Historical stand reconstruction in ponderosa pine

forests to guide silvicultural prescriptions

Richy J. Harrod*, Bradner H. McRae, William E. Hartl

Leavenworth Ranger District, 600 Sherbourne, Leavenworth WA 98826, USA

Abstract

We reconstructed the historical stand structure and spatial patterning of ®re-maintained ponderosa pine forests in the Eastern

Cascades of Washington to develop and design silvicultural prescriptions to restore historical structure and composition. The

structure of the dominant overstory was inferred from the size and spatial patterning of stumps, logs, snags, and live trees

(>140 years of age) within 48 0.5 ha plots. Size class distributions, basal area, and spatial distribution of historical trees were

compared among plant association groups representing a range of environmental conditions. Using spatial point pattern

analysis, we found that signi®cant clumping at ®ne scales (0±15 m) existed historically. Spatial patterning of present day and

historical trees of four comparable plots suggests that while strong clumping exists in present day stands, the largest trees

today exhibit less clumping than did large historical trees. Historical SDI (260) for dominant overstories was nearly the same

as threshold for serious beetle mortality (263) for ponderosa pine. Cut-tree marking was carried out within 15 m radius circles,

as guided by the spatial patterning analysis, and using a sliding scale of trees per circle by quadratic mean diameter. # 1999

Elsevier Science B.V. All rights reserved.

1. Introduction

Public land managers are faced with ever-increas-

ing complexity in forest management. Establishing a

balance between ecological diversity and system resi-

liency and providing goods and services to a growing

human population is a central theme in present-day

land management. The term `ecosystem management'

has been used to emphasize management of systems

rather than their component parts, while integrating

economic and social values (Bormann, 1993; Jensen

and Everett, 1994; Harrod et al., 1996). An important

component of ecosystem management is achieving

sustainability of ecological systems as de®ned by the

historical range of variability of ecosystem patterns

and processes at relevant spatial scales (Jensen and

Everett, 1994). Knowledge of historical variability is

useful to resource managers for several reasons: (1) it

provides an understanding of conditions and processes

prior to alteration by European settlers; (2) it allows us

to identify the causes and consequences of change in

ecosystem characteristics; (3) it permits us to estimate

departure of current vegetation conditions from those

that would have been present in the absence of man-

agement; (4) it may be used to predict potential

changes in forest composition and structure; and (5)

it can be used to develop objectives or targets for

restoring forest composition, structure, and spatial

pattern (Kaufmann et al., 1994; Morgan et al.,

1994; Swanson et al., 1994; Foster et al., 1996;

Hessburg et al., 1997).

Forest Ecology and Management 114 (1999) 433±446

*Corresponding author.

0378-1127/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.

P I I : S 0 3 7 8 - 1 1 2 7 ( 9 8 ) 0 0 3 7 3 - 9

Retrospective studies of forest composition or struc-

ture can be employed to infer historical patterns of

ecosystem development (Foster et al., 1996). For

example, reconstruction of forest structure at the stand

level has been used to describe successional develop-

ment, disturbance history, changes in species compo-

sition and spatial patterning, and changes in climate

(e.g. Henry and Swan, 1974; Oliver and Stephens,

1977; Morrow, 1985; Duncan and Stewart, 1991;

Savage, 1991; Taylor and Halpern, 1991; Taylor,

1993). Reconstructions of stand structure develop-

ment in ponderosa pine (Pinus ponderosa)-dominated

forests are limited (see Morrow, 1985; Savage, 1991).

Retrospective studies in P. ponderosa (or others)

ecosystems have not been conducted with the speci®c

objective of guiding silvicultural prescriptions that

restore past forest structure and composition.

This study focuses on restoration management of a

dry forest landscape in the Eastern Cascades of

Washington. The historical ®re regime in these dry

forests has been described as high frequency, low

intensity and severity that maintained an open,

park-like forest dominated by P. ponderosa (Hall,

1976, 1980, 1984; Johnson, 1994; Agee, 1994; Hess-

burg et al., 1997 and others). However, quantitative

data on a speci®c historical stand structure in these

forests are lacking. The purpose of this project was to

collect data on the structural and spatial characteristics

of past P. ponderosa forests to guide the design of

current silvicultural prescriptions. Due to insuf®cient

time and lack of physical evidence, we limited our

reconstruction to the dominant overstory trees. How-

ever, silvicultural prescriptions were designed for the

entire range of size classes.

We addressed the following questions: (1) What

diameter distributions were historically present within

P. ponderosa and Douglas-®r (Pseudotsuga menziesii)

plant associations (Lillybridge et al., 1995) within the

Fig. 1. Vicinity map of study area.

434 R.J. Harrod et al. / Forest Ecology and Management 114 (1999) 433±446

study area? (2) What were the historical ranges of

densities of trees within the study area? (3) How have

spatial patterns of trees changed within the study area?

(4) How can these data be used to guide restoration

treatments?

2. Study area

The study area is located approximately 17 km due

south of Cashmere, Chelan County, Washington,

within the Wenatchee National Forest (Fig. 1).

Forested plant associations are within the P. ponderosa

and P. menziesii series and include the following

seven types: P. ponderosa/Purshia tridentata/Agro-

pyron spicatum (PIPO/PUTR/AGSP), P. menziesii/P.

tridentata/A. spicatum (PSME/PUTR/AGSP), P. men-

ziesii/Symphoricarpos oreophilus (PSME/SYOR), P.

menziesii/S. albus/Calamagrostis rubescens (PSME/

SYAL/CARU), P. menziesii/Spirea betulifolia/C.

rubescens (PSME/SPBEL/CARU), P. menziesii/C.

rubescens (PSME/CARU), and P. menziesii/Carex

geyeri (PSME/CAGE) (Lillybridge et al., 1995). Pinus

ponderosa is the dominant of all tree-types, even in the

P. menziesii series for which P. menziesii is the climax.

For simplicity in implementing silvicultural prescrip-

tions, we then grouped these associations into the `hot

dry shrub/grass' (PIPO/PUTR/AGSP and PSME/

PUTR/AGSP), `warm dry shrub/herb' (PSME/

SYOR), `warm mesic shrub/herb' (PSME/SYAL/

CARU), and `cool dry grass' (PSME/SPBEL/CARU,

PSME/CARU, and PSME/CAGE) plant association

groups (PAGs) (Lillybridge et al., 1995).

Stands within the study area are currently fully

stocked, mostly even-aged, and dominated by a 70±

90-year-old age class, with diameters at breast

height (dbh) generally below 51 cm. Trees larger

than or equal to 51 cm dbh can be found throughout

the study area. Some of these trees are residual

trees of the presettlement overstory, some are larger

individuals of the ®re exclusion age class, but many

are the larger members of presettlement understory

clumps which, because of ®re exclusion, are at rela-

tively high clump densities. Bark beetles are active

throughout the study area and populations appear to be

increasing.

The study area was extensively grazed between the

turn of the century and ca.1940; extensive logging

occurred during the late 1920s and early 1930s

(USDA, 1995). Fire suppression by the U.S. Forest

Service also became effective in the 1930s in this area

and has continued to the present (USDA, 1995). Given

these anthropogenic in¯uences, we choose 1935 as the

date to reconstruct historical stand composition and

structure.

3. Methods

3.1. Sampling

Historical stand structure was determined by mea-

suring and mapping stumps, logs, snags, and live trees

greater than 140 years of age within 48 0.5 ha

(50�100 m2) plots. Each plot was subjectively located

to fall within a single plant association and to maintain

constant aspect and slope within the plot. Diameter at

breast height (dbh, 1.4 m) and species were recorded

for snags and naturally downed logs that were likely to

be of 1935 vintage, based on comparison of decay

classes with those of cut stumps and logs. As stumps

could not be measured at breast height, an empirically

derived conversion factor (c) was used to transform

ground-level diameter (g) to dbh (g�c�dbh). We

measured 53 P. ponderosa and 70 P. menziesii trees

of various sizes to calculate c. Conversion factors for g

to dbh measurements were 0.66 (SD�0.065) for P.

menziesii and 0.77 (SD�0.065) for P. ponderosa.

Trees greater than 140 years of age were only used

because we assumed trees that were less than 80 years

of age in 1935 were not part of the dominant overstory.

This assumption was based upon observations of

current stands with similar densities that were thought

to exist historically. Trees greater than 140 years of

age were cored to determine radial growth r since

1935; 1935 dbh was calculated as 1935 dbh�current

dbhÿ2�r.

Stumps, logs, snags, and live trees greater than 140

years of age were mapped to the nearest meter to

describe the spatial patterning of historical trees. In

addition, four of the plots within the PIPO/PUTR/

AGSP plant association were surveyed intensively and

all live trees were measured and mapped to the nearest

0.1 m for comparison of current and historical tree

patterning. Tree density, basal area, and diameter

distribution by plant association were calculated for

1935 stands.

R.J. Harrod et al. / Forest Ecology and Management 114 (1999) 433±446 435

3.2. Spatial patterning

Spatial patterning of stems was analyzed using the

K(t) function (Ripley, 1977, 1981; Diggle, 1983;

Bailey and Gatrell, 1995) as described in Duncan

(1991, 1993). The K(t) function provides a measure

of spatial dependence between mapped points over a

range of spatial scales. For each plot, K(t) was calcu-

lated for inter-stem distances, t, of 1±25 m. Ninety-

®ve percent con®dence envelopes were generated

from maximum and minimum K(t) values calculated

for 19 randomly generated stem maps, each with the

same number of coordinate pairs as the original plot.

Values of K(t) falling outside the upper or lower

envelopes indicate signi®cantly (p<0.05) clumped

or regular patterning, respectively.

Results from these analyses were aggregated across

plots using the L(t) function described in Bailey and

Gatrell (1995), a transformation of K(t):

L�t� � �K�t�=��1=2 ÿ t

Positive values of L(t) indicate clumped patterns,

while negative values indicate regular patterns. Where

there is clumping, peaks in the L(t) function cor-

respond with average clump radii. For each sampled

plot, L(t) values were normalized for the stem maps

and con®dence envelopes by calculating the ratio of

L(t) to the mean magnitude of the upper and lower

con®dence envelopes. The resulting normalized L(t)

values for each distance t were then averaged across

all plots within each PAG, permitting an evaluation of

average plot conditions for each PAG.

Spatial patterning was examined in this manner for

historical trees in 32 plots across all plant associations,

and for present-day trees in the four PIPO/PUTR/

AGSP plots. For these four plots, maps of historical

stems, present-day stems, and the n largest present-day

stems (n being the number of historical trees mapped

for each plot), were analyzed. Spatial patterning was

compared for equal numbers of stems because the

power to detect deviations from spatial randomness

increases with increasing numbers of coordinate pairs.

3.3. Restoration prescription

Prescriptions were designed to alter stand density

and composition, diameter distribution, and spatial

arrangement. The primary management objective

for lands within the study area is to restore ®re-

maintained structure and disturbance regimes similar

to those that existed prior to European settlement

while maintaining bark beetle populations at endemic

levels. Since these were long-term objectives, any

silvicultural treatment performed in the short-term

should be designed to move stands toward the long-

term condition. Historical stand structure and compo-

sition were used to guide development of silvicultural

prescriptions. These objectives were to be applied to

all aspects except the more mesic north slopes (largely

occurring within the warm dry shrub/herb and warm

mesic shrub/herb PAGs) where dense-layered struc-

tures were to be retained for spotted owl habitat.

3.3.1. Assumptions and decisions

Several assumptions or decisions were made at the

early stages of prescription development: (1) preset-

tlement structure and composition were assumed to be

sustainable (maintaining natural ecological patterns

over time (Bourgeron and Jensen, 1994)); (2) thinned

small diameter stands should be at low risk to beetle

attack for at least 20 years (applies primarily to the 70-

to-90-year age class) so that stands would not require

management during that period of time; (3) density

management prescriptions would not be developed for

individual plant associations but for the hot dry shrub/

grass and cool dry grass PAGs because the ®ne-

textured mosaic of plant associations throughout the

study area makes management by plant association

impractical; (4) spacing of trees would be prescribed

as guided by the results of the spatial patterning

analysis, especially in the larger diameter stands;

(5) remnant overstory trees should not be thinned.

Trees 51 cm or larger that are not remnants of the

presettlement overstory can be thinned in an attempt to

approximate ®re-maintained spatial patterns.

3.3.2. Bark beetle management

We used the methods of Cochran (1992) and

Cochran et al. (1994) to estimate critical stand den-

sities, above which beetle mortality can be serious.

Seventy stands within the study area were sampled to

estimate average full stocking stand density index

(SDI) for P. ponderosa. Only stands composed of at

least 75% P. ponderosa were used in this calculation.

Average full stocking SDI (Reineke, 1933) for P.

ponderosa-dominated stands within the study area is

436 R.J. Harrod et al. / Forest Ecology and Management 114 (1999) 433±446

about 516, a relatively low stocking capacity given

that normal or full stocking for P. ponderosa is 902

(Demars and Barrett, 1987). Average full stocking SDI

and site index were then used to calculate critical

densities for beetle mortality (see Cochran et al.,

1994).

Using multiple radial growth projections (Hall,

1987), we found that reducing average stand density

to approximately 63% of critical densities would

maintain low beetle mortality risk for 20 years. A

range of residual densities was determined for the

clump scale based on growth projections, which sug-

gested that small diameter stands would accumulate

basal area faster than larger diameter stands with the

same starting basal area.

4. Results

4.1. Historical stand structure

Historical stand structure varied among plant asso-

ciations. The warm dry shrub/herb PAG had the high-

est density and basal area, and the warm mesic shrub/

herb PAG the lowest. Pseudotsuga menziesii was most

abundant by basal area in the warm dry shrub/herb and

cool dry grass PAGs, and relatively uncommon in the

hot dry shrub/grass PAG (Table 1).

Bell-shaped diameter distributions were observed

across all PAGs (Fig. 2), but this may be an artifact of

excluding trees greater than 140 years of age (see

Section 5). Larger mean diameters and lower overall

densities were observed in the cooler, more mesic

PAGs (Fig. 2(c), (d)).

In the four PIPO/PUTR/AGSP plots (Fig. 3), there

are fewer trees in the 70±80 cm size class today than

existed historically, and trees above 80 cm are absent.

Both present-day and historical populations approx-

imate bell-shaped distributions, although historical

distributions are biased by exclusion of trees greater

than 140 years of age.

4.2. Spatial patterning

Clumping of historical trees was observed across all

spatial scales (Table 2). A majority of plots contained

signi®cant (p<0.05) clumping of trees at distances

greater than 15 m. Three plots showed regular spacing

of trees at scales at or above 14 m: two in the cool dry

grass PAG and one in the warm mesic shrub/herb PAG

(Table 2). Although high variability in patterning was

observed within individual PAGs, normalized L(t)

values indicate average plot conditions with strong

clumping at nearly all scales in the driest PAG, and

progressively less clumping at larger scales in more

mesic PAGs (Fig. 4). These values suggest a general

trend of decreasing average clump sizes and decreas-

ing inter-stem distances at which patterns begin to

repeat as plots become increasingly mesic. Historical

clump sizes ranged from 0.005 ha in the most mesic

plots to 0.2 ha in the driest plots.

Comparison of spatial patterning of present-day and

historical trees in the four PIPO/PUTR/AGSP plots

suggests that while strong clumping exists in present-

day stands, the largest trees today exhibit less clump-

ing than did large trees historically (Table 3, Fig. 5).

Present-day trees in general are most strongly

clumped at smaller spatial scales, while present-day

and historical large trees are most strongly clumped at

larger scales (Table 3, Fig. 5).

Fig. 6 illustrates differences in clumping between

historical and current trees in one plot (HD1).

Although current trees are clumped at ®ne scales in

this plot, the largest current trees exhibit far less

clumping than did historical overstory trees. L(t)

values for all current trees in this plot indicated

Table 1

Mean values for density, basal area and percentage of Pseudotsuga menziesii by basal area for historical plots by plant association group

Plant association group Number plots Density (trees haÿ1) Basal (m2 haÿ1) area P. menziesii (percent)

Hot dry shrub/grass 10 61 (27) 15.6 (5.5) 0.5 (1.6)

Warm dry shrub/herb 7 68 (43) 22.3 (11.7) 12.3 (12.3)

Cool dry grass 25 45 (26) 15.8 (8.4) 11.7 (16.2)

Warm mesic shrub/herb 6 27 (23) 10.3 (7.4) 7.0 (10.4)

All 48 50 (31) 16.0 (8.7) 9.0 (13.4)

Standard deviations are shown in parentheses.

R.J. Harrod et al. / Forest Ecology and Management 114 (1999) 433±446 437

signi®cant clumping up to inter-stem distances of

17 m, with a peak in the values corresponding to an

average clump radius of 7 m. However, analysis of the

largest 27 current trees showed statistically random

patterning at most inter-stem distances. In compari-

son, L(t) values for the 27 historical overstory trees

indicated signi®cant clumping at most scales, with

peaks in L(t) values corresponding to clumps with

average radii of 2 and 19 m.

4.3. Restoration prescription

The important aspects considered in designing the

silvicultural prescriptions included the critical density

threshold for serious beetle mortality, the scale of tree

clumping to manage, maximum leave±tree density at

the stand scale and ®ner, and practical tree marking

guidelines.

The SDI threshold value for serious beetle mortality

was about 263. Using density and basal area from

stump plots, SDI in 1935 was estimated to be 260.

Since the estimated average 1935 density of the

dominant overstory trees is equal to the critical thresh-

old for beetle activity, it seems reasonable to assume

that, historically, densities within clumps ranged on

both sides of the threshold.

Maximum residual tree densities were prescribed at

the 30 m diameter circular scale which was considered

large enough to account for site occupancy of ponder-

osa pine and was a practical size for marking crews.

Thirty meters was also suf®ciently larger than the

average clump radius of 5 m found in the cool dry

PAG (the most common PAG in the study area) to

allow marking that could preserve natural clump sizes

(Fig. 4(c)).

In reviewing the one-half hectare plot maps, histor-

ical overstory density at the clump scale was highly

variable. For each plot, one circle (30 m diameter)

with the highest historical stem density was identi®ed

and we found the following distribution: 38% of plots

Fig. 2. Historical diameter distributions among plant association groups (PAGs): (a) hot dry shrub/grass, (b) warm dry shrub/herb, (c) cool dry

grass, and (d) warm mesic shrub/herb.

438 R.J. Harrod et al. / Forest Ecology and Management 114 (1999) 433±446

Fig. 3. Diameter distributions of (a) present-day, and (b) historical trees for four plots in the PIPO/PUTR/AGSP association.

Fig. 4. Mean values of normalized L(t) function among plant association groups (PAGs) as a function of inter-stem distance (t): (a) hot dry

shrub/herb, (b) warm dry shrub/herb, (c) cool dry grass, (d) warm mesic shrub herb. Simulation envelopes represent plot means and are shown

for reference only.

R.J. Harrod et al. / Forest Ecology and Management 114 (1999) 433±446 439

had 1±5 trees 46 cm or larger; 38% of plots had 6±9

trees 46 cm or larger; and 24% of plots had 10 or more

trees 46 cm or larger. For trees smaller than 51 cm

dbh, a maximum leave density (SDI) of 63% of critical

density for beetle mortality, which was prescribed

expected 20 years protection from beetle attack,

resulted in stem densities per circle similar to the

historical distribution. For clumps of trees greater than

51 cm, however, maximum leave density (SDI) per

circle was prescribed at approximately 150% of the

average historical density (Table 1) in order to provide

a similar historical distribution of trees (as above).

Maximum leave densities were again converted to

trees per circle for each 5 cm diameter class involved.

As a result, maximum residual stem densities per

circle would range from 8.5 to 3.2 trees for average

diameters ranging from 51±86 cm dbh. For trees

greater than 51 cm dbh, maximum residual stem

densities per circle would range from 9.0 to 4.7

trees.

Trees greater than 51 cm would be managed at a

variety of clump (30 m diameter) densities and rela-

tive risk to beetle infestation. Concentrations of trees

greater than 51 cm located in no-treatment areas

Table 2

Spatial patterning for historic trees in all plots by plant association group

PAG Plot t (meters)

1 5 10 15 20 25n

Hot dry shrub/grass Hd1 . C C C C..... C C C C C C C C C C C C C C C 27

HD2 C C C C C C C C C C.. C C C C C C.... C C 36

HD3 ....... C......... C C C C C C C C 10

HD4 ....... C.. C C. C C C C. C...... 20

HD5 C C C C C C C C C C C C C C C C C C C C C C C C C 47

HD6 . C C...................... 30

HD7 .... C C C C C C C C C C C C C C C C C C C C C 40

HD8 .C C C C C C C C C C C C C C C C C C C C C C C. 54

HD9 ......... C C. C C C C. C C C..... 20

Warm dry shrub/herb WD1 .. C C C.................... 26

WD2 . C C C C C C C C C C C C.... C C...... 63

WD3 . C. C.... C C C C C C C C......... 49

WD4 ....... C. C C C C C C. C C C C C C C C C 39

WD5 C C C C C C C C C C C C C C C C C C C C C C C C 63

Cool dry grass CD1 . C C C C C C C C... C C C C C........ 31

CD2 .... C C C C C C C C C C C C C C C C..... 42

CD3 .. C C C C C C C C C C............. 49

CD4 .. C C C C C C C. C C C C C C...... C.. 39

CD5 .. C C C C C C C C C C............. 42

CD6 . C C C......... U U U......... 12

CD7 . C C C C C C C C C C.......... U U U. 28

CD8 . C C C C C C C C C C C C C C C C C C C..... 47

CD9 ......................... 14

CD10 . C C C C C C C C C C C C C C C C C C C C C C C C 18

CD11 ......................... 12

CD12 .... C.. C C C C.. C........... 19

CD13 . C C C C C C C C C C C C C C C C C.... C. 40

CD14 C C C. C.................... 23

Warm mesic shrub/herb WM1 ..... C C..... C C C C C C C C C C C C C 17

WM2 .. C C C C................... 21

WM3 ......................... 10

WM4 . C C C C C..............U U U U U 35

Letters indicate significant (p<0.05) deviation from random distribution at distance t: C, clumped distribution; U, regular; . random. For each

plot, n is the number of stems.

440 R.J. Harrod et al. / Forest Ecology and Management 114 (1999) 433±446

would be retained at existing high densities. Within

treatment areas, small concentrations of trees greater

than 51 cm would be thinned up to a maximum of

150% of the 1935 density at the clump scale, a density

considered of moderate-to-high risk to beetle attack.

In areas where trees greater than 51 cm are more

scattered, younger aged clumps containing one,

two, or three of these larger trees would be reduced

to densities prescribed for the younger age class,

which is below the 1935 average and beetle mortality

threshold. A summary of the prescription for trees

greater than 51 cm is given in Table 4.

For trees greater than 51 cm dbh, density targets

were prescribed at the 30 m diameter clump scale. For

tree clumps containing 25 cm trees, residual stem

densities similar to historical stands were obtained

by marking to thin clumps to approximately 48% of

the critical beetle mortality threshold. Clumps of

45 cm trees were marked to thin clumps to approxi-

mately 70% of the critical beetle mortality threshold.

Table 3

Spatial patterning for present-day and historical trees in four PIPO/PUTR/AGSP plots

Plot Pattern analyzed t (meters) n

1 5 10 15 20 25

HD1 1935 ± Large trees . C C C...... C C C C C C C C C C C C C C C 27

1996 ± Large trees ......................... 27

1996 ± All trees C C C C C C C C C C C C C C C C C..... U U U 131

HD5 1935 ± Large trees ....... C......... C C C C C C C C 10

1996 ± Large trees ......................... 10

1996 ± All trees .. C C C C C C C C C C C C C C C C....... 29

HD6 1935 ± Large trees ....... C.. C C. C C C C. C...... 20

1996 ± Large trees C......... C C....... C C C C C C 20

1996 ± All trees C C C C C C C C C C C C C C C C C C C C C C C C C 80

HD7 1935 ± Large trees C C C C C C C C C C C C C C C C C C C C C C C C C 47

1996 ± Large trees C C C C C C C C C C C C C C C C C C C C C C C C C 47

1996 ± All trees C C C C C C C C C C C C C C C C C C C C C C C C C 63

Letters indicate significant (p<0.05) deviation from random distribution at distance t: C, clumped distribution; U, regular; ., random. For each

plot, n is the number of stems.

Fig. 5. Mean values of normalized L(t) function in the PIPO/PUTR/AGSP plant association: (a) large historical trees, (b) largest present-day

trees, (c) all present-day trees.

R.J. Harrod et al. / Forest Ecology and Management 114 (1999) 433±446 441

To simplify tree marking, SDI values were converted

to trees per circle by 5 cm diameter classes. Marking

crews were directed to thin from below leaving the

best trees (based on crown form) regardless of spacing.

In addition, P. ponderosa was to be favored because it

was dominant in historical stands (Table 1). The pre-

scription for trees lesser than 51 cm dbh is summar-

ized in Table 5.

5. Discussion

5.1. Historical stand structure

The bell-shaped size frequency distribution re¯ects

uncertainty about smaller size classes, since trees that

were less than 80 years old in 1935 were excluded

from the study. Historically, seedling establishment

Fig. 6. Stem map for plot HD1. This plot had 27 inventoried historical overstory trees and 131 current trees, with the 27 largest current trees

and the remaining 104 live trees labeled separately on the map. Three of the largest current trees were more than 140 years old and were also

considered part of the 1935 overstory.

Table 4

Marking guidelines for trees 51 cm or greater given both on a per

hectare basis and a per implementation circle basis

QMDa TPH BA/HA SDI TPC BA/C SDI/C

51 116 23 405 9 4 73

56 99 24 405 7 4 73

61 82 24 405 6 4 73

66 72 25 405 5 4 73

71 64 25 405 5 4 73

76 57 25 405 4 5 73

81 52 26 405 4 5 73

86 44 26 405 3 7 73

These values are 1.5 times that of presettlement density.aQMD�Quadratic mean diameter (cm); TPH�Trees per hectare;

BA/HA�Basal area per hectare (m2 haÿ1); SDI�Stand density

index (metric); TPC�Trees per 30 m diameter circle; BA/C�Basal

area per 30 m diameter circle (m2); SDI/C�Stand density index per

30 m circle (metric).

Table 5

Marking guidelines for trees less than 51 cm given both on a per

hectare basis and a per implementation circle basis

QMDa TPH BA/HA SDI TPC BA/C SDI/C

20 146 5 101 11 1 18

25 124 6 127 9 1 23

30 111 8 157 8 1 28

36 91 9 170 7 2 31

41 77 10 180 6 2 32

46 64 11 187 5 2 34

These values are below that of presettlement density so that trees

could grow for 20 years without high risk to beetle mortality.aQMD�Quadratic mean diameter (cm); TPH�Trees per hectare;

BA/HA�Basal area per hectare (m2 haÿ1); SDI�Stand density

index (metric); TPC�Trees per 30 m diameter circle; BA/C�Basal

area per 30 m diameter circle (m2); SDI/C�Stand density index per

30 m circle (metric).

442 R.J. Harrod et al. / Forest Ecology and Management 114 (1999) 433±446

was likely to be rare due to the low-intensity ®re

regime and subsequent response of grasses (Cooper,

1960; White, 1985). The current high density of

smaller trees has likely resulted from a pulse of P.

ponderosa establishment in the 1920s from heavy

grazing shortly after the turn of the century. Intense

grazing would provide suitable sites for seedling

establishment similar to ®re `hot spots' and this

combined with ®re exclusion would have allowed

an unusually high density of trees to become estab-

lished and persist (Cooper, 1960; White, 1985). This

density would have included any trees that became

established after the last ®re event prior to European

in¯uence. In a nearby area, Camp (1995) suggests that

following ®re exclusion, new cohorts initiated until

growing space was fully utilized, then recruitment

decreased abruptly.

Covington and Moore (1994) found 43 trees haÿ1

historically, similar to our 50 trees haÿ1, in the South-

west. East of the Cascades in Oregon, Morrow (1985)

showed a similar age class distribution that approx-

imates our size class distribution for the dominant

overstory. Avery et al. (1976) measured 14 m2 haÿ1

within a P. ponderosa stand in Arizona in 1920, which

was similar to our results.

The changes in forest structure during this century

observed in this study have consequences relative to

current and future forest management. The high

density of small P. ponderosa, presence of ladder

fuels, and high surface fuel loading can lead to

large-scale crown ®re (Agee, 1996) which would kill

even the large residual trees remaining in the study

area and eliminate late-successional wildlife

habitat currently being maintained on the northerly

aspects. The historical structure we measured is simi-

lar to the forest structure modeled and discussed in

Harrod et al. (in press) and would have resulted in low-

intensity ground ®res. In addition, current stands are

susceptible to widespread bark beetle infestations

because of high stand density relative to site capacity

(Larsson et al., 1983; Hall, 1987; Cochran et al.,

1994). Our ability to manage a forest structure domi-

nated by high density, small trees is also diminished.

The majority of trees in the study area are of

small (<30 cm) diameter and are growing slowly

due to intense competition. The cost of removing

this material is often more than the value of the wood

®ber.

5.2. Spatial patterning

Reduced clumping at larger scales in the cool dry

grass and warm mesic PAGs indicate smaller clump

sizes relative to other PAGs, and smaller scales at

which clustering patterns begin to repeat across the

landscape. These results may re¯ect the higher pro-

ductivity of these sites. These PAGs currently support

higher densities of trees, with few large openings in

the canopy. Strong historical clumping observed at all

scales in the hot dry shrub/herb group may be

explained by the more xeric nature of these sites, with

limited seedling establishment resulting in larger

openings between clumps of trees. Fuel would build

up below widely spaced trees creating favorable sites

for seedling establishment following low-intensity

burns. In general, signi®cant clumping existed up to

the 15 m scale in a majority of plots sampled, equiva-

lent to clump sizes of approximately 0.09 ha; this

result is similar to Cooper's (1960) ®ndings of clump

sizes of 0.06±0.13 ha, and White's (1985) ®ndings of

clump sizes from 0.02±0.29 ha (average 0.1 ha).

These patterns are consistent with the process of stand

development described by Cooper (1960) and White

(1985), in which seedlings are established in a patchy

fashion as result of frequent ®re and occasional `hot

spots' that result from accumulations of fuel where

patchy mortality has occurred. Morrow (1985) also

found signi®cant clumping at scales between 0.1±

0.3 ha depending on the age of individual trees.

In reviewing marked stands, it appears that the

prescription will result in recognizable clumps at

the 0.1 ha scale primarily in the larger diameter stands

(QMD>36 cm dbh) where stem densities are less than

smaller diameter stands. Stands with QMDs of 20±

25 cm tend to support more uniform distribution of

trees and may require extra efforts to create clumps in

the short-term.

Differences observed between clumping patterns of

current and historical trees in the four intensive PIPO/

PUTR/AGSP plots must be interpreted with consid-

eration to differences in numbers of stems sampled.

Differences in signi®cance of K(t) and L(t) values

between all current trees and large historical trees

may be a statistical artifact of varying numbers of

coordinate pairs (stems) analyzed. The important dif-

ferences between current and historical patterning are

found in comparing the shapes of all L(t) functions, as

R.J. Harrod et al. / Forest Ecology and Management 114 (1999) 433±446 443

well as the signi®cance levels of these functions for

the largest present-day trees and historical large trees,

for which there are identical numbers of coordinate

pairs.

Present-day trees in general are most strongly

clumped at smaller spatial scales whereas large trees

in present and historical plots are most strongly

clumped at the largest measured scales. This differ-

ence may re¯ect different processes acting upon the

spatial distributions of small and large trees (e.g.

processes acting at upon seedling establishment pat-

terns vs. mortality patterns as cohorts mature). The

less clumped distribution of the largest present-day

trees relative to historical large trees may be due to an

arti®cial regularity imposed on remnant large trees by

past logging. Alternatively, these differences may

indicate that changes in the processes that in¯uence

tree patterning have occurred since historical times, or

that processes producing clumping of large trees

intensify as the trees become larger. One might predict

that processes producing clumping through patchy

mortality patterns would tend to increase clumping

throughout the development of a given cohort.

5.3. Restoration prescription

The almost identical SDI values for estimated

historical stands and the estimated beetle mortality

threshold support the idea that presettlement forests on

average were low-risk to crown ®re and bark beetle

attack. Agee (1994) and others (Hall, 1976, 1980,

1984; Johnson, 1994; Hessburg et al., 1997) note that

the P. ponderosa-dominated forests were the most

stable in landscape pattern. Prescribed stem densities

at the clump scale (30 m diameter) are very similar to

those found in the 1935 forest and should be compa-

tible with the long-term development of ®re-main-

tained stand structures, which on average, are at low-

risk to beetle attack.

Implementing the average prescribed density in

every stand may at ®rst seem to simplify the structure

within the study area. However, it may actually

increase structural diversity when applied over a wide

range of site qualities. Tree vigor, growth, and beetle

risk would vary over time where a range of sites are

thinned to the same density.

It is interesting to note that we found the lowest

historical stand density in the warm dry shrub/herb

PAG, an area not proposed for treatment under the

current land management direction. These more mesic

sites are largely on northerly aspects within the study

area, which may have had variable ®re intensity over

time historically (Agee, 1994), thus leading to overall

lower densities of dominant overstory trees. The cur-

rent stand structure may be dif®cult to sustain into the

future.

Acknowledgements

The authors would like to acknowledge the ®eld

assistance of the Leavenworth Ranger District 1995

Pre-sale and 1995 and 1996 Botany/Ecology crews.

Lauri Malmquist did the tedious tree core dating.

Programs for analyzing spatial patterns were gener-

ously provided by Richard Duncan. Helpful comments

on earlier drafts of the manuscript were provided by

Charles B. Halpern, James K. Agee, Richard Duncan,

and two anonymous reviewers. Figures were made by

Dan O'Connor.

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