Mountain pine beetle outbreaks in the Rocky Mountains: Regulators of primary productivity?

TLDR: The results do not support the hypothesis that mountain pine beetles function as cybernetic regulators of primary productivity in ecosystems of lodgepole pine forest through their selective killing of dominant trees and the subsequent redistribution of resources, and the beetle-pine system shows great resilience.

Abstract: We consider the hypothesis that mountain pine beetles function as cybernetic regulators of primary productivity in ecosystems of lodgepole pine forest through their selective killing of dominant trees and the subsequent redistribution of resources. Following a recent major beetle outbreak in Yellowstone and Grand Teton national parks, surviving trees did grow significantly faster (P < .1); wood production was redistributed among canopy, subcanopy, and understory trees; and annual wood production per hectare usually returned to pre-attack levels or exceeded them within 10-15 yr. However, reconstructions of annual wood production over the last 70-80 yr indicate that the beetle outbreak did not reduce the variation in productivity; rather, the beetles introduced more variation than would have existed in their absence. Hence, our results do not support the hypothesis that the beetles function as cybernetic regulators (in the strict sense). Nevertheless, the beetle-pine system that we studied shows great resil...

Figures

FIG. 3.-Reconstructed history of mean annual bole volume increment in a beetle-affected stand with a well-developed understory of subalpine fir.

FIG. 2.-Reconstructed history of mean annual bole volume increment in a beetle-affected stand with little understory.

FIG. 1. -Mean annual bole volume increment in four stands during the 5 yr before a beetle outbreak (B), 1-5 yr after the outbreak (5), 6- 10 yr after (1 0), 11I- 14 yr after (1 4; stand 3 only), 11-15 yr after (15; stands 1, 2, 4), and 15-20 yr after (20; stand 1 only). Mean annual increments during comparable 5-yr periods are also shown for the two control stands. The open portions of the columns represent understory production; the patterned portions represent canopy and subcanopy production.

TABLE 1

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Quinney Natural Resources Research Library,
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1986
Mountain Pine Beetle Outbreaks in the Rocky Mountains: Mountain Pine Beetle Outbreaks in the Rocky Mountains:
Regulators of Primary Productivity? Regulators of Primary Productivity?
W. H. Romme
D. H. Knight
J. B. Yavitt
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Romme, W.H., D.H. Knight, and J.B. Yavitt. 1986. Mountain pine beetle outbreaks in the Rocky Mountains:
Regulators of primary productivity? Am. Nat. 127(4):484-494
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1986
Mountain pine beetle outbreaks in the Rocky
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W. H. Romme
D. H. Knight
J. B. Yavi#
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productivity? Am. Nat. 127(4):484-494

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Mountain Pine Beetle Outbreaks in the Rocky Mountains: Regulators of Primary Productivity?
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Source:
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Vol. 127, No. 4 (Apr., 1986), pp. 484-494
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Vol. 127,
No. 4 The American Naturalist
April 1986
MOUNTAIN
PINE
BEETLE
OUTBREAKS
IN
THE
ROCKY MOUNTAINS:
REGULATORS OF
PRIMARY
PRODUCTIVITY?
WILLIAM H.
ROMME,
DENNIS H.
KNIGHT,*
and JOSEPH
B.
YAVITT*t
Department of
Biology,
Fort
Lewis
College, Durango, Colorado
81301; *Department
of
Botany,
University
of
Wyoming, Laramie,
Wyoming
82071
Submitted November
7, 1983;
Revised
May 20,
1985; Accepted August
26,
1985
Major
outbreaks of the
mountain pine beetle
(Dendroctonus ponderosae
Hopk.)
have killed
millions of trees over
thousands of
square kilometers in
the central and
northern
Rocky
Mountains
during
recent
years.
This native bark beetle
feeds
on
the
phloem of
various species of
Pinus,
introducing several species
of sapwood
fungi
in the
process (Amman
1978; Mitton and
Sturgeon 1982). The
beetles, which
tend
to
selectively attack larger
trees having
thicker phloem, may kill
50% or more
of the
canopy
density and basal area over a
period
of
a few
years.
Understory
and
small
canopy
trees
usually are not
killed,
apparently because
they
lack
an ade-
quate
food
supply
for the
beetles
(Roe
and Amman
1970;
Amman
and Baker
1972;
Cole and
Amman 1980). Although the
population
dynamics of
the beetle
and
its
effects
on
stand
structure have
been studied, little
is
known about the effects of a
beetle
outbreak
on ecosystem processes such as
productivity,
cycling,
and suc-
cession.
Our
study was designed
to
examine
the
effects of beetle outbreaks on
primary
productivity
in
forests
dominated by
lodgepole pine (Pinus
contorta var.
latifolia
Engelm.)
in northwestern
Wyoming.
Although it is
commonly
assumed that
phytophagous-insect
outbreaks cause a
reduction in
primary
productivity, such
reductions may
be of
short duration.
Mattson
and
Addy (1975) presented
evidence
suggesting
that
certain insects
may
stimulate
primary productivity
in
forest ecosystems
by
selectively killing
less
productive plants or
plant parts,
thus enhancing
light, water,
and nutrient
avail-
ability for survivors that had
been suppressed
by larger
individuals.
In
this
manner, the
insects help to
maintain a more even
distribution
of energy
flow
through the
various ecosystem
components, plant or
animal,
and
probably con-
tribute to
maintaining
a
near-maximum level of
primary
productivity in
the
system
(Berryman
1981). Similarly,
McNaughton
and
Coughenour
(1981)
used
the
in-
teraction
between
Dendroctonus brevicomis LeConte
and
Pinus
ponderosa
Laws.
as an
example
illustrating the
cybernetic nature
of ecosystems.
In one aspect
of
our
investigation,
we
consider
the
hypothesis
that the mountain
pine
beetle
t
Present
address:
Department of
Biology, West Virginia
University,
Morgantown, West Virginia
26506.
Am. Nat.
1986. Vol.
127,
pp. 484-494.
?
1986
by
The
University
of Chicago.
0003-0147/86/2704-0006$02.00.
All rights
reserved.
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BARK
BEETLES AND
FOREST PRIMARY
PRODUCTIVITY
485
functions as a
cybernetic regulator of
primary
productivity in lodgepole
pine
forests.
Ecologists
sometimes use the term
"regulation"
loosely, referring to any
mech-
anism that changes or
influences an
organism or process in
any way. In the
precise
definition of
cybernetic theory, a
regulator responds to the
state of a process
or its
output and, through
some feedback
mechanism, reduces the
variation in the
state
or
output of that
process (Ashby 1963;
Mattson and
Addy 1975). Thus, to
con-
clude
that bark beetles
function as
cybernetic regulators
(in the strict
sense) of
primary productivity
we must
demonstrate (1) that the
insects can
detect, and
their
behavior is
modified by, the
current level of
primary productivity;
and (2)
that productivity is
more constant or
stable in the
presence of the beetles
than it is
or
would be in their
absence. The first
requirement appears
to
have
been
met at
least partially by
recent work
showing that mountain
pine beetles
selectively
attack trees of lower
physiological
vigor when the insects
are at endemic
popula-
tion
levels and in the
early stages of
outbreaks (Sartwell
1971; Berryman
1976,
1982; Larsson et al.
1983; Waring and
Pitman 1983,
1985; Stuart 1984) and that
outbreaks
usually
occur
in
mature or overmature stands
that presumably
have
passed
their period
of peak
productivity (Mitchell
et
al. 1983; Shrimpton and
Thomson 1983;
Waring and Pitman
1983).
The
observation that beetles
may
kill
even
vigorous,
fast-growing trees at the
height
of an
outbreak (Berryman
1976,
1982; Amman 1978)
does not
detract, in our opinion,
from the feedback or
detection-response
requirement. (But
see Cole et al. 1985
for the argument that
tree
diameter, not
vigor,
is the
major
variable
controlling
mountain
pine
beetle
dynamics.)
The
second
requirement,
that of less-variable
productivity,
is more
troublesome,
however,
and it is
the focus of our
paper.
We ask whether forest
productivity is more
stable in the
presence of periodic
outbreaks than it would be
without them.
The
situation that
we
studied differs in two
important
respects
from most of
the
examples
of insect
regulation proposed
by
Mattson and
Addy (1975). First,
they
primarily examined
light grazing by
insects, rather than the substantial
mortality
associated
with
a
beetle outbreak.
They
did
suggest,
however,
that
regulation
may
occur even in the
more severe outbreak conditions.
Second,
unlike the
defoliating
insects
emphasized
by Mattson
and
Addy,
the mountain
pine
beetle does not
feed
directly on
the
leaves, and because the beetles exist
inside the
bark, they
do not
immediately
accelerate nutrient cycling
by creating
nutrient-rich
litter
fall
(insect
bodies, excrement,
etc.).
Nevertheless, successfully
infested
trees are
usually
dead
within
a
year.
Resource
distribution occurs
as a
result
of
a
leaf-fall
episode
lasting
a few
months
and
the
toppling of dead trees over
a
period
of more
than
20
yr.
METHODS
Our
study
focused on a
series of
10
stands
affected
by
a
major
beetle
outbreak
from
1-20
yr ago. Each
stand
originated in a destructive fire
80-125 yr
earlier and
was
dominated by an
even-aged
lodgepole pine canopy.
All stands were
located
within
80
km
of each
other in
southwestern Yellowstone
National Park,
northern
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Frequently asked questions

Q1. What are the patterned portions of the columns?

The open portions of the columns represent understory production; the patterned portions represent canopy and subcanopy production. 

Q2. What is the effect of the beetle outbreak on the tree?

Waring and Pitman (1985) concluded from a thinning and fertilization experiment in Oregon that tree growth improved after a beetle outbreak mainly because of increased light availability. 

Q3. What is the effect of the beetle on the forest?

In turn, the biomass and productivity of herbivores, decomposers, and other organisms associated with each of the subdominant strata may be increased by the beetle-induced changes. 

Q4. What is the effect of the beetle outbreak on the forest?

It appears that before an outbreak, biomass and productivity are overwhelmingly concentrated in the dominant canopy stratum, but afterward they are distributed somewhat more evenly among the three strata. 

Q5. How did the authors estimate the bole volume of each tree?

Using radius measurements from their increment cores and field measurements of tree height, the authors estimated the bole volume of each sampled tree 5 yr before the beetle outbreak and at 5-yr intervals after the outbreak. 

Q6. How long after the outbreak did the wood production decrease?

Annual wood production per hectare decreased during the first 5 yr after the outbreak, returning to its former level after only 6-10 yr. 

Q7. How did the authors measure the understory and canopy of the beetle-affected stands?

To do this the authors sampled understory and canopy tree density before and after the beetle outbreak in four stands, using belt transects that covered about 30% of the stand area. 

Q8. What is the first requirement of the paper?

The first requirement appears to have been met at least partially by recent work showing that mountain pine beetles selectively attack trees of lower physiological vigor when the insects are at endemic population levels and in the early stages of outbreaks (Sartwell 1971; Berryman 1976, 1982; Larsson et al. 

Q9. What are the results of the two simulation studies?

Their results resemble those of Mattson and Addy (1975) and Moore and Hatch (1981), who showed through simulation studies that outbreaks of spruce budworm and Douglas fir tussock moth, respectively, led to a redistribution of wood production from canopy to understory or from host to nonhost species. 

Q10. What is the main variable controlling mountain pine beetle dynamics?

(But see Cole et al. 1985 for the argument that tree diameter, not vigor, is the major variable controlling mountain pine beetle dynamics.) 

Q11. What is the effect of beetle outbreaks on the forest?

From this perspective, the effects of the beetle outbreaks are similar to the effects of other natural disturbances and predators, which tend to reduce the abundance of the more competitive species in a community (Paine 1974; Connell 1978; Huston 1979).