North Central Washington

Problems with the determination of fire regime condition class: Shrub-steppe ecosystems

by George Wooten
Twisp, Washington

The concept of fire regime is that a given area is normally subject to periodic wildfires of a given severity to the dominant overstory. The normal case refers to the average periodicity in North America prior settlement ca. 100 years ago. The concept of fire regime has been extended to allow a "condition class" rating, which represents the degree of divergence from the "normal" pattern.

Determination of fire regime condition class (frcc) is predicated on five criteria described below, and discussed for the case of shrub-steppe lands.

1. Knowledge of normal conditions, or ability to reference fire periodicity to a given time period.

a. Presettlement fire regimes have most frequently been determined from fire scars on trees. Although a number of good research studies have been published on fire regimes, the conclusions are typically limited to fires in the recent past.

For non-forested areas, fire frequencies are typically assumed to be similar to that of forested areas nearby. This assumption has been hotly debated, particularly by researchers of shrub-steppe ecosystems.

b. Welch and Criddle (2003) debunk the myth that mountain big sagebrush evolved in an environment with a mean fire interval of 20 to 30 years (e.g., Winward 1984). The authors state, "Of all the axioms we have challenged in this paper, none is more speculative, that is not based on scientific investigation, than this one. In an often cited article by Winward (1991, p. 4) he, in reference to the big sagebrush ecosystem, states: 'These ecosystems, which have developed with an historical 10-40 year fire interval, were dependent on this periodic removal or thinning of sagebrush crowns to maintain their balanced understories.' "

Wright and Bailey (1982, p.159) suggested a fire interval of 50 years based on the vigorous response of horsebrush (Tetradymia canescens) to fire in eastern Idaho.

Welch and Criddle summarize, "Thus it appears that an estimated fire interval of 20 to 30 years for mountain big sagebrush is too low and that the natural or normal fire interval is much longer, perhaps 50 years or more."

c. The choice of the time period to represent historic fire regimes is problematic, but is typically assigned to the year 1900, which is a compromise of many factors.

c.1. One factor for choosing year 1900 is climate. The principle of determining condition class is based on the general assumption that since European settlement of the Americas, we have had a magnitude of effects on fire regimes that were relatively unchanged for many years prior to that. This assumption is dependent on having a stable climate and vegetative cover since the historic period. However, this assumption is unlikely to hold over thousands of years, although it appears to be valid for periods of perhaps several hundred years.

c.2. Another factor for choosing the year 1900 for the historic period is that it is recent enough that the scarred trees preserving the record have not all rotted out.

c.3. Another factor for choosing the year 1900 for the historic period is that a few written narratives exist to aid in the description of fire behavior and fuel conditions.

c.4. Another factor for choosing the year 1900 for the historic period is that this year has also been frequently assigned as the baseline year for several other changing ecological systems: noxious weeds and forest stand condition.

c.5. Thus the choice of the year 1900 has serious drawbacks. This was a period of rapid change, in which fire regimes had already been altered, most likely significantly. Fur trappers from Hudson's Bay Company and the Northwest Company had already depleted beaver populations and brought smallpox to the native peoples by the mid-1800s. It is likely that the disruption of native life and relocation to reservations played a significant role in the of anthropomorphic use of fire well before the year 1900. It is likely, although difficult to prove, that the loss of an estimated 90% of beaver dam habitats in North America also changed fire behavior.

2. The intervals between fires must be regular enough to substantiate the assigned frequencies.

3. There should be an independent a means of assigning and determining severity for different types of wildfire.

4. Data for making accurate determination of current conditions that would result in a given frequency and severity of wildfire.

5. Ability to map current and historic fire regimes over large areas without loss of resolution critical for determining fire regimes and condition classes.

References