Fig. 3. Frequencies, thresholds, and alternate steady states.

Fig. 3 Note: Records of fire scars (moderate surface fires) from many sites in the southwest United States show the effects of different combinations of historical grazing regimes and modern fire suppression. Most show a reduction in fire scars from the end of the 19th century, largely as a result of intensive grazing. Exceptions strengthen this inference, for example: a) Spanish colonists on traditional Navajos pasturelands since the early 19th century; b) remote area with no grazing, but fire suppression in the 20th century; c) no intensive grazing nor fire suppression in the 20th century. Contemporary statistics show that many sites are now at risk from catastrophic “stand-replacing” fires as dense scrub thickets fuel non-natural fire patterns. After Swetnam et al. (1999).