Region Specific Findings Concerning Climate Variability Historical Climatic Trends and Fire Regimes

Two studies conducted by Balling et al. (1992a, b) and focusing directly on the GYE, concluded that trends toward both warming and increased drought have already occurred in this region. Through examination of weather station data, a mean annual increase in summer temperatures of 0.87°C over the last century was detected (Balling et al. 1992a, 1992b). In addition, employing the Palmer Drought Severity Index (PDSI), Balling et al. (1992a,b) were able to correlate years of large fires with periods of severe to extreme drought, confirming the relationship between assumed fire-related climate variables and on the ground burns. When the index was computed seasonally for the GYE, statistically significant trends toward increasing drought were discovered for both the summer fire season and the antecedent season spanning the period from January to June of each year. This trend occurred despite concurrent increases in summer precipitation (Balling et al. 1992a, 1992b). Such a scenario may also arise when precipitation events are concentrated into fewer, more intense bouts, with long stretches of dry weather in between. An additional century-long study employing the PDSI revealed the five-state area including Montana, North Dakota, Colorado, Nebraska, and Wyoming had exhibited the largest nationwide trend toward increased aridity. Of these states, Wyoming displayed the most significant trend (Balling 1996). This finding is consistent with climate change predictions for warming and drying of mid-latitude continental interiors (Overpeck et al. 1990, Rind et al. 1990).

The PDSI is a measure developed by the National Weather Service for estimating drought (Rind et al. 1990). The index is computed monthly for a particular location, and incorporates temperature, potential evapotranspiration, and precipitation values to determine relative moisture levels in relation to historical means. Values predominantly fall within the range from 6.0 to -6.0, with values of -4.0 corresponding to an extreme drought, and values of 4.0 marking an unusually wet year. An examination of drought trends conducted as part of this study for the Yellowstone National Park area also revealed a tendency toward increased drought in recent years. This trend is illustrated from 1895 through 2000 for the months of January and July (Figs. 8.7a and 8.7b). By establishing thresholds at 2 and -2, especially in January, it becomes evident that at least from the mid-1970s, and possibly earlier, there have been fewer very wet years, and more years of moderate to extreme drought. In July, with the exception of 1997, a wet year, a similar trend is evident. A longer period of observation and PDSI values computed for additional weather stations after the 1970s might confirm or reject this trend.

Paleoclimatic investigations lend additional evidence to forecasts of increased wildfire hazard with rising temperatures. The link g Q

January Values of the Palmer Drought Index for Yellowstone National Park

1970 * 1990

July Values of the Palmer Drought Index for Yellowstone National Park








Figure 8.7. (A) January and (B) July values of the Palmer Drought Severity Index for the years 1895 to 1996 for Yellowstone National Park.

between climate and fire cycles is supported by stratigraphic methods examining the abundance of charcoal particles in varved lake sediment layers over a single or multiple drainages, and den-drochronological studies of fire-scarred trees that allow for approximate regional fire history reconstruction (Patterson and Backman 1988, Clark 1990). In their investigation of the lake sediment record from several small lakes in Yellowstone National Park, and the West Thumb portion of Lake Yellowstone, Millspaugh and Whitlock (1995) discovered intervals of large fires mimicked recorded climate fluctuations (Millspaugh and Whitlock 1995). The findings of Millspaugh and Whitlock (1995) correspond well to those determined from dendrochronological investigations conducted by Romme and Despain for the same time period (Romme and Despain 1989b).

Additional and unpredictable hazards can arise in the GYE through a variety of fire-mediated mechanisms that may be unpredictable. One such potential consequence considers an increase in blister rust susceptibility for the region through a proliferation of Ribes bushes in recently burned areas. Post-fire succession can favor the establishment of herbaceous cover and species that reproduce vegetatively from root systems unaffected by fire (Turner et al. 1997). Ribes seeds can remain viable for up to 200 years in cool moist duff until liberated by disturbance. Ribes lacustre, capable of establishing on hillsides in addition to valley bottoms, is able to reproduce vegetatively (Hagle et al. 1989). In the wake of disturbance, Ribes population presence generally peaks within 3 to 4 years. In the event that this limited period of increased Ribes population presence corresponds to a period of climatic conditions especially conducive to blister rust spread, large increases in infection levels of whitebark pine could result. Ribes presence in the GYE may currently be near historical highs as a result of the 1988 fires. More research is needed to examine the population-level changes in Ribes in relationship to whitebark pine and climate change.

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