United States Department of Agriculture

Forest 

Service

Pacific Northwest 

Research Station

Research Paper

PNW-RP-606

March

2016

Stand Dynamics of Douglas-fir 

20 Years After Precommercial 

Thinning and Nitrogen Fertilization 

on a Poor-Quality Site

Richard E. Miller, Timothy B. Harrington, and Harry W. Anderson 
In accordance with Federal civil rights law and U.S. Department of Agriculture (USDA) 

civil rights regulations and policies, the USDA, its Agencies, offices, and employees, and 

institutions participating in or administering USDA programs are prohibited from discriminating 

based on race, color, national origin, religion, sex, gender identity (including gender 

expression), sexual orientation, disability, age, marital status, family/parental status, income 

derived from a public assistance program, political beliefs, or reprisal or retaliation for prior 

civil rights activity, in any program or activity conducted or funded by USDA (not all bases 

apply to all programs). Remedies and complaint filing deadlines vary by program or incident. 

Persons with disabilities who require alternative means of communication for program 

information (e.g., Braille, large print, audiotape, American Sign Language, etc.) should 

contact the responsible Agency or USDA’s TARGET Center at (202) 720-2600 (voice and 

TTY) or contact USDA through the Federal Relay Service at (800) 877-8339. Additionally, 

program information may be made available in languages other than English. 

To file a program discrimination complaint, complete the USDA Program Discrimination 

Complaint Form, AD-3027, found online at http://www.ascr.usda.gov/complaint_filing_cust.html 

and at any USDA office or write a letter addressed to USDA and provide in the letter all of the 

information requested in the form. To request a copy of the complaint form, call (866) 632-9992. 

Submit your completed form or letter to USDA by: (1) mail: U.S. Department of Agriculture, 

Office of the Assistant Secretary for Civil Rights, 1400 Independence Avenue, SW, Washington, 

D.C. 20250-9410; (2) fax: (202) 690-7442; or (3) email: program.intake@usda.gov. 

USDA is an equal opportunity provider, employer, and lender.

Authors

Richard E. Miller is a retired soil scientist, Timothy B. Harrington is a research 

forester, and Harry W. Anderson is a volunteer forester, Forestry Sciences Laboratory, 3625 93rd Avenue SW, Olympia, WA 98512-1101.

Cover photo: Researchers apply nitrogen fertilizer in spring 1969 to a 30-yearold thinned stand of Douglas-fir near Brinnon, Washington. In this Rocky Brook 

study, stand dynamics of thinned and nonthinned stands of Douglas-fir were 

compared for 20 years after application of three rates of nitrogen fertilizer. 

Photo by Richard E. Miller.
Abstract

Miller, Richard E.; Harrington, Timothy B.; Anderson, Harry W. 2016.

Stand dynamics of Douglas-fir 20 years after precommercial thinning and 

nitrogen fertilization on a poor-quality site. Res. Pap. PNW-RP-606. Portland, 

OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest 

Research Station. 66 p. 

Amendment of soil nutrients through fertilization is used to increase wood production 

of forest stands. Subsequent allocation of growth to individual trees and the resulting 

increase in stand volume and value, however, depend on stand density at the time of 

treatment. Our primary research question was: To what extent can volume growth per 

acre in precommercially thinned or nonthinned portions of a poor-site, 30-year-old 

coast Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco var. menziesii) plantation be 

increased by a single application of 200 or 400 lb nitrogen (N)/acre as urea? Each fertilizer treatment was replicated on three 0.2-acre plots in thinned and nonthinned stands 

located in Rocky Brook drainage on the Olympic Peninsula in Washington. All trees 

in the interior 0.1-acre plots with a diameter at breast height (d.b.h.) of 1.6 inches and 

larger were measured for stem diameter, and a subset of trees was measured for total 

height and height to live crown. We anticipated that poor site quality, prolonged overstocking, and a subsequent severe thinning would influence the responses we observed. 

In the 20 years after fertilization, net growth in total stem volume on fertilized plots 

averaged 50 percent greater than on nonfertilized plots in the previously thinned stand 

and 31 percent greater in the nonthinned stand. In both stands, mortality losses were 

relatively small, averaging 1 to 15 percent of gross annual growth. Diameter growth of 

the 100 largest trees per acre was improved by both thinning and fertilization. Present 

net value (PNV) (in 2014 dollars) at the hypothetical final harvest of the thinned stand 

was increased by delaying the harvest from 10 years (age 40) until 20 years after fertilization (age 50), and by fertilization, although differences between the 200 and 400 lb 

N/acre treatments were not significant. Present net value of the nonthinned stand also 

was increased by delaying final harvest until 20 years after fertilization; however, PNV 

did not differ significantly among the 0, 200, and 400 lb N/acre treatments, indicating 

no financial benefit from fertilizing this nonthinned stand. The 90-percent confidence 

interval for PNV of nonfertilized plots in the thinned stand ($919 ± $278 per acre) did 

not overlap with that of nonfertilized plots in the nonthinned stand ($2,639 ± $498 per 

acre), suggesting that the thinning prescription reduced the economic value of this overstocked stand at Rocky Brook. Clearly, the severe thinning at age 30 reduced stocking 

to the point at which the stand was slow to reoccupy this poor quality site.

Keywords: Douglas-fir plantation, precommercial thinning, forest fertilization, 

nitrogen, stand growth, economic analysis.
Summary

Amendment of soil nutrients through fertilization is often used to increase wood 

production and overall vigor of forest stands. However, the allocation of growth to 

individual trees and the resulting increase in stand volume and value depends on 

initial density and tree mortality. Our research addressed the following questions: 

• To what extent can volume growth per acre in precommercially thinned 

or nonthinned portions of a 30-year-old poor-site coast Douglas-fir 

(Pseudotsuga menziesii [Mirb.] Franco var. menziesii) plantation be 

increased by a single application of 200 or 400 lb nitrogen (N)/ac as urea? 

• Does inclusion of phosphorus (P), potassium (K), and sulfur (S) amendments with 400 lb N/ac provide additional response? 

• How are stem and crown growth of crop trees (i.e., the 100 largest trees 

per acre by diameter at breast height [d.b.h.]) influenced by fertilization in 

thinned and nonthinned stands? 

• What are the effects of thinning, fertilizer rate, and timing of final harvest 

(10 vs. 20 years after fertilization) on yield and present net value? 

Located in the Rocky Brook drainage on the east side of the Olympic Peninsula 

in Washington, our plantation originated after clearcut logging to salvage an earlier, 

storm-toppled stand of Douglas-fir, western hemlock (Tsuga heterophylla [Raf.] 

Sarg.), and western redcedar (Thuja plicata [Donn ex D. Don]). At about stand 

age 25, a portion of this plantation was precommercially thinned at high intensity; 

approximately 75 percent of stand basal area was felled and left on the plots. Five 

years later, 0.1-ac square plots for measuring tree growth were established within 

the 0.2-ac treatment areas in the thinned stand. Nine plots were used for our fertilizer trial, and three plots (replications) each were treated in spring of 1969 with 

either 200 lb N/ac (200N) or 400 lb N/ac (400N) as urea (46 percent N). Three plots 

remained nonfertilized. Two years later in an adjacent nonthinned portion of the 

same plantation, six additional and equal-size plots were established and uniformly 

fertilized. Three additional nonthinned plots of the same size from the nearby 

levels-of-growing-stock (LOGS) study were used for nonfertilized comparisons. 

Urea fertilizer markedly increased live-stand volume of both thinned and 

nonthinned portions of this 30-year-old plantation in the subsequent 20-year period. 

Per acre gains in live-stand volume after the 200N application averaged about 1,500 

ft3 in the thinned stand and 1,800 ft3 in the nonthinned stand. Among fertilized 

plots in both thinned and nonthinned stands, gains in merchantable volume of trees 

5.6 inches and larger to a 4-inch top (CV4) exceeded gains in cubic volume total 

stem (CVTS). This contrast is important for financial returns to the landowner and 

for creating wildlife habitat that includes large trees.
In a separate trial, fertilization with 400 lb N/ac plus amendments of phosphorus (P), potassium (K), and sulfur (S) increased diameter growth of individual 

Douglas-fir relative to nonfertilized trees, but the corresponding 400N-alone 

treatment provided similar increases in tree growth. The addition of P, K, and S—a 

costlier treatment than 400N alone—was not justifiable on this poor quality site at 

Rocky Brook. 

In both thinned and nonthinned stands, mean d.b.h. and height of crop trees 

(i.e., the 100 largest trees per acre by d.b.h.) increased with fertilization. At stand 

age 50 years, live crown ratio (i.e., crown length/tree height) for crop trees ranged 

from 0.45 to 0.49 in the thinned stand and from 0.37 to 0.42 in the nonthinned stand.

Our economic analysis compared relative effects of precommercial thinning, N 

fertilizer rate, and timing of final harvest (i.e., 10 vs. 20 years after fertilization) on 

present net value (PNV). To remove the effects of inflation, all costs and revenues 

were adjusted to 2014 dollars using producer price indices from the U.S. Bureau of 

Labor Statistics. These standardized values were then discounted at an assumed 5 

percent interest rate. Real increases in the value of wood products were assumed 

to be zero. The PNV of thinned stands was greater after fertilization and when 

timber harvest occurred 20 years after treatment (i.e., at age 50); however, there 

was no significant difference in PNV between the 200N and 400N treatments. The 

PNV of nonthinned stands was not affected by fertilization but was greater when 

final harvest occurred 20 years after treatment. Because formal statistical testing 

between thinned and nonthinned stands was not appropriate, we instead estimated 

90-percent confidence intervals for mean values of PNV that bracketed the range 

of responses. The 90-percent confidence interval for PNV of nonfertilized plots in 

the thinned stand ($919 ± $278 per acre) did not overlap with that for nonfertilized 

plots in the nonthinned stand ($2,639 ± $498 per acre), suggesting that the thinning 

prescription reduced economic value of this overstocked stand at Rocky Brook. 

Poor site quality, excessive reduction in stand stocking, and removal of some of the 

larger trees likely contributed to this negative effect from thinning.
Contents

1 Introduction

2 Research Questions 

2 Methods

2 Study Area

4 Plot Installation and Fertilization Treatments

6 Tree Tagging and Measurements

7 Experimental Design and Analysis

9 Results

9 Effects of the Operational Thinning 

10 Effects of Urea-N in the Thinned Stand 

20 Effects of Urea-N in the Nonthinned Stand

28 Stand Density and Species Composition by Trees per Acre 

29 Species Composition by Basal Area

32 Relative Density

34 The 100 Largest Trees Per Acre

45 Discussion

45 Initial Variability in Stand and Site Characteristics

46 Growth and Mortality

51 Growth and Response to Fertilizer Depend on Stand Stocking

53 Changes in Basal Area and Species Composition

54 Relative Density

54 Changes in the 100 Largest Trees per Acre

55 An Economic Analysis

56 Methods for Economic Analysis

57 Results and Discussion of Economic Analysis

62 Conclusions

63 Acknowledgments

63 Metric Equivalents

64 Literature Cited
1

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

Introduction

On formerly glaciated sites in the Puget Sound region of Washington, slow growth 

and chlorotic foliage are typical for young stands of coast Douglas-fir (Pseudotsuga 

menziesii [Mirb.] Franco var. menziesii). To increase growth and obtain earlier and 

greater utilizable yield from these slow-growing, generally overstocked stands, 

foresters of Olympic National Forest initially prescribed precommercial thinning. 

In the period 1960–1963, the former Hoodsport Ranger District thinned several 

hundred acres of an extensive plantation that originated in 1940 in the Rocky Brook 

drainage. Because of dense stocking from volunteer regeneration and steep terrain, 

thinning costs averaged about $60 per acre at that time.1 Assuming a 5-percent rate 

of inflation, current costs would exceed $400 per acre. The removal of excess trees, 

primarily from lower crown classes, initially resulted in very little increase in diameter growth of residual trees (300–450 trees per acre [TPA]), and their chlorotic 

appearance and low vigor persisted for at least 3 years after thinning. These early 

results observed by district personnel generated a desire to determine causes of 

poor tree growth in this glaciated area and, if possible, apply a corrective treatment 

that would supplement or possibly substitute for expensive precommercial thinning.

In the Rocky Brook drainage, a short growing season probably limits tree growth. 

At the nearby levels-of-growing-stock (LOGS) study (located at approximately 2,500 

ft elevation on a southerly aspect) (Williamson 1976), height growth started about 1 

month later than that observed near sea level at Olympia, Washington.2 Short, yellowish-green needles on Douglas-fir from this and other local stands indicated nitrogen 

(N) deficiency, which was confirmed when foliage samples collected in June 1964 

near the LOGS study area were chemically analyzed at the University of Washington.3

The glacial origin of soils at Rocky Brook also suggested N deficiency. Early 

research by staff of the University of Washington indicated a general N deficiency 

in glaciated areas, as repeatedly demonstrated by an increase in growth of Douglasfir after N fertilization (Gessel et al. 1965). Although magnitude of response 

differed among their study locations because of differences in site quality, stand 

condition, treatment year, and amount of N applied, a single application of 200 lb N/

ac generally resulted in at least a 20-percent increase in gross volume increment in 

the first 5-year period after fertilization. 

1 Grubb, J. 1965. Personal communication. District ranger, Hoodsport Ranger District, 

Olympic National Forest, Hoodsport, WA 98548.

2 Williamson, R.L. 1968. Personal communication. Mensurationist (deceased), Forestry 

Sciences Laboratory, 3625 93rd Avenue SW, Olympia, WA 98512.

3 Gessel, S. 1960. Personal communication. Professor, College of Forestry, University of 

Washington, Seattle, WA 98195.
2

RESEARCH PAPER PNW-RP-606

Research Questions 

This report presents the 20-year results of a study designed to address the following 

research questions: 

• To what extent can volume growth per acre in precommercially thinned or 

nonthinned portions of a poor-site, 30-year-old coast Douglas-fir plantation 

be increased by a single application of 200 or 400 lb N/ac as urea? 

• Does inclusion of phosphorus (P), potassium (K), and sulfur (S) amendments with 400 lb N/ac provide additional growth response?

• How are stem and crown growth characteristics of crop trees (i.e., the 100 

largest trees per acre by diameter at breast height [d.b.h.]) influenced by fertilization in the thinned and nonthinned stands?

• What are the effects of thinning, fertilizer rate, and timing of final harvest 

(10 vs. 20 years after fertilization) on yield and present net value? 

Answers to these questions have practical implications. If responses to fertilization are strong and lasting, then application of N fertilizer to similar stands and sites 

in the general study area would be justified, either: (1) in combination with precommercial thinning to increase growth and hasten recovery of thinning costs, or (2) in 

lieu of thinning to increase growth and hasten mortality in the lower crown classes. 

Methods

Study Area

Our study included three separate areas in a coast Douglas-fir plantation located 

in Township 26N, Section 18 in Range 2W, and Section 13 in Range 3W, Willamette Meridian (fig. 1) . The study area is located in the upper Rocky Brook 

drainage on the east side of the Olympic Peninsula near Brinnon, Washington. 

During the last glaciation (Vashon) approximately 13,000 years ago, water-sorted 

sediments were deposited in the valley (fig. 2). The uniformly fine texture of these 

sediments indicates deposition in the deep, still water of a temporary lake. Subsequently, nonsorted materials—gravel through large boulders—were deposited 

on these indurated (i.e., hardened) sediments mostly by colluvial transport from 

higher slopes. 

Soils of the general study area were mapped as two slope phases of the Hoodcanal-Sawpeak complex (USDA NRCS, n.d.). The Hoodcanal series is mapped in 

glaciated valleys and formed in glacial residuum and colluvium over dense glacial 

till. The series consists of medial-skeletal, ferrihydritic, frigid Typic Durudands. 

This soil drains moderately well to the depth of a cemented horizon above indurated 

till, which seasonally perches water. Textures range from very gravelly loams to 

Can nitrogen fertilization be used on poor 

quality sites to 

increase stand 

growth and value?
3

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

Figure 1—Study area in a Douglas-fir plantation established in 1937–1938 at Rocky Brook, Washington. Treatment plots were located in blocks 1 and 2 of the levels-of-growing-stock (LOGS) study and 

in both thinned and nonthinned portions of the adjacent plantation.

Figure 2—Indurated glacial till underlies the Rocky Brook, Washington, study areas. 

Richard E. Miller
4

RESEARCH PAPER PNW-RP-606

sandy loams. The Sawpeak series lies on higher slopes and is formed in colluvium 

from marine basalt. The series consists of medial-skeletal, ferrihydritic, frigid Alic 

Hapludands (USDA NRCS, n.d.). This soil drains well to basalt bedrock. Typical 

texture is extremely gravelly sandy loam. The LOGS study area and most of our 

fertilizer study are situated on the 5- to 30-percent slope phase of this soil complex. 

A few plots are on the 30- to 60-percent slope phase. Growing season precipitation 

(April–September) was estimated from rain gauges located on site and at the nearby 

Quilcene Ranger Station.

Our stand originated from a 1937–1938 planting established after clearcut 

logging to salvage an earlier, wind- or snow-toppled stand of Douglas-fir, western 

hemlock (Tsuga heterophylla [Raf.] Sarg.), and western redcedar (Thuja plicata

Donn. ex D. Don). Logging slash was broadcast-burned in fall 1937. Two-year-old 

coast Douglas-fir seedlings were planted at a 10-ft-by-10-ft spacing. 

The eastern-most portion of the plantation was precommercially thinned in 

fall 1963 (figs. 1 and 3). Thinning removed trees primarily from lower crown 

classes and created a relatively uniform 11-ft-by-11-ft spacing (fig. 3). Based on 

stump counts and diameters, thinning intensity was severe, removing about 75 

percent of basal area and leaving an average of 351 TPA. As enumerated later, 

numerous volunteer seedlings of local conifers increased stand density and 

eventually created a two-story stand in the thinned portions. Height and age 

measurements on individual plots indicated that 50-year site class ranged from 

IV to V (King 1966).

Plot Installation and Fertilization Treatments

Thinned stand—

Ten 0.2-ac square plots were established in the thinned stand, each with an 

interior 0.1-ac plot for measuring tree growth. Volume growth was measured on 

each plot for a 2-year period (1967 and 1968 growing seasons), and based on this 

calibration growth period, nine plots were selected for the fertilizer study. Treatments were assigned using a completely randomized design, and in spring 1969, 

urea fertilizer (46 percent N) was uniformly broadcast by hand on the designated 

0.2-ac plots. The following treatments were applied to three plots (replications) 

each: 0, 200, or 400 lb N/ac, designated by the treatment codes 0N, 200N, or 

400N, respectively. 

To test the effects of other nutritional elements, additional trees were selected 

to be treated with fertilizer containing N, phosphorus (P), potassium (K), and 

sulfur (S). Ten dominant trees were systematically chosen outside the cluster 

of thinned-stand plots by projecting a compass line either diagonally from plot 
5

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

Figure 3—The fertilization study at Rocky Brook, Washington, was installed in 1968 in a portion 

of a 30-year-old Douglas-fir plantation that had been precommercially thinned 5 years earlier by 

Quilcene District personnel. Portions of the planation remained nonthinned. Thinned plot 12 (top) 

and nonthinned plot 29 (bottom) of the levels-of-growing-stock study. 

Richard E. Miller Richard E. Miller
6

RESEARCH PAPER PNW-RP-606

corners or perpendicularly from the midpoint of one side. The closest, nondamaged 

dominant tree located at least 33 ft from any plot was tagged. To provide spacing 

comparable to that of other fertilized trees, two trees were felled near each of the 

trees numbered 8, 9, and 10. Fertilizer was concurrently and uniformly broadcast 

around these 10 exterior trees (within an 18.6 ft radius=0.025 ac). The complete 

fertilizer treatment provided N at 400 lb/ac as urea (the largest dosage tested on 

the larger plots), P at 150 lb/ac as single and treble superphosphate, K at 100 lb/

ac as potassium chloride, and S at 50 lb/ac contained in the phosphate fertilizer. 

Because tree spacing was about 11 ft, this fertilized area included the subject tree 

and its nearest competitors. Consequently, growing conditions for the fertilized, 

individual subject trees should closely simulate those of d.b.h.-matched trees in the 

0.1-ac plots. For comparison, 10 similar-sized dominant trees were selected in plots 

treated with 400 lb N/ac and from nonfertilized plots.

Nonthinned stand—

Six additional, equal-size plots were installed in 1968 in the adjacent, nonthinned 

portion of the same plantation. The same 200N and 400N treatments were each 

randomly assigned to three of the six 0.2-ac plots. Three nonthinned plots in the 

nearby LOGS study located in the same plantation (Williamson 1976) were used 

for the 0N control plots. We acknowledge that this was not a randomly assigned 

control treatment.

Application— 

All fertilizer treatments were applied on May 9, 1969, to the 12 designated 0.2-ac 

treatment areas and exterior trees. Weather at application and in the preceding and 

subsequent 5 days was clear, dry, and warm. We suspect volatilization losses of 

urea were minimal. 

Tree Tagging and Measurements

All trees within the 0.1-ac measurement plots with a stem d.b.h. of 1.6 inches 

and larger were identified with numbered aluminum tags, and their d.b.h. was 

measured to the nearest 0.1 inch. Crown class and damage were also recorded. 

Trees were measured at 1- to 10-year intervals, specifically after the growing 

seasons of 1968, 1969, 1970, 1972, 1974, 1976, 1978, and 1988. Within each 

plot, total height and height to live crown to the nearest foot were measured 

with an Abney level on 20 randomly chosen trees; two-thirds of these height 

trees had a d.b.h. greater than the quadratic mean d.b.h. of the plot. Concurrently, d.b.h. and height to the nearest foot were measured on the 10 exterior 

trees selected for the NPKS study. 

Does the addition 

of phosphorus, 

potassium, and 

sulfur increase tree 

response to nitrogen 

fertilization?
7

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

Stand volumes for 1968 through 1988 were computed from tarif equations 

(Brackett 1973). Cubic volume total stem (CVTS) was computed for all trees 

measured for height using the equation derived by Bruce and DeMars (1974) from 

which tarifs were calculated. Individual tree tarifs were averaged for each plot, and 

mean tarifs for each plot and measurement year were smoothed over the 20-year 

period. Total stem volume inside bark of each tree 1.6 inches d.b.h. and larger 

(CVTS) was computed from d.b.h. and tarifs, summed to give volume per plot, and 

expanded to give volume per acre. Total stem volumes were converted to merchantable volumes to a 4-inch top (CV4) for trees 5.6 inches d.b.h. and larger using 

equations from Brackett (1973). Relative density (RD), a measure of stand stocking, 

was computed from basal area per acre, TPA, and quadratic mean d.b.h. (QMD) 

using the following equation (Curtis 1982): RD = BA/sqrt(QMD).

Experimental Design and Analysis

We estimated effects of N treatment on gross periodic annual increment (PAI) of 

live-stand volume (CVTS), merchantable volume (CV4), d.b.h., height, and crown 

variables for the 100 largest trees per acre (by d.b.h.). We also calculated fertilizer 

response efficiency as the additive gain in CVTS from fertilization (relative to the 

0N control) divided by N dosage (lb/ac).

Lack of randomization among all plots constrained some statistical analyses of 

this 20-year study. Fertilizer treatments in the thinned stand were randomized, so 

these data were analyzed separately. Likewise, the fertilizer study in the nonthinned 

stand was analyzed separately, although randomization of fertilizer treatments was 

constrained because all nonfertilized plots were restricted to the nearby LOGS 

study. Because the thinning treatment was not randomized, statistical tests of 

significance were not appropriate for comparing the main effects of thinning and 

the thinning-by-fertilizer interaction. Instead, inferences will be made based on 

contrasting the results of separate statistical analyses of data from the thinned and 

nonthinned stands. 

Data for each level of thinning were subjected to repeated-measures analysis 

of variance (ANOVA) using PROC MIXED, a mixed-model approach in SAS 

software (SAS Institute, Inc. 2013), to test for significant fixed effects of decadal 

period (10 or 20 years after fertilization), N fertilizer rate (0, 200, or 400 lb N/ac), 

and their interaction. A first-order autoregressive covariance structure was used 

to adjust each repeated measures ANOVA for serial correlation. When a given 

model failed to converge, an unstructured covariance structure was assumed. 

Plots within thinned and especially nonthinned portions had a wide range of 

initial stand volumes; therefore, our preliminary statistical analyses of fertilizer 
8

RESEARCH PAPER PNW-RP-606

effects tested the utility of using initial stand volume (CVTS) as a covariate to 

adjust for such differences. Scatter-grams documented the overall pattern of the 

relation between growth and initial volume and slopes of individual treatments so 

we could determine if covariance adjustment of treatment means along a common 

slope would be appropriate (Quinn and Keough 2002). We found that we were able 

to analyze the data with the covariate because the individual treatments shared a 

common slope for each of the growth variables. 

Data from the thinned and nonthinned stands were analyzed separately as 

completely randomized designs according to the following analysis of variance: 

Source of variation Degrees of freedom

N fertilizer (F) 2

Replication within F (Error 1) 5

Period (P) 1

P × F 2

Error 2 26

Covariate 1

Total (18-1) 17

Statistical significance was assigned at α = 0.10; however, probability values are 

provided so readers can make their own interpretations. If an F test in the ANOVA 

indicated a significant main effect of N fertilizer rate, multiple comparisons of 

means were conducted with Bonferroni adjusted probabilities to control the Type 

I error rate (Quinn and Keough 2002). If a significant interaction was detected 

between period and N fertilizer rate, multiple comparisons were conducted with 

Bonferroni-adjusted probabilities to identify differences among N fertilizer rates 

within a given period or between periods within a given N fertilizer rate (i.e., the 

simple main effects). Potential linear effects of N fertilizer rate were tested for each 

period using orthogonal contrasts and Bonferroni-adjusted probabilities (Sokal and 

Rohlf 1981).

The combined effect of P, K, and S additions was evaluated initially by graphically comparing basal area increments of the 10 exterior trees with those of plot 

trees. Covariance analysis of individual tree growth using initial tree basal area as 

the covariate was conducted to test for statistical differences using α = 0.10 to judge 

statistical significance.

Similar analyses were conducted for stem and crown size attributes of the 100 

largest trees per acre (by d.b.h.) to determine potential effects of thinning and fertilization on crop trees, which constitute the primary component of stand structure 

and wildlife habitat.

Data from the thinned 

and nonthinned 

stands were 

analyzed separately 

as completely 

randomized designs
9

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

Results

Effects of the Operational Thinning 

The belated and severe thinning at stand age 25 years felled about 75 percent of 

stand volume (table 1). Five years later at fertilization, relative density (Curtis 1982) 

averaged 19 (range of 18 to 20) in thinned plots compared to 67 (range of 56 to 81) 

in nonthinned plots. Note that RD 65 equates to a normal, fully stocked stand of 

Douglas-fir based on the seminal study by McArdle et al. (1961). Among the several 

species of conifers, Douglas-fir averaged 90 percent by basal area in the thinned 

stand compared to 76 percent in the nonthinned stand. 

Table 1—Stand statistics for trees with a diameter at breast height of 1.6 inches and larger at time of 

fertilization in 1968 in thinned and nonthinned stands at Rocky Brook, Washington

Treatmenta Plot Trees QMD H40 CVTS

Relative 

density

50-year site 

index

Douglas-fir 

BAb

Number/acre Inches Feet Ft3/acre Feet Percent

Thinned stand:

0N 8 400 4.3 35 501 19 70 100

9 330 4.8 41 577 19 78 96

17 280 5.4 41 631 19 81 92

200N 10 410 4.1 34 467 19 70 98

12 350 4.7 38 598 19 66 59

13 320 5.1 51 794 20 83 88

400N 11 380 4.6 42 635 20 76 94

15 360 4.4 33 478 18 66 87

16 330 4.8 35 534 19 71 100

Mean 351 4.7 39 579 19 73 90

Nonthinned stand:

0N 14 1,550 3.8 37 1,520 62 79 73

27 1,180 4.3 53 2,296 58 91 80

29 1,250 4.2 45 1,849 58 90 68

200N 1 1,190 4.2 46 1,914 56 88 71

4 1,700 4.3 48 2,923 81 91 80

5 1,520 4.3 43 2,689 74 81 78

400N 2 1,650 4.2 52 2,897 78 82 67

3 1,680 4.2 49 2,808 77 88 87

6 980 4.8 50 2,333 57 94 80

Mean 1,411 4.2 47 2,359 67 87 76

QMD = quadratic mean diameter at breast height; H40 = average height of 40 largest trees per acre, by diameter; CVTS = cubic volume of the 

total stem, inside bark; N = nitrogen;

a Application of 0, 200, or 400 lb N/ac.

b

 Percentage of stand basal area (BA) in Douglas-fir.
10

RESEARCH PAPER PNW-RP-606

Initial quadratic mean diameter averaged 4.7 and 4.2 inches in the thinned and 

nonthinned stands, respectively (table 1). Height of the 40 largest trees per acre 

by d.b.h. (H40) averaged 39 ft in the thinned stand versus 47 ft in the nonthinned 

stand. Based on estimated H40 and documented age counts from 91 increment 

cores extracted at plot establishment, estimated site index (King 1966) averaged 

only 73 ft in the thinned stand compared to 87 ft in the nonthinned stand. This 

difference may be due to lower site quality in the thinned stand, the likelihood that 

some potential site-index trees were felled during thinning, or some combination 

of both effects.

In both thinned and nonthinned stands, species composition among plots at 

fertilization did not differ in basal area per acre or in percentage of total basal area 

by species (tables 2 and 3). Total basal area per acre of the nonthinned stand averaged almost three times that of the thinned stand, and the percentage of basal area 

in Douglas-fir was less in the nonthinned stand. 

Effects of Urea-N in the Thinned Stand 

Live-stand volume—

Two years before fertilization (1966), mean live-stand volumes (CVTS) were similar 

among the three pending treatments (fig. 4). In the 20 years after the 2-year calibration period (1966–1968), accumulation of live-stand volume differed markedly 

among the fertilizer treatments. Live-stand volume more than doubled in period 2 

(the second decade after fertilizer treatment), especially on fertilized plots; the ratio 

of period 2 to period 1 (first decade after fertilizer treatment) values (P2/P1) ranged 

between 2.0 and 2.2 (table 4). Live-stand volume differed significantly among the N 

dosages. In each period, stand volume increased linearly with N dosage, indicating 

a proportionate response to the treatment. Fertilizer response efficiency (gain in 

CVTS from fertilization/N dosage) did not differ significantly between 200N and 

400N for either of periods 1 and 2 (table 5). 

Gross periodic annual increment— 

Throughout the 20-year study, PAI in the thinned stand increased gradually over 

time, and growth was roughly proportional to fertilizer rate (fig. 5). Annual growth 

or PAI did not appear to be related to growing season precipitation as measured 

on site and at the local ranger station in Hoodsport. Covariance analysis indicated 

that mean gross PAI differed significantly among fertilizer treatments and between 

periods, but the fertilizer-by-period interaction was also significant (P = 0.001); 

therefore, generalizations about main effects are not justified (table 6). In period 1, 

gross PAI for the 0N control averaged 116 ft3/ac/yr compared to 168 and 200 ft3/ac/

yr for the 200N and 400N treatments, respectively. Responses to the two N dosages 

In thinned stands, 

wood volume more 

than doubled during 

the second decade 

after fertilization.
11

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

Table 3a—Analysis of variance of mean basal area per acre by species before fertilization in the 

nonthinned stand at Rocky Brook, Washington

Douglas-fir Hemlock Other All

Source of variation D.f. F-value P F-value P F-value P F-value P

N fertilizer (F) 2 0.5 0.653 0.3 0.772 3.1 0.117 0.6 0.586

Error 6 — —

Total (9-1) 8

D.f. = degrees of freedom; N = nitrogen; — = not appropriate.

Table 3b—Mean basal area per acre by species (mean ± standard error) before fertilization in the 

nonthinned stand at Rocky Brook, Washington

Treatmenta Douglas-fir Hemlock Other All Douglas-fir Hemlock

- - - - - - - - - - - - - - - - - - - Ft2/acre - - - - - - - - - - - - - - - - - - - - - - Percentage of all - - -

0N 97.1 ± 9.4 a b 24.5 ± 4.9 a 5.4 ± 1.1 a 127.0 ± 9.0 a 76 19

200N 106.6 ± 9.4 a 28.8 ± 4.9 a 2.3 ± 1.1 a 137.7 ± 9.0 a 77 21

400N 109.2 ± 9.4 a 29.0 ± 4.9 a 1.6 ± 1.1 a 139.7 ± 9.0 a 78 21

Note: A similar ANOVA of starting stems per acre by species demonstrated no statistically significant differences among N dosage means for 

Douglas-fir, hemlock, other, and all species.

a Application of 0, 200, or 400 lb N/ac.

b Means within a column followed by a different letter differ significantly (P < 0.10).

Table 2a—Analysis of variance of mean basal area per acre by species before fertilization in the thinned 

stand at Rocky Brook, Washington

Douglas-fir Hemlock Other All

Source of variation D.f. F-value P F-value P F-value P F-value P

N fertilizer (F) 2 1.5 0.304 1.6 0.279 1.5 0.296 0.1 0.888

Error 6 — —

Total (9-1) 8

D.f. = degrees of freedom; N = nitrogen; — = not appropriate.

Table 2b—Mean basal area per acre by species (mean ± standard error) before fertilization in the thinned 

stand at Rocky Brook, Washington

Treatmenta Douglas-fir Hemlock Other All Douglas-fir Hemlock

- - - - - - - - - - - - - - - - - - - Ft2/acre - - - - - - - - - - - - - - - - - - - - - - Percentage of all - - -

0N 39.8 ± 2.6 ab 1.8 ± 2.5 a 0 ± 0 a 41.6 ± 1.4 a 96 4

200N 33.7 ± 2.6 a 7.6 ± 2.5 a 0 ± 0 a 41.4 ± 1.4 a 81 18

400N 38.2 ± 2.6 a 2.5 ± 2.5 a 0 ± 0 a 40.7 ± 1.4 a 94 6

Note: A similar ANOVA of starting trees per acre (TPA) by species demonstrated no statistically significant differences among fertilizer rates for 

Douglas-fir, other species, and all species. For hemlock, however, TPA of the 200N treatment (73 TPA) greatly exceeded that of the 0N and 400N 

treatments (13 and 20 TPA, respectively) (P = 0.087).

a Application of 0, 200, or 400 lb N/ac.

b Means within a column followed by a different letter differ significantly (P < 0.10).
12

RESEARCH PAPER PNW-RP-606

Table 4a—Analysis of covariance of mean live-stand 

volume (CVTS) by period after fertilization in the thinned 

stand at Rocky Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 130.2 <0.001

Rep. within F (Error 1) 5 — —

Period (P) 1 2,319.2 <0.001

F × P 2 48.1 <0.001

Linear, period 1a 1 145.9 <0.001

Linear, period 2 1 214.7 <0.001

Covariate (starting CVTS) 1 98.6 <0.001

Error 2 6 — —

Total (18-1) 17

D.f. = degrees of freedom; N = nitrogen; — = not appropriate.

a

 Linear contrasts with P ≤ 0.05 are considered statistically significant; period 

1 = 1 to 10 years and period 2 = 11 to 20 years after fertilization.

Figure 4—Trends of observed live-stand volume (CVTS, cubic volume-total stem) over time, by thinning and fertilizer treatments (T = 

thinned and NT = nonthinned; 0N, 200N, and 400N indicate urea fertilizer rates of 0, 200, and 400 lb N/ac, respectively), Rocky Brook, 

Washington. 
13

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

Table 5a— Analysis of covariance of fertilizer response 

efficiency by period after fertilization in the thinned stand 

at Rocky Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 1 2.7 0.197

Rep. within F (Error 1) 3 — —

Period (P) 1 339.7 <0.001

F × P 1 9.9 0.034

Covariate (starting CVTS) 1 36.6 0.004

Error 2 4 — —

Total (12-1) 11

D.f. = degrees of freedom; N = nitrogen; — = not appropriate.

Table 5b—Adjusted mean fertilizer response efficiency (mean ± standard error) 

by period after fertilization in the thinned stand at Rocky Brook, Washington

Relative to 200N

Treatmenta P1 P2 P1 P2 P2/P1

- - - - Ft3/ac ÷ dosage - - - - - - - - Percent - - -

200N* b 3.0 ± 0.44 a 7.1 ± 0.44 a 2.4

400N* 2.6 ± 0.44 a 5.5 ± 0.44 a 87 77 2.1

Fertilizer response efficiency = CVTS gain ÷ dosage; CVTS = cubic volume of the total stem, inside bark; 

N = nitrogen; P1 = years 1 to 10; P2 = years 11 to 20.

a

 Application of 0, 200, or 400 lb N/ac.

b

 Asterisks (*) indicate period means for a given fertilizer rate that differ significantly (P ≤ 0.10). Fertilizer 

means within a given period followed by a different letter differ significantly (P ≤ 0.10).

Table 4b—Adjusted mean live-stand volume (mean CVTS ± standard error) by 

period after fertilization in the thinned stand at Rocky Brook, Washington

Relative to 0N

Treatmenta P1 P2 P1 P2 P2/P1

- - - - - - - Ft3/acre - - - - - - - - - - Percent - - -

0N*b 1,690 ± 50 c 3,441 ± 98 c 100 100 2.0

200N* 2,244 ± 52 b 4,825 ± 99 b 133 140 2.2

400N* 2,549 ± 51 a 5,475 ± 99 a 151 159 2.1

CVTS = cubic volume of the total stem, inside bark; N = nitrogen; P1 = years 1 to 10; P2 = years 11 to 20. 

a Application of 0, 200, or 400 lb N/ac.

b

 Asterisks (*) indicate period means for a given fertilizer rate that differ significantly (P ≤ 0.10). Fertilizer means 

within a given period followed by a different letter differ significantly (P ≤ 0.10).
14

RESEARCH PAPER PNW-RP-606

Figure 5—Trends of observed gross periodic annual increment over time, by thinning and fertilizing treatments (T = thinned and 

NT = nonthinned; 0N, 200N, and 400N indicate urea fertilizer rates of 0, 200, and 400 lb N/ac, respectively) and histograms indicating 

growing season (April-September) precipitation at Rocky Brook, Washington. 

Table 6a—Analysis of covariance of mean annual gross volume 

growth (PAI) by period after fertilization in the thinned stand at 

Rocky Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 110.6 <0.001

Rep. within F (Error 1) 5 — —

Period (P) 1 1,260.9 <0.001

F × P 2 31.7 0.001

Linear, period 1a 1 171.0 <0.001

Linear, period 2 1 186.4 <0.001

Covariate (starting CVTS) 1 53.2 <0.001

Error 2 6 — —

Total (18-1) 17

D.f. = degrees of freedom; N = nitrogen; — = not appropriate; PAI = periodic annual increment.

a Linear contrasts with P ≤ 0.05 are considered statistically significant; period 1 = 1 to 10 years 

and period 2 = 11 to 20 years after fertilization.
15

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

differed, indicating 45- and 72-percent gains (52 and 84 ft3/ac/yr) as a result of N 

fertilization with 200N and 400N, respectively. 

In period 2, mean PAI increased for all three treatments (table 6). For the 0N 

control, mean PAI (182 ft3/ac/yr) was 60 percent greater in period 2 than in period 1, 

whereas PAI for 200N and 400N (280 and 313 ft3/ac/yr) averaged 70 and 60 percent 

greater, respectively. Orthogonal contrasts indicated that gross PAI increased 

linearly with increasing N dosage in each decade. Gross PAI of fertilized plots in 

period 2 exceeded that of nonfertilized plots by 54 and 72 percent for 200N and 

400N, respectively. 

Mortality losses—

Cubic volume per acre—Although scant CVTS volume was lost to mortality in 

period 1 in the thinned stand, volume losses on fertilized plots increased in period 

2 (table 7). In period 1, mean mortality losses among the three treatments were 

small and similar (0.0 to 1.8 ft3/ac/yr). In period 2, however, mortality volume was 

significantly greater after fertilization (22 and 12 ft3/ac/yr for 200N and 400N, respectively) than in the nonfertilized control (2 ft3

/ac/yr). The higher after-thinning 

density of plot 13 (794 trees per acre) compared to other plots in the thinned stand 

(467–635 trees per acre) may account for the observed greater mortality in the 200N 

treatment (table 1). Increased mortality in period 2 along with differences among N 

dosages explains the fertilizer-by-period interaction. Despite a greater volume loss 

to mortality in period 2, mortality losses in the thinned stand averaged only about 1 

to 8 percent of gross PAI among the three treatments (table 7 vs. table 6).

Number and volume of dead trees—In period 1, adjusted mean losses of trees were 

small, and values were similar among the three treatments, ranging from 0 to 3 

TPA per decade (table 8). Significantly more trees died in period 2 (second decade 

after treatment), averaging 71, 94, and 215 TPA per decade in the 0N, 200N, and 

Table 6b—Adjusted mean annual gross volume growth (mean ± standard error) 

by period after fertilization in the thinned stand at Rocky Brook, Washington

Relative to 0N

Treatmenta P1 P2 P1 P2 P2/P1

- - - - - - Ft3/ac/year - - - - - - - - - Percent - - -

0N*b 116 ± 4.5 c 182 ± 6.8 b 100 100 1.6

200N* 168 ± 4.7 b 280 ± 6.9 a 145 154 1.7

400N* 200 ± 4.6 a 313 ± 6.8 a 172 172 1.6

N = nitrogen; P1 = years 1 to 10; P2 = years 11 to 20.

a Application of 0, 200, or 400 lb N/ac.

b

 Asterisks (*) indicate period means for a given fertilizer rate that differ significantly (P ≤ 0.10). Fertilizer 

means within a given period followed by a different letter differ significantly (P ≤ 0.10).

Fertilization of thinned 

stands increased 

mortality volume.
16

RESEARCH PAPER PNW-RP-606

400N treatments, respectively. Despite these apparent differences in tree losses in 

period 2, the three N dosages did not differ statistically. Moreover, tree losses did 

not increase linearly with N dosage.

We wondered whether the large losses in mortality volume in period 2 were 

related to the loss of more trees or to the loss of more volume per tree. Note that 

prior to fertilizer treatments, the starting density among all plots averaged 351 TPA 

(table 1). Over time, trees were added through ingrowth, and some of these died 

and were counted as mortality. We compared average volume of both dead and live 

trees from observed (unadjusted by covariance analysis) metrics of CVTS and TPA 

Table 7a—Analysis of covariance of mean annual losses in 

CVTS per acre by period after fertilization in the thinned stand 

at Rocky Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 1.1 0.416

Rep. within F (Error 1) 5 — —

Period (P) 1 149.8 <0.001

F × P 2 35.7 <0.001

Linear, period 1a 1 2.5 0.165

Linear, period 2 1 18.9 0.005

Covariate (starting CVTS) 1 3.8 0.100

Error 2 6 — —

Total (18-1) 17

D.f. = degrees of freedom; CVTS = cubic volume of the total stem, inside bark; 

N = nitrogen; — = not appropriate.

a

 Linear contrasts with P ≤ 0.05 are considered statistically significant; period 1 = 1 to 

10 years and period 2 = 11 to 20 years after fertilization.

Table 7b—Adjusted mean annual losses in CVTS per acre (mean ± standard error) 

by period after fertilization in the thinned stand at Rocky Brook, Washington

Relative to 0N

Treatmenta P1 P2 P1 P2 P2/P1

- - - - - - Ft3/ac/year - - - - - - - - - Percent - - -

0N 1.8 ± 0.6 a 2.3 ± 6.1 b 100 100 1.3

200N*b 0 ± 0.6 a 22.4 ± 6.1 a 0 974 —

400N* 0.5 ± 0.6 a 11.9 ± 6.1 a 28 517 23.8

CVTS = cubic volume total stem, inside bark; — = not appropriate; P1 = years 1 to 10; P2 = years 11 to 20.

a Application of 0, 200, or 400 lb N/ac. 

b Asterisks (*) indicate period means for a given fertilizer rate that differ significantly (P ≤ 0.10). Fertilizer 

means within a given period followed by a different letter differ significantly (P ≤ 0.10).
17

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

(table 9). In both periods after fertilization, and in nearly all plots, average dead-tree 

volume was much less than the average volume of surviving trees. On average, 

smaller trees died. Both number and total volume of dead trees increased markedly 

in period 2, especially on fertilized plots. Average total volume of mortality was 

greatest on 200N plots (297 ft3/ac/decade and 103 TPA) and was notably high in 

plots 12 and 13 where the death of some large trees was attributed to storm damage. 

We doubt that these losses were caused by fertilization at the 200N dosage. Also in 

period 2, losses on control plots averaged 93 TPA and 0.27 ft3 per tree compared to 

213 TPA and 0.56 ft3 per tree after 400 lb N/ac was applied. We infer that application of 400 lb N/ac in the thinned stand increased both number and average volume 

of trees lost to mortality. 

Table 8—Analysis of covariance of mean 10-year losses in trees 

per acre (TPA) by period after fertilization in the thinned stand at 

Rocky Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 2.0 0.234

Rep. within F (Error 1) 5 — —

Period (P) 1 162.6 <0.001

F × P 2 3.4 0.103

Linear, period 1a 1 1.2 0.313

Linear, period 2 1 3.7 0.102

Covariate (starting TPA) 1 2.8 0.147

Error 2 6 — —

Total (18-1) 17

D.f. = degrees of freedom; N = nitrogen; — = not appropriate.

a Linear contrasts with P ≤ 0.05 are considered statistically significant; period 1 = 1 to 10 

years and period 2 = 11 to 20 years after fertilization.

Table 8b— Adjusted mean 10-year losses in trees per acre (mean TPA ± standard 

error) by period after fertilization in the thinned stand at Rocky Brook, Washington

Relative to 0N

Treatmenta P1 P2 P1 P2 P2/P1

- - Trees/acre/decade - - - - - Percent - - -

0Nb 3 ± 2.0 71 ± 32 100 100 24

200N 0 ± 2.0 94 ± 32 0 132 —

400N 1 ± 2.0 215 ± 32 33 303 215

N = nitrogen; P1 = years 1 to 10; P2 = years 11 to 20.

a Application of 0, 200, or 400 lb N/acre.

b Fertilizer-by-period means shown for information only. Main effect differences are described in the text.
18

RESEARCH PAPER PNW-RP-606

Table 9— Observed mean ending volume (CVTS) of live and dead trees by period after fertilization in the thinned stand at Rocky Brook, Washington

Ending live CVTS Dead tree count Ending dead tree CVTS

CVTS dead/live 

ratio

Per acre Per tree Trees Per acre Per tree Per tree

Treatmenta Plot P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2

- - - - - - - - Cubicfeet - - - - - - - - Number/acre - - - - - - - - Cubicfeet - - - - - - - -

0N 8 1,454 2,912 2.64 5.29 0 60 0 10 — 0.17 — 0.03

9 1,635 3,400 2.60 5.40 10 170 37 54 3.70 0.32 1.42 0.06

17 1,888 3,919 3.85 8.00 10 50 34 16 3.40 0.32 0.88 0.04

Mean 1,660 3,410 3.03 6.23 7 93 24 27 3.55 0.27 1.15 0.04

200N 10 1,785 4,190 2.55 5.98 0 170 0 125 — 0.74 — 0.12

12 2,400 5,032 5.45 11.44 0 40 0 227 — 5.68 — 0.50

13 2,925 5,631 6.80 13.09 0 100 0 539 — 5.39 — 0.41

Mean 2,370 4,951 5.78 10.17 0 103 0 297 — 3.94 — 0.34

400N 11 2,579 5,520 4.23 9.05 0 130 0 78 — 0.60 — 0.07

15 2,321 5,151 3.36 7.46 0 270 0 97 — 0.36 — 0.05

16 2,461 5,468 4.64 10.32 10 240 16 170 1.60 0.71 0.34 0.07

Mean 2,450 5,380 4.08 8.94 3 213 5 115 1.60 0.56 0.34 0.06

CVTS = cubic volume of the total stem, inside bark; P1 = years 1 to 10; P2 = years 11 to 20; N = nitrogen.aApplication of 0, 200, or 400 lb N/acre.
19

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

Effects of additional nutrient elements in the thinned portion—

Prior to fertilization, average arithmetic mean d.b.h. was greatest for the 10 trees 

that were systematically selected for fertilization with NPKS (5.2 inches) and least 

for the trees selected in 400N plots (4.6 inches). This initial ranking of treatment 

means gradually reversed after fertilization. Basal area growth per tree was clearly 

enhanced after fertilization, and the 400N dosage as a single fertilizer was as 

stimulating as that same amount of N combined with PKS.

Covariance analysis indicated that the 400N application increased mean d.b.h. 

of subject trees both 10 and 20 years after treatment, but d.b.h. in the NPKS application did not differ significantly from the 0N treatment (table 10). An additional 

covariance analysis used mean annual growth in cross-sectional area at breast 

height (i.e., basal area increment) as the response variable. This more sensitive 

metric of response indicated a decline for the NPKS treatment in both growth and 

Table 10a—Analysis of covariance of mean diameter at breast height (d.b.h.) 10 and 20 years 

after fertilization and basal area increment (BAI) after individual tree fertilization in the 

thinned stand at Rocky Brook, Washington

D.b.h. (inches) Basal area increment (inch2)

Source of variation D.f. F-value P F-value P

N fertilizer (F) 2 7.6 0.003 6.1 0.007

Rep. within F (Error 1) 26 — — — —

Period (P) 1 571.4 <0.001 4.3 0.049

P × F 2 2.8 0.076 11.5 <0.001

Covariate (starting d.b.h.) 1 123.3 <0.001 36.7 <0.001

Error 2 27 — —

Total (60-1) 59

N = nitrogen; — = not appropriate.

Table 10b—Adjusted mean diameter at breast height (mean d.b.h. ± standard error) 10 and 20 years after 

fertilization and basal area increment (BAI ± standard error) after individual tree fertilization in the thinned 

stand at Rocky Brook, Washington

d.b.h. Relative to 0N BAI

Treatmenta P1 P2 P1 P2 P2/P1 P1 P2 P1 P2 P2/P1

- - - - Inches - - - - - - Percent - - - - Square inches per year - - - - Percent - - 

0N*b 7.4 ± 0.3 b 9.5 ± 0.3 b 100 100 1.28 2.5 ± 0.3 b 3.0 ± 0.3 b 100 100 1.2

400N* 8.5 ± 0.3 a 11.1 ± 0.3 a 115 117 1.31 3.8 ± 0.3 a 4.1 ± 0.3 a 152 137 1.1

400NPKS* 8.1 ± 0.3 b 10.2 ± 0.3 b 109 107 1.26 3.3 ± 0.3 b 2.9 ± 0.3 b 132 97 .9

d.b.h. = diameter at breast height; NPKS = nitrogen plus phosphorus, potassium, and sulfur; P1 = years 1 to 10; P2 = years 11 to 20.

a

Application of 0 or 400 lb N/ac or 400 lb N/ac plus phosphorus, potassium, and sulfur.

b

Asterisks (*) indicate period means for a given fertilizer rate that differ significantly (P ≤ 0.10). Year means for BAI only differ significantly for 0N. 

Fertilizer means within a given period followed by a different letter differ significantly (P ≤ 0.10).

Addition of phosphorus, 

potassium, and sulfur 

did not increase stand 

response to nitrogen 

fertilization.
20

RESEARCH PAPER PNW-RP-606

response relative to control trees in the second decade when the P2/P1 ratio for the 

NPKS treatment averaged 0.9 compared to 1.2 for the control and 1.1 for the 400N 

treatment. We suspect that the smaller fertilized area around our individually fertilized NPKS trees was too limited to provide a treatment effect that was as lasting as 

that of the N-only trees located within a fertilized area of 0.2 ac.

Effects of Urea-N in the Nonthinned Stand

Live-stand volume—

Note there was no calibration period before fertilization in the nonthinned stand. 

Starting volume (CVTS) in the three nonthinned, nonfertilized plots located in the 

nearby LOGS study averaged slightly less than the means of fertilized plots. Average RD was also less (table 1). Live-stand volume of the nonthinned stand increased 

linearly in the 20 years since study establishment with a strong divergence between 

fertilized and nonfertilized treatments, but little differentiation between the 200N 

and 400N dosages (fig. 4). Stand volume in the nonthinned stand increased by about 

50 percent in the second decade (P2/P1 = 1.4 to 1.5) (table 11). CVTS was greater 

in fertilized plots than in the nonfertilized control in each period, but there was no 

significant difference between N dosages. Both N dosages increased net accumulation of live-stand volume over that of the 0N control by about 30 percent at the end 

of both period 1 and period 2. In each decade, fertilizer response efficiency (gain 

in CVTS from fertilization/N dosage [lb/ac]) in the 200N treatment was over twice 

that observed in the 400N treatment, indicating more gain in live-stand volume per 

pound of applied N (table 12).

Gross periodic annual increment—

In the nonthinned stand, gross PAI of fertilized plots exhibited some variability and 

increased considerably in period 1, whereas PAI of nonfertilized plots was similar 

to that of the thinned stand (fig. 5). Covariance analysis indicated a significant 

fertilizer-by-period interaction (table 13). The covariate, initial CVTS, was highly 

significant (P < 0.001), and observed mean PAI for the 0N treatment was increased 

by about 13 percent after mathematical adjustment from the covariance analysis. 

This adjustment also lowered observed means for the fertilized plots, and thereby 

reduced estimated response to N. 

In period 1 in the nonthinned stand, gross PAI for the 0N control averaged 

223 ft3/ac/yr compared to 320 and 330 ft3/ac/yr for the 200N and 400N treatments, 

respectively. The 43 to 48 percent increases in gross growth from these N treatments, relative to the control, were statistically significant but not different from 

each other (table 13). The gross PAI response to fertilizer rate was linear in period 1 

but not in period 2. 
21

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

In period 2, gross growth in the nonfertilized plots increased by 10 percent 

but declined in the 200N and 400N treatments by 10 and 20 percent, respectively 

(table 13). The 20-percent reduction for the 400N treatment was statistically 

significant. In contrast, recall that gross PAI in the thinned stand accelerated in 

period 2 in both nonfertilized and fertilized plots (table 6). In the nonthinned 

stand, gross PAI in the 400N treatment (265 ft3/ac/yr) did not differ significantly 

from either of the 0N or 200N treatments (246 and 290 ft3/ac/yr, respectively). 

Gain in gross PAI from fertilization averaged 44 ft3/ac/yrfor 200N compared to 

19 ft3/ac/yr for 400N.

Table 11a—Analysis of covariance of mean live-stand volume 

(CVTS) 10 and 20 years after fertilization in the nonthinned stand 

at Rocky Brook, Washington 

Source of variation D.f. F-value P

N fertilizer (F) 2 16.7 0.006

Rep. within F (Error 1) 5 — —

Period (P) 1 897.0 <0.001

F × P 2 6.2 0.035

Linear, period 1a 1 19.4 0.004

Linear, period 2 1 26.7 0.002

Covariate (starting CVTS) 1 19.4 0.005

Error 2 6 — —

Total (18-1) 17

D.f. = degrees of freedom; CVTS = cubic volume of the total stem, inside bark; 

N = nitrogen; — = not appropriate.

a Linear contrasts with P ≤ 0.05 are considered statistically significant; period 1 = 1 to 10 

years and period 2 = 11 to 20 years after fertilization.

Table 11b—Adjusted mean live-stand volume (mean CVTS ± standard error) 

10 and 20 years after fertilization in the nonthinned stand at Rocky Brook, 

Washington

Relative to 0N

Treatmenta P1 P2 P1 P2 P2/P1

- - - - - - Cubic feet/ac - - - - - - - - - - - - - Percent - - - - - - Ratio

0N*b 4,094 ± 193 b 6,018 ± 229 b 100 100 1.5

200N* 5,280 ± 165 a 7,840 ± 206 a 129 130 1.5

400N* 5,379 ± 179 a 7,771 ± 217 a 131 129 1.4

CVTS = cubic volume of the total stem, inside bark; N = nitrogen; P1 = years 1 to 10; P2 = years 11 to 20.

a Application of 0, 200, or 400 lb N/acre. 

b

Asterisks (*) indicate period means for a given fertilizer rate that differ significantly (P ≤ 0.10). Fertilizer 

means within a given period followed by a different letter differ significantly (P ≤ 0.10).
22

RESEARCH PAPER PNW-RP-606

Table 12a—Analysis of covariance of fertilizer response 

efficiency by period after fertilization in the nonthinned stand 

at Rocky Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 1 42.3 0.007

Rep. within F (Error 1) 3 — —

Period (P) 1 21.9 .009

F × P 1 4.7 .096

Covariate (starting CVTS) 1 39.3 .003

Error 2 4 — —

Total (12-1) 11

D.f. = degrees of freedom; N = nitrogen; — = not appropriate; CVTS = cubic volume of 

the total stem, inside bark.

Table 12b— Mean fertilizer response efficiency (mean ± standard error) by period 

after fertilization in the nonthinned stand at Rocky Brook, Washington

Relative to 200N

Treatmenta P1 P2 P1 P2 P2/P1

- - - - - Ft3/ac ÷ dosage - - - - - - - - Percent - - - Ratio

200N*b 9.6 ± 0.7 a 12.8 ± 0.7 a 1.3

400N 4.4 ± 0.7 b 5.6 ± 0.7 b 46 44 1.3

Fertilizer response efficiency = CVTS gain ÷ dosage; CVTS = cubic volume of the total stem, inside bark; 

P1 = years 1 to 10; P2 = years 11 to 20.

a Application of 0, 200, or 400 lb N/ac. 

b Asterisks (*) indicate period means for a given fertilizer rate that differ significantly (P ≤ 0.10). Fertilizer 

means within a given period followed by a different letter differ significantly (P ≤ 0.10).

Table 13a—Analysis of covariance of mean annual gross volume 

growth (PAI) by period after fertilization in the nonthinned stand 

at Rocky Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 10.9 0.015

Rep × F (Error 1) 5 — —

Period (P) 1 6.8 .040

F × P 2 7.5 .023

Linear, period 1a 1 16.6 .006

Linear, period 2 1 2.9 .138

Covariate (starting CVTS) 1 49.4 <.001

Error 2 6 — —

Total (18-1) 17

D.f. = degrees of freedom; N = nitrogen; — = not appropriate; PAI = periodic annual 

increment; CVTS = cubic volume of the total stem, inside bark.

a Linear contrasts with P ≤ 0.05 are considered statistically significant; period 1 = 1 to 10 

years and period 2 = 11 to 20 years after fertilization.
23

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

Mortality losses—

Cubic volume per acre—Covariance analysis indicated that the difference between the two periods was the only significant response for annual mortality in 

CVTS (table 14). In period 1, adjusted means of annual mortality in CVTS in the 

nonthinned stand were small and similar, ranging from about 10 to 17 ft3/ac/yr 

among the three treatments, which did not differ significantly. Mortality losses 

in period 2 increased two- to three-fold for all treatments, but losses did not differ among the three fertilizer treatments. We suspect that high variability among 

replicate plots of each treatment constrained detection of significant differences 

among N dosages in either period. As a percentage of gross PAI, mortality losses 

in the second decade were 12, 13, and 15 percent of gross production of the 0N, 

200N and 400N treatments, respectively. These are larger percentages than in the 

thinned stand, where CVTS mortality averaged only 1 to 8 percent of gross PAI 

(tables 6 and 7).

Number and volume of dead trees—Note that mean starting density among these 

treatments in the nonthinned stand averaged 1,411 TPA in 1968 (table 1), and 

ingrowth trees—some of which died and were counted as mortality—were later 

recruited into the starting TPA. As expected, many trees died in the overstocked 

nonthinned stand in both 10-year periods (table 15). Losses were related linearly 

to N dosage in the first decade, but unrelated in the second. Covariance analysis 

indicated a significant fertilizer-by-period interaction (P = 0.021). Tree losses in 

nonfertilized plots more than doubled in the second decade and this increase was 

statistically significant. Tree losses in 400N plots exceeded those in the 0N control 

plots in the first decade, but not in the second decade when adjusted losses averaged 260 and 307 TPA, respectively. 

Table 13b—Adjusted mean annual gross volume growth (mean PAI ± 

standard error) by period after fertilization in the nonthinned stand at 

Rocky Brook, Washington

Relative to 0N

Treatmenta P1 P2 P1 P2 P2/P1

- - - - - Feet3/acre/year - - - - - - - - Percent - - -

0 223 ± 18 b 246 ± 7 b 100 100 1.1

200 320 ± 18 a 290 ± 6 a 143 118 0.9

400* b 330 ± 18 a 265 ± 7 ab 148 108 0.8

a Application of 0, 200, or 400 lb N/ac; P1 = years 1 to 10; P2 = years 11 to 20.

b Asterisks (*) indicate period means for a given fertilizer rate that differ significantly (P ≤ 0.10). 

Fertilizer means within a given period followed by a different letter differ significantly (P ≤ 0.10).

Fertilization of the 

nonthinned stand did 

not increase mortality 

volume.
24

RESEARCH PAPER PNW-RP-606

We compared the average volume of dead and live trees as derived from 

CVTS and TPA (table 16). Among all treatments and in both decades, smaller than 

average-sized trees died. Similar numbers died on fertilized plots, but dead trees 

had more volume per tree in the second decade, especially on fertilized plots. In the 

first decade, dead tree volume averaged 0.67, 0.67, and 0.70 ft3, respectively, for 0N, 

200N, and 400N, and in the second decade volume averaged 0.90, 1.38, and 1.76 ft3. 

This greater volume per tree, rather than greater numbers of dead trees, probably 

explains the greater losses in CVTS after fertilization of the nonthinned stand.

Considerably more mortality occurred in the nonthinned stand than in the thinned 

stand (fig. 6). Volume mortality in the nonthinned stand increased in proportion to 

Table 14a— Analysis of covariance of mean annual losses 

in CVTS per acre by period after fertilization in the 

nonthinned stand at Rocky Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 3.1 0.132

Rep. within F (Error 1) 5 — —

Period (P) 1 105.2 <0.001

F × P 2 0.6 0.576

Linear, period 1a 1 4.1 0.088

Linear, period 2 1 5.6 0.056

Covariate (starting CVTS) 1 10.1 0.019

Error 2 6 — —

Total (18-1) 17

D.f. = degrees of freedom; CVTS = cubic volume of the total stem, inside bark; 

N = nitrogen; — = not appropriate; P1 = years 1 to 10; P2 = years 11 to 20.

a

 Linear contrasts with P ≤ 0.05 are considered statistically significant; period 

1 = 1 to 10 years and period 2 = 11 to 20 years after fertilization.

Table 14b—Adjusted mean annual losses in CVTS per acre (mean ± 

standard error) by period after fertilization in the nonthinned stand at 

Rocky Brook, Washington

Relative to 0N

Treatmenta P1 P2 P1 P2 P2/P1

- - Cubic feet/acre/year - - - - - Percent - - -

0Nb 9.8 ± 0.8 29.4 ± 4.2 100 100 3.0

200N 12.8 ± 0.8 38.8 ± 4.2 131 132 3.0

400N 17.2 ± 0.8 40.6 ± 4.2 176 138 2.4

CVTS = cubic volume of the total stem, inside bark; N = nitrogen; — = not appropriate; P1 = years 

1 to 10; P2 = years 11 to 20.

a Application of 0, 200, or 400 lb N/ac. 

b Fertilizer-by-period means shown for information only. Main effect differences are described in 

the text.
25

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

the RD present at the start of each decade. This trend was especially apparent in the 

second decade when RD equaled or exceeded 100 in several of the fertilized plots. 

Mortality volume of the thinned stand was relatively independent of RD, probably 

because initial values at the start of each decade never exceeded RD 50. 

Net periodic annual increment after mortality losses— 

In the two decades after fertilization, volume lost to mortality was generally less 

than 5 percent of total gross growth in the thinned stand, but it was 8 to 10 percent 

in the nonthinned stand (fig. 7). Both mortality and net PAI averages were greater 

on fertilized plots. 

Table 15a—Analysis of covariance of mean 10-year losses 

in trees per acre by period after fertilization in the 

nonthinned stand at Rocky Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 3.0 0.142

Rep. within F (Error 1) 5 — —

Period (P) 1 16.4 0.007

F × P 2 7.9 0.021

Linear, period 1a 1 20.2 0.004

Linear, period 2 1 0.9 0.376

Covariate (starting TPA) 1 17.8 0.006

Error 2 6 — —

Total (18-1) 17

D.f. = degrees of freedom; N = nitrogen; — = not appropriate.

a Linear contrasts with P ≤ 0.05 are considered statistically significant; period 

1 = 1 to 10 years and period 2 = 11 to 20 years after fertilization.

Table 15b—Adjusted mean 10-year losses in trees per acre (mean ± 

standard error) by period after fertilization in the nonthinned stand at 

Rocky Brook, Washington

Relative to 0N

Treatmenta P1 P2 P1 P2 P2/P1

- - - - Trees/acre/decade - - - - - - - Percent - - - Ratio

0N*b 128 ± 12 b 307 ± 28 a 100 100 2.4

200N 189 ± 12 ab 277 ± 28 a 148 90 1.5

400N 280 ± 12 a 260 ± 28 a 219 85 0.9

N = nitrogen; P1 = years 1 to 10; P2 = years 11 to 20.

a Application of 0, 200, or 400 lb N/acre.

b

Asterisks (*) indicate period means for a given fertilizer rate that differ significantly (P ≤ 0.10). 

Fertilizer means within a given period followed by a different letter differ significantly (P ≤ 0.10).
26

RESEARCH PAPER PNW-RP-606

Table 16—Observed ending mean volume (CVTS) of live and dead trees by period after fertilization in the nonthinned stand at Rocky Brook, Washington

Ending live (CVTS) Ending mortality Dead/live ratio

CVTS CVTS

Per acre Per tree Trees Per acre Per tree Per tree

Treatmenta Plot P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2

- - - - - - - - - - Cubic feet - - - - - - - - - - Number/acre - - - - - - - - - - Cubic feet - - - - - - - - - - - - - - Ratio - - - -

0N 14 3,545 5,189 2.20 3.93 80 310 65 228 0.81 0.74 0.37 0.19

27 3,888 6,036 3.16 6.35 170 280 96 295 0.56 1.05 0.18 0.17

29 3,470 5,451 2.99 6.05 270 260 175 236 0.65 .91 0.22 0.15

Mean 3,630 5,560 2.78 5.44 173 283 112 253 0.67 .90 0.24 0.17

200N 1 4,428 6,583 4.02 8.33 140 310 121 346 0.86 1.12 0.21 0.13

4 6,098 8,706 4.15 7.57 250 330 158 575 0.63 1.74 0.15 0.235 5,704 8,621 4.32 8.06 240 260 125 333 0.52 1.28 0.12 0.16

Mean 5,410 7,970 4.16 7.99 210 300 135 418 0.67 1.38 0.16 0.17

400N 2 5,731 8,080 4.55 8.69 410 330 392 503 0.96 1.52 0.21 0.17

3 6,182 8,771 4.65 8.77 390 340 190 526 0.49 1.55 0.11 0.186 5,213 7,450 6.52 10.49 140 160 93 352 0.66 2.20 0.10 0.21

Mean 5,710 8,100 5.24 9.32 313 277 225 460 0.70 1.76 0.13 0.19

CVTS = cubic volume of the total stem, inside bark; N = nitrogen; P1 = years 1 to 10; P2 = years 11 to 20.a Application of 0, 200, or 400 lb N/acre.
27

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

700

600

500

400

300

200

100

0

M

ortality (ft 3/acre)

Starting RD at stand age 30

0 10 20 30 40 50 60 70 80 90 100 110 120

A

T-0N

T-200N

T-400N

NT-0N

NT-200N

NT-400N

700

600

500

400

300

200

100

0

M

ortality (ft 3/acre)

Starting RD at stand age 40

0 10 20 30 40 50 60 70 80 90 100 110 120

B

Figure 6—Total volume of dead trees in a 10-year period compared to relative density (RD) at the 

start of the period (A) the first decade after fertilization and (B) the second decade after fertilization, by thinning and fertilizer treatments (T = thinned and NT = nonthinned; 0N, 200N, and 400N 

indicate urea fertilizer rates of 0, 200, and 400 lb N/ac, respectively), Rocky Brook, Washington.

Mortality volume per 

decade was related to 

initial relative density.
28

RESEARCH PAPER PNW-RP-606

Stand Density and Species Composition by Trees per Acre 

Thinned stand—

Initial TPA in the thinned stand at fertilization averaged 337, 360, and 357, respectively, for the 0N, 200N, and 400N treatments (table 17). Despite mortality losses, 

especially in period 2, unadjusted tree density on all plots increased in the 20 years 

after treatment as volunteers attained the 1.6-inch d.b.h. threshold for measurement. 

Stem density at the end of the second decade increased by 181 percent in the nonfertilized treatment compared to 128 percent after 200N and 146 percent after 400N. 

Although TPA in 0N plots increased in both periods, TPA increased in the first 

decade but decreased in the second decade for 200N and 400N plots, presumably 

the result of increasing competitive stress among trees. Percentage of Douglas-fir by 

TPA in 1968 averaged 96, 82, and 94, respectively, for the 0N, 200N, and 400N treatments (table 17). This percentage decreased slightly on most thinned plots over the 

next 20 years as mostly western hemlock volunteers were recruited to our inventory.

Nonthinned stand—

Before fertilization in the nonthinned stand, live trees per acre averaged 1,327,

1,470, and 1,437, respectively, for the 0N, 200N, and 400N treatments (table 

17). In contrast to increasing tree numbers for all treatments in the thinned 

stand, tree numbers decreased over the next 20 years on all treatments in the 

400

350

300

250

200

150

100

50

0

0 200 400 0 200 400

Periodic annual increment (ft3/year)

Thinned Nonthinned

Nitrogen fertilizer rate (lb N/ac)

Mortality

Net growth

Figure 7—Mean annual mortality and net growth increment in live-stand volume (CVTS) in the two 

decades after fertilization, by thinning and fertilizing treatments, Rocky Brook, Washington. Note 

that mortality plus net increment equals gross periodic annual increment (PAI).
29

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

nonthinned stand. Net loss of TPA averaged 20 percent on 0N plots compared 

to 32- and 39-percent losses on 200N and 400N plots, respectively. Initially, 

percentage of Douglas-fir by TPA averaged 74, 77, and 78, respectively, for the 

0N, 200N, 400N treatments. As in the thinned plots, the percentage of Douglasfir in trees per acre decreased slightly on most plots over the next 20 years as the 

percentage of hemlock increased.

Species Composition by Basal Area

Thinned stand— 

At stand age 30 years and 5 years after thinning, basal area per acre in the thinned 

stand averaged less than half that in the nonthinned stand. Of this total, Douglas-fir 

averaged 90 percent by basal area among the nine plots in the thinned stand and 

averaged 76 percent in the nonthinned stand, presumably because other species 

were not felled by thinning (table 1). 

Total basal area per acre and Douglas-fir composition by basal area increased 

in both periods after treatment, especially in period 2 (table 18). Basal area of 

Douglas-fir increased by 60 to 70 percent in the second decade over the first 

decade (P2/P1 = 1.6 to 1.7). Increases in the percentage of basal area in western 

hemlock were smaller in the second decade (P2/P1 = 0.9 to 1.2). As a portion of 

total basal area for all species, the percentage of Douglas-fir by age 50 averaged 

84 percent on control plots, but it was 5 to 7 percent greater on 200N and 400N 

plots, respectively. 

Table 17—Observed means of starting and ending trees per acre (TPA) by period after 

fertilization in the thinned and nonthinned stands at Rocky Brook, Washington

All live trees Douglas-fir/all

P1 P2 P1 P2

Treatmenta Start End End

20-year 

change Start End End

20-year 

change

- - - - Number/acre - - - - - - - - - - - - - - - - - Percent - - - - - - - - - - - - - -

Thinned:

0N 337 557 610 181 96 92 89 -7

200N 360 523 460 128 82 78 83 1

400N 357 610 523 146 94 91 93 -1

Nonthinned:

0N 1,327 1,333 1,057 -20 74 72 71 -4

200N 1,470 1,297 1,003 -32 77 77 77 0

400N 1,437 1,150 880 -39 78 79 79 1

N = nitrogen; P1 = years 1 to 10; P2 = years 11 to 20.

a Application of 0, 200, or 400 lb N/acre. 
30

RESEARCH PAPER PNW-RP-606

After the first decade, average basal area of Douglas-fir on fertilized plots 

exceeded that on nonfertilized plots, but it did not differ between 200N and 400N (table 

18). After the second decade, all three treatments differed statistically, and BA was the 

greatest in the 400N treatment for Douglas-fir only and for all species combined. In 

each decade, Douglas-fir basal area increased linearly with fertilizer rate. Basal area 

of western hemlock averaged less than 15 percent of total basal area per acre in both 

periods. Variation in hemlock basal area was not related to N application or period. 

Nonthinned stand—

In the nonthinned stand, the amount of Douglas-fir and hemlock calculated as a 

percentage of total basal area for all species changed very little in two decades 

(table 19). By age 50, Douglas-fir averaged 75 to 79 percent and hemlock 

Table 18a—Analysis of covariance of mean basal area per acre (BA) by species 

and period after fertilization in the thinned stand at Rocky Brook, Washington

Douglas-fir Hemlock

Source of variation D.f. F-value P F-value P

N fertilizer (F) 2 213.5 <0.001 0.2 0.789

Rep × F (Error 1) 5 — — — —

Period (P) 1 567.7 <0.001 0.5 0.492

F × P 2 9.0 0.016 0.4 0.669

Linear, period 1a 1 81.1 <0.001 0.8 0.401

Linear, period 2 1 246.3 <0.001 0.2 0.676

Covariate (starting BA) 1 222.5 <0.001 604.4 <0.001

Error 2 6 — — — —

Total (18-1) 17

D.f. = degrees of freedom; N = nitrogen; — = not appropriate; P1 = years 1 to 10; P2 = years 11 to 20.

a Linear contrasts with P ≤ 0.05 are considered statistically significant; period 1 = 1 to 10 years and period 2 = 11 

to 20 years after fertilization.

Table 18b—Adjusted mean basal area per acre (mean BA ± standard error) by species and period after 

fertilization in the thinned stand at Rocky Brook, Washington

All species Douglas-fir Hemlock

Treatmenta P1 P2 P1 P2 P1 P2 P2/P1 P1 P2 P1 P2 P2/P1

- - - - - - - - - - - Feet2/acre - - - - - - - - - - - - Percent - - - Feet2/acre - - - Percent

0N*b 88 c 141 c 72 ± 2.6 b 118 ± 2.6 c 82 84 1.6 12 ± 2.1 15 ± 2.1 14 11 1.2

200N* 107 b 165 b 93 ± 2.7 a 147 ± 2.7 b 87 89 1.6 12 ± 2.1 15 ± 2.1 11 9 1.2

400N* 122 a 193 a 105 ± 2.6 a 176 ± 2.6 a 86 91 1.7 15 ± 2.0 14 ± 2.0 12 7 0.9

a Application of 0, 200, or 400 lb N/ac; P1 = years 1 to 10; P2 = years 11 to 20.

b Asterisks (*) indicate all-species and Douglas-fir period means for a given fertilizer rate that differ significantly (P ≤ 0.10). All-species and Douglas-fir 

fertilizer means within a given period followed by a different letter differ significantly (P ≤ 0.10).
31

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

averaged 19 to 22 percent of the total basal area in the nonthinned stand. The 

percentage of Douglas-fir averaged 2 to 4 percent greater on fertilized plots than 

nonfertilized plots. 

As also measured in the thinned stand, total basal area per acre for all species combined increased in both decades, especially in the second decade (table 19). Although 

the fertilizer-by-period interaction was not significant for Douglas-fir basal area, both 

fertilizer and period main effects were significant. Averaged across periods, basal area 

of Douglas-fir was 175, 191, and 191 ft2/ac for the 0N, 200N, and 400N treatments, 

respectively, and it was significantly greater in fertilized plots than in the nonfertilized 

control. Averaged across fertilizer rates, Douglas-fir basal area was significantly greater 

in period 2 (203 ft2

/ac) than in period 1 (169 ft2/acre). Only period main effects were 

significant for hemlock basal area (44 vs. 54 ft2/ac in periods 1 and 2, respectively).

Table 19a—Analysis of covariance of mean basal area per acre (BA) by 

species and period after fertilization in the nonthinned stand at Rocky 

Brook, Washington

Douglas-fir Hemlock

Source of variation D.f. F-value P F-value P

N fertilizer (F) 2 4.1 0.09 1.4 0.339

Rep × F (Error 1) 5 — — — —

Period (P) 1 218.3 <0.001 57.4 <0.001

F × P 2 1.1 0.404 0.8 0.511

Linear, period 1a 1 5.3 0.062 0.0 0.982

Linear, period 2 1 5.2 0.064 1.2 0.312

Covariate (starting BA) 1 71.5 0.001 80.1 <0.001

Error 2 6 — — — —

Total (18-1) 17

D.f. = degrees of freedom; N = nitrogen; — = not appropriate; P1 = years 1 to 10; P2 = years 11 to 20.

a Linear contrasts with P ≤ 0.05 are considered statistically significant; period 1 = 1 to 10 years and 

period 2 = 11 to 20 years after fertilization.

Table 19b—Mean basal area per acre (mean BA ± standard error) by species and period after fertilization in 

the nonthinned stand at Rocky Brook, Washington

All species Douglas-fir Hemlock

Treatmenta P1 P2 P1 P2 P1 P2 P2/P1 P1 P2 P1 P2 P2/P1

- - - - - - Square feet per acre - - - - - - Percent Square feet per acre Percent

0Nb 208 254 160 ± 4.9 191 ± 4.9 77 75 1.2 43 ± 2.4 55 ± 2.4 21 22 1.3

200N 224 271 172 ± 4.8 210 ± 4.8 77 77 1.2 47 ± 2.4 57 ± 2.4 21 21 1.2

400N 223 263 176 ± 4.9 207 ± 4.9 79 79 1.2 43 ± 2.4 51 ± 2.4 19 19 1.2

a Application of 0, 200, or 400 lb N/acre; P1 = years 1 to 10; P2 = years 11 to 20.

b Fertilizer-by-period means shown for information only. Main effect differences are described in the text.
32

RESEARCH PAPER PNW-RP-606

Relative Density

Recall that relative density (RD) is computed from basal area per acre, TPA, and 

quadratic mean d.b.h. (QMD), and that QMD is derived from TPA and basal area 

per acre (Curtis 1982). Consequently, changes in tree numbers and stand basal 

area over time result in changes in RD.

Thinned stand—

Relative density in 1968 was similar among thinned plots and averaged 19 for 

pending 0N, 200N, and 400N treatments (table 1). Although this narrow range 

of starting RDs before fertilization resulted in a nonsignificant covariate (P = 

0.98), we still used covariance to adjust for small differences among plots in 

prefertilization RD (table 20). In period 1, mean initial RD doubled to 38 on 

Table 20a—Analysis of covariance of mean relative density 

(RD) by period after fertilization in the thinned stand at Rocky 

Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 81.9 <0.001

Rep. within F (Error 1) 5 — —

Period (P) 1 1,397.1 <0.001

F × P 2 3.7 0.091

Linear, period 1a 1 109.0 <0.001

Linear, period 2 1 120.4 <0.001

Covariate (starting RD) 1 0 0.980

Error 2 6 — —

Total (18-1) 17

D.f. = degrees of freedom; N = nitrogen; — = not appropriate. 

a Linear contrasts with P ≤ 0.05 are considered statistically significant; period 1 = 1 to 10 

years and period 2 = 11 to 20 years after fertilization.

Table 20b—Adjusted mean relative density (mean RD ± standard error) by 

period after fertilization in the thinned stand at Rocky Brook, Washington

Relative to 0N

Treatmenta P1 P2 P1 P2 P2/P1

- - - - Relative density - - - - - - - - Percent - - - 

0N*b 38 ± 0.8c 55 ± 0.8 b 100 100 1.4

200N* 43 ± 0.8b 58 ± 0.8 b 113 105 1.3

400N* 49 ± 0.8a 67 ± 0.8 a 129 122 1.4

N = nitrogen; P1 = years 1 to 10; P2 = years 11 to 20.

a Application of 0, 200, or 400 lb N/ac. 

b Asterisks (*) indicate period means for a given fertilizer rate that differ significantly (P ≤ 0.10). 

Fertilizer means within a given period followed by a different letter differ significantly (P ≤ 0.10).
33

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

nonfertilized plots. Concurrently, mean RDs averaged 13 and 29 percent greater 

than that after the 200N and 400N treatments, respectively. Relative density differed significantly among each of the three fertilizer treatments, and there was a 

linear trend with increasing fertilizer rate. In the second period, RD increased in 

all treatments (P2/P1= 1.3 to 1.4). By the end of period 2, RD was 67 in the 400N 

plots, and this exceeded the RD of both the 0N control and the 200N treatment 

(RD of 55 and 58, respectively). The lower RD at stand age 50 years for the 200N 

treatment is probably explained by storm-caused mortality losses of some large 

trees in period 1 in plots previously fertilized with 200N (fig. 6b). The 22 percent 

greater RD in the 400N treatment was statistically significantly different from the 

control treatment and notably equaled the starting RD in the nonthinned stand 20 

years earlier (table 1).

Nonthinned stand—

In 1968 and before fertilization, nonthinned stand RD averaged 59, 70, and 71, 

respectively, for the 0N, 200N, and 400N treatments (table 1). Note that RD in 

nonfertilized plots initially averaged less than in fertilized plots, and the covariance analysis adjusted for this difference. Large starting RDs in the nonthinned 

stand forecasted subsequent loss of trees and their cumulative volume in subsequent decades (fig. 6).

Covariance analysis indicated that starting RD was a significant covariate 

and the fertilizer-by-period interaction was nonsignificant (table 21; P = 0.346). 

Real differences were observed between the two periods but not among fertilizer 

treatments. Averaged across fertilizer rates, RD was significantly greater in period 

2 than in period 1 (RD = 96 and 89, respectively). Adjusted mean RD for fertilized plots were similar to control plots in both 10-year periods. To explain the net 

effect of fertilization on RD, urea increased growth in basal area (the numerator 

in RD) (tables 18 and 19), but it also increased QMD (the denominator in RD) 

through growth and change in tree numbers, especially for trees of below-average 

d.b.h. (tables 8 and 15). 

By stand age 50 years, adjusted RDs on nonfertilized plots averaged 94 in the 

nonthinned stand, compared to 55 in the thinned stand (tables 20 and 21). For the 

200N and 400N treatments, RDs in the nonthinned stand averaged RD 100 and 94, 

respectively, compared to RD 58 and 67 in the thinned stand. 

Twenty years after 

fertilization, relative 

density of the thinned 

stand equaled the 

starting density of the 

nonthinned stand.
34

RESEARCH PAPER PNW-RP-606

The 100 Largest Trees Per Acre

Diameter at breast height—

Before fertilization at stand age 30 years, mean d.b.h. of the 100 largest trees per 

acre in the three treatments in both thinned and nonthinned stands did not differ 

(tables 22 and 23). Surprisingly, d.b.h. averaged about 1 inch smaller in the thinned 

stand than in the nonthinned, probably because some large trees were felled in the 

intensive precommercial thinning about 5 years earlier.

In the thinned stand, fertilization increased average d.b.h. of the 100 largest 

trees by 11 to 18 percent over the two decades after treatment (table 22). In both 

decades, the 400N treatment stimulated a greater increase in average d.b.h. than 

did 200N treatment. The relationship of d.b.h. to fertilizer rate was linear in both 

Table 21a—Analysis of covariance of mean relative 

density (RD) by period after fertilization in the nonthinned 

stand at Rocky Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 1.1 0.411

Rep × F (Error 1) 5 — —

Period (P) 1 77.9 <0.001

F × P 2 1.3 0.346

Linear, period 1a 1 0.3 0.621

Linear, period 2 1 0.0 0.990

Covariate (starting RD) 1 38.4 <0.001

Error 2 6 — —

Total (18-1) 17

D.f. = degrees of freedom; N = nitrogen; — = not appropriate; RD = relative density.

a Linear contrasts with P ≤ 0.05 are considered statistically significant; period 1 = 1 

to 10 years and period 2 = 11 to 20 years after fertilization.

Table 21b—Adjusted mean relative density (RD ± standard error) by period 

after fertilization in the nonthinned stand at Rocky Brook, Washington

Relative to 0N

Treatmenta P1 P2 P1 P1 P2/P1

- - - Percent - - -

0Nb 87 ± 2.7 94 ± 2.7 100 100 1.1

200N 92 ± 2.4 100 ± 2.4 106 106 1.1

400N 89 ± 2.4 94 ± 2.4 102 100 1.1

N = nitrogen; P1 = years 1 to 10; P2 = years 11 to 20.

a Application of 0, 200, or 400 lb N/ac. 

b Fertilizer-by-period means shown for information only. Main effect differences are described in the text.
35

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

decades. Regardless of treatment, d.b.h. increased about 30 percent more in the 

second decade than in the first decade. 

In the nonthinned stand, increases in average d.b.h. of the 100 largest trees 

after fertilization were less than in the thinned stand (tables 22 and 23). In contrast to the results from the thinned stand, the fertilizer-by-period interaction was 

not significant in the nonthinned stand. Instead, both period and fertilizer main 

effects were significant. Regardless of treatment, adjusted mean d.b.h. increased 

about 20 percent more in the second decade than in the first, which is a lower 

percentage and absolute increase than in the thinned stand. When averaged across 

both periods, mean d.b.h. for trees treated with 400 lb N/ac (10.8 inches) was 

Table 22a—Analysis of covariance of mean diameter at breast 

height (d.b.h.) of the 100 largest trees per acre (by d.b.h.) by period 

after fertilization in the thinned stand at Rocky Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 79.5 <0.001

Rep × F (Error 1) 5 — —

Period (P) 1 2,430.4 <.001

F × P 2 11.3 .009

Linear, period 1a 1 88.8 <.001

Linear, period 2 1 178.8 <.001

Covariate (starting d.b.h.) 1 95.8 <.001

Error 2 6 — —

Total (18-1) 17

Note: 100 TPA = 10 trees/0.1 ac plot. Of these 10 trees, the size of five to nine was measured, 

not estimated.

D.f. = degrees of freedom; N = nitrogen; — = not appropriate.

a Linear contrasts with P ≤ 0.05 are considered statistically significant; period 1 = 1 to 10 years 

and period 2 = 11 to 20 years after fertilization.

Table 22b—Adjusted mean diameter at breast height (mean d.b.h. ± standard 

error) of the 100 largest trees per acre (by d.b.h.) prior to treatment and by period 

after fertilization in the thinned stand at Rocky Brook, Washington

Relative to 0N

Treatmenta P0 P1 P2 P1 P2 P2/P1

- - - - - - - - - - Inches - - - - - - - - - - - - - Percent - - -

0N*b 6.0 a 8.1 ± 0.1 c 10.3 ± 0.1 c 100 100 1.3

200N* 5.6 a 9.0 ± 0.1 b 11.5 ± 0.1 b 111 112 1.3

400N* 5.7 a 9.5 ± 0.1 a 12.2 ± 0.1 a 117 118 1.3

N = nitrogen; P0 = prior to treatment; P1 = years 1 to 10; P2 = years 11 to 20.

a Application of 0, 200, or 400 lb N/ac.

b Asterisks (*) indicate period means for a given fertilizer rate that differ significantly (P ≤ 0.10). Fertilizer means 

within a given period followed by a different letter differ significantly (P ≤ 0.10).
36

RESEARCH PAPER PNW-RP-606

greater than the nonfertilized control trees (10.1 inches) by 7 percent, but did not 

differ from trees treated with 200 lb N/ac (10.5 inches). As in the thinned stand, 

the relation between mean d.b.h. of the 100 largest trees and N dosage was linear 

in both decades.

For the total 20-year period, mean values for these crop trees increased by 4.3 

to 6.5 inches in the thinned stand compared to 3.7 to 4.9 inches in the nonthinned 

stand. Fertilizer increased d.b.h. by 12 to 18 percent in thinned stands compared to 

3 to 6 percent in nonthinned stands.

Table 23a—Analysis of covariance of mean diameter at breast height 

(d.b.h.) of the 100 largest trees per acre (by d.b.h.) by period after 

fertilization in the nonthinned stand at Rocky Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 5.2 0.061

Rep × F (Error 1) 5 — —

Period (P) 1 2,061.3 <0.001

F × P 2 0.6 0.576

Linear, period 1a 1 8.5 0.027

Linear, period 2 1 11.0 0.016

Covariate (starting d.b.h.) 1 91.4 <0.001

Error 2 6 — —

Total (18-1) 17

Note: 100 TPA = 10 trees/0.1 ac plot. Of these 10 trees, the size of five to nine was 

measured, not estimated.

D.f. = degrees of freedom; N = nitrogen; — = not appropriate.

a Linear contrasts with P ≤ 0.05 are considered statistically significant; period 1 = 1 to 10 

years and period 2 = 11 to 20 years after fertilization.

Table 23b—Adjusted mean diameter at breast height (mean d.b.h. ± standard 

error) of the 100 largest trees per acre (by d.b.h.) by period after fertilization in 

the nonthinned stand at Rocky Brook, Washington

Relative to 0N

Treatmenta P0 P1 P2 P1 P2 P2/P1

- - - - - - - - - Inches - - - - - - - - - - - - Percent - - -

0Nb 7.3 a 9.3 ± 0.1 11.0 ± 0.1 100 100 1.2

200N 6.5 a 9.6 ± 0.1 11.4 ± 0.1 103 104 1.2

400N 7.0 a 9.8 ± 0.1 11.7 ± 0.1 105 106 1.2

N = nitrogen; P0 = prior to treatment; P1 = years 1 to 10; P2 = years 11 to 20.

a Application of 0, 200, or 400 lb N/ac. 

b Fertilizer-by-period means shown for information only. Main effect differences are described in the text.

Fertilizer increased 

the d.b.h. of the 

largest trees by 12 to 

18 percent in thinned 

stands but only 3 to 6 

percent in nonthinned 

stands.
37

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

Height—

Before fertilization in both thinned and nonthinned stands, mean height of the 100 

largest trees per acre did not differ among the pending fertilizer treatments (tables 

24 and 25). Starting height of these 100 largest trees at age 30 years averaged 7 

to 13 ft taller in the nonthinned stand than in the thinned stand, probably a consequence of somewhat poorer soil quality in the thinned stand and the removal of 

some larger trees when the stand was intensively thinned.

Table 24a—Analysis of covariance of mean height of the 100 largest 

trees per acre by period after fertilization in the thinned stand at 

Rocky Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 9.7 0.019

Rep × F (Error 1) 5 — —

Period (P) 1 2,866.0 <0.001

F × P 2 35.8 <0.001

Linear, period 1a 1 3.6 0.105

Linear, period 2 1 32.4 0.001

Covariate (starting d.b.h.) 1 56.7 <0.001

Error 2 6 — —

Total (18-1) 17

Note: 100 TPA = 10 trees/0.1 ac plot. Of these 10 trees, the size of five to nine was measured, 

not estimated.

D.f. = degrees of freedom; N = nitrogen; — = not appropriate.

a Linear contrasts with P ≤ 0.05 are considered statistically significant; period 1 = 1 to 10 years 

and period 2 = 11 to 20 years after fertilization.

Table 24b—Adjusted mean height (mean ± standard error) of the 100 largest 

trees per acre prior to treatment and by period after fertilization in the thinned 

stand at Rocky Brook, Washington

Relative to 0N

Treatmenta P0 P1 P2 P1 P2 P2/P1

- - - - - - - - - - - Feet - - - - - - - - - - - - - - Percent - - -

0N*b 36 a 53 ± 1.1 a 67 ± 1.1 b 100 100 1.3

200N* 38 a 56 ± 1.1 a 75 ± 1.1 a 106 112 1.3

400N* 35 a 56 ± 1.1 a 75 ± 1.1 a 106 112 1.3

N = nitrogen; P0 = prior to treatment; P1 = years 1 to 10; P2 = years 11 to 20.

a Application of 0, 200, or 400 lb N/ac. 

b Asterisks (*) indicate period means for a given fertilizer rate that differ significantly (P ≤ 0.10). Fertilizer means 

within a given period followed by a different letter differ significantly (P ≤ 0.10).
38

RESEARCH PAPER PNW-RP-606

The fertilizer-by-period interaction was significant for thinned stands because 

average height of the 100 largest trees did not differ among treatments in period 1 

but did differ in period 2 (table 24). Fertilized trees averaged about 12 percent taller 

than nonfertilized trees after the second decade, but there was no significant difference between the 200N and 400N dosages. Regardless of treatment, mean height in 

the second decade was about 30 percent greater than in the first decade. Change in 

mean height of crop trees in 20 years averaged 31, 37, and 40 ft, respectively, for the 

0N, 200N and 400N treatments.

 In the nonthinned stand, the fertilizer-by-period interaction was not significant; however, main effects of fertilizer and period were significant (table 25). 

Table 25a—Analysis of covariance of mean height of the 100 

largest trees per acre by period after fertilization in the 

nonthinned stand at Rocky Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 4.6 0.074

Rep × F (Error 1) 5 — —

Period (P) 1 307.4 <0.001

F × P 2 0.5 0.636

Linear, period 1a 1 9.5 0.022

Linear, period 2 1 4.9 0.069

Covariate (starting d.b.h.) 1 18.1 0.005

Error 2 6 — —

Total (18-1) 17

Note: 100 TPA = 10 trees/0.1-ac plot. Of these 10 trees, size of five to nine was measured, 

not estimated.

D.f. = degrees of freedom; N = nitrogen; — = not appropriate.

a Linear contrasts with P ≤ 0.05 are considered statistically significant; period 1 = 1 to 

10 years and period 2 = 11 to 20 years after fertilization.

Table 25b—Adjusted mean height ( mean ± standard error) of the 100 largest 

trees per acre prior to treatment and by period after fertilization in the 

nonthinned stand at Rocky Brook, Washington

Relative to 0N

Treatmenta P0 P1 P2 P1 P2 P2/P1

- - - - - - - - Feet - - - - - - - - - - - Percent - - - Ratio

0b 44 61 ± 1.4 76 ± 1.4 100 100 1.2

200 45 64 ± 1.4 79 ± 1.4 105 104 1.2

400 48 67 ± 1.4 81 ± 1.4 110 107 1.2

N = nitrogen; P0 = prior to treatment; P1 = years 1 to 10; P2 = years 11 to 20.

a Application of 0, 200, or 400 lb N/ac. 

b Fertilizer-by-period means shown for information only. Main effect differences are described in the text.
39

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

Averaged across periods, heights of the largest Douglas-fir were 69, 71, and 74 ft 

for the 0N, 200N, and 400N treatments, respectively, and height differed significantly between 0N and 400N. Averaged across fertilizer rates, height was significantly greater in the second decade (79 ft) than in the first decade (64 ft). The 

percentage increase in mean heights in the second period was slightly less than in 

the thinned stand.

At stand age 50 years, mean heights of crop trees in the thinned stand (67, 75, 

and 75 ft tall for 0N, 200N and 400N, respectively) averaged 4 to 9 ft shorter than 

those in the nonthinned stand (76, 79, and 81 ft, respectively). In the 20-year period, 

means of crop tree height increased by 31 to 40 ft in the thinned stand and by 32 to 

34 ft in the nonthinned stand. In the two decades after fertilization, mean height of 

fertilized plots in the thinned stand increased by 24 percent (mean of the 200N and 

400N treatments) more than nonfertilized. The gain over initial height from fertilization in the nonthinned stand averaged only 5 percent, in part because starting 

height was greater. 

Height to live crown—

Prior to fertilization in both the thinned and nonthinned stands, the mean height 

to live crown (HLC) did not differ among the three N fertilizer treatments, and 

ranged from 8 to 11 ft in the thinned stand (table 26) and 12 to 13 ft in the nonthinned stand (table 27) (P = 0.281 and 0.844, respectively; ANOVA output not 

shown). Thus, crown base averaged 2 to 4 ft higher from the ground in the more 

densely stocked, nonthinned stand. 

In the thinned stand, average HLC of the 100 largest trees elevated nearly 

three-fold more rapidly in the second decade than in the first (table 26). Specifically, 

mean crown height elevated 4 to 6 ft among all treatments in period 1 and 23 to 

26 ft in period 2. Although the fertilizer-by-period interaction was significant (P = 

0.031), fertilization had no detectable effect on mean HLC. 

Ten years after fertilization of the nonthinned stand, height to live crown base 

of the 100 largest trees per acre already averaged 10 to 16 ft higher than those in the 

thinned stand (table 27 vs. table 26). Crowns receded faster in the second decade 

than the first, but not as rapidly as in the thinned stand. The P2/P1 ratio in the nonthinned stand was 1.6 to 2.0 compared to 2.7 to 2.9 in the thinned stand (table 27 

vs. table 26). Covariance analysis for the nonthinned stand indicated a marginally 

significant fertilizer-by-period interaction (P = 0.098), probably because fertilization increased the rate of crown recession in the second decade, particularly in the 

200N treatment. However, as was found for the thinned stand, multiple comparisons of HLC in the nonthinned stand failed to detect a significant effect attributable 

to fertilization.

In both thinned 

and nonthinned 

stands, fertilization 

increased the height 

of the largest trees.
40

RESEARCH PAPER PNW-RP-606

At stand age 50 years, height to live crown in the nonthinned stand averaged 

43 ft for 0N and 49 ft for both the 200N and 400N treatments compared to 35 ft for 

0N and 40 ft for both the 200N and 400N treatments in the thinned stand. In the 

20-year period after fertilization, means of crop tree HLC increased by 31 to 37 ft 

in the nonthinned and by 27 to 32 ft in the thinned stand. This faster recession of 

crowns in the nonthinned stand is expected and consistent with denser stocking of 

trees and greater RD. 

Table 26a—Analysis of covariance of mean height to live crown 

(HLC) of the 100 largest trees per acre by period after 

fertilization in the thinned stand at Rocky Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 2.1 0.219

Rep × F (Error 1) 5 — —

Period (P) 1 2,963.5 <0.001

F × P 2 6.6 0.031

Linear, period 1a 1 1.1 0.340

Linear, period 2 1 4.6 0.075

Covariate (starting HLC) 1 7.0 0.038

Error 2 6 — —

Total (18-1) 17

Note: 100 TPA = 10 trees/0.1 ac plot. Of these 10 trees, size of five to nine was measured, 

not estimated.

D.f. = degrees of freedom; N = nitrogen; — = not appropriate.

a Linear contrasts with P ≤ 0.05 are considered statistically significant; period 1 = 1 to 10 

years and period 2 = 11 to 20 years after fertilization.

Table 26b—Adjusted mean height to live crown (mean HLC ± standard error) of 

the 100 largest trees per acre prior to treatment and by period after fertilization 

in the thinned stand at Rocky Brook, Washington

Relative to 0N

Treatmenta P0 P1 P2 P1 P2 P2/P1

- - - - - - - - - - Feet - - - - - - - - - - - - - Percent - - -

0N*b 8 a 12 ± 1.3 a 35 ± 1.9 a 100 100 2.9

200N* 11 a 15 ± 1.3 a 40 ± 1.9 a 125 114 2.7

400N* 8 a 14 ± 1.3 a 40 ± 1.9 a 117 114 2.9

N = nitrogen; P0 = prior to treatment; P1 = years 1 to 10; P2 = years 11 to 20.

a Application of 0, 200, or 400 lb N/ac. 

b Asterisks (*) indicate period means for a given fertilizer rate that differ significantly (P ≤ 0.10). Fertilizer means 

within a given period followed by a different letter differ significantly (P ≤ 0.10).
41

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

Live crown ratio—

Change in live crown ratio (LCR) expresses the net result of differential increases in 

tree height and HLC. Before fertilization in the thinned stand, mean LCR was 0.77, 

0.71, and 0.77 for the 0N, 200N, and 400N treatments, respectively (table 28). An 

ANOVA of pre-fertilization values of LCR suggested that potential differences existed 

among treatments (P = 0.080; ANOVA output not shown); however, multiple comparisons failed to detect them. After period 1, mean ratios ranged from 0.74 to 0.76 but 

declined to 0.45 to 0.49 by stand age 50 years (table 28). Only the main effect of period 

was statistically significant (P < 0.001); LCR declined much more rapidly in the second 

decade than in the first. Fertilization had no real effect on LCR in either decade. 

Table 27a—Analysis of covariance of mean height to live crown 

(HLC) of the 100 largest trees per acre by period after 

fertilization in the nonthinned stand at Rocky Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 6.6 0.039

Rep × F (Error 1) 5 — —

Period (P) 1 354.7 <0.001

F × P 2 3.5 0.098

Linear, period 1a 1 4.1 0.089

Linear, period 2 1 11.2 0.016

Covariate (starting HLC) 1 1.9 0.221

Error 2 6 — —

Total (18-1) 17

Note: 100 TPA = 10 trees/0.1 ac plot. Of these 10 trees, size of five to nine was measured, 

not estimated.

D.f. = degrees of freedom; N = nitrogen; — = not appropriate.

a Linear contrasts with P ≤ 0.05 are considered statistically significant; period 1 = 1 to 10 

years and period 2 = 11 to 20 years after fertilization.

Table 27b—Adjusted mean height to live crown (mean HLC ± standard error) of 

the 100 largest trees per acre prior to treatment and by period after fertilization 

in the nonthinned stand at Rocky Brook, Washington

Relative to 0N

Treatmenta P0 P1 P2 P1 P2 P2/P1

- - - - - - - - - Feet - - - - - - - - - - - - Percent - - -

0N*b 12 a 26 ± 1.5 a 43 ± 1.2 a 100 100 1.7

200N* 13 a 25 ± 1.5 a 49 ± 1.2 a 96 114 2.0

400N* 12 a 30 ± 1.5 a 49 ± 1.2 a 115 114 1.6

N = nitrogen; P0 = prior to treatment; P1 = years 1 to 10; P2 = years 11 to 20.

a Application of 0, 200, or 400 lb N/ac. 

b Asterisks (*) indicate period means for a given fertilizer rate that differ significantly (P ≤ 0.10). Fertilizer 

means within a given period followed by a different letter differ significantly (P ≤ 0.10).
42

RESEARCH PAPER PNW-RP-606

In the nonthinned stand before fertilization, mean LCR did not differ among 

the three treatments, which ranged from 0.71 to 0.74 (P = 0.835; ANOVA output not 

shown). After period 1, mean LCR ranged for 0.56 to 0.61 but averaged only 0.37 to 

0.42 after period 2 (table 29). Again, fertilization had no real effect in either decade. 

The marginally significant fertilizer-by-period interaction is likely explained by the 

trend of mean LCR for the 200N treatment, which had the largest LCR in period 1 

and the smallest in period 2. 

At stand age 50 years, means of LCR ranged from 0.45 to 0.49 in the thinned 

stand and from 0.37 to 0.42 in the nonthinned stand (fig. 8). The larger LCR in the 

Table 28a—Analysis of covariance of mean live crown ratio (LCR)

 of the 100 largest trees per acre by period after fertilization in the 

thinned stand at Rocky Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 0.4 0.681

Rep × F (Error 1) 5 — —

Period (P) 1 1,374.3 <0.001

F × P 2 1.8 0.245

Linear, period 1a 1 .6 0.460

Linear, period 2 1 .5 0.507

Covariate (starting LCR) 1 5.1 0.066

Error 2 6 — —

Total (18-1) 17

Note: 100 TPA = 10 trees/0.1 ac plot. Of these 10 trees, size of five to nine was measured, 

not estimated.

D.f. = degrees of freedom; N = nitrogen; — = not appropriate.

a Linear contrasts with P ≤ 0.05 are considered statistically significant; period 1 = 1 to 10 

years and period 2 = 11 to 20 years after fertilization.

Table 28b—Adjusted mean live crown ratio (mean LCR ± standard error) of the 

100 largest trees per acre prior to treatment and by period after fertilization in 

the thinned stand at Rocky Brook, Washington

Relative to 0N

Treatmenta P0 P1 P2 P1 P2 P2/P1

- - - - - - - - - - Ratio - - - - - - - - - - - - - Percent - - -

0Nb 0.77 a 0.76 ± 0.02 0.47 ± 0.02 100 100 0.6

200N 0.71 a 0.75 ± 0.02 0.49 ± 0.02 99 104 0.7

400N 0.76 a 0.74 ± 0.02 0.45 ± 0.02 97 96 0.6

N = nitrogen; P0 = prior to treatment; P1 = years 1 to 10; P2 = years 11 to 20.

a Application of 0, 200, or 400 lb N/ac. 

b Fertilizer-by-period means shown for information only. Main effect differences are described in the text.

Fertilization had no 

effect on live crown 

ratio.
43

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

thinned stand likely resulted from the combination of greater height growth and 

slower upward recession of the crown (tables 24 and 26 vs. tables 25 and 27). Larger 

LCR in the less densely stocked thinned stand is likely to support faster growth of 

crop trees in the future. 

Sample trees used for the 100 largest trees per acre changed somewhat between 

stand age 30 and 50 years because some of the original trees were damaged and 

replaced by other trees. Average values for total height, height to live crown, and 

live crown ratio also changed (fig. 8). Initial variation among means of fertilizer 

treatments at age 30 is evident, especially in the thinned stand. 

Table 29a—Analysis of covariance of mean live crown ratio (LCR) 

of the 100 largest trees per acre by period after fertilization in 

the nonthinned stand at Rocky Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 0.4 0.699

Rep × F (Error 1) 5 — —

Period (P) 1 195.1 <.001

F × P 2 3.5 .097

Linear, period 1a 1 .4 .558

Linear, period 2 1 .6 .460

Covariate (starting LCR) 1 3.1 .127

Error 2 6 — —

Total (18-1) 17

Note: 100 TPA = 10 trees/0.1 ac plot. Of these 10 trees, size of five to nine was measured, 

not estimated.

D.f. = degrees of freedom; N = nitrogen; — = not appropriate.

a Linear contrasts with P ≤ 0.05 are considered statistically significant; period 1 = 1 to 10 

years and period 2 = 11 to 20 years after fertilization.

Table 29b—Adjusted mean live crown ratio (mean LCR ± standard error) of the 

100 largest trees per acre prior to treatment and by period after fertilization in 

the nonthinned stand at Rocky Brook, Washington

Relative to 0N

Treatmenta P0 P1 P2 P1 P2 P2/P1

- - - - - - - Ratio - - - - - - - - - - Percent - - - Ratio

0N*b 0.72 a 0.58 ± 0.02 a 0.42 ± 0.02 a 100 100 0.7

200N* 0.71 a 0.61 ± 0.02 a 0.37 ± 0.02 a 105 88 0.6

400N* 0.74 a 0.56 ± 0.02 a 0.40 ± 0.02 a 97 95 0.7

N = nitrogen; P0 = prior to treatment; P1 = years 1 to 10; P2 = years 11 to 20.

a Application of 0, 200, or 400 lb N/ac. 

b Asterisks (*) indicate period means for a given fertilizer rate that differ significantly (P ≤ 0.10). Fertilizer 

means within a given period followed by a different letter differ significantly (P ≤ 0.10).
44

RESEARCH PAPER PNW-RP-606

90

80

70

60

50

40

30

20

10

0

0 200 400

Mean height (ft)

Age 30 Age 40 Age 50

Nitrogen fertilizer rate (lb N/ac)

0 200 400 0 200 400

HLC

Crown length

0.77 0.71 0.77

0.76 0.75 0.74

0.47

0.49 0.45

90

80

70

60

50

40

30

20

10

0

0 200 400

Mean height (ft)

Age 30 Age 40 Age 50

Nitrogen fertilizer rate (lb N/ac)

0 200 400 0 200 400

HLC

Crown length

0.73 0.71

0.75

0.58 0.61

0.56

0.42 0.37 0.40

A

B

Figure 8—Average total height (top of bars), height to live crown (HLC), and live crown ratio (numbers on top of bars) of the 100 largest trees per acre 20 years after fertilization, by fertilizer treatment 

in (A) thinned stands and (B) nonthinned stands, Rocky Brook, Washington. Crown length is the 

difference between total height and HLC.

Both crown length 

and live crown ratio 

declined during the 

20-year period of 

measurement.
45

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

Discussion

Initial Variability in Stand and Site Characteristics

Our 0.1-acre measurement plots sampled a coast Douglas-fir plantation initially 

established in 1937-1938. Although originally planted at an average density of about 

436 TPA, density in the nine nonthinned plots 30 years later ranged from 980 to 

1,700 TPA. Percentage of Douglas-fir by basal area in these nonthinned plots ranged 

from 67 to 87 percent (table 1). Because of volunteer western hemlock and western 

redcedar, this Rocky Brook plantation was not a monoculture. Similarly, in the Snow 

Creek plantation of Douglas-fir that was successfully established in 1927–1928 on 

the nearby former Quilcene District, natural regeneration of hemlock averaged 28 

percent of the total number of trees at age 60 years (Miller and Anderson 1995).

Site quality based on estimated mean 50-year site index (SI) varied greatly (66 

to 94 ft; table 1) among the 18 plots. Some differences in site index could be related 

to a variation in the thickness or depth of soil above an indurated, fine-textured 

glacial deposit that underlies most of our study area. Tree roots cannot penetrate 

this hard lake sediment or till, and on-site and upslope sources of groundwater 

accumulate seasonally on this compact layer. For example, after snow melt in early 

May 1975, LOGS study plot 26 (located next to one of our 0N control plots) had 

standing water on the soil surface, and that plot was the only one containing 

devilsclub (Oplopanax horridus [(Sm.] Miq.).4 In addition to site conditions related 

to soil depth and topographic positions that accumulate or shed subsurface flow, 

another source of variation in site quality relates to the proportion and size of 

gravel- through boulder-sized material. We did not, however, attempt to quantify 

relations among these soil factors and site index.

Additionally, we suspect that some among-plot variation in mean 50-year SI 

arises from the selection of trees measured for tree height and breast-height age, and 

then used to estimate SI. As a substitute for the sample selection specified by King 

(1966) for estimating SI, we used mean height of the 40 largest trees per acre by 

d.b.h. (H40). According to Curtis and Clendenen (1994), H40 is commonly used as 

the basis for site index estimates in the region. For our 0.1-acre measurement plots, 

H40 corresponded to the average height of the four largest d.b.h. trees per plot. Surprisingly, our random sample of 20 tree heights per plot, of which two-thirds were 

restricted to being larger than the mean diameter, seldom included all four of the 

largest trees, nor did our 91-tree sample of breast-height ages. To some extent, these 

sampling deficiencies for estimating H40 were mitigated by estimating missing 

4 Williamson, R.L. 1975. Field work report. On file with: Forestry Sciences Laboratory, 

3625 93rd Avenue SW, Olympia, WA 98512.
46

RESEARCH PAPER PNW-RP-606

heights from height-d.b.h. curves for individual plots and by substituting the overall 

mean breast-height age of 20 years for missing ages. Nonetheless, we surmise that 

these sampling deficiencies contributed to the wide range of values estimated for 

H40 and SI among our study plots.

Initial SI in the thinned stand averaged about 14 ft less than in the nonthinned 

stand (73 vs. 87 ft) (table 1). Differences in soil attributes and tree sampling may 

explain some of this variation, although the severe thinning 5 years earlier probably removed some large trees in attempts to create uniform spacing. We estimate 

that about 75 percent of the original trees were felled in the belated precommercial 

thinning at plantation age 25 years. The unexpected smaller mean d.b.h. of the 100 

largest trees per acre in the thinned stand compared to the nonthinned stand also 

suggests that some large trees were felled which were potentially suitable for site 

index estimation. Moreover, reduced height growth by residual trees for several 

years after intensive thinning was previously reported at this Rocky Brook location 

(Williamson 1976) and at other low-quality sites (DeBell et al. 2002, Harrington 

and Reukema 1983).

Growth and Mortality

Individual trees in well-stocked stands compete for light, soil moisture, and 

nutrients. On N-deficient sites such as our Rocky Brook study site, N fertilizer 

enhances growth of some trees more than others (Miller et al. 1981). Larger trees, 

which already have larger crowns and rooting volumes, are more likely to capture 

and respond to added N. Fertilizer-enhanced growth of these larger trees further 

increases their competitive advantage over smaller trees. Consequently, loss of 

smaller trees can be accelerated by N fertilization as occurred in our nonthinned 

stand, but only in the first decade (table 16). The ultimate effect of fertilization 

on tree stocking in our thinned or nonthinned stands is uncertain. The net of tree 

losses and additions to an arbitrary minimum d.b.h. threshold, like our 1.6 inch 

minimum, will change over time. In this general area of the Olympic Peninsula, 

numbers and species composition of understory trees shift to shade-tolerant western hemlock and western redcedar, which are more likely to survive under shade 

than Douglas-fir. In our study area and the neighboring LOGS study area (Curtis 

and Marshall 2009), thinned stands gradually develop a two-layer structure with a 

Douglas-fir overstory and an understory of younger, mostly shade-tolerant species. 

Typically, seed crops of western hemlock are frequent and heavy, which produce 

recurring pulses of volunteer trees, some of which eventually attain diameters for 

recruiting as ingrowth. For example, our 20-year inventory of the nonthinned stand 

documented a decrease in the percentage composition of Douglas-fir as more trees 
47

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

of other species were included in our inventory. This change in species composition in our study area was evident in both tree numbers and as a percentage of total 

basal area. 

We anticipated that applying urea fertilizer to this N-deficient site would enhance 

native site quality and tree growth. We used rates of gross volume PAI of trees 1.6 

inches d.b.h. and larger as our biological measure of response to fertilization. We 

anticipated that gain in stem volume (CVTS) from fertilizing a recently thinned 

portion of this plantation could differ from gain in the nonthinned portion. Although 

fertilization would have enhanced content of soil N in both thinned and nonthinned 

portions, we expected starting differences in stocking between the thinned and 

nonthinned stand would affect subsequent volume growth per acre and volume of 

mortality. Because gross growth per acre is related positively to initial growing 

stock, we expected and documented less gross growth per acre in the first decade in 

the thinned stand. Conversely, volume lost to mortality was less in the thinned stand. 

Of particular relevance are results from a thinning and fertilizer study at a 

poor-quality site near Shawnigan Lake, B.C. This study was installed in 1971–1972 

as a 3-by-3 factorial design in a 24-year-old Douglas-fir plantation. The most severe 

thinning treatment tested removed two-thirds of starting basal area, which is 

similar to the estimated 75 percent removal at our Rocky Brook location. Moreover, 

urea fertilizer was applied at corresponding dosages of 200 and 400 lb N/ac. Results 

24 years after fertilization were reported by McWilliams and Therien (1996). The 

32-year results extend trends reported for the 24-year period (Omule et al. 2011). 

In general, results and conclusions from the Shawnigan Lake study and our Rocky 

Brook study were similar and will be discussed later.

Because thinning treatments at Rocky Brook had restricted randomization, 

statistical tests to compare effects of urea-N in thinned and nonthinned portions of 

this plantation were not justified. Instead, we could only conduct informal contrasts 

of growth responses in these two stands. 

Gross growth—

Application of urea fertilizer increased volume growth in the first year in both 

thinned and nonthinned stands (fig. 4). We were unable to detect any clear relation 

between gross PAI and growing season precipitation as estimated from on-site measurements in the first decade and supplemented with measurements at the Quilcene 

Ranger Station about 10 miles away. In the first decade after fertilization of the 

under-stocked thinned stand (mean RD = 19), gross volume growth (gross PAI) of 

nonfertilized plots averaged about half that in the nonthinned stand, and growth 

increased by 45 and 72 percent compared to the nonfertilized control after the 200N 

or 400N treatments, respectively (tables 6 and 13). Both control and fertilized plots 
48

RESEARCH PAPER PNW-RP-606

in the thinned stand accelerated PAI by 60 percent in the second decade compared 

to their respective PAIs in the first decade, presumably because crown size and 

photosynthetic efficiency increased in response to available growing space and 

added N. Relative to the nonfertilized control, response to N in the second decade 

averaged about the same as in the first decade. Despite accelerated losses in tree 

numbers and CVTS in the second decade, ending RD in the thinned stand at age 50 

years averaged 55, 58, and 67, respectively, for the 0N, 200N, and 400N treatments 

(table 20). This recovery to nearly full stocking was attained about 25 years after 

the intense precommercial thinning at stand age 25. In contrast, the nonthinned 

stand had already attained RD of 67 by age 30 years.

In the first decade after application of 200 and 400 lb N/ac in the nonthinned 

stand, we estimated 43 and 48 percent increases, respectively, in gross volume 

growth (table 13). In the second decade, gross volume PAI slowed on fertilized 

plots but increased by 10 percent on nonfertilized plots. This decline in volume 

growth was especially noticeable after fertilization with 400 lb N/ac where growth 

averaged 20 percent less than in the first decade. We ascribe this growth reduction 

on fertilized plots in the second decade to progressive over-stocking and greater 

mortality losses. 

At the end of the second decade, RD averaged 94, 100, and 94, respectively, 

for the 0N, 200N, and 400N treatments in the nonthinned stand (table 21). Such 

large values of RD probably resulted from a mixture of shade-tolerant species and 

the inclusion of small diameter, ingrowth trees when calculating RD (Curtis 2010). 

Small trees reduce QMD, the divisor in the RD calculation, because they contribute 

little to basal area per acre but add fully to TPA in calculation of QMD. Had we 

calculated RD using the summation method (Curtis 2010), our RD values likely 

would have been lower.

In the second decade of the study, gross PAI was similar in the thinned and 

nonthinned stands (fig. 7), but greater volume was lost to mortality in the nonthinned stand. In the 20 years after fertilization in both thinned and nonthinned 

stands, urea fertilizer clearly increased net volume growth and live-stand volume. 

Mortality volume— 

In the thinned stand, mortality losses were much less than in the nonthinned stand, 

probably because starting RDs in the nine plots were only 18 to 20. Consequently, 

fewer and smaller trees died, total volume of mortality was small, and net gains 

in live-stand volumes were similar to those in gross PAI. Also the thinned stand, 

volume lost to mortality was minimal in years 1-10 but increased substantially in 

Gross volume growth 

in thinned stands was 

60 percent greater in 

the second decade 

than in the first.
49

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

years 11-20, especially on fertilized plots (table 7). This can be explained in part by 

a more rapid increase in RD and the larger size of dead trees.

Starting RDs in the nonthinned stand ranged from 56 to 81 among the nine 

plots (table 1). Cubic volume of subsequent mortality was weakly related to starting 

RD (fig. 6). Although mortality losses in stem volume accelerated about threefold in the second decade, fertilization had no real effect in either decade (table 

14). Nonetheless, relationships shown in figure 6 and the consistently larger mean 

values suggest that fertilization did increase mortality losses in stem volume. For 

all treatments and both periods, average volume per dead tree was much less than 

average volume of surviving tress. In general, smaller than average trees died in 

both nonthinned and thinned stands. 

Depending on bole size, dead trees that are physically sound can be salvaged 

when live trees are harvested. In the second decade after fertilization in the 

nonthinned stand, volume per dead tree averaged 0.90, 1.38, and 1.76 ft3 in the 

0N, 200N, and 400N treatments, respectively (table 16). For corresponding treatments in the thinned stand, volume per dead tree averaged 0.27, 3.94, and 0.56 ft3 

(table 9). Surprisingly, dead trees in the thinned stand averaged less volume than 

nonthinned stands, except for a few large trees that were wind-thrown on two 

200N plots. 

Net yield—

Urea fertilization increased live-stand volume in both thinned and nonthinned 

stands. In the thinned stand, net increase in live CVTS was greater in period 2 

than period 1 (table 4). This is expected because initial stocking at age 30 years 

was less than a RD of 20 as a result of intensive thinning about 5 years earlier. 

In contrast, the nonthinned stand at the same age had an average RD of 67, a 

stocking only attained by the thinned stand after fertilization and two decades of 

growth. At Shawnigan Lake, stand volume at 24 years after a similarly intensive 

thinning also remained less than nonthinned plots, but the gap narrowed over 

time. By 32 years after treatment, live-stand volume in thinned plots that were 

not fertilized remained less than that in the nonthinned stand, but average stand 

volume was greater on plots fertilized with 200N (McWilliams and Therien 1996, 

Omule et al. 2011). 

Doubling N dosage from 200 to 400 lb N/ac resulted in about 20 percent 

more net volume gain in the thinned stand but failed to provide additional gain 

in the nonthinned stand. In the thinned stand, net gains in live-stand volume 20 

years after fertilization averaged about 1,380 ft3/ac (40-percent gain) after 200 lb 
50

RESEARCH PAPER PNW-RP-606

N/ac and about 2,040 ft3/ac (59-percent gain) after 400 lb N/ac (table 4). Despite 

a greater loss of volume in mortality in the nonthinned stand, application of 200 

lb N/ac resulted in average gains of about 1,870 ft3/ac (31 percent) versus about 

1,820 ft3/ac (30 percent) after 400 lb N/ac (table 11). In both thinned and nonthinned portions of this plantation, a single application of urea fertilizer greatly 

increased stand productivity. Similarly at Shawnigan Lake, fertilization increased 

stand total and merchantable volume in both thinned and nonthinned stands 

(McWilliams and Therien 1996, Omule et al. 2011). Although the 400N treatment 

resulted in more volume production than the 200N treatment, the lower dosage 

was more efficient for the nonthinned stand in terms of gain in CVTS per pound 

of N (table 12). 

Large absolute and especially large percentage increases in stand volume 

growth have been reported after N fertilization of Douglas-fir on poor-quality sites 

in western Washington (Miller and Tarrant 1983, Miller et al. 1986) and at Shawnigan Lake in British Columbia (McWilliams and Therien 1996, Omule et al. 2011). 

Growth-projection models that synthesize results at numerous locations sampling 

a wide range of site qualities also estimate larger gains in total bole volume after 

fertilizing lower versus higher quality sites (Hann 2011). 

Assuming a harvest of all live trees at age 50, additional yield in CVTS 20 

years after application of 200 lb N/ac is about 500 ft3/ac less in the thinned stand 

than in the nonthinned stand (tables 4 and 11). Note that percentage gains in yield 

over nonfertilized growth are similar for the thinned and nonthinned stands, at 

30 and 40 percent, respectively. We observe that both nonfertilized growth and 

response to fertilization were related positively to initial stocking (live-stand 

volume and RD), which was substantially less in the thinned stand.

Unadjusted net gains in merchantable volume—

Most importantly for wood utilization and for creating habitat with large trees, 

observed gains in merchantable stand volume (CV4) 20 years after fertilization 

averaged nearly 70 percent greater for the 200N treatment than for the 0N treatment 

in both thinned and nonthinned stands (table 30). Gains over nonfertilized plots 

after the 400N treatment were about 95 percent in both stands. Thus, doubling the 

dosage did not double the gain in CV4. Larger gains in CV4 than CVTS have at 

least two explanations: (1) biologically, more rapid diameter growth and ingrowth 

into the 5.6 in and larger diameter classes, and (2) arithmetically, more net loss of 

small-diameter trees in fertilized plots in both the thinned (table 9) and nonthinned 

stand (table 16). Although loss of small trees reduces CVTS volume, such losses 

have little or no effect on CV4. 
51

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

Growth and Response to Fertilizer Depend on Stand Stocking

Although we could not statistically compare gains from fertilizing thinned vs. 

nonthinned portions of the plantation, we review and comment on this important 

and practical topic. Results from the nearby Rocky Brook installation of the LOGS 

study are particularly relevant (Curtis and Marshall 2009). Volume production in 

thinned plots in the LOGS study increased with growing stock, expressed as basal 

area. Results at this poor-quality site are qualitatively consistent with those from 

other LOGS study installations, although growth has been much slower than for 

installations on more productive sites. Timing of the initial calibration thinning in 

1964 corresponds to the thinning of our study area in 1963. This calibration thinning was intended to create comparable stand densities among the LOGS study 

plots. All trees less than one-half the initial stand quadratic mean diameter were 

cut. This left 400 trees per acre (average spacing 10.4 ft) compared to a mean of 351 

trees per acre after thinning in our study. In both studies, all trees 1.6 inches d.b.h. 

and larger were tagged and measured. Subsequent thinning treatments in the LOGS 

study differed in the amount of basal area allowed to accumulate in growing stock. 

Average residual basal area of thinned plots after the calibration thinning was the 

starting point for calculating future growing-stock accumulation. Thereafter, the 

basal area retained in any thinning was that retained in the previous thinning plus 

a predetermined percentage of the gross increase found in the nonthinned plots 

since the previous thinning. Retained growth was 10, 30, 50, and 70 percent of 

nonthinned growth. Gross growth of nonthinned plots was assumed to represent the 

productive potential of the site at full stocking. 

Trends in live-stand basal area over time for both studies illustrate the consequences severe thinning at about age 30 years has on subsequent growth (fig. 9). 

Nonthinned stands in both studies had very similar basal area development, with a 

gradually decreasing rate of growth over time. Plots in our study lagged somewhat 

behind those of the LOGS study, in part due to lower initial stocking. Thinned 

stands in both studies had a similar starting point. Repeated thinnings in the LOGS 

study resulted in somewhat linear development of basal area, with the steepness 

of slopes increasing with the amount of retained growth. Without any additional 

thinnings, basal area development in our study demonstrated a gradually increasing 

rate of growth. Despite the growth acceleration, final trends suggested that thinned 

plots in our study would need an additional 10 to 20 years to achieve the basal area 

found in nonthinned plots. 

Overall, the various LOGS study installations have clearly demonstrated 

that in these relatively young stands, volume increment is strongly related to 

growing-stock level. High volume increment requires at least moderately high 

In these relatively 

young stands, volume 

increment is strongly 

related to growingstock level.
52

RESEARCH PAPER PNW-RP-606

growing stock. Results are similar at the two other LOGS study installations 

located on poor-quality sites (Beddows 2002).

For younger plantations and on higher quality sites than Rocky Brook, Li 

et al. (2007) reported effects of stand density management and fertilization on 

subsequent growth of 6- to 13-year-old Douglas-fir plantations at nine locations 

in western Washington and Oregon. Site quality at eight of these locations was 

Site Class II or better; only one was Site Class IV. Before precommercial thinning 

average density, calculated as the initial stems per acre (ISPA), among the nine 

plantations ranged from 289 to 690 TPA. Three density treatments were imposed 

at each location: (1) retain all trees (ISPA); (2) retain 50 percent of ISPA (ISPA/2); 

and (3) retain 25 percent of ISPA (ISPA/4). The RD of these three initial density 

treatments averaged 14, 7, and 4, respectively; therefore they were clearly understocked compared to a RD of 65 to 70, the density at which full stocking occurs 

and density-related mortality begins to occur for Douglas-fir. In the subsequent 12 

300

250

200

150

100

50

0

Stand basal area (ft2/acre)

1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990

Year

LOGS, nonthinned

LOGS, 70% BA

LOGS, 50% BA

LOGS, 30% BA

LOGS, 10% BA

Rocky Brook, NT-0N

Rocky Brook, T-0N

Figure 9—Comparison of stand basal area development in the LOGS study and in unfertilized treatments of the current study in Rocky 

Brook, Washington. Legend indicates the percentage of gross basal area (BA) growth retained after each thinning treatment of the LOGS 

study relative to the nonthinned treatment (NT = nonthinned, T = thinned, and 0N = nonfertilized).
53

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

years, two plots at each location in the ISPA density treatments were repeatedly 

thinned to a RD of 35 to 40 whenever RD reached 55 to 60. Plots in ISPA/2 were 

thinned once when a RD of 55 was attained. Plots in ISPA/4 required no thinning 

in the first 12 years because average RD did not exceed 30 and plots remained 

understocked. At each location and for each of the three density levels, one of the 

two 0.5-acre measurement plots (each within 1 acre treatment areas) was initially 

fertilized with 200 lb N/ac as urea and was fertilized again after years 4 and 8 for a 

cumulative total of 600 lb N/ac in 12 years.

As reported by Li et al. (2007), volume growth per acre was more strongly 

affected by stand density treatments than by fertilization. Thinning reduced peracre growth for 4 or more years. Averaged across the three densities, fertilization 

increased volume growth by about 12, 14, and 7 percent, respectively, in the first, 

second, and third 4-year growth periods. Although statistical significance of the 

interaction between density and fertilization was not reported, trends of volume 

PAI are similar to those in our study: less response from fertilizer in understocked 

stands (Li et al. 2007) (fig. 5). Denser stands (lightly thinned) responded more 

quickly to the first fertilization than did the less dense stands. However, response in 

the denser stands decreased over time, while the response in the initially less dense 

stands increased.

Li et al. (2007) suggest that reducing initial spacing to 50 percent or 25 percent of starting TPA is relatively intense, and growth in such spaced stands had 

not recovered from the spacing treatment during the first 12 years. Our results 

were similar at Rocky Brook, where delayed thinning at age 25 probably retained 

about 25 percent of starting TPA, and where 5 years after thinning RD averaged 

only 19. Our conclusions are similar: sufficient TPA or RD must be present to 

occupy a site to exploit fully its native and amended nutrients and water. Moreover, both before and after a site is fully occupied by trees, nutrient shortages 

can develop and become apparent when the site demonstrates a positive response 

to fertilization.

Changes in Basal Area and Species Composition

The operational, precommercial thinning of our stands left about 25 percent of 

the original basal area per acre and favored the retention of the originally planted 

Douglas-fir. At stand age 30 years and 5 years after thinning, basal area per acre in 

the thinned stand averaged less than half that in the nonthinned stand. Of this total, 

Douglas-fir averaged 90 percent by basal area among the nine plots in the thinned 

stand and averaged 76 percent in the nonthinned stand, presumably because other 

species were not removed (table 1). 

Moderate levels of 

growing stock are 

required if stands are 

to respond robustly to 

fertilization.
54

RESEARCH PAPER PNW-RP-606

Thinned stand—

Basal area of Douglas-fir and of all species combined increased in both periods, 

especially in the second decade and after the 400N treatment (table 18). At stand 

age 50 years, the percentage Douglas-fir averaged 84, 89, and 91 percent, respectively, for the 0N, 200N, and 400N treatments. Basal area of hemlock averaged less 

than 15 percent of total basal area and showed no relation to N treatments.

Nonthinned stand—

Basal area of Douglas-fir and of all species combined accelerated less rapidly in the 

nonthinned stand (table 19). Unlike results in the thinned stand, the 400N treatment 

was not more effective than the 200N for increasing basal area growth. Moreover, 

percentage of total basal area in Douglas-fir in the second decade decreased by 2 

percent in 0N plots and stayed unchanged for both fertilization treatments. At stand 

age 50 years, the percentage of Douglas-fir by basal area averaged 75, 77, and 79, 

respectively, for the 0N, 200N, and 400N treatments. Hemlock basal area averaged 

20 to 22 percent of total basal area and showed no relation to N treatment.

Relative Density

Relative density (Curtis 1982) is an index of stand density that is commonly used 

to schedule and prescribe intensity of thinning to manage density in Douglas-fir 

stands. Changes in RD over time reflect changes in stand basal area, TPA, and their 

integration to QMD. Therefore, our fertilizer treatments that accelerated basal area 

growth arithmetically increased RD. However, this potential increase in RD could 

be offset by a greater net loss of smaller than average d.b.h. trees that occurred in 

our thinned stand and especially in the nonthinned stand with 400N application. 

Such losses of small trees would arithmetically increase QMD and lower RD. 

Recall that we documented greater losses of small trees in the first decade after fertilization in the nonthinned stand and in the second decade in the thinned stand. In 

short, large increases in RD in the second decade in both thinned and nonthinned 

stands resulted from a combination of accelerated basal area growth and loss of 

small trees as influenced by thinning and N fertilizer. 

Changes in the 100 Largest Trees per Acre

Our assessment was based on changes in adjusted mean values, by decade. Mean 

values represented the 100 largest d.b.h. trees per acre at stand ages 30, 40, and 50 

years, but these means seldom represented the same trees in each decade. In short, 

changes in these means over time integrated both tree growth and some changes in 

sample trees.
55

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

In the thinned stand, changes in both mean d.b.h. and height of the 100 largest TPA (i.e., crop trees) were accelerated by N additions, especially in the second 

decade after fertilization. After the application of 200 lb N/ac, mean d.b.h. was 

about 12 percent greater than that for the 0N treatment in both decades; after the 

application of 400 lb N/ac means averaged about 18 percent more than that for the 

0N treatment (table 22). Fertilizer affected diameter growth more than height growth 

in both decades after fertilization, with both dosages causing a similar gain in height 

over nonfertilized plots of 6 percent in the first decade and 12 percent in the second 

decade (table 24). Increases in tree height were concurrent with increases in height to 

live crown, as lower branches died and live crown receded upward. Fertilizer had no 

detectable effect on HLC or LCR. At stand age 50 years and two decades after fertilization, means of LCR in the thinned stand were 0.47, 0.49, and 0.45 for 0N, 200N, 

and 400N, respectively (table 28). We infer that N fertilization had no real effect on 

LCR and that adequate crown was present for sustained growth in the next decade.

In the nonthinned stand, absolute and percentage changes in mean d.b.h. and 

mean height of crop trees were clearly less than in the thinned stand (tables 23 

vs. 22 and 25 vs. 24). As in the thinned stand, both diameter and height growth 

accelerated in the second decade. Mean d.b.h. increased by only 4 to 6 percent after 

fertilization; both dosages had similar effects. Fertilizer effects on height growth 

mirrored those of diameter growth; both dosages had a similar effect of less than a 

10-percent gain over nonfertilized means.

Increases in mean tree height (32 to 34 ft) of crop trees in the 20-year period 

were similar to increases in height to live crown (31 to 37 ft), suggesting a similar 

length of live crown in the two decades. At stand age 50 years and two decades 

after fertilization, mean LCR in the nonthinned stand were 0.42, 0.37, and 0.40 for 

0N, 200N, and 400N, respectively (table 29). We infer that N fertilization had no 

real effect on LCR and that a slightly less favorable LCR was present in the nonthinned stand for sustained growth in the next decade (fig. 8b). 

An Economic Analysis

The extra volume of wood gained from fertilization and how soon this wood can 

be harvested strongly affects gross financial return from this investment. Profit is 

attained when revenue from selling the wood at the mill exceeds the total cost of 

precommercial thinning, fertilizing, and harvesting discounted at an assumed interest rate. The following economic analysis compares the effects of precommercial 

thinning, fertilization, and timing of final harvest (i.e., 10 or 20 years after fertilization) on present net value (PNV). 
56

RESEARCH PAPER PNW-RP-606

Methods for Economic Analysis

To estimate financial return from precommercial thinning, fertilization, and timing of harvest at Rocky Brook, we first estimated live-stand merchantable volume 

and tonnage yields for each of the 18 study plots (i.e., nine each in the thinned and 

nonthinned stands) at 10 and 20 years after fertilization (our assumed years of harvest). These metrics were based on tree numbers and sizes for each plot. Further, we 

assumed that volume in dead trees was not usable at harvest. Merchantable volume 

and dollar value paid at a mill were estimated from our tree records that documented 

species, d.b.h., height, and live/dead status of each tree by plot and measurement date.5

Our first step for estimating financial value per acre was to subdivide total bole volume into merchantable volume in logs. For all live trees, diameter inside bark (DIB) at 

the small end of each log was calculated from the known tree d.b.h. and the estimated 

average tarif number of each plot at years 10 and 20 after fertilization. Using a tarif 

taper equation, an iterative process assigned upper stem DIB of each 32-ft log. The last 

log was 12 to 32 ft long with a minimum DIB of 5 inches. This procedure included any 

tree that had a full log or partial log to a 5 inch top, but estimates of volume of potential 

pulpwood less than 5 in DIB were not made. Normally, trees less than 7 inches d.b.h. 

would not contain any merchantable volume and would be left on the harvested site.

The following procedure estimated total harvested volume and green weight 

in three categories: saw logs, smaller chip-and-saw, and pulp. The selection of the 

three grades was based on what mills in western Washington were purchasing in 

2008. All logs 8 inches and larger at top end were considered number 3 saw logs. 

All logs with a DIB of 6 to 7 inches at the small end were considered chip-and-saw 

grade logs. Logs with a DIB of 5 inches and a length less than 24 ft (top log) were 

classified as pulp logs. Note that a more accurate estimate of grade and value likely 

would have resulted if trees had been graded in the field. All estimated product 

values were current as of 2008. Log prices were based on a moving average of 

6-month periods covering 5 years (from July 2003 through January 1, 2008) published in “The Washington Oregon Log Market Report.”6

We developed the following practical approach for comparing our experimental 

treatments using PNV.7 Precommercial thinning costs at Rocky Brook in 1963 were 

$60 per acre. Based on a survey of commercial forest land owners in 2005, cost of 

fertilization with 200 lb N/ac averaged $97 per acre with a standard error of $8 per 

5 Chambers, C.J. 2008. Personal communication. Biometrician (rertired), West Mason 

Consulting, SE 101 Binns-Swigert Loop Rd, Shelton, WA 98584.

6Report available from John Lindberg, logmkt@comcast.net.

7 Hotvedt, J.E. 2014. Personal communication. Forest economist (retired), Washington State 

Department of Natural Resources, 1111 Washington St. SE, Olympia, WA 98504.
57

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

acre (Briggs 2007). Note that costs of fertilization per acre generally decline as the 

size of the contract increases. Therefore, we assumed fertilization costs of $100 and 

$190 per acre for the 200N and 400N treatments, respectively.

To remove the effects of inflation from our analysis, we adjusted all costs and 

revenues to 2014 dollars using annual values of the producer price index (USDOL 

BLS, n.d.). To simplify our economic analysis, we assumed no real change in the 

price of wood products over time. In addition, we did not account for logging 

and hauling costs, which typically consume about half of the revenue generated 

from sale at the mill, thus leaving the remaining revenue as stumpage to the 

landowner. In addition, we did not consider potential financial consequences of 

alternative investments of revenue generated from timber harvest. In our calculations of PNV we assumed a 5 percent discount rate because such a conservative 

value was considered appropriate for public lands and small acreages owned by 

nonindustrial private landowners. The equation for calculating PNV for a given 

level of precommercial thinning, fertilization, and timing of final harvest is:

PNV = R/(1+i)

t1 - CPCT/(1+i)t2 – CFERT/(1+i)t3(1)

where R is the revenue generated from the sale of logs at the mill, i is the discount rate (5 

percent), t1-t3 are the number of years in which revenues or costs are discounted, CPCT is 

the cost of precommercial thinning, and CFERT is the cost of nitrogen fertilization.

For each thinned and nonthinned stand, we subjected our estimates of PNV to 

covariance analysis as described previously for the growth and mortality variables. To 

account for differences in initial stocking that existed prior to fertilization, we adjusted 

the analysis for total cubic volume of live trees (CVTS) immediately before fertilization in 1968. The same analytical approach as described previously for the growth 

variables was used to assess potential treatment effects and interactions on PNV.

Results and Discussion of Economic Analysis

Estimated mill values clearly accelerated in the second decade because more livestand volume accumulated and more higher-valued saw logs and chip-and-saw logs 

were harvested (table 30). Overall, estimated mill value at harvest was greater for 

nonthinned stands compared to thinned stands, and fertilizer increased estimated 

values in both thinned and nonthinned stands (fig. 10a). 

Present net value of the thinned stand varied as a result of the interaction of 

fertilization and timing of final harvest (table 31; fig. 10b). For each fertilizer treatment, greater PNV resulted when the stand was harvested 20 years versus 10 years 

after fertilization. PNV increased with nitrogen fertilization for each timing of 

final harvest, but there were no significant differences between the 200N and 400N 

treatments within each timing.
58

RESEARCH PAPER PNW-RP-606

In the nonthinned stand, PNV only differed significantly between the two 

timings of final harvest (table 32; fig. 10b). Averaged across nitrogen fertilizer rates, 

PNV was 60 percent greater when the stand was harvested 20 years ($2,795 per 

acre) versus 10 years ($1,743 per acre) after fertilization. Fertilization had no detectable effect on PNV of the nonthinned stand. 

To estimate the financial benefit of precommercial thinning, we calculated 

90-percent confidence intervals for PNV of nonfertilized plots in both the thinned 

and nonthinned stands harvested 20 years after fertilization. The confidence 

interval for the thinned stand ($919 ± $278 per acre) did not overlap with that for 

the nonthinned stand ($2,639 ± $498 per acre), suggesting that thinning reduced the 

economic value of this overstocked stand at Rocky Brook. We suspect the delayed 

and intensive thinning at this location reduced growing stock too severely to allow 

the stand to fully benefit from thinning. Note also that differences in apparent site 

index between thinned (73 ft) and nonthinned stands (87 ft) probably had a substantial impact on the estimated volumes at final harvest, which likely also affected the 

results of our economic analysis (table 1). 

Of the factors studied, timing of final harvest (i.e., 10 vs. 20 years after fertilization) had the greatest impact on PNV of both thinned and nonthinned stands, 

probably because accumulation of merchantable volume was delayed by the poor 

quality of the site, the prolonged understocked condition of thinned stands, and the 

overstocked condition of nonthinned stands. Surprisingly, fertilization increased 

PNV of thinned stands but had no detectable effect on nonthinned stands. Clearly, 

overstocking of the nonthinned stand limited the benefits of fertilization.

Table 30—Observed mean net increase in live-stand and merchantable volumes and apparent gain from 

fertilization in the total 20-year period after fertilization at Rocky Brook, Washington

CVTS CV4

Treatmenta Start End Increase Gain Start End Increase Gain

- - - - - - - Cubic feet per acre - - - - - - - Percent - - - - - - - Cubic feet per acre - - - - - - - Percent

Thinned stand:

0N 570 3,410 2,840 — — 0 2,430 2,430 — —

200N 620 4,950 4,330 1,490 52 0 4,100 4,100 1,670 69

400N 549 5,380 4,830 1,990 70 0 4,720 4,720 2,290 94

Nonthinned stand:

0N 1,890 5,560 3,670 — — 344 3,080 2,740 — —

200N 2,510 7,970 5,460 1,790 49 82 4,700 4,620 1,880 69

400N 2,680 8,100 5,420 1,750 48 170 5,520 5,350 2,610 95

N = nitrogen; — = not appropriate; CVTS is cubic volume of the total stem, inside bark; CV4 is cubic volume of the stem to a 4 inch top, inside bark.

a Application of 0, 200, or 400 lb N/ac. 

Fertilization increased 

present net value of 

thinned stands, but not 

for nonthinned stands.
59

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

10,000

9,000

8,000

7,000

6,000

5,000

4,000

3,000

2,000

1,000

0

T-0N T-200N T-400N NT-0N NT-200N NT-400N

Mill value, all species (dollars per acre)

Treatment

10

20

Years after 

fertilization

A

3,500

3,000

2,500

2,000

1,500

1,000

500

0

T-0N T-200N T-400N NT-0N NT-200N NT-400N

Present net value (dollars per acre)

Treatment

B

10

20

Years after 

fertilization

Figure 10—Estimated mean dollar value of (A) bole tonnage as delivered to a mill (2008 dollars) 

and (B) present net value (2014 dollars) to a landowner at final harvest 10 or 20 years after fertilization, by thinning and fertilizing treatment (T = thinned and NT = nonthinned; 0N, 200N, and 400N 

indicate urea fertilizer rates of 0, 200, and 400 lb N/ac, respectively), Rocky Brook, Washington.
60

RESEARCH PAPER PNW-RP-606

Table 31a—Analysis of covariance of present net value (PNV) by 

harvest time after fertilization in the thinned stand at Rocky 

Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 35.4 0.001

Rep × F (Error 1) 5 — —

Harvest time (H) 1 322.1 <0.001

F × H 2 15.1 0.004

Linear, harvest at 10 yearsa 1 46.8 <0.001

Linear, harvest at 20 years 1 54.0 <0.001

Covariate (starting CVTS) 1 141.6 <0.001

Error 2 6 — —

Total (18-1) 17

D.f. = degrees of freedom; PNV = present net value (dollars per acre) at final timber harvest 

in 2014 dollars; N = nitrogen; — = not appropriate.

a Linear contrasts with P ≤ 0.05 are considered statistically significant.

Table 31b—Adjusted mean present net value (mean PNV ± standard error) by 

harvest time after fertilization in the thinned stand at Rocky Brook, Washington

Relative to 0N

Treatmenta H1 H2 H1 H2 H2/H1

- - - - - Dollars/acre - - - - - - - - Percent - - -

0N*b 67 ± 45 b 919 ± 143 b 100 100 13.7

200N* 335 ± 46 a 1,804 ± 143 a 500 196 5.4

400N* 502 ± 46 a 2,406 ± 143 a 749 262 4.8

PNV = present net value (dollars per acre) at final timber harvest in 2014 dollars; H1 = harvest at 10 years after 

fertilization; H2 = harvest at 20 years after fertilization; N = nitrogen.

a Application of 0, 200, or 400 lb N/acre. 

b Asterisks (*) indicate harvest time means for a given fertilizer rate that differ significantly (P ≤ 0.10). Fertilizer 

means within a given harvest time followed by a different letter differ significantly (P ≤ 0.10).
61

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

Table 32a—Analysis of covariance of present net value (PNV) by 

harvest time after fertilization in the nonthinned stand at Rocky 

Brook, Washington

Source of variation D.f. F-value P

N fertilizer (F) 2 0.8 0.507

Rep × F (Error 1) 5 — —

Harvet time (H) 1 156.0 <0.001

F × H 2 2.0 0.212

Linear, harvest at 10 yearsa 1 0.1 0.720

Linear, harvest at 20 years 1 1.0 0.365

Covariate (starting CVTS) 1 6.7 0.042

Error 2 6 — —

Total (18-1) 17

D.f. = degrees of freedom; PNV = present net value (dollars per acre) at final timber harvest 

in 2014 dollars; N = nitrogen; — = not appropriate.

a Linear contrasts with P ≤ 0.05 are considered statistically significant.

Table 32b—Adjusted mean present net value (PNV ± standard error) by harvest 

time after fertilization in the nonthinned stand at Rocky Brook, Washington

Relative to 0N

Treatmenta H1 H2 H1 H2 H2/H1

- - - Dollars/acre - - - - - - Percent - - -

0Nb 1,801 ± 273 2,639 ± 256 100 100 1.5

200N 1,473 ± 240 2,727 ± 220 82 103 1.9

400N 1,955 ± 256 3,018 ± 238 109 114 1.5

PNV = present net value (dollar per acre) at final timber harvest in 2014 dollars; H1 = harvest at 10 years after 

fertilization; H2 = harvest at 20 years after fertilization; N = nitrogen.

a Application of 0, 200, or 400 lb N/ac. 

b Fertilizer-by-period means shown for information only. Main effect differences are described in the text.
62

RESEARCH PAPER PNW-RP-606

Conclusions

Thirty-year-old Douglas-fir and other coniferous species at this Site Class IV-V 

location responded strongly to urea fertilizer. Adding 200 lb N/ac as urea increased 

cumulative live-stand volume 20 years later by about 1,900 ft3/ac in the nonthinned 

portion and by about 1,400 ft3/ac in the understocked portion that had been belatedly and intensively thinned about 5 years before fertilization. Doubling fertilizer 

rate to 400 lb N/ac resulted in lesser marginal gain (gain per lb N applied) in 

nonthinned stands. 

Growth in stand basal area and volume in the thinned stand accelerated greatly 

in the second decade, especially after N fertilization. Change in RD also accelerated so that by stand age 50 years, mean RD in the thinned stand ranged from 55 

to 67 after a mean starting RD of 19. In the nonthinned stand at age 50 years, RD 

ranged from 94 to 99 after a mean starting RD of 67. As basal area and RD stocking increased, live crown ratios of the 100 largest trees per acre narrowed to about 

0.47 in the thinned stand and to about 0.40 in the nonthinned stand.

Volume lost to mortality was generally less than 4 percent of total gross 

growth in the thinned stand, but it was 4 to 15 percent in the nonthinned stand. 

The d.b.h. of dead trees averaged much smaller than average d.b.h. of live trees 

in both thinned and nonthinned stands. Mortality losses in both volume and 

tree numbers accelerated markedly in the second decade. Fertilization increased 

mortality volume in both the 200N and 400N treatments in the thinned stand but 

not in the nonthinned stand. Addition of P, K, and S to the 400 lb N/ac treatment 

provided no additional increase in d.b.h. or basal area growth of crop trees compared to the sole application of 400 lb N/ac.

Both thinning and fertilization accelerated diameter growth of the 100 largest 

d.b.h. trees. Height to live crown increased rapidly in the second decade, especially 

in the nonthinned stand; concurrently, live crown ratio decreased to about 0.40. 

However, fertilization had no measureable effect on HLC or LCR. 

For both thinned and nonthinned stands, delaying a hypothetical final harvest 

until 20 years after fertilization (age 50) substantially increased PNV over that 

resulting when harvest was assumed to occur after only 10 years. On these juvenile, 

N-deficient soils, both merchantable volume and PNV of the stand were clearly 

increased by fertilization, but the increases were statistically significant only in 

thinned stands where adequate growing space was available to allow trees to grow 

into the larger, more valuable size classes. Overstocking in the nonthinned stand 

prevented realization of any financial benefits from fertilization. However, the 

lowest PNV was detected for stands receiving a precommercial thinning because 

felling about 75 percent of the original basal area—including removal of some of 

Overstocking in 

nonthinned stands 

reduced financial 

benefits from 

fertilization.
63

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

the larger trees—severely reduced growing stock and subsequent growth. Results of 

our study confirm that improved growth and financial benefits are attainable when 

silvicultural investments such as precommercial thinning and nitrogen fertilization 

are applied to a poor quality site. On poor quality sites like Rocky Brook, growth 

potential (capacity) is much less then on higher quality sites. Slower rates of growth 

extend the time of harvest after thinning or fertilization, which then reduces the 

financial return from these treatments.

Acknowledgments

The former Quilcene Ranger District and the Olympic National Forest initially 

supported this research with partial funding and field personnel. For many years, 

Harlow Scott and numerous forestry technicians diligently and reliably implemented our study plan. Our former colleagues at the Forestry Sciences Laboratory 

in Olympia, WA, Richard Williamson (deceased) and Don Reukema (retired), 

provided sound technical advice and assistance in planning, executing, and coauthoring an earlier publication. We also acknowledge and thank Robert Curtis for 

advice and review of an earlier draft of this report. Subsequent reviews by Harry 

Bell, Steve Duke, and Robert Curtis were also helpful. We greatly appreciate the 

work of Charles J. Chambers, retired biometrician for the Washington State Department of Natural Resources, for adapting and implementing routine programs that 

estimated tonnage by log grades and current market values. Sincere thanks also to 

James E. Hotvedt, retired forest economist for the Washington State Department of 

Natural Resources, for skillful guidance and reviews of our economic analysis.

Metric Equivalents

When you know: Multiply by: To find:

Inches (in) 2.54 Centimeters

Feet (ft) .305 Meters

Acres .405 Hectares

Square feet per acre (ft2/ac) .229 Square meter per hectare

Cubic feet (ft3) .0283 Cubic meters

Cubic ft per acre (ft3/ac) .07 Cubic meters per hectare

Trees per acre (TPA) 2.47 Trees per hectare

Pounds (lb) .454 Kilograms

Pounds per acre (lb/ac) 1.12 Kilograms per hectare
64

RESEARCH PAPER PNW-RP-606

Literature Cited

Beddows, D. 2002. Levels-of-growing-stock cooperative study in Douglas-fir: 

report no. 16—Sayward Forest and Shawnigan Lake. Info. Rep. BC-X-393. 

Victoria, British Columbia, Canada: Canadian Forest Service, Pacific Forestry 

Centre. 67 p.

Brackett, M. 1973. Notes on tarif tree volume computation. Resource Management 

Rep. 24. Olympia, WA: Washington State Department of Natural Resources. 26 p.

Briggs, D. 2007. Management practices on Pacific Northwest west-side industrial 

forest lands, 1991–2005: with projections to 2010. SMC Working Paper No. 6. 

Seattle, WA: University of Washington, College of Forest Resources, Stand 

Management Cooperative. 79 p.

Bruce, D.; Demars, D.J. 1974. Volume equations for second-growth Douglasfir. Res. Note PNW-239. Portland, OR: U.S. Department of Agriculture, Forest 

Service, Pacific Northwest Forest and Range Experiment Station. 5 p.

Curtis, R.O. 1982. A simple index of stand density for Douglas-fir. Forest Science. 

28(1): 92–94.

Curtis, R.O. 2010. Effect of diameter limits and stand structure on relative density 

indices: a case study. Western Journal of Applied Forestry. 25: 169–175.

Curtis, R.O.; Clendenen, G.W. 1994. Levels-of-growing stock cooperative study 

in Douglas-fir: report no. 12—the Iron Creek study, 1966–89. Res. Pap. PNWRP-475. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific 

Northwest Research Station. 67 p.

Curtis, R.O.; Marshall, D.D. 2009. Levels-of-growing-stock cooperative study in 

Douglas-fir: report no. 18—Rocky Brook, 1963–2006. Res. Pap. PNW-RP-578. 

Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest 

Research Station. 91 p.

DeBell, D.S.; Harrington, C.A.; Shumway, J. 2002. Thinning shock and response 

to fertilizer less than expected in young Douglas-fir stand at Wind River 

Experimental Forest. Res. Pap. PNW-RP-547, Portland, OR: US. Department of 

Agriculture, Forest Service, Pacific Northwest Research Station. 20 p.

Gessel, S.P.; Stoate, T.N.; Turnbull, K.J. 1965. Growth behavior of Douglasfir with nitrogenous fertilizer in western Washington, a first report. Res. Bull. 

No. 1. Seattle, WA: University of Washington, College of Forestry. 204 p., 

plus illustrations.
65

Stand Dynamics of Douglas-fir 20 Years After Precommercial Thinning and Nitrogen Fertilization on a Poor-Quality Site

Hann, D.W. 2011. ORGANON user’s manual. Edition 9.1. Corvallis, OR: 

Oregon State University, Department of Forest Engineering, Resources, 

and Management. 134 p.

Harrington, C.A.; Reukema, D.L. 1983. Initial shock and long-term stand 

development following thinning in a Douglas-fir plantation. Forest Science. 

29: 33–46.

King, J.E. 1966. Site index curves for Douglas-fir in the Pacific Northwest. 

Weyerhaeuser Forestry Paper 8. Centralia, WA: Weyerhaeuser Company. 49 p.

Li, Y.; Turnblom, E.C.; Briggs, D.G. 2007. Effects of density control and 

fertilization on growth and yield of young Douglas-fir plantations in the Pacific 

Northwest. Canadian Journal of Forest Research. 37: 449–461.

McArdle, R.E.; Meyer, W.H.; Bruce, D. 1961. The yield of Douglas-fir in 

the Pacific Northwest. Technical Bulletin 201. Rev. Washington, DC: U.S. 

Department of Agriculture. Forest Service. 65 p.

McWilliams, E.R.G.; Therien, G. 1996. Fertilization and thinning effect on a 

Douglas-fir ecosystem at Shawnigan Lake: 24-year growth response. FRDA 

Report 269. Victoria, BC: Canadian Forest Service, Pacific Forestry Centre. 22 p. 

Miller, R.E.; Anderson, H.W. 1995. Stand characteristics of 65-year-old planted 

and naturally regenerated stands near Sequim, Washington. Res. Pap. PNWRP-482. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific 

Northwest Research Station. 11 p.

Miller, R.E.; Barker, P.R.; Peterson, C.E.; Webster, S.R. 1986. Using nitrogen 

fertilizers in management of coast Douglas-fir: I. Regional trends of response. In: 

Oliver, C.D.; Johnson, D.D.; Johnson, J.A., eds. Douglas-fir: stand management 

for the future. Proceedings of a symposium. Seattle, WA: University of 

Washington, College of Forest Resources: 290–303.

Miller, R.E.; Reukema, D.L.; Williamson, R.L. 1981. Response to fertilization 

in thinned and nonthinned Douglas-fir stands. In: Gessel, S.P.; Kenady, R.M.; 

Atkinson, W.A., eds. Proceedings, forest fertilization conference. Seattle, WA: 

University of Washington, College of Forest Research: 150–157.

Miller, R.E.; Tarrant, R.F. 1983. Long-term growth response of Douglas-fir to 

ammonium nitrate fertilizer. Forest Science. 29(1): 127–137. 
66

RESEARCH PAPER PNW-RP-606

Omule, A.Y.; Mitchell, A.K.; Wagner, W.L. 2011. Fertilization and thinning 

effects on a Douglas-fir ecosystem at Shawnigan Lake: 32-year growth response. 

Info. Rep. FI-X-005. Victoria, BC: Canadian Forest Service, Canadian Wood 

Fibre Centre. 21 p. 

Quinn, G.P.; Keough, M.J. 2002. Experimental design and data analysis for 

biologists. Cambridge, United Kingdom: Cambridge University Press: 49–50, 

349–350.

SAS Institute Inc. 2013. The SAS system for Windows. Version 9.4. Cary, NC. 

Sokal, R.R.; Rohlf, F.J. 1981. Biometry. 2nd ed. New York: W.H. Freeman and 

Company: 677–683.

U.S. Department of Agriculture, National Resources Conservation Service 

[USDA NRCS]. [N.d.]. Official soil series descriptions. http://www.nrcs.usda.

gov/wps/portal/nrcs/detail/soils/survey/class/?cid=nrcs142p2_053587. (February 

13, 2015).

U.S. Department of Labor, Bureau of Labor Statistics [USDOL BLS]. [N.d.].

Producer Price Indexes. http://www.bls.gov/ppi/tables.htm. (March 7, 2015). 

Williamson, R.L. 1976. Levels-of-growing-stock cooperative study on Douglas-fir. 

Report No. 4—Rocky Brook, Stampede Creek, and Iron Creek. Res. Pap. PNW210. Portland, OR: U.S. Department of Agriculture, Forest Service, 

Pacific Northwest Range and Experiment Station. 39 p.


Pacific Northwest Research Station

Website http://www.fs.fed.us/pnw/

Telephone (503) 808–2592

Publication requests (503) 808–2138

FAX (503) 808–2130

E-mail pnw_pnwpubs@fs.fed.us

Mailing address Publications Distribution

Pacific Northwest Research Station

P.O. Box 3890

Portland, OR 97208–3890
U.S. Department of Agriculture

Pacific Northwest Research Station

1220 SW 3rd Ave., Suite 1400

P.O. Box 3890

Portland, OR 97208–3890

Official Business

Penalty for Private Use, $300