The Forest Ecosystem - Mountains and Plains: The Ecology of Wyoming Landscapes - page 210

in the Rocky Mountains have a variety of impressive
mechanisms for conserving nutrients. As noted, micro-
bial organisms, notably bacteria and fungi, sequester
them in their biomass. Also, some of the first plants to
grow back after fire absorb nitrogen and other nutrients
in excess of what they actually need for growth—a phe-
nomenon referred to as luxury consumption. 35 As a result
of microbial immobilization and uptake by recovering
vascular plants, the amount of nutrients lost is less than
might be expected. 36
over a longer time frame, the nitrogen lost to a fire
in a lodgepole pine forest is typically replenished within
40-70 years through local nitrogen fixation and inputs
from rain, snow, and dust . 37 As the forest regrows, nitro-
gen and other nutrients are ever more tightly bound in
the living and dead biomass, both above- and below-
ground. the forest floor, including downed wood,
immobilizes the largest amounts of nitrogen, phospho-
rus, calcium, and magnesium—at least during the first
decade of post-fire stand development. Living biomass
appears to be the second most important factor in nutri-
ent immobilization, especially after 60-80 years. Rates
of nutrient sequestration remain positive even in very
old stands . 38
With regard to insects, the extensive bark beetle out-
breaks now sweeping across the West have important
effects on ecosystem processes (see chapter 11 for bark
beetle natural history and effects on forest structure).
By selectively killing the larger trees in a stand, beetles
reduce total tree growth for several years after the peak
of an outbreak. However, the smaller trees in the stand
often are not killed, and, with fewer large trees compet-
ing for water, nutrients, and light, they grow faster than
before the outbreak. Understory shrubs and herbs also
grow faster, resulting in recovery of total stand pro-
ductivity and carbon uptake and storage within a few
decades or even a few years. 39 L ooking at the effects of
bark beetles and other native insects on ecosystem pro-
cesses over long time frames, some ecologists have sug-
gested that plant-feeding insects actually help maintain
high levels of total plant growth in forests by killing
older, slow-growing trees and freeing resources for young
individuals. this intriguing idea requires testing, along
with other possible effects of insects on ecosystems.
the death of dominant canopy trees during a beetle
outbreak can result in a substantial reduction in total

16 7

6

12

5

4

8

3

2

4

1

10 0 10 1 10 2 10 3

Pine saplings/hectare

10 4 10 5

Fig. 12.9. total aboveground plant growth rate, also known as
aboveground net primary productivity (AnPP), and leaf area
index (LAi), defined as square meters of leaf area per square
meter of soil surface, in relation to tree density (number of
saplings per hectare) in 10-year-old lodgepole pine forests
developing after the 1988 fires in Yellowstone national Park.
Both AnPP and LAi tend to increase with increasing sapling
density until extremely high densities of 40,000 saplings per
acre are reached. LAi in such places is almost as high as in
some mature forests. AnPP and LAi appear to level off or even
decrease in stands of even greater density (not shown here),
but the data do not permit this prediction. See fig. 12.1 for
unit conversion. Adapted from turner et al. (2004).

the surviving aspen root system. Rocky Mountain coni-
fers lack that ability.
All kinds of fire affect nutrient cycling, with more
severe burns causing greater effects. 32 Some nitrogen is
volatilized by the heat, but most persists in the large
amount of unburned soil organic matter. A portion of
other nutrients (like phosphorus) remains in the ash,
which becomes incorporated into the soil within the
first year after the fire if not carried away by wind or
water. With more water moving through the soil profile
as a result of reduced leaf area and transpiration, the
probability of nutrient losses from the soil is enhanced—
with subsequent nutrient gains to streams . 33 After the
1988 Yellowstone fires, streams flowing from several
severely burned watersheds contained elevated levels of
nitrogen and phosphorus for at least 5 years. Similarly,
nitrate concentrations were elevated for at least 5 years
in streams draining heavily burned watersheds after the
2002 Hayman fire in colorado. 34
Although it would seem that nutrient losses asso-
ciated with fire, especially losses of nitrogen, might
impair post-fire recovery, conifer forest ecosystems