The Forest Ecosystem - Mountains and Plains: The Ecology of Wyoming Landscapes - page 216

61

Spring-Summer
Te mperature

100

59

57

0

55

1970 1975 1980 1985 1990 1995 2000

Fig. 12.15. trends in wildfire frequency and average spring-
summer temperature (March through August) in the western
United States since 1970. Fire frequency is the number of fires
greater than 1,000 acres in size per year. temperature and

fire frequency are correlated, and both increased beginning
around 1985. climate models suggest these trends are likely
to continue during the twenty-first century. Adapted from
Westerling et al. (2006).

water would be seen only in years of average or above-
average precipitation, which may become infrequent.
Because the regrowth of high-elevation forests is slow
and such forests are highly valued for other reasons—
wildlife, recreation, and aesthetics—intensive harvest
may not be a socially acceptable strategy for increasing
water supplies. other approaches, such as improving
water conservation measures and augmenting water
storage facilities, may be more effective.
With regard to fires, an upsurge of large fires dur-
ing the past 25 years has already occurred across most
of western north America (fig. 12.15). in addition,
extensive bark beetle outbreaks are now a continental
phenomenon. 62 Several factors may be responsible for
this recent upswing in forest disturbance. one is the
extensive cover of dense, mature conifer forests that
developed during the twentieth century, when fires and
other natural disturbances were relatively infrequent,
and when human-caused disturbances (such as timber
harvest) affected only a small portion of the forested
land. Dense, mature forests generally are more sus-
ceptible to fire and insect outbreaks than are younger
or more open forests. it is tempting to attribute such
problems to inadequate forest management, that is, too
much fire suppression or too little timber harvesting.
However, the increase in fire and bark beetle activity
is almost ubiquitous, occurring in forests with a great
variety of management histories throughout most of
the western and northwestern states and up into Alberta
and British columbia. climate change is the most likely
explanation ; 63 c urrent climate models suggest that con-
ditions favorable for beetles and fire will become more
frequent in the future.

Have Past Fires Increased the Probability of
Beetle Outbreaks?

For many years ecologists thought that fire-injured trees
provided loci for subsequent mountain pine beetle out-
breaks into surrounding unburned forests. the results
of one study in the Greater Yellowstone ecosystem re-
vealed that the number of new adult beetles in the next
generation was greater in trees that sustained moderate
fire injury than in severely injured or uninjured trees. 64
However, moderately injured trees are usually scarce
after fire in lodgepole pine forests, as most trees touched
by fire are severely injured and die. Few new adult
beetles emerge from the severely injured trees because
of various factors, including damage to the inner bark
tissues on which the beetles depend.
Drought or other nonfire stresses are more likely
initiators of mountain pine beetle outbreaks, simply
because they cause healthy trees to be more susceptible
to beetle attack without damaging the food resource
used by the beetle larvae. Also, in contrast to the moun-
tain pine beetle, spruce beetle outbreaks are known
to begin in windthrown timber or large-diameter log-
ging slash—though drought or other conditions that
stress living trees are necessary to sustain a widespread
outbreak. 65

Do Beetle Outbreaks Increase the Probability of Fire?

After a beetle epidemic, forests are mostly reddish-
brown or gray and they appear alarmingly flammable.
Research suggests that this association is not as simple
as it seems. Jeffrey Hicke and his colleagues from the
University of idaho developed a model that character-