United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

Research Paper 

RMRS-RP-54

August 2005

Mountain Pine Beetle-Caused Tree 

Mortality in Partially Cut Plots 

Surrounded by Unmanaged Stands

J.M. Schmid and S.A. Mata
Rocky Mountain Research Station

Natural Resources Research Center

2150 Centre Avenue, Building A

Fort Collins, CO 80526

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Schmid, J.M.; Mata, S.A. 2005 Mountain pine beetle-caused tree mortality in partially cut plots 

surrounded by unmanaged stands. Res. Pap. RMRS-RP-54. Fort Collins, CO: U.S. Department 

of Agriculture, Forest Service, Rocky Mountain Research Station. 11 p.

Abstract—Mountain pine beetle activity was monitored in one set of 2.5 acre plots in the southern portion of the Black Hills National Forest over a 17-year period. Beetles attacked 77 percent of the trees 

in the uncut control, 48 percent of the trees in the growing stock level (GSL) 100/110, 53 percent of the 

trees in the GSL 80/90, and 9 percent of the trees in the GSL 60/70. The percentages of MPB-attacked 

trees in each 1 in diameter class were lowest in the GSL 60/70, intermediate in the GSL 80/90 and GSL 

100/110, and highest in the uncut control. Live basal area was significantly reduced in the GSL 80/90, 

GSL 100/110, and control but not in the GSL 60/70. Partial cutting to reduce beetle-caused mortality may 

be ineffective for partially cut parcels of <10 acres if the partially cut stands are surrounded by unmanaged susceptible stands. To increase the effectiveness of partial cutting, stands should be managed on 

a landscape basis. A 100-ft-wide strip with stand density of ≤GSL 70 between unmanaged and managed stands may be sufficient to limit the spread of beetle-caused mortality from unmanaged stands to 

adjacent partially cut stands.

Keywords: Mountain pine beetle, ponderosa pine, stand management

Acknowledgments

K.K. Allen, D.M. Hardesty, D.F. Long, J. Negron, and J. Popp helped monitor tree conditions on the plots. 

Critical but helpful reviews of the initial draft of this manuscript were provided by R.R. Kessler and L. 

Hebertson.
USDA Forest Service RMRS-RP-54. 2005 1

Introduction

The mountain pine beetle (Dendroctonus ponderosae Hopkins) causes high levels of tree mortality in 

ponderosa pine (Pinus ponderosa Lawson) and lodgepole pine (Pinus contorta Douglas) stands. Epidemics 

of the mountain pine beetle (MPB) have caused extensive ponderosa pine (PP) mortality in the Black Hills 

during most decades in the last century (see Thompson 

1975; Pasek and Schaupp 1992; Allen and McMillin 

2001; and Johnson and Long 2003). Since 1996, forest-wide MPB-caused tree mortality increased from 

about 1500 trees in 1996 to an excess of 300,000 trees 

per year in 2001 (Johnson and others 2001).

In 1984, the Rocky Mountain Forest & Range 

Experiment Station of the U.S. Forest Service began 

a study to determine the relationship between stand 

density and MPB-caused tree mortality in both PP and 

lodgepole pine (LP) stands. From 1985 to 1992, 10 

sets of 2.5 acre plots were established in PP stands at 

various locations on the Black Hills National Forest 

(BHNF) in South Dakota. Each set of plots was usually 

composed of four 2.5 acre plots, although one location 

had only three plots and one location had five plots. In 

the four plot set, three of the plots were partially cut 

to various growing stock levels (GSL) while the fourth 

plot was left uncut to serve as the control.

At the time sets of plots were being installed on the 

BHNF, MPB populations were epidemic in 1987 in 

Bear Basin just north of Bear Mountain in the southern Black Hills. Two sets of plots were installed in the 

northern and eastern portions of the infested area; one 

set in 1989 and one set in 1991. As MPB populations 

and, thus, the infested area increased in subsequent 

years, trees in the plots were increasingly exposed to 

MPB attack. Unfortunately, silvicultural activities by 

the BHNF in 1992 and unusually cold temperatures 

in October/November 1991 (see Schmid and others 1993) drastically reduced MPB populations, and 

thereby reduced potential information yield. However, 

preliminary results from the set of plots established in 

1989 suggested the critical threshold for MPB infestation of unmanaged stands should be lowered from 

basal area 150 ft2/acre to basal area 120 ft2/acre (see 

Schmid and Mata 1992).

In 1997, MPB populations began to increase 

throughout the Black Hills and particularly in the vicinity of Bear Mountain in the southern Black Hills 

(McMillin and Allen 1999). Simultaneously, the number of MPB-infested trees began to increase on a set of 

plots located southwest of Bear Mountain (hereafter 

called the Bear Mountain I plots) that were surrounded 

by unmanaged stands. These plots were not exposed 

to the MPB populations associated with the previous 

MPB epidemic in Bear Basin north of Bear Mountain. 

This report summarizes the MPB activity to date on 

that set of plots in the southern Black Hills and discusses the results in relation to PP management.

Methods

In June 1986, a set of four 2.5 acre growing stock 

level (GSL) plots were installed on the west side of the 

Bear Mountain Lookout road about 0.5 mile southwest 

of the Lookout. The Bear Mountain Lookout is on the 

BHNF about 12 miles northwest of Custer, SD. Three 

of the plots were partially cut to GSLs of 60, 80, and 

100 in June-July 1987 while the fourth plot was left 

uncut to serve as the control. Each of the partially cut 

plots bordered on the control; the GSL 80 on the south 

side of the control, the GSL 100 on the west side, and 

the GSL 60 on the north side. The plots were generally surrounded by unmanaged PP stands with tree 

densities and diameter classes the same as within the 

plots when the plots were being installed. In 2004, the 

BHNF conducted a sanitation/salvage timber sale in 

the area surrounding the plots.

Each 2.5 acre plot was subdivided into two parts: 

a central inventory plot (CIP) equal to 1.25 acres and 

the buffer strips surrounding it which also equaled 

Mountain Pine Beetle-Caused Tree Mortality in Partially Cut Plots 

Surrounded by Unmanaged Stands

J.M. Schmid1 and S.A. Mata

1

 Retired Entomologists, Rocky Mountain Research Station, Fort 

Collins, Colorado.
2 USDA Forest Service RMRS-RP-54. 2005

1.25 acres. When the appropriate plot size for this 

study was being discussed, a 2.5 acre plot was considered as possibly too small to evaluate the effect of 

partial cutting because epidemic MPB populations 

might inundate a plot and prevent detection of treatment effect. While this disadvantage was recognized, 

a 2.5 acre plot size was chosen because it was more 

conducive for plot installation and long-term record 

keeping. To address the potential problem of overwhelming MPB populations, the central one-half of 

the plot (CIP) was designated for record keeping. The 

CIP was thus surrounded by strips that would “buffer” the influence of MPB infestation from adjacent 

plots of different stocking levels or uncut stands that 

bordered the plots.

After the plot boundaries were delineated, the diameter at breast height (DBH) of each tree within the 

CIP was measured and recorded. Other characteristics 

such as forked boles, presence of diseases, and scars 

were also recorded. Using the tree diameters, the GSL 

of each plot was computed. Following computation of 

GSLs within all of the CIPs, the CIPs within the three 

plots chosen for cutting were marked to GSLs of 60, 

80, or 100. We tried to hold the GSL of each partially 

cut plot within +1 of the designated level (i.e., a GSL 

80 stand would be between 79 and 81).

Leave trees were selected on the basis of DBH, 

spacing, tree form, crown development, and visually apparent good health. Tree selection emphasized 

leaving the best and largest PP as evenly spaced as 

possible. Although tree selection tended to favor trees 

with larger diameters and discriminate against trees 

with smaller diameters, not all of the largest trees were 

retained nor were all of the smallest trees marked to 

be cut. Metal tags were placed at DBH on all leave 

trees to facilitate record keeping with regard to MPB 

activity and the determination of diameter growth in 

subsequent years.

The buffer strips for each plot were marked to the 

same GSL as their CIP but tree diameters were not 

permanently recorded nor did the leave trees receive 

metal tags.

The plots were reinventoried in August 1997, at 

which time all live trees within the CIPs were remeasured. Using those tree diameters, the current GSLs 

of each plot were computed. Using this information, 

each plot was marked for cutting in May 2000 and 

then cut in November and December 2000. Because 

the susceptibility of GSLs between 80 and 120 was 

still questionable (Schmid and others 1994), and additional information regarding susceptibility of such 

stands was desirable, we increased the GSL in each of 

the partially cut stands during marking in 2000. The 

GSL 60 plot was raised to GSL 70, the GSL 80 to GSL 

90, and the GSL 100 to GSL 110. The increased levels were attained through the diameter growth on the 

existing trees and not by the addition of more leave 

trees.

The plots were surveyed at one or two year intervals 

from 1988 to 1997 to assess MPB activity. From 1997 

to 2004, surveys were at two to three year intervals. 

During each survey, each tree was examined for the 

presence or absence of MPB attacks as well as other 

possible mortality factors such as Ips spp., Armillaria, 

and physical damage. Examinations for insect activity 

were confined to the lower 7 ft of the bole. If insect activity as evidenced by woodpecker-caused debarking 

was observed on the bole above 7 ft, it was noted but 

the bole was not examined. Records were maintained 

for each tree in the CIP as to its health or cause of 

death during each survey.

MPB-attacked trees were classified as successfully attacked (tree likely to die) or as a pitchout (tree 

unlikely to die because MPB attacks appeared insufficient to kill it). Some trees were pitchouts one year 

but were successfully attacked in subsequent years. 

Other pitchouts were not as yet attacked one or two 

years later. Because some pitchouts survived one or 

more years after attack, they could not be considered 

as MPB-caused mortality. Thus, the question arose as 

to whether pitchouts should be considered together 

with successfully attacked trees or ignored as MPBattacked trees. We chose to combine pitchouts with 

successfully attacked trees because the combined 

number of MPB-attacked trees is more indicative of 

the level of beetle activity. In addition, because some 

pitchouts were attacked and killed by another generation of MPBs in subsequent years, such trees could be 

considered as two MPB-attacked trees. However, that 

accounting procedure would represent double counting in such instances. Therefore, pitchouts that were 

attacked again at a later date were counted as MPB-attacked trees only once, but the incidence of reattacks 

in a plot is discussed.

Sources of tree mortality other than MPB were not 

always accurately assessed because they often act in 

concert with each other. For example, a tree may have 

had Ips galleries present as well as Armillaria. Whether 

the Ips killed the tree and the Armillaria developed after Ips attack or the Armillaria essentially killed the 

tree and the Ips attacked as the tree was dying could not 

be determined. Similarly, wind damaged the crowns of 

some trees but whether the wind-damage killed the tree 

or only predisposed it to some other mortality factor 
USDA Forest Service RMRS-RP-54. 2005 3

was not determined. A few MPB-attacked trees were 

predisposed by lightning, Armillaria, or wind damage. 

Thus, two or more sources of mortality should be recognized as possibly acting in concert.

Numbers of MPB-attacked trees are based on the 

CIP records from 1987 to 2004. Data from the buffer strips is used to supplement the CIP information 

but is not incorporated in the CIP records. Percentages 

of trees attributed to the various mortality factors are 

based on the total number of trees existing in the CIPs 

after the 1987 cutting.

To determine the distribution of MPB-attacked 

trees by diameter class, the diameters recorded in 

1986 were used for trees attacked from 1987 through 

1997 because we did not measure tree diameters the 

year each tree was killed. From 1998 through 2004, 

diameters recorded in 1997 were used for the MPBattacked trees.

Basal area (BA) in ft2/acre was computed for the 

CIP of each GSL from the diameters of the live trees 

on the plots in 1986 and 1997. Because diameters were 

not remeasured after 1997, BAs for 2004 were computed by determining the average annual growth rate 

from 1986 to 1997 for those trees still alive in 2004. 

The average annual growth rate for each tree was then 

multiplied by seven and the result added to the 1997 

diameter to derive the 2004 diameter. The estimated 

2004 diameters were then used to compute the 2004 

BAs.

Results and Discussion

Control

Seventy-seven percent (272 trees) of the trees in 

the CIP were MPB-attacked (table 1). If pitchouts 

that were attacked a second time are counted as two 

MPB-attacked trees, then the percentage of MPBattacked trees would increase. For example, seven 

pitchouts in 2003 were attacked again in 2004.

No MPB-attacked trees were found in the CIP between 1987 and 1997. MPB-attacked trees were first 

observed in the CIP in 1998 (figure 1). The number of 

MPB-attacked trees in the CIP increased in 1999 and 

again in 2000. The numbers in the CIP in 2001 were 

the same as in 2000. MPB-attacked trees decreased 

in 2002 but increased 50-fold in 2003 as compared to 

2002 (figure 1).

Within the buffer strips, six MPB-attacked trees 

were present in the south buffer in 1991. Fifteen trees 

were attacked in buffer strips in the southwest corner 

of the plot in 2000. Sixteen trees were attacked in the 

north buffer in 2001. Fifty trees were also attacked in 

2003 in the north, east, and west buffer strips.

Ips, Armillaria, wind, and unknown caused 3 percent of the tree mortality in the CIP since 1987 (table 

1). One tree had both Ips and Armillaria present while 

another tree had a wind-broken top and Ips present. Ips

may have attacked the trees after the Armillaria and 

wind had affected tree health.

GSL 60/70

Nine percent (10 trees) of the trees in the CIP 

were MPB-attacked from 1987 to 2004 (table 1). No 

pitchouts were observed. From 1987 to 1997, MPBs 

attacked one tree (figure 2). After 1997, no new MPBattacked trees were evident in the CIP until nine trees 

were attacked in 2003.

Within the north buffer, one MPB-attacked tree was 

found in 1990, one in 1991, six in 2001, and 17 in 

2003. Just outside the north buffer, one MPB-attacked 

tree was found in 2000 and 21 in 2001. Outside the 

Table 1—Tree conditions on the Bear Mountain I growing stock level plots after 17 years. Under each 

GSL, the percent of the total number of trees in the CIP is listed for each condition category and is followed by the total number of trees (in parentheses) in each category.

Growing stock level (GSL)

Tree condition GSL 60/70 GSL 80/90 GSL 100/110 Control

Live 78% (90) 34% (61) 41% (86) 20% (71)

MPB-attacked 9% (10) 53% (96) 48% (100) 77% (272)

Cut 10% (12) 11% (20) 9% (18)

Ips 1% (1) 1% (2)

Scolytid 1% (1)

Armillaria 1% (2) <1% (1) 1% (2)

Unknown/Wind 1% (1) 1% (1) 2% (5) 1% (4)

Total 100 (115) 100 (180) 100 (210) 100 (351)
4 USDA Forest Service RMRS-RP-54. 2005

west buffer, one MPB-attacked tree was found in 1999, 

three in 2000, and one in 2001.

Ips, other scolytids, and unknown factors caused a 3 

percent loss in the total number of trees (table 1).

GSL 80/90

Fifty-three percent (96 trees) of the trees in the CIP 

were MPB-attacked since 1987 (table 1). From 1987 

to 1997, one tree was attacked in 1988 and one in 1995 

(figure 3). From 1996 through 2000, no MPB-attacked 

trees were found in the CIP. MPB-attacked trees in the 

CIP numbered one in 2001, 29 in 2002, one in 2003, 

and 63 in 2004 (figure 3). Three pitchouts were present among the trees attacked in 2002 and five pitchouts 

among the 2004 trees. One 2002 pitchout was attacked 

again in 2004.

In the buffer strips, four trees were MPB-attacked 

in the north strip in 2001. Trees attacked in the south 

buffer strip numbered two in 2000, three in 2002, and 

10 in 2003. In the west buffer, two trees were attacked 

in 2002 and eight trees in 2003.

Armillaria and unknown factors accounted for 

2 percent of tree loss in the CIP from 1987 to 2004 

(table 1). One of the MPB-attacked trees had evidence 

of Armillaria resinosis and another had a broken top in 

addition to the MPB attacks.

GSL 100/110

Forty-eight percent (100 trees) of the trees in the 

CIP were MPB-attacked since the plot was established 

(table 1). During the first 10 years, one tree was attacked 

in 1991 (figure 4). Additional attacked trees were not 

Figure 1—Number of MPB-attacked trees per year 

in the CIP of the control plot from 1987 through 

2004.

Figure 2—Number of MPB-attacked trees per year in 

the CIP of the GSL 60/70 plot from 1987 through 

2004.
USDA Forest Service RMRS-RP-54. 2005 5

found until one tree was attacked in 2001. The numbers 

of MPB-attacked trees increased to 30 in 2002, more 

than doubled to 63 in 2003, and then decreased to six in 

2004 (figure 4). Of the 63 MPB-attacked trees in 2003, 

12 were considered pitchouts and two of the 2003 pitchouts were attacked a second time in 2004.

In the buffer strips, four trees were attacked in the 

west buffer in 1997. One of the four trees had a broken 

top that may have caused it to become a primary focus 

tree. Eight trees were attacked in the west buffer in 

2000. Two trees were attacked in the west buffer and 

22 trees were attacked in an unmanaged stand just outside the west buffer in 2001.

About 3 percent of the trees were lost to Armillaria 

or unknown causes (table 1). Several of the trees had 

broken tops that may have killed them or caused them 

to succumb to Armillaria.

Sequence of Infestation History

During the first 10 years (1987-1996) the Bear 

Mountain I plots existed, the number of MPB-attacked 

trees in each of the four CIPs was either zero or limited 

to a single tree for one or two years. In the partially cut 

plots, one tree was found in the GSL 60/70 and GSL 

100/110 and two trees were found in the GSL 80/90 

during the period. No MPB-attacked trees were found 

in the control CIP. While MPB-attacked trees were 

relatively scarce in the CIPs during the first 10 years, 

small groups of trees (four to six trees per group) were 

Figure 3—Number of MPB-attacked trees per year 

in the CIP of the GSL 80/90 plot from 1987 

through 2004.

Figure 4—Number of MPB-attacked trees per year in 

the CIP of the GSL 100/110 plot from 1987 through 

2004.
6 USDA Forest Service RMRS-RP-54. 2005

evident in the buffers of the GSL 100/110 and control 

as well as outside the plots.

As MPB populations began to increase in the vicinity 

of Bear Mountain in 1997 (McMillin and Allen 1999), 

MPB-attacked trees also increased in and around the 

Bear Mountain I plots. The initial increases of MPBattacked trees in the CIPs occurred in the control in 

1998 (figure 1). Increasing numbers of MPB-attacked 

trees became evident in the CIPs of the partially cut 

plots in 2001 (figures 3 and 4). Some of the initial 

MPB-infested spots were created by primary focus 

factors—factors that predispose trees to attack by the 

MPB (see Eckberg and others 1994). For example, 

a lightning-struck tree in the control CIP centered a 

group of five MPB-attacked trees in 1998 and one of 

four MPB-attacked trees in the west buffer of the GSL 

100/110 had a recently broken top.

As MPB populations further enlarged in the control 

CIP (figure 1) and in surrounding stands, more extensive infestations appeared in the partially cut plots. In 

the GSL 80/90, attacked trees appeared in the north 

buffer that was adjacent to the south boundary of the 

control and in the west buffer that was adjacent to an 

unmanaged stand. In the GSL 100/110, attacked trees 

appeared in the northern part of the CIP and in the 

west buffer; both areas were near infestations in adjacent unmanaged stands to the north and west. In the 

GSL 60/70, attacked trees were first evident in the 

north buffer that was adjacent to an unmanaged stand. 

Eventually, the MPB populations residing within the 

plots plus those from surrounding stands concentrated 

within the plots to create the substantial numbers of 

MPB-attacked trees evident in 2003.

Distribution of MPB-attacked Trees by 1 inch 

Diameter Class

MPB-attacked trees in the CIPs of the four plots 

were only present in diameter classes <10 in from 1987 

to 1997 even though each CIP had substantial numbers 

of trees in diameter classes >11 in (tables 2-5). From 

1998 to 2004, MPB-attacked trees were present in 

nearly all diameter classes in the GSL 80/90, 100/110, 

and control except for the two lowest diameter classes 

(tables 3, 4, and 5). In contrast, the GSL 60/70 had 

no MPB-attacked trees in the <9 and >15 in diameter 

classes (table 2).

Percentage-wise, MPB-attacked trees in respective diameter classes were generally lowest in the 

GSL 60/70, intermediate in the GSL 80/90 and GSL 

100/110, and highest in the control (tables 2, 3, 4, and 

5). However, percentages in the GSL 80/90 and GSL 

100/110 were variable such that percentages in the 9, 

10, and 15 in classes of the GSL 80/90 were greater 

than the same classes in the GSL 100/110 but the situation was reversed for the 12, 13, 14, and 16 in classes 

(tables 3 and 4). The highest percentage loss in the 

GSL 60/70 was 22 percent in the 13 in class (table 2) 

while losses >95 percent were present in the 13, 15, 

16, and 19 in classes in the control (table 5, figure 5).

Table 2—Number and percent of MPB-attacked trees by 1 inch diameter class in the CIP of the GSL 60/70. Numbers in parentheses are the 1998 leave trees that were cut in 2000.

 Diameter Number of Number of Percent of Number of Number of Percent of

 class leave-trees MPB-attacked trees-attacked leave-trees MPB-attacked trees-attacked

 (inches) 1986 1987-1997 1987-1997 1998 1998-2004 1998-2004

 7 5 1 20 0 0 0

 8 9 0 0 2 (2) 0 0

 9 19 0 0 6 (3) 0 0

 10 26 0 0 15 (3) 1 7

 11 23 0 0 17 (1) 1 6

 12 20 0 0 26 (1) 1 4

 13 3 0 0 18 4 22

 14 4 0 0 15 (1) 2 13

 15 2 0 0 4 0 0

 16 1 0 0 2 0 0

 17 1 0 0 3 (1) 0 0

 18 1 0 0 1 0 0

 19 0 0 0 0 0 0

 20 1 0 0 2 0 0

Total 115 1 111 9 
USDA Forest Service RMRS-RP-54. 2005 7

Table 3—Number and percent of MPB-attacked trees by 1 inch diameter class in the CIP of the GSL 80/90.

 Diameter Number of Number of Percent of Number of Number of Percent of

 class leave-trees MPB-attacked trees-attacked leave-trees MPB-attacked trees-attacked

 (inches) 1986 1987-1997 1987-1997 1998 1998-2004 1998-2004

 7 4 0 0 0 0 0

 8 30 0 0 2 0 0

 9 44 1 2 22 11 50

 10 44 1 2 46 30 65

 11 29 0 0 47 23 49

 12 18 0 0 22 11 50

 13 5 0 0 20 12 60

 14 4 0 0 10 4 40

 15 1 0 0 4 2 50

 16 0 0 0 2 1 50

 17 0 0 0 0 0 0

 Total 180 2 175 94

Table 4—Number and percent of MPB-attacked trees by 1 inch diameter class in the CIP of the GSL 100/110. Numbers in 

parentheses are the 1998 leave trees that were cut in 2000.

 Diameter Number of Number of Percent of Number of Number of Percent of

 class leave-trees MPB-attacked trees-attacked leave-trees MPB-attacked trees-attacked

 (inches) 1986 1987-1997 1987-1997 1998 1998-2004 1998-2004

 7 4 0 0 1 0 0

 8 30 0 0 5 (3) 0 0

 9 35 1 3 29 (6) 9 31

 10 49 0 0 30 (1) 11 37

 11 48 0 0 48 (4) 24 50

 12 22 0 0 43 (1) 25 58

 13 16 0 0 21 (1) 14 67

 14 4 0 0 19 13 68

 15 2 0 0 5 2 40

 16 0 0 0 1 1 100

 17 0 0 0 0 0 0

Total 210 1 202 99

On first thought, the decrease in the number of MPBattacked trees in the GSL 60/70, GSL 100/110, and 

control in 2004 (figures 1, 2, and 4) may have resulted 

because previous MPB-caused mortality reduced the 

number of larger diameter trees available for MPB 

attack. While this may be true for the control where 80 

percent or more of the trees were lost in several diameter classes (table 5), it is not the case for the GSL 60/70 

and 100/110 (tables 2 and 4). Moreover, the number 

of MPB-attacked trees increased in the GSL 80/90 in 

2004 and substantial numbers of 2003 pitchouts were 

reattacked in 2004. Thus, while the number of MPB-attacked trees in those three plots decreased in 2004, the 

decrease was probably not due to a lack of available 

larger diameter trees but to the shifting of the population 

from the three plots to the GSL 80/90, reattack of pitchouts, and sanitation/salvage efforts by the BHNF.

The distribution of MPB-attacked trees by diameter class indicates that MPBs do not always attack 

the largest diameter trees in PP stands. During the first 

10 years, no trees >11 in were attacked even though 

substantial numbers of trees >11 in existed. When the 

MPB epidemic commenced in 1997, only three trees 

>14 in were attacked from 1997 to 2001. Thus, while 

the MPB in LP usually selects the largest trees to infest during the few years preceding and during a major 

epidemic (Amman and Cole 1983), that is not necessarily the case in PP stands. As suggested by Olsen 

and others (1996), large trees are not always the first 

trees to be attacked in PP stands.
8 USDA Forest Service RMRS-RP-54. 2005

Table 5—Number and percent of MPB-attacked trees by 1-inch diameter class in the CIP of the control.

 Diameter Number of Number of Percent of Number of Number of Percent of

 class leave-trees MPB-attacked trees-attacked leave-trees MPB-attacked trees-attacked

 (inches) 1986 1987-1997 1987-1997 1998 1998-2004 1998-2004

 5 1 0 0 0 0 0

 6 1 0 0 1 0 0

 7 21 0 0 12 3 25

 8 67 0 0 39 22 56

 9 82 0 0 74 57 77

 10 68 0 0 73 62 85

 11 53 0 0 64 53 83

 12 28 0 0 37 32 86

 13 20 0 0 26 25 96

 14 3 0 0 11 7 64

 15 6 0 0 5 5 100

 16 0 0 0 5 5 100

 17 1 0 0 0 0 0

 18 0 0 0 0 0 0

 19 0 0 0 1 1 100

Total 351 0 349 272

Changes in Basal Areas and Mean 

Diameters

BAs in all plots increased from 1986 to 1997 

(table 6). The control increased about 20 ft2 while 

the GSLs 60/70, 80/90, and 100/110 increased 17 to 

18 ft2. MPB-caused tree mortality was insignificant 

during this period so BAs were not significantly 

altered.

BAs decreased on all plots from 1998 to 2004. The 

decrease in the GSL 60/70 was only 2.3 ft2 but the GSLs 

80/90, 100/110, and control lost at least 53, 60.4, and 

139.9 ft2, respectively (table 6). Percentage-wise, the 

GSL 80/90 and GSL 100/110 lost 53 percent and 51 

percent of their respective BAs while the control lost 

80 percent. The actual and percentage losses in BAs 

are greater than reported above because most trees 

in each plot grew for several years before the MPB 

population reached epidemic status. The substantial 

loss in BAs during the 1998 to 2004 period coincides 

with the onset and increase in the MPB population.

Figure 5—Number and percent of MPB-attacked trees 

in each diameter class in the CIP of the control
USDA Forest Service RMRS-RP-54. 2005 9

The 2004 BAs are, however, somewhat misleading 

because MPB-caused tree mortality was not uniformly 

distributed throughout the CIPs. Trees were attacked 

in groups or portions of the plots such that BAs in the 

attacked areas were zero while BAs in the unattacked 

portions were considerably greater than the 1997 BAs. 

The unattacked portions of the stands are similar to 

the microcosm stands of Olsen and others (1996) and, 

therefore, of concern to forest managers. Left to grow 

without subsequent management, the unattacked portions of the GSL 100/110 and GSL 80/90 in the Bear 

Mountain I plots could reach the susceptibility threshold of GSL 120 within five and 15 years, respectively. 

The stands then become potential sites for future MPB 

infestations.

Mean diameters increased about 1 inch in the GSL 

60/70, 80/90, and 100/110 from 1986 to 1997 while 

the mean diameter in the control increased about 0.5 

inches (table 6). From 1998 to 2004, mean diameters 

increased 1.1 inches in the GSL 60/70 and 80/90; increased 0.4 inches in the GSL 100/110; and decreased 

0.6 inches in the control. The increases in mean diameters in the GSL 60/70, 80/90, and 100/110 indicate 

that the MPB was not always attacking the trees of 

largest diameter further supporting the hypothesis that 

the largest trees are not always attacked first during 

MPB epidemics in PP stands.

Management Implications

The effectiveness of partial cutting unmanaged PP 

stands to GSLs from 80 to 120 seems questionable 

in light of the MPB-caused tree mortality in the GSL 

80/90 and 100/110. With mortality averaging about 

50 percent, forest managers will question whether 

stands with GSLs ranging from 80 to 120 are really 

moderately susceptible and whether partial cutting is 

really reducing susceptibility. However, the presence 

of unmanaged stands surrounding the Bear Mountain 

I plots may mitigate these detrimental conclusions regarding the effectiveness of partial cutting to GSLs 

of 80 to 120. As noted in the methods, the partially 

cut plots were surrounded by unmanaged stands and 

MPB populations were evident in those stands before 

MPB-caused tree mortality substantially increased 

in the partially cut plots. MPB populations also began causing substantial mortality in the control plot 

before such mortality was evident in the partially cut 

plots. Eventually, MPB populations in the control and 

the unmanaged adjacent stands increased to the point 

where their numbers began to overwhelm the partially cut stands. Had the unmanaged stands not existed 

adjacent to the partially cut stands, MPB-caused tree 

mortality in the cut stands may have been different.

The development of MPB infestations in partially 

cut stands surrounded by unmanaged stands supports 

the need to manage susceptible stands on a landscape 

basis. As primary focus trees are attacked in unmanaged stands at or near the boundaries of partially cut 

stands, MPB pheromones released from these trees 

attract additional MPBs to the general area. When epidemic MPB populations are present, such as existed in 

and around the Bear Mountain I plots, in the immediate vicinity of the initially attacked tree(s), thousands 

of beetles could be attracted to the vicinity of the primary focus tree. MPBs will then attack trees adjacent 

to the focus tree within the unmanaged stand but may 

eventually attack trees within the partially cut stands. 

This occurs because the pheromone’s influence may 

extend beyond the boundary of the unmanaged stand 

and into the adjacent partially cut stand. The influence 

of the pheromones may override the positive benefits 

of increased spacing and improved tree growth derived 

from partial cutting. As former research entomologist 

William McCambridge stated in past years, “Given 

sufficient MPBs, any tree no matter what its condition can be successfully attacked.” Thus, while partial 

cutting can eliminate substantial MPB-caused mortality, zero mortality should not be expected in partially 

cut stands adjacent to unmanaged stands, especially in 

the vicinity of their common boundaries. Further, as 

evidenced by the MPB-caused tree mortality in this set 

of plots, management of a single stand or a few stands 

Table 6—Basal areas (BA) and mean diameters (DBH) in the CIPs of the partially cut plots and control in 

1986, 1997, and 2004. Basal areas and mean diameters are estimated for 2004.

GSL 60/70 GSL 80/90 GSL 100/110 Control

Year BA DBH BA DBH BA DBH BA DBH

1986 61.1 11.0 80.8 10.1 101.5 10.5 154.7 10.0

1997 77.3 12.6 98.4 11.3 117.3 11.5 174.0 10.6

2004 75.0 13.7 46.0 12.4 56.9 11.9 34.1 10.0
10 USDA Forest Service RMRS-RP-54. 2005

within an unmanaged landscape may not provide 

long-term reduction in MPB-caused tree mortality in 

the cut stands. Reduced long-term tree mortality will 

be accomplished when sufficient area is managed so 

that partially cut stands are separated from unmanaged 

stands by natural buffers and/or buffers of low GSLs.

While it is not desirable to manage only one or a few 

susceptible stands in a landscape of susceptible stands, 

it is equally undesirable from a MPB management 

standpoint to leave one or a number of unmanaged 

MPB-susceptible stands scattered throughout a managed landscape. Cover plots, presumably cover for 

wildlife, are commonly left among managed stands on 

the BHNF (R.R. Kessler, 2005, personal communication). These cover plots are high density stands with 

DBHs generally >8 in. As such, the cover plots function as focal points for MPB infestations (R.R. Kessler, 

2005, personal communication) in a manner similar 

to the “microcosm stands” in unmanaged stands (see 

Olsen and others 1996). Leaving these “cover plots” 

among a managed landscape may mean that forest 

managers will have to return to the plots to manage 

MPB populations before the usual time for reentry.

If susceptible stands are partially cut in and adjacent to other susceptible stands, what GSL levels can 

be expected to have the least amount of MPB-caused 

mortality? The GSL 60/70 plot was the only GSL level 

sustaining <10 percent mortality (table 1). The GSL 

80/90 and 100/110 sustained about 50 percent tree 

mortality (table 1). Thus, GSLs <80 would be more 

appropriate for situations where partially cut stands 

are to exist next to unmanaged stands.

While GSLs <80 are best for reducing MPB-caused 

mortality in the long-term, such stands usually become 

heavily stocked with seedlings and saplings in the 

Black Hills. Eventually, two-storied susceptible stands 

evolve and managers are faced with stand conditions 

highly conducive for MPB infestations. If management objectives prefer GSLs >80 in partially cut stands 

adjacent to unmanaged stands, then a combination of 

a GSL ≤70 in a buffer strip adjacent to the unmanaged stand and GSLs >80 in the remainder of the stand 

might provide a desirable solution to minimizing the 

infestation of the partially cut stand adjacent to the unmanaged stand while carrying a higher GSL in the rest 

of the stand.

The threshold for highly susceptible stands was 

lowered from GSL 150 to GSL 120 as the result of the 

work of Schmid and Mata (1992). Stands with GSLs 

<120 but >80 are considered moderately susceptible stands (Schmid and others 1994). The data from 

this study also suggest that the threshold for highly 

susceptible stands may need further lowering to GSL 

100 or lower. However, before that action is taken, 

additional evidence should be gathered. Mortality in 

the GSL 80/90 and 100/110 Bear Mountain I plots 

may represent an anomaly because of the epidemic 

MPB populations and unmanaged susceptible stands 

surrounding the partially cut stands. Other sets of plots 

in the Black Hills may confirm or deny reducing the 

high susceptibility threshold because they are generally not surrounded by unmanaged stands.

The number of MPB-attacked trees within the CIP 

and buffer strips of the GSL 60/70 has implications regarding what width of buffer should be provided along 

the edge of unmanaged stands to minimize mortality 

in adjacent managed stands. In unmanaged stands, 84 

percent of the new MPB-infested spots were found on 

average within 330 ft of the previous year’s infested 

spot (Knight and Yasinski 1956). Thus, a logical buffer would be 330 ft or <.1 mile. However, the lack of 

MPB-attacked trees in the CIP of the GSL 60/70 suggests that a narrower buffer could be used. The buffer 

on the GSL 60/70 was about 50 ft so a buffer of 100 ft 

would provide an additional measure of reduced risk.

References Cited

Allen, K.K.; McMillin, J.D. 2001. Evaluation of mountain 

pine beetle activity on the Black Hills National Forest. 

Biological Evaluation R2- 01-01. Lakewood, CO: U.S. 

Department of Agriculture, Forest Service, Rocky 

Mountain Region, Renewable Resources, Forest Health 

Management. 27 p.

Amman, G. D.; Cole, W.E. 1983. Mountain pine beetle 

dynamics in lodgepole pine forests Part II: Population 

dynamics. Gen. Tech. Rept. INT-145. Ogden, UT: 

Department of Agriculture, Forest Service, Intermountain 

Forest and Range Experiment Station. 59 p.

Eckberg, T.B.; Schmid, J.M.; Mata, S.A.; Lundquist, J.E. 

1994. Primary focus trees for the mountain pine beetle 

in the Black Hills. Res. Note RM-531. Fort Collins, CO: 

U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Forest and Range Experiment Station. 10 p.

Johnson, E.; Schaupp, W.E.,Jr.; Long, D.F. 2001. [Letter to 

the Forest Supervisor]. October 12. 3 p. On file at: U.S. 

Department of Agriculture, Forest Service, Black Hills 

National Forest, Custer, SD.

Johnson, E.; Long, D.F. 2003. [Letter to the Forest 

Supervisor]. January 7. 3 p. On file at: U.S. Department 

of Agriculture, Forest Service, Black Hills National 

Forest, Custer, SD.

Knight, F.B.; Yasinski, F.M. 1956. Incidence of trees 

infested by the Black Hills Beetle. USDA Forest Service 

Res. Note RM-21. Fort Collins, CO: U.S. Department of 
USDA Forest Service RMRS-RP-54. 2005 11

Agriculture, Forest Service, Rocky Mountain Forest and 

Range Experiment Station. 4 p.

McMillin, J.D.; Allen, K.K. 1999. Evaluation of mountain 

pine beetle activity in the Black Hills National Forest. 

Biological Evaluation R2-00-03. Lakewood, CO: U.S. 

Department of Agriculture, Forest Service, Rocky 

Mountain Region, Renewable Resources, Forest Health 

Management. 18 p.

Olsen, W.K.; Schmid, J.M.; Mata, S.A. 1996. Stand 

characteristics associated with mountain pine beetle 

infestations in ponderosa pine. Forest Science 42: 310-

327.

Pasek, J.E.; Schaupp, W.C., Jr. 1992. Status and trends of 

mountain pine beetle populations in the Bear Mountain 

and White House Gulch areas on the Harney Ranger 

District, Black Hills National Forest, South Dakota. 

Biological Evaluation R2-92-04. Lakewood, CO: U.S. 

Department of Agriculture, Forest Service, Rocky 

Mountain Region, Renewable Resources, Forest Health 

Management. 17 p.

Schmid, J.M.; Mata, S.A. 1992. Stand density and mountain 

pine beetle-caused tree mortality in ponderosa pine stands. 

Res. Note RM-515. Fort Collins, CO: U.S. Department 

of Ariculture, Forest Service, Rocky Mountain Forest 

and Range Experiment Station. 4 p.

Schmid, J.M.; Mata, S.A., Olsen, W.K.; Vigil, D.D. 1993. 

Phloem temperatures in mountain pine beetle-infested 

ponderosa pine. Fort Collins, CO: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Forest and 

Range Experiment Station. Res. Note RM-521. 10 p.

Schmid, J.M.; Mata, S.A.; Obedzinski, R.A. 1994. Hazard 

rating ponderosa pine stands for mountain pine beetles 

in the Black Hills. Res. Note RM-529. Fort Collins, CO: 

U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Forest and Range Experiment Station. 4 p.

Thompson, R.G. 1975. Review of mountain pine beetle 

and other forest insects active in the Black Hills. Special 

Report R2-75-1. Lakewood, CO: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Region, 

Forest Pest Management. 35 p.
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RMRS

ROCKY MOUNTAIN RESEARCH STATION

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diseases. Studies are conducted cooperatively, and applications may 

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