Yellowstone National Park, established in 1872, is the oldest national park in the world. In 1988, there was a massive fire that made headline news and stimulated a good deal of ecological interest. The Turner et al. (2003) Frontiers in Ecology and the Environment article focuses on spatial heterogeneity resulting from fires like this one and application of lessons learned to fire management generally.
This article is a synthesis of the research on vegetation and ecosystem processes over 15 years in the Yellowstone National Park (YNP). The authors of the article begin by contrasting low intensity understory fires (intervals of years to decades, easily suppressed, resulting in an open forest) and high intensity, stand-replacing fires (kills most of the canopy, long intervals from 50 to hundreds of years). According to the authors, stand-replacing fires characterize boreal forests such as those in the YNP in the northern Rockies. These rarer extensive fires occur under severe drought conditions and are little influenced by variations in fuel and therefore by fire suppression policies. As described below, the 1988 fire was a result of severe drought and high winds. Turner et al. clearly state that weather-related factors rather than fuel availability were responsible for this fire. This is important because of controversies around the “let it burn” policies.
The authors emphasize the heterogeneity of the burned landscape created by the 1988 fire and rapid plant establishment post-fire. They describe (p. 352-353) how the fires created a mosaic of burned and unburned patches and the great increases in plant cover four years after the fire. They were surprised to find that the most of the viable seeds came from plants that survived the fire and not from nearby, unburned sites. Several years post-fire, plant communities were similar in composition in burned and unburned areas.
The Figures section of this Issue includes two of the figures found in Turner et al. 2003 Frontiers (figure 3 and figure 5). Figure 3 concerns effect of patch size and fire severity on post-fire plant cover and species richness. The figure shows that small patches have more forbs, grasses, and total cover compared to large patches. In addition, cover of forbs, grasses, and shrubs is lower in more severely burned locations. However, Turner et al. state that “the effects of environmental composition on species richness and community composition are becoming more pronounced (figure 3), and post-fire communities are similar in composition to nearby forests that did not burn. The influence of the abiotic template on vegetation is, therefore, becoming more evident as succession proceeds.”
Figure 5 focuses on lodgepole pine. Adult lodgepoles do not survive fire and regeneration depends on release of seeds from serotinous cones at high temperatures (see background information below). Seedlings grow well in the bright, burnt forest, resulting in dense, even-aged stands. Figure 5 shows influences of fire severity, patch size, and geographic location on densities of pine seedlings post-fire.
Nearly all plant communities in Yellowstone have experienced fire, but the response of the vegetation differs, resulting in a complicated pattern of responses and effects. For example, Douglas-fir trees have thick, insulating bark; mature Douglas-firs are not often killed by fire. In contrast, other trees (lodgepole pine, Engelmann pine, subalpine fir) have thin bark but have other adaptations to fire. Some lodgepole pines have serotinous cones “glued” tight with resin that only melts at high temperatures, releasing seeds onto the well-burned landscape. Engelmann pine and subalpine fir grow in cooler, wetter habitats where fire is less likely; therefore they “escape” fire. Aspen has thin bark and the above-ground trees burn fairly readily; belowground is a network of cloned roots which sprout after fires. In contrast to the much more frequent intervals of historic fires in shrub grasslands (about 25 years), “natural” intervals for lodegpole pine forests in Yellowstone may be 300 years or more (Houston 1973).
The 1988 Yellowstone fire attracted a lot of attention in part because it was so large – about a third of the park (roughly 800,000 acres) burned. Over 9000 firefighters from all over the country tried to suppress the fire at a cost of 140 million dollars; in the end rainfall and snow stopped the fires. In addition to its size, the economic cost and damage to property near the park resulted in heated debates about the National Park Service’s “let burn” policies.
There are a number of websites with detailed accounts of the fire (e.g., http://www.x98ruhf.net/yellowstone/fire.htm). When lightening started the fire in late June officials assumed that summer rains would eventually extinguish it; however, that summer turned out to be historically dry and windy (data are in above website).
Although stories appeared in the popular press about the “death” of Yellowstone, the forest regenerated faster than expected because of the patchiness of the burn. "Even close to the center of the largest burn, there were areas that were relatively unburned, that served as sources of propagules," says William Hargove, a research associate at Oak Ridge National Laboratory who also studied the fire (http://whyfiles.org/018forest_fire/main2.html). Wildflowers and shrubs survived because soils were affected only a few centimeters deep on average. By 1990, wildflowers were abundant. Finally, surprisingly few large animals such as elk and bison were impacted with rates of mortality at most about 10% above expected by winter weather (ibid).