, and 1970s.

B. Western spruce budworm: 1940s, 1960s, 1976-1983

C. Douglas fir bark beetle: early 1950s, mid-1980s

All of these insects are native species and experienced epidemics prior to the effects of fire suppression or any other forest management.

II. Changes in disturbance regimes and forest conditions over the past c. 150 years.

Sources of evidence:

Repeat photography;

Tree population age structures describing stand development patterns;

Tree-ring dating of past disturbances

1. Logging impacts

Widespread elimination of large ponderosa pine and Douglas fir; near elimination of old-growth stands.

Creation of dense stands of young trees.

2. Post-fire stand development

A. Mostly recovery to original species composition; there is very little evidence of shifts from dominance by Douglas fir in the 19^th century to ponderosa pine in the 20^th century following burning or logging.

B. At harshest sites, original dominants replaced initially by limber pine (possibly facilitation?)

3. Consequences of changes in fire regimes (consider differences between lower montane versus the mid- to upper montane zone)

A. Changes related to decline in fire frequency in 20th century:

1. Ponderosa pine invasions of grasslands; increased stand density in low elevation, open woodland areas due to the suppression of formerly frequent surface fires. Disturbance by livestock, logging, mining, road construction probably also favored tree establishment in former grasslands. Furthermore, ponderosa pine seedling survival at low elevations is dependent on periodic episodes of greater moisture availability (e.g. higher spring precipitation in association with El Niño events. Overall, however, these tree seedlings would not have survived without suppression of formerly more frequent fires. This has resulted in an accumulation of woody fuels so that sites which formerly could support only surface fires now can support crown fires.

Based on fire history studies, stand reconstruction, and GIS-based modeling of fire regime/habitat associations, we estimate that less than 20% of the ponderosa pine zone of Boulder County fits this model of fire suppression, fuels accumulation, and shift from open woodlands to dense stands. Less than 20% of the ponderosa pine zone had surface fires occurring more frequently than c. 30 years (R. Sherriff and T. Veblen, in progress). It is in this habitat type (for example on much of City of Boulder Open Space land), that ecological restoration through thinning and prescribed surface fires would be appropriate.

2. In the mid- and upper montane zone (above c. 2100 m) tree population age structures do not indicate that large numbers of trees have established since fire suppression in c. 1920. In other words, we do not see the expected invasion of young trees if formerly frequent surface fires had been suppressed. Instead, fire suppression has resulted in a decline in the occurrence of severe fires which implies that today the area of young (e.g. < 80 years old) is probably less than what it would have been in the absence of fire suppression. However, stand age data from Forest Service surveys indicate that young stands are relatively common (probably due to logging in the early 1900s). Furthermore, recent fires in the montane zone have created young stands. So, although the extent of burning at a landscape scale in the 20^th century was much less than in the 19^th century the current forest age structures are not necessarily outside the historic range of variability. This is because the mid- and upper-montane zone historically was characterized by widespread severe fires that varied greatly in their occurrence over centennial time scales. Thus, episodes of widespread fires such as in the second half of the 19^th century have occurred previously and resulted in a high degree of variability in stand ages/structures at a landscape scale. In the upper montane zone, ecological restoration would require prescribed stand-replacing fires because there is abundant evidence that the pre-20th century fire regime was characterized by widespread stand-replacing fires.

Although there is widespread agreement among researchers that severe fires played a major role in shaping the historic landscape in the mid- and upper montane zone, there is uncertainty about the size of those fires. Patch sizes would have been at least several to thousands of hectares in extent. It is not clear if fire suppression has changed fuel conditions to such an extent that future fires would burn significantly larger areas more completely (e.g. with 100% tree mortality) than they did in the past. This is because under exceptionally extreme fire weather, fuel structures play only a minimal role in fire behavior.

B. Changes related to extensive burning of the upper montane zone during the mid- and late- 1800s due to climatic conditions that were more favorable to fire and to an increase in human-set fires after c. 1858.

1. Extensive even-aged stands of uniform age structure. Current abundance of suppressed understory Douglas fir in these post-fire stands. These small Douglas-fir are in stands where they occurred prior to fires too. So, the presence of Douglas-fir should not be regarded as unnatural or atypical.

2. High tree densities in ponderosa pine and Douglas-fir stands. This is the natural consequence of stand development following severe fires in the late 19^th century. This high tree density should not be interpreted as a consequence of fire suppression and conversion from open woodlands to dense stands.

3. Probable decline in fire hazard during early 1900s, followed by increased fire hazard as the even-aged stands become more fire susceptible in the 1980s. However, the effects of stand ages on fire hazard are probably much less than the effects of climatic variation such as the 1998-2002 drought.

4. Decline in insect outbreak (especially budworm) during early 1900s due to a lack of moderately old stands (i.e., over much of the landscape stands were less than 50 years old). Insect outbreaks increased in mid-1900s as a larger percentage of the landscape supported trees older than c. 80 years. However, the existing tree-ring reconstructions of budworm outbreaks indicate no major differences between the late 20^th century and the pre-historic period. Overall, budworm outbreaks appear to be more controlled by climatic variation than by stand ages.