USDA United States 

=- Department of 

Agriculture 

Forest Service 

Rocky Mountain 

Research Station 

Growth of Ponderosa Pine Stands 

in Relation to 

Mountain Pine Beetle Susceptibility 

General Technical 

Report RMRS-GTR-28 

April 1999 

R.A. Obedzinski 

J.M. Schmid 

S.A. Mata 

W.K. Olsen 

R.R. Kessler 

This file was created by scanning the printed publication.

Errors identified by the software have been corrected;

however, some errors may remain.
h 

Abstract 

Obedzinski, R.A.; Schmid, J.M.; Mata, S.A.; Olsen, W.K.; Kessler, R.R. 1999. Growth of ponderosa 

pine stands in relation to mountain pine beetle susceptibility. Gen. Tech. Rep. RMRS-GTR-28. 

Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 

13 p. 

Ten-year diameter and basal area growth were determined for partially cut stands at 4 locations. Average 

diameters in the partially cut plots generally increased by 1 inch or more, while average diameter in the uncut 

controls increased by 0.9 inches or less. Individual tree growth is discussed in relation to potential 

susceptibility to mountain pine beetle infestation. Basal area increases ranged from 0.9 to 1.9 ft2/acre/ 

yr in partially cut plots, while basal area increases in the control plots ranged from 0.4 to 1.4 ft2/acre/yr. 

Endemic mountain pine beetle infestations and snow breakage accounted for most of the mortality on the 

plots, which decreased the residual basal area and basal area growth. Increases in basal area are used 

to estimate the length of time required for various stand densities to reach the susceptibility thresholds for 

mountain pine beetle infestation. Stand marking may influence future susceptibility to beetle infestations. 

Keywords: Ponderosa pine, basal area growth, diameter growth, mountain pine beetle, stocking levels 

The Authors - - 

R.A. Obedzinski is a Silviculturist, Fremont National Forest, Lakeview, Oregon 97630. 

J.M. Schmid is an Entomologist (retired) Rocky Mountain Forest and Range Experiment Station, 

Fort Collins, Colorado 80526. 

S.A. Mata is a Forestry Technician, Rocky Mountain Forest and Range Experiment Station, 

Fort Collins, Colorado 80526. 

W.K. Olsen is a Forestry Consultant, Advanced Forestry, 247 Falls Creek Drive, Bellvue, Colorado 8051 2. 

R.R. Kessler is a Consulting Forester, Route 1, Box 50B, Custer, South Dakota 57730. 

You may order additional copies of this publication by sending your mailing 

information in label form through one of the following media. Please specify 

the publication title and General Technical Report number. 

Fort Collins Service Center 

Telephone (970) 498-1 71 9 

FAX (970) 498-1 660 

E-mail rschneider/rmrs@fs.fed.us 

Web site http://www.xmission.com/-rmrs 

Mailing Address Publications Distribution 

Rocky Mountain Research Station 

3825 E. Mulberry Street 

Fort Collins, CO 80524 

Cover: Unthinned stands (top) usually infested by the mountain pine beetle, although thinned stands 

(bottom) are less susceptible. (Photos from the Black Hills, SD.) 
Growth of Ponderosa Pine Stands in Relation 

to Mountain Pine Beetle Susceptibility 

RIAm Obedzinski 

JIM. Schmid 

SIAm Mata 

WmKm Olsen 

R.R. Kessler 

Contents 

Introduction .................................................................................................................. 

Methods ...................................................................................................................... 

Results and Discussion ............................................................................................... 

Diameter Growth ............................................................................................ 

Basal Area Growth ......................................................................................... 

MPB Susceptibility Estimates ......................................................................... 

Management Implications ........................................................................................... 

Literature Cited ............................................................................................................ 

Introduction 

Knowledge of tree growth under various stand and 

site conditions is essential for managing ponderosa 

pine, Pinus ponderosa Lawson, stands. Such knowledge is absolutely necessary for the development of 

growth and yield models that are used to predict 

future outcomes for different silvicultural scenarios. 

This knowledge is also useful fbr predicting when 

partially cut stands may become susceptible to infestation by the mountain pine beetle (MPB), 

Dendroctonus ponderosae Hopkins. 

High susceptibility of ponderosa pine stands has 

been associated with basal areas ,150 ft2/acre 

(Sartwell and Stevens 1975), but recent evidence 

suggests that the basal area threshold for high susceptibility may be as low as 120 ft2/acre (Schmid and 

Mata 1992). Lacking actual growth data for partially 

cut stands of susceptible size, Schmid (1987) used a 

growth and yield program for even-aged stands 

called RMYLD (see Edminster 1978) to estimate 

when partially cut ponderosa pine stands of specific 

densities would reach basal area of 150 ft2/acre. Based 

on the number of trees in each experimental stand 

and the site indices for the stands, RMYLD estimated 

that a ponderosa pine stand with basal area of 60 

took 76 yr, a stand with basal area of 80 took 51 yr, 

and a stand with basal area of 100 required 37 yr to 

reach basal areas equal to 150 ft2/acre (Schmid 1987). 

This paper reports on the 10-yr diameter and basal 

area growth of partially cut stands at 4 locations in 

the Black Hills of South Dakota. The results are 

discussed in relation to future susceptibility of ponderosa pine stands to MPB populations, future management schedules for keeping stands below the high 

susceptibility threshold, and the microcosm stand 

concept of Olsen et al. (1996). 

Methods 

During a study regarding the relationship between 

stand density and MPB-caused tree mortality, sets of 

growing stock level (GSL) plots were installed in 

susceptible-size ponderosa pine stands in the Black 

Hills of South Dakota (Schmid et al. 1994). Each set 

typically consisted of 4 2.47-acre plots; 3 plots partially cut to different GSLs and the fourth plot left 

uncut to serve as a control. Leave trees within cut plots 

were selected on the basis of diameter, spacing, crown 

development, and visually apparent good health. 

Tree selection emphasized leaving the best and largest trees as evenly spaced as possible. The marlung 

crew tried to leave the basal area at k1 of the designated level (i.e., a GSL 80 stand would be between 79 

and 81). 

When the plots were installed, the ce*tral 1.25 

acres of each plot were designated as the central 

inventory plot (CIP). Diameter at breast height 

(DBH) was measured on all trees within the CIP. Tree 

diameters and information on the presence or absence 

of MPB attacks, crown form, defects, and diseases 

were recorded. Metal tags were placed 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 basal area and diameter 

growth statements for the 2.47-acre plot. 

Trees in the plots were also examined for the presence of MPB attacks during the annual surveys after 

cutting. Trees with MPB attacks were classified as 

successfully attacked or pitchout (a tree that has 

external evidence of MPB attacks but usually survives 

the attacks); the trees were examined the following 

year to verify the classification. 

In 1985, the Brownsville plots (BRN) were installed in the northern Black Hills about 9 miles 

southeast of Deadwood. Three plots were partially cut 

to GSL 60, GSL 80, and GSL 100 in May 1986, while 

the fourth plot was left uncut as the control at GSL 

146. Site index for the GSL 60 was 64, the GSL 80 was 

58, the GSL 100 was 56, and for the control was 68. 

The plots were remeasured in September 1990 and 

September 1985. In 1995,l tree in the GSL 60 and 

1 tree in the GSL 80 had been attacked by MPBs. 

These trees were included in computations because 

they were not dead when the plots were inventoried. 

In 1985, the Crook Mountain plots (CRK) were 

installed in the northern Black Hills about 6 miles 

northeast of Deadwood. Three plots were cut to GSL 

80, GSL 100, and GSL 120 in December 1986, while 

the fourth plot was left uncut at GSL 158. Site index 

for the GSL 80 was 71, the GSL 100 was 72, the GSL 

120 was 66, and for the control was 69. Loggers 

mistakenly cut 1 leave tree in the GSL 120 plot but 

also left trees in the GSL 80 and GSL 100 plots. This 

increased the GSLs in these 2 plots to more than the 

designed GSL +l; specifically, to GSL 84.8 and 104.5. 

This paper will refer to these plots as GSL 80 and GSL 

100 although stocking levels were above the design 

level. The GSL 80 plot sustained MPB-caused mortality of3 trees, the GSL 100 had 4 MPB-killed trees, 

and the GSL 120 plot sustained 1 MPB-killed tree 

within their CIPs in 1985 after they were inventoried, 

but before they were cut. The MPB-attacked trees 

were not cut by the loggers because they were marked 

as leave trees. Beetle activity ceased after the cutting 

and was not evident until the 1996 inventory. The 

plots were not remeasured after 5 yr, but they were 

remeasured for 10-yr growth on September 4, 1996. 

In 1985, a 2.47-acre plot (C-C plot) was established 

in a ponderosa pine stand about 8.5 miles south of 

Deadwood. The stand and surrounding area had been 

thinned under a commercial timber sale some years 

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-28. 1999 
prior. The Black Hills National Forest (BHNF) normally thins to a basal area of about 80 ft2/acre but 

permits the basal area of leave trees to be k10 ft2 of 

the designated level (for example, 80 +lo). The plot's 

basal area at the time of plot installation was about 

83 ft2/acre. Considering basal area growth rates 

and the BHNF's marking guideline of 80 f 10 ft2, the 

basal area after thinning was probably below 80 ft2 

and closer to 70 ft2/acre. In addition, tree densities 

within different parts of the plot were not as uniform 

as they are in the Brownsville, Border, and Crook 

Mountain plots because the basal area in subsections of the plot ranged from 66 to 100 ft2/acre. 

All trees within the 2.47-acre plot were inventoried in 

September 1985 and again on May 16, 1996. One 

1984 MPB-attacked and 4 1985 MPB-attacked trees 

were present when the plot was inventoried in 1985. 

The C-C plot was about 0.5 mile north of the BRN 

plots, which allowed evaluation of MPB infestations and growth under different levels of marking 

precision (i.e., BA 80+10 of this plot versus the BA 

80+1 of the BRN GSL 80). Because the plot was within 

a large thinning area and no uncut stands were available in the immediate vicinity, a control plot was not 

installed. 

In 1986, the Border plots (BOR) were installed 

along the Wyoming-South Dakota border on Schoolhouse Gulch Road about 6.5 miles southwest of Savoy, 

South Dakota. Three of the plots were cut to GSLs of 

60, 80, and 100 in July 1987 while the fourth plot 

was left uncut at GSL 207. Site index for the GSL 60 

was 73, for GSL 80 was 71, for GSL 100 was 76, and 

for the control was 71. The GSL 60 and GSL 80 had 

been partially cut some years prior so their precutting GSLs were in the 140 to 160 range, whereas the 

GSL 100 and the control had original GSLs of over 

200. The plots and stands in the area surrounding the 

plots were subject to a wet snow accompanied by 

strong winds in March or April 1996. Although 

windthrown and broken-top trees were present in 

the cut plots, especially the GSL 100, none of the 

damaged trees within the plots were salvaged. The 

plots were remeasured on August 26, 1997. 

Diameter growth was based on trees within the 

CIP and was calculated for only those trees alive at 

the time of remeasurement. The diameter growth of 

each tree was calculated by subtractingits diameter at 

the time the plots were installed from its diameter 

at the time of remeasurement. 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 measurement error and 

assigned such trees zero growth. Trees dying during 

the measurement period were not included in the 

calculations of mean diameter growth. 

Basal area values for the partially cut GSLs at 

the 4 locations were used to estimate when each GSL 

might reach the MPB-susceptibility thresholds of 

150 and 120 ft2/acre. For each plot, the basal area at 

the time when the plots were installed was considered the initial basal area when time equaled zero; 

the basal area approximately 10 yr later was considered the basal area 10 yr after cutting. Trees that were 

dead when the plots were remeasured were not included in the 10-yr computations ofbasal area. Straight 

lines were drawn through the 2 points in the respective figures, and extended until they intercepted the 

120 and 150 basal area values. 

As noted, the CRK and C-C plots had live but MPBattacked trees at the time of inventory or cutting. If 

the basal area for these trees is incorporated into the 

initial basal area for the plots, straight-line projections for the plots would be misrepresented because 

these trees did not grow during the years following 

MPB attack. Thus, the basal area associated with the 

MPB-attacked trees should be withdrawn from the 

computations. However, these trees were generally 

alive at the time of initial inventory so their basal area 

can be considered part of the initial basal area and 

incorporated into the computations. Because comparison of these 2 situations provides insight into the 

influence of endemic MPB-caused mortality on stand 

growth and basal area growth, susceptibility estimates are presented and discussed for both situations. 

Straight-line projections may underestimate the 

time required to reach the susceptibility thresholds 

because they do not include subsequent tree mortality. To determine if our projections differed substantially from those derived from a growth and yield 

model, we compared the predictions from the straightline projections to those of GENGYM. GENGYM is a 

computerized growth and yield model used for projecting future stand conditions in ponderosa pine 

stands (Edminster et al. 1990). GENGYM supercedes 

RMYLD, which was used by Schmid (1987) to estimate the time for stands in the Brownsville plots to 

reach susceptibility thresholds. 

Results and Discussion 

Diameter Growth 

Mean diameter increased in all plots; even those 

with MPB-caused tree mortality (table 1). Mean diameter growth in most of the cut plots generally 

exceeded 1 inch during the first 10 yr, while mean 

diameters in the control plots grew ~0.9 inch (table 1). 

Brownsville. Average growth rates by diameter class 

were very similar among diameter classes within each 

cut plot at BRN except for the smallest and largest 

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-28. 1999 
Table 1-Periodic annual increment (P.A.I.) in mean diameter in inches by growing stock level 

(GSL). 

Mean diameter P.A.I. 

GSL~ Start 5 Yr 10 yr 5 Yr 10 yr 

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

BRN 60 12.4 13.1 13.8 0.14 0.1 4 

BOR 60 10.9 NA~ 13.1 N A 0.22 

BRN 80 11.5 12.0 12.5 0.1 0 0.1 0 

BOR 80 10.8 N A 12.9 N A 0.21 

CRK 80W 13.7 N A 15.3 N A 0.1 6 

CRK 80W0 13.7 NA 15.5 N A 0.1 8 

C-C 80 W 

C-C 80 WO 

BRN 100 

BOR 100 

CRK l00W 

CRK 100 WO 

CRK 120W 

CRK 120 WO 

BRN Control 

BOR Control 

CRK Control 12.6 N A 13.3 N A 0.07 

a~~~ = Brownsville plots, BOR = Border plots, CRK = Crook Mtn. plots, C-C = Black Hills National Forest 

standard thinning plot. GSLs followed by a W (with) have MPB-killed trees incorporated into the diameter 

values, while those followed by a WO (without) have these trees withdrawn. 

b~~ = data unavailable. 

classes where the diameter class was represented by the 10-yr growth within the diameter classes also 

only 1 or 2 trees (table 2). Within the control, average decreased from the GSL 60 to the control (table 3). 

growth rates increased withincreasing diameter class. Except for the lone 9 inch tree (table 2), the smallest 

Among GSLs, the mean growth rate within each 10-yr growth for trees in the other diameter classes in 

diameter class decreased from the GSL 60 to the the GSL 60 was 0.6 inch and the largest was 2.2 inches 

control (table 2). The smallest and largest values for (table 3); the majority of the trees grew 1 inch or more. 

Table 2-Mean diameter growth (XfSD) in inches by 1 -inch diameter class for different 

GSLs at Brownsville (BRN) based on 10-yr growth data. 

Diameter GSL 60 GSL 80 GSL 100 Control 

al tree sample \ 

b2 tree sample 

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-28. 1999 
Table 3-Range of 10-yr growth rates by diameter class for different GSLs at 

Brownsville (BRN). 

Diameter GSL 60 GSL 80 GSL 100 Control 

--- 

a~ zero value for the low end of the range indicates that the diameter of at least 1 tree did not 

change measurably. 

In contrast, no measurable growth or growth of only 

0.1 inch.110-yr was common in diameter classes in 

the BRN control. The largest 10-yr growth for any 

tree in the control was 1.7 inches, but the majority of 

trees in all diameter classes grew less than 1 inch. 

Crook Mountain. Average growth rates by diameter 

class were similar among diameter classes within the 

CRK cut plots except for those classes represented by 

only 1 or 2 trees (table 4). Average growth rates within 

the control increased as diameter class increased. 

Among GSLs, mean growth rate within each diameter 

class was greatest in the GSL 80 and lowest in the 

control (table 4). Although mean growth rate within 

the same diameter class decreased from the GSL 80 

to the control in several diameter classes, this trend 

was not as uniform in all CRK diameter classes as it 

was in the BRN plots (tables 3 and 5). 

Three growth characteristics for the CRK plots contrast sharply with the same characteristics for the 

BRN plots. Some trees in each of the CRK plots grew 

more than 2.0 inches in diameter in 10 yr (table 5) 

whereas trees growing more than 2 inches in the 

BRN plots were confined to the BRN GSL 60 and 80 

(table 3). Similarly, some trees in each of the CRK 

plots showed no measurable increase in growth or 

Table 4-Mean diameter growth (Xf SD) in inches by 1 -inch diameter class for different 

GSLs at Crook Mountain (CRK) based on 10-yr growth data. 

Diameter GSL 80 GSL 100 GSL 120 Control 

-- - 

\ 

al tree sample 

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-28. 1999 
Table 5-Range of 10-yr growth rates by diameter class for different GSLs at Crook 

Mountain (CRK). 

Diameter GSL 80 GSL 100 GSL 120 Control 

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

a~ zero value for the low end of the range indicates that the diameter of at least 1 tree did not 

change measurably. Ranges are not presented for diameter classes represented by only 1 tree. 

grew only 0.1 inches in 10 yr (table 5) whereas trees 

with no measurable growth in the BRN plots were 

mainly confined to the BRN control. Within the same 

diameter class at BRN, the upper and lower ends of 

the range in growth generally declined from the lowest 

GSL to the highest GSL but this trend was not 

consistent among all the diameter classes in the CRK 

GSLs (tables 3 and 5). An explanation is not readily 

evident, but the CRK plots have a greater number of 

diameter classes in respective GSLs, which indicates 

that the CRK stands were more diverse than the 

relatively homogeneous BRN stands. Whether this 

diversity led to more variability in diameter growth 

is unknown. 

C-C. Average growth rates increased with increased 

diameter in the C-C plot (table 6). Diameter growth 

rates for trees >10 inches were similar to those for 

the BRN GSL 80, which is about 0.5 miles away. The 

range in diameter growth by diameter class in the 

C-C plot was also similar to that in the BRN GSL 80 

except that the C-C plot had more diameter classes 

with no measurable increase (tables 3 and 6). 

Table 6-Range of 10-yr growth rates and mean diameter 

growth by diameter class for the Black Hills National 

Forest standard thinning plot (C-C). 

Diameter class Range Mean + SD 

dl tree sample 

b~ zero value for the low end of the range indicates that the diameter 

of at least 1 tree did not change measurably. 

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-28. 1999 
Table 7-Mean diameter growth (XfSD) in inches by 1 -inch diameter class for different 

GSLs at the Border location (BOR) based on 10-yr growth data. 

Diameter GSL 60 GSL 80 GSL 100 Control 

al tree sample 

Border. Average diameter growth rates were similar 

among diameter classes within each partially cut 

BOR plot but increased with increased diameter in the 

BOR control (table 7). This result corresponds to that 

in the BRN and CRK plots. Nearly all trees in the cut 

plots grew at least 0.5 inches or more, while some 

trees in 8 of the diameter classes in the BOR control 

showed no measurable growth (table 8). Within a 

specific diameter class, the high and low ends of the 

range decreased from the GSL 80 to the control. These 

changes in the diameter growth ranges for the BOR 

plots also followed a pattern similar to ranges for the 

BRN and CRL plots. 

What do these growth rates mean in regard to 

individual tree susceptibility and what trees become 

primary focus trees for the MPB in unmanaged and 

managed stands? In previous work on White House 

Gulch plots in the Black Hills, Olsen et al. (1996) noted 

that high density clumps of trees exhibiting a range of 

diameters existed within a relatively homogeneous 

unmanaged stand. These clumps, which they called 

microcosm stands, contained some trees disadvantaged by competition and disease. Growth rates for 

these trees were probably low regardless of their 

diameter. Because of their physiological condition, 

these trees are more susceptible to MPB attack and 

are more likely to function as primary focus trees. 

Table 8-Range of 10-yr growth rates by diameter class for different GSLs at the 

Border location (BOR). 

Diameter GSL 60 GSL 80 GSL 100 Control 

a~ zero value for the low end of the range indicates that the diameter of at least 1 tree did not 

change measurably. Ranges are not presented for diameter classes represented by only 1 tree. 

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-28. 1999 
Within the control plots at BRN, CRK and BOR, 

some trees within all diameter classes exhibited 

minimal or no measurable growth over the 10-yr 

period (tables 3,5, and 8). Thus, if diameter growth in 

these plots is an indicator of susceptibility for trees in 

unmanaged stands, highly susceptible trees are 

found in a wide range of diameter classes. 

In managed stands represented by the cut plots, 

competition is less noticeable because the number of 

diameter classes having minimal or no measurable 

growth is less frequent than in the unmanaged 

stands. In addition, the average rate of diameter 

growth is similar among the diameter classes in each 

GSL, whereas it increased as diameter increased in 

the unmanaged stands (tables 2,4, and 7). However, 

comparison of the average growth rates in the 

various diameter classes in plots with the lowest 

stocking to those of plots with greater stocking at the 

same location indicates that competition may be occurring in the higher density cut plots. For example, 

comparison of the growth rates in the BRN GSL 60 to 

the growth rates in the respective diameter classes in 

the other BRN GSLs (table 2) indicates that trees in 

the GSL 80 and GSL 100 are apparently competing 

even though partial cutting increased spacing. As the 

stockmg level increases from the GSL 80 to the GSL 

100, average diameter growth is reduced further as 

spacing is reduced. Although this trend is less evident 

in the CRK and BOR plots, similar relationships 

appear to exist (tables 4 and 7). In general, competition increases as stand density increases (as GSL 

increases). 

More importantly, 10-yr growth rates for individual trees in the cut plots at BRN, CRK, C-C, and 

BOR show considerable variability within all diameter classes (tables 3, 5, 6, and 8), as they did in the 

control plots, although the number of trees with minimal increases in diameter is less than that in the 

unmanaged situation. Despite the increased spacing 

and decreased competition, some trees in most diameter classes grew rather poorly and thus, may be more 

susceptible to MPB attack. Future inventories of 

these plots representing managed stands will determine if trees with low growth rates become the primary focus trees for the MPB and if the higher density 

areas within the C-C plot become the microcosm stands 

as suggested by Olsen et al. (1996). 

Situations within the BRN and C-C plots provide 

contrasting arguments for and against the Olsen et al. 

hypothesis. First, 3 trees in the BRN cut plots were 

attacked by MPBs in the summers of 1995 and 1996. 

One tree grew 0.8 for the 10 yr. This tree grew 0.6 

inches during the first 5 yr but only 0.2 inches in the 

second 5 yr. Secondly, MPB-caused mortality in the 

C-C plot generally occurred in subsections of the plot 

with higher stocking levels. These 2 results support 

the Olsen et al. hypothesis. 

In contrast, the evidence from the other 2 MPBattacked trees in the BRN GSL 80 is not supportive 

because both grew over 1.2 inches for the 10 yr, grew 

relatively well in the second 5 yr, and neither had 

evidence of Armillaria in the bole at ground level. 

Growth rate did not apparently influence MPB attacks on these trees. Concurrent with these 2 MPBattacked trees, a tree in the BRN GSL 60 died in 1996 

with evidence of Armillaria and Ips but without evidence of MPB (J.M. Schmid 1996, personal observation). During the first 5 yr of growth after partial 

cutting, this tree grew 0.7 inches but thereafter, 

measurable growth was not evident. The evidence 

suggests that Armillaria invaded the root systems 

about 5 yr into the 10-yr growth period and sharply 

curtailed growth during the next 5 yr. Presumably, 

this tree would be highly susceptible to MPB attack if 

growth rate is an indicator of susceptibility. Why 

the tree was not attacked is not readily explainable 

except that MPB populations in the vicinity of the 

BRN plots were endemic and the 1995 attacks on 

other trees just outside the plots may have drawn 

beetles away from this tree. This suggests that every 

predisposed or more susceptible tree may not always 

be attacked by the MPB because endemic MPB populations may be insufficient to attack them. Such trees 

may die from the effects of the Armillaria infection, 

Ips attacks, or other agents before MPBs attack. 

Olsen et al. (1966) suggested that large diameter 

ponderosa pine trees are not always the primary 

focus trees. Although the numbers of MPB-killed 

trees in each of the CRK plots were small compared to 

the total number, the increases in mean diameter 

(table 1) indicates the MPB was not exclusively infesting the largest diameter trees. If only the largest 

diameter trees were being killed, mean diameter 

would not be appreciating at a rate similar to comparable noninfested plots. Selection of a range of tree 

diameters by the MPB further supports the claim by 

Olsen et al. (1996) that large diameter trees are not 

always the primary focus trees. 

Basal Area Growth 

Basal area increased in all plots in the 4 locations; 

MPB-caused tree mortality during the 10 yr was 

insufficient in those plots where it occurred to decrease total basal area below the initial level (table 9). 

Basal area increased uniformly in the BRN cut plots 

(ca. 13 ft2/acre/10 yr), while increases were more 

variable among the CRK and BOR cut plots (table 9). 

When MPB-caused mortality is withdrawn from the 

starting basal area in the CRK plots, basal area 

increased more in the CRK and BOR plots than in the 

BRN plots (table 9); probably because both locations 

were better sites. Basal area growth was about the 

same in the BRN and C-C GSL 80 plots when MPBcaused mortality is deleted from the C-C plot. 

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-28. 1999 
Table 9--Periodic annual increment (P.A.I.) in basal area by growing stock level (GSL). 

Basal area P.A.I. 

GSL~ Start 5 Yr 10 yr 5 Yr 10 yr 

-----------------------fiZperacre---------------------- 

BRN 60 60.5 66.8 73.6 1.26 1.31 

BOR 60 60.1 NA~ 78.3 N A 1.82 

BRN 80 80.8 86.9 93.8 1.22 1.30 

BOR 80 80.1 N A 97.7 NA 1.76 

CRK 80W 84.8 NA 100.2 N A 1.54 

CRK 80W0 82.2 N A 100.2 NA 1.80 

C-C 80 W 83.6 NA 92.9 N A 0.93 

C-C 80 WO 81.7 NA 92.9 NA 1.12 

BRN 100 100.7 106.6 1 13.4 1.18 1.27 

BOR 100 98.3 N A 108.2 N A 0.99 

CRK 100W 104.5 N A 115.1 NA 1.06 

CRK 100 WO 98.6 N A 115.1 N A 1.65 

CRK 120W 119.1 NA 136.8 N A 1.77 

CRK 120 WO 1 17.4 NA 136.8 N A 1.94 

BRN Control 146.1 149.3 155.3 0.64 0.92 

BOR Control 206.6 N A 21 1.1 N A 0.45 

CRK Control 158.1 NA 172.1 NA 1.40 

a~R~ = Brownsville plots, BOR = Border plots, CRK = Crook Mtn. plots, C-C = Black Hills National Forest 

standard thinning plot. GSLs followed by a W (with) have MPB-killed trees incorporated into the basal area 

values while those followed by a WO (without) have these trees withdrawn. 

b~~ = data unavailable 

Brownsville. During the first 5 yr after cutting, basal 

area in the 3 cut plots at BRN increased 5.9 to 6.3 ft2/ 

plot while basal area in the control plot increased 3.2 

(table 9). For the second 5-yr period, basal area increased 6.8 to 6.9 ft2 in the cut plots, while the control 

plot increased 6.0 ft2 (table 9). The difference in the 

growth rates for the control plot appears to be influenced primarily by precipitation patterns because 3 

of the first 5 yr had below average precipitation 

(Schmid et al. 1991), while 3 of the second 5 yr had 

above average precipitation, which exceeded 138% in 

2 of the 3 yr (EarthInfo 1996). Thus, although basal 

area growth for the control plot was and should be 

expected to be less than for the cut plots during both 

periods, basal area growth in an "unthinned stand" 

may almost equal growth in "thinned stands" during 

periods of above-average precipitation. 

Crook Mountain. Basal area in the CRK plots increased 15.4 ft2/acre in the GSL 80, 10.6 in the GSL 

100, and 17.7 in the GSL 120, when the basal area of 

the MPB-attacked trees was retained in the initial 

basal areas (table 9). When this basal area is withdrawn, the increases were 18.0 ft2/acre in the GSL 80, 

16.5 in the GSL 100, and 19.4 in the GSL 120 (table 9). 

The CRK control increased 14.0 ft2/acre (table 9). 

In general, the P.A.1.s for the CRK plots appears 

better than P.A.1.s for comparable plots from the other 

locations. However, because of data variabilit,~, the 

influence of site remains circumstantial until future 

10-yr measurements can be made. 

C-C. Basal area increased about 9 ft2/acre/10 yr in 

the C-C plot when the basal area associated wth 

MPB-attacked trees was retained and about 11 ft2 

when it was withdrawn (table 9). The latter increase is 

slightly less than the increase in the comparable BRN 

GSL 80 plot (table 9). In the absence of MPB-caused 

mortality, marking precision did not apparently affect 

basal area growth. However, when MPB-caused mortality is considered, marking precision may have had 

an indirect influence. Basal area in different parts of 

the plot ranged from 66 to 100 ft2/acre in 1985 as a 

result of marking, and 4 of the 5 MPB-attacked trees 

were in the higher density areas. Because MPBs are 

known to attack microcosm stands of higher density 

(Olsen et al. 1996), the mortality may be partially 

attributable to the denser clumps left by the marking 

crews. This suggests that marking crews must be 

careful not to leave such clumps or they risk creating 

future pockets of MPB infestations in stands that 

otherwise are below the susceptibility threshold. 

Border. Basal area increased 18.2 ft2 in the GSL 60, 

17.6 in the GSL 80,9.9 in the GSL 100, and 4.5 ft2 in 

the control at the BOR location (table 9). The low 

increase in basal area in the GSL 100 was caused by 

the 1996 snow/wind storm which uprooted or snapped 

off more trees in this plot than in the other plots. Bereft 

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-28. 1999 
of this mortality event, these plots would have increased more than 20 ft2 over the 10 yr for an average 

of more than 2 ft2/yr. 

MPB Susceptibility Estimates 

Based on the 10-yr growth in basal area in the BRN 

plots, straight-line projections (SLP) indicate the 

GSL 60 would take about 68 yr, while the GSL 80 

would take about 53 yr, and the GSL 100 about 39 yr 

to reach the basal area threshold of 150 ft2/acre 

(figure 1). Previous computations using RMYLD estimated periods of 76, 51, and 37 yr for these plots to 

reach the same threshold (Schmid 1987); the 2 sets 

of estimates are very similar for the GSL 80 and GSL 

100. GENGYM estimates that the GSL 60 would take 

90 yr, the GSL 80 would take 59 yr, and the GSL 100 

about 43 yr before they reach the basal area level of 

150 ft2 (table 10). From any of the 3 estimationmethods, 

the cut stands appear unsusceptible for relatively 

long periods, if 150 is the critical level. However, if 

the threshold is reduced to 120 ft2/acre, the SLP 

suggest the GSL 60 would take about 46 yr, while the 

GSL 80 would take about 31, and the GSL 100 about 

16 yr to reach the threshold (figure 1). GENGYM 

estimates that the GSL 60 would take 50 yr, the GSL 

80 would take 31 yr, and the GSL 100 about 16 yr. 

Based on 10-yr growth rates in the CRK plots, SLP 

indicate the GSL 80 would take about 42 yr, while 

the GSL 100 would take 42 yr, and the GSL 120 

would take about 17 yr to grow to the basal area 

threshold of 150 ft2/acre if the basal area associated 

with MPB-attacked trees is retained as part of the 

initial basal area (figure 2). If the MPB-caused mortality is withdrawn from the initial basal area, the 

same plots would reach the same threshold in about 

38,3l, and 17 yr. GENGYM projections indicate these 

3 plots would reach the 150 threshold in 38, 32, and 

17 yr with MPB mortality incorporated and 41, 31, 

and 17 yr with MPB mortality deleted. 

If the susceptibility threshold is reduced to 120 ft2 

and the basal area associated with MPB-attacked 

trees is retained as part of the initial basal area, the 

SLP indicates that the CRK GSL 80 would reach this 

threshold in about 23 yr, while the GSL 100 would 

take 15 yr (figure 2). GENGYM predicts the GSL 80 

would take about 19 yr and the GSL 100 about 11 yr. 

The GSL 120 was essentially at the threshold immediately after partial cutting, and the MPB-caused mortality did not change the stocking level enough to 

Figure 1-Years required for stand densities in the Brownsville plots to reach MPB susceptibility 

thresholds. 

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-28. 1999 
Table 10-Number of years for various GSLs to reach the susceptibility thresholds of 

120 and 150 as derived from straight line and GENGYM projections. 

Basal area 120 Basal area 150 

LocationIGSL Line Proj. GENGYM Line Proj. GENGYM 

BRN 60 46 50 68 90 

BOR 60 32 25 48 39 

BRN 80 3 1 3 1 53 59 

BOR 80 23 13 40 24 

CRK 80W 23 19 42 38 

CRK 80W0 2 1 2 1 38 4 1 

C-C 80 W 34 34 6 1 70 

C-C 80 WO 3 1 36 55 73 

BRN 100 16 16 39 43 

BOR 100 23 9 53 23 

CRK 100W 15 11 42 32 

CRK 100 WO 13 12 3 1 3 1 

CRK 120 NA~ N A 17 17 

a~~ = data unavailable 

warrant developing with and without MPB mortality Based on the 10-yr growth, the C-C plot would take 

estimates. If the MPB-caused mortality is withdrawn, about 61 yr to reach the 150 ft2 level and about 34 yr 

SLP indicates the GSL 80 and GSL 100 would reach to reach the 120 ft2 level if the basal area associated 

the threshold about 2 yr sooner. GENGYM predicts with MPB-attacked trees is retained as part of the 

threshold crossing 1 to 2 yr later than their respective initial basal area (figure 3). For the same conditions, 

projections when mortality is retained. GENGYM indicates about 70 and 34 yr (table 10). If 

+ 

GSL BOW GSL BOW0 GSL 100W GSL 100WO GSL120W GSL120WO --...- '& --.- 0 ---*--- ---I.--- -.--A .--- 

Figure 2-Years required for stand densities in the Crook Mountain 

plots to reach MPB susceptibility thresholds. GSLs followed by a W 

represent the projection when the MPB-attacked trees are incorporated 

into the initial basal area. GSLs followed by a WO represent the projection 

when the MPB-attacked trees are deleted from the initial basal area. 

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-28. 1999 
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 

Years 

Figure 3-Years required for the C-C plot to reach MPB susceptibility 

thresholds. C-C 80W line represents the projection when the MPB-attacked 

trees are incorporated into the initial basal area. C-C 80W0 line represents the 

projection when the MPB-attacked trees are deleted from the initial basal area. 

the MPB-caused mortality is withdrawn, the plot may 

reach the 150 ft2 level in about 55 yr and the 120 ft2 

level in about 31 yr. MPB-caused mortality thus, 

increases the time to reach the 150 ft2 level by about 

6 yr and the time to reach the 120 fl2 level by 3 yr. 

Comparing GSLs of the same level with endemic 

MPB-caused mortality excluded, the CRK GSL 100 

would reach the critical thresholds for MPB epidemics 

sooner than the BRN GSL 100 probably because the 

CRK plot was on a better site. Similarly, the CRK GSL 

80 would reach the critical thresholds sooner than the 

BRN and C-C GSL 80s probably because the CRK plot 

was on a better site and the initial diameters were 

generally greater. The BRN and C-C GSLs 80 reached 

the thresholds in almost identical times; apparently 

marking precision did not influence stand performance in regards to growth (figures 1 and 3). It 

remains to be seen, however, if the 2 plots will function 

similarly during future MPB infestations or whether 

the microcosm stands created in the C-C thinned 

stand by less precise markmg will increase susceptibility of the C-C stand and cause it to function like an 

unthinned stand. 

Straight-line projections indicate the BOR GSL 60 

will cross the 150 threshold in 48 yr, while the BOR 

GSL 80 will take 40 yr, and the BOR GSL lOOwill take 

53 yr (figure 4). GENGYM estimates the 3 plots will 

cross the same threshold in 39,24, and 23 yr. For the 

120 threshold, SLP indicate the GSL 60 will cross the 

threshold in 32 yr, while both the GSL 80 and GSL 100 

will take 23 yr. GENGYM estimates the 3 plots will 

cross the same threshold in 25, 13, and 9 yr. The 

substantial disparity between comparable estimates 

for each BOR GSL occurs because ofthe way GENGYM 

computes future conditions. GENGYM uses the initial stand conditions (in this case, the stand conditions 

existing after the stands were partial cut) and 10-yr 

growth rates. GENGYM assumes a minimal amount 

of tree mortality over the duration of the simulation 

but does not incorporate substantial mortality events 

such as MPB infestations or snow/wind storms. In the 

BOR plots, the April 1996 storm killed 10 or more trees 

in each plot; more than 30 in the GSL 100. This 

mortality causes the disparity between the 2 methods 

and also explains why the BOR GSLs did not cross the 

2 thresholds in timeframes similar to comparable 

GSLs from the other locations. 

Except for the BOR plots, comparable estimates 

from the SLP and GENGYM methods exhibit the 

greatest disparity when the 150 threshold is being 

considered (table 10). Most of the disparity appears to 

be caused by differences between stand growth/ 

mortality relationships in the 2 methods. The straightline method (SLP) assumes no tree mortality over the 

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-28. 1999 
0 10 20 30 40 50 60 

Years 

Figure $-Years required for stand densities in the Border plots to reach MPB 

susceptibility thresholds. Snow breakage decreased the number of trees on the 

partially cut plots but was especially deleterious to the stocking on the GSL 100. 

time of the projection, while GENGYM incorporates 

mortality in the timeframe of the projection. No tree 

mortality is unrealistic because trees die during the 

life of a stand. The assumption of no mortality is 

probably insignificant in the 20 to 30 yr timeframe 

that exists when the 120 threshold is used and explains why the estimates from the 2 methods are so 

similar when this threshold is being considered. 

However, when projections are made for 40 to 50 or 

more yr, as is the case when the 150 threshold is 

used, the no mortality assumption in the SLP method 

will cause it to underestimate the time required to 

cross the threshold. 

SLP projections also underestimate the time required 

to reach the susceptibility thresholds because basal 

area growth (P.A.I.) will decrease in thinned plots over 

time (W.K. Olsen 1996, personal communication), but 

the method assumes the same rate of growth throughout the length of the projection. In addition to the loss of 

basal area attributed to treemortality, basal area growth 

will decrease because it decreases as age increases (see 

Boldt and Van Deusen 1974). As tree diameter increases with age, the annual increment in diameter 

becomes less. Thus, our stands and similar stands in 

the Black Hills National Forest would reach critical 

susceptibility levels slightly later than our gtraight 

line projections indicate. 

Management Implications 

Even though some disparity occurs in the estimates, forest managers should be aware of the relationships between threshold levels for epidemic MPB 

populations, stand growth, and reentry intervals for 

partially cut stands. Stand growth in terms of basal 

area per year varies among stands, but growth rates of 

1.3 or more fi2/yr are possible in partially cut stands 

(table 9). If 150 ft2 of basal area is considered the 

threshold level for epidemic MPB populations, stands 

increase at >1.5 fi21yr, and reentry is scheduled about 

every 20 yr after the original partial cutting, then 

stands with GSLs >I20 could exceed the threshold 

level before the first reentry at 20 yr (table 10). Similarly, stands with GSLs <I20 but >80 (table 10) may 

exceed the threshold level between the first and second reentry period (40 yr) and should be cut. However, 

if the threshold for susceptibility is lowered to 120 ft2/ 

acre, as Schmid and Mata (1992) suggest, then most 

GSL 100 stands would reach this susceptibility 

threshold before the first scheduled reentry (20 yr) 

(table 10, figures 1, 2, and 3). Thus, reentry in the 

GSLs >I00 may be necessary before the first scheduled reentry at 20 yr, while reentry in the GSL 80 may 

be necessary halfway between the first and second 

reentrys. 

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-28. 1999 
These relationships between thresholds for epidemic 

MPB populations and reentry schedules do not mean 

that a specific stand must be automatically reentered 

just because the stand basal area has exceeded the 

threshold. While it is good management practice to do 

so, stands exceeding the threshold could be left for a 

few years as long as managers increase their vigilance 

for MPB activity in such stands and are able to readily 

adjust their silvicultural activities if epidemic MPB 

infestations appear. History indicates, however, that 

stands allowed to exceed the thresholds become the 

ignition points for subsequent MPB epidemics. Ignoring such stands and leaving them until the normal 

reentry occurs is inviting a MPB epidemic. 

Whether SLP or GENGYM projections are most 

accurate will eventually be resolved when partially 

cut susceptible-size stands grow for 50 or more yr, 

reach the critical susceptibility thresholds, and the 

amount of tree mortality attributable to endemic MPB 

populations during the time span is identified. Endemic MPB mortality obviously influences growth 

projections for both methods. In a GSL 80 stand, 1 ft2 

of MPB-caused mortality may alter SLP estimates for 

crossing the 150 threshold 4 to 6 yr (figures 2,3). The 

same mortality in the GENGYM model will alter its 

estimates by 3 yr (table 10). Until the influence of 

endemic MPB mortality is determined, forest managers should be cognizant that different stand growth 

scenarios can be derived from the 2 methods. 

Forest managers should also realize that although 

endemic MPB populations may kill a few trees and 

thereby reduce the overall basal area of a stand as well 

as the time to reach the critical susceptibility threshold in the short term, the long-term result may be 

development of relatively homogeneous stands containing microcosm stands of greater densities as described by Olsen et al. (1996). Such stands are highly 

conducive to MPB epidemics. This situation is not 

likely to occur when trees are uniformly spaced. However, if the original marking tends to leave variable 

tree densities, as is evident in the C-C plot, then the 

potential for the development of relatively homogeneous stands containing microcosm stands is greater. 

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-28. 1999 

Overall, the GSL may be below the susceptibility 

threshold but, within the microcosm stands, it may 

exceed the threshold. Thus, managers should not be 

lulled into thinking that because overall stocking is 

below the threshold, substantial time exists before 

MPB infestations may become evident. 

Literature Cited 

Boldt, C.E.; Van Deusen, L.L. 1974. Silviculture of ponderosa pine 

in the Black Hills. Res. Paper RM-124. Fort Collins, CO: USDA 

Forest Service, Rocky Mountain Forest & Range Experiment 

Station, 43 p. 

EarthInfo. 1996. NCDC summary of the day information for Lead, 

South Dakota. CD ROM diskette on file at the Atomspheric 

Sciences Library of Colorado State University, Foothills Campus, 

Fort Collins, Colorado, 80521. 

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

even-aged and two-storied stands. Res. Paper RM-199. Fort 

Collins, CO: USDA Forest Service, Rocky Mountain Forest and 

Range Experiment Station, 26 p. 

Edminster, C.E.; Mowrer, H.T.; Schuler, T.M.; Olsen, W.K.; 

Hawksworth, F.G. 1990. GENGYM: a variable density stand 

table projection system calibrated for mixed conifer and ponderosa pine stands in the Southwest. Res. Paper RM-297. Fort 

Collins, CO: USDA Forest Service, Rocky Mountain Forest and 

Range Experiment Station, 32 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. 

Sartwell, C.; Stevens, R.E. 1975. Mountain pine beetle in ponderosa 

pine: prospects for silvicultural control in second-growth stands. 

Journal of Forestry 73:136-140. 

Schmid, J.M. 1987. Partial cutting in MPB susceptible stands: will 

it work and for how long? In Troendle, C.A.; Kaufmann, M.R.; 

Hamre, R.H.; Winokur, R.P./ tech. coordinators. Management of 

subalpine forests: Building on 50 years of research. Proceedings 

of a technical conference, 6-9 July 1987, Silver Creek, CO. Fort 

Collins, CO: USDA Forest Service, Rocky Mountain Forest and 

Range Experiment Station. Gen. Tech. Rep. RM-149: 243-245. 

Schmid, J.M.; Mata, S.A.; Edminster, C.B. 1991. Periodic annual 

increment in basal area and diameter growth in partial cut 

stands of ponderosa pine. Res. Note RM-509. Fort Collins, CO: 

USDA Forest Service, Rocky Mountain Forest and Range Experiment Station. 3 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: USDAForest Service, Rocky Mountain 

Forest and Range Experiment Station. 4 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: USDA Forest Service, 

Rocky Mountain Forest and Range Experiment Station. 4 p. 

13 

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