Response of Ponderosa Pine Stands 

to Pre-Commercial Thinning 

on Nez Perce and Spokane Tribal Forests 

in the Inland Northwest, USA

Dennis E. Ferguson, John C. Byrne, 

William R. Wykoff, Brian Kummet, and Ted Hensold

United States Department of Agriculture / Forest Service

Rocky Mountain Research Station

Research Paper RMRS-RP-88

June 2011
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Ferguson, Dennis E.; Byrne, John C.; Wykoff, William R.; Kummet, Brian; Hensold, Ted. 2011. Response of ponderosa pine stands to pre-commercial thinning on Nez Perce and Spokane Tribal forests in the Inland 

Northwest, USA. Res. Pap. RMRS-RP-88. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, 

Rocky Mountain Research Station. 33 p.

Abstract

Stands of dense, natural ponderosa pine (Pinus ponderosa var. ponderosa) regeneration were operationally, precommercially thinned at seven sites—four on Nez Perce Tribal lands in northern Idaho and three on Spokane Tribal 

lands in eastern Washington. Five spacing treatments were studied—control (no thinning), 5x5 ft, 7x7 ft, 10x10 ft, and 

14x14 ft. Sample trees were measured pre- and post-thinning, 3 yrs after thinning, and 5 yrs after thinning, starting 

in 1997 (Nez Perce sites) and 1998 (Spokane sites). Models of diameter and height growth, change in crown ratio 

and height to crown base, and probability of western pine shoot borer (Eucosma sonomana Kearfott) infestation were 

developed. Little mortality occurred after thinning, and the trees that died had small crowns. Diameter growth for all of 

the spacings was significantly different, with widely spaced trees growing faster than narrowly spaced trees. Trees with 

larger initial diameters or crown ratios had faster post-thinning diameter growth. Spacing had no influence on height 

growth. Height growth increased with increasing initial tree size, larger crown ratios, and increasing pre-thinning height 

growth, but decreased with increasing post-thin basal area. Height/diameter ratios (an indication of tree slenderness 

and stability) increased over time on the narrower spacings, indicating trees were still crowded. We found that thinnings 

of at least 10x10 ft are needed to stimulate a change in height/diameter ratio. Wider spacings (10x10 ft and 14x14 ft) 

resulted in lengthening crowns because height to crown base remained closer to the ground, which had implications 

for fire management. Shoot borer damage occurred on about one-fourth of the sample trees and was negatively correlated with elevation and positively correlated with tree vigor. There was no dramatic non-tree vegetation response 

to the thinnings, which may be related to herbivory or the lack of ground disturbance from the thinning operations.

Keywords: Pinus ponderosa, height growth, diameter growth, height/diameter ratios, thinning from below, crown 

ratios, competing vegetation

Authors 

Dennis E. Ferguson, Research Silviculturist (retired), USDA Forest Service, Rocky 

Mountain Research Station, Moscow, Idaho.

John C. Byrne, Forester, USDA Forest Service, Rocky Mountain Research Station, 

Moscow, Idaho.

William R. Wykoff, Research Forester (retired), USDA Forest Service, Rocky Mountain 

Research Station, Moscow, Idaho.

Brian Kummet, Fee Lands Forester, Nez Perce Tribe, Lapwai, Idaho.

Ted Hensold, Forester (retired), Bureau of Indian Affairs, Spokane Agency, Wellpinit, 

Washington.
Acknowledgments 

We thank the following for their assistance with this study:

•	 Nez Perce and Spokane Tribal Executive Committees for access to the study sites;

•	 John DeGroot, Nez Perce Forestry Director, for his support and direction;

•	 Melinda Moeur for her help with study design and installation;

•	 the tribal forestry technicians that implemented the thinning regimes on their respective areas, especially 

Larry Geffre on the Nez Perce sites, and George Hill, Chuck Kieffer, Marty Andrews, and Harvey Flett on 

the Spokane sites;

•	 those who helped with measurements, including Anna McClintick, Laurie Clark-Blackmore, Ben Poulter, 

Ryan Fogelberg, Marcus Warwell, Dave Hamilton, Kate Schneider, Carson Sprenger, and Marc Antonietti;

•	 Jerry Rehfeldt for generating spline climate predictions for the seven study sites; and

•	 Stephen Fitzgerald and David Affleck for reviewing earlier drafts of this manuscript.

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Post-Thinning Densities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Severe Lean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Mortality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Diameter growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Height growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Height/diameter ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Change in crown ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Height to crown base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Shoot borer damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Non-Tree Vegetation and Thinning Slash . . . . . . . . . . . . . . . . . . . 17

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Severe lean and mortality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Diameter growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Height growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Height/diameter ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Change in crown ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Height to crown base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Shoot borer damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Growth anomaly and climate . . . . . . . . . . . . . . . . . . . . . . . . . 28

Non-Tree Vegetation and Thinning Slash . . . . . . . . . . . . . . . . . . . 28

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

USDA Forest Service Res. Pap. RMRS-RP-88. 2011 1

Introduction_____________________________________________________________

Ponderosa pine (Pinus ponderosa) is one of the most widely distributed conifers in 

western North America (Oliver and Ryker 1990). Ponderosa pine forests provide tangible and intangible goods and services such as timber, water, wildlife habitat, forage, 

carbon sequestration, recreation, biodiversity, and aesthetics. In the Inland Northwest, 

ponderosa pine (Pinus ponderosa var. ponderosa) grows in a wide variety of conditions, 

ranging from very dry forests where it is the only conifer species to moist forests where 

it grows with up to nine other conifer species (Pfister and others 1977; Williams and 

others 1995; Rehfeldt and others 2008).

Genetic variation of ponderosa pine in the Inland Northwest occurs along relatively 

steep elevational clines, but with gentle clines for changes in latitude and longitude 

(Rehfeldt 1991). Growth decreases with increasing elevation, and populations separated 

by about 1300 ft in elevation tend to be genetically different—that is, they are adapted 

to different portions of the environmental gradient.

Ponderosa pine often regenerates prolifically, which results in dense stands. Several 

studies of the effects of thinning naturally regenerated ponderosa pine regeneration 

have been conducted (e.g., Van Deusen 1968; Barrett 1982; Ronco and others 1985; 

Cochran and Barrett 1999). These studies have quantified the tradeoffs between greater 

individual tree growth at lower stand densities versus greater volume production at 

higher stand densities.

In the Inland Northwest, there is insufficient information on response of ponderosa 

pine regeneration to thinning. In addition to the question of tree size versus stand volume, 

tradeoffs of various thinning levels include snow damage, economics, crown dynamics 

that relate to wood quality, and development of non-conifer understory vegetation. Growth 

data for ponderosa pine regeneration are especially lacking for our driest ponderosa pine 

sites. Such data can be used to calibrate ponderosa pine growth in the Forest Vegetation 

Simulator (FVS, Wykoff and others 1982; Crookston and Dixon 2005). In addition, data 

from thinning trials can be used to improve prediction of height/diameter ratios in FVS, 

which are poorly represented (Wonn and O’Hara 2001).

The Nez Perce and Spokane Tribes own and manage ponderosa pine stands that occur 

on the dry end of the distribution for ponderosa pine in the Inland Northwest. Dense 

regeneration often occurs as even-aged patches following harvesting or in abandoned 

fields. The Tribes have multiple objectives for these dry sites, yet data on response to 

thinning are lacking. The objective of this study was to gather the basic data for evaluating tradeoffs among different post-thin densities on Nez Perce and Spokane Tribal lands.

Response of Ponderosa Pine Stands to 

Pre-Commercial Thinning on Nez Perce and 

Spokane Tribal Forests in the Inland Northwest, USA

Dennis E. Ferguson, John C. Byrne, William R. Wykoff, 

Brian Kummet, and Ted Hensold
2 USDA Forest Service Res. Pap. RMRS-RP-88. 2011

Methods ________________________________________________________________

This study was established on seven sites—four on Nez Perce Tribal and three on 

Spokane Tribal lands (Table 1). Initially, a fourth site was installed on the Spokane 

Tribal lands, but it was burned by a wildfire shortly after thinning. Each site was an 

area of dense, natural ponderosa pine regeneration in need of pre-commercial thinning. 

Data were gathered at each site for elevation, slope, aspect, and habitat type (Cooper 

and others 1991).

Five 250x50 ft experimental units were located at each site to create the greatest 

uniformity of initial conditions. The long axes of units were oriented parallel with stand 

or environmental conditions, such as gradients from taller trees to shorter trees, higher 

to lower topography, or higher to lower seedling density. Because large, uniform contiguous areas of regeneration were not available, installing buffer strips between units 

was not possible.

One of five treatments was randomly assigned to each experimental unit—control 

(no thinning), 5x5 ft spacing (~1740 trees/acre), 7x7 ft (~890 trees/acre), 10x10 ft 

(~435 trees/acre), and 14x14 ft (~220 trees/acre). Ten 1/400-acre circular plots (radius 

5.9 ft) were evenly spaced at 45-ft intervals along two lines that transected the long axis 

of the unit (Figure 1). Each transect was 17 ft from the edge of the unit, and plots were 

offset within units by starting from opposite ends of the unit. With this plot arrangement, 

an area about 11 ft wide existed between the edge of the circular plot and the closest 

boundary between units, which was deemed suitable for studying regeneration-size trees 

for 5 yrs after thinning. All units had competition at the edges because units did not 

border non-forested areas. Reukema and Smith (1987) noted no differences in growth 

rates between interior versus edge trees in plantations that had competition at the edges.

Each increment in spacing from 5x5, 7x7, 10x10, to 14x14 ft doubled the area available for tree growth. We felt a progression of spacings would help evaluate interactions 

among mortality, lean, crown development, and diameter and height growth.

Table 1. Description of the seven study sites.

 Elevation Slope Aspect Habitat Pre-thin density Latitude

Tribe Name (ft) (%) (°) type1 (trees/acre) Longitude

Nez Perce Cold Springs 3430 9 150 PSME/SYAL 8280 46.3161

 116.4218

Nez Perce Reubens 3490 15 355 PSME/SYAL 2469 46.3553

 116.5585

Nez Perce West Talmaks 4310 10 60 PIPO/SYAL 9807 46.1597

 116.5733

Nez Perce North Talmaks 4330 16 80 PSME/SYAL 7244 46.1642

 116.5663

Spokane South Castle Rock 1700 0 Flat PIPO/SYAL 4840 47.9354

 118.3349

Spokane East Castle Rock 1840 18 270 PIPO/SYAL 10,128 47.9404

 118.3307

Spokane Mosquito Heights 2540 11 60 PSME/PHMA 3140 47.9866

 118.1819

1

 PSME/SYAL = Pseudotsuga menziesii/Symphoricarpos albus

PIPO/SYAL = Pinus ponderosa/Symphoricarpos albus

PSME/PHMA = Pseudotsuga menziesii/Physocarpus malvaceus
USDA Forest Service Res. Pap. RMRS-RP-88. 2011 3

Pre-thinning measurements were taken at the Nez Perce sites in 1997 and at the 

Spokane sites in 1998 after the cessation of height and diameter growth. At each 1/400-

acre circular plot, height, diameter at breast height (dbh), and species of all trees were 

measured and recorded. Heavily stocked plots were subsampled using quarter plots as 

the sampling unit. The quarter plots were delineated using a perpendicular sampling 

frame. For the first quarter plot, the frame was located with the vertex at plot center so 

that one leg pointed at, and was perpendicular to, the nearest plot boundary. The other 

leg was parallel to the boundary and was aimed toward the next plot in the sequence 

(i.e., for plot 1, the frame was aimed at plot 2). If fewer than 15 trees were found in the 

first quarter, the frame was moved one quarter in the clockwise direction until a total 

of 15 trees was obtained. Once any quarter was started, all of the trees in that quarter 

were measured. The quarters that were measured had to be representative of the plot 

as a whole in terms of density and tree size; otherwise the whole plot was measured. 

Figure 1. Example plot layout showing 5 experimental units and 10 plots per site.
4 USDA Forest Service Res. Pap. RMRS-RP-88. 2011

Trees sampled on the quarter plots were weighted inversely to the number of quarters 

sampled to calculate density statistics.

Sites were operationally thinned from below under the direction of Tribal foresters 

in the fall of 1997 (Nez Perce) and 1998 (Spokane). Thinned trees were left where they 

fell, except for a few plots at the West Talmaks site, where the thinning was delayed until 

the spring of 1998 and trees were removed to eliminate the possibility of an outbreak 

of pine engraver beetle (Ips pini) (Livingston 1979).

The first post-thinning measurements were taken in the fall immediately after thinning or in the next spring before trees initiated growth. Density measurements were 

determined by measuring trees on the 1/400-acre circular fixed-area plot and a concentric 

variable radius plot (10 ft2

/acre basal area factor). Trees less than or equal to 2.3 inch dbh 

and greater than 1 ft in height were sampled with the fixed-area plot; trees greater than 

2.3 inch dbh were sampled with the variable radius plot to quantify competition from 

trees surrounding the 1/400-acre plot. Basal area/acre and trees/acre were calculated 

at each plot location based on the measured dbh using standard expansion formulas 

for each plot type (Avery and Burkhart 1983) and then were summed. If a fixed-area 

plot was subsampled, then the fixed-area plot density measures were appropriately 

weighted. Calculated densities from the 10 plots in each treatment were averaged to 

obtain treatment-level measurements.

Three sample trees were located using the center of each 1/400-acre plot. Sample 

points were located at the following positions relative to plot center: 1 ft at 60°; 3 ft at 

180°; and 5 ft at 300°. Sample trees were the nearest tree to the sample point in each 

120° sector, which meant that the sample tree could be outside the 1/400-acre plot. All 

sample trees were numbered and tagged. Trees were measured at the time of thinning 

and at 1, 3, and 5 yrs after thinning, following completion of growth for that year. The 

purpose of measurements at yr 1 and 3 was to detect early responses to thinning, such 

as mortality, severe lean, physiological shock, or insect problems.

Initially, sample trees in the controls were selected by different rules in the Nez Perce 

and Spokane sites. At the Nez Perce sites, the three sample trees per plot were chosen 

by adhering to the rule of the closest tree to the sample points. This sampling rule 

often resulted in sample trees that would not have been a leave tree in an operational 

thinning, thus biasing the sample toward small or slender trees. We corrected this bias 

in the Spokane sites by selecting the closest tree to the sample points that would be a 

leave tree if the area was to be thinned. During the yr 5 measurement at the Nez Perce 

sites, we corrected the earlier bias by rating each tree in the control to determine if it 

would have been a leave tree in an operational thinning. Those trees that would have 

been a leave tree were used in the analyses, which resulted in the elimination of about 

one-third of the control trees from the Nez Perce sites. Therefore, sample trees in the 

control at both locations are comparable.

For each tagged tree, dbh was measured with a tape or caliper to the nearest 0.1 inch. 

Heights of trees up to 40 ft tall were measured with a telescoping height pole, and a 

few of the tallest trees were measured using a clinometer. Height was measured to the 

top of the terminal bud to the nearest 0.1 ft. In order to maximize consistency between 

sequential measurements on the same tree, all height measurements used the breast 

height tag as a reference point for 4.5 ft. Height increment for the 5 yrs prior to thinning 

was determined by measuring height five whorls below the tree top. A subjective ocular 

estimate of crown ratio was made to the nearest 10 percent. Height to the crown base 

was measured from the uphill side of the tree to the lowest whorl such that the crown 

was continuous down to that whorl and at least two branches in the whorl were alive. 

Tree damages were recorded in marginal notes.

Even-numbered plots were used to record non-tree vegetation and ponderosa pine 

thinning slash in order to relate thinnings to species occurrence, coverage, and size. A 
USDA Forest Service Res. Pap. RMRS-RP-88. 2011 5

species occurred on a plot if any aerial part of the plant was present within the cylinder 

defined by a vertical projection of the 1/400-acre plot boundary. These measurements 

were made 1, 3, and 5 yrs after thinning when vegetation was fully developed (July and 

August). Depending on species, both the line intercept and canopy coverage methods 

were used (Canfield 1941; Daubenmire 1959; Chambers and Brown 1983). For the line 

intercept method, a random azimuth was chosen for each plot, and the transect was laid 

out to run through the plot center. The transect was divided at the plot center into two 

radii for recording the length of the radii covered by grasses, forbs, and ponderosa pine 

thinning slash. The number of forb species per radii was recorded; individual forb species were also recorded, but length of individual forbs along the radii was not recorded. 

The randomly chosen azimuth for line transects remained the same through subsequent 

measurements.

A modification of Daubenmire’s (1959) canopy coverage method was used to record 

shrub species. For each shrub species on the 1/400-acre plot, percent cover (ocular 

estimate) and measured average height were recorded. A value of 1% was used to note 

occurrence only; otherwise, percent cover classes were 5, 10, 20, 25, 30, 40, 50, 60, 70, 

75, 80, 90, 95, and 100%.

Data Analysis____________________________________________________________

Data were analyzed using methods for mixed models in SAS 9.2 using PROC MIXED 

(Littell and others 1996). Event data (tree mortality and shoot borer occurrence) were 

analyzed with PROC GLIMMIX using a dichotomously distributed dependent variable 

(1 if the event occurs, 0 otherwise). The predicted probability is continuous and bounded 

in the interval [0,1].

Tree data were analyzed for diameter and height growth, height/diameter (H/D) ratios, 

height to crown base, and crown ratio change.

Data for shrubs, forbs, and ponderosa pine thinning slash were analyzed for occurrence and percent cover, and height for shrubs. These three variables show if species are 

expanding by becoming established on more plots (occurrence), expanding horizontally 

(cover), or expanding vertically (height). Conversely, species could decrease in occurrence, cover, or height.

For all analyses, the statistical significance of independent variables was assessed at 

the P = 0.05 level. LSMEANS was used to detect statistical differences between means. 

The SUBJECT statement was used to account for the correlation among trees on the 

same plots; SAS then accounted for these correlations to construct the appropriate test 

statistics. Transformations of variables were explored to achieve homogeneity of error 

variance and normality, and to obtain additivity of effects (Kirk 1982). The seven study 

sites were used as a random variable because results of these analyses should be applicable to similar sites in the Inland Northwest. The significance of independent variables 

was evaluated using t-ratios and F-values. Akaike’s information criteria (AIC, smaller 

is better), as output by SAS, was used to compare alternative equations that had different independent variables but the same dependent variable. Graphs were prepared to 

visually inspect whether residuals were centered about the 0 line, with no abnormalities.

Results _________________________________________________________________

Post-Thinning Densities

The data for this study came from naturally regenerated, operationally thinned ponderosa pine stands and should show what can be expected under actual silvicultural 

prescriptions. Leave trees were not pre-selected. Crews were instructed to thin to the 
6 USDA Forest Service Res. Pap. RMRS-RP-88. 2011

approximate spacing, leaving the best tree in the vicinity specified by the spacing, but 

with no expectation of rigid spacing. Thus, some variation in spacing did occur.

Table 2 shows mean post-thin spacing by study site. Mean spacings are 3.1 ft between trees in the control, 5.8 ft in the 5x5 ft spacing, 7.4 ft in the 7x7 spacing, 9.4 ft in 

the 10x10 spacing, and 13.5 ft in the 14x14 spacing. All of the 5x5 spacings exceeded 

5 ft between trees, and trees in the Mosquito Heights control averaged 5.1 ft between 

trees. Even though the spacings were not exact, crews did a reasonable job of creating 

conditions needed for this study.

Table 2. Mean post-thin ponderosa pine spacing (ft).

Spacing

Site Control 5x5 7x7 10x10 14x14

Cold Springs 2.1 5.7 7.7 8.6 13.2

Reubens 4.6 5.9 8.0 10.1 12.0

West Talmaks 2.3 5.7 7.0 9.1 13.1

North Talmaks 1.8 6.7 8.2 11.3 16.3

South Castle Rock 3.9 5.3 7.2 8.8 11.6

East Castle Rock 1.8 5.1 6.4 8.7 15.6

Mosquito Heights 5.1 6.3 7.4 9.0 12.8

Spacing mean 3.1 5.8 7.4 9.4 13.5

Trees

A total of 973 trees were available for analyses. Five trees were not ponderosa pine, 

so they were eliminated from analyses. One tree was much larger than other trees (10.5 

inches dbh at the beginning of the study) and was eliminated as an outlier. Twenty-three 

trees killed by a wildfire on a few plots at East Castle Rock in 2001 were also eliminated 

from analyses. Fifteen trees developed severe lean after thinning and were considered 

unable to recover and grow vertically. Six of these 15 trees subsequently died. An additional 24 trees died from other causes during the study period. Data summaries for 

the 905 live trees are given in Table 3, and Table 4 shows average post-thin height (H0), 

dbh (D0), and crown ratio (CR0) by spacing.

Severe lean

Of the 15 trees that developed severe lean, none occurred in the control. Severe lean 

trees were mid-size trees, always between heights of 10 and 24 ft, with high H0/D0

ratios (slender trees) between 60 and 130. An important finding is that only 1.6% of 

the sample trees fell over following thinning. Severe lean was first observed at yr 1 for 

about one-third of the 15 trees, and at yr 3 for the other two-thirds of these trees. Six of 

the 15 trees died by yr 5 and are discussed in the following section on mortality.

Mortality

Thirty trees (3.1%) died during the 5-yr study. PROC GLIMMIX models (not shown) 

determined that trees with smaller crown ratios (CR0); slower 5-yr, pre-thin height growth 

(H-5grow); and higher H0/D0 ratios had a higher probability of mortality. Figure 2 is a 

frequency distribution of crown ratios at the beginning of the study by live/dead status at 

yr 5. This figure shows that (1) few trees died, and (2) most of the dead trees had small 

crown ratios. Of the 30 trees that died, 4 were dead at yr 1, 11 more died by yr 3, and 

15 more died by yr 5. Mortality was attributed to a wide variety of causes, including 

pine engraver beetles, animal damage, poor health, severe lean, mechanical damage, 

and unknown. Thinning stands in the fall of the year allowed slash to dry and avoided 

a buildup of pine engraver beetle populations.
USDA Forest Service Res. Pap. RMRS-RP-88. 2011 7

Table 3. List of variables used in analysis of data and their means and extreme values.

Abbreviation Explanation Mean Min/Max

Tree variables

(unless otherwise noted, N = 905 trees)

CL0 Crown length at the time of thinning (H0-HCB0). (ft) 9.66 0.3 to 23.8

CL0/H-5grow Ratio of crown length to 5-yr height increment prior 

 to thinning. 904 trees. 1.41 0.3 to 3.6

CR0 Ocular crown ratio the year of thinning. 0.58 0.2 to 0.9

CR5 Ocular crown ratio at yr 5. 902 trees. 0.60 0.2 to 0.9

CRHCB Crown ratio calculated using height to crown base. 0.68 0.23 to 0.96

CR∆	 5-yr change in CRHCB. 900 trees. -0.035 -0.46 to 0.32

D0 Diameter at breast height the year of thinning. (inches) 2.75 0.0 to 7.9

D5 Diameter at breast height at yr 5. 903 trees. (inches) 3.97 0.0 to 10.3

Dgrow 5-yr diameter increment (D5 -D0). 868 trees. (inches) 1.21 0.0 to 3.2

H0 Tree height the year of thinning. (ft) 14.47 1.0 to 36.5

H5 Tree height at yr 5. 900 trees. (ft) 19.46 2.0 to 43.3

H0/D0 Height/Diameter ratio the year of thinning, in identical 

 units (H0/[D0/12.0]). 867 trees. 67.04 34.7 to 184.0

H5/D5 Height/Diameter ratio at yr 5. 892 trees. 62.72 34.4 to 180.0

Hgrow 5-yr post-thin height increment (H5-H0). 900 trees. (ft) 5.0 0.3 to 9.7

H-5grow 5-yr pre-thin height increment. 904 trees. (ft) 6.9 0.8 to 14.7

HCB0 Height to crown base the year of thinning. (ft) 4.8 0.5 to 16.3

HCB5 Height to crown base at yr 5. 901 trees. (ft) 7.0 1.0 to 21.0

HCB∆	 Change in height to crown base (HCB5 - HCB0). 901 trees. (ft) 2.2 0.0 to 9.1

Plot variables

(N = 350)

BAp Post-thin plot basal area. (ft2

/acre) 41.3 0 to 224

SLO Plot slope. (%) 11.3 0 to 22

TPAp Post-thin plot density (trees per acre) 2048 0 to 37,260

Experimental unit variables

(N = 35)

BA0 Pre-thin treatment basal area. (ft2

/acre) 78.8 26.7 to 149.0

BAt Post-thin treatment basal area. (ft2

/acre) 41.3 5.5 to 140.9

SPAC Class variable for nominal spacings of none (control), 

 5x5 ft, 7x7 ft, 10x10 ft, and 14x14 ft. n/a n/a

TPAt Post-thin treatment density. (trees per acre) 2048 164 to 13,997

Site variables

(N = 7 )

E Elevation. (ft) 3091 1700 to 4330

Table 4. Post-thin means for sample trees by spacing (ft). Within columns, means that are followed 

by the same letter are not statistically different at the P = 0.05 level.

H0 (ft) D0 (inches) CR0

Spacing Mean Std. Dev. Mean Std. Dev. Mean Std. Dev. N

Control 14.7a 7.5 2.77ab 1.63 0.59ab 0.16 126

5x5 13.5a 7.5 2.34a 1.48 0.55b 0.14 190

7x7 14.2a 6.9 2.67ab 1.53 0.58b 0.14 189

10x10 14.7a 6.0 2.83bc 1.33 0.57b 0.15 201

14x14 15.2a 6.0 3.13c 1.54 0.62a 0.16 199
8 USDA Forest Service Res. Pap. RMRS-RP-88. 2011

Diameter growth

The dependent variable used to model diameter growth is the natural logarithm of 

the 5-yr change in squared inside bark diameter [ln(dds)], based on the merits of this 

transformation (model behavior, accuracy, and validity of statistical assumptions) as 

discussed by Wykoff (1990). Diameter was measured at breast height, outside bark, but 

was converted to inside bark by dividing by 1.245 (Johnson 1956). Initial screening of 

the data indicated possible relationships between ln(dds) and many of the variables listed 

in Table 3, especially those related to tree size, vigor, and amount of growing space. 

Data screening also suggested potential dependent variable transformations to explore.

We calculated mean ln(dds) for the five spacings (Table 5). Spacings were significantly different from each other. The influence of more available growing space created 

by the different thinning intensities was explored in two separate analyses: (1) using 

actual post-thin density measures (plot and treatment trees/acre and basal area/acre) and 

(2) using spacing as a class variable. Variables were added into the equation based on 

improvements in AIC. The equation with density measures can be used to make predictions for any density within the range of our data, while the equation using spacing 

classes can be used to make predictions for operationally thinned stands that have the 

same nominal spacings as our study.

Equation with density measures:

ln(dds) = 1.6892 +0.7725*ln(D0) +0.1994*D0 -0.3758*sqrt(BAt+1) 

+0.2062*sqrt(BAt+1)*CR0 Eq. 1a1

1 A value of 1.0 is added to BAt to avoid a zero BAt*CR0 interaction when BAt is 0.0.

Figure 2. Frequency distribution of crown ratios at the beginning 

of the study by live/dead status at 5 yrs.

Crown ratio

250

200

150

100

50

0

0.1

Status Dead Live

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Frequency
USDA Forest Service Res. Pap. RMRS-RP-88. 2011 9

Equation with spacing classes:

ln(dds) = 0.6112 +1.0768*ln(D0) +0.2733*(D0) +SPAC +INTER*CR0

-1.3607*ln(H0) +0.5962*ln(H-5grow) Eq. 1b

where SPAC and INTER are adjustments to the intercept coefficient for each spacing:

Spacing SPAC INTER

Control 0.0 0.1381

5x5 0.0929 0.1973

7x7 0.8154 0.1162

10x10 1.0561 0.1135

14x14 2.0266 -0.0090

The log bias correction factor for Eq. 1a and 1b is 1.527.2

Both equations include tree size variables, with predicted diameter growth increasing as tree size increases. Density measures were significant in both equations (BAt in 

Eq. 1a and spacing in Eq. 1b), with increasing diameter growth as basal area decreases 

or as spacings became wider.

Also in both equations, CR0 is a measure of tree vigor and is included in the equation as an interaction with density. This interaction is easier to interpret in Eq. 1b. The 

spacing coefficients show that diameter growth increases for all trees with increases 

in spacing. In addition, the interaction term (INTER*CR0) shows that some spacings 

have better growth rates as crown ratio increases. The 5x5 spacing has the most rapid 

increase in diameter growth as crown ratio increases, followed by the control, and then 

the 7x7 and 10x10. Trees in the 14x14 spacing have the best diameter growth attributable to spacing, but the interaction between crown ratio and spacing is not significant. 

The interaction between spacing and crown ratio results in the differentiation in tree 

sizes within some spacings.

2

 When transforming predicted values to the original units of measurement, predictions must be multiplied by a log bias correction factor 

(LBCF) to avoid underestimations (Baskerville 1972). The formula is LBCF = EXP[X2

/2], where X is the standard deviation of the transformed 

variable. For this equation, average ln(dds) = 1.4352 and the standard deviation is 0.92005; therefore, LBCF = EXP[(0.92005)2

/2] = 1.527.

Table 5. Mean 5-yr diameter growth (Dgrow) and ln(dds)1 by 

spacing (ft). Within columns, means followed by 

the same letter are not statistically different at the 

P = 0.05 level.

Spacing Dgrow (inches) ln(dds) N

Control 0.71a 0.78a 120

5x5 0.89b 0.90a 179

7x7 1.15c 1.41b 175

10x10 1.41d 1.66c 199

14x14 1.77e 2.12d 195

1

 ln(dds) = natural logarithm of the 5-yr change in squared insidebark dbh.
10 USDA Forest Service Res. Pap. RMRS-RP-88. 2011

Height growth

The natural log of 5-yr height growth [ln(Hgrow)] was the dependent variable in the 

analysis of height growth. The best independent variables were tree size (D0), beginning crown ratio [ln(CR0], pre-thin height growth [ln(H-5grow)], and post-thin treatment basal area (BAt). The coefficients predicting height growth are shown in Eq. 2. 

Increases in height growth are associated with increasing initial tree size, larger crown 

ratios, and increasing 5-yr height growth prior to thinning. Decreases in height growth 

are associated with increases in post-thin treatment basal area. Shoot borer occurrence 

was not significant. An alternative equation to predict height growth from spacing was 

not developed because spacing was not significant.

ln(Hgrow)= 1.4856 +0.0917*D0 +0.2789*ln(CR0) +0.2335*ln(H-5grow) 

-0.1372*ln(BAt+1) Eq. 2

The LBCF for Eq. 2 is 1.079.

Height/diameter ratios

Inter-tree competition restricts diameter growth more severely than height growth, 

which results in trees that are tall relative to their diameter (Ritchie 1997; Henry and 

Aarssen 1999). These slender trees are more susceptible to wind and snow damage (Wonn 

and O’Hara 2001). A measure of slenderness is the height/diameter (H/D) ratio. In this 

study, initially slender trees reflect the degree of crowding experienced before thinning. 

Changes in H/D ratios following thinning reflect adjustments to the additional growing 

space, which could help managers choose the appropriate spacing for their objectives. 

H/D ratios were calculated using identical units in the numerator and denominator and 

using dbh. Higher H/D ratios indicate slender trees.

Figure 3 shows H0/D0 ratios for all sites. Very short trees have unrealistically high 

ratios, which indicate that H0/D0 ratios are not stable across a range of tree sizes. This 

Figure 3. Height/diameter (H0/D0) ratios by tree height at the time of 

thinning.

Height (ft)

190

180

170

160

150

140

130

120

110

100

90

80

70

60

50

40

30

0 5 10 15 20 25 30 35 40

H0/D0 ratio
USDA Forest Service Res. Pap. RMRS-RP-88. 2011 11

instability is caused by the numerator of the ratio beginning at 4.5 ft as trees grow past 

the dbh threshold, while the denominator begins at zero. For this reason, we restricted 

analyses of H/D ratios to trees ≥8 ft tall at the time of thinning.

Mean H0/D0 ratios for sample trees varied from 60.5 for the 14x14 spacing to 69.6 

for the 5x5 spacing (Table 6). At narrow spacings, crews left higher numbers of slender 

trees (higher H0/D0 ratios), but as spacing increased, the proportion of trees with lower 

ratios increased. This is a reasonable result because crews were instructed to leave the 

best available trees at the approximate spacing for the experimental unit. As spacing 

decreases, a higher proportion of poor trees must be left to obtain the desired spacing.

Table 6. Mean H/D ratios for sample trees ≥8.0 ft tall at yr 0

(H0/D0) and yr 5 (H5/D5) by spacing (ft). Means 

followed by the same letter are not statistically 

different at the P = 0.05 level.

Spacing Year Mean Std. Dev. N

Control 0 65.7ab 13.7 108

5 69.6a 16.6 108

5x5 0 69.6a 13.7 151

5 69.4a 15.8 150

7x7 0 64.8bc 12.5 163

5 61.2cde 12.8 162

10x10 0 63.9bcd 12.8 183

5 57.9e 12.7 181

14x14 0 60.5de 12.2 184

5 51.2f 8.7 183

Five yrs after thinning, mean H5/D5 ratios had changed across spacings (Table 6). At 

narrower spacings, the ratios or variation increased over time, indicating the trees were 

still crowded and were becoming more slender. At the 10x10 and 14x14 spacings, the 

ratios and variation decreased.

Graphical results of H5/D5 ratios are shown in Figure 4. As spacing increases, there 

is a large increase in the percentage of trees at a ratio of 60 and lower. Many of the trees 

in the tighter spacings were still slender, while trees at wider spacings were increasing 

in diameter relative to height.

We also analyzed ratios for 5-yr height growth divided by 5-yr diameter growth 

(Hgrow/Dgrow), because this ratio may be a more sensitive indicator of tree response to 

thinning. All trees ≥4.5 ft were used in this analysis. The best predictors of Hgrow/Dgrow

ratios are spacing, post-thin basal area (BAt), and beginning crown ratio (CR0). Mean 

Hgrow/Dgrow ratios decreased with increasing spacing, from 101.5 in the control to 38.2 

in the 14x14 treatment (Table 7). The largest change was 31.5 points (50% of the total 

decrease) between the 5x5 and 7x7 spacings. Similarly, Hgrow/Dgrow ratios decreased 

as beginning crown ratio (CR0) increased (Table 8). The trees with small crown ratios 

tended to occur at closer spacings, which meant they were growing at high densities 

before thinning and were still at relatively high densities after thinning (Table 8).

Change in crown ratios

Crown ratio is calculated using height to crown base and total tree height. Generally, 

trees released from competition would increase in crown ratio, as the lower branches 

persist longer and total height increases, unless trees have been so affected by dense 

conditions that they respond slowly to additional space (Oliver and Larson 1990). We 

calculated mean change in crown ratio (CR∆) for the five spacings (Table 9). The spacings were significantly different, except for the 10x10 and 14x14.
12 USDA Forest Service Res. Pap. RMRS-RP-88. 2011

Figure 4. Distribution of height/diameter (H5/D5) ratios at yr 5.

spacing=7x7

H5/D5 ratio

50

40

30

20

10

0

30 40 50 60 70 80 90 100 110 120

Percent

spacing=10x10

H5/D5 ratio

50

40

30

20

10

0

30 40 50 60 70 80 90 100 110 120

Percent

spacing=14x14

H5/D5 ratio

50

40

30

20

10

0

30 40 50 60 70 80 90 100 110 120

Percent

spacing=Control

H5/D5 ratio

50

40

30

20

10

0

30 40 50 60 70 80 90 100 110 120

Percent

spacing=5x5

H5/D5 ratio

50

40

30

20

10

0

30 40 50 60 70 80 90 100 110 120

Percent
USDA Forest Service Res. Pap. RMRS-RP-88. 2011 13

Table 7. Mean Hgrow/Dgrow ratio by spacing (ft). Means followed by the 

same letter are not statistically 

different at the P = 0.05 level.

Spacing Hgrow/Dgrow N

Control 101.5a 123

5x5 93.8a 183

7x7 62.3b 184

10x10 52.0bc 198

14x14 38.2c 198

Table 8. Mean Hgrow/Dgrow ratio by beginning crown ratio 

(CR0). Means followed by the same letter are not 

statistically different at the P = 0.05 level.

 Pre-thin Post-thin

 basal area basal area

CR0 Hgrow/Dgrow (BA0) (BAt) N

0.2 118a 94 35 5

0.3 104a 112 50 57

0.4 86a 102 48 124

0.5 82a 97 44 156

0.6 59b 68 36 232

0.7 53b 56 30 191

0.8 47b 41 24 106

0.9 41b 25 27 15

Table 9. Mean CR∆ by spacing 

(ft). Means followed by 

the same letter are not 

statistically different at 

the P = 0.05 level.

Spacing CR∆	 N

Control -0.117a 122

5x5 -0.065b 183

7x7 -0.037c 174

10x10 -0.002d 198

14x14 0.010d 194
14 USDA Forest Service Res. Pap. RMRS-RP-88. 2011

We modeled change in crown ratio (CR∆) based on independent variables determined 

from initial screening of the data. The influence of increased growing space after thinning was explored with separate analyses, one with measures of post-thin density and 

one with spacing as a class variable.

Equation with density measures:

CR∆ = 0.5211 -0.5027*CRHCB -0.06347*ln(BAt +1) Eq. 3a

Equation with spacing classes:

CR∆ = 0.1979 -0.4485*CRHCB + SPAC Eq. 3b

where SPAC is an adjustment to the intercept coefficient specific to each spacing level:

SPAC Coefficient

Control 0.0

5x5 0.0296

7x7 0.0710

10x10 0.1028

14x14 0.1291

Both equations include a coefficient for beginning crown ratio (CRHCB) and a measure 

of density. For both equations, as CRHCB increases, CR∆ decreases. And for any fixed 

CRHCB, tighter spacing results in a decrease in CR∆.

Height to crown base

The height to crown base (HCB) is defined as the lowest whorl in the crown that has 

at least two live branches. HCB is important because low crown bases increase the risk 

of tree torching and crown fires, while higher HCB values mean there is greater distance 

between the ground and base of the crown. A low canopy base height makes crown 

fire initiation easier (Scott and Reinhardt 2001). Also, crown retention has important 

implications for wood quality. We analyzed height to base of crown to (1) determine if 

HCB changed following spacing (HCB∆); (2) determine if HCB tended to change by 

spacing or, conversely, if there was large tree-to-tree variation in HCB within spacings; 

and (3) to predict HCB∆ for the first 5 yrs after thinning.

Table 10 shows attributes for HCB by spacing, and Figure 5 shows the decline in 

HCB∆ as spacing increases for the seven sites. There is a clear increase in HCB∆ at 

denser spacings, tapering off to smaller increases at wider spacings. When all sites are 

combined, HCB∆ increased 3.5 ft in the control, 2.7 ft in the 5x5, 2.2 ft in the 7x7, 1.7 ft 

in the 10x10, and 1.3 ft in the 14x14. Therefore, narrower spacings resulted in higher 

crown bases than wider spacings.

Table 10. Mean change in 5-yr height to crown base (HCB∆) and heights 

to crown base by year and spacing (ft). Within columns, means 

followed by the same letter are not statistically different at the 

P = 0.05 level.

Spacing Mean HCB∆ (ft) Mean HCB0 (ft) Mean HCB5 (ft)

Control 3.5a 4.6a 8.0a

5x5 2.7b 4.9a 7.6ab

7x7 2.2c 4.8a 7.0abc

10x10 1.7d 4.9a 6.6bc

14x14 1.3e 4.8a 6.1c
USDA Forest Service Res. Pap. RMRS-RP-88. 2011 15

Figure 5. Average change in height to crown base (HCB∆) by site and 

spacing: A = all sites, C = Cold Springs, E = East Castle Rock, M = Mosquito Heights, N = North Talmaks, R = Reubens, S = South Castle Rock, 

W = West Talmaks.

We calculated and plotted cumulative HCB0 and HCB5 distributions by spacing 

(Figure 6), which shows what can be expected for these types of operationally thinned 

stands. The control had the largest increase in HCB, followed by progressively smaller 

increases at wider spacings. Differences between HCB0 and HCB5 can be used to estimate the percentage of crowns that will be above a given height as a result of thinning 

to a particular spacing, which should be useful in determining spacings that have HCB 

values above wildfire or prescribed fire torching heights.

Regression analysis showed that beginning crown ratio (CR0), post-thin treatment 

basal area (BAt), and beginning diameter (D0) were the best variables for predicting 

HCB∆. Increases in HCB∆ were associated with larger initial crown ratios, increasing 

treatment basal area or spacing, and larger initial tree diameter.

Equation with density measures:

HCB∆ = -0.7507 +2.5305*CR0 +0.0301*BAt +0.2077*sqrt(D0) Eq. 4a

Equation with spacing classes:

HCB∆ = 1.4645 +1.1790*CR0 +0.8525*sqrt(D0) +SPAC Eq. 4b

where SPAC is an adjustment to the intercept coefficient for each spacing level:

SPAC Coefficient

Control 0.0

5x5 -0.6414

7x7 -1.2283

10x10 -1.8309

14x14 -2.3880

Spacing (sq.ft./tree)

5

4

3

2

1

0

0 25 50 75 100 125 150 175 200

HCBdelta

Site A

N

C

R

E

S

M

W
16 USDA Forest Service Res. Pap. RMRS-RP-88. 2011

Figure 6. Cumulative percentage height to crown base (HCB) distributions.

spacing=7x7

Height to crown base (ft)

100

80

60

40

20

0

0 5 10 15 20 25

Cumulative percent

year 0 5

spacing=10x10

Height to crown base (ft)

100

80

60

40

20

0

0 5 10 15 20 25

Cumulative percent

year 0 5

spacing=14x14

Height to crown base (ft)

100

80

60

40

20

0

0 5 10 15 20 25

Cumulative percent

year 0 5

spacing=Control

Height to crown base (ft)

100

80

60

40

20

0

0 5 10 15 20 25

Cumulative percent

year 0 5

spacing=5x5

Height to crown base (ft)

100

80

60

40

20

0

0 5 10 15 20 25

Cumulative percent

year 0 5
USDA Forest Service Res. Pap. RMRS-RP-88. 2011 17

Shoot borer damage

A total of 245 trees (27.1%) had shoot borer damage during the 5 yrs after thinning. 

The probability of shoot borer damage is predicted as a continuous value bounded in the 

interval [0,1] using the logistic equation 1/(1+e-∑ßiXi) where “e” is the base of natural 

logarithms, ß are regression coefficients, and X are independent variables.

Study site elevation (E) had the highest predictive value, with the probability of 

shoot borer damage decreasing with increasing elevation. The next best predictor 

was 5-yr height growth prior to thinning (H-5grow), which was positively associated 

with shoot borer damage. Beginning crown ratio (CR0) was also positively associated 

with shoot borer damage. Finally, post-treatment basal area (BAt) was inversely related 

to the probability of shoot borer damage. The equation is:

Prob = 1/(1+e-(-0.5467 -0.00075*E +0.6612*ln(H-5grow) +1.2225*CR0

-0.0085*BAt)) Eq. 5

An alternative equation to predict shoot borer damage from spacing was not developed 

because spacing was not significant.

Non-Tree Vegetation and Thinning Slash

The forbs and shrubs that occurred on the study sites are very typical for these habitat 

types. There are differences in species composition between the Nez Perce and Spokane 

Tribal lands, but these differences relate to habitat types. Species acronyms, common 

names, and scientific names are given in Table 11, while occurrence for forb and shrub 

species is shown in Table 12 by study site. Species that are fairly common across all 

study sites are common yarrow, clustered frasera, thistles, vetch, serviceberry, black 

hawthorn, roses, spiraea, and snowberry. Forbs common on wetter sites are strawberry, 

northern bedstraw, sweetscented bedstraw, old man’s whiskers, sticky purple geranium, 

hawkweed, Brewer’s mitrewort, mountain sweet-cicely, and penstemon. Field bindweed, 

coast tarweed, and spotted knapweed have high occurrence at the two Castle Rock sites, 

which are the driest sites in the study. The decrease in spotted knapweed at the two 

Castle Rock sites between yrs 1 and 5 (Table 12) is attributed to release of biological 

control agents (Bangasternus fausti in 2001 and Larinus minutus in 2002). Both agents 

are weevils that feed on flower heads of spotted knapweed.

The forb species that had the highest occurrences are shown for yr 1 and 5 in Table 13. 

Some species had increased occurrence at wider spacings, some species had decreased 

occurrence at narrower spacings, some species increased at all spacings, and some species had no discernable pattern.

Epilobium (species other than fireweed, Epilobium angustifolium) had increased 

occurrence as spacing increased. There was no change in the control, +5% in the 5x5 

spacing, +14% in the 7x7, +14% in the 10x10, and +23% in the 14x14. Similar responses 

of increased occurrence at wider spacings were found for vetch and, to a lesser extent, 

for field bindweed, Gairdner’s yampa, cinquefoil, and penstemon.

Strawberry had a 14% decrease from yr 1 to yr 5 in the control, but was fairly stable 

over time at all spacings. Thistle had 8 to 11% decreases in the 5x5, 10x10, and 14x14 

spacings, but no change in the 7x7 and control. Coast tarweed was not found in any 

spacings in yr 1 after the thinning but had the highest occurrence of any forb by yr 5. 

Coast tarweed occurred more often at the three wider spacings (29% in the 7x7, 20% in 

the 10x10, and 29% in the 14x14), but less often in the control (11%) and 5x5 (14%). 

Also, dandelion had increases at all spacings, varying from 11 to 20%.
18 USDA Forest Service Res. Pap. RMRS-RP-88. 2011

Table 11. Acronym, common name, and scientific name for plant species.

Acronym Common name Scientific name

Forbs

ACMI Common yarrow Achillea millefolium

CEST Spotted knapweed Centaurea stoebe

COAR Field bindweed Convolvulus arvensis

EPAN Fireweed Epilobium angustifolium

EPSP Epilobium Epilobium species other than EPAN

ERSP Daisy Erigeron species

FORB Forbs All forb species

FRAG Strawberry Fragaria virginiana (92%) and F. vesca (8%)

FRFA Clustered frasera Frasera fastigiata

GABO Northern bedstraw Galium boreale

GATR Sweetscented bedstraw Galium triflorum

GETR Old man’s whiskers Geum triflorum

GEVI Sticky purple geranium Geranium viscosissimum

GRAS Grass and sedge species Grasses and sedges

HIAL Hawkweed Hieracium albertinum and H. albiflorum

LUPN Lupine Lupine species

MASA Coast tarweed Madia sativa

MIBR Brewer’s mitrewort Mitella breweri

OSCH Mountain sweet-cicely Osmorhiza chilensis

PESP Penstemon Penstemon species

PEGA Gairdner’s yampa Perideridia gairdneri

POGR Cinquefoil Potentilla gracilis

TAOF Dandelion Taraxacum officinale

THIS Thistle Thistle (Cirsium) species

THOC Western meadowrue Thalictrum occidentale

TRDU Western salsify Tragopogon dubius

VTCH Vetch Vetch (mostly Vicia americana)

Shrubs

AMAL Serviceberry Amelanchier alnifolia

BEAQ Tall Oregon grape Berberis aquifolium

CEVE Shinyleaf ceanothus Ceanothus velutinus

CRDO Black hawthorn Crataegus douglasii

HODI Oceanspray Holodiscus discolor

PHMA Ninebark Physocarpus malvaceus

PRVI Common chokecherry Prunus virginiana

RICE Wax current Ribes cereum

ROSA Rose species Rosa species

SALX Willow species Salix species

SPBE Spiraea Spiraea betulifolia

SYAL Common snowberry Symphoricarpos albus

Other

SLAS Thinned p-pine slash n/a
USDA Forest Service Res. Pap. RMRS-RP-88. 2011 19

Table 12. Average percent occurrence by species and site. The first number in each box is for yr 1 and the second number 

is for yr 5. The definitions for species acronyms are given in Table 11.

Cold West North South East Mosquito

Species Springs Reubens Talmaks Talmaks Castle Rock Castle Rock Heights

Forbs

ACMI 60 76 16 28 72 64 4 12 16 32 16 8 4 4

CEST 0 0 0 0 0 0 0 0 88 52 96 16 0 0

COAR 0 0 0 0 0 0 0 0 76 88 36 52 0 0

EPAN 4 4 0 0 36 32 0 4 0 0 0 0 0 0

EPSP 0 4 0 0 4 4 0 0 8 20 16 72 0 4

ERSP 0 12 4 16 0 8 0 0 0 0 0 0 0 0

FORB 100 100 100 100 100 100 100 100 100 100 100 100 100 100

FRAG 64 44 0 8 76 80 24 8 0 0 0 0 28 40

FRFA 28 28 0 0 20 40 20 24 44 52 8 8 12 0

GABO 64 88 0 0 72 80 8 32 0 0 0 0 0 0

GATR 0 0 0 0 0 0 0 0 0 0 0 0 0 4

GETR 16 24 0 0 28 48 0 12 0 0 0 0 0 0

GEVI 12 44 0 0 68 60 16 20 0 0 0 0 4 0

GRAS 100 100 100 100 100 100 100 100 96 100 48 100 88 100

HIAL 12 16 0 0 28 16 0 4 0 0 0 0 0 0

LUPN 0 20 0 0 0 24 0 0 0 12 0 4 0 8

MASA 0 0 0 0 0 0 0 0 0 52 0 68 0 24

MIBR 0 0 0 0 24 16 8 0 0 0 0 0 0 0

OSCH 20 16 0 4 8 0 0 0 0 0 0 0 0 0

PESP 40 60 0 0 64 56 24 12 0 0 0 0 0 12

PEGA 0 16 0 0 0 32 12 20 0 0 0 0 0 0

POGR 44 16 4 4 12 60 4 24 0 0 0 0 0 8

TAOF 0 0 0 48 0 16 0 12 0 24 0 0 8 12

THIS 32 36 8 0 20 32 12 8 12 0 0 0 24 0

THOC 0 0 0 0 4 16 0 12 0 0 0 0 0 0 

TRDU 0 8 4 8 4 4 0 0 4 32 0 4 4 0

VTCH 60 56 4 12 0 0 20 36 4 32 12 60 0 0

Shrubs

AMAL 4 24 56 76 72 88 12 32 0 24 4 4 36 76

BEAQ 0 0 0 0 0 0 0 0 0 0 0 0 4 4

CEVE 0 0 0 0 0 0 0 0 0 0 0 0 16 8

CRDO 0 8 28 28 0 12 4 4 0 4 0 4 24 64

HODI 12 20 0 0 0 0 0 0 0 0 0 0 12 20

PHMA 4 0 0 0 0 0 0 0 0 0 0 0 4 4

PRVI 0 0 0 0 0 0 20 20 0 0 0 0 32 36

RICE 32 36 0 0 0 0 0 0 0 0 0 0 0 0

ROSA 64 84 8 12 92 96 76 72 0 0 0 0 40 64

SALX 0 0 0 0 0 0 4 0 0 0 0 0 0 4

SPBE 0 0 8 12 12 4 72 68 0 4 4 0 44 60

SYAL 100 100 4 8 96 100 96 100 44 56 12 28 100 100
20 USDA Forest Service Res. Pap. RMRS-RP-88. 2011

Only six shrub species were abundant enough for analysis of occurrence, percent 

cover, and height; and few of these attributes changed with spacings from yr 1 to yr 5 

(Table 14). Serviceberry occurrence varied from 20 to 37% in yr 1, increasing to 40 to 

54% in yr 5, but these changes were not significant. The average increase in occurrence 

across all spacing for serviceberry was 20%, but the control increased about the same. 

Cover and height of serviceberry changed little between yrs 1 and 5.

Black hawthorn occurrence varied from 9 to 14% in yr 1, increasing to 11 to 31% 

in yr 5, but these changes were not significant (Table 14). Cover and height of black 

hawthorn changed little between yrs 1 and 5. Chokecherry occurrence varied from 0 

to 14% in yrs 1 and 5, with essentially no increase in occurrence. Percent cover did 

increase consistently for the three wider spacings, but by less than 4% (not significant). 

Roses had overall high occurrence, which varied from 31 to 49% in yr 1 and increased 

slightly to 40 to 57% in yr 5, but these changes were not significant. Average change 

in rose cover showed some significant differences between spacings, but increases in 

height were not significantly different.

Spiraea increased slightly in occurrence, cover, and height between yrs 1 and 5, but 

there were no significant differences between spacings. Snowberry had the highest occurrence of all shrub species, averaging 60 to 69% in yr 1, with minor increases to 66 

to 77% in yr 5. Percent cover of snowberry was also the highest of all shrub species, 

averaging 19 to 24% in yr 1 and 20 to 25% in yr 5. Changes in snowberry occurrence, 

cover, and height were not significantly different between spacings.

The bottom of Table 14 shows a summary for all shrubs, which includes species 

other than the six listed in the upper part of the table. Percent cover for all shrubs is the 

summation of cover for each shrub species on the plot, which could exceed 100% due 

to overlapping crowns. There is a small increase in shrub occurrence (averaging 7.8 %) 

and percent cover (averaging 3.7%) but no patterns related to spacing.

Table 13. Average percent occurrence of forb species along line intercept transects having a minimum of 20 occurrences by 

spacing (ft). Definitions for species acronyms are given in Table 11.

Control 5x5 7x7 10x10 14x14

Species 1 5 5-11 1 5 5-1 1 5 5-1 1 5 5-1 1 5 5-1

ACMI 14 20 6 23 29 6 31 37 6 31 29 -2 34 49 14

CEST2 26 11 -15 26 9 -17 26 6 -20 26 11 -15 29 11 -17

COAR 23 20 -3 20 26 6 20 20 0 11 17 6 6 17 11

EPAN 3 6 3 6 9 3 11 9 -2 9 6 -3 0 0 0

EPSP 6 6 0 6 11 5 0 14 14 3 17 14 3 26 23

FRAG 31 17 -14 26 23 -3 29 29 0 34 29 -5 31 31 0

FRFA 17 23 6 34 26 -8 17 23 6 9 23 14 17 14 -3

GABO 11 23 12 23 26 3 20 31 11 26 34 8 23 29 6

GETR 6 11 5 3 11 8 11 20 9 6 9 3 6 11 6

GEVI 9 17 8 11 14 3 17 17 0 14 20 6 20 20 0

MASA 0 11 11 0 14 14 0 29 29 0 20 20 0 29 29

PEGA 0 3 3 0 11 11 0 9 9 0 14 14 9 11 3

PESP 23 17 -6 31 29 -2 23 29 6 6 9 3 9 17 8

POGR 0 6 6 11 20 9 11 26 15 14 14 0 9 20 11

TAOF 3 17 14 3 23 20 0 11 11 0 17 17 0 11 11

THIS 14 14 0 14 6 -8 11 11 0 17 9 -8 26 14 -11

VTCH 14 17 3 11 20 9 9 23 14 17 37 20 23 40 17

FORB 80 90 10 97 94 -3 87 90 3 91 94 3 91 97 6

GRAS 100 93 -7 100 97 -3 100 90 -10 100 100 0 100 100 0

SLAS n/a n/a n/a 81 78 -3 87 90 3 94 94 0 94 94 0

1 Yr 5 minus yr 1.

2 Decline is attributed to two introduced biological control insects. See text for details.
USDA Forest Service Res. Pap. RMRS-RP-88. 2011 21

Table 14. Average occurrence, cover, and height of shrub species by spacing (ft). Within species and variable 

(occurrence, cover, or height), means followed by the same letter are not statistically different at the 

P = 0.05 level. Definitions for species acronyms are given in Table 11.

Occurrence (%) Cover (%)2 Height (ft)2

Species Spacing 1 5 5-11 1 5 5-11 1 5 5-11

AMAL Control 20 43 23a 1.87 3.80 1.93a 1.10 1.52 0.42a

5x5 37 54 17a 2.00 2.42 0.42a 1.11 1.18 0.07a

7x7 26 40 14a 2.50 3.43 0.93a 1.01 1.82 0.81a

10x10 20 49 29a 1.65 3.59 1.94a 1.52 2.24 0.72a

14x14 29 46 17a 1.63 3.44 1.81a 1.03 2.07 1.04a

CRDO Control 9 20 11a 2.43 4.29 1.86a 1.97 3.60 1.63a

5x5 9 11 2a * * * * * *

7x7 14 31 17a 0.45 1.00 0.55a 0.51 1.25 0.74a

10x10 9 11 2a * * * * * *

14x14 9 14 5a 0.60 1.00 0.40a 0.56 1.18 0.62a

PRVI Control 3 6 3a * * * * * *

5x5 0 0 0a

7x7 14 14 0a 5.20 8.40 3.20a 1.84 2.66 0.82a

10x10 6 6 0a * * * * * *

14x14 14 14 0a 4.40 8.20 3.80a 2.67 2.33 -0.34a

ROSA Control 40 40 0a 1.86 3.36 1.50a 1.24 1.51 0.27a

5x5 49 57 8a 1.45 1.20 -0.25c 0.67 0.98 0.31a

7x7 43 46 3a 1.63 1.50 -0.13bc 0.93 1.09 0.16a

10x10 34 40 6a 1.07 1.86 0.79abc 0.77 1.08 0.31a

14x14 31 51 20a 1.22 2.44 1.22ab 0.76 1.27 0.51a

SPBE Control 23 29 6a 3.30 4.00 0.70a 0.67 0.67 0.0a

5x5 20 23 3a 2.38 3.00 0.62a 0.55 0.64 0.09a

7x7 20 23 3a 6.50 7.50 1.00a 0.43 0.78 0.35a

10x10 26 20 -6a 1.43 1.57 0.14a 0.68 0.75 0.07a

14x14 11 11 0a * * * * * *

SYAL Control 66 77 11a 21.74 21.15 -0.59a 1.55 1.42 -0.13a

5x5 69 69 0a 22.25 24.71 2.46a 1.39 1.52 0.13a

7x7 69 69 0a 18.75 20.33 1.58a 1.48 1.48 0.0a

10x10 60 66 6a 24.43 25.09 0.66a 1.25 1.34 0.09a

14x14 60 71 11a 21.84 19.40 -2.44a 1.27 1.31 0.04a

Shrubs Control 74 86 12 22.47 27.61 5.14

5x5 83 89 6 21.67 22.77 1.10

7x7 80 83 3 21.16 25.65 4.49

10x10 74 80 6 19.61 23.83 4.22

14x14 74 86 12 24.27 27.63 3.36

1 Yr 5 minus yr 1.

2 Mean for those plots on which the species occurred by yr 5; if the species did not occur on the plot in yr 1, cover and height 

were assigned a value of zero for yr 1.

*Fewer than 5 plots available to calculate the mean.
22 USDA Forest Service Res. Pap. RMRS-RP-88. 2011

Table 15. Average percent cover of forbs, grasses, and ponderosa 

pine thinning slash along line intercept transects for 

plots where the species occurs by spacing (ft).

Variable Spacing Yr 1 (%) Yr 5 (%) 5-11 (%)

Forb Control 7 5 -2

5x5 7 9 2

7x7 9 9 0

10x10 7 7 0

14x14 7 6 -1

Grass Control 25 15 -10

5x5 23 12 -11

7x7 19 14 -5

10x10 19 12 -7

14x14 17 8 -9

SLAS Control n/a n/a n/a

5x5 9 4 -5

7x7 11 6 -5

10x10 17 8 -9

14x14 13 7 -6 

1

Yr 5 minus yr 1.

As shown in Table 13, forb and grass occurrence was always high, averaging at least 

80% occurrence for forbs and at least 90% for grasses. Forb coverage did not change 

appreciably (≤2%) between yr 1 and yr 5 (Table 15). Grass coverage decreased in all 

spacings between yr 1 and yr 5, from -5% in the 7x7 spacing to -11% in the 5x5. The 

control and 5x5, which had the highest values at yr 1, had the largest declines in coverage of grass.

Ponderosa pine thinning slash was recorded on the line transects. Highest occurrences 

were in the wider spacings, starting at ~80% for the 5x5, ~90% for the 7x7, and 94% 

for the 10x10 and 14x14 (Table 13). The measurement of slash in yr 1 included stems, 

branches, and foliage. By yr 5, foliage had fallen off the branches and was mostly incorporated into the duff layer. Therefore, average percent cover of ponderosa pine thinning 

slash decreased between 5 and 9% from yr 1 to yr 5 (Table 15, Figure 7).

Discussion ______________________________________________________________

Data for this study came from naturally regenerated, operationally thinned-frombelow stands. Thinning crews chose the best available trees at the approximate spacing 

that was randomly chosen for the experimental unit. Thinning from below results in the 

“selection effect” (discussed by Hynynen 1995) or the “chainsaw effect,” meaning that 

leave trees in stands that are thinned from below grew better before thinning compared to 

those removed during thinning. In our study, there is a second component to the selection 

effect because of pre-thin heterogeneity in tree sizes and vigor. With a variety of tree 

sizes and vigor to choose from, it is expected that there would be a higher proportion of 

larger, better trees at wider spacings than at narrower spacings. This happened because 

at narrower spacings, crews had no choice but to leave smaller, low vigor trees; while 

at wider spacings, there were generally enough large, better vigor trees to meet spacing requirements. Therefore, mean tree characteristics vary with spacing immediately 
USDA Forest Service Res. Pap. RMRS-RP-88. 2011 23

after thinning. Our analyses that used individual tree characteristics allow prediction 

for stands that may differ from ours in terms of tree sizes, H/D ratios, crown ratios, etc.

Thinning studies differ from plantation spacing studies because in plantation spacing studies, tree size and vigor are similar across spacings, and spacings can be rigidly 

controlled. Thinning studies are less likely to meet the designated densities. Even if 

dense areas are rigidly spaced, understocked or non-stocked patches within treatments 

will result in wider average spacings than specified, which appeared to be the case in 

the 5x5 and 7x7 spacings (Table 2).

Trees

Severe lean and mortality

Trees that were classified as having severe lean following thinning were those we 

considered unable to recover and grow vertically. Only 15 trees (1.6%) developed severe 

lean, showing that the vast majority of trees did not have problems remaining upright 

after thinning, even though pre-thin densities averaged from 2,469 trees/acre at Reubens 

to 10,128 trees/acre at East Castle Rock. Few trees died following the thinning (30 trees 

or 3.1%). Those trees that died had smaller crown ratios, were growing slower relative 

to other trees, and had higher H0/D0 ratios (slender trees). Therefore, severe lean or 

tree death following thinning does not seem to be a major concern for ponderosa pine 

on these dry sites where snowloads are not generally a problem. However, our results 

should be used with caution because snowloads can differ between areas and between 

different years.

Our results agree with other research. Ronco and others (1985) found little tree damage and mortality in a sapling- and pole-size ponderosa pine thinning experiment in 

northern Arizona, except for some snow damage the first spring after thinning. About 

12% of the trees had snow damage and were removed; these trees were the smaller 

Figure 7. Percent cover of ponderosa pine thinning slash over time 

by spacing.

Years since thinning

20

15

10

5

0

1 3 5

Percent cover

spacing 5x5 7x7 10x10 14x14
24 USDA Forest Service Res. Pap. RMRS-RP-88. 2011

trees in their study. Cochran and Barrett (1999) attributed all mortality in their study to 

mountain pine beetle (Dendroctonus ponderosae) in a pole-size ponderosa pine thinning study in central Oregon. Those naturally regenerated trees averaged 48 ft tall at the 

time of treatment, compared to 14.5 ft tall in our study. Even though the trees sampled 

by Cochran and Barrett (1999) only averaged 25% crown ratio, the authors reported no 

snow or wind damage following thinning. Barrett (1982) reported that mortality was 

minor in thinned ponderosa pine trees that were about 1 inch dbh and 8 ft tall at the time 

of thinning. No mortality occurred at the widest spacings, and most of the mortality was 

in the smallest diameter class.

Diameter growth

Diameter growth (Dgrow) was greatly influenced by spacing—widely spaced trees 

grew faster in diameter than narrowly spaced trees. Also, larger trees, as measured by 

beginning dbh (D0), had faster diameter growth, and trees with larger beginning crown 

ratios (CR0) had faster diameter growth. Our results showing higher diameter growth 

being associated with larger trees and trees with larger crown ratios agrees with other 

research (e.g., Wykoff 1986; Wagner and Radosevich 1991; Hynynen 1995). Larger 

trees are in a more dominant position than smaller trees. Beginning crown ratio is an 

indicator of pre-thin conditions, likely serving as a surrogate for stand density (Garber 

and others 2008). Trees with larger crown ratios have the photosynthetic capacity to 

support faster growth rates.

There is good agreement that lower densities result in faster diameter growth for 

ponderosa pine (Meyer 1934; Oliver 1979; Barrett 1982; Ronco and others 1985; Oliver 

1997) and for other species that can grow in the Inland Northwest (Johnstone 1981; 

Reukema and Smith 1987; Harrington and others 2009).

Height growth

Five-yr post-thin height growth was not related to spacing. Height growth was better for larger diameter trees (D0), trees with larger crown ratios (CR0), and trees with 

better pre-thin height growth (H-5grow). Height growth decreased with increasing postthin treatment basal area (BAt). For seedlings and saplings in this study, it was typical 

that larger trees in the size classes grew more rapidly than smaller trees (Wykoff 1986; 

Reukema and Smith 1987; Ferguson 1988; Wagner and Radosevich 1991; Uzoh and 

Oliver 2006). Smaller pre-thin crown ratios indicated competition that had prevented 

trees from developing fuller crowns, which meant these trees needed time to build crowns 

following thinning in order to support normal growth. Lower pre-thin height growth 

was an indication of competition, poor microsites, or genetic variation within species.

Post-thin treatment basal area (BAt) was a significant predictor of height growth, 

but spacing (SPAC) was not. SPAC and BAt are two different measures of competition. 

SPAC indicates the approximate amount of growing space for each tree, regardless of 

tree size. BAt includes the basal area attributed to competing trees on the 1/400-acre 

plot, plus any trees in the variable radius plot. Trees in the variable radius plot were 

greater than or equal to 2.3 inches dbh and contribute to BAt more than small-diameter 

trees on the 1/400-acre plot because the contribution of each tree to BAt is proportional 

to diameter squared. Therefore, higher BAt values indicate that the subject tree was 

likely receiving competition from larger trees, whereas SPAC does not imply anything 

about size of competing trees.

There is fairly good agreement in the scientific literature that height growth of trees 

is not influenced by spacing, except at very high or very low densities. Lanner (1985) 

reviewed literature for several species and provided explanations for height growth 

insensitivity to spacing and diameter growth sensitivity to spacing. Specific to ponderosa pine, Oliver (1984) found that height growth of trees in a thinned plantation of 
USDA Forest Service Res. Pap. RMRS-RP-88. 2011 25

saplings in northern California was not influenced by spacing, and Oliver (1990) found 

that height growth of planted ponderosa pine was not influenced by spacing on the west 

slope of the Sierra Nevada Mountains. Meyer (1934) noted that in cutover ponderosa 

pine forests of the Pacific Northwest, increased diameter growth was not accompanied 

by increased height growth.

However, there is also contradictory evidence on the generality that spacing does not 

influence height growth. Ronco and others (1985) found that annual height growth of 

ponderosa pine decreased with increasing basal area during the first 10-yr period after 

thinning in northern Arizona, but basal area was not significant during the second 10-yr 

period. They attributed the significant negative association between height growth and 

basal area to the removal of shorter trees of poor form during thinning, rather than to 

increased height growth with lower stand densities. This is an example of the selection 

effect cited by Hynynen (1995).

Barrett (1982) found that thinned ponderosa pine had better height increments at wider 

spacings in central Oregon. Although significant, Barrett cautioned that the trees had 

been very suppressed by high sapling densities and an overstory, and were growing only 

a few inches in height per year before thinning. It may have been possible that released 

saplings were able to respond more rapidly at the wider spacings, while adjustments 

were taking longer at narrower spacings.

Van Deusen (1968) reported that average height growth of ponderosa pine in the Black 

Hills of South Dakota was always greater for trees in thinned stands versus unthinned 

stands. The length of the height growth period was similar between thinned and unthinned 

stands; however, density could be confounded with tree size at the time of treatment 

because trees in the thinned stand averaged 5.2 inch dbh while trees in the unthinned 

stand averaged 3.5 inch dbh. This, too, appears to be an example of the selection effect.

Results from plantation spacing trials may be relevant to detecting density effects on 

height growth. Oliver (1997) found that height growth was weakly related to density 

in a ponderosa pine plantation thinned to five stocking levels. There was a decline in 

height growth from low density to high density, but this relationship was only significant 

in the first two of the five 5-yr growth periods. Reukema and Smith (1987) found that 

plantation spacing had little effect on heights of Douglas-fir (Pseudotsuga menziesii), 

western redcedar (Thuja plicata), or western hemlock (Tsuga heterophylla). Harrington 

and others (2009) found no spacing differences in Douglas-fir plantations for 3-yr height 

or 25-yr top height. Reukema (1979) found that, on better sites, plantation spacing had 

little effect on height growth; but on poor sites, better growth at wide spacings versus 

close spacings may be common.

The response of ponderosa pine to various densities is complicated by interacting 

factors—response to thinning, differences in initial tree size, interactions between tree 

size and density levels, competing tree sizes, shrub competition, site productivity, and 

the selection effect. Studies that consider many factors together are insightful. Uzoh 

and Oliver (2006) analyzed data from long-term permanent research plots in even-aged, 

pure stands of planted and natural ponderosa pine. There were six levels of density in 

stands throughout the species range in the western United States. Ponderosa pine height 

increment decreased with increasing Stand Density Index (SDI, a measure of density), 

but SDI was the least important of the seven variables in the equation. Other variables 

predicting height growth were site index, dbh, dbh2

, the interaction of slope and aspect, 

elevation, and basal area in larger trees (BAL). Because the equation included BAL, 

which is a measure of the tree’s position relative to other trees in a plot or stand, Uzoh 

and Oliver felt that SDI represents the effect of density on height growth of all trees in 

the stand regardless of tree size.

Thus, while there is no consensus on the effect of spacing on ponderosa pine height 

growth, it is clear from the literature that the effect of spacing on height growth is not 
26 USDA Forest Service Res. Pap. RMRS-RP-88. 2011

as pronounced as the effect of spacing on diameter growth. Our finding of no spacing 

effect on height growth likely results from choosing sample trees that were leave trees 

in operationally thinned-from-below stands or, in the case of the controls, that would 

have been leave trees.

Height/diameter ratios

An H/D ratio is the quotient quantifying a unit of height per unit of diameter. 

These ratios have been used to determine when trees are susceptible to snow and 

wind damage. For example, Wonn and O’Hara (2001) found that four northern 

Rocky Mountain conifer species (including ponderosa pine) were more prone to stem 

bending and breakage when H/D ratios were 80:1 (ft:ft) or higher. Slender trees develop 

in higher stand densities; thus, H/D ratios can be controlled silviculturally by managing 

stand density. Increased stand density results in height growth at the expense of diameter growth as trees try to attain or retain apical dominance (Henry and Aarssen 1999; 

Wonn and O’Hara 2001). Evidence has accumulated that trees “perceive” competition 

from neighboring plants through the phytochrome system (e.g., Ritchie 1997; Henry 

and Aarssen 1999). The light reflected from competitor foliage has a reduced red:far red 

ratio compared to solar radiation, which provides trees with a signal of nearby competition even before shading occurs.

Because H/D ratios are a function of density, they can be used to study response to 

thinning and as an indicator of competition. Our data show that H/D ratios increased in 

the 5 yrs after thinning in the control plots (Table 6), indicating that trees may be responding to inter-tree competition. Average H/D ratios remained statistically unchanged in 

the 5x5 and 7x7 spacings but decreased significantly in the 10x10 and 14x14 spacings. 

To obtain a significant decrease in H/D ratios, thinning to at least 10x10 spacing is appropriate for ponderosa pine seedlings and saplings on these types of sites.

Hgrow/Dgrow ratios decreased rapidly with increasing spacing (Table 7), from 101.5 

in the control to 38.2 in the 14x14. The ratio declined from 101.5 to 93.8 between the 

control and 5x5, followed by a decline of 31.5 from the 5x5 to the 7x7 spacing. Between 

the 7x7 and 10x10 spacings, the average ratio declined by 10.3, followed by a decline 

of 13.8 between the 10x10 and 14x14 spacings. Because we found that spacing affected 

dbh growth but not height growth, we conclude that at least a 7x7 spacing is necessary 

to stimulate significant diameter growth response in these types of sites.

Reukema and Smith (1987) found that height/diameter ratios were influenced by 

spacing in plantations in British Columbia. Douglas-fir, western redcedar, and western 

hemlock had higher ratios (slender trees) at closer spacings, which agree with our results 

for ponderosa pine.

Change in crown ratios

The two variables important in predicting 5-yr change in crown ratio (CR∆) are beginning crown ratio (CRHCB) and density. Density can be expressed as either spacing 

or residual basal area (BAt). Wider spacings or lower basal area result in lengthening 

crown ratios.

Reukema and Smith (1987) found in plantation spacing trials that wider spacings 

resulted in higher crown ratios for Douglas-fir and western redcedar. However, spacing 

had no effect on crown ratios for western hemlock.

Crown ratios are a morphological indicator of individual tree vigor and have been 

used to predict tree growth (e.g., Wykoff 1986; Ferguson 1988; Wykoff 1990). In forest 

growth models (Wykoff and others 1982; Crookston and Dixon 2005), crown ratio can 

be used to differentiate tree growth through successive time periods.
USDA Forest Service Res. Pap. RMRS-RP-88. 2011 27

Height to crown base

Height to crown base (HCB) has become increasingly important for rating stand risk 

to wildfire and prescribed fire, for predicting wood quality, and for including in forest 

growth models such as the Fire and Fuels Extension to the Forest Vegetation Simulator 

(FFE-FVS, Reinhardt and Crookston 2003). To date, information is generally lacking 

for this tree attribute and how it changes through management. Fortunately, HCB can 

be calculated if tree height and crown ratio are known.

HCB changes with post-thin density, beginning tree crown ratio (CR0), and initial 

tree diameter (D0). Trees within spacings tended to respond similarly. HCB increased 

more at higher densities than at lower densities, showing that the distance between the 

ground and the base of crowns can be silviculturally managed through density control. 

Also, HCB values had less variation at wider spacings than at narrower spacings, showing that trees in narrower spacings were differentiating more so than those in wider 

spacings. Tradeoffs exist between having high-density stands with high crown bases 

versus low-density stands with low crown bases because high crowns decrease fire 

hazard to trees, but high density increases fire hazard. Conceivably, dense stands could 

be thinned enough to lower H/D ratios, and then thinned again when crown bases reach 

a pre-determined height.

The cumulative percentage distributions of HCB5 shown in Figure 6 may be helpful in assessing the tradeoffs between HCB and density. For example, the cumulative 

distributions for the control (Figure 6a) show that about 30% of the trees had an HCB 

greater than 5 ft at the time of thinning. Five yrs later, about 75% of the trees had an 

HCB greater than 5 ft. Therefore, if an HCB of 5 ft or more was the target, an additional 

45% of the trees attained this value after 5 yrs.

Hann and Hanus (2004) developed HCB∆ equations for untreated Douglas-fir stands in 

southwestern British Columbia, western Washington, and northwestern Oregon. Crown 

ratio and stand density were also important in their equations, as well as stand age. In 

a study of thinned loblolly pine (Pinus taeda) in Virginia, Peterson and others (1997) 

found that lower branches remained alive longer in thinned stands. While live crowns 

in control stands decreased 30%, crown ratios in thinned stands decreased only 14%. 

The slower rate of decrease in the thinned stands was attributed to survival of lower 

branches. Crown recession occurs when branches at the base of the crown are shaded 

by competitors, which is primarily a function of density. Garber and others (2008) developed crown recession models for three conifer species in northern Idaho, including 

ponderosa pine. Crown ratio was the most important predictor of HCB∆, with crown 

recession increasing with greater crown ratios, which agrees with our study results.

Shoot borer damage

About one-quarter of the trees had shoot borer damage within the 5-yr period after 

thinning. Site elevation was the best predictor of shoot borer damage, with lower elevations correlated with an increasing probability of shoot borer occurrence. While we 

had only seven sites, and therefore seven elevations, we left elevation in the equation 

because of its known importance for predicting shoot borer damage. Other authors 

(Stoszek 1973; Robertson and Dewey 1983) have noted that dry, lower-elevation sites 

have a higher incidence of shoot borer damage than more mesic, higher-elevation sites. 

Within the range of elevations and site conditions in this study, we feel the use of elevation in Eq. 5 should work well.

Shoot borer damage is positively correlated with tree characteristics (H-5grow and 

CR0) and negatively correlated with post-thin treatment basal area (BAt). In northern 

Idaho and western Montana, Robertson and Dewey (1983) also found that trees with 

better height growth rates had higher infestation levels; however, unlike our results, 

they found a negative correlation between infestation rate and height growth. Sower 
28 USDA Forest Service Res. Pap. RMRS-RP-88. 2011

and Shorb (1984) explained that shoot borer infestation may appear to have no effect on 

height growth because larger trees or larger terminal buds are more likely to be selected 

by females as oviposition sites. Potential elongation of the terminal on larger trees or 

buds is greater than on smaller trees or buds, but actual elongation is decreased by bud 

mining. Therefore, direct comparison of infested and uninfested shoots may show no 

difference in growth.

Even though we found statistical significance for the prediction of shoot borer damage and the equation makes biological sense, trees that had shoot borer damage often 

appeared no different than undamaged trees. Perhaps factors other than tree and site 

characteristics were controlling shoot borer dispersion, survival, and colonization success.

Growth anomaly and climate

An anomaly became apparent when examining height growth data. As shown 

in Table 3, average 5-yr pre-thin height growth (H-5grow) was 6.9 ft, while average 

5-yr post-thin height growth (Hgrow) was 5.0 ft. This is counter-intuitive because the 

thinning-from-below should have made more resources available for tree growth. A 

further breakdown of the data showed that average height growth at the Nez Perce 

sites was 5.8 ft before thinning and 5.4 ft after thinning. Average height growth at the 

Spokane sites was 8.2 ft before thinning and 4.5 ft after thinning.

We examined the data for possible explanations for the growth decline. The decline 

was across all spacings, including the control, so it was not a response to thinning. Also, 

the decline was not related to shoot borer damage.

We were able to examine site climate patterns using techniques of Rehfeldt (2006) 

and Rehfeldt and others (2009). Data from western weather stations were used to 

develop thin plate spline surfaces to estimate monthly climate variables for the seven 

study sites. The monthly predictions were then averaged to produce yearly temperature 

and precipitation estimates, as well as growing season precipitation and temperature. 

Eighteen variables were predicted, as defined by Rehfeldt (2006).

Analyses of the climate data showed that the Spokane sites had an elevated growing season dryness index (GSDI) following thinning, while the Nez Perce sites did not 

(Figure 8). GSDI is calculated as the square root of growing season degree days that are 

greater than 5 oC divided by growing season precipitation. It has been used to predict 

conifer occurrence and abundance in the Inland Northwest (Rehfeldt and others 2008). 

Higher GSDI values indicate drier conditions during the growing season. Average GSDI 

for the 5-yr pre-thin period at the three Spokane sites was 0.263, and it was 0.386 for the 

5-yr post-thin period. Average GSDI for the 5-yr pre-thin period at the four Nez Perce 

sites was 0.108, and it was 0.135 for the 5-yr post-thin period.

Because annual shoot growth is determined by the number of needle primordia formed 

in the bud in the previous year, moisture stress during the growing season could result in 

poor terminal bud formation, which translates into less shoot elongation the following 

growing season (Lanner 1985). Therefore, high GSDI values at the Spokane sites could 

account for the slower-than-expected height growth. We feel that slower height growth 

after thinning at the Spokane sites did not affect our conclusions.

Non-Tree Vegetation and Thinning Slash

The forbs, grasses, and shrubs recorded during the course of this study are typical 

for these types of sites. There were no dramatic vegetation responses to the thinnings 

between yrs 1 and 5, with a few exceptions. For example, epilobium and vetch had 

steady increments in occurrence from the control to larger increases at wider spacings, 

and strawberry had a substantial decrease in the control with no corresponding decrease 

in the thinned areas (Table 13). Coast tarweed and dandelion had large increases across 

all spacings.
USDA Forest Service Res. Pap. RMRS-RP-88. 2011 29

Figure 8. Average growing season dryness index (GSDI) by year for the 

Nez Perce and Spokane study sites.

The invasive species spotted knapweed actually decreased across spacings, but this 

may have been due to the apparently successful introduction of two biological control 

insects that were released in 2001 and 2002 near the two Castle Rock sites.

Our separate analyses of shrub occurrence, cover, and height helped determine if 

species were becoming established on additional plots, increasing in coverage, and/or 

growing in height. Only serviceberry had consistently large increases in occurrence in 

thinned areas; however, there was also a large increase in the control, suggesting favorable conditions during the time period of this study (Table 14).

Although generally positive, no shrub species had increases in coverage greater than 

5%, and increased coverage in the controls was often as good as, or better than, in the 

thinned areas (Table 14). Shrub species did not respond much in percent cover between 

yrs 1 and 5 after thinning.

Shrub heights generally increased between yr 1 and yr 5, but again not significantly 

(Table 14). Serviceberry grew 0.81, 0.72, and 1.04 ft at the three wider spacings; black 

hawthorn had the best height growth in the control; and roses increased between 0.16 

and 0.51 ft in height at all spacings. Snowberry was, by far, the most dominant shrub 

species on these sites, but the response to thinning was nil (Table 14).

Griffis and others (2001) found little difference in understory vegetation between 

unthinned and thinned stands in Arizona. Unlike our young stands, their stands were 

mature, even-aged ponderosa pine. Their thinned stands averaged 81 ft2

 basal area/acre 

and 772 stems/acre, while the unthinned stands averaged 139 ft2

 basal area/acre and 

1968 stems/acre. The authors speculated that the thinnings, which removed at least 

30% of the basal area, may not have reduced density enough to result in a response in 

the understory.

Vegetation responses to thinning are confounded by a number of factors, including 

variation in weather from year to year, herbivory, secondary succession, and site productivity. For example, we noted that small black hawthorn plants were repeatedly browsed 

Year

0.5

0.4

0.3

0.2

0.1

0.0

1990 1995 2000 2005

GSDI

Location Nez Perce Spokane
30 USDA Forest Service Res. Pap. RMRS-RP-88. 2011

to groundline. Whatever response black hawthorn may have had to thinning was masked 

by intense preferential browsing of this species. Other species may be similarly limited 

in their ability to respond to thinnings. Animal exclosures would be useful to assess the 

effect of herbivory on vegetation response following thinning.

Although we did not control competing vegetation in this study, other researchers 

have noted an important tradeoff that may influence the timing and amount of thinning. 

In a northern California study of thinning 11-yr-old ponderosa pine, Oliver (1984) found 

no trend of increasing shrub growth as stand density decreased, which agrees with our 

results. However, he found that increasing shrub density restricted tree diameter and 

height growth. Above a threshold value of about 30% cover, shrub competition negated 

any positive effects of spacing on tree growth. The tradeoff is that stem quality and 

form factors can be improved at wider spacings with shrub competition (Oliver 1990). 

Ponderosa pine branch diameters in plots with shrubs averaged 71% of branch diameters in plots without shrubs. Busse and others (1996) found that understory vegetation 

negatively affected growth of ponderosa pine for the first 20 yrs after thinning to release 

suppressed ponderosa pine saplings in central Oregon. The authors also noted a longterm benefit to the upper soil horizon associated with maintaining understory vegetation 

versus long-term vegetation removal.

We feel that the generally mild responses of vegetation to thinning in this study are 

typical for sites where trees are felled with chainsaws. Ground disturbance in this study 

was minimal, which reduced potential sites for establishment of competing and invasive 

species. Other types of thinning, such as the use of heavy equipment or fire, would 

increase the amount of soil disturbance and, therefore, the potential for establishment 

of vegetation. Still, it was somewhat surprising that already established shrubs did not 

expand more in response to the thinning. Perhaps ungulate browsing prevented shrub 

response in these relatively small treatment units, whereas larger treatment units may 

have resulted in browsing being spread across larger areas. Perhaps the size of shrub 

plants was already near maximum at the time of thinning, so little response could be 

expected. Oliver’s (1984) finding that shrub growth did not decrease with increasing 

stand density may provide an explanation. He felt that perhaps shrubs were better at 

exploiting site resources than ponderosa pine saplings in northern California, thus the 

trees had little effect on the shrubs.

Ponderosa pine thinning slash had higher occurrences along line transects at wider 

spacings (Table 13). This was expected because more trees were cut and left on sites at 

wider spacings (except for a few plots at West Talmaks, where the slash was removed 

to eliminate problems with pine engraver beetles). Occurrences varied from 81% in the 

5x5 spacing to 94% in the 14x14 spacing. Percent cover increased at wider spacings 

(9% in the 5x5 to 17% in the 10x10; Table 15). Percent cover dropped rapidly by yr 3 

as the needles and fine twigs decomposed following thinning, then remained constant 

through yr 5 (Figure 7). An increase in fine fuels following thinning, which decay and 

are quickly incorporated into forest litter, was expected and has been documented by 

others (Youngblood and others 2008).

Conclusions _____________________________________________________________

These are naturally regenerated, operationally thinned ponderosa pine stands on relatively dry sites. Seedling- and sapling-size trees were thinned from below to leave the 

largest and most vigorous trees as evenly spaced as possible. Managers should expect 

similar results when pre-commercially thinning stands like those in this study.

Prior to thinning, these stands were very dense (2469 to 10,128 trees/acre). Trees 

were maintaining height growth at the expense of diameter growth, resulting in high 

H/D ratios (slender trees) and small crown ratios. After thinning, a few trees (1.6%) 
USDA Forest Service Res. Pap. RMRS-RP-88. 2011 31

with high H/D ratios developed severe lean. Only 3% of the trees died during the 5-yr 

period after thinning; these trees had high H/D ratios, small pre-thin height growth, and 

small crown ratios.

Fortunately, there are pre-thin morphological indicators of tree vigor that can be used 

to predict post-thin response. Larger trees and trees with larger crown ratios at the time 

of thinning are associated with larger post-thin diameter and height growth. Larger prethin height growth is associated with larger post-thin height growth.

Lowering the density of stands by thinning results in important changes in tree 

growth. Wider spacings were associated with larger diameter growth, which resulted in 

lower H/D ratios because spacing did not influence height growth. Wider spacings were 

also associated with larger crown ratios and crown bases closer to the ground because 

lower branches did not die as rapidly at wide spacings as they did at narrow spacings. 

At least a 10x10 ft spacing was required to obtain a significant lowering of H/D ratios 

during the 5-yr period.

Western pine shoot borer damage was recorded on one-quarter of the trees, but there 

was no statistical difference in height or diameter growth between infested and uninfested 

trees. Increased shoot borer damage was associated with larger crown ratios and larger 

pre-thin height growth. Damage was higher at lower densities and lower elevations.

Other vegetation growing in association with these ponderosa pine trees did not 

markedly change in response to the thinnings. Also, there was not an increase in nonnative invasive plant species. The lack of shrub, grass, and forb response was perhaps 

due to a combination of herbivory and the lack of ground disturbance during the thinning operations.

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