logged forest would require a windstorm of over 200 km hr−1 for a fire to spread from crown to crown across the forest (which might be strong enough to topple the trees). With heavy cattle grazing and fire suppression for eight decades, the grass meadow matrix was replaced by a high density of closely packed trees (white arrow points to the same rock;
Source: photo by Andrew Sanchez‐Meador),
and a wind of only 50 km hr−1 could spread a crown fire. For a related, spatially explicit example, see Figure 11.14.
(Source: D.W. Huffman, J.D. Bakker, D.M. Bell, and M.M. Moore, unpublished).
Fires Depend on Temperature, Water, Winds
Most forests on Earth have been shaped by fires in the past, and the behavior and impacts of the flames depend on the fuel structure of the forest, on the water content of the fuels, the temperature and humidity of the air, and the speed of the wind. No single one of these factors would be very useful in understanding fires. Many forests experience fires multiple times within the lifespan of dominant trees, including ponderosa pine forests of western North America. Repeated fires lead to ecosystem structures with low densities of trees among small meadows (Figure 2.21). During hot, dry periods, fires may burn readily through the meadows and around the bases of the trees, and a few trees may even have fires reach into their crowns. Strong winds enable fires to race through canopies, but when canopies are very patchy, the necessary wind speeds would be so high that trees would topple over before burning. If fires are absent from such systems for the span of a human lifetime, the forest structure and fuels change so much that surface fires may be less likely, and only moderate wind speeds would be needed to fan flames from tree crown to tree crown. The details of interactions of forest structure, winds, temperature, and humidity would be different for other forests, but the importance of understanding these interactions would be important for all types of forest fires.
Droughts Affect Trees, Beetles, Forest Structure and Fire Intensity
Insects have coevolved with tree species, and favorable environmental conditions can change insect populations from low background levels to incredibly high numbers that overwhelm the ability of trees (and other vegetation) to cope. Most conifer species are hosts to one or more specialized species of bark beetles, and fungal spores spread by the beetles can kill trees (Chapter 10). The success of bark beetles may be higher in forests with trees that have experienced a strong drought, or winds that toppled large numbers of trees (where dying trees succumb to the beetles). These changes in forest structure result from interactions of drought, wind, and insects also change the potential of fire to affect the forest, with legacies that last for decades. In the Sierra Nevada mountains of California, droughts increase stress in pine trees, increasing the success of beetles. The beetles' fungal symbiont plugs the trees' water‐conducting sapwood, and needles dry and die. Dry needles ignite more easily and release more energy when burned than green needles, and forests with large numbers of recently killed pines have a high potential for running crown fires (Figure 2.22). The dry needles fall off within a few years, and the reduction in crown fuels reduces the potential for crown fires. After the woody material of dead trees falls to the ground, the high fuel loads can support high intensity surface fires.
The actual “story” of the environmental drought effects on the trees and forests would be only partially about water supply and tree physiology, and more about beetles, fires, and all the interacting legacies that shape forest changes over time. Most forests do not burn soon after beetle outbreaks, so the variety of post‐beetle forests that develop across landscapes can be quite broad.
FIGURE 2.22 Severe droughts can foster outbreaks of bark beetle populations, with legacies that last for decades or centuries. If a fire occurs shortly after the trees are killed, the dry needles can support a fire that leaps from crown to crown (upper), killing many (or all) of the surviving trees. A few years later, the dead needles have mostly fallen, and the risks of a spreading crown fire declines. Dead tree stems fall after a decade or so, just as small trees and shrubs are increasing in biomass. The accumulation of fuels near the ground can support surface fires that burn at high intensities.
Source: Stephens et al. 2018/Oxford University press.
Weather Events Can Matter More than Averages
The two examples above underscore the importance of weather extremes in shaping forests. The year‐to‐year ecophysiology of trees in response to daily and seasonal weather is important (see Chapter 4). However, the longer‐term changes in forests often result from big events that may, or may not, happen in any given year. And once a major event has happened, the responses of vegetation and animals can set up patterns in forest composition and structure that continue to shape forests in ways that diverge from trends that would have occurred if the major event had not.
Ecological Afterthoughts
The major tree species of the Rocky Mountains in the USA can be plotted in two‐dimensional space of average annual temperature, and average annual precipitation (Figure 2.23). The points represent the environment where each species currently shows its highest frequency of occurrence, and the oval clouds represent the full range of species occurrences (based on 9500 plots across the region). The overlap of ranges makes it hard to see separate species, but it paints a picture of how similar (but not identical) the distributions of species may be. The environment where each species has its peak of occurrence is not in the middle of the species' oval (lower graph). Plots like these represent how two features covary, but not whether one or both factors actually drive the pattern (correlation should not be confused with strong evidence of causation). Do the species ranges likely result from direct effects of temperature and precipitation? If not, what other factors might be important? The ranges for the species could be coupled with climate simulation models, with the output showing the geographic locations where each species might be expected to be found. What limitations would be important in how well such stories would actually map onto real landscapes? How would the factors on such a list of limitations interact, and what would the implications be for the interactions?
FIGURE 2.23 The major tree species in the Rocky Mountains, USA can be plotted in relation to where they are most frequent on axes of temperature and precipitation (upper left, based on 9500 locations). The full range of occurrence can be plotted as overlapping clouds (which are difficult to decipher for any single species in the upper‐right graph). The points of maximum frequency for each species does not fall in the middle of each species' range (lower graph;
Source: Data from Martin and Canham 2020).
