United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

Research Paper 

RMRS-RP-42

September 2003

Growth of Lodgepole Pine 

Stands and Its Relation 

to Mountain Pine Beetle 

Susceptibility

S. A. Mata, J. M. Schmid, and W. K. Olsen 
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Mata, S.A.; Schmid, J.M.; Olsen, W.K. 2003. Growth of lodgepole pine stands and its relation to 

mountain pine beetle susceptibility. Res. Pap. RMRS-RP-42. Fort Collins, CO: U.S. Department 

of Agriculture, Forest Service, Rocky Mountain Research Station. 19 p.

Abstract

Periodic diameter and basal area growth were determined for partially cut stands of lodgepole pine 

at five locations over approximately 10 year periods. After cutting, average diameters in the partially 

cut plots generally increased by 0.8 inches or more, while average diameter in the uncut controls 

increased by 0.6 inches or less. Diameter growth in the partially cut plots was generally significantly 

greater than diameter growth in the controls. Individual tree growth is discussed in relation to potential susceptibility to mountain pine beetle infestation. Basal area decreased in three of the four 

GSL (growing stock level) 40 stands because of windthrow. Basal area generally increased >1.0 ft2/

acre/year in partially cut plots except in the GSL 40 stands with substantial windthrow and one GSL 

100 with an Armillaria infection pocket. Basal area increases in the control plots ranged from 0.2 to 

1.1 ft2/acre/year, although the one control with a BA growth rate of 1.1 ft2/acre/year had a relatively 

low initial BA. Data from the stands are employed in the susceptibility rating methods of Amman 

et al. (1977), Shore and Safranyik (1992), and Anhold et al. (1996) to determine stand susceptibility 

and the results discussed in terms of general applicability of these methods to partially cut stands. 

Basal area growth is used to estimate the length of time required for various stand densities to reach 

specific susceptibility thresholds for mountain pine beetle infestation. Several of the GSL 40 stands 

are not projected to reach the susceptibility thresholds in <100 years because of windthrow. Barring 

mortality >1%, GSL 80 stands are estimated to reach the basal area threshold of 120 ft2 per acre in 

<25 years. GSL 120 stands, essentially at the basal area 120 threshold, are generally projected to 

reach the basal area threshold of 150 ft2 per acre in <35 years.

Keywords: Lodgepole pine, basal area growth, diameter growth, mountain pine beetle, stocking 

levels

Acknowledgments

Critical but helpful reviews were provided by: K. E. Gibson, Forest Health Management, Missoula, 

MT; S. Munson, Forest Health Management, Ogden, UT; L. Safranyik, Canadian Forest Service, 

Victoria, BC; J. J. Witcosky and C. L. Jorgensen, Forest Health Management, Lakewood, CO. The 

authors greatly appreciate their comments and suggestions for improving this report.
USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 1

Introduction

Knowledge of tree growth under various stand and 

site conditions is essential for managing stands of 

lodgepole pine (LP), Pinus contorta Douglas. Such 

knowledge is necessary for developing growth and 

yield models that are used to predict future outcomes for 

different silvicultural prescriptions. Knowledge of tree 

growth and stand conditions is also useful for predicting 

when unmanaged stands and partially cut stands may 

become susceptible to infestation by the mountain pine 

beetle (MPB), Dendroctonus ponderosae Hopkins.

Susceptibility of LP stands to MPB attack has been 

defined by Shore and Safranyik (1992) as “the inherent 

characteristics or qualities of a stand of trees that affect 

its likelihood of attack or damage by a MPB population.” Their definition conceptualizes susceptibility in 

terms of the probability of MPB infestation and not in 

terms of subsequent tree mortality as some prior authors 

assumed in their susceptibility rating methods. Although 

the Shore and Safranyik susceptibility-rating method 

provides a long-term index of potential basal area loss 

in the event of a MPB infestation (Shore et al. 2000), its 

primary function is the probability of MPB infestation. 

This paper discusses hazard rating methods wherein susceptibility may have been defined with some measure of 

tree mortality, but it follows Shore and Safranyik’s 1992 

definition for susceptibility without the connotations of 

subsequent mortality, i.e., just the probability of MPB 

infestation.

Methods for estimating the susceptibility of unmanaged LP stands to MPB infestation have been developed 

by Amman et al. (1977), Anhold et al. (1996), Berryman 

(1978), Mahoney (1978), and Shore and Safranyik 

(1992) among others. Most of the various methods are 

based on stand characteristics (Bentz et al. 1993) such 

as diameter at breast height (DBH), stand density expressed in terms of basal area (BA) or number of stems 

per ha, age in years, percentage of susceptible pine BA, 

a periodic growth ratio, or a crown competition factor as 

they interact with the combined influence of elevation, 

latitude, and longitude for the particular stand. More importantly, of the numerous methods developed for determining susceptibility, nearly all are primarily designed 

for unmanaged stands, are not designed to rate managed 

stands, or have limitations if used to determine when 

managed stands may become susceptible.

Three methods previously or currently used to estimate the susceptibility of LP stands have been developed by Amman et al. (1977), Shore and Safranyik 

(1992), and Anhold et al. (1996). In the Amman et 

al. (1977) method, susceptibility of lodgepole pine 

stands is a function of elevation and latitude, average 

tree age, and average DBH. Susceptibility is greatest when: elevation is below 9500 ft, latitude is less 

than 490, average age >80 years, and average DBH >8 

inches (Amman et al. 1977). In the Shore and Safranyik 

(1992) method, susceptibility is a function of the percentage of susceptible pine basal area, age of the stand 

in years, density of the stand in terms of stems per ha, 

and location (a combination of longitude, latitude, and 

elevation). Susceptibility is greatest when: the stand 

contains a high percentage of pine with diameters >15 

cm DBH (6 inches), the average age of dominant trees 

>80 years, the number of stems per ha is between 750 

and 1000 (304 and 405 trees per acre, respectively), and, 

generally, elevation, latitude, and longitude are at the 

lower values for their respective ranges. In the Anhold 

et al. (1996) method, susceptibility is defined by the 

Growth of Lodgepole Pine Stands and 

Its Relation to Mountain Pine Beetle 

Susceptibility 

S.A. Mata1, J.M. Schmid1, and W.K. Olsen2

1 Forestry Technician and Forest Entomologist (retired), respectively, Rocky Mountain Research Station, Fort Collins, Colorado 

80526. 2 Forestry Consultant, W.K. Olsen & Associates, L.L.C., Bellvue, 

Colorado 80512.
2 USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 

relative density of the stand compared to maximum stand 

density as expressed by the stand density index (SDI). 

Stands with densities of 20% to 35% of the maximum 

SDI and a quadratic mean diameter >8” define a “zone” 

of high susceptibility. Stands with densities of <20% and 

>35% of maximum SDI, or a quadratic mean diameter 

<8 inches are assumed to have low susceptibility. More 

pertinent for comparison to our data, the “zone” of high 

susceptibility equivocates to specific numbers of trees 

per acre for quadratic mean diameters >8 inches with 

the lower (20% SDI) and upper (35% SDI) limits of the 

“zone” decreasing as diameter increases.

This paper reports on the diameter and basal area 

growth of partially cut and uncut LP stands at five locations in southern Wyoming and northern Colorado. The 

stand conditions of each plot before and after partial 

cutting are evaluated in terms of the susceptibility rating criteria of Amman et al. (1977), Shore and Safranyik 

(1992), and Anhold et al. (1996) to estimate their initial 

and future susceptibility. The stand conditions and tree 

growth are also used in straight-line projections and a 

growth and yield model to estimate when the stands 

might enter susceptibility thresholds as determined for 

the MPB in ponderosa pine, P. ponderosa Lawson. The 

results are discussed in relation to the future susceptibility of these stands to MPB infestations, and their implications for management.

Methods

During a study of the relationship between stand 

density and MPB-caused tree mortality, sets of 

growing stock level (GSL)3 plots were installed in 

susceptible-size LP stands in southern Wyoming and 

northern Colorado between 1984 and 1987. Each set 

typically consisted of four 1-ha (2.47-acre) plots; 3 plots 

partially cut to different GSLs and the fourth plot left 

uncut to serve as a control. Occasionally, an additional 

1-ha (2.47-acre) plot was installed and partially cut 

when stand conditions allowed.

Leave trees within cut plots were selected on the basis 

of DBH, spacing, tree form, crown development, visually apparent good health, and species. Tree selection 

emphasized leaving the best and largest LP as evenly 

spaced as possible. Other tree species such as aspen, 

Populus tremuloides Michaux, were marked to be cut. 

The process of tree selection attempted to leave the GSL 

within +1 of the designated level (i.e., a GSL 80 stand 

would be between 79 and 81).

When installed, the central 0.5 ha (1.24 acres) of each 

plot were designated as the central inventory plot (CIP). 

DBH was measured on all trees within the CIP. Tree 

diameters and information on crown form, defects, the 

presence or absence of MPB attacks, and diseases were 

recorded. Metal tags were placed at about breast height 

on the designated leave trees in the CIP to facilitate record keeping in regard to MPB infestation and the determination of individual tree growth in subsequent years. 

Trees in the CIP provided the growth information and 

were the basis for statements regarding diameter and BA 

growth on the 1-ha (2.47-acre) plots.

Trees were also examined for the presence of MPB 

attacks and physical damage during surveys subsequent 

to cutting. Trees with MPB attacks were classified as 

“successfully attacked” or “pitchout” (a pitchout is a 

tree that has external evidence of MPB attacks but survives the attacks). Trees were examined the following 

year to verify the classification. 

Sets of plots were installed on the Medicine Bow 

National Forest in southern Wyoming and on the 

Colorado State Forest in northern Colorado. Two sets 

of plots (Divide Peak [DPEAK]) and North Savery 

[NSAV]) were installed about 24 miles southwest of 

Saratoga, WY and one set (Brush Creek [BCRK]) was 

installed about 21 miles southeast of Saratoga. Two 

sets of plots were installed near Gould, CO: one set 

(Colorado State Forest [CSF]) about 5 miles northeast 

of Gould and one set (KOA) 1 mile northwest of Gould. 

Two of the CSF plots were superimposed on stands that 

had been partially cut prior to our establishment of the 

plots. Additional plot information with respect to dates 

of installation, cutting, and remeasurement, elevation, 

site index, etc., is presented in tables 1 and 2. MPBcaused mortality was observed in the Wyoming plots 

prior to cutting but no MPB-caused mortality has been 

observed since cutting. Plots at the Colorado locations 

have not been exposed to MPB infestations.

Diameter growth was based on trees within the CIP 

and was calculated for only those trees alive at the time 

of remeasurement. Diameter growth for each tree was 

calculated by subtracting its DBH at installation from 

its DBH at remeasurement. As calculated in this manner 

3

Growing stock level (GSL) is defined as the residual square feet 

of BA when the average diameter of the stand is >10 inches 

(Alexander and Edminster 1981). When the average diameter 

is <10 inches, BA is numerically less than GSL. While BA is 

more commonly used and preferrable than GSL, the plots were 

installed under the GSL concept so the results are reported as 

such. Further, statements regarding specific GSL levels become 

awkward when reported in BA equivalents because a range of 

BAs must be used if the average diameter is less than 10 inches. 
USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 3

for these sets of plots, diameter growth or periodic annual increment for the partially cut stands represents a 

mixture of annual increments of growth under the uncut 

and partially cut stand conditions because the trees grew 

in the uncut situation for one to three growing seasons 

before the stands were partially cut. The consequences 

of this situation are discussed later.

Some trees had smaller diameters at the time of 

remeasurement than they had when the plots were 

installed. We considered this situation to result from 

differences in the manner in which the diameters were 

measured and assigned such trees zero growth. Trees dying 

between plot installation and plot remeasurement were not 

included in the calculations of mean diameter growth.

Table 2—Elevation, site index, trees per acre, and percent mortality in the CIP between installation and 

remeasurement by location and GSL.

Trees per acre

Before After CIP

Location/GSLs Elevation (ft) Site index cut cut mortality

Brush Creek (WY)

GSL 40 8480 67 674 78 28%

GSL 60 8460 71 283 77 2%

GSL 80 8460 67 567 150 6%

GSL 120 8480 69 683 268 5%

Control (GSL 234) 8480 67 535 3%

Colorado State Forest (CO)

GSL 40 8900 58 279 104 0%

GSL 80 8910 62 227 187 1%

GSL 120 8850 71 572 288 4%

Control (GSL 197) 8930 68 526 7%

Divide Peak (WY)

GSL 40 8340 72 520 73 40%

GSL 100 8320 78 560 184 18%

GSL 120 8320 80 529 206 10%

Control (GSL 197) 8280 78 267 5%

KOA (CO)

GSL 80 8820 62 452 206 2%

GSL 100 8820 64 364 224 0%

GSL 120 8820 64 364 281 1%

Control (GSL 155) 8820 61 416 2%

North Savery (WY)

GSL 40 8340 72 371 49 15%

GSL 80 8340 67 984 223 5%

GSL 120 8340 68 917 299 2%

Control (GSL 270) 8340 73 589 11%

Table 1—Dates for installation, cutting, and remeasurement; number of plots; and age of site index trees for the plots at the five 

locations.

Plot name

Characteristics Brush Creek Divide Peak North Savery CO State Forest KOA

Date installed August 1985 September 1984 June 1985 August 1984 September 1987

Number of plots 5 4 4 4 4

GSL’s of cut plots 40, 60, 80, 120 40, 100, 120 40, 80, 120 40, 80, 120 80, 100, 120

Date cut July 1987 August 1986 August 1986 July 1985 July 1990

Date remeasured July 1997 August 1997 August 1997 July 1997 August 1999

Age of site index >90 yrs >130 yrs >105 yrs >90 yrs >85 yrs

trees
4 USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 

One-way analysis of variance was used to determine 

if mean diameter growth in the various diameter classes 

varied significantly among GSLs within each diameter 

class at each location and among diameter classes within 

each GSL at each location. If significant variation was 

found, Tukey’s test was used to determine which means 

were significantly different, a = 0.05.

The increase in periodic annual increment (PAI) in 

BA was calculated in the same manner as for diameter 

growth. Total BA after marking was subtracted from 

total BA at remeasurement. The resulting value was 

divided by the number of years between installation and 

remeasurement to derive the unadjusted PAI. However, 

the plots grew in the uncut condition before they were 

cut (see previous paragraphs on diameter growth) so 

that the BA after cutting was actually greater than the 

BA after marking. The time interval between tree 

selection/marking and tree cutting varied among locations; it was 1 yr for CSF, 2 yr for BCRK, DPEAK, 

and NSAV, and 3 yr for KOA. Therefore, to derive a 

more realistic PAI for BA growth, it was assumed that 

the partially cut plots grew at the same rate as their 

respective control during the years preceeding their 

cutting and a percentage of this growth was added 

to their BA after marking. The amount of the BA 

growth for the control added to the BA after marking 

for each of the partially cut plots depended on what 

percentage of the original BA existed after marking. 

For example, if the BA after marking was 40 and the 

BA at plot installation was 200, the BA after marking represented 20% of the original BA (40/200) so 

20% of the BA growth for the control was added to 

the after-marking BA for the partially cut plot. The 

difference between the adjusted BA and the BA at 

remeasurement was divided by the number of years between tree cutting and plot remeasurement to derive the 

adjusted BA PAI. 

The susceptibility of each GSL at each location to 

MPB infestation was determined according to the methods of Amman et al. (1977), Shore and Safranyik (1992), 

and Anhold et al. (1996).

The current and future susceptibility of each partially 

cut GSL was also estimated by determining when its BA 

might reach the BA thresholds of 150 and 120 ft2/acre 

(see Obedzinski et al. 1999). In previous work in ponderosa pine, Sartwell and Stevens (1975) believed that 

BA >150 ft2/acre was a critical level for MPB infestation. The BA 150 level was commonly accepted as 

the critical threshold until Schmid and Mata (1992) 

suggested the critical threshold might be as low as 

BA 120 ft2/acre in ponderosa pine. Although a basal 

area threshold(s) for lodgepole pine has not been 

determined, information in McGregor et al. (1987) and 

Gibson (1989) suggest that 120 ft2/acre may also be the 

critical threshold for the MPB in LP. BA 120 is routinely 

considered the threshold for high hazard LP stands in 

current evaluations (K.E. Gibson 2002, personal communication). Thus, for comparative purposes, the 150 

and 120 ft2/acre values were used to determine how long 

it would take the partially cut LP stands to reach these 

levels. The estimates, based on growth data obtained 

from each plot, were derived from straight-line projections (SLP) and from a growth and yield model.

For straight-line projections, two BA values were 

used: the BA after tree marking was completed (i.e., 

when the plots were installed and leave trees were selected), and the BA at remeasurement. Ordinarily, the 

BA after marking was plotted for time (year) equal 

to zero because the non-leave trees were cut soon 

after selection and marking. However, as indicated in 

the previous paragraph regarding the determination 

of BA growth, the trees were not cut within a short 

time after tree selection and marking so trees in the 

partially cut stands grew in the uncut condition for 1 

to 3 years depending on the location. Therefore, the 

BAs after marking were adjusted for their growth 

in the uncut condition as indicated in the previous 

paragraph regarding the determination of BA growth. 

The BA at remeasurement was unaffected by the difficulty with the determination of the BA after marking. 

BA at remeasurement was plotted for when time (year) 

equalled the number of years between tree cutting and 

remeasurement. Straight lines were drawn through the 

two BA values for each GSL and extended until they either intercepted the 120 and 150 BA values or exceeded 

100 years.

To determine if our straight-line projections differed 

substantially from those derived from a growth and 

yield model, we also estimated the time required 

to reach the susceptibility thresholds by inputting 

the stand and growth data for each GSL plot into 

the growth and yield model GENGYM. GENGYM 

is a computerized growth and yield model used for 

projecting future stand conditions in mixed conifer 

and ponderosa pine stands (Edminster et al. 1991). 

GENGYM supercedes RMYLD (see Edminster 1978 

regarding RMYLD), which was used by Schmid 

(1987) to estimate the time for ponderosa pine stands 

to reach susceptibility thresholds.

Some problems inherent in using the straight-line 

projection and GENGYM methods for estimating future stand conditions are discussed in more detail when 

the projection estimates are made in the Results and 

Discussion and Management Implications sections. 
USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 5

Results and Discussion

Diameter Growth

Mean diameter increased in all plots with increases 

ranging from 0.3 to 3.2 inches (table 3). Increases in 

mean diameter in the partially cut stands resulted from 

tree selection criteria and growth. Because smaller diameter trees were generally but not exclusively discriminated against in our design, more smaller trees were 

generally cut than larger diameter trees. Thus, the initial 

increase in mean stand diameter immediately after cutting was not a matter of growth, but the removal of a 

high percentage of the smaller trees. Subsequent to the 

partial cutting, increases in average diameters resulted 

from growth. Mean diameter growth in most of the 

partially cut plots generally exceeded 0.8 inches during 

the first 10 yr after cutting, while mean diameters in the 

control plots increased <0.6 inches (table 3).

Brush Creek. Among GSLs, mean growth rates 

within the 7- to 12-inch diameter classes in the partially 

cut GSLs (GSLs 40, 60, 80, and 120) were significantly 

greater than the mean growth rates for comparable diameters in the control (table 4). In the 13-inch class, 

the mean growth rate in the GSL 60 was significantly 

greater than the rates in the GSL 120 and control. In 

the 14-inch class, the mean growth rate in the GSL 60 

was significantly greater than that of the control. Mean 

growth rates in the GSL 40 and GSL 60 were significantly greater in the 10- to 11-inch diameter classes than 

mean growth rates in the GSL 120 (table 4). 

Within each partially cut GSL, mean growth rates 

were not significantly different among diameter classes 

(table 4). Within the control, mean growth rates were 

significantly different among diameter classes and increased with increasing diameter class. Mean growth 

rates in the 8- to 14-inch classes were generally significantly greater than mean growth rates in the 4- to 7-inch 

classes. 

Table 3—Increases in mean diameter in inches by growing stock level (GSL).

Mean diameter Increase between:

Location After After At Installation & Marking &

/GSL installation marking remeasurement marking remeasurement

-------------------------------------------------------inches-------------------------------------------

Brush Creek (WY)

GSL 40 7.8 9.7 11.0 1.9 1.3

GSL 60 10.5 12.0 13.4 1.5 1.4

GSL 80 8.4 10.0 11.1 1.6 1.1

GSL 120 7.6 8.9 9.7 1.3 0.8

Control 8.8 8.8 9.1 0.0 0.3

Colorado State Forest (CO)

GSL 40 7.3 8.0 9.7 0.7 1.7

GSL 80 8.5 8.6 10.0 0.1 1.4

GSL 120 7.6 8.5 9.4 0.9 0.9

Control 7.8 7.8 8.3 0.0 0.5

Divide Peak (WY)

GSL 40 8.9 10.6 11.3 1.7 0.7

GSL 100 8.7 9.9 10.9 1.2 1.0

GSL 120 9.2 10.4 11.3 1.2 0.9

Control 11.6 11.6 12.0 0.0 0.4

KOA (CO)

GSL 80 7.4 8.3 9.1 0.9 0.8

GSL 100 8.3 8.9 9.7 0.6 0.8

GSL 120 8.2 8.6 9.1 0.4 0.5

Control 7.8 7.8 8.2 0.0 0.4

North Savery (WY)

GSL 40 10.4 12.4 13.2 2.0 0.8

GSL 80 7.1 8.4 9.5 1.3 1.1

GSL 120 7.4 8.6 9.5 1.2 0.9

Control 9.0 9.0 9.6 0.0 0.6
6 USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 

No measurable growth or growth insufficient enough 

to provide a mean value greater than 0.05 was observed 

in only the 4- and 5-inch diameter classes in the control 

(table 4). The greatest growth for any tree in the GSL 40, 

GSL 60, and GSL 80 was >2 inches. Only one tree in the 

control grew 1.4 inches while the rest of the trees in all 

diameter classes grew less than 1 inch.

Colorado State Forest. Among GSLs, mean growth 

rates in the 7- to 9-inch diameter class were signicantly 

greater in the partially cut GSLs (GSL 40, 80, and 120) 

than mean growth rates in the control (table 5). In the 

10- and 11-inch clases, mean growth rates in the GSL 80 

were greater than those of the GSL 120 and control. In 

other diameter classes, data was insufficient to provide 

satisfactory tests among the 4 plots. 

Within each GSL, mean growth rates were not significantly different among diameter classes in the partially cut GSLs but were significantly different in the 

control (table 5). Except where diameter classes were 

represented by only one or two trees, mean growth rates 

in the 9- to 12-inch diameter clases within the control 

were significantly greater than growth rates for lesser diameter classes but were not significantly different from 

each other. 

Growth rates for individual trees in the GSL 40 and 

GSL 80 commonly exceeded 2 inches while individual 

trees in the control rarely exceeded 0.9 inches. While 

the difference can be attributed to the different stocking 

levels in the partially cut plots versus the control, the difference is also attributable to previous stand treatments. 

Table 4—Mean diameter growth (X±SD) in inches by 1-inch diameter class for different GSLs 

at Brush Creek (BCRK) based on 12-yr growth data.

Diameter GSL 40 GSL 60 GSL 80 GSL 120 Control

-----------------------------------------------------inches----------------------------------------------------

4 0.0±0.06

5 0.0±0.08

6 1.1a 0.8±0.27 0.1±0.14

7 1.1±1.04 1.1±0.39 0.8±0.27 0.2±0.16

8 0.9±0.44 2.2a 1.1±0.37 0.8±0.29 0.2±0.18

9 1.0±0.54 1.2±0.37 1.1±0.38 0.8±0.31 0.3±0.19

10 1.4±0.60 1.5±0.74 1.0±0.48 0.8±0.25 0.3±0.18

11 1.2±0.50 1.1±0.48 1.1±0.39 0.8±0.32 0.4±0.23

12 1.2±0.70 1.3±0.44 1.0±0.39 0.8±0.22 0.4±0.18

13 1.3a 1.2±0.43 0.8±0.29 0.6±0.22 0.4±0.28

14 1.0a 1.3±0.40 1.3±0.46 1.1±0.38 0.6±0.13

15 1.2±0.38 0.4±0.07

16 1.1a 0.6a

a

 One tree sample

Table 5—Mean diameter growth (X±SD) in inches by 1-inch diameter class for different 

GSLs at Colorado State Forest (CSF) based on 12-yr growth data.

Diameter GSL 40 GSL 80 GSL 120 Control

----------------------------------------------inches------------------------------------------------

3 0.1±0.15

4 0.1±0.10

5 0.7a 0.6±0.13 0.2±0.17

6 1.6±0.26 1.2±0.44 0.8±0.40 0.3±0.20

7 1.7±0.29 1.2±0.41 0.9±0.34 0.4±0.24

8 1.7±0.38 1.4±0.38 0.9±0.31 0.5±0.25

9 1.7±0.33 1.5±0.40 0.9±0.28 0.6±0.22

10 1.7±0.21 1.5±0.47 0.9±0.35 0.6±0.23

11 1.4a 1.4±0.47 0.9±0.40 0.7±0.30

12 1.7a 1.0±0.25 0.7±0.23

13 1.4±0.64 0.8a

14 0.3±0.35

15 0.5a

a One tree sample
USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 7

The CSF GSL 40 and GSL 80 plots were superimposed 

on an area that had previously been partially cut to about 

GSL 80 years prior to plot installation. Thus, trees in 

these 2 plots had adjusted to the reduced competition 

and were already growing at an increased rate before we 

installed our plots. Our additional partial cutting probably further enhanced the growth rates. The combined 

influence of the 2 partial cuts explains why trees in these 

plots generally exhibited growth rates superior to the 

growth rates of trees in comparable GSLs from the other 

locations.

Divide Peak. Mean growth rates for 9- to 13-inch 

diameter classes in the partially cut plots (GSL 40, 

100, and 120) were significantly greater than those of 

the control; growth rates in the partially cut plots were 

generally double those in comparable diameter classes 

in the control (table 6). Mean growth rates in the 7-inch 

class were not significantly different among GSLs. In 

the 8-inch diameter class, the mean growth rate in the 

GSL 120 was greater than that of the control but rates 

for the GSL 40 and 100 were not significantly different 

(table 6).

Within GSLs, mean growth rates were significantly 

different among diameter classes in all GSLs (table 6). 

In the GSL 40, mean growth rates in the 6- and 7-inch 

diameter classes were significantly less than the rate in 

the 15-inch class; all other diameter classes were not 

significantly different from each other. In the GSL 100, 

the rate for the 8-inch class was significantly less than 

the rate for the 13-inch class; rates for all other diameter 

classes were not significantly different from each other. 

In the GSL 120, the mean growth rate for the 7-inch 

diameter class was significantly less than the rates 

for the 9- to 13-inch diameter classes; other diameter 

classes were not significantly different from each other. 

Mean growth rates for the 4- to 8-inch and 10-to 15-inch 

diameter classes in the control were significantly less 

than the growth rate for the 19-inch class. 

KOA. Among GSLs, mean diameter growth rates 

were significantly variable within the 7- to 10-inch diameter classes but not significantly different in the 11- to 

13-inch classes (table 7). Mean growth rates in the GSL 

80 were significantly greater than the rates for the other 

GSLs in the 7- to 10-inch classes while mean growth 

rates among the other GSLs were not significantly different in the same classes. 

Mean diameter growth rates within GSLs were 

highly variable. Although significant differences were 

present in each GSL, significant differences usually 

existed between only 2 diameter classes in each GSL 

while the other diameter classes were not signicantly 

different. Only the control exhibited a trend in mean 

growth rates; growth rates increased with increasing 

diameter (table 7). 

North Savery. Among GSLs, mean diameter growth 

varied significantly. Generally, mean diameter growth 

in the 7- to 12-inch diameter classes in the partially cut 

plots was significantly greater than those in the same 

diameter classes in the control; only in the 13-inch class 

were differences nonsignificant (table 8). In most cases, 

growth rates in the partially cut plots were more than 

double those in comparable diameter classes in the control (table 8). Average diameter growth rates exceeded 1 

inch in some diameter classes in each of the partially cut 

Table 6—Mean diameter growth (X±SD) in inches by 1-inch diameter class for different 

GSLs at Divide Peak (DPEAK) based on 13-yr growth data.

Diameter GSL 40 GSL 100 GSL 120 Control

---------------------------------------------inches-----------------------------------------------

4 0.2±0.21

5 1.2±0.78 0.8a 0.3±0.17

6 0.8±0.32 0.7±0.42 0.2±0.25

7 0.3±0.16 0.6±0.33 0.4±0.26 0.4±0.32

8 0.8±0.31 0.7±0.29 0.8±0.39 0.4±0.29

9 1.3±0.71 0.8±0.38 0.9±0.39 0.4±0.35

10 1.0±0.29 0.9±0.38 0.8±0.34 0.4±0.29

11 1.0±0.41 0.9±0.47 0.8±0.33 0.4±0.30

12 1.5±0.31 0.8±0.21 0.9±0.38 0.4±0.24

13 1.9±0.14 1.3±0.18 0.9±0.40 0.4±0.25

14 1.8a 1.2a 0.8±0.39 0.4±0.26

15 1.8±0.90 1.0±0.23 0.3±0.26

16 1.0a 0.5±0.27

17 0.5a 0.5±0.23

18 0.4±0.21

19 1.0±0.60

a

 One tree sample
8 USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 

GSLs while 0.6 inch was the highest average growth rate 

in the control (table 8).

Mean diameter growth varied significantly among 

diameter classes within the GSL 80, 120, and control 

but nonsignificantly in the GSL 40. In the GSL 40, mean 

growth ranged from 0.7 to 1.2 inches in the various diameter classes with no distinctive trend (table 8). In the 

GSL 80, GSL 120, and control, mean diameter growth 

increased from the lowest diameter classes to the highest diameter classes (table 8). Mean growth rates in the 

5- and 6-inch classes in the GSL 80, GSL 120, and control were significantly less than rates in larger diameter 

classes in their respective GSLs (table 8). 

What do these growth rates mean in regard to individual tree and stand susceptibility to MPB infestation 

in unmanaged and managed stands? Because a specific 

growth rate threshold that separates susceptible LP trees 

from nonsusceptible LP trees has not been determined 

in terms of a fraction of an inch per year, or inches per 

10-year period, it is difficult to say which growth rates 

are characteristic of susceptible trees and which are 

characteristic of nonsusceptible trees. That is, are trees 

growing at the rate of <0.5 inches per 10 years, more 

susceptible than trees growing at 0.6 or 0.7 inches per 

10 years, or some larger rate? Although the literature 

does not directly address susceptible vs. nonsuceptible 

growth rates, some indirect evidence is pertinent.

Mahoney’s periodic growth ratio (PGR) is the ratio of the current 5-year radial increment divided by 

the previous 5-year radial increment of dominant and 

Table 7—Mean diameter growth (X±SD) in inches by 1-inch diameter class for different 

GSLs at KOA (KOA) based on 12-yr growth data.

Diameter GSL 80 GSL 100 GSL 120 Control

-----------------------------------------------------inches----------------------------------------

4 0.0±0.09

5 0.6±0.21 0.1a 0.3±0.22 0.1±0.11

6 0.7±0.20 0.5±0.21 0.4±0.22 0.3±0.17

7 0.9±0.31 0.4±0.20 0.4±0.25 0.3±0.17

8 0.9±0.28 0.5±0.23 0.5±0.23 0.5±0.18

9 0.9±0.35 0.5±0.22 0.5±0.20 0.5±0.19

10 1.0±0.36 0.5±0.36 0.5±0.24 0.6±0.15

11 0.7±0.33 0.6±0.25 0.6±0.24 0.6±0.19

12 0.5±0.21 0.9±0.28 0.4±0.28 0.8±0.17

13 0.7a 0.6a

14 1.1a 0.4a

15

16 0.7a

 

a

 One tree sample

Table 8—Mean diameter growth (X±SD) in inches by 1-inch diameter class for different 

GSLs at North Savery (NSAV) based on 13-yr growth data.

Diameter GSL 40 GSL 80 GSL 120 Control

-----------------------------------------------------inches----------------------------------------

4 0.4a 0.2±0.15

5 0.5±0.24 0.5±0.20 0.1±0.13

6 0.6±0.41 0.6±0.31 0.1±0.16

7 1.0±0.39 0.8±0.32 0.2±0.17

8 1.1±0.38 0.8±0.27 0.3±0.21

9 1.3±0.37 0.8±0.36 0.4±0.22

10 0.9±0.28 1.3±0.49 1.0±0.31 0.4±0.23

11 0.8±0.31 1.3±0.34 1.1±0.47 0.4±0.26

12 0.8±0.19 1.3±0.53 1.1±0.39 0.5±0.23

13 0.7±0.49 0.9±0.31 0.4±0.27

14 1.0±0.30 0.4±0.25

15 1.2±0.49 0.5±0.23

16 1.0a 0.6±0.17

17 0.6a

18 0.7a

a

 One tree sample
USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 9

co-dominant LP trees, i.e., growth for years 1-5 divided by growth for years 6-10. If the average PGR for 

the stand is <0.9, the stand is considered susceptible 

(Mahoney 1978). While we did not calculate PGRs for 

the various plots, the growth rates in the partially cut 

plots were generally greater than those of the respective controls. PGRs for these plots would probably be 

>1.0 and would thus indicate nonsusceptible status. The 

growth rates for the controls probably didn’t change 

over the measurement period. Their PGRs are probably 

<1.0 so the controls would be considered susceptible.

On first thought, the reader might conclude that future 

PGRs for partially cut stands would be continually >1.0 

because the trees in those stands would exhibit increasing growth rates as they take advantage of the reduced 

competition. However, the trees will eventually reach a 

point where their annual radial increment is no longer 

increasing and may even decrease in relation to previous annual increments. If growth rates for 5 years are 

equal or less than those of the previous years, the PGRs 

for the partially cut plots probably become 1.0 or <1.0 

so the stands would be considered susceptible. Thus, if 

MPB infestations do not develop in partially cut stands 

despite <1.0 PGRs, Mahoney’s system wouldn’t appear 

applicable to managed stands.

While a specific growth rate threshold is not available, Amman (1989) suggests that radial growth rates 

of 0.02 to 0.04 inches/yr (0.20 to 0.40 inches/10 yr) 

are too small to reduce MPB-caused tree mortality in 

partially cut LP stands. If such rates are characteristic 

of susceptible stands, then the control stands in our five 

locations would be susceptible (tables 4-8) and most of 

the partially cut stands except the the KOA GSLs and 

some GSL 120s would be nonsusceptible. However, 

within all the partially cut stands except the CSF GSL 40 

and GSL 80, diameter growth rates <0.50 inches per 12 

to 13 years (radial growth = 0.25 inches) were evident 

in some susceptible-size trees. While most of the trees 

are exhibiting nonsusceptible growth rates, some trees, 

especially those in low diameter classes, are exhibiting 

susceptible growth rates and, therefore, may function as 

primary focus trees. Future inventories of these plots 

will determine if trees with low growth rates become the 

primary focus trees for the MPB.

Large diameter trees are usually the first trees in a 

stand to be attacked by the MPB (Cole and Amman 

1969, Mitchell & Preisler 1991). Whether such trees 

are attacked because (1) they represent a visual size 

attractive to the MPB (Shepherd 1966), or (2) they 

have low growth rates and therefore issue some physiologically susceptible signal, or (3) size, growth rate, 

and physiological condition act in unison, remains to 

be determined. Future inventories of these plots could 

also determine whether diameter, growth rate, or a 

combination of diameter, growth rate, and physiological 

condition plays the most important role in creating the 

primary focus tree. If diameter growth is more important 

than tree diameter for attracting attacking MPBs, then 

trees of all diameters should be attacked because highly 

susceptible trees are found in a wide range of diameter 

classes.

Basal Area Growth

Basal area increased in all plots in the five locations 

except in the BCRK, DPEAK, and NSAV GSL 40s, and 

the DPEAK GSL 100 (table 9). BA growth generally 

appeared greater in the partially cut plots than in their 

respective controls (table 9). Apparent increases in BA 

were greatest in either the GSL 80 or GSL 120 plots. The 

largest numerical increase in basal area was in the CSF 

GSL 80, probably because this plot was superimposed 

on a stand that had been previously cut (table 9).

Total BA decreased in the BCRK, DPEAK, and 

NSAV GSL 40s where 80% or more of the original 

BA was removed because a substantial number of the 

residual trees were windthrown. Removal of more than 

30% of the original BA can cause significant loss in the 

residual stand because the residual trees lack windfirmness and are easily toppled (Alexander 1972). The CSF 

GSL 40 did not suffer substantial windthrow because 

this stand had been cut to about BA 80 before we superimposed a GSL 40 on it. Apparently, a GSL 40 can be 

developed if an uncut stand is initially cut to a BA >80, 

allowed to grow at that level for 10 to 20 years, and then 

cut to a GSL 40. 

The DPEAK GSL 100 was the only partially cut plot 

of higher stocking that did not exhibit an increase in BA. 

This plot has an Armillaria pocket in the CIP and had 

lost 12 trees to Armillaria by July 1997. Trees were also 

lost to windthrow and scolytids which may have been 

directly related to the Armillaria presence. The loss in 

BA from these causes was sufficient enough to negate 

increases in BA in the healthy trees. 

MPB Susceptibility Estimates

Amman et al. (1977) method. Based on the Amman 

et al. (1977) stand susceptibility system, all of the plots 

at the Wyoming locations rated moderate in susceptibility when the plots were installed, after cutting, and at 

remeasurement (table 10). Some DPEAK and NSAV 

plots might have been ranked high rather than moderate 

because the values for the latitude-elevation factor for 
10 USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 

Table 9—Unadjusted and adjusted periodic annual increment (PAI) in basal area by growing stock level (GSL). Unadjusted 

values represent the average increment per year between the BA after marking and the BA at remeasurement and assumes 

the plots were cut immediately after installation. Adjusted values represent the average increment per year between the BA 

after marking and BA at remeasurement with the BA after marking adjusted for growth in the uncut condition (see Methods 

section for how the adjustments were made).

Basal area PAI

Location After At

/GSL Installation marking remeasurement Unadjusted Adjusted

----------------------------------------------ft2 per acre-----------------------------------------

Brush Creek (WY)

GSL 40 223.6 40.3 37.1 -0.27 -0.35

GSL 60 170.2 60.7 73.8 1.09 1.26

GSL 80 216.0 81.2 94.4 1.10 1.27

GSL 120 214.3 116.1 131.9 1.32 1.50

Control 224.1 224.1 232.9 0.73 NAa

Colorado State Forest (CO) 

GSL 40 81.8 36.6 53.2 1.28 1.35

GSL 80 88.6 75.2 100.3 1.93 2.03

GSL 120 181.6 113.7 133.7 1.54 1.62

Control 175.9 175.9 186.4 0.81 NA

Divide Peak (WY)

GSL 40 222.6 44.9 30.4 -1.12 -1.33

GSL 100 229.7 98.9 99.0 0.01 -0.01

GSL 120 245.0 122.9 128.9 0.46 0.52

Control 197.4 197.4 200.4 0.23 NA

KOA (CO)

GSL 80 135.9 76.4 91.4 1.25 1.47

GSL 100 136.9 97.9 114.2 1.36 1.55

GSL 120 132.9 113.8 125.6 0.98 1.00

Control 138.4 138.4 151.3 1.08 NA

North Savery (WY)

GSL 40 218.1 40.6 39.0 -0.12 -0.17

GSL 80 273.1 86.8 105.3 1.42 1.83

GSL 120 267.3 121.6 142.9 1.64 2.11

Control 262.6 262.6 266.0 0.28 NA

a

 NA equals not applicable.

these plots intersected near the line separating the moderate risk from the high risk but we conservatively chose 

this risk factor as moderate. 

Similarly, about one-half of the Colorado plots rated 

moderate while the other half rated high at the time 

of plot installation (table 10). Most of the Colorado 

plots rated high after cutting and at remeasurement 

(table 10). The rating changed for the partially cut plots 

because average diameter became >8 inches after cutting (table 3) as the result of our discrimination against 

smaller diameter trees. The Colorado plots rated higher 

than the Wyoming plots because the elevation-latitude 

factor rated high rather than moderate.

If the susceptibility ratings under the Amman et al. 

(1977) system accurately portray the susceptibility of 

the various GSLs, then partial cutting does not modify 

stand susceptibility in LP stands where the average 

age >80 years, the average DBH >8 inches, and larger 

diameter trees are selected as leave trees. This conclusion differs with results from partial cutting in ponderosa 

pine stands where partial cutting appears to lower susceptibility even though larger diameter trees are retained 

(see Schmid and Mata 1992).

Shore & Safranyik (1992) method. Based on the 

Shore and Safranyik (1992) susceptibility rating system, 

the stand ratings ranged from 14 to 70 at installation 

(table 11). None of the GSLs rated >70 because they 

either had <100% of the stand in trees >6 inches, the 

density was below 751 stems per ha, the location factor 

was not optimal, or a combination of less than optimal 

ratings for all three of these factors. 

After marking and at remeasurement, the ratings 

for the partially cut GSLs ranged from 7 to 35 while 

the controls remained essentially the same (table 11). 

Even though the percentage of susceptible LP increased 

in most of the partially cut GSLs, the decrease in the 
USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 11

number of stems lowered the ratings for most of the 

partially cut GSLs. 

In contrast to the Amman et al.(1977) rating system, 

the Shore and Safranyik (1992) rating system indicates 

that stand susceptibility is substantially lowered by partial cutting. If their range for stand susceptibility (0 to 

100) is arbitrarily subdivided into low (0 to 35), moderate (36 to 70), and high (71 to 100) classes, the partially 

cut GSLs fall in the low susceptibility class. This occurs 

because the tree density and location factors are less 

than optimal for MPB infestation.

Susceptibility ratings for the partially cut GSLs are 

not likely to change significantly in next 20 years, and 

perhaps longer, under the Shore and Safranyik (1992) 

system. After partial cutting, the percentage of susceptible pine is near or at 100% because our design 

discriminated against the smaller diameter lodgepole 

and other species; therefore, the rating for this factor will not change. Unless the density factor is at 

or near threholds where it changes (primarily, 250 or 

750 stems per ha), the rating for this factor will also 

not change in the 20 to 40 years. If anything, the density factor will fall into a lower susceptibility class 

as individual trees die and tree density decreases. In 

40 or more years, the density factor could increase if 

ingrowth occurs. However, GENGYM estimates that 

GSL 80 stands will not have ingrowth for more than 

40 years and stands with greater GSLs may not have 

ingrowth, so the density factor will not increase significantly. Because the various susceptibility factors 

in the Shore and Safranyik method will not significantly change with future growth of the existing trees 

in the GSLs, the future susceptibility of the partially 

cut GSLs appears to remain essentially constant until 

substantial ingrowth occurs or the larger diameter 

trees die. This differs sharply with the situation in 

ponderosa pine wherein the susceptibility of partially 

cut stands with BAs >80 are thought to increase with 

BA growth on the residual stand (see Obedzinski et al. 

1999). However, because BA growth on residual trees 

is not part of the Shore and Safranyik system for LP, 

such growth does not influence future susceptibility. It 

remains to be seen if partially cut LP stands become 

more susceptible in the future as a result of growth on 

existing trees or the susceptibility remains unchanged as 

the Shore and Safranyik system predicts.

Table 10—Susceptibility ratings for each GSL plot based on the rating method of Amman et al. (1977).

At After After

Location/GSL installation marking remeasurement

Brush Creek (WY)

GSL 40 Moderate Moderate Moderate

GSL 60 Moderate Moderate Moderate

GSL 80 Moderate Moderate Moderate

GSL 120 Moderate Moderate Moderate

Control Moderate Moderate Moderate

Colorado State Forest (CO) 

GSL 40 Moderate High High

GSL 80 High High High

GSL 120 Moderate High High

Control Moderate Moderate High

Divide Peak (WY)

GSL 40 Moderate Moderate Moderate

GSL 100 Moderate Moderate Moderate

GSL 120 Moderate Moderate Moderate

Control Moderate Moderate Moderate

KOA (CO)

GSL 80 Moderate High High

GSL 100 High High High

GSL 120 High High High

Control Moderate Moderate High

North Savery (WY)

GSL 40 Moderate Moderate Moderate

GSL 80 Moderate Moderate Moderate

GSL 120 Moderate Moderate Moderate

Control Moderate Moderate Moderate
12 USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 

Anhold et al. (1996) method. Based on the Anhold 

et al. (1996) method, all stands except the CSF GSL 80 

rated low in susceptibility at installation because either 

the number of trees per acre was greater than the upper 

limit of the “zone” of high susceptibility or the quadratic 

mean diameter was <8 inches (table 12). The CSF GSL 

80 rated high susceptibility because the number of trees 

per acre fell within the “zone” of high susceptibility as a 

result of the plot being superimposed on a stand that had 

been partial cut prior to our establishment of the plot.

After cutting and at remeasurement, all GSL 40, GSL 

60, and control stands rated low in susceptibility. The 

partially cut GSL 40 and 60 stands rated low because the 

number of trees per acre fell below the lower limit of the 

“zone” of high susceptibility. Control stands rated low 

because the number of trees per acre remained greater 

than the upper limit of the “zone” of high susceptibility.

After cutting and at remeasurement, all GSL 80, 

GSL 100, and GSL 120 stands rated high because the 

number of trees per acre fell within the “zone” of high 

susceptibility (table 12). The high susceptibility ratings 

for the GSL 80 to GSL 120 stands is contrary to the results of partial cutting in LP wherein tree mortality (and 

presumably susceptibility) was reduced for stands with 

BA <120 (see McGregor et al. 1987) and in ponderosa 

pine wherein stands with GSL <120 were considered 

less susceptibile (see Schmid and Mata 1992). If the 

Anhold et al. method accurately estimates susceptibility 

in LP stands, then partial cutting to GSLs of 80 to 120 

would be unwise.

Straight-line & GENGYM projections. Straight-line 

projections (SLP) indicate that GSL 40 stands will take 

from 85 to more than 100 years to reach the BA threshold of 150 ft2

 per acre while GENGYM projections 

indicate the same GSL 40 stands will take from 68 to 

more than 100 years (table 13). Similar time periods are 

estimated for most of the GSL 40 stands with respect to 

the BA threshold of 120 ft2 per acre (table 13). However, 

the timeframes of >100 years for the BCRK, DPEAK, 

and NSAV GSL 40s (figures 1, 3, and 5 and table 13) are 

misleading. Because these GSLs lost trees to windthrow 

between when they were cut and when they were remeasured, the stocking levels decreased below the design 

levels. Therefore, both BA growth and total BA was 

much less than what might be expected if no windthrow 

had occurred. This causes the SLP method to predict the 

eventual demise of these stands (see figures 1, 3, and 

5) and the GENGYM method to predict shorter time 

Table 11—Susceptibility ratings for each GSL plot based on the rating method of Shore 

and Safranyik (1992).

At After After

Location/GSL installation marking remeasurement

Brush Creek (WY)

GSL 40 51 7 7

GSL 60 35 7 7

GSL 80 65 35 35

GSL 120 49 35 35

Control 67 67 68

Colorado State Forest (CO) 

GSL 40 14 15 15

GSL 80 15 15 15

GSL 120 27 15 15

Control 27 27 28

Divide Pk (WY)

GSL 40 67 7 7

GSL 100 70 35 35

GSL 120 68 35 35

Control 35 35 35

KOA (CO)

GSL 80 63 32 32

GSL 100 68 35 35

GSL 120 66 35 35

Control 65 65 67

North Savery (WY)

GSL 40 69 7 7

GSL 80 34 34 34

GSL 120 34 34 35

Control 67 67 69
USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 13

periods to reach the BA 150 level than will actually occur. When the BA growth and total BA are adjusted for 

this mortality (figures 1, 3, and 5, and table 13), the estimates are more realistic although they still project these 

GSL 40s to take >100 years to reach the two thresholds.

In contrast to the BCRK, DPEAK, and NSAV GSL 

40s, the CSF GSL 40 (figure 2) is estimated to reach the 

BA 150 threshold in 68 to 85 years and the 120 threshold 

in 49 to 62 years (figure 2 and table 13). Although these 

estimates represent more realistic estimates for the times 

required by a GSL 40 to reach the BA thresholds of 150 

and 120 ft2 per acre, they are also somewhat misleading. Because the CSF GSL 40 was superimposed on a 

previously thinned area, the growth rate may be greater 

than what would be expected after an initial partial cut 

in an unmanaged stand with BA >200 ft2/acre. Despite 

this, the timeframe for the CSF GSL 40 is probably more 

typical for a windfirm stand than the time frames for the 

windthrown GSL 40s. 

The time intervals for the GSL 80 plots to reach the 

BA 150 threshold ranged from 30 to 56 years (table 

13) with time estimates differing substantially between 

estimation methods for the BCRK and CSF GSL 80s. 

The longest time interval (55-56 years) occurred in the 

BCRK and KOA GSL 80s.

The difference between the SLP and GENGYM 

estimates for the BCRK GSL 80 is most likely caused 

by the tree mortality. Because this GSL 80 had 6% tree 

mortality, the SLP estimate deflected the projection to a 

longer time period and overestimated the time (56 years) 

to reach BA 150. Under the same conditions, GENGYM 

did not take into account tree mortality and applied the 

growth data to the stand as if no trees had died. This 

caused the GENGYM estimate to understate the time 

required to reach BA 150.

The difference between the two estimates for the 

CSF GSL 80 probably results from the compound effect of growth rate and the incorporation of that growth 

rate into the GENGYM model. While the CSF GSL 80 

had 1% mortality, the amount of mortality was probably 

an insignificant contributor to the difference in the two 

estimates. More importantly, the GSL 80 was superimposed on a stand that had been partially cut years 

before the plot was installed. The growth rates for 

the individual trees were greater than what might be 

expected from an uncut stand. When these growth 

rates are incorporated into the GENGYM model on 

a 10-year basis, the increase in BA is greater than 

for the SLP method. Thus, the GENGYM estimate is 

considerably less than the SLP estimate. 

Table 12—Susceptibility ratings for each GSL plot based on the rating method of Anhold et al. (1996).

At After After

Location/GSL installation marking remeasurement

Brush Creek (WY)

GSL 40 Low Low Low

GSL 60 Low Low Low

GSL 80 Low High High

GSL 120 Low High High

Control Low Low Low

Colorado State Forest (CO) 

GSL 40 Low Low Low

GSL 80 High High High

GSL 120 Low High High

Control Low Low Low

Divide Peak (WY)

GSL 40 Low Low Low

GSL 100 Low High High

GSL 120 Low High High

Control Low Low Low

KOA (CO)

GSL 80 Low High High

GSL 100 Low High High

GSL 120 Low High High

Control Low Low Low

North Savery (WY)

GSL 40 Low Low Low

GSL 80 Low High High

GSL 120 Low High High

Control Low Low Low
14 USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 

Table 13—Approximate number of years for various GSLs to reach the susceptibility thresholds of basal areas of 120 and 150 as derived from 

straight-line and GENGYM projections.

GSL 120 threshold

GSL 40 GSL 60 GSL 80 GSL 100 GSL 120 Control

Line GENGYM Line GENGYM Line GENGYM Line GENGYM Line GENGYM Line GENGYM

proj. proj. proj. proj. proj. proj.

Brush Creek >100 (>100)a >100 (>79)a 49 42 32 22 2 3 0 0

 (WY)

Colorado State 62 49 22 19 4 3 0 0

 Forest (CO)

Divide Peak >100 (>100) >100 (>73) 24 (>100) 26 (12) 0 0 0 0

 (WY)

KOA (CO) 32 33 13 21 4 6 0 0

North Savery >100 (>100) >100 (>100) 18 18 (19) 0 0 0 0

 (WY)

GSL 150 threshold

GSL 40 GSL 60 GSL 80 GSL 100 GSL 120 Control

Line GENGYM Line GENGYM Line GENGYM Line GENGYM Line GENGYM Line GENGYM

proj. proj. proj. proj. proj. proj.

Brush Creek >100 (>100) >100 (>100) 74 64 56 38 23 17 0 0

 (WY)

Colorado State 85 68 38 30 22 15 0 0

 Forest (CO)

Divide Peak >100 (>100) >100 (>100) 41 (>100) 43 (28) 50 22 (15) 0 0

 (WY)

KOA (CO) 55 54 34 46 33 29 11 11

North Savery >100 (>100) >100 (>100) 35 30 (33) 14 15 (14) 0 0

 (WY)

a

 For some GSLs, two time estimates are given because straight-line projections and GENGYM do not adjust for significant tree mortality evident in these GSLs 

subsequent to cutting, i.e., between cutting and remeasurement. The first time estimate represents the number of years wherein the estimate was adjusted 

for the mortality. The numbers in parentheses represent an estimate based on the unadjusted tree numbers. See figures 1-5.

Figure 1—Years required for the partially 

cut stands at the Brush Creek location 

to reach basal area (BA) susceptibility thresholds of 150 and 120 ft2

/acre 

based on straight-line projections. 

Because mortality caused the BA at 

remeasurement to be less than the BA 

after marking for the GSL 40, the initial 

straight-line projection indicates the demise of the stand. However, unless further mortality occurs, the trees existing 

at remeasurement will grow and BA will 

increase, the line below the “adj” represents the adjusted projection for the 

GSL when a BA growth rate is applied 

to the BA at remeasurement.
USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 15

Figure 2—Years required for the partially cut 

stands at the Colorado State Forest location to 

reach basal area (BA) susceptibility thresholds 

of 150 and 120 ft2/acre based on straight-line 

projections. 

Figure 3—Years required for the partially cut 

stands at the Divide Peak location to reach 

basal area (BA) susceptibility thresholds of 

150 and 120 ft2/acre based on straight-line projections. Because mortality caused the BA at 

remeasurement to be less than or equal to the 

BA after marking for the GSL 40 & GSL 100, 

the initial straight-line projections indicate the 

stands will not increase in BA. However, unless 

further mortality occurs, the trees existing at 

remeasurement will grow and BA will increase, 

the line below the “adj” represents the adjusted 

projection for the GSL when a BA growth rate is 

applied to the BA at remeasurement. 

Figure 4—Years required for the partially cut 

stands at the KOA location to reach basal 

area (BA) susceptibility thresholds of 150 

and 120 ft2/acre based on straight-line projections.
16 USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 

The NSAV GSL 80 attained BA 150 in 33 to 35 years 

(table 13) with the unadjusted SLP and GENGYM estimates almost identical. However, when the GENGYM 

estimate was corrected for the 5% tree mortality, its estimate is several years less than the SLP estimate (table 13). 

If the basal area threshold is lowered to 120 ft2, then 

the time interval for the different GSL 80s to reach 

that level ranges from 18 to 33 years depending on the 

method of estimation. Again, the BCRK and KOA GSL 

80s took the longest (32 years), while the other GSL 80s 

will take about 18 to 22 years depending on the method 

of estimation. The problems inherent in the estimates 

for the BCRK and KOA GSL 80s when the BA equaled 

150 are also present in the time estimates when the BA 

threhold equaled 120.

Comparing time intervals for LP GSL 80s with those 

for ponderosa pine GSL 80s (see Obedzinski et al. 1999), 

the LP GSL 80s will reach the basal area threshold of 

150 ft2 before the ponderosa pine GSL 80s do. Similarly, 

the LP GSL 80s will reach the threshold of 120 ft2 about 

the same time or sooner than ponderosa pine GSL 80s. 

This relationship probably results from higher site indicies for most of the LP GSLs than for the ponderosa 

pine GSLs, and a greater number of trees per acre on 

the LP sites than on the ponderosa pine plots. Thus, even 

though average DBHs on the LP sites are less than on 

the ponderosa pine sites, the greater site productivity 

coupled with the greater number of trees equates to a 

more rapid increase in BA on the LP sites. 

The time intervals for the GSL 120s to reach the BA 

threshold of 150 ft2 ranged from 14 to 50 years (table 

13) although the majority of the GSL 120s took <25 

years. The DPEAK SLP estimate of 50 years resulted 

from the 10% tree mortality which created a longer estimate when incorporated into the SLP. The KOA GSL 

120 is estimated to take 29 to 33 years to achieve the 150 

threshold (figure 4, table 13) probably because the average DBH was 8.5 and the basal area PAI was <1.

In comparison to the lone PP GSL 120 (see Obedzinski 

et al 1999), most of the LP GSL 120s achieved the 150 

threshold before the ponderosa pine GSL 120. As with 

the GSL 80s, the lesser time intervals for the LP GSL 

120s are probably attributable to greater site productivity and the greater number of trees per acre even though 

the trees are generally smaller than those on the comparable ponderosa pine site.

Management Implications

Although the partial cutting of unmanaged LP stands 

to a GSL 40 (or perhaps between GSL 40 and GSL 60) 

with one cutting may be desirable from a MPB management perspective, the option is undesirable because of 

windthrow risk. As Alexander (1972, 1974) suggests, the 

initial cutting in single-storied old-growth stands in low 

windfall risk situations could remove up to 30% of the 

BA in overstory trees. While stands in this study may not 

be considered old-growth, where more than 80% of the 

original BA was cut in the GSL 40 plots and Alexander’s 

recommendation was exceeded by about 50%, three of 

the four GSL 40s lost trees to windthrow and their stocking levels decreased below the designated level. Thus, 

creating a windfirm GSL 40 via a single partial cut in 

an unmanaged stand with BA >200 seems unlikely because >75% of the original BA is removed. However, if 

a GSL 40, or any GSL <60, is deemed desirable from a 

MPB management perspective and time is not a limiting factor, a windfirm GSL 40 might be achieved with 

Figure 5—Years required for the partially cut 

stands at the North Savery location to reach 

basal area (BA) susceptibility thresholds of 

150 and 120 ft2/acre based on straight-line 

projections. Because mortality caused the BA 

at remeasurement to be less than the BA after 

marking for the GSL 40, the initial straight-line 

projection indicates the demise of the stand. 

However, unless further mortality occurs, the 

trees existing at remeasurement will grow 

and BA will increase, the line below the “adj” 

represents the adjusted projection for the 

GSL when a BA growth rate is applied to the 

BA at remeasurement.
USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 17

two partial cuts in a manner similar to the cuttings in 

the CSF GSL 40. In that GSL 40, the unmanaged stand 

was initially cut to about GSL 80. After windfirmness 

developed, the stand was further cut to the GSL 40 and 

windthrow has not been evident since the second cutting. While the CSF GSL 40 may be an exceptional case, 

the silvicultural prescriptions creating it offer a possible 

means of achieving a windfirm GSL 40. 

In the reports of Amman (1989) and Gibson (1989), 

partial cutting reduced the susceptibility of LP stands to 

MPB infestations. How long the partially cut stands remain relatively nonsusceptible depends on the threshold 

for MPB infestation and the residual stocking level of 

the stand. If a BA 120 threshold for MPB infestation is 

valid for LP as it appears to be for the MPB in ponderosa pine (see Gibson’s 2002 personal communication, 

Schmid and Mata 1992, Schmid et al. 1994), then partial 

cutting an unmanaged stand to a GSL 120 will reduce 

stand density and improve tree growth but will still leave 

it in a susceptible condition. If the threshold for infestation is considered to be BA 150 and reentry into partially 

cut stands occurs about every 25 years, then most GSL 

120 stands may reach that level in <25 years (table 13), 

well before the scheduled reentry.

Stands cut to GSLs <100 should be less susceptible 

than GSL 120 stands but may enter the susceptible category rather quickly. Stands of GSL 80 may reach the BA 

120 level in <25 years (table 13), well before scheduled 

reentry unless tree mortality exceeds 5%. If the threshold for infestation is considered to be BA 150 and reentry occurs every 25 years, then most GSL 80 stands will 

probably take more than 30 years to reach this threshold 

or well after reentry. Thus, forest managers should be 

aware of the relationships between threshold levels for 

epidemic MPB populations, stand growth, and reentry 

intervals for partially cut stands.

The relationships between MPB infestation thresholds, stand densities, and reentry schedules does not 

mean that specific stands must be automatically reentered just because stand BAs have exceeded the BA 120 

threshold. While it is good management practice to do 

so, stands exceeding the threshold need not be reentered 

on the specific reentry date as long as managers increase 

their vigilance for MPB activity in such stands and are 

able to readily adjust their silvicultural activities if MPB 

infestations appear. Past epidemics indicate, however, 

that stands allowed to exceed the susceptibility threshold 

become the ignition points for subsequent MPB epidemics. Ignoring susceptible stands or leaving them until well 

beyond the normal reentry date invites a MPB epidemic.

Both the SLP and the GENGYM methods will give 

erroneous estimates of future stand conditions if the 

stands are subject to substantial mortality during the first 

10 years after cutting. The SLP method will exaggerate 

the time required to reach specific BA levels, or in some 

GSLs, indicate a stand heading toward nonexistence. 

How much the time estimate will deviate from the actual 

trend of stand conditions is influenced by the percent of 

tree mortality, the GSL, and the number of trees per acre 

in that GSL. In GSL 40s, mortality sufficient to lower the 

10-year BA to less than the original BA will cause the 

SLP method to predict the eventual demise of the stand 

as evidenced in the BCRK, DPEAK and NSAV GSL 40s 

(figures 1, 3, and 5). For greater GSLs, mortality rates 

may have to exceed 20% in order for the SLP method to 

indicate stand loss. In the DPEAK 100, a mortality rate 

of 17% yields a static condition for 100 years (figure 

3). Realistically, however, the existing GSL 40 stands 

will not be lost as the SLPs project unless the mortality 

continues. Instead they will continue to exist and grow, 

albeit at a rate unlikely to project them crossing the GSL 

120 threshold in less than 100 years.

In contrast to the SLP estimates, the time estimates 

from GENGYM will underestimate the actual time 

needed to reach the susceptibility thresholds when substantial mortality occurs. While minimal mortality is 

incorporated in the GENGYM equations, that mortality 

is substantially less than the mortality observed in the 

BCRK, DPEAK and NSAV GSL 40s. Because abnormal 

mortality is not incorporated into the original stand data 

and the growth information is applied to the stand data 

as if no trees had died, the time estimates are much less 

than the time actually needed. 

Neither method will provide an accurate estimate if 

a stand is subject to substantial mortality in the first 10 

years and adjustments are not made. Because both methods were strongly influenced by the degree of mortality 

observed in this study, they may be applicable only to 

situations where mortality is limited to the death of a 

few trees from competition or endemic MPB populations. Even then, the loss of 1 ft2

 of BA to MPB populations in a GSL 80 stand may alter the SLP estimate for 

crossing the GSL 150 threshold by 4 to 6 years and the 

GENGYM estimate by 3 years (Obedzinski et al. 1999). 

However, endemic MPB mortality may not be as important in LP stands as windthrow, especially like the 

windthrow observed in some of our GSL 40s. In several 

GSLs, windthrow decreased the stocking level and created SLP estimates that suggest an ever decreasing BA. 

While windthrow may decrease the stocking level in the 

short term, it seems unlikely that this would continue as 

the SLP projection forecasts. At some point, the residual 

trees would become windfirm and BA would start to increase. Thus, the SLP estimates would be less realistic 
18 USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 

than the GENGYM estimates. Until the influence of endemic MPB mortality and wind events are determined, 

forest managers should be cognizant that different stand 

growth scenarios can be derived from the two methods and 

both methods may provide inaccurate estimates if the stands 

are subject to substantial mortality soon after cutting. 

In marking unmanaged stands to specific stand densities in preparation for partial cutting, spacing was one 

of several factors considered in the selection of leave 

trees. On some occasions, spacing was reduced in order 

to leave a more desirable tree. Thus, although spacing 

between leave trees was generally uniform throughout 

most of a stand, some higher density clumps may have 

existed. If tree selection tends to leave clumps of trees, 

pockets of greater tree densities embedded in otherwise 

relatively homogeneous stands may be problematic in 

the future. As long as average diameters remain below 

8 to 9 inches, such stands may not be conducive to 

MPB infestation. As these stands develop, however, 

the susceptibility threshold may be exceeded within 

the pockets while most of the stand remains below the 

susceptibility threshold (i.e., a situation similar to the 

microcosm stands within unmanaged ponderosa pine 

stands as found by Olsen et al. 1996). As McGregor et 

al. (1987) noted, tree losses in partially cut stands were 

associated with unequal tree distributions, so relatively 

equal spacing needs to be maintained. Thus, managers 

should realize that the process of tree selection may create embedded pockets or microcosm stands of greater 

densities within a stand that may exhibit MPB infestations well before most of the stand reaches the susceptibility threshold.

In LP, large diameter trees are thought to be the first 

trees attacked by the MPB (Cole and Amman 1969) and 

are the most susceptible (Safranyik et al. 1974). If this is 

the case, then the susceptibility of the partially cut stands 

in this study may not have been altered substantially for 

the long term because larger diameter trees were generally retained as the residual trees while the smaller diameter trees were cut. As noted by McGregor et al. (1987), 

partial cutting may not be as effective in stands with 

many large diameter trees as in stands with smaller average diameters. If the MPB attacks these stands and kills 

a substantial portion of the trees in each stand regardless 

of its stand density, then partial cuts that favor larger diameter trees will not be a viable MPB management option for forest managers. If the MPB kills relatively few 

trees within the partially cut stands, then favoring large 

diameter trees in a partial cut is a viable option and the 

statement regarding beetle preference for large diameter 

trees may be applicable only when dealing with unmanaged stands.

Literature Cited

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the Central and Southern Rocky Mountains: the status 

of our knowledge. Res. Pap. RM-121. Fort Collins, CO: 

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

Mountain Forest and Range Experiment Station. 88 p.

Alexander, R. R., Edminster, C. B., 1981. Management of 

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Res. Pap. RM-228. Fort Collins, CO: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Forest and 

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Amman, G. D. 1989. Why partial cutting in lodgepole 

pine stands reduces losses to mountain pine beetle. In: 

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of lodgepole pine to minimize losses to the mountain pine 

beetle. Proceedings of a symposium: 1988 12-14 July; 

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Amman, G. D.; McGregor, M. D.; Cahill, D. B.; Klein, W. H. 

1977. Guidelines for reducing losses of lodgepole pine to 

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Mountains. Gen. Tech. Rep. INT-36. Ogden, UT: U.S. 

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Forest and Range Experiment Station. 19 p.

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Bentz, B. J.; Amman, G. D.; Logan, J. A. 1993. A critical 

assessment of risk classification systems for the mountain 

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Berryman, A. A. 1978. A synoptic model of the lodgepole 

pine/mountain pine beetle interaction and its potential 

application in forest management. In: Berryman, A. A.; 

Amman, G. D.; Stark, R. W., tech. eds. Theory and practice 

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forests. Proceedings of a symposium; 1978 25-27 April; 

Pullman, WA. Moscow, ID: University of Idaho: 98-105.

Cole, W. E., Amman, G. D. 1969. Mountain pine beetle 

infestations in relationship to lodgepole pine diameters. 

Res. Note INT-95. Ogden, UT: U. S. Department of 

Agriculture, Forest Service, Intermountain Forest and 

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Edminster, C. E. 1978. RMYLD: computation of yield tables 

for even-aged and two-storied stands. Res. Pap. RM-199. 

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M.; Olsen, W. K.; Hawksworth, F. G. 1991. GENGYM: a 

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USDA Forest Service Res. Pap. RMRS-RP-42. 2003. 19

Gibson, K. E. 1989. Partial cutting (sanitation thinning) to reduce 

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rating ponderosa pine stands for mountain pine beetles 

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