Comparative Effects of 

Urea Fertilizer and Red 

Alder in a Site III, Coast 

Douglas-Fir Plantation 

in the Washington 

Cascade Range

Richard E. Miller, Harry W. Anderson, Marshall Murray, and Rick Leon

United States 

Department of 

Agriculture

Forest Service

Pacific Northwest 

Research Station

Research Paper

PNW-RP-565

June 2005
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Authors

Richard E. Miller is an emeritus soil scientist, Harry W. Anderson and

Marshall Murray are volunteers, U.S. Department of Agriculture, Forest Service, 

Pacific Northwest Research Station, Forestry Sciences Laboratory, 3625 93d Ave. 

SW, Olympia, WA 98512-9193; and Rick Leon is a forester, Weyerhaeuser Co., 

Tacoma, WA 98477.
Abstract

Miller, Richard E.; Anderson, Harry W.; Murray, Marshall; Leon, Rick. 2005.

 Comparative effects of urea fertilizer and red alder in a site III, coast Douglasfir plantation in the Washington Cascade Range. Res. Pap. PNW-RP-565. 

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

Northwest Research Station. 28 p.

Five randomly assigned treatments were used to quantify effects of adding varying 

numbers of red alder (Alnus rubra Bong.) or nitrogen (N) fertilization on growth of 

a 10-year-old conifer plantation at a medium-quality site in the western Washington 

Cascade Range. Zero, 20, 40, and 80 alder trees per acre were retained along with 

about 300 conifers per acre. Nearly all conifers were coast Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco). A fifth treatment substituted N fertilizer for N-fixing alder. Changes in average tree height, and in numbers of trees, 

basal area, and volume per acre between plantation ages of 10 and 27 are compared. 

In pure conifer plots, gross volume growth averaged 26 percent greater on fertilized 

than nonfertilized plots, indicating measurable benefits of additional N. On both 

fertilized and nonfertilized plots, an average of 13 percent of the original conifers 

died. Retaining 20, 40, or 80 alder per acre (7, 13, and 27 percent of the associated 

conifer trees per acre, respectively) was associated with reduced numbers of 

Douglas-fir by about 19, 5, and 17 percent, respectively, in the next 17 years. 

Mortality and growth of Douglas-fir were not related to alder density, but losses of 

Douglas-fir were especially large on plots where relatively large red alder (20 per 

acre) were retained. Neither total stand nor conifer yields were changed by retaining 

alder. Additional comparisons are needed at other locations, especially those with 

known N deficiency.

Keywords: Mixed stands, competition (plant), Douglas-fir, Pseudotsuga 

menziesii, red alder, Alnus rubra, thinning, nitrogen fertilization, volume growth.

Summary

We compared 17-year survival and growth of Douglas-fir (Pseudotsuga menziesii

var. menziesii (Mirb.) Franco) and other species after installing five silvicultural 

treatments in a 10-year-old, site III Douglas-fir plantation in western Washington. 

Conifer density was thinned to about 300 trees per acre (TPA); nearly all were 

planted Douglas-fir. Treatments 1 to 4 tested the effects of retaining 0, 20, 40, or 

80 red alder (Alnus rubra Bong.) per acre, respectively. Treatment 5 retained 300 

conifers but no alder, and substituted 200 lb nitrogen (N) per acre as urea fertilizer 
for N biologically fixed by admixed alder. Our purpose was to quantify the effects 

of these treatments on size and yield of conifers and red alder. We installed two or 

three replicates of each treatment. 

Nitrogen fertilization increased tree and stand growth in the 17 years of observation. Retaining red alder in the plantation had no effect on conifer or total stand 

yields. Retaining 20, 40, or 80 red alder per acre had no measurable positive but 

some negative effects on associated Douglas-fir. Admixed alder increased losses of 

Douglas-fir, especially where relatively large red alder were retained. Yield of the 

Douglas-fir component may have been reduced slightly. Combined yield of all 

species, however, was similar for admixtures of 0 through 80 red alder per acre.

During stand development, crowns of Douglas-fir can be mechanically damaged or suppressed by a neighboring alder, yet, conversely, individual red alder can 

be dominated by one or more Douglas-fir before contributing much N-rich organic 

matter to the soil. In our study, the 80-alder-per-acre treatment corresponds to an 

average spacing of about 23 ft between alder; 300 TPA of Douglas-fir equals an average spacing of 12 ft. Where these average spacings exist, each red alder neighbors 

four Douglas-fir. We assert that intensity and consequences of competition between 

the two species is site-specific and dynamic. Red alder was less competitive at this 

inland location than at the more moist, coastal site II location investigated earlier. 

To ensure timely control of alder in alder-Douglas-fir mixtures, foresters should 

periodically assess damage to conifer crowns. Controlling the stocking of either 

or both species is advisable. Moreover, we infer from our study that planting alder 

from an off-site source may be another way to reduce alder competition.

We suggest that retaining 80 red alder per acre with 300 Douglas-fir per 

acre at plantation age 10 is too many as an operational treatment. Moreover, 

future research should consider using a replacement design in which each red 

alder substitutes for or replaces one Douglas-fir. New testing should use large 

plots for both treatment areas and tree measurement plots. Application of our 

alder treatment to provide biologically fixed N was much more variable than 

our application of urea because sizes of individual alder differed greatly. Additional comparisons are needed at other locations, especially those with known 

N deficiency where the N-fixing capacity of associated red alder is more likely 

to enhance yield of Douglas-fir. 
Contents

 1 Introduction

2 Materials and Methods

 2 Location

 2 Experimental Treatments

 3 Experimental Design

 3 Treatment History

 5 Fertilization

 8 Tree Measurement and Volume Computation

 8 Data Summarization and Statistical Analysis

 9 Results

 9 Trees Per Acre and Average Diameter

12 Mean Height

14 Basal Area Per Acre

14 Volume Per Acre

19 Yield at Plantation Age 27

21 Discussion

21 Effect of Fertilizing

22 Effect of Increasing Numbers of Red Alder

24 Implications for Silvicultural Researchers

25 Implications for Land Managers

26 Conclusions

26 Acknowledgments

26 Metric Equivalents

26 Literature Cited 

Introduction

Additions of nitrogen (N) have special significance to coast Douglas-fir 

(Pseudotsuga menziesii (Mirb.) Franco) in the Pacific Northwest because growth 

frequently is limited by insufficient quantities of available N in the soil. Across the 

range of site qualities, about 70 percent of N-fertilization trials in western Oregon, 

Washington, and British Columbia show response to 150 to 200 lb N per acre

(Chappell and others 1992, Miller and others 1986). Applying urea fertilizer (46-0-

0) is the conventional way to supply N for increased stand growth. Using N-fixing 

plants has also been considered, especially red alder (Alnus rubra Bong.), an extensively distributed native hardwood with increasing commercial value (Hibbs and 

DeBell 1994). Published estimates of the amount of N fixed annually by red alder in 

pure and mixed stands range from 20 to 300 lb N per acre (Binkley 1992, Binkley 

and others 1994). Because of its rapid juvenile growth, red alder in mixed stands 

frequently threatens full and uniform distribution of associated, higher valued 

Douglas-fir (Newton and Cole 1994). This risk is generally greater on highquality, mesic sites (where early height growth of red alder is more rapid than that 

of Douglas-fir) compared to lower quality, drier sites (where relative growth is 

slower) (Newton and others 1968). Thus, the silvicultural option of admixing red 

alder in Douglas-fir stands is more complicated than using fertilizer (Miller and 

Murray 1979).

Development of the red alder option for supplying N to Douglas-fir requires (1) 

quantification of both positive and negative effects of admixed red alder on yield 

and value of associated Douglas-fir at a wide range of site qualities and (2) comparison of the cost-effectiveness of retaining or planting alder vs. applying fertilizer. Thus, optimum density of alder admixture for Douglas-fir needs definition and 

comparison with optimum fertilizer dosage. Currently, this optimum fertilizer 

dosage seems to lie between 150 and 300 lb N per acre applied several times per 

rotation. For N-deficient sites, an economically optimum alder density in mixed 

stands could be about 40 trees per acre (TPA) maintained in a dominant/codominant position until about one-half of the red alder and associated Douglas-fir are 

removed at the first commercial thinning (Miller and Murray 1979). 

We started such quantification about two decades ago on a good (site II) and a 

poorer (site III) quality site assuming that the balance between negative competitional effects of red alder and beneficial effects of its N-fixing capacity are likely to 

differ by site quality. Our controlled-treatment study was designed to test two working hypotheses about the consequences of red alder retained with Douglas-fir:

Comparative Effects of Urea Fertilizer and Red Alder in a Site lll, Coast Douglas-Fir Plantation in the Washington Cascade Range

1

We sought to determine 

the optimum density 

of nitrogen-fixing alder 

for improving growth of 

Douglas-fir and compared growth to that 

obtained by adding 

fertilizer.
RESEARCH PAPER-PNW-RP-565

2

1. Benefits of soil improvement by red alder to Douglas-fir yield will be similar 

in magnitude to the benefits of N fertilization. We anticipated that fertilization 

with 200 lb N per acre would increase short-term volume growth less than 10 

percent at our site II location and about 20 percent at our site III location. 

2. With increasing alder stocking, negative competitive effects of red alder on

 associated Douglas-fir will gradually offset potentially positive effects of 

increased N and improved organic matter conditions in the soil. Thus, we 

speculated that yield of Douglas-fir on N-deficient sites would increase at 

lower densities of admixed alder, but progressively decline as increasing 

numbers of red alder damage or overtop more Douglas-fir. 

Here we report results at our site III location and will compare these with 

results reported earlier for the better site (Miller and others 1999).

Materials and Methods

Location

The trial is located in a Douglas-fir plantation at 1,800 ft elevation about 4 mi 

east of Enumclaw, Washington. The current plantation was established in spring 

1971 when 2+1 Douglas-fir seedlings were hand planted at an average density of 

600 TPA. The preceding conifer stand had been clearcut, then yarded by tractor. 

Logging slash was piled, but not burned. The deep, well-drained soil at this site 

is mapped as Pitcher sandy loam, 8 to 30 percent slopes, which developed from a 

mixture of volcanic ash and andesitic rock (Goldin 1992). This soil is classified as 

loamy-skeletal, mixed frigid family of Typic Haplorthods. It has a spodic horizon 

but no underlying argillic horizon. We observed some rounded surficial gravel in 

the study area and suspect this originated from local alpine glaciation. Douglas-fir 

site index on the Pitcher soil averages 112 ft at 50-year breast-height age (site III). 

Experimental Treatments

To test our two hypotheses, we compared tree and stand growth of conifers (practically all Douglas-fir) and red alder on plots representing varying densities of red 

alder with a constant nominal conifer density of 300 TPA (after precommercial 

thinning 10 years after planting). As shown in the following tabulation, the no-alder 

treatment (treatment 1) corresponds to a conventional regime that removes all red 

alder by herbicide or cutting. Admixtures of 20, 40, and 80 alder per acre straddle 

the assumed financially optimum number of alder to retain in Douglas-fir stands 

(Miller and Murray 1979). A code number for each treatment follows:
Comparative Effects of Urea Fertilizer and Red Alder in a Site lll, Coast Douglas-Fir Plantation in the Washington Cascade Range

3

Treatments 1 to 4 define a red alder response surface; treatment 5 simulates a 

conventional regime that eliminates red alder and periodically uses urea fertilizer 

to increase stand growth.

Experimental Design

Our experimental design was treated in the analysis as a completely random assignment of five treatments, although plots of treatment 5 were installed beside and 4 

years after plots of the other treatments. Treatments 1 and 5 (in pure conifer) have 

three replications, and treatments 2, 3, and 4 have two replications. Treatments were 

applied to twelve 0.20-acre areas. A 0.10-acre tree-measurement plot was centered 

in each treatment area, which left a 13-ft-wide treated buffer. 

Treatment History

Our test included nine plots from an earlier investigation of interplanting local 

versus coastal sources of red alder. Those preliminary regimes were applied 3 and 

7 years after Douglas-fir were planted; our test treatments were applied in year 10 

after planting (table 1).

At age 3, the Douglas-fir plantation also contained volunteer trees of red alder, 

Douglas-fir, and western hemlock (Tsuga heterophylla (Raf.) Sarg.). Three preliminary regimes (A, B, C) started at age 3. On three randomly selected plots, all volunteer alder were cut (regime A). Two years later, alder stump sprouts and new alder 

seedlings were cut and treated with Tordon1 herbicide to maintain these three plots 

in an alder-free condition. Likewise in year 3 on six additional plots, all volunteer 

alder were cut, but two 2-year-old red alder wildlings were interplanted near each 

planted Douglas-fir, one at a distance of 2 ft from a Douglas-fir and the other at 4 ft. 

Density of the interplanted alder averaged about 1,200 TPA. Alder from a local 

Red alder retained (No. per acre)

 Urea (lb N per acre) 0 20 40 80

Treatment number

 0 1 2 3 4

 200 5 - - -

1 Use of trade or firm names in this publication is for reader information and does not imply 

endorsement by the U.S. Department of Agriculture of any product or service.
RESEARCH PAPER-PNW-RP-565 4 Table 1—Schedule of original and final treatment regimes in a Douglas-fir plantation Original regime Item A B C D

Years after planting: 3 (1973) All volunteer RA cut Same as regime A; 2 local Same as regime A; 2 nonlocal No treatment

RA wildlings planted beside RA wildlings planted beside

each planted DF each planted DF

 5 (1975) RA sprouts and new No treatment No treatment No treatment

seedlings cut; stumps treated with herbicide

 7 (1977) RA sprouts and new Most RA cut except 20, 40, or Same as regime B All volunteer RA cut

seedlings cut 80/acre retained

 10 (1980) Conifer density reduced to Same as regime A Same as regime A Same as regime A;

300/acre; RA sprouts and 200 lb N/acre asnew seedlings cut urea applied

Final treatment No. 1 2, 3, 4 2, 3, 4 5Years with RA: Volunteers 3 3 3 7 Planted 0 24 24 0Years after thinning 17 17 17 17Years after 200 lb N/acre 0 0 0 17DF = Douglas-fir, RA = red alder.
Comparative Effects of Urea Fertilizer and Red Alder in a Site lll, Coast Douglas-Fir Plantation in the Washington Cascade Range

5

source were planted on three of the six plots (regime B), and alder from a nonlocal 

source near sea level at Aberdeen, Washington, were planted on three other plots 

(regime C). The objective of this alder interplanting was to compare with no-alder 

(regime A) and to learn if one source of transplants grew faster, hence, was more 

competitive to associated Douglas-fir (regime A and B). Weyerhaeuser Company 

initiated this original investigation, which was expanded in 1977 to a cooperative 

study between Weyerhaeuser Co. and the USDA Forest Service, Pacific Northwest 

Research Station. The remaining untreated plantation constituted regime D.

In 1977 (plantation age 7, total age 10), three new plots sampling regime D were 

installed. On new and original plots, future crop trees were selected and tagged. 

Conifer density on all 12 plots was targeted at 300 Douglas-fir per acre. On three 

plots where suitable Douglas-fir were not available, a few western hemlock were 

substituted as crop trees. Alder were retained on plots and buffers of regimes B and 

C at randomly assigned densities of 20, 40, or 80 TPA within each alder source (to 

create treatments 2, 3, and 4). Surplus red alder were cut from all plots including 

regimes A and D (no-alder treatments).

 Final thinning of conifers in 1980 occurred 10 years after planting (figs. 1a 

and 1b). Surplus conifers and alder seedlings missed earlier were cut as were recent 

stump sprouts. Average numbers of stems before and after cutting at ages 7 and 10 

years are shown in table 2.

Fertilization

The three plots of treatment 5 were fertilized on February 27, 1981 (10 growing 

seasons after planting). Urea (46-0-0) was uniformly spread within string-bounded 

lanes to control distribution of the amount allocated to each lane and plot. Volatilization losses were unlikely because the surface soil was moisture-saturated when 

fertilized and weather remained cool with rain in the weeks after fertilization.
RESEARCH PAPER-PNW-RP-565

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Figure 1—Stand on plot 9 (local alder source): (A) before and (B) after reduction of alder density at 

plantation age 7 (1977).

A

B
Comparative Effects of Urea Fertilizer and Red Alder in a Site lll, Coast Douglas-Fir Plantation in the Washington Cascade Range

7

Table 2—Average number of stems per acre 0.1 inch d.b.h. and larger before and after cutting (in parentheses) of surplus alders and conifers at plantation ages 7 and 10 years 

Douglas-fir Other conifers Red alder

Planted Volunteer Volunteers Planted Volunteers

Original regime 7 yra 10 yr 7 yr 10 yr 7 yr 10 yr 7 yr 7 yr 10 yr

Number per acre

Volunteer alder:  A. Alder cut at age 3 503 493 (307) 33 (0) 0 117 (37) 37 (10) – 113 (0) 

–

 D. Alder cut at age 7 516 506 (313) 76 (0) 0 43 (17) 17 (4) 

– 453 (0) 

–

Planted alder:  B. Local alder 443 433 (307) 50 (0) 0 97 (40) 40 (0) 723b 80 (0) 

–

 C. Nonlocal alder 476 473 (307) 30 (0) 0 13 (13) 13 (0) 894b 67 (0) 

–

a No planted Douglas-fir were cut at age 7; difference between number of trees before cutting at 10 years and at 7 years is mortality.b Alder were reduced to 20, 40, or 80 per acre on one plot each in both regimes B and C at year 7; these alder were retained at age 10.
RESEARCH PAPER-PNW-RP-565

8

Tree Measurement and Volume Computation

Tree diameters at breast hight (d.b.h.) were measured (to nearest 0.1 in) at 4.5 ft 

above ground. Heights of all trees exceeding 4.5 ft were measured to the nearest 

0.5 ft by using graduated-height poles initially and either Abney-tape or Vertex 

hypsometer after tree heights exceeded about 35 ft. Diameters and heights were 

measured after the 1977, 1980, 1983, 1986, 1989, 1993, and 1997 growing seasons 

(plantation ages 7 through 27).

Total stem volume (inside bark), including tip and stump, was calculated for 

each tree from height-d.b.h. equations and regional volume equations for the appropriate species. Regional volume equations that were used included those for 

Douglas-fir (Bruce and DeMars 1974), western hemlock (Wiley and others 1978), 

and red alder (Browne 1962). Heights of a few nonmeasured trees were calculated 

from iteratively fitted height-diameter relations in metric units by using the equation form:

 Est. ht. (in meters) = A*EXP(B*DBH**-1.0) + 1.37 m, 

in FORTRAN language 

(or mathematically, y = aeb/x + 1.37, where x = d.b.h. in cm)

Height-diameter relations for Douglas-fir were fitted for individual plots, but those 

for red alder were fitted to pooled data from plots having the same treatment.

Data Summarization and Statistical Analysis

Individual tree data were summarized for each 3- to 4-year growth period after 

thinning and fertilization: 1981-83 (inclusive), 1984-86, 1987-89, 1990-93, 1994-97, 

and for the total 17-year period. Growth of conifers (mostly Douglas-fir) in basal 

area and volume per acre, as well as change in average height of Douglas-fir, were 

compared by covariance analysis, by using initial (at year 10, 1980) basal area, volume, or mean height, respectively, as covariates (SAS Institute, Inc. 1988). Height 

and volume growth of red alder and of all species were compared by ANOVA; 

covariance adjustment was inappropriate because the potential covariate (starting 

volume of red alder) was affected by treatment. 

Because the number of red alder retained in the Douglas-fir plantation is a 

continuous variable, we used the method of polynomials to fit an approximate 

response function to the number of alder retained in treatments 1 through 4. In both 

covariance and ANOVA, we used nonorthogonal contrasts to separate the following 

treatments: 
Comparative Effects of Urea Fertilizer and Red Alder in a Site lll, Coast Douglas-Fir Plantation in the Washington Cascade Range

9

Contrast Treatment numbers Indicates the effects of

1 1 vs. 5 N fertilization

2 1 through 4 Increasing alder density, linear relation

3 1 through 4 Increasing alder density, lack of fit

By reusing treatment 1 in these contrasts, we affected to some unknown extent 

the probability levels at which we can ascribe statistical significance. Unadjusted 

means from ANOVA analysis of red alder and all-species volume growth were 

separated by Bonferroni multiple-comparison procedures (SAS Institute, Inc. 1988). 

All tests of statistical significance were made at p < 0.10.

Results

Trees Per Acre and Average Diameter

After thinning —Density of Douglas-fir among the five treatments ranged from 

300 to 313 TPA after thinning at plantation age 10 (table 3). Inclusion of a few 

naturally regenerated hemlock as crop trees in treatments 1 and 5 increased the 

combined density of conifer crop trees to 317 TPA on these plots. Red alder density 

ranged from 0 to 80 TPA.

Quadratic mean d.b.h. (Dq) of residual Douglas-fir averaged 3.5 to 3.7 in among 

the five treatments. By chance, admixed alder in the 20 TPA treatment (treatment 2) 

averaged larger in both d.b.h. (table 3) and height (table 4) than those in the 80-TPA 

(treatment 4) and especially the 40-TPA (treatment 3) admixture.

Trends with time—Subsequent 17-year losses of planted Douglas-fir averaged 

from 0 to 140 TPA among the 12 plots (fig. 2). No hemlock crop trees died in treatments 1 and 5, where as many as 20 TPA hemlock were retained to provide uniform 

spacing of crop trees. Causes of mortality included disease (41 percent), suppression (25 percent), mountain beaver (Aplodontia rufa), or bear (Ursus americanus 

altifrontalis) (10 percent), windthrow (10 percent), and unspecified (14 percent). In 

treatment 1, which had no competing alder, an average of 50 TPA of Douglas-fir 

died (16 percent of the original 307 TPA of coniferous crop trees); however, losses 

among the three plots differed greatly (0 to 140 TPA) creating a standard error of 

45 TPA. Mean d.b.h. of dead trees averaged less than that of live trees in each 

measurement period. An average of 50 (16 percent) of the original 313 TPA also 

died on fertilized plots in this 17-year period. Retention of 20, 40, or 80 alder per 

acre (representing 6, 13, and 27 percent, respectively, of the density of conifer crop 

trees) was related to reduction in numbers of associated Douglas-fir of about 22, 8, 

and 17 percent, respectively. The largest alder inadvertently were retained in the 

Fewer Douglas-fir died 

where associated with 

40 alder per acre than 

with 20 or 80 alder per 

acre.
RESEARCH PAPER-PNW-RP-565

10

Table 3—Average stand statistics after final thinning at plantation age 10, trees 0.1 inch d.b.h. and larger, by treatment and species

Treatment Stems Quadratic d.b.h. CVTSb

Plantation number DF RA All DF RA All Site heighta DF RA All

 ---- Number per acre ---- ---------- Inches ------------ Feet ----- Cubic feet per acre-----

Fertilizer:

0 N 1 307 0 317 3.6 0 3.6 26.9 236 0 242

 200 lb N per acre 5 313 0 317 3.5 0 3.6 25.8 210 0 210Mixed (RA per acre):  20 2 310 20 330 3.6 4.6 3.7 27.4 243 33 276 40 3 310 40 350 3.6 3.4 3.7 28.0 245 28 273

 80 4 300 80 380 3.7 4.1 3.8 26.4 238 87 325

DF = Douglas-fir, RA = red alder, All = all species (includes other species beside DF and RA).a Average height of the 80 largest (by diameter at breast height [d.b.h.]) Douglas-fir per acre.b CVTS = cubic volume total stem, including tip and stump.
Comparative Effects of Urea Fertilizer and Red Alder in a Site lll, Coast Douglas-Fir Plantation in the Washington Cascade Range

11

Table 4—Mean height and height ratio of live Douglas-fir (DF) and red alder (RA), by treatment, plantation age, and alder source

 Age (years) and species

Treatment 10 13 16 22 27

Alder retained number DF RA DF RA DF RA DF RA DF RANo. per acre -- ------------------------------------------------------- Height in feet -------------------------------------------------------- 0 1 22.4 – 32.7 – 40.4 – 60.0 – 73.7 20 2 22.3 28.8 32.4 33.0 39.8 38.9 58.2 53.4 72.4 60.240 3 22.6 23.7 32.9 27.4 40.8 34.0 60.0 48.2 73.4 57.480 4 22.7 26.2 33.3 31.2 41.8 37.8 61.1 51.6 75.0 62.9

---------------------------------------------------- Height ratio (DF/RA) -------------------------------------------------- 

20: 2 0.77 0.98 1.02 1.09 1.20  Local 0.62 0.77 0.83 0.97 1.06 Coastal .98 1.28 1.30 1.22 1.3640: 3 .95 1.20 1.20 1.24 1.28  Local .94 1.17 1.12 1.17 1.18 Coastal .96 1.24 1.29 1.33 1.3980: 4 .87 1.06 1.10 1.18 1.19  Local .87 .98 1.03 1.08 1.15 Coastal .87 1.17 1.20 1.31 1.24
RESEARCH PAPER-PNW-RP-565

12

20-TPA admixture; therefore, loss of 70 Douglas-fir TPA or about three Douglasfir crop trees per retained alder is not surprising. Doubling the number of alder to 

40 TPA, however, resulted in fewer losses of Douglas-fir (25 TPA vs. 70), probably 

because alder in treatment 3 averaged much smaller d.b.h. than those in the 20 TPA 

treatment (3.4 vs. 4.6 in; table 3). At each alder admixture, more Douglas-fir died 

where associated with the local source of alder transplants, especially in treatment 

2 (fig. 1).

Loss of coniferous crop trees averaged the same for control plots (treatment 1) 

and fertilized plots, but increased with alder additions of 20 and 80 TPA (fig. 2). 

Mean Height

After thinning —Ten years after planting, mean height of residual Douglas-fir was 

similar among the five treatments (22.3 to 22.7 ft; table 4). Alder in the 20-TPA 

alder treatment, however, averaged taller than those in the 40- and 80-TPA admixtures. Moreover, height of Douglas-fir in treatment 2 averaged only 0.77 that of 

Red alder (number per acre)

0 20 40 60 80

17-year loss of Douglas-fir (number per acre)

0

20

40

60

80

100

120

140

160

Nonfertilized

Fertilized

Local source

Coastal source

2

Figure 2—Number of Douglas-fir crop trees per acre that died (from the original of about 300 per acre) in the 17 years 

after thinning at plantation age 10 years, by alder density and source. Note the two overlapping plotting points indicated 

by “2.”
Comparative Effects of Urea Fertilizer and Red Alder in a Site lll, Coast Douglas-Fir Plantation in the Washington Cascade Range

13

associated red alder (height ratio: Douglas-fir/red alder = 0.77), compared to height 

ratios of 0.95 and 0.87 in treatments 3 and 4, respectively (table 4). This suggests 

that some suppression of Douglas-fir by the larger alder in treatment 2 might have 

occurred by year 10 after planting.

Trends with time—Mean height of both Douglas-fir and alder progressed in subsequent years (fig. 3). Douglas-fir initially averaged shorter than associated red alder. 

By plantation age 13, the Douglas-fir/red alder height ratio exceeded 1.0 with 40 and 

80 alder per acre (table 4). This implied shift of Douglas-fir to dominance occurred 

later (year 16) in the 20-TPA alder treatment, probably because the ratio was only 

0.77 at year 10. By year 27 after planting, Douglas-fir averaged about 20 percent 

taller than associated red alder in all admixtures. We infer from the DF/RA ratio of 

mean heights that the local source of red alder was more competitive with Douglasfir than was the coastal source (table 4). Consequently, Douglas-fir associated with 

the local source required more years to exceed mean height of associated alder.

17-year change in mean height—Change in mean height represents (1) net height 

growth of individual trees and (2) change in the number and size of sample trees 

Years after planting

5 10 15 20 25 30

Mean total height (feet)

0

10

20

30

40

50

60

70

80

Douglas-fir, nonfertilized

Douglas-fir, 200 lb N per acre

Douglas-fir, 20 alder per acre

Douglas-fir, 40 alder per acre

Douglas-fir, 80 alder per acre

Alder, 20 per acre

Alder, 40 per acre

Alder, 80 per acre

Figure 3—Trends of mean height of Douglas-fir (solid symbols) and red alder (open symbols) surviving each growth 

period, by treatment.
RESEARCH PAPER-PNW-RP-565

14

when some trees died. In this study, heights of all trees were measured, so no replacement of dead trees was possible. Tree losses in pure-conifer plots represented 

13 percent of the original trees. Because Dq of dead trees averaged less than that for 

the entire stand, that loss of smaller trees arithmetically increased mean height and 

its 17-year change.

Observed annual change in mean height in the 17-year period after thinning, 

ranged from 2.94 to 3.17 ft per yr (table 5). To better isolate the effect of retaining 

red alder on height of associated Douglas-fir, treatment means of observed change 

in mean height were adjusted for initial differences in starting height among treatments. After covariance adjustment, Douglas-fir in fertilized plots (treatment 5) 

averaged 6 percent greater change in mean height than those on nonfertilized plots 

(p = 0.07; tables 5 and 6). A linear fit of adjusted means for the four treatments (0, 

20, 40, and 80 alder/acre) was nonsignificant (p = 0.35) and unlikely to be improved 

by a curvilinear fit (table 6). We infer that mean height of Douglas-fir was not affected by increasing numbers of admixed alder (table 5). As in contrast 1, however, 

the arithmetic effect of mortality losses could have influenced these means.

Basal Area Per Acre

Adjusted gross basal area growth of fertilized crop trees in the 17-year period after 

thinning exceeded that of nonfertilized plots (table 5). The 29-percent greater basal 

area growth on fertilized plots, however, was statistically nonsignificant (p = 0.12; 

table 6). The pattern of basal area growth of Douglas-fir crop trees with increasing 

numbers of admixed alder was erratic and nonsignificant.

Volume Per Acre

To avoid misinterpretations, we define our terms as follows: (1) Net PAI is the estimated change in live stand volume between measurements plus the volume of small 

trees (in-growth) that attained minimum d.b.h. (0.1 in). This net volume growth, 

expressed on an annual basis, includes volume accumulated on trees that survived 

specified 3- or 4-year periods between measurements. (2) Periodic annual mortality (PAM) is the volume in trees that died in specific periods, also expressed on an 

annual basis. This mortality volume includes their volume at the start of the period 

and any growth added before death. (3) The sum of net PAI and PAM is the gross 

annual change in stand volume in specified periods.
Comparative Effects of Urea Fertilizer and Red Alder in a Site lll, Coast Douglas-Fir Plantation in the Washington Cascade Range

15

Table 5—

Mean annual change (years 10 to 27 after planting) in mean height, and stand basal area and volume on fertilized vs. nonfertilized plots, 

by speciesa

Douglas-fir All species

Stand and Height Gross basal area Gross volume Net volume Gross volumetreatmentb Treatment Observed Adjustedc Observed Adjustedc Observed Adjustedc Observed Adjustedc Observed Adjustedd Number ------- Feet ------ Square feet per acre ------------------------------ Cubic feet per acre ---------------------------------Fertilizer: 0 N 1 3.02 3.01 b 9.33 9.24 305 305 298 294 326 – 200 N 5 3.17 3.18 a 11.43 11.88 368 388 364 388 376 

–

 Difference .15 .17 2.10 2.64 60 83 66 94 50 

–

Fertilized trees (105) (106) (122) (129) (119) (127) (122) (132) (115) 

–

(percentage of nonfertilized height)Mixed (RA per acre):  20 2 2.94 2.94 9.05 8.86 301 292 284 273 319 

–

 40 3 2.99 2.98 10.35 10.08 336 325 334 321 350 

–

 80 4 3.08 3.07 8.45 8.39 286 282 277 272 334 

–

a Within columns, adjusted means with different letters are significantly different (p 

< 0.10); see table 6. b Pounds of N per acre (in nearly pure Douglas-fir plots) or number of red alder per acre (in mixed-species plots). c Adjusted means from covariance analysis, where covariate = postthinning mean height, basal area, or volume per acre, respectively. 

The linear relation was significant (p 

< 0.10) for each variable. d Covariance adjustment of all species volume growth is inappropriate because the covariates were affected by treatment. ANOVA was used
RESEARCH PAPER-PNW-RP-565

16

Table 6—Degrees of freedom, mean squares, and p values from analyses of covariance of the mean annual changes (years 10 to 27 after planting) in mean height and stand basal area and volume of conifers  Mean annual change, in years 10-27 after planting, conifers only

 Volume

Mean height Gross basal area Gross Net

Source of variation D.f. MSE p MSE p MSE p MSE pCovariatea 1 0.0025 0.574 10.382 0.117 12,553 0.082 1,8256 0.081Contrasts:b 4  1 = Fertilizer (1) .0347 .068 10.025 .123 9,766 .116 12,421 .135 2 = Linear (1) .0073 .348 .515 .698 332 .746 196 .835 3 = Lack of fit (2) .0058 .484 1.319 .672 898 .744 1,509 .710Error 6 .0071 – 3.108 2,891 – 4,160 –

Total 11 

a Covariate = postthinning height, basal area, or volume, respectively. b Contrast 1 = fertilized vs. nonfertilized; contrast 2 = linear fit to adjusted means of treatments 1 through 4; contrast 3 = lack of fit (to linear) of treatments 1 through 4.
Comparative Effects of Urea Fertilizer and Red Alder in a Site lll, Coast Douglas-Fir Plantation in the Washington Cascade Range

17

Trends of gross PAI—In both pure and mixed stands, mean gross PAI of conifer 

volume averaged greatest in years 23 through 27 after planting (table 7). In the first 

3 years after fertilization, mean PAI of fertilized plots was similar to that of nonfertilized, but thereafter was consistently more than that of nonfertilized plots. We 

infer about a 3-year delay in response to fertilization. Within mixed stands, gross 

PAI of red alder volume was not proportional to number of retained alder; doubling 

alder density from 20 to 40 TPA had little effect on gross PAI, probably because 

alder were much smaller in the 40-TPA plots. Largely because of differences in tree 

size, doubling alder density from 40 to 80 increased PAI by a factor of 3 to 4 (table 

7). Volume PAI of associated conifers increased over time and, not surprisingly, 

totaled most with 40 alders per acre, the treatment in which alders averaged the 

smallest mean size and the least growth (table 7).

Gross and net PAI in the 17-year period—In the 17-year period after thinning and 

fertilizing, adjusted gross and net volume PAI of Douglas-fir were both significantly 

related to initial volume of Douglas-fir (p = 0.08; table 6). We used covariance to 

adjust observed treatment means to a common (average) postthinning stand volume 

(table 5). Adjusted gross and net PAI of Douglas-fir on fertilized plots averaged 

greater than PAI on nonfertilized plots, but differences were statistically nonsignificant (p = 0.12 and 0.14, table 6). Adjusted gross volume growth was increased 

by 27 percent on fertilized plots (table 5). Gross and net volume growth of 

conifers was least where admixed with 80 TPA of red alder, but most with 40 alder 

per acre. A linear fit of adjusted means, however, was nonsignificant (p = 0.75 for 

gross PAI and 0.84 for net) as was a lack of fit (table 6). Thus, these current data 

provide no reliable evidence that alder admixture increased volume growth of 

associated conifers.

Periodic annual mortality (PAM)—Over the 17-year period, volume of dead 

Douglas-fir annually averaged 4 to 10 ft3 per acre in the pure plantation (less than 4 

percent of gross PAI) compared to 1 to 6 percent of gross PAI in the mixed stands. 

As expected, mortality of Douglas-fir averaged greater with 20 TPA of red 

alder than in the 40- and 80-TPA admixtures (table 7). Mortality of red alder was 

limited mostly to the highest density of alder admixture.
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Table 7—Observed mean gross annual increment (PAI) and mortality (PAM) in stand 

bole volume, by treatment, age, and species 

Stand and Treatment PAI Plantation age (inclusive years)

treatmenta number PAM 11-13 14-16 17-22 23-27 11-27

 ---------------- Cubic feet per acre ---------------

Douglas-fir:

Pure– 

 0 N 1 PAI 188 219 355 376 308

 PAM 0 8 14 18 10

 200 N 5 PAI 186 250 433 472 368

 PAM 2 8 3 4 4

 Mixed– 

 20 2 PAI 179 226 354 355 301

 PAM 1 8 8 42 17

 40 3 PAI 182 250 396 410 336

 PAM 4 0 1 5 2

 80 4 PAI 171 202 326 360 286

 PAM 2 11 8 15 10

 Red alder:

 Mixed– 

 20 2 PAI 15 16 17 9 17

 PAM 0 0 1 0 1

 40 3 PAI 10 12 17 14 14

 PAM 0 0 0 0 0

 80 4 PAI 42 49 56 41 48

 PAM 0 0 0 6 2

a Pounds N per acre (in pure-conifer plots) or number of red alder per acre 

(in mixed-species plots).
Comparative Effects of Urea Fertilizer and Red Alder in a Site lll, Coast Douglas-Fir Plantation in the Washington Cascade Range

19

Yield at Plantation Age 27

Cumulative live-stand yield of Douglas-fir at age 27 after planting declined slightly 

with increasing density of admixed alder (fig. 4); the reduction was not significantly 

related to increasing numbers of retained alder (p = 0.84, table 6). Clearly, size of 

retained alder also contributed to the effects of alder on associated Douglas-fir 

(fig. 5). Decrease in Douglas-fir yield was offset by corresponding increase in 

alder volume, so that yield of combined species was similar across all densities of 

red alder admixture (fig. 6). Among the three mixed-stand treatments, yield of red 

alder was roughly proportional to alder density (fig. 4). 

Red alder (number per acre)

0 20 40 60 80 100

Mean yield (cubic feet per acre)

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

Conifer, nonfertilized

Conifer, 200 lb N per acre

Red alder

All species

Figure 4—Mean live-stand volume at plantation age 27, by species and treatment.

Yield of combined 

species was similar 

across all densities of 

red alder admixtue.
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20

Figure 5—Red alder transplanted from a local source proved more competitive than those from 

an off-site, coastal source. Note the relative size of the red alder (ribboned) of local source in 1994 

(20 growing seasons after transplanting): (A) 20 alder per acre (treatment 2); (B) 40 alder per acre 

(treatment 3).

A

B
Comparative Effects of Urea Fertilizer and Red Alder in a Site lll, Coast Douglas-Fir Plantation in the Washington Cascade Range

21

Discussion

Effect of Fertilizing

Changes in mean height, basal area, and volume of conifers on fertilized plots exceeded those on nonfertilized plots, although some differences were marginally significant (p = 0.07 to 0.14). For example, 17-year adjusted gross volume growth was 

increased by about 27 percent after 200 lb N per acre was applied. This surprisingly 

large response exceeded our expectation (20 percent) and totaled about 1,410 ft3per 

acre. Because volume of mortality was less on fertilized plots (smaller trees died), 

gain in net volume averaged 32 percent or 1,600 ft3 per acre in the 17-year period 

after thinning and fertilization. Key statistics that summarize response of 

Douglas-fir after covariance adjustment (from tables 5 and 6) follow:

Red alder volume (cubic feet per acre)

0 200 400 600 800 1,000 1,200

Conifer volume (cubic feet per acre)

3,500

4,000

4,500

5,000

5,500

6,000

6,500

7,000

Conifer nonfertilized

Coastal alder

Local alder

2

Figure 6—Conifer yield related to red alder yield at year 27, by alder source. Note the two overlapping plotting points 

indicated by “2.”

Changes in mean height 

basal area, and volume 

of conifers on fertilized 

plots exceeded those on 

nonfertilized plots.
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22

In contrast to this strong response to N fertilization at this site III location, no 

response to the same 200 lb N per acre treatment occurred at our site II plantation 

near coastal Oregon (Miller and others 1999).

Douglas-fir mortality averaged 50 TPA (16 percent) on both nonfertilized plots 

and fertilized plots. The larger variation in crop tree losses among the three nonfertilized plots weakens any inference about the effect of fertilization on mortality. 

The general reliability of our mortality data is uncertain because our plots are small 

and replications few.

Effect of Increasing Numbers of Red Alder

In nearly pure Douglas-fir plots (treatments 1 and 5), 17-year mortality of Douglasfir averaged 50 TPA; no hemlock died. The surprisingly large number of trees lost 

on one plot (140 TPA) clouds comparisons with Douglas-fir losses in mixed stands 

(fig. 2). The relatively large loss of Douglas-fir associated with 20 TPA of alder (70 

TPA or 3 Douglas-fir per alder) compared to that with the doubled alder density of 

40 TPA (25 TPA or 0.6 Douglas-fir per alder) is not surprising. Residual alder on 

the 20-TPA treatment initially averaged larger than those in the 40-TPA treatment 

(1.6 vs. 0.7 ft3 per alder tree) and much taller than associated Douglas-fir (30 vs. 5 

percent taller).

To comprehend spatial relations in our mixed-species stands, one should consider between-tree distances. With 300 TPA, Douglas-fir spacing averages about 13 

ft. With 80 red alder per acre, alder spacing averages 23 ft. Where average spacing 

exists, each red alder adjoins four Douglas-fir. We suspect that Douglas-fir nearest 

vigorous red alder are most likely to be damaged or killed. At 80-tree alder retention, it is likely that individual Douglas-fir eventually will be competing with several red alder, especially if a rapid-growing source of red alder were used. At this 

study area, the local source of transplants grew faster and proved more competitive 

to associated Douglas-fir than did the coastal source. Because more Douglas-fir 

trees died when associated with increasing numbers of red alder, we infer that red 

alder densities of 80 TPA and greater reduced yield of Douglas-fir at this location.

 Treatment Difference

Adjusted annual change Fertilized Nonfertilized Absolute Percent p

Mean height (ft) 3.18 3.01 0.17 5.5 0.07

Gross basal area (ft2 per acre) 11.88 9.24 2.64 28.6 .12

Volume (ft3 per acre): 

Gross 388 305 83 27.0 .12

Net 388 294 94 32.0 .14
Comparative Effects of Urea Fertilizer and Red Alder in a Site lll, Coast Douglas-Fir Plantation in the Washington Cascade Range

23

Results in this Washington Cascade plantation contrast with those from a 

coastal Oregon plantation of similar age, but more fertile soil. At that site II coastal 

location, growth was not stimulated by N fertilization, but losses of associated 

Douglas-fir clearly increased as more red alder were admixed (Miller and others 

1999). At that coastal location, our equation for predicting Douglas-fir losses in the 

17-year period of observation was Y = 19 + ATPA (0.719), where ATPA is red alder 

TPA. Accordingly, an average of 76 Douglas-fir trees per acre could be expected to 

be lost in 17 years at that coastal site if 80 red alder were retained with 300 Douglas-fir per acre. Tree losses reduced stand yields (28 years after planting); average 

yield of Douglas-fir was greatest in pure stands and gradually declined as alder 

density increased. Results at that coastal location supported our second working 

hypothesis: by increasing alder density, negative effects of red alder on associated 

Douglas-fir will gradually offset the potentially positive effects of increased N and 

improved organic matter status.

Results at our Cascade site also contrast with those of an earlier investigation 

at a yet poorer (site V) location in the Cascade Range in southwest Washington. 

At that very N-deficient location, a 1929 Douglas-fir plantation grew taller and 

produced more bole volume in response to both N-fertilizer (Miller and Tarrant 

1983) and a nearly 2:1 alder-fir planting mix (Miller and Murray 1978). This large 

complement of planted alder explained the 30-percent increase of N (about 1,000 lb 

N per acre) in the duff and mineral soil of the mixed stand compared to the adjacent pure Douglas-fir stand about 30 years after planting (Tarrant and Miller 1963). 

Not only were benefits to Douglas-fir of improved N status at that location clearly 

established, but competitive effects of alder were minimized because (1) alder was 

interplanted 4 years after the Douglas-fir were planted and (2) the newly planted alder (a nonlocal, low-elevation source) were severely frost damaged in the first year. 

Consequently, at plantation age 58, yield of Douglas-fir in the mixed plantation 

exceeded that in the pure Douglas-fir plantation, and combined yield of both species 

was nearly twice that of the pure stand (Miller and Murray 1978). 

As a followup to that early study, Weyerhaeuser Co. scientists compared nonlocal vs. local sources at our site III location. Both sources were transplanted as 

2-year-old volunteer seedlings into this 3-year-old Douglas-fir plantation. Through 

year 7, there was no apparent advantage in using nonlocal red alder to reduce competition with Douglas-fir at this location (Murray and Miller 1986). Our continued 

measurements through year 27, however, clearly show that trees from the local 

source eventually grew much more in height than those from the nonlocal source. 

Consequently, survival and growth of associated Douglas-fir were reduced more by 

the local source of admixed alder.

Survival and growth of 

associated Douglas-fir 

were reduced more by 

the locally derived admixed alder than by the 

nonlocal alder.
RESEARCH PAPER-PNW-RP-565

24

Implications for Silvicultural Researchers 

Despite strong response to N fertilizer at our Cascade site, the benefits of red alder 

are not readily inferred. For several reasons, we have a less reliable test of our hypothesis at the Cascade site: smaller plots, two instead of three replications, and the 

varied size of admixed alder (in part related to delayed planting of two contrasting 

alder sources instead of retaining volunteer alder as at the site II study area.

The experimental design of both our trials does not permit separating the 

effects of increasing total stand density from increasing proportion of red alder. 

Our adding 20, 40, and 80 TPA of red alder to a constant 300 TPA of Douglas-fir 

changes both total stand density and proportion of red alder (7, 13, and 27 percent, 

respectively). This additive design limits understanding of competitive relations 

in mixed stands (Harper 1977, Hibbs and DeBell 1994, Peterson and others 1996). 

Two sets of more discriminating substitutive (replacement) designs have been 

installed in the Northwest to quantify competing relations within red alder/Douglas-fir mixtures. In one of these (Shainsky and others 1994), plots are small and not 

adequate for comparing stand yields beyond the sapling stage of development. The 

other set installed in the mid-1990s at six locations by the Hardwood Silviculture 

Cooperative at Oregon State University has suitably large plots2, but results remain 

unpublished.

Further testing of our working hypotheses should (1) consider using a replacement design and (2) use much larger plots for both treatment areas and tree measurement plots. The 0.2-acre treatment areas and 0.1-acre measurement plots at our 

Cascade site were too small, especially with only two to three replications per treatment. Small plots are inadequate for characterizing stand dynamics occurring in 

mixed-species stands. “Treatment integrity” (Lipsey 1990) also proved difficult to 

attain because size of the retained alder differed greatly among the plots and treatments. For example, alder in the 20-TPA treatment averaged 4.6 in d.b.h. compared 

to 3.4 in d.b.h. in the 40-TPA treatment. Consequently, application of our alder 

treatment was much more variable than our application of urea. Installing larger 

plots is a likely solution to this potential problem, as well as to that caused by the 

large variation in mortality that we experienced among our control plots; this loss 

of “control group integrity” also complicated interpretation of our results.

2

 Hibbs, D. 2001. Personal communication. Professor, Department of Forest Science, 

Oregon State University, Corvallis, OR, 97331-5752.
Comparative Effects of Urea Fertilizer and Red Alder in a Site lll, Coast Douglas-Fir Plantation in the Washington Cascade Range

25

Implications for Land Managers

Numerous N-fertilization trials in coast Douglas-fir forests of the Pacific Northwest demonstrate where N fertilization is likely to increase tree growth. Similar 

areas could be candidates for using N-fixing species like red alder as an alternative 

source of N. Conversely, where a lack of response to N-fertilizer is predicted, we 

infer that N fixed by bacteria associated with alder roots also would be unlikely to 

stimulate growth of associated Douglas-fir. As noted by Binkley (1992), only on 

N-limited sites is growth of associated non-N-fixing trees in mixed stands generally 

greater than in pure stands.

We anticipate that growth of Douglas-fir on N-deficient soil would increase 

from either N-fertilizer or N fixed by red alder. To enhance N status, however, 

alder must survive in dominant or codominant positions so that photosynthesis can 

supply energy for both alder growth and associated N-fixing actinomycetes. Where 

moisture or temperature relations are less favorable to red alder (Cascades sites vs. 

coastal sites), we observed that red alder height and crown growth was less vigorous, hence, less likely to overtop or damage nearby Douglas-fir. The ratio of Douglas-fir to red alder heights is a useful measure of their relative development. Among 

plots at our site III, Cascade study area, Douglas-fir averaged 19 to 28 percent taller 

than associated alder (20 to 80 TPA) at plantation age 27 years. At 26 years on the 

coastal Oregon site, Douglas-fir averaged 0 to 14 percent taller, depending on alder 

density (Miller and others 1999).

Although we deliberately tested 80 red alder per acre retained with about 300 

Douglas-fir per acre at plantation age 10, we suspect this is too many for an operational treatment designed to enhance Douglas-fir yields. Even at our lower quality 

Cascade site, which is less favorable to red alder growth, too many Douglas-fir were 

damaged or killed. With close spacing of admixed alder, individual Douglas-fir are 

likely to be mechanically damaged or suppressed by neighboring alder or, conversely, individual red alder are likely to be dominated by Douglas-fir before contributing N-rich organic matter to the stand. Delaying planting or using an off-site source 

of planted alder are other ways to reduce competition. Periodic assessment of crown 

damage in mixed stands is necessary to ensure timely reductions of stocking of red 

alder or Douglas-fir or both.
RESEARCH PAPER-PNW-RP-565

26

Conclusions

• Fertilization with 200 lb N per acre as urea at this site enhanced volume growth 

of Douglas-fir by about 30 percent. 

• Alder size as well as numbers can affect survival and growth of associated 

Douglas-fir. Adding 80 red alder per acre likely reduced numbers, growth, 

and yield of associated Douglas-fir.

• Additional comparisons of fertilizer and red alder for enhancing growth of 

Douglas-fir are needed at other locations, especially those with known 

N deficiency.

Acknowledgments

We thank numerous people of the Olympia Forestry Sciences Laboratory and 

Weyerhaeuser Co. for helping install and thin study plots and measure trees. We 

thank Steve Duke, Tim Max, and Karen Hyer for biometrics advice, Rod Meade for 

coordinating contractors, and Robert O. Curtis, David Hibbs, and William Scott for 

reviewing our draft manuscript.

Metric Equivalents

1 inch (in) = 2.54 centimeters

1 foot (ft) = 0.3048 meter

1 mile (mi) = 1.609 kilometers

1 square foot (ft2) = 0.0929 square meter

1 cubic foot (ft3) = 0.0283 cubic meter

1 acre (ac) = 0.4047 hectare

1 square foot per acre = 0.2296 square meter per hectare

1 cubic foot per acre = 0.06993 cubic meter per hectare

1 pound (lb) = 453.59 grams

1 pound per acre (lb/acre) = 1.12 kilograms per hectare

1 tree per acre = 2.47 trees per hectare

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Pacific Northwest Research Station

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Pacific Northwest Research Station

P.O. Box 3890

Portland, OR 97208-3890
U.S. Department of Agriculture

Pacific Northwest Research Station

333 SW First Avenue

P.O. Box 3890

Portland, OR 97208-3890

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