Fire Climate

Climate, which in this context refers to the antecedent conditions prior to fire, works largely through changing fuels. The most immediate effect of climate is when drought conditions cause the drying of fuels to the point where living foliage combusts almost as readily as dead foliage. Under drought conditions there is commonly an increased incidence of fire in the subsequent fire season (Figure 4). Climate, however, can have longer-term impacts by affecting primary production and the quantity of fuels. This is most evident in forests where herbaceous plants are the primary fuels responsible for fire spread. When very high rainfall years produce luxuriant herb growth, this flush of herbs will eventually die and increase the probability of burning 1 or 2 years later (Figure 4b). Forests with surface fire regimes driven by dead leaves and branches on the forest floor do not exhibit this delayed response to anomalously high rainfall years (Figure 4a).

Historical climate signals are responsible for regional synchrony in fire activity evident in increased fire frequency and fire size during the Medieval Warm Period (1000-650 yr BP) in the western US. The cooler and drier conditions of the Little Ice Age (500-100 yr BP) may be responsible for greater fire frequency compared to the less-frequent fires of the warmer moister conditions of the twentieth century.

A number of ocean-atmosphere patterns (e.g., El Nino southern oscillation (ENSO) or the southern oscillation) that have been discovered in recent decades by climatol-ogists have been implicated in long-term patterns of fire behavior. These patterns are often evident over decades, and suggest some level of periodicity in fire behavior that may be useful in interpreting the past and planning for the future.

Mixed conifer

Ponderosa pine

Mixed conifer

Ponderosa pine

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Fire year (b) Fire year t t

Fire year (b) Fire year

Figure 4 Relationship of fire to climate in two forest types with surface fire regimes driven by (a) dead litter and (b) herbaceous fuels. Climate diagrams plot the Palmer Drought Index on the y-axis (positive numbers indicate moist years and negative numbers drought years). Bars represent the degree of drought and the x-axis the fire year (0) or years prior to or after fire. Horizontal dotted, dashed, and solid lines are 99.9, 99.0, and 95.0 confidence intervals, respectively. From Swetnam TW and Betancourt JL (1998) Mesoscale disturbance and ecological response to decadal climatic variability in the American Southwest. Journal of Climate 11: 3128-3147.

Figure 4 Relationship of fire to climate in two forest types with surface fire regimes driven by (a) dead litter and (b) herbaceous fuels. Climate diagrams plot the Palmer Drought Index on the y-axis (positive numbers indicate moist years and negative numbers drought years). Bars represent the degree of drought and the x-axis the fire year (0) or years prior to or after fire. Horizontal dotted, dashed, and solid lines are 99.9, 99.0, and 95.0 confidence intervals, respectively. From Swetnam TW and Betancourt JL (1998) Mesoscale disturbance and ecological response to decadal climatic variability in the American Southwest. Journal of Climate 11: 3128-3147.

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