55

— Red Alder: A State of Knowledge —

The Value of Red Alder as an 

Integrated Pest Management Tool

 For Controlling Weevil Damage 

to Sitka Spruce

Lyle Almond2

2Lyle Almond is a forester with the Makah Tribe Forestry Department, P.O. Box 116, Neah Bay, WA 98357, email: lylealmond@olypen.com

Abstract

Among the numerous benefits they provide, mixedspecies forests are often extolled for their greater resilience 

than monocultures to disease and insect outbreaks. Evidence 

of this resistance mechanism was investigated by observing 

variation in the intensity of leader damage inflicted by 

spruce weevils (Pissodes strobi [Peck] [Coleoptera: 

Curculionidae]) on Sitka spruce (Picea sitchensis [Bong.] 

Carr.) terminals under a broad gradient of canopy closure 

densities in stratified mixtures of red alder (Alnus rubra

Bong.) and Sitka spruce. Analysis using all tree species 

failed to reveal a substantial correlation between weevil 

damage and canopy closure, but a very high level of 

association emerged when all tree species except red alder 

were removed from the regression model. Further analysis 

of the whiplash damage that upper stratum red alder 

imposes on Sitka spruce terminals attempting to emerge into 

the upper canopy resulted in predictably greater incidence 

of terminal damage at higher densities of red alder crown 

This paper was published in: Deal, R.L. and C.A. Harrington, eds. 2006. Red alder—a state of knowledge. General Technical 

Report PNW-GTR-669. Portland, OR: U.S. Department of Agriculture, Pacific Northwest Research Station. 150 p.

closure. As an attempt to address these two inextricably 

linked concerns, the hypothesis of this research analysis is 

that, at some ideal spacing of overstory red alder, a balance 

can be achieved whereby minimally acceptable levels of 

weevil damage occur without drastically suppressing leader 

growth. Superimposed linear trends of increasing whiplash 

damage and decreasing weevil infestation intersected at 

a red alder canopy closure value of 88% closure. This 

observational study of pest-host interactions under a broad 

range of canopy densities establishes an optimal range of 

threshold values for managing canopy closure that could 

be useful in managing mixed stands of red alder and Sitka 

spruce for better commodity production and ecosystem 

diversity.

Keywords: integrated pest management, Sitka spruce, Alnus 

rubra, red alder, spruce weevil, mixed-species silviculture.
56

Introduction

Sitka spruce (Picea sitchensis [Bong.] Carr.) is a 

vital component of the Pacific Northwest’s temperate rain 

forest, set in a unique coastal maritime zone with growing 

conditions which maintain such high rates of productivity 

by Sitka spruce that it reaches the largest size of any spruce 

on Earth, making it the fourth tallest tree in the world (Van 

Pelt 2001).

Sitka spruce and its primary pest, the spruce-tip weevil 

(Pissodes strobi [Peck] [Coleoptera: Curculionidae]), have 

successfully coexisted over millennia. Since the advent of 

single-species plantation management in the 20th century, 

the spruce-tip weevil population has exploded to epidemic 

proportions, reaching such levels of infestation that 

regeneration commitments have begun shifting away from 

Sitka spruce production altogether. However, avoidance 

cannot be considered an integrated pest management 

approach, especially when the tree species in question is as 

densely distributed and as ecologically and economically 

valuable as Sitka spruce.

An integrated pest management (IPM) approach is 

necessary to allow the continued use of Sitka spruce for 

reforestation and to protect those stands already established, 

but prone to continued damage from weevil attack. With 

the possible exception of avoidance, the range of available 

control options has met with limited success and lacks 

sufficient coordination. At present, few, if any, practical 

options are available to minimize weevil damage in 

susceptible stands. Avoidance of planting spruce obviously 

works, but at the expense of encouraging lesser valued 

trees on prime spruce sites. While a successful IPM system 

should be predicated on studying the biology, population 

dynamics, genetics, and impacts of the weevil and its host, 

the overall objective of all lines of research should be to 

develop forest management practices that will produce and 

maintain productive forest stands on the riparian floodplain 

and terrace sites that have proven to be most ecologically 

suited for Sitka spruce.

The resurgence of heightened research interest in 

the ecology and silviculture of mixed-species forest 

communities has recognized, among numerous benefits they 

provide, that forest mixtures often exhibit greater resistance 

than monocultures to disease and insect outbreaks (Kelty 

1992, Perry 1994, Smith et al. 1997). Evidence of this 

resistance mechanism is exemplified by natural stands of 

even-aged red alder (Alnus rubra Bong.) and Sitka spruce, 

a stratified mixture commonly found growing in the riparian 

floodplains of coastal river systems in the Pacific Northwest. 

Under the expansive canopies of mature red alder growing 

along river bottoms and streambanks, shade-tolerant Sitka 

spruce trees are generally observed to be virtually free 

of weevil damage. The use of deciduous overstory trees, 

principally red alder, in mixture with Sitka spruce in the 

understory could have great value in an integrated pest 

management program to deter spruce-tip weevil invasion. 

Initial research trials in British Columbia in a shade versus 

no-shade study design have shown encouraging evidence 

that the shade provided by red alders overtopping spruce 

trees in a nurse-tree shelterwood system can significantly 

reduce weevil damage. (McLean 1989). This research 

analysis elaborates upon those findings by investigating the 

response in spruce-tip weevil damage intensity to a wide 

range of red alder overstory shade levels.

Dense overstory canopies of red alder may substantially 

reduce levels of weevil infestation, yet those same canopies 

may also impose barriers impeding the growth rate of 

terminal development as spruce trees attempt to emerge 

through the red alder canopy. Whiplash damage to Sitka 

spruce terminals abraded by red alder branches breaks down 

epinastic control, causing primary meristem development 

of the terminal bud to produce several small shoots that 

each attempt to assume apical dominance. This results in 

arrested height growth, unduly extending the rotation period 

or allowing other tree species to outcompete spruce trees. 

Whiplashed spruce trees that do emerge above the red alder 

canopy expose this cluster of terminal shoots to the risk of 

multiple weevil attacks.

This research investigation was partially intended 

to construct a predictive tool as a means to assist forest 

managers in selectively cultivating a red alder canopy as an 

integrated pest management approach in protecting Sitka 

spruce from weevil attacks. 

The primary research goal in this research study was 

to determine what optimal level of red alder canopy closure 

will produce the greatest reduction in weevil attack without 

overly suppressing growth of Sitka spruce due to whiplash 

damage. By using a regression model approach, this 

research investigation also provided a look at the sensitivity 

with which changes in the level of spruce weevil activity 

might correspond with differences in the level of overstory 

canopy closure.

Weevils respond directly to changes in the level of 

ambient air temperature, not to canopy closure per se, 

preferring to oviposit at high temperatures. Although 

weevils will oviposit after emerging from hibernation and 

before consistently warm weather, they do not do so readily 

until temperatures above 60°F become common in British 

Columbia (Silver 1968). In southwestern Washington, 

egg-laying activity commenced at approximately 75°F and 

peaked at 90° F (Gara et al. 1971). The air temperature for 

maximum oviposition in the field was 84.9°F (Overhulser 

1973). Maximum rates of oviposition have also been 

recorded to occur between 50.5° to 79.3°F (Holsten 1977). 

McMullen (1976) demonstrated that the accumulated heat 

required for brood development on Vancouver Island, from 

egg to emergence, is at least 888-degree days above 45°F 

(Holsten 1977). Oviposition most commonly occurs only at 

bark temperatures between 77°F and 84°F associated with 
57

20% to 35% relative humidity (Beleyea and Sullivan 1956, 

Gara et al. 1980). 

In conditions where heat accumulation is insufficient 

for completion of larval development, a Sitka spruce 

stand cannot sustain viable weevil populations (Holsten 

1977). Regulation of understory microclimate temperature 

is clearly a determining factor in the success of any 

silvicultural approach to arrest spruce weevil damage.

Methodology and 

Research Models

Do increasing levels of red alder canopy closure 

influence the level of spruce weevil damage to Sitka spruce 

growing in the understory? If so, is red alder the only tree 

capable of producing this nurse-tree function? Is there some 

ideal level of canopy closure that can optimize reductions in 

the hazard of weevil damage as well as the whiplash damage 

that these canopies may also inflict on developing spruce 

terminals? To answer these questions, a set of hypotheses 

was formulated:

Research hypothesis 1: Sitka spruce weevil damage 

diminishes under canopies of red alder and continues to 

decrease along a gradient sensitive to the increasing density 

of canopy closure.

Research hypothesis 2: reductions in spruce weevil damage 

are uniquely associated with the presence of a red alder 

canopy in the overstory; the overstory presence of other 

coniferous canopies has no significant effect on the intensity 

of spruce weevil damage.

Research hypothesis 3: at some ideal spacing of overstory 

red alder, a balance can be achieved whereby minimally 

acceptable levels of weevil damage occur without 

drastically suppressing leader growth. 

Field data collection 

Twenty circular quarter-acre plots were established 

in a 59-acre stand at Merrill and Ring’s Pysht Tree Farm 

located on the north coast of the Olympic Peninsula in 

western Washington. The stand was planted in 1986 with 

Sitka spruce. During the intervening years, red alder seeded 

naturally and became a vigorous component in the stand. 

Canopy closure was calculated by measuring and, 

in many cases, estimating with Schumacher equations, 

the diameters, heights, and crown lengths of all trees in 

each plot. This data was then entered into least crown 

width (LCW) equations developed for each tree species 

(Hann 1997). The radius of this width was then squared 

and multiplied by pi to calculate each tree’s horizontal 

crown area. Summation of tree crown areas was compiled 

individually for each species (reduced model) and for all 

species (full model) in each plot. Percent canopy closure 

was calculated as the ratio of square-foot canopy closure 

within the total square footage of the quarter-acre plot 

(10,890 ft2). 

With the assistance of binoculars, each Sitka spruce 

tree was evaluated for leader deformities caused by weevil 

damage. A four-point damage rating system was developed 

to identify the morphological characteristics of the damaged 

terminal. This rating system simulates a chronological 

sequence of weevil damage over time, whereby a shepherd’s 

crook indicates most recent damage (one year), a broken top 

indicates two to three years since weeviling, one or more 

upturned laterals suggest three to four years since weeviling, 

and a forked top represents the longest time since weeviling, 

approximately four or more years.

Data analysis 

To compare the relative benefits of red alder to other 

tree species canopies in weevil protection, a full regression 

model and a partial model were both constructed. Included 

in the full model were the combined canopy closure values 

of red alder, western hemlock, Douglas-fir, and Sitka spruce 

itself (weeviled and unweeviled) that were measured in 

each plot. All species except red alder were eliminated to 

formulate the partial model.

The response variable of spruce weevil damage was 

determined by comparing the ratio of weeviled spruce trees 

to all spruce trees, resulting in a percentage of spruce weevil 

damage for each plot. 

Polynomial equations, based on a method of curve 

fitting (Sit and Poulin-Costello 1994), were calculated using 

SPSS© for constructing a set of linear regression models 

to best explain the change in weevil damage under various 

levels of canopy closure, one for total canopy closure (full 

model), the other for red alder alone (reduced model).

Results

Weevil damage

Polynomial regression analysis comparing percent 

canopy closure with percent of Sitka spruce damaged by 

spruce-tip weevils resulted in the graphs and equations 

displayed in figures 1 and 2.

Preliminary analysis of total canopy closure by all tree 

species within each quarter-acre research plots, including 

mixtures of red alder, western hemlock, Douglas-fir, and 

Sitka spruce itself (weeviled and unweeviled), revealed 

a very poor relationship (r2=0.0626) to the percentage of 

weevil damage.

When all other tree species were removed from 

the model except red alder, a conspicuous well-defined 

relationship emerged between levels of red alder canopy 

closure and the intensity of weevil damage, indicating that 
58

73% of the change in weevil infestation could be explained 

solely by the amount of red alder canopy shade present in 

the overstory (r 2=0.7301). ANOVA testing confirmed a very 

high level of significance to red alder canopy closure as a 

predictor of weevil damage (p<0.001).

Whiplash damage

The percent of spruce trees whiplashed in each plot 

was compared to the density of red alder canopy. The 

slope of this linear trend conforms to the expectation that 

the incidence of whiplash damage rises with an increasing 

density of red alder canopy closure (fig. 3).

Balancing levels of weevil damage 

vs. whiplash damage

Is there a balance that can be achieved between 

minimally acceptable levels of weevil damage and spruce 

leader whiplash damage in a nurse-tree shelterwood 

system? As an attempt to address these two inextricably 

linked concerns, the linear trend of the impacts made by 

various levels of red alder canopy closure on weevil damage 

reduction was superimposed with the linear trend of height 

growth suppression measured as a function of whiplash 

damage. The intersection of these two lines in the resulting 

model indicated that a threshold target of 88% red alder 

canopy closure would best optimize control of both weevil 

attack and whiplash damage (fig. 4).

Relative density of unweeviled 

Sitka spruce

Another finding was revealed by graphical comparison 

of the relative stem density by various species in each 

plot. Under a broad range of different species mixtures, 

including Douglas-fir, red alder, western hemlock, and 

weeviled spruce, the stocking level of unweeviled spruce 

trees consistently remained at around 25% of the plot stem 

density.

Figure 5 shows three representative distributions of 

plot species composition; in all three, unweeviled Sitka 

spruce comprised about 25% of the plot’s stem density. In 

Plot MR13 (fig. 5A), red alder comprised half of the stem 

density; the remaining plot area was shared equally between 

weeviled and unweeviled spruce. Plot MR07 (fig. 5B) was 

comprised of red alder, weeviled and unweeviled spruce, 

as well as western hemlock and Douglas-fir. Even with 

the introduction of a more diverse tree species mixture, 

unweeviled spruce maintained a relative density of 25%. 

Under ideal conditions for managing an unweeviled Sitka 

spruce stand, red alder comprised 75% of Plot MR19 (fig. 

5C), leaving only 25% of the growing space for unweeviled 

spruce.

Conclusion

The high coefficient of determination produced by 

the linear regression model constructed for this research 

analysis illustrates the sensitivity with which spruce weevil 

activity changes with corresponding shifts in the level of red 

alder canopy closure. By analyzing this phenomenon under 

a broad continuum of canopy closure levels, compelling 

evidence revealed the enormous—and unique—impact 

that red alder canopy shade exerts on the reduction in 

spruce weevil damage. As a natural resistance mechanism 

to facilitate healthy spruce leader development, red alder 

creates a thermal barrier that significantly reduces spruce-tip 

weevil infestation levels on young Sitka spruce by lowering 

the high ambient temperatures required by the spruce weevil 

to complete larval development and carry out much of its 

subsequent life history.

To optimize reductions in spruce terminal damage 

by maintaining a threshold range of 88% canopy closure, 

a carefully executed red alder management plan must be 

pursued, both in initial stem density and in intermediate 

density management, particularly during that period in 

stand development when Sitka spruce is most susceptible 

to weeviling, approximately between seven and twentyFigure 1—Comparison of weevil damage to the amount of total canopy 

closure.

Figure 2—Comparison of weevil damage to changing levels of red alder 

canopy closure.

POLYNOMIAL REGRESSION EQUATION

y = -0.0985x2 + 0.2359x + 0.3684

r

2

 = 0.0626

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 50% 100% 150% 200% 250% 300%

Total canopy closure (%)

) %( ega madli vee W

POLYNOMIAL REGRESSION EQUATION

y = 0.3223x3 - 0.9529x2 + 0.3946x + 0.5407

r

2

 = 0.7301

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 20% 40% 60% 80% 100% 120% 140% 160% 180% 200%

Red alder canopy closure (%)

) %( ega madli vee W
59

five years of age. The 88% threshold recommended by this 

research analysis should be thought of as a range, not a fixed 

point, to provide direction for continued investigation of 

interspecies interactions between Sitka spruce, the spruce 

weevil, and red alder within the complex dynamics of 

mixed-species forest stand development.

Results from this retrospective, primarily descriptive, 

case study of spruce-tip weevil damage occurring in a single 

Sitka spruce stand leave many questions to be answered. A 

controlled and more sophisticated research effort must be 

undertaken to measure the effects of managing various red 

alder canopy closure levels to achieve crown characteristics 

that are most effective in facilitating healthy Sitka spruce 

leader development. 

Mixtures of red alder planted at, and thinned to, various 

densities in a range of proportions with Sitka spruce would 

provide the ideal opportunity to analyze variations in stand 

development patterns that maximize overall production by 

Sitka spruce and red alder. Empirical analysis or theoretical 

modeling could be carried out to establish what initial 

spacing of red alder seedlings will produce a canopy at 

88% closure at a given age of fifteen years. A review of 

the red alder data gathered for this research indicates that 

dominant 17-year old red alders have a largest crown width 

of approximately 25 feet, based on LCW equations devised 

by Hann (1997). Red alder seedlings could be interplanted 

with Sitka spruce at a relative density of 1:1 on a 12’ x 12’ 

spacing. As alders mature and overtop the spruce seedlings, 

their crowns may close at age 15.

 However, graphical comparison of the relative stem 

density by various species in each plot suggests that healthy, 

unweeviled Sitka spruce will consistently comprise no 

more than 25% of the stocking density in any mixture. 

The significance of this finding indicates that initial and 

intermediate density management of a spruce-alder forest 

stand should maintain a 1:3 balance of spruce-to-alder. 

Greater red alder density will accelerate height growth 

competition, promote better self-pruning, and force alder 

branching to assume a more acute upright angle, all of 

which are important canopy design features for reducing the 

hazard of whiplash damage to understory spruce.

Figure 3—Comparison of whiplash damage to the amount of red alder 

canopy closure. Figure 4—Superimposed trends of weevil and whiplash damage indicating 

best practices in managing red alder canopies to protect spruce leader 

development.

Figure 5—Relative density of component species in a weeviled spruce 

stand.

LOGARITHMIC REGRESSION EQUATION

y = 0.2708Ln(x) + 0.412

r

2

 = 0.5583

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 20% 40% 60% 80% 100% 120% 140% 160% 180% 200%

Red alder canopy closure (%)

) (% ega mad hsal pi h W

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 20% 40% 60% 80% 100% 120% 140% 160% 180% 200%

Red alder canopy closure (%)

) %( ega madli vee W

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

) %( ega mad hsal pi h W

% Weevil damage % Whiplash damage

88%

A

Red alder

Sitka spruce (weeviled)

Sitka spruce (unweeviled)

B

Red alder

Sitka spruce (weeviled)

Sitka spruce (unweeviled)

Western Hemlock

Douglas-fir

C

Red alder

Sitka spruce (weeviled)

Sitka spruce (unweeviled)
60

Successful innovation in mixed-species stand 

management is a critical requirement to meet increasing 

multiple resource demands for improved wood utilization 

and greater biodiversity on a shrinking land base. 

Developing silvicultural methods that are congruent with 

the ecological compatibility of red alder and Sitka spruce, 

such as the nurse-tree two-cropping system of management 

recommended by this study, will lead to increased 

commodity production in an environmentally acceptable 

manner.

Literature Cited

Almond, L.E. 2004. An analysis of mixed red alder-sitka 

spruce stand development patterns and structures that 

effectively reduce damage to sitka spruce by spruce-tip 

weevil. Unpublished M.S. Thesis. 113 p.

Belyea, R.M. and C.R. Sullivan. 1956. The white pine 

weevil: a review of current knowledge. Forest Chronicle 

32:59-67.

Gara, R.I., R.L. Carlson, and B.F. Hrutfiord. 1971.

Influence of some physical and host factors on 

the behavior of the Sitka spruce weevil, Pissodes 

sitchensis, in southwestern Washington. Annals of the 

Entomological Society of America, 64(2), pp. 467-471.

Gara, R.I., T. Mehary, and C.D. Oliver. 1980. Studies 

on the integrated pest management of the Sitka spruce 

weevil. 1980 Annual Report to the Sitka Spruce 

Committee, University of Washington College of Forest 

Resources, Seattle, 43 pp.

Hann, D.W. 1997. Equations for predicting the largest 

crown width of stand-grown trees in western Oregon. 

Research Publication 17. Forest Research Laboratory, 

Oregon State University, Corvallis, Oregon. 14 p.

Holsten, E.H. 1977. Control of the Sitka spruce weevil, 

Pissodes strobi (Peck) (Coleoptera:Circulionidae): a 

feasibility study. University of Washington, Seattle. 

Unpublished report.

Kelty, J.M. 1992. Comparative productivity of 

monocultures and mixed-species stands. The Ecology 

and Silviculture of Mixed-Species Forests Edited by M.J. 

Kelty, B.C. Larson, and C.D. Oliver. Kluwer Academic 

Publishers. Dondrecht. pp. 125-141.

McLean, J.A. 1989. Effect of red alder overstory on 

the occurrence of Pissodes strobi (Peck) during the 

establishment of a Sitka spruce plot. In: Insects Affecting 

Reforestation: Biology and Damage. Proceedings 

of a IUFRO Symposium Held on July 3-9, 1988, in 

Vancouver, B.C., Under the Auspices of the XVIII 

International Congress of Entomology (R.I. Alfaro and S. 

Glover, editors), Forestry Canada, Victoria, BC, pp. 167-

176.

McMullen, L.H. 1976. Spruce weevil damage, ecological 

basis and hazard rating for Vancouver Island. Canadian 

Forest Service, Informational Report BC-X-141. Victoria, 

BC.

Overhulser, D.L. 1973. Flight and seasonal activity of the 

Sitka spruce weevil, Pissodes strobi (Peck) (Coleoptera: 

Circulionidae), in western Washington. Masters Thesis, 

University of Washington, Seattle. 71p.

Perry, D. 1994. Forest Ecosystems. Johns Hopkins 

University Press: Baltimore. 649 p.

Silver, G.T. 1968. Studies on the Sitka spruce weevil, 

Pissodes sitchensis, in British Columbia. Canadian 

Entomologist. 100:93-110.

Sit, V. and M. Poulin-Costello. 1994. Catalogue of curves 

for curve fitting. Biometrics Information Handbook 

Series No.4; BC Ministry of Forests. 110 p.

Smith, D.M., B.C. Larson, M.J. Kelty, and P.M.S. 

Ashton. 1997. The Practice of Silviculture: Applied 

Forest Ecology., ninth edition. John Wiley and Sons: 

New York.. 537 p.

Van Pelt, R. 2001. Forest giants of the Pacific coast. 

Global Forest Society: San Francisco, 200 p.
61

1Kevin R. Brown is a consultant, K.R. Brown and Associates , Victoria, BC V8Z 4W2 Canada, email: kevinlouise@pacificcoast.net

2Paul J. Courtin is regional pedologist, BC Ministry of Forests, Coast Forest Region, 2100 Labieux Road, Nanaimo, BC V9T 6E9 Canada, email: 

paul.courtin@gems7.gov.bc.ca

— Red Alder: A State of Knowledge —

Experimental Studies of Red Alder Growth 

and Nutrition on Vancouver Island

Kevin R. Brown1 and Paul J. Courtin2

Abstract

Correlative field studies suggest that productivity 

in red alder stands on Vancouver Island increases with P 

availability. We have examined this in glasshouse and field 

experiments.

In the glasshouse, triple superphosphate (TSP) and 

dolomite were added to seedlings potted in soil from alluvial 

alder stands. Growth increased with P additions, less so 

in soils with high Bray-P levels and in higher-pH soils. 

Dolomite additions did not affect growth. We then added 

P and a blend of other elements to 10 young plantations 

on eastern Vancouver Island. P, as TSP, was added with or 

without other elements. The initial experiments applied 0, 

20, or 40 g P per tree to plantations ranging from 2-4 years 

old. The second set applied 0, 10, 20 or 30 g P per tree 

within one year of planting. Additions of P increased growth 

in two of five older and in all five younger plantations 

through three years. Adding other elements did not increase 

growth. 

This paper was published in: Deal, R.L. and C.A. Harrington, eds. 2006. Red alder—a state of knowledge. General Technical 

Report PNW-GTR-669. Portland, OR: U.S. Department of Agriculture, Pacific Northwest Research Station. 150 p.

We are now assessing the effects of P additions on stand 

development and carbon and nitrogen accretion at a slightly 

dry site. Seedlings were planted in fall 1999, with additional 

seedlings fill-planted in spring 2001. P (0, 15, 30 g per tree 

as TSP) was added in 2001; additional P has been added 

subsequently (cumulative additions of 0, 41, 88 g per tree 

through 2004) in order to maintain differences in foliar P 

concentrations. Through fall 2004, individual stem volumes 

have increased by 52 % (1999 cohort) or 164 % (2001 

cohort). Mortality increased in 2003, but is not yet related to 

P treatment. 

Growth of very young red alder on eastern Vancouver 

Island appears often limited by P deficiencies. How long 

effects of P additions persist and how responses vary with 

stand development remain unknown. 

Keywords: Growth, nutrition, phosphorus, Alnus rubra, red 

alder, Vancouver Island.
62

Introduction

Red alder (Alnus rubra Bong.) is the most widespread 

deciduous broadleaved tree species in low-elevation forests 

of coastal British Columbia. In B.C., as in Oregon and 

Washington, red alder was long considered an undesirable 

competitor with its coniferous associates. More recently, 

management for alder growth has been encouraged, in view 

of its rapid juvenile growth rates (Harrington 1990), ability 

to fix atmospheric nitrogen (N2) (Binkley et al. 1994), 

immunity to laminated root rot, Phellinus weirii (Thies and 

Sturrock 1995), suitability for a variety of valuable products 

(Tarrant et al. 1994) and contribution to habitat diversity in 

a conifer-dominated forest landscape. In B.C., low-elevation 

sites on eastern Vancouver Island and the Sunshine Coast 

have the greatest potential for intensive management of red 

alder (Massie et al. 1994). 

Increased harvest of mature alder has led to concerns 

that the current inventory of red alder will not meet 

projected demands (Rural Technology Initiative, Tarrant et 

al. 1994, Weyerhaeuser BC Coastal Group 2005). In B.C., 

the reliability of inventory data for red alder is questionable, 

but there is insufficient alder in the 1-20 year age class to 

supply projected demands without plantation establishment 

and intensified management. In anticipation of these needs, 

studies have been conducted in B.C. to assess the effects of 

site characteristics (Courtin 1992; Harrington and Courtin 

1994), proportions of conifers (Comeau et al. 1997, Courtin 

and Brown 2001), and spacing, thinning and pruning on tree 

growth and log quality (Courtin et al. 2002). 

The extent to which elemental deficiencies limit the 

growth of red alder is relatively unknown and unstudied 

(previously reviewed by Radwan and DeBell 1994), perhaps 

because alder fixes atmospheric N2 and deficiencies of N 

most often limit the growth of Pacific Northwest coastal 

forests (e.g., Weetman et al. 1992). Indeed, a significant 

amount of research has examined how much N is added to 

soil at the scale of stands (reviewed by Binkley et al. 1994) 

and watersheds (Compton et al. 2003). Growth responses 

to supply of phosphorus (P) have been of interest because 

P is required in relatively high quantities in all plants 

and requirements may be greater in N2-fixing species in 

association with nodulation and N2-fixation (Marschner 

1995). In earlier research, conducted in western Washington, 

additions of P to potted seedlings increased growth (Radwan 

and DeBell 1994) and either increased (Radwan and DeBell 

1994) or did not affect growth (Harrington and DeBell 

1995, Hurd and DeBell 2001) of young plantations. Alder 

planted on a site previously containing alder grew less and 

had lower tissue concentrations of P, calcium (Ca), and 

magnesium (Mg) than did alder planted on a site previously 

containing Douglas-fir (Cole et al. 1990, Compton et al. 

1997), possibly as a consequence of N2-fixation. 

The purpose of this paper is to review recent and 

ongoing experiments examining the effects of nutrient 

supply on growth of red alder on Vancouver Island. Our 

reasons for initiating these studies in the mid-1990s were: 

(1) site index of red alder on Vancouver Island and the 

adjacent mainland increased with foliar and available soil 

P concentrations on low-pH soils, even on sites classified 

as rich or very rich (Courtin 1992; Harrington and 

Courtin 1994); (2) experimental confirmation of nutrient 

deficiencies on such sites was lacking, except for a single 

study with potted seedlings (Binkley 1986) which inferred 

deficiencies of P and S in a single soil; (3) red alder was 

increasingly being replanted on sites from which mature 

alder stands had been harvested, raising the question of 

whether nutrient supply might limit the growth of a second 

consecutive rotation of red alder (e.g., Compton et al. 

1997); (4) there was increasing experimental evidence of P 

deficiencies in other tree species (Brown 2004; Brown and 

van den Driessche 2005, Zabek and Prescott 2001) on sites 

considered suitable for red alder. 

The long-term objectives of our studies have been 

to determine: (1) what mineral nutrients are deficient for 

red alder; (2) on what sites deficiencies are most likely 

and, presumably, correctable, and (3) long-term effects 

of additions of limiting nutrients on stand growth and 

site characteristics, particularly carbon and nitrogen 

accumulation. Such information should assist in site 

selection and refine the use of fertilization for red alder 

management. 

Throughout the paper, we refer to soil moisture regimes 

(SMR) and soil nutrient regimes (SNR) in discussing 

variation of response to nutrient additions on different sites. 

SMR and SNR are integral parts of the B.C. biogeoclimatic 

ecosystem classification system and are identified through 

presence and abundance of indicator plant species, 

topographic (e.g., slope position) and soil (e.g., humus form, 

soil depth, soil texture and coarse fragment content, type of 

A horizon, and soil color) characteristics (Green and Klinka 

1994). SNR is linked to measures of nutrient availability, 

particularly for N (Kabzems and Klinka 1987); SMR 

has been related to soil water balance (Giles et al. 1985). 

Combined, SMR and SNR indicate the site series (site 

classification), which is linked to site index for different 

species (Anonymous 1997), including alder (Courtin, 1992) 

and is used to guide silvicultural decision-making (Green 

and Klinka 1994).

The initial nutrition experiments were conducted 

with potted seedlings in glasshouses, followed by shortterm field experiments employing single-tree plots, and 

currently, long-term field experiments with multi-tree plots. 

The experimental approach varied with the question asked 

and the availability of appropriate sites and plantations. 

Studies employing potted seedlings were used first because 

appropriate plantations were unavailable and because 

they allowed assessment of which essential nutrients 

were limiting when other factors (e.g., light, moisture, 

competition, herbivory) were not. Subsequently, 
63

single-tree plot experiments were conducted because 

available young plantations were small and of variable size 

and tree spacing; this approach allowed for experimental 

confirmation of elemental deficiencies. As larger plantations 

became available, we initiated multi-tree plot experiments. 

The primary objective in the latter studies is to assess 

growth responses of young red alder to a restricted (and 

manageable) number of nutritional treatments over time at a 

stand level. 

Nutrient analyses were conducted on all foliage (potted 

seedlings) or on recently-matured leaves from the upper 

crown (field studies). Collected leaves were oven-dried at 

70°C for 48 hours, and ground using a coffee grinder. Total 

N and C were determined by micro-Dumas combustion 

using an automated NCS analyzer. For other elements, 

samples were digested in a microwave digestor with a 

mixture of 30% H2O2 and concentrated HCl and HNO3

(Kalra and Maynard 1991), then analyzed by inductively 

coupled argon plasma emission spectrometry (ICAP). 

Soil extractable P was determined colorimetrically by 

autoanalyzer following extraction with Bray P1 extractant 

(Kalra and Maynard 1991). 

Experiments

Glasshouse: Effects of phosphorus 

fertilization and liming on red alder 

seedlings grown in soils from mature 

alluvial alder stands

The impetus for this experiment (Brown and Courtin 

2003a) came from field data relating alder site index to 

foliar and soil available P concentrations in low pH soils 

(Courtin 1992), data indicating that growth of alder, soil 

pH and availability of P and other elements all decreased 

when alder was grown in consecutive rotations (Compton 

et al. 1997), and recognition that alder might be preferred 

for reforesting recently-harvested and potentially brushy 

alluvial sites. The objectives were to determine if: (1) 

seedling growth was limited by elemental deficiencies in 

soil from alluvial alder stands; (2) seedling growth response 

to growth of alder seedlings to P additions differed in higher 

and lower pH alluvial soils; and (3) liming could alleviate 

deficiencies of P (or other elements) by increasing soil pH. 

Soils were collected from 0-15 cm depth (mainly 

Ah horizon) in six mature alluvial red alder stands on 

Vancouver Island, screened and potted into 3 L capacity pots 

in the glasshouse. Soils from four stands were classified as 

low pH (mean = 4.4) and two were classified as high pH 

(mean = 5.3); soil nutrient regimes (SNR) were classified 

as “very rich” at all sites. Three levels (0, 0.4, or 0.8 g P 

pot-1, equivalent to 0, 225, or 450 kg P ha-1) of triple super 

phosphate (0-45-0) and two levels of dolomitic lime (0 or 

8.8 g pot-1, equivalent to 5 t ha-1) were applied in factorial 

combination. Seedlings were grown for 54 days, dried and 

weighed, and analyzed for tissue nutrient concentrations. 

Growth in unfertilized soils increased with P 

availability (fig. 1). Additions of P increased growth, 

but the response decreased with increased P availability 

(fig. 1) and was greater in low-pH soils. Liming did not 

increase growth, but increased pH and whole-plant and 

foliar concentrations of Mg. The increases in growth with 

P additions were accompanied by increases in whole-plant 

and foliar concentrations of P, but also of N, Ca, and S. 

Growth increases were probably due mainly to increased 

uptake of P, because: (1) concentrations of P were lower 

than those previously suggested as deficient (Brown 2002) 

and (2) the correlations of growth and P concentrations were 

consistent across soils. Conversely, Ca concentrations were 

high compared with concentrations previously reported 

(Radwan and DeBell 1994) and increases in N, Ca, and S 

concentrations with P additions or liming were not always 

associated with increased growth. P additions increased 

masses of all plant parts, but increased root mass the least 

and branch mass the most. Consequently, P additions 

resulted in decreased root weight ratios and increased 

branch weight ratios, without affecting allocation to stems 

or leaves. 

Figure 1—Effects of soil Bray-1 P concentrations on (A) whole-plant dry 

mass of unfertilized red alder seedlings (B) response (%) of whole-plant 

mass to P additions, potted seedling experiment (adapted from Brown and 

Courtin 2003).

Bray-P (mg kg-1)

0 5 10 15 20 25 30

Response (%)

0

100

200

300

400

P1

P2

Whole-plant mass (g)

2

3

4

5

6

7

8

9

10

11

KL

CL

SL

SY BT

NI

ns

A

B
64

The data suggested that P deficiencies might limit the 

growth of alder in “very rich” soil from mature alluvial 

alder stands. Interpretation of such bioassay results is 

problematic. Glasshouse conditions should maximize 

uptake of added nutrients and expression of potential growth 

responses to nutrient additions (i.e., elemental deficiencies 

should be more obvious). Conversely, soil processing might 

increase nutrient availability and lessen growth response to 

nutrient additions. In short, field experiments are required to 

confirm whether deficiencies exist and to assess magnitude 

of response to nutrient additions. 

Field: Responses of young red alder 

plantations on southeastern Vancouver 

Island to additions of P and other 

elements using a single tree plot design

This study was initiated in 1997 (Brown 1999); 

ultimately, ten experiments were established from 1997–

1999 (table 1), mainly in the coastal western hemlock very 

dry maritime subzone (CWHxm) of eastern Vancouver 

Island between Duncan and Campbell River. Soil moisture 

regimes (SMR) ranged from moderately dry to very moist 

(preferred, Courtin et al. 2002); soil nutrient regimes (SNR) 

ranged from poor – very rich (fig. 2). Site series ranged 

from 03 - 07 where 07 is considered optimal for red alder 

(Courtin et al. 2002) and 03 and 05 are drier than optimal.

 Plantation age at time of fertilization ranged from 0 

(within one month of planting) – 4 years. Older plantations 

(> 2 years old) were fertilized in 1997 or 1998; younger 

plantations (fertilized within one year of planting) were 

fertilized in 1998 or 1999. The types of plantations available 

necessitated use of single-tree plot designs, for reasons 

discussed previously. Only healthy trees were selected 

for treatment and a minimum distance of 8 m (> one tree 

height) was maintained between treatment trees. The basic 

treatments consisted of three levels of P (P0, P1, P2), added 

as triple super phosphate (TSP, 0-45-0), with or without the 

“C” fertilizer (C0, C1), a blend of potassium magnesium 

sulphate (0-0-22K-11Mg-22S) and fritted micronutrients 

(table 2). Treatments were either randomly assigned across 

the plantation or within blocks. The number of trees per 

treatment ranged from 14 to 25.

Experiments differed between older and younger plantations 

as follows:

(1) Older plantations were located on sites generally 

classified as less fertile and drier than were younger 

plantations (fig. 2). 

(2) Of five older plantations, four were established in 

infection centers of laminated root rot (Phellinus 

weirii) requiring the removal of infected Douglasfir; the fifth also previously contained Douglas-fir. 

In contrast, four of five younger plantations were 

planted following the harvest of an alder stand. 

Hence, previous stand history and plantation age at 

the time of treatments were confounded. 

(3) Older plantations were generally smaller in 

area and numbers of potential treatment trees; 

consequently, only three treatments (P0C0, i.e., 

control; P1C1 and P2C1) were applied in two 

experiments. Conversely, three younger plantations 

were large enough to allow adding four levels of P, 

with or without C.

(4) All plantations received the same balance of 

nutrients in a given treatment; however, the 

maximum mass of nutrients added per tree was less 

in younger plantations (table 2) because trees were 

smaller.

Table 1—Selected site and plantation characteristics, alder single-tree plot fertilization experiments.

Site Age Bray-P(mg kg-1) CEC(cmol kg-1) Previous Stand

Malaspina (M) 4 37.8 5.6 P. menziesii

Bowser (B) 4 11.4 4.3 P. menziesii

French Creek (FC) 4 48.5 3.5 P. menziesii

Quinsam River (QR) 3 91.6 2.8 P. menziesii

Hillcrest (Hill) 2 118.5 4.1 P. menziesii

Fanny Bay Dry (FBD) 1 3.0 7.5 A. rubra

Fanny Bay-Wet (FBW) 1 6.8 10.8 A. rubra

Campbell River Lower (CRL) 1 3.5 5.7 A. rubra /P. trichocarpa

Campbell River Upper (CRU) 1 124.9 2.3 P. menziesii

Harry Road (HR) 0 6.8 9.1 A. rubra

Note: age represents years since planting. Previous stand refers to dominant tree species on site prior to plantation establishment. 
65

(5) In older plantations, vegetation was removed to 1 

m from the base of each tree and fertilizers applied 

in a band 0.6 – 0.8 m from the tree base. In younger 

(0-2 year old) plantations, vegetation was removed 

to ca. 0.3 m distance from the seedling. Fertilizer 

was applied in 2 (P1), 4 (P2), or 6 (P3) dibble holes 

0.2 – 0.3 m from (and spaced equidistant around) 

the seedling. The objective in both was to ensure 

that added fertilizer was available for uptake by the 

target tree. 

Height and basal diameters were measured in the spring 

at the time experiments were established, in the fall after 

one and two growing seasons and, in younger plantations, 

after three years. Individual tree stem volume was estimated 

assuming the stem was a cone and using basal diameter (bd) 

to estimate basal area of the cone. An earlier study (Brown 

2002) indicated that this approach reasonably estimated 

stem volumes of red alder to the sizes seen in younger 

plantations, in the two years of measurement at two older 

plantations, and in the first year of treatment at two other 

older plantations.

Growth responses to P additions were significant 

(α=0.05) in the five youngest plantations (fig. 3). Averaged 

across the five plantations, P additions increased foliar 

concentrations of P from 1.3 (unfertilized) to 2.0 g kg-1, 

within the range considered deficient (Brown 2002). P 

additions also increased foliar N and decreased foliar Zn 

in four of five young plantations and increased (Ca, Mg, 

S) or decreased (K, B, Cu, Mn) in one to three plantations, 

depending on the element. The “C” fertilizer increased foliar 

concentrations of K, S, and B in all five plantations and N 

Mg, and Zn in three of five plantations without increasing 

growth; increases in S and Mg were of the same or greater 

magnitude as those resulting from P additions (Brown 

and Courtin, submitted). The data indicate that, on eastern 

Vancouver Island, low availability of P limits growth of 

young (within one year of planting) alder plantations on 

sites classified as rich – very rich and suitable for alder 

management. This is consistent with results of the potted 

seedling trial discussed above and with correlative data 

presented for mature alder stands (Courtin 1992). 

SNR

SMR VP Poor Med Rich VR

Very Dry 02

Mod-Dry 03 M FC

 Hill CRU 04

Slightly-Dry 01 B 05 

Fresh Q 

Moist 06 07 FBD, HR,

Very Moist FBW, CRL

Wet 11 12

Figure 2—Site series (italics), soil moisture regime (SMR) and 

soil nutrient regime (SNR) of alder plantations in the single-tree 

plot fertilization study. A site series of 07 (CWHxm subzone) is 

considered optimal for red alder (Courtin et al. 2002).

Table 2—Nutrient additions by treatment (Trt) for fertilized plots. P0 and C0 treatments did not receive P or the C 

blend, respectively.

Element Added (g tree-1)

Site Trt P K Ca Mg S Fe Mn Zn B Cu Mo

M, B, QR, FC P1 20 12

P2 40 24

C1 11 7 13 6.3 2.6 2.4 1.1 1.1 0.03

Hill, FBD,FBW, P1 10 6

CRL,CRU,HR P2 20 12

P3 30 18

C1 5.5 3.5 6.5 3.2 1.3 1.2 0.6 0.6 0.015

Note: Site abbreviations are as shown in Table 1.

Figure 3—Effect of P addition on stem volumes and heights of young red 

alder (< 2 year old at time of fertilization) after 3 years, single-tree plot P

fertilization experiment.

0

1

2

3

4

5

P0

P1

P2

P3 

0

1

2

3

4

5

CRU FBW CRL FBD HR

Stem volume (dm3

) Height (m)
66

In the older plantations, growth increased with addition 

of combined P and C fertilizers at Bowser and French 

Creek, the latter only when in combination with understory 

vegetation removal around the tree base (Brown 1999). 

Volume responses at both sites were significant through two 

growing seasons following fertilization (Brown and Courtin, 

submitted). The Bowser plantation had relatively low soil 

Bray-P concentrations, compared with other plantations > 

two years old at the time of fertilization. 

Several factors might explain the more frequent 

growth response of younger plantations to P additions. 

Foliar P concentrations in unfertilized trees were much 

less in younger than in older plantations (mean of 1.3 and 

2.1 g kg-1, respectively) and much more likely deficient 

(Brown 2002). With the exception of the CRU site, soil 

Bray-P concentrations were also less in younger plantations 

(table 1). The greater soil and foliar P concentrations in 

older plantations may have been due to differences in site 

characteristics (e.g., parent materials, climate) or stand 

history (e.g., Compton et al. 1997), as most older alder 

plantations in the study were established after harvest of 

a Douglas-fir stand, rather than after harvest of an alder 

stand. If P was deficient in the older plantations, insufficient 

amounts of P may have been added to the older and larger 

trees to elicit any detectable increase in P uptake and 

growth. Tree volumes in plantations four years old at the 

time of fertilization were 75 times greater, on average, than 

in plantations fertilized one year after planting. However, 

trees in older plantations received only two times as much 

P as did the corresponding trees in younger plantations and 

foliar P concentrations did not increase with P additions. 

Multi-tree plot experiments: 

Stand-level responses of young 

plantations to P additions

In 2001, we established a multi-tree plot field 

experiment in a young alder plantation on eastern 

Vancouver Island (Brown and Courtin 2003b). The longterm objectives are to assess effects of P additions on stand 

growth and ecosystem properties (specifically, carbon and 

nitrogen accumulation). The site is relatively dry; SNR was 

classified as medium-rich and soils were relatively low in 

Bray-1 extractable P (mean = 10.7 mg kg-1), suggesting 

that P might be deficient. The site previously contained a 

Phellinus-infected Douglas-fir stand. Although the greatest 

growth potential for red alder is on moist and nutrientrich sites, its immunity to Phellinus may make alder 

appropriate to plant on sites that are less moist and fertile 

than considered optimal. Such sites are common on eastern 

Vancouver Island. We felt P additions might be beneficial to 

red alder under such conditions because growth responses of 

alder to P additions were greater when moisture supply was 

suboptimal than when optimal (Radwan and DeBell 1994) 

and P additions increased instantaneous water use efficiency 

on a unit leaf area basis (Brown 2002; Brown and Courtin 

2003a). 

Following harvest of the existing Douglas-fir stand, 

the site was stumped and a portion of the site planted with 

alder seedlings in fall 1999. Survival was patchy and the 

site was fill-planted in March 2001 to a target density of 

1300 seedlings ha-1. Fifteen plots, each 45 x 45 m, were 

established in March 2001. Each plot consisted of a 25 x 

25 m inner plot surrounded by a 10 m wide buffer and was 

separated from adjacent plots by an untreated buffer.

 Soil was sampled at the beginning of the experiment 

from 0-30 cm depth at two points randomly selected along 

each of four lines extending from the plot center to a corner 

and analyzed for Bray-P, exchangeable cations, total N and 

P. Coarse fragment contents were estimated in each plot 

near the plot center and pits were excavated for description 

of soil profiles. 

Table 3—Foliar concentrations of N, P, and Zn of red 

alder over time in relation to P treatment (Trt) at the 

McColl Road experiment. Trees were planted in fall 1999 

and initially fertilized in spring 2001. 

Year

Element Trt 2000 2001 2003 2004

P (g kg-1) P0 1.41 1.53 1.44 1.44

P1 ---- 1.95 1.44 1.52

P2 ---- 1.99 1.48 1.60

N (g kg-1) P0 29.3 29.7 28.8 28.0

P1 ---- 32.1 30.5 29.2

P2 ---- 32.7 30.0 29.6

Zn (mg kg-1) P0 62.0 37.0 28.0 24.0

P1 --- 32.0 22.0 20.0

P2 --- 31.0 20.0 21.0

Figure 4—Stem volume prior to fertilization in relation to soil Bray-P

concentrations, multi-tree plot fertilization experiment. Stem volume (c

m3

)

Bray-P (mg kg-1)

2 4 6 8 10 12 14 16 18 20 22 24

0

5

10

15

20

25

30

35

40

45

Y = 1.496P + 4.730 r

2

 = 0.49
67

An objective of the treatments is to maintain differing 

foliar concentrations of P, while maintaining adequate 

concentrations of other elements. In May 2001, P was added 

at rates of 0, 15, or 30 g P tree-1 as triple superphosphate 

and placed in dibble holes 15-30 cm from the tree. P has 

been added in subsequent years, based on projected rates 

of growth from year to year (determined from annual 

measurements), measured allocation of P to foliage (Brown 

2002), and conservative assumptions of uptake of added P 

and rates of resorption of internal P prior to leaf senescence. 

Cumulative additions of P through 2004 (start of experiment 

year 4) in the P0, P1 and P2 treatments total 0, 41, and 88 g 

P tree-1 (0, 58, 124 kg P ha-1), respectively. 

Heights and diameters (basal in years one, two, and 

three; dbh in years three and four) were measured at the start 

of the experiment (spring of 2001, year one) and again in 

the fall of years one, two, three, and four. Foliage has been 

analyzed yearly as described above. 

Similar to the relationship demonstrated earlier across 

sites, stem volumes of unfertilized seedlings one growing 

season after planting increased with soil Bray-P levels 

(fig. 4). Through four growing seasons, P additions have 

increased individual stem volumes by 56 % in the 1999 

cohort and 156 % in the 2001 cohort (fig. 5); absolute 

increases due to P additions have been greater in the 1999 

cohort. To-date, absolute effects of P addition have also 

increased each year since fertilization (fig. 5). Effects of P 

additions have been greater for dbh than for height in both 

the 1999 (24 vs. 16%) and 2001 (60 vs. 31%) cohorts.

Over the experiment, P additions have most 

consistently increased foliar concentrations of P and 

Figure 5—Four-year stem volume growth of red alder in relation to P 

treatment and cohort, multi-tree plot fertilization experiment.

Figure 6—Mortality (% of initial number of seedlings per plot) of red 

alder in relation to cohort, P treatment, and year, multi-tree fertilization 

experiment. Vertical bars represent standard errors about the mean.

Figure 7—Precipitation, 2001–2004 growing seasons, multi-tree plot 

fertilization experiment. 

decreased concentrations of Zn (table 3). Nonetheless, 

foliar P concentrations have decreased over time despite our 

refertilizing treatment plots. The foliar P concentrations are 

within the range of P concentrations felt to be deficient in 

both cohorts of seedlings. Concentrations of elements other 

than P and Zn have generally been unaffected by P additions 

during the study period and have been within ranges 

observed in the single-tree plot experiments. 

Mortality in this relatively dry site has been significant 

through four years (fig. 6). Mortality has been greater in the 

fill-planted 2001 seedlings, possibly because of increased 

competition at the time of establishment, or because fill 

planting occurred mainly on poorer microsites. There is 

no clear evidence that P treatments have affected mortality 

rates. Mortality was greater in 2003 and 2004 than 2002 

and may have been associated with drought during those 

growing seasons (fig. 7). A simple estimate of stand level 

Stem volume (d

m3

) 0

1

2

3

4

5

6

7

8

Y0 

Y1

Y2

Y3

Y4

planted 1999 planted 2001

fall 2004

fall 2003

P0 P1 P2 P0 P1 P2

1999 cohort

0

5

10

15

20

25

2002

2003

2004

2001 cohort

0

5

10

15

20

25

30

P0 P1 P2

Mortality (%) Mortality (%)

Month

3 4 5 6 7 8 9 10

0

20

40

60

80

100

normal

2001

2002

2003

2004

Apr May June July Aug Sept

Precipitation (mm)
68

volume, based on individual stem volume responses and 

mortality rates suggests that P additions have increased 

volume by 2.1 (46%) and 2.5 m3 ha-1 (56%) in the P1 and P2 

treatments, respectively, through 4 years of treatments.

Discussion

Our studies suggest that the early growth of red alder 

on eastern Vancouver Island is limited by insufficient P 

supply. Deficiencies of other elements have not been clearly 

demonstrated, which might indicate that supplies of those 

elements are sufficient or additions of those elements were 

insufficient to increase growth. 

The effects of P additions have been most pronounced 

at low soil Bray-1 P levels (< 12 mg kg-1) and low foliar 

P concentrations (< 2 g kg-1) and in plantations fertilized 

within one year of planting. Responses have been measured 

for up to three years following fertilization, but it is 

unknown how long growth responses might persist. In 

the sites studied, soil nutrient regimes classified as rich 

or very rich had low Bray-P concentrations. At this point, 

the relative effects of plantation age, stand history, and 

site characteristics on growth response to P additions 

are difficult to untangle, because the less-responsive 

older plantations were on drier sites with higher BrayP concentrations and were planted on sites previously 

occupied by Douglas-fir, not red alder. 

Growth responses in the multi-tree plot experiment 

discussed here have been largely consistent with results 

observed in the single-tree plot studies, as are first-year 

responses to 0, 30, and 60 g P tree-1 in a moist, very 

rich site near Powell River. Continued growth and soil 

measurements in these and similar newly-initiated multi-tree 

plot P fertilization experiments should provide insight into 

long-term effects of P availability on stand growth and soil 

development.

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young Alnus rubra. Tree Physiol. 15:427-432. 

Hurd, P.D.; DeBell, D.S. 2001. Growth and early stand 

development of intensively cultured red alder plantings. 

New Forests 21:71-87. 

Kabzems, R.D.; Klinka, K. 1987. Initial quantitative 

characterization of soil nutrient regimes. I. Soil 

properties. Can. J. For. Res. 17:1557-1564.

Kalra, Y.P.; Maynard, D.G. 1991. Methods manual for 

forest soil and plant analysis. For. Can. Inf. Rep. NORX-319.

Marschner, H. 1995. Mineral nutrition of higher plants. 

2nd ed. Academic Press.

Massie, M.R.C.; Peterson, E.B.; Peterson, N.M.; Enns, 

K.A. 1994. An assessment of the strategic importance 

of the hardwood resource in British Columbia. Can. For. 

Serv. / B.C. Min. For. FRDA Rep. 221.

Radwan, M.A.; DeBell, D.S. 1994. Fertilization and 

nutrition of red alder. In: Hibbs, D.E.; DeBell, D.S.; 

Tarrant, R.F.; eds. The biology and management of red 

alder. Corvallis, OR: Oregon State University Press: 216-

228.

Rural Technology Initiative. [N.d.]. After decades of 

Douglas-fir plantations, is it time for forest landowners 

to consider planting alder and cedar? Rural Technology 

Initiative, Univ. Washington. RTI Fact Sheet # 22.

Tarrant, R.F.; Hibbs, D.E.; DeBell, D.S. 1994. Red alder 

and the Pacific Northwest. In: Hibbs, D.E.; DeBell, D.S.; 

Tarrant, R.F.; eds. The biology and management of red 

alder. Corvallis, OR: Oregon State University Press: ix-xi.

Weetman, G.F.; McWilliams, E.R.G.; Thompson, W.A. 

1992. Nutrient management of coastal Douglas-fir and 

western hemlock stands: the issues. In: Chappell, H.N.; 

Weetman, G.F.; Miller, R.E.; eds. Forest fertilization: 

Sustaining and Improving Growth of Western forests. 

Contribution No. 73. Seattle: University of Washington, 

Institute of Forest Resources: 17-27.

Zabek, L.M.; Prescott, C.E. 2001. Nutrition and 

fertilization response of T x D hybrid poplar. In: 

Schroeder, W.; Kort, J.; eds. Temperate agroforestry: 

adaptive and mitigative roles in a changing physical and 

socio-economic climate. Proceedings Seventh Biennial 

Conference on Agroforestry in North America: 291-297. 
70
71

Management/Silviculture
72
73

1Andrew A. Bluhm (email: Andrew.Bluhm@orst.edu) and David E. Hibbs (email: David.Hibbs@orst.edu) are with the Department of Forest Science, 

Oregon State University, Corvallis, OR 97331.

— Red Alder: A State of Knowledge —

Red Alder: 

Its Management and Potential

Andrew A. Bluhm and David E. Hibbs1

Abstract

Red alder is now recognized as a valuable tree species 

in the Pacific Northwest because of recent increases in wood 

value, its fast growth and therefore its ability to produce 

sawlogs on short rotations. As compared to conifers, red 

alder has extremely fast juvenile growth rates but good 

growth is limited to a narrower range of site conditions. 

Red alder is also resistant to many of the diseases afflicting 

conifers, improves site productivity due to its nitrogen-fixing 

ability, and enhances wildlife and plant diversity across the 

landscape. Diameter and height growth of trees in managed 

plantations of red alder are greater than that of naturally 

regenerated stands. This apparent increase in growth is 

primarily due to the control of competing vegetation and 

planting density, and intermediate silvicultural activities 

(i.e. thinning). This faster growth results in a shorter time 

required to achieve trees of any given size. Tree form is also 

improved with management due to more uniform stocking. 

Results are presented here on the effect of initial planting 

density and various thinning treatments on twelve-yearold variable-density red alder plantations. Initial planting 

density influenced early diameter and height growth. Up 

through age 6, diameter growth increased with increasing 

density until a crossover occurred between ages 7 through 

11. Diameter increment ranged from 1.4 cm/year to 1.8 

cm/year. Height growth also increased with stand density 

except at extremely high densities. Height increment ranged 

from 1.1 m/year to 1.4 m/year. Optimal alder height and 

This paper was published in: Deal, R.L. and C.A. Harrington, eds. 2006. Red alder—a state of knowledge. General Technical 

Report PNW-GTR-669. Portland, OR: U.S. Department of Agriculture, Pacific Northwest Research Station. 150 p.

diameter growth was maximized around 1400-1500 tph until 

about age 10, at which time optimal diameter growth shifted 

to lower densities. Early stand volumes of close to 80 

m3/ha were achieved in the intermediate planting densities 

mainly as a function of absolute tree number and minimum 

merchantability limits. Thinning increased diameter growth 

response as compared to the control plots and thinning at 

age five resulted in greater diameters than thinning at age 

eight. By age 12, diameters were 21% and a 14% greater 

than the control for the early thinned and later thinned plots, 

respectively. Post thinning annual diameter increments were 

25% and 38% greater for the early thin and the late thin, 

respectively. Thinning had no effect on tree height. Thinning 

increased individual tree volume 14% and 4% over the 

control plots for the early thin and the late thin, respectively. 

Thinning reduced the number of merchantable trees per 

hectare. Volume per hectare was greatest in the early thin 

plots (89.0 m3/ha), followed by the control plots (81.2 m3/

ha) and the late thinned plots (68.0 m3/ha). These results 

illustrate that the careful control of planting density and 

thinning regimes provide opportunities to achieve higher 

yields of better quality logs in a relatively short time.

Keywords: red alder, Alnus rubra, management, 

silviculture, planting density, precommercial thinning, tree 

growth, stand yield
74

Introduction

“What is a weed? A plant whose virtues 

 have not been discovered.”

– Ralph Waldo Emerson (1803-1882)

Red alder (Alnus rubra; hereafter referred to simply as 

alder) is the major hardwood tree in the Pacific Northwest 

(PNW), comprising approximately two-thirds of the 

hardwood growing stock in the region. Although always 

present in the landscape, alder has historically been a lowvalue and under-utilized timber resource. The origin of 

most stands was accidental (natural regeneration after fire, 

logging or failed conifer plantations) and management 

efforts focused on eradication or conversion. This is 

changing rapidly due to the increased commercial value and 

recognition of its ecological benefits across the landscape. 

Alder’s easy regeneration, rapid growth rates and log 

price trends make it an attractive species to manage in the 

Pacific Northwest. However, this has not always been the 

case. Until recently, alder has been considered a “weed” 

by most foresters in the region. In fact, even recently, 

debate still surrounds the desirability of managing alder. 

For instance, the OSU Extension Service released, in the 

same year, two publications, “Managing Red Alder” (Hibbs 

1996) and “Converting Western Oregon Red Alder Stands to 

Productive Conifer Forests” (Bondi and Emmingham 1996). 

While some were trying to grow alder, others were trying to 

get rid of it.

Alder is not without its merits. It has always been a 

small component of the PNW conifer-based timber industry 

and has long been recognized as a desirable component 

in riparian systems. But as much as people used alder, the 

scientific understanding of this species lagged far behind 

that of its associated conifers. Recognizing the virtues of 

alder and the serious lack of silvicultural understanding, the 

Hardwood Silviculture Cooperative (HSC) was formed to 

improve the understanding, management, and production 

of alder. Due to the efforts of the HSC and other research 

organizations, the acceptance of alder is increasing along 

with (or due to) the growing knowledge base. This chapter 

describes the state of knowledge of managing alder 

plantations. Focus is primarily on four topics:

1) Characteristics of alder compared to conifers and 

other hardwoods

2) Reasons to manage alder

3) Differences between alder plantations and natural 

stands

4) Responses of alder to stand density management 

activities (initial spacing and thinning).

The HSC in brief

The Hardwood Silviculture Cooperative (HSC), begun 

in 1988, is a multi-faceted research and education program 

focused on the silviculture of alder and mixtures of alder 

and Douglas-fir (Pseutotsuga menzeisii). The goal of the 

HSC to improve the understanding, management, and 

production of alder. The activities of the HSC have already 

resulted in significant gains in understanding regeneration 

and stand management, and have highlighted the potential 

of alder to contribute to both economic and ecological forest 

management objectives.

To understand the response of alder to intensive 

management, the HSC has installed 26 variable-density 

plantations from Coos Bay, Oregon to Vancouver Island, 

British Columbia. The plantation distribution covers a 

wide range of geographic conditions and site qualities. At 

each site, cooperators planted large blocks of alder at four 

specific densities. Each block is subdivided into several 

treatment plots covering a range of thinning and pruning 

options.

Since the HSC was established, they (and many others) 

have learned a great deal about seed zone transfer, seedling 

propagation, stocking guidelines, identification of sites 

appropriate for alder, and the effects of spacing on early 

tree growth. Much of this information is available to the 

public and found on the HSC web-page http://www.cof.orst.

edu/coops/hsc. Furthermore, the data set is now complete 

enough to begin analyzing the growth response of alder after 

thinning and/or pruning. However, much still needs to be 

accomplished. The ultimate goal is a better understanding 

of the effects of stand density on alder growth and yield and 

wood quality, and to develop a robust alder growth model.

How does alder stack up against conifers 

and other hardwoods?

There are obvious physical differences between 

alder and other conifers in the region, but perhaps the 

most important to managers is that there is relatively 

little knowledge about alder management as compared 

to conifers. This lack of information is a result of a 

combination of many factors, including inconsistent markets 

and the region-wide conifer-forestry mindset. Some of 

the important differences, with management implications, 

between alder and conifers are discussed below.

Alder is not what usually comes to mind when imaging 

the forests of the PNW. Conifers dominate the landscape, 

in abundance, size and stature. Hardwoods comprise only 

12% of the growing stock in the region (Raettig et al. 1995) 

and pale in comparison to conifers in tree size and lifespan. 

However, alder has impressive juvenile height growth rates, 

accumulating more than two-thirds of its mature height 

by age 30 (Harrington and Curtis 1986, Worthington et al. 

1960). This rapid growth is attractive to foresters. However, 

this fast growth dictates that management activities must 
75

be performed in a narrower time window and much earlier 

than in conifer stands for at least two reasons. One, stand 

characteristics are determined primarily by early stand 

conditions (i.e. diameter and height increment, live crown 

ratio). Two, vertical crown expansion is the primary means 

of crown response following thinning. Therefore early 

thinning is needed whereas thinning older stands (with 

minimal live crown) may not be beneficial. 

Good performance of alder is limited to a narrower 

range of site conditions than most of its associated conifers. 

Despite the fact that alder is (incorrectly) perceived as a 

wet site species, it grows across a wide variety of sites. 

However, what is a good site for conifers may not be a good 

site for alder, and vice versa. For consistent, risk-minimizing 

management, species requirements (Harrington 1990) and 

proper site identification (Harrington 1986) is critical. Alder 

survival and growth is also more sensitive than conifers to 

small-scale, within-stand site variations.

At full stocking, alder usually has lower basal area 

than conifers (Puettman 1994), ranging from 20-30 m2/ha. 

Volume is likewise lower than conifer stands, ranging from 

300-500m3/ha at 50-70 years. However, alder has similar 

rates of annual wood production as many conifers, with 

annual cubic volume growth rates of 21 m3/ha in pulpwood 

rotations of 10 to 12 years, and 14 m3/ha in saw-log 

rotations of 30 to 32 years (DeBell et al. 1978).

Today, alder is managed primarily for high-grade 

lumber/veneer. This fact means the quality of the resource 

(i.e. the log) is as important (or more so) than the quantity of 

the resource. Furthermore, unlike conifers, alder is a diffuse 

porous species. This trait allows uniform wood properties 

and wood quality regardless of growth rate (Lei et al. 1997). 

These two traits have far-reaching impacts on management 

strategies.

 Although alder is a minor component of the total 

timber volume in the PNW, it is, by far the most abundant 

hardwood in the region and comprises approximately 60% 

of the total hardwood resource (Raettig et al. 1995). It has 

a wide geographic range (1.9 million hectares) and a large 

volume of approximately 30 billion board feet (Glenn 

Aherns, personal communication). So while other hardwood 

species may be locally important, alder is the most 

important region-wide hardwood species.

 A few factors other than the limited availability 

of other hardwoods favor the management of alder. Except 

for black cottonwood (Populus trichocarpa), alder growth 

rates far exceed other hardwoods of the region. Fast growth 

means short rotations. The only other region-wide hardwood 

species, bigleaf maple (Acer macrophyllum) has been 

found to be notoriously hard to grow. Alder seedlings are 

less susceptible to browsing than those of bigleaf maple. 

Furthermore, compared to many other hardwood species, 

living alder has surprisingly little decay. Allen (1993) 

indicates that merchantable volume losses to decay average 

4% in alder 60-80 years old.

Why manage alder?

First and foremost, alder exhibits very rapid juvenile 

growth rates (Harrington and Curtis 1986, Nigh and Courtin 

1998, Peterson et al. 1996) making it an appealing species 

to manage in short rotations. Through about age 25, alder 

height growth exceeds that of all associated conifers (fig. 

1). Therefore, a primary management objective is to capture 

this difference. Although height growth rates decline rapidly 

after 20 years, a short-rotation, high-value crop can be 

achieved. With today’s quick-return emphasis on plantation 

management, this short rotation length is economically 

appealing.

 Although volume yields for alder are lower than 

those for conifers at typical conifer final rotation ages, alder 

volume is greater than the associated conifers at these short 

rotations (25-35 years). For instance, empirical data from 

natural stands in British Columbia indicate that for the 

first 25 years, the rapid volume growth of alder surpasses 

all other conifers. After age 25, well-stocked, undamaged 

(i.e. no weevil) Sitka spruce (Picea sitchensis) stands may 

surpass the alder. Western hemlock (Tsuga heterophylla) 

may catch up to alder by age 35, Douglas-fir by age 40, and 

western red cedar (Thuja plicata) not until age 75 (Peterson 

et al. 1996).

The resistance of alder to many diseases afflicting 

conifers offers another reason for interest. Although alder 

has diseases of its own, it is immune to two widespread 

diseases in the region. The high incidence of laminated root 

rot (Phellinus weirii) in the region causes untold amounts 

Figure 1—Comparison of height growth from seed to age 100 for red alder 

and associated coniferous species. Height curves and site index values are 

based on median values for conifers on Site productivity class II (from 

“Red Alder manager’s Handbook for British Columbia,” Peterson et. al. 

1996).

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50 60 70 80 90 100

Height Growth Comparison

Alder S127

Douglas-fir S130

Hemlock S126

Redcedar S118

Sitka spruce S129

Source: Green and Klinka, 1994

Height (m)

Age (yr from seed)
76

of conifer growth losses. Conifer species show various 

levels of resistance, but all hardwoods are immune. On 

appropriate sites, alder is the species of choice in reforesting 

lands infected with laminated root rot (Nelson et al. 1978). 

Swiss needle cast, first observed in the Oregon Coast Range 

in the 1990’s, affects approximately 50,000 hectares. In 

some heavily infected areas, it may cause up to 50% volume 

loss in Douglas-fir stands. Alder and western hemlock are 

alternative species to manage on lands heavily infected with 

this disease.

Alder produces more aboveground litterfall than 

do associated conifers. This litterfall has higher nutrient 

concentrations and decomposes more rapidly, resulting in 

improved nutrient cycling rates and leading to enhanced 

nutrient availability on a site. Furthermore, alder is a 

partner in a three-way symbiosis among roots, nitrogenfixing actinomycetes in root nodules (Frankia spp.), and 

mycorrhizal fungi. This contributes to alder’s rapid growth 

and positively influences soil structure and fertility. Rates of 

nitrogen fixation vary, but can range between 10-150kg/ha/

yr (see table 1 in Bormann et al. 1994). Because nitrogen is 

the commonly limiting nutrient in the PNW, alder is likely 

to be important in the ecosystem productivity. Clearly, 

alder has the ability to improve soils not only for the next 

rotation, but long-term as well.

Studies in Alaska have shown that understory 

vegetation abundance, wildlife browse, songbird and aquatic 

invertebrate abundance, terrestrial arthropod abundance 

(Wipfli et al. 2002), and plant species richness (Deal 1997) 

increased with increasing proportion of alder in the stand. 

Furthermore, mixed alder-conifer stands provided greater 

complexity than stands dominated by either conifer or alder 

(Deal et al. 2004), enhancing the productivity and biological 

function of headwater streams (Wipfli et al. 2002).

Plantations vs. natural stands

The activities of the HSC have shown that managed 

plantations of alder can dramatically out-produce natural 

stands, resulting in a short-rotation, high-value crop. This 

apparent increase in productivity can be attributed primarily 

to three management activities: 1) proper selection of 

planting sites, 2) control of competing vegetation and initial 

planting density, and 3) intermediate stand treatments 

(i.e. thinning and/or pruning). Proper site selection and 

competition control are covered in other chapters. Planting 

density and thinning will be covered later in this chapter.

Perhaps the greatest advantage of managed stands of 

alder as compared to natural stands is the improvement in 

tree form. Most of the stems in older, natural alder stands 

have considerable lean and sweep, usually the result of 

uneven stocking, alder’s high degree of phototropism, 

and differential growth rates (Wilson 1984). Such traits 

are considered defects and result in lower log value. 

Observations of planted stands indicate that the stems are 

much straighter than in unmanaged natural stands (Bormann 

1985). Because the greatest value is in high-quality sawlogs, 

increases in log quality will result in direct increases in 

returns.

Alder plantations exhibit improved diameter and height 

growth rates across all site qualities and greater height 

growth across most planting densities except for extremely 

low densities. Research results indicate that the growth 

and yield of managed alder plantations will exceed that 

of natural stands (fig. 2). According to Worthington et al. 

(1960), 17 year old trees growing on site with a site index 

of 35 m would have a mean diameter of 15.3 cm. The mean 

diameter from one 17-year-old managed alder plantation of 

the same site quality was 23.3 cm, a 66% increase.

Height growth is also improved in plantations as 

compared to natural stands both across site quality classes 

and planting densities. Taking height growth data from 13 

alder plantations, dividing into site quality class, and then 

overlaying these height growth curves on the site index 

curves from natural stands (Harrington and Curtis 1986) 

indicated that observed alder height growth in plantations 

was greater than predicted height growth across all site 

quality classes. Observed total height at age 12 was 

improved 1.4 m (9.3%) and 1.1 m (8.9%) for the high and 

low site quality classes, respectively (data not shown).

Alder growth is affected by stand density, and by 

controlling stand density it is possible to affect long-term 

stand trajectory. Figure 3 shows that observed tree height at 

year 12 was dramatically improved in all densities except 

of the extremely wide spacing (290 trees per hectare [tph]). 

Using the same data as the previous comparisons, observed 

height was improved 1.4 m (10.4%), 3.0 m (22.4%), and 

2.0 m (14.9%) for the 680 tph, 1480 tph, and the 2800 

tph densities, respectively, with the 1480 tph density 

approximating desired operational planting densities. 

However, tree height started to decline at extremely high 

densities (2800 tph). One would expect to see even greater 

declines in height growth with increasing density until it 

reached the levels of natural stands (which usually establish 

at very high densities).

So what are reasonable yield targets for managed alder 

plantations? Since no plantations have yet reached harvest 

age, one cannot say with absolute certainty. However, 

management of any species will increase yield. This pattern 

of increasing yield with management for Douglas-fir show 

that yields of intensively managed plantations can be double 

that of unmanaged natural stands. There is no reason to 

believe alder is any different. 

Research results so far clearly indicate an increase 

in yield with alder management. Estimates of increased 

volume yield due to management range from 10-40% 

(Peterson et al. 1996, Puettman 1994) and increases in 

basal area of 14% (DeBell and Harrington 2002). Thinning 

can maintain diameter growth rates as much as 30-80% 
77

higher than those in unmanaged stands, at least until age 

25. Thinning can maintain diameter growth rates of 0.75-

1.0 cm/year for at least 10-15 years (Peterson et al. 1996). 

In another study, thinning increased the diameter growth 

of crop trees by 54% and net basal area growth was 60% 

greater, compared to unthinned stands (Hibbs et al. 1989).

Plantations would also have shorter rotations than 

natural stands. Managed stands are expected to attain an 

average diameter by 30 cm by age 30 years or earlier; an 

average natural stand would take 45 years (Peterson et al. 

1996). Using the data from fig. 2 and assuming a constant 

diameter increment of 1.37 cm/year, the time required to 

produce a stand with mean diameter of 30 cm would be 22 

years. DeBell and Harrington (2002) concluded that 20-

year-old managed stands reached an equivalent basal area 

as that of a fully-stocked unmanaged stand of 27 years old. 

Other research (Barri Hermann, personal communication) 

has shown a halving of the time it takes managed plantations 

to reach the same yield (approximately 300 m3/ha) as 

unmanaged stands (25 years vs. 50 years, respectively).

A serious concern for foresters is the lack of control 

one has over both density and stocking of natural stands. Of 

all the tools a silviculturist has, the manipulation of stand 

density holds the most promise; for density management 

provides opportunities to influence stand yield and tree 

size, form, and quality (Puettmann et al. 1993). Sawtimber 

yields and economic return can be greatly improved 

with management of most tree species—both alder and 

conifer—but the return for alder may be greater. With alder, 

management is needed to make the difference between good 

and poor results. The next section will describe experimental 

results of density management activities in alder plantations. 

Specifically, it will address the effect of initial planting 

density and thinning on individual tree and stand growth.

Density Management 

in Alder Stands

Methods

Site Description

The data in this analysis are from the 13 variabledensity plantations of pure stands of alder in western 

Oregon and Washington that have reached 12 years old. 

The majority of plantations are in the Coast Range, with 

the remainder on the Olympic Peninsula, the Puget Trough, 

and the Western slopes of the Cascade Range (fig. 4). The 

climate is maritime and characterized by mild temperatures, 

wet, mild winters, cool, dry summers, and heavy 

precipitation (Franklin and Dyrness 1973). Soil types are 

silty loams, clay loams, gravelly loams, and cobbly loams. 

Elevation ranged from 46 m to 548 m, slopes ranged from 

5% to 60%, and annual precipitation ranged from 115 cm to 

330 cm. 

Plantations were established on previously harvested 

sites of at least 6 ha and reasonably uniform ground 

Figure 2—Comparison of diameter growth between an alder plantation 

(HSC Site #4201), planted at 1300 trees/hectare (tph) and thinned to 565 

tph at age 4 with natural alder stands of the same site index (base age 50 

years) of 35 m (from Worthington et. al. [1960]).

Figure 3—Comparison of height growth curves between managed 

plantations and natural stands of red alder. Plantation data is from 13 sites 

with a mean site index (base age 50 years) of 27.0 m (calculated from 

Harrington 1986). Height was calculated by planting density and was 

derived from the largest 247 trees per hectare. Natural stand data is from 

Harrington and Curtis (1986).
78

conditions. Climatic (annual and growing season 

precipitation, length of growing season), and soils 

information was determined and site index was determined 

using the soil-site method of Harrington (1986). Mean 

site index (base age 50 years) was 30.9 m and ranged 

from 26-35 m. Treatment activities and data collection are 

administered by the Hardwood Silviculture Cooperative, 

Forest Science Department, Oregon State University, 

Corvallis, OR.

Measurements

At each site, blocks of inoculated, local alder nursery 

stock were planted with target spacings of 5.8 x 5.8 m, 

3.8 x 3.8 m, 2.5 x 2.5 m, and 1.7 x 1.7 m (247, 568, 1297, 

2967 tph). Site preparation methods used were the standard 

operating methods for the region and included normal 

competition reduction practices. Sites were planted using 

operational planting crews. Each treatment plot is 0.50 

ha containing a 0.13 ha measurement plot. In addition to 

the four control plots, thinning and pruning treatments are 

present at each site. Seedling survival was evaluated in year 

one and year two and plots were interplanted if mortality 

exceeded 30 percent. All invading trees and any overtopping 

shrubs were controlled

Two thinning treatments were performed on the two 

highest planting densities (1297 and 2967 tph). The first 

treatment was thinning when the tree crowns closed and 

lower branch mortality commenced (hereafter referred to 

as the “early thin”). The second thinning treatment was 

thinning when the average height to the live crown was 

between 4.5 and 6.0 m (hereafter referred to as the “late 

thin”). Residual target density for all thinning treatments 

was 568 tph. Leave trees were selected based on spacing, 

form, and dominance and marked with flagging. DBH was 

measured on all trees (both trees to be “cut” and “saved”) 

and height and height to live crown was measured on all 

“save” trees. Measurements and thinning were done in the 

dormant season. Thinning was done with a chainsaw.

At age 3, 6, and 9, data on permanently tagged 

individual trees was collected in the dormant season. For 

every tree, stem diameter at 1.37 m (DBH), stem defect 

(fork, lean, sweep) and presence or absence of damage 

(animal, weather, etc) was recorded. Height was measured 

on a subsample of 40 trees spatially well-distributed over 

the plot that included the 10 trees of smallest diameter,10 

of largest diameter, and 20 mid-range trees (based on 

diameter). Mean tree diameter was calculated as quadratic 

mean diameter. Plot means were calculated for diameter, 

height, and height to live crown for 1) the sample of trees 

on the plot that would represent the 247 trees per hectare 

with the largest diameter (used for initial planting density 

comparisons), or 2) all the trees on the plot (used for thinned 

versus unthinned comparisons). Actual density differed from 

target density and was calculated as the number of trees 

alive at age three, averaged across all sites. Individual tree 

volume was estimated from Skinner (1959) and volume 

per hectare was calculated by multiplying mean individual 

tree volume by the number of merchantable trees per acre 

(diameter greater than 15 cm).

Results

Planting Density

Early in stand development (through age 6), diameter 

increased with increasing density (fig. 5). After age 6, 

reduction of diameter with increasing density was observed 

primarily for the highest density. By age 9, DBH for the 

2800 tph plots was 2.9 cm less (16% less) than the DBH in 

the lowest density (290 tph). DBH in the two intermediate 

densities was practically equal to the lowest density at age 

9. Annual increment peaked at age 6 for all densities and 

then declined. At age 12, annual DBH increment ranged 

from 1.4 cm/yr to 1.8 cm/yr across all densities. By age 12, 

the typical reduction in DBH with increasing density was 

observed.

Figure 4—Location of Hardwood Silviculture Cooperative (HSC) pure 

alder plantations at least 12 years old.
79

The influence of planting density on height was similar 

to that of DBH. Generally, height increased with density 

(fig. 6). This positive relationship was observed through age 

6, after which height growth reductions were greater for 

the 2800 tph plots as compared to the 1480 tph plots. From 

age 6 through 12 the relative rankings did not change. As 

compared to the lowest density, total tree height increased 

13%, 25%, and 19% for trees planted at 680 tph, 1480 tph 

and 2800 tph, respectively. For every measurement date, 

the lowest density (290 tph) had the shortest trees. By age 

12, tree height in this density was at least 1.7 m less than 

the next closest planting density. At age 12, annual height 

increment ranged from 1.1 m/yr to 1.4 m/yr across all 

densities.

Across all densities, individual tree volume averaged 

0.16 m3 and ranged slightly from 0.14 m3 to 0.18 m3 (data 

not shown). A pattern of decreased individual tree volume 

with increasing density was observed. The number of 

merchantable trees varied substantially by planting density 

(data not shown). The intermediate densities, 680 tph and 

1480 tph, had two to three times (432 tph and 494 tph, 

respectively) the number of merchantable trees per acre than 

that of both the high (202 tph) and low (158 tph) extremes. 

The proportion of merchantable trees to total trees was 

0.56 and 0.63 for the 290 tph and the 680 tph densities, 

respectively, then declined to 0.33 for the 1480 tph and 

dropped sharply to 0.07 for the 2800 tph density.

The sharp differences in the number of merchantable 

trees per acre accounted for large differences in volume per 

hectare. Volume per hectare for the 680 tph and the 1480 tph 

was 73.3 m3/ha and 81.2 m3/ha, respectively. These volumes 

were almost three times that of the two density extremes. 

Volume per hectare for the 290 tph and the 2800 tph was 

28.5 m3/ha and 29.6 m3/ha, respectively The 290 tph density 

had the largest individual trees; there were just not that 

many of them. The 2800 tph density had a lot of trees, just 

not many yet of merchantable size.

Live crown ratio, an important indicator of tree vigor, 

differed predictably by density. Figure 7 illustrates the 

decrease in crown ratio through time and with increasing 

density. Reductions in crown ratio commenced immediately 

and were of greatest magnitude in the highest density. 

Reductions occurred later and were of less magnitude as 

density decreased. No appreciable drop in crown ratio 

occurred in the lowest density through age 12. Although 

reductions in crown ratio are seemingly severe, it is 

important to note that even in the highest density, crown 

ratios of the crop trees are still greater than 50%. These 

results imply that although interspecific competition is 

undoubtedly occurring, dominant tree vigor remains high. 

Thinning

Table 1 describes the tree and stand characteristics, both 

pre-and post-thinning, for all four treatments. In the 1480 

tph density (both the early thin and the late thin treatments), 

thinning removed about half of the basal area and for the 

2800 tph density, approximately two-thirds of the basal area. 

There were similar reductions in relative density.

Tree and stand responses to thinning are presented 

only for the 1480 tph densities with some comparisons to 

the 2800 tph densities. Two main factors affect tree and 

stand response to thinning; the timing and the intensity of 

thinning. Therefore, the similar response patterns observed 

between the two densities reveals a notable result, intensity 

(i.e. proportion of trees or basal area removed) has little 

effect on tree and stand growth following thinning at least 

within these timings and intensities. However, the timing of 

thinning noticeably affects tree and stand growth responses. 

Figure 5—Comparison of diameter at breast height (DBH) by initial 

planting density for the 247 largest trees per hectare (crop trees). Data is 

derived from 13 red alder plantations throughout the Pacific Northwest 

with an average site index of 30.9 m (base age 50 years) calculated from 

Harrington (1986).

Figure 6—Comparison of total tree height by initial planting density for the 

247 largest trees per hectare (crop trees). Data is derived from 13 red alder 

plantations throughout the Pacific Northwest with an average site index of 

30.9 m (base age 50 years) calculated from Harrington (1986).
80

Thinning increased diameter growth response as 

compared to the control plots; however, thinning at age 

five resulted in greater diameters than thinning at age eight 

(fig. 8). At age 12, diameters in the early thinned plots were 

3.8 cm greater than the control while diameters in the late 

thinned plots were 2.3 cm greater. This corresponds to a 

21% and a 14% increase, respectively. Annual diameter 

increments since thinning were 2.0 cm/yr and 1.5 cm/yr for 

the early thin and control, respectively and 1.3 cm/yr and 

0.8 cm/yr for the late thin and control, respectively.

Thinning had virtually no effect on tree height (data not 

shown). At age 12, tree height ranged between 14.1 m and 

15.0 m. Control trees were the tallest. There was only a 3% 

and a 6% reduction in tree height for the early thin and late 

thin, respectively.

Thinning increased individual tree volume. Individual 

tree volume at age 12 in plots thinned at age five was 0.19 

m3, a 14% increase compared to unthinned plots (0.16 

m3). The volume of trees thinned at age eight was only 4% 

greater (0.17 m3) than unthinned trees (data not shown). 

Thinning reduced the number of merchantable trees per 

hectare. There were 494 tph of merchantable size in the 

unthinned plots as compared to 469 tph in the early thin and 

400 in the late thin (data not shown). The minor differences 

in individual tree volume and merchantable trees per hectare 

resulted in slight differences in volume per hectare. The 

greatest volume occurred in the early thin plots (89.0 m3

/ha), 

followed by the unthinned plots (81.2 m3/ha) and finally the 

late thinned plots (68.0 m3/ha). This corresponded to a 9% 

increase in volume for the early thin and a 16% reduction in 

volume for the late thin compared to the controls.

Discussion

 Planting Density

Contrary to conventional wisdom and much experience, 

the results presented here show that early diameter and 

height increase with stand density. Up through age 6, 

diameter growth increases with density until a crossover 

effect occurs between ages 7 through 11. Up through age 

9, no penalty occurs with increasing density except at 

extremely high densities. Optimal diameter growth was 

maximized in the intermediate densities through about 10 

years of age, after which optimal diameter growth shifted to 

the lowest density (290 tph). Only by age 12 was the pattern 

of decreasing diameter with increasing density observed. 

It could be argued that these results are not unique at all 

since the typical relationship of diameter and density did 

occur and (most likely) will continue and intensify as the 

stands age. Also, this crossover effect could be due to a 

combination of factors since early tree growth is a result of 

many factors in addition to density (Puettman 1994).

As discussed earlier in this chapter, one goal of 

managing alder is to capture the difference in early growth 

between alder and conifers. Therefore it follows that a 

manager should then continue to capture the difference 

in early growth between alder densities. These early 

differences are important for at least three reasons. First, due 

to the short rotation ages predicted, 10 years old is about 

half to a third of a rotation. It would be unwise to ignore 

this phase of growth. Second, since thinning can maintain 

diameter growth rates and current research indicates the 

possible need to thin alder plantations before age 10, one 

could not only take advantage of the positive relationship 

Table 1—Stand characteristics of thinned HSC plots.

Early Thin Late Thin

Treatment 1480 tph 2800 tph 1480 tph 2800 tph

Age at thinning 5.09 5.27 8.73 8.20

Pre DBH (cm) 6.15 6.20 11.62 9.15

Post DBH (cm) 6.79 6.99 12.70 10.81

Post Live Crown Ratio 0.87 0.78 0.63 0.52

Pre Density (tph) 1332 2474 1371 2646

Post Density (tph) 555 573 584 624

% Density Removed 57% 72% 57% 76%

Pre Basal Area (m2/ha) 4.07 7.36 14.34 17.22

Post Basal Area (m2/ha) 2.09 2.26 7.43 5.78

% Basal Area Removed 48% 65% 48% 67%

Pre Relative Density 0.14 0.25 0.37 0.49

Post Relative Density 0.07 0.07 0.18 0.15

Relative Density Removed 0.07 0.18 0.19 0.34
81

between diameter and initial density, but possibly continue 

to build off of these increased growth rates. Third, since 

no penalty is incurred with increasing density but a huge 

improvement in tree form occurs with increasing density, 

log quality and thus value is maximized. 

Mean annual diameter growth rates reported here (1.64 

cm/yr, averaged across all densities) compare favorably 

to those reported in DeBell and Harrington (2002). They 

report annual diameter growth rates of 1.2 and 1.0 cm/yr 

for 20-year-old trees grown in a plantation on a comparable 

site. Across their lowest density plots (within the range of 

densities presented here and potential operational planting 

densities), the similar pattern of decreasing diameter 

increment with increasing density was detected.

Comparisons of the results presented here with other 

spacing trials are difficult due to the natural regeneration 

origin of previous spacing studies as well as the extremely 

high (outside the range of operational forestry) densities 

used. However, in a review of density management studies, 

Puettman (1994) reported annual diameter growth rates 

between 0.6 and 1.2 cm/yr. He then extrapolates that under 

optimal management regimes “good” sites could average 

trees 38 cm in diameter in approximately 27 years. Using 

these same assumptions and the mean diameter increment 

across all densities (1.64 cm/yr) reported here, it would take 

only 23 years to attain trees of comparable size.

Contrary to conventional wisdom, dominant height 

growth was not independent of stand density. Generally, 

height increased with stand density except at extremely 

high densities. Trees were consistently shorter in the lowest 

density plots. Will the crossover effect occur for height as it 

did for diameter? It did for the highest density but looking 

at the height growth curves of the three lower densities, it 

remains unclear. All curves seem to have parallel trajectories 

at age 12 and since height growth for alder decreases 

dramatically around age 20, one could conclude that early 

differences in tree height due to planting density may 

maintain themselves through an entire rotation.

The rapid decline in height growth for only the 

densest plots raises an interesting question about potential 

differences in height growth patterns between plantations 

and natural stands. All published height growth curves are 

based on natural stands and natural stands usually establish 

at densities equal to, or more often, much higher, than 

2800 tph. Height growth reductions of the lower plantation 

densities were less than the highest density and seem to 

remain fairly constant. These results further indicate tree 

growth in plantations is distinct from that of natural stands 

and elucidates the need to draw new height growth curves 

for plantation-grown trees.

The negative effect of density on diameter combined 

with the positive effect on height resulted in similar 

mean individual, merchantable tree volume across all 

planting densities. However, the number of merchantable 

trees per acre varied considerably, mainly as a function 

of the absolute number of trees per acre and minimum 

merchantability limits. Trees in the lowest densities were 

generally larger in diameter than the other densities and 

therefore had a very large percentage (but not number) of 

merchantable trees. Conversely, the densest plots had lots 

of trees but few above the merchantable diameter limit. At 

this time, the best balance between tree size and number 

is found in the intermediate densities. Stand volume was 

considerably greater at intermediate densities as compared 

to the two density extremes.

Individual crop tree volumes reported here (0.16 m3) 

were comparable but slightly greater than that reported 

by Hibbs et al. (1989) for 14-year-old individual crop 

Figure 7—Comparison of live crown ratio (LCR) by initial planting density 

for the 247 largest trees per hectare (crop trees). Data is derived from 13 

red alder plantations throughout the Pacific Northwest with an average site 

index of 30.9 m (base age 50 years) calculated from Harrington (1986).

Figure 8—Comparison of diameter at breast height (DBH) for unthinned 

(control) trees, trees thinned at age 4 and trees thinned at age 8. All plots 

were planted at approximately 1480 trees per hectare (tph) and thinned 

to approximately 570 tph. Data is derived from 13 red alder plantations 

throughout the Pacific Northwest with an average site index of 30.9 m 

(base age 50 years) calculated from Harrington (1986).
82

tree volumes found in a natural stand (0.15 m3). Per 

hectare volumes of the two intermediate densities in 

this study (approx. 77.3 m3/ha) was much greater than 

what Worthington et al. (1960) predicted for 15 year-old 

unmanaged alder of equivalent site productivity (31.4 m3/

ha).

These volume estimates are only a snapshot during 

stand development. Using these values at age 12 to 

predict volume at final rotation age is unwise for multiple 

reasons. First, as the stands age, growth rates change 

in different patterns for the different planting densities. 

These differential diameter and height growth rates 

are not accounted for and will result in major errors in 

extrapolation. Second, these are merchantable volume 

estimates (trees greater than 15 cm DBH). Not accounted 

for are the trees just below this limit that would become 

merchantable in the future. Third, stand growth (and thus 

volume) will be affected by any changes in site or stand 

growing conditions. Precipitation, storm events, insects, 

etc. all affect final stand volume.

The growth patterns observed here are unexpected 

results according to current theory (Grey 1989, Smith et al. 

1997). Density-dependent height growth has been observed 

for southern pines (MacFarlane et al. 2000, Quicke et al. 

1999) and for Douglas-fir (Scott et al. 1998). For alder, this 

effect has either not been observed (Bormann 1985, Smith 

and DeBell 1974) or observed for alder planted at extremely 

high densities (DeBell and Harrington 2002, Hurd and 

DeBell 2001), well outside the range of operational planting 

levels.

This research agrees with previous research (Bormann 

and Gordon 1984, Cole and Newton 1987, DeBell and 

Giordano 1994, Hibbs et al. 1989, and Knowe and 

Hibbs 1996) that optimum height growth is maintained 

at intermediate levels of density and reduced at either 

extreme. But a word of caution is needed when attempting 

to compare natural stand growth with plantation growth. 

Puettman (1994) recognized that site index curves are based 

on natural stands and natural stands come in at very high 

initial densities. This would result in reductions in potential 

height growth and an overestimation of productivity when 

applied to plantations. Furthermore, will this enhanced 

height growth continue to final rotation? Regardless, it is 

apparent that just the simple act of planting trees at lower 

densities than those found in natural stands can increase 

diameter and height growth through at least age 12.

Log quality is extremely important in determining 

value for alder since clear wood has much greater value 

than unclear wood. Perhaps what is more important than 

the differential effect of density on growth is the effect of 

density on tree form. Figure 9 illustrates the stem form 

differences found across planting densities. Increasing 

planting density reduces multiple stems, tree lean and 

sweep and increases the length of the branch-free bole; 

resulting in increased log quality. This research and others 

has found that long-term diameter growth will be reduced 

somewhat at these higher densities. Wide spacings, however, 

lead to large branches on the lower bole that reduces log 

grade and value of lumber recovered. Such branching can be 

reduced by growing the trees at denser spacing to facilitate 

natural pruning (Hibbs and DeBell 1994), and then thinning 

once a desired branch-free bole is obtained. This research 

indicates planting at intermediate densities maximizes tree 

form with minimal deductions in growth. Furthermore, a 

clear understanding of the effect of live crown ratio on tree 

growth and log quality is absolutely essential in choosing 

and timing silvicultural treatments.

Thinning

The crossover effect observed in diameter due to 

planting density begs the question of whether or not to 

thin alder plantations (and if one should, then when and to 

what residual density). Clearly, diameter growth rates are 

optimized in intermediate densities through age ten, then 

they start to decline. This leads to the question; can these 

declines in diameter (and by extension, volume) growth be 

prevented with thinning? This question will be discussed in 

the framework of four issues: intensity of the thin, timing of 

the thin, tree vigor, and stem form. 

Although not specifically tested in this project, it 

seemed that the intensity or magnitude of removal with 

thinning had little effect on tree growth response. As 

mentioned earlier, thinning 2800 tph plots to the same 

residual density as the 1480 tph plots (i.e. a much heavier 

thin, see table 1) resulted in the same patterns of individual 

tree response. However, stand responses would differ in 

that the leave trees in these denser plots would be smaller 

than the intermediate plots, would have reduced diameter 

increments, and would thus take longer to reach equivalent 

size. Secondly, it has been observed that alder is susceptible 

to weather damage following thinning, especially 

with drastic reductions from very high densities. The 

experimental design here had all thinning treatments to one 

residual density (approximately 570 tph). Therefore, until 

more research is done testing specific residual densities, 

specific recommendations on the intensity of precommercial 

thinning cannot be accurately made. However, within the 

range of the densities used in this project, thinning resulted 

in increased diameter response, had no effect on tree height, 

and thus increased individual tree volume regardless of preor post-thinning densities. 

Results presented here indicate that thinning elevated 

diameter increment levels as compared to not thinning. 

Furthermore, thinning early offset or prevented the gradual 

decline in diameter increment with age and thus resulted in 

greater diameter increment when compared to thinning later. 

At age 12, the mean diameter of plots thinned at age five (on 

average) was 1.5 cm greater than plots thinned at age eight. 

This increase would likely be maintained throughout the 
83

rotation as seen by the apparent parallel diameter trajectories 

of the thinned plots illustrated in fig. 8. 

The question of when to thin can also be addressed by 

using this data and placing it into the context of relative 

density. Relative density was calculated for each of the 

four planting densities and plotted in relation to the density 

management guide (Puettman et al. 1993) in fig. 10. Line 

B (relative density of 0.65) is the average maximum or 

the “self-thinning” line that stands approach as trees grow 

and mortality reduces their numbers. Line C (relative 

density of 0.45) is the operating maximum which is the 

line above which considerable mortality occurs. Line D 

(relative density of 0.25) is the competition threshold which 

is the line below which the site resources are not fully 

utilized. The zone between lines D and C is considered the 

recommended management zone because 1) below line D 

individual tree diameter growth is maximized but stand 

productivity is reduced and 2) above line C substantial 

mortality and reduction in tree growth occurs. This 

management zone compromises individual tree growth and 

stand yield.

Therefore, according to the diagram, trees in the 

1480 tph density would start suffering growth losses (i.e. 

competition) at age six and significant mortality at age 

ten. This timeframe of the management zone underlies the 

importance of early management and illustrates the narrow 

“window” of management activities of alder stands as 

compared to conifers. However, the data used here implies 

that diameter growth reductions may occur even before age 

6 (i.e. Line D) and waiting until age ten (i.e. Line C) would 

result in not only mortality but significant losses in diameter 

growth.

According to Puettman et al. (1993), the location of the 

competition threshold (Line D) was determined not only 

by diameter growth reductions due to competition but also 

by the observation that some stands thinned below this line 

showed reductions in height growth (also see Hibbs et al. 

1989) and that trees growing in stands with densities below 

this line had poor height growth and stem form. Therefore, 

the recommendation is not to thin below a relative density 

of 0.25. The results presented here are contrary to this 

recommendation. First, as stated above, it seemed that 

substantial diameter growth losses in plantations occurred 

near or even below Line D. Second, no reductions in height 

growth after thinning occurred in any of the four thinning 

treatments, all of which reduced relative density far below 

Line D. Third, trees in these stands had already achieved 

desirable stem form and form did not deteriorate after 

thinning. The discrepancies between what was observed 

here and the recommendations from the stand density 

diagram (thin earlier and heavier) emphasize the need for 

more research on tree growth in plantations versus natural 

Figure 9—Comparison of tree form between trees planted at planted at 1480 trees per hectare (left) and planted at 290 trees per hectare (right). Both photos 

taken on the same day, from the same plantation, from the same location.
84

stands, underscore the benefits of stand management and 

illustrate the importance of tree vigor and stem form in 

determining when to thin.

When alder is planted at densities high enough to 

rapidly occupy the site and (as shown earlier) to maximize 

early tree growth, crown recession occurs rapidly. Thus, 

it would seem that thinning early is crucial since thinning 

must favor trees with good growth potential (i.e. large 

crown size). Thinning early will capture the early rapid 

growth phase and maintain a good live crown ratio but 

reduce stem quality because of the presence of branches on 

the lower stem. Trees thinned late would have better wood 

quality but reduced vigor (as indicated by reduced crown 

ratios) and thus, less growth response following thinning. 

Therefore, as with all species, the goal of thinning is to 

balance the maintenance of vigorous individual tree growth 

with overall stand growth and, more exclusively with alder, 

optimal stem quality.

Thinning in alder plantations increased individual tree 

diameter with minimal effect on height growth. Volume 

data at age 12 indicated that the reduction in stand density 

through thinning is offset or will be overcome by the 

increased diameter growth rates associated with thinning. 

Furthermore, the accelerated diameter growth rates would 

result in shorter rotations because the remaining trees would 

reach commercial size sooner. However, the long-term 

effects of the time and intensity of thinning on plantation 

yield and wood quality are still unknown.

Conclusion

Red alder (Alnus rubra) is the major hardwood tree in 

the Pacific Northwest yet, until recently, management efforts 

have been practically non-existent. However, the rapid 

growth rates and the recent increase in alder prices have 

resulted in increased interest in managing alder.

Due to a number of biological factors, managing 

alder is different from managing conifers. First, alder has 

impressive juvenile growth rates. It is not until about age 40 

that the associated conifers surpass alder in height. This not 

only reduces rotation lengths but creates a narrower window 

in which to perform management activities. Second, good 

performance of alder is limited to a narrower range of sites 

than most conifers. For instance, alder is very susceptible 

to summer drought stress as well as unseasonable frosts. 

This makes site selection and site preparation an extremely 

important first step in alder plantation establishment. Third, 

alder is resistant to many diseases that afflict conifers such 

as laminated root rot and Swiss needle cast. Fourth, alder 

is an important element of healthy forests and biodiversity. 

Alder fixes nitrogen (improving site productivity), furnishes 

wildlife habitat and is an important component in aquatic 

food webs. Fifth, alder is not a commodity product; it 

occupies a special niche due to its uniform wood properties. 

This places special emphasis on log quality as well as 

overall yield.

Previous research has indicated that managed stands of 

alder can out-perform natural stands. Not only are growth 

rates increased, but tree/stem form are improved as well. 

This improvement is mainly due to proper site selection, 

control of competing vegetation and planting density, and 

silvicultural treatments.

Figure 10—Relative density by initial 

planting density. Data is derived from 13 

red alder plantations throughout the Pacific 

Northwest with an average site index of 

30.9 m (base age 50 years) calculated from 

Harrington (1986). Lines B, C, and D are 

taken from the density management guide 

developed by Puettman et al. (1993).
85

When deciding on whether, when, and how much 

to thin, a manager must weigh individual tree growth 

and vigor, stand yield, and log quality. Regardless of 

specific ownership objectives, it is recommended that 

precommercial thinning should be done early in stand 

development. Thinning too early negates the growth 

benefits associated with higher densities and would reduce 

stem quality due to branches. Thinning too late results in 

improved wood quality (due to self-pruning) but the trees 

are smaller with reduced vigor and thus do not respond well 

to the thinning.

More research into the long-term effects of initial 

plantation density and the timing and intensity of thinning 

treatments on alder tree growth, wood quality, and stand 

yield is needed. Once this is achieved and an alder growth 

and yield model is developed, alder can contribute to both 

economic and ecological forest management objectives.

Acknowledgements

This research was supported by the dues paying 

members of the Hardwood Silviculture Cooperative. We 

would like to thank the following organizations for their 

contributions to this research:

British Columbia Ministry of Forests

Bureau of Land Management

Goodyear-Nelson Hardwood Lumber Co.

Oregon Department of Forestry

Pacific Northwest Research Station, USFS

Siuslaw National Forest

Washington Department of Natural Resources

Washington Hardwoods Commission

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Forests. 124 p.

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The biology and management of red alder. Edited by: 

Hibbs, D.E.; DeBell, D.F.; Tarrant, R.F. Oregon State 

University Press, Corvallis, OR. pps. 229-242.

Puettman, K.J.; DeBell, D.S.; Hibbs, D.E. 1993. Density 

management guide for red alder. Research Contribution 2. 

Corvallis, OR: Oregon State University Forest Research 

Laboratory. 6 p.

Quicke, H.G.; Glover, G; Glover, R. 1999. Loblolly pine 

growth response to herbaceous vegetation control at 

different planting densities. Canadian Journal of Forest 

Research 29: 960-967.

Raettig, T.L.; Connaughton, K.P.; Ahrens, G.R. 1995.

Hardwood supply in the Pacific Northwest: A policy 

perspective. Research paper PNW-RP-478. U.S. 

Department of Agriculture, Forest Service, Pacific 

Northwest Research Station. 80 p.

Scott, W.R.; Meade, R.; Leon, R.; Hyink, D.; Miller, R. 

1998. Planting density and tree-size relations in coast 

Douglas-fir. Canadian Journal of Forest Research 28: 74-

78.

Skinner, E.C. 1959. Cubic volume tables for red alder 

and Sitka spruce. Res. Note 170. Portland, OR: U.S. 

Department of Agriculture, PNW Forest and Range 

Experiment Station. 4 p.

Smith, D.M.; Larson, B.C.; Kelty, M.J.; Ashton, P.M.S. 

1997. The practice of Silviculture: applied forest ecology. 

New York. John Wiley and Sons, Inc. 537 p.

Smith, J.H.G.; DeBell, D.S. 1974. Some effects of stand 

density on biomass of red alder. Canadian Journal of 

Forest Research 4:335-340.

Wilson, B.F. 1984. The growing tree. Amherst, MA: The 

University of Massachusetts Press.

Wipfli, M.S.; Deal, R.L.; Hennon, P.E.; Johnson, A.C.; 

DeSanto, T.L.; Hanley, T.A.; Schultz, M.E.; Bryant, 

M.D.; Edwards, R.T.; Orlikowska, E.H.; Gomi, T. 

2002. Managing young upland forests in Southeastern 

Alaska for wood products, wildlife, aquatic resources, 

and fishes: problem analysis and study plan. USDA Forest 

Service, General Technical Report PNW-GTR-558. 

Pacific Northwest Research Station, Portland, OR.

Worthington, N.P.; Johnson, F.R.; Staebler, G.R.; Lloyd, 

W.S. 1960. Normal yield tables for red alder. Res. Paper 

36. Portland, OR: US Department of Agriculture, Forest 

Service, Pacific Northwest Forest and Range Experiment 

Station.
87

1

Alex Dobkowski is a research forester, Weyerhaeuser Timberlands Research & Development, 505 North Pearl Street, Centralia, WA 98531, 

email: alex.dobkowski@weyerhaeuser.com.

— Red Alder: A State of Knowledge —

Red Alder Plantation Establishment: 

Site Selection, Site Preparation, 

Planting Stock, and Regeneration

Alex Dobkowski1

Abstract 

The information to be presented is based on research 

results from experimentation done on red alder (Alnus rubra 

Bong.) from 1987 to 2001 by Weyerhaeuser Company, and 

anecdotal evidence from almost 20-years of experience 

growing red alder at an operational-scale on Weyerhaeuser 

lands in western Washington. 

As with any tree species, the keys to successful red 

alder plantations establishment are: (1) site selection; (2) 

quality planting stock; (3) site preparation; (4) out plant 

timing; and (5) planting quality. However, red alder is 

generally less forgiving of sub-optimums for these factors 

than other commercial tree species grown in the Pacific 

Northwest.

 Frost, very poor soil drainage, drought stress and 

exposure can negatively affect establishment success, 

site productivity and wood quality (from top-breakage 

and sun-scald) of red alder plantations. Sites with these 

characteristics are not suitable for growing of red alder 

commercially. Frost/cold is the number one killer of planted 

red alder—avoid frost prone sites and within-site frost 

pockets. Generally, the best sites biologically for growing 

red alder are also some of the best sites for growing 

Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco).

All stock types have been used to establish red 

alder plantations successfully. Seedling morphological 

characteristics and physiological readiness at time of 

planting, and site conditions over the first 3-years, determine 

field performance success. Seedling survival and early 

growth is predicted by the following factors: (1) stem basal 

caliper (measured 25-mm above the root-collar); (2) height; 

(3) height on the stem to the lowest healthy bud; (4) percent 

This paper was published in: Deal, R.L. and C.A. Harrington, eds. 2006. Red alder—a state of knowledge. General Technical 

Report PNW-GTR-669. Portland, OR: U.S. Department of Agriculture, Pacific Northwest Research Station. 150 p.

of the stem with healthy buds; (5) total vegetation ground 

cover in years 1 and 2; and (6) root system fullness. Basal 

caliper appears to be the most significant factor for seedling 

survival and growth—presumably because more stored 

carbohydrates are available for new root growth early in the 

growing season. This allows the tree to develop an active 

root system before the demands from rapid leaf growth 

begins. Seedlings with a basal caliper of 6-10 mm, height 

of 60-90 cm, a root mass that is dense enough to indicate 

a good balance with the shoot, and with abundant Frankia

nodules throughout the root system will survive, grow fast 

and capture the site quickly.

Weed competition > 30% cover decreases growth and 

> 90% decreases survival. Currently there are few selective 

herbicides for release of planted red alder from weed 

competition—most broadcast vegetation control must be 

done prior to planting.

The spring planting period begins when the probability 

of a killing frost is low and ends before there is an 

appreciable seasonal drying of the soil. The recommended 

planting window for western Washington, at elevations less 

than 300 meters, is mid-March to mid-April.

Red alder plantation establishment can be very 

successful on “the right sites,” when the “correct” 

silviculture is practiced. Establishment success and 

subsequent tree growth can be highly variable on “suboptimal” sites, or when sub-optimal silviculture is practiced.

Keywords: Red alder (Alnus rubra Bong.), site selection, 

planting stock, site preparation and planting.
88

Introduction

Red alder (Alnus rubra Bong.) is a valuable hardwood 

tree species that competes with many other hardwoods in 

the marketplace (Western Hardwood Association 2005). Red 

alder is used principally for the production of lumber and 

veneer—high-grade lumber for fine furniture, cabinets, and 

turnings; lower-grade lumber for furniture frames/interior 

parts and pallets; face veneer for cabinets; and core veneer 

for high-quality paneling. When red alder is compared 

to other important North American hardwoods—yellow 

birch (Betula alleghaniensis Britton), black cherry (Prunus 

serotina Ehrh.), sugar maple (Acer saccharum Marshall)—

its workability index reflects that it is very competitive for 

finishing, machining, sanding, polishing, color uniformity 

and gluing, and it has the lowest specific gravity/strength. 

A very important characteristic of red alder is that the wood 

readily accepts stain enabling red alder to mimic more 

expensive woods very convincingly. Lower grade alder logs 

have value as a desirable pulpwood species. 

Because of the expectation of a continuing strong 

hardwood industry in the Pacific Northwest, some 

landowners are investing in growing red alder saw-logs to 

provide future raw material for that industry. This paper will 

focus on what we know about the requisites for successful 

red alder plantation establishment. The information 

presented is based on results from experimentation, and 

anecdotal evidence from almost 20-years of research and 

operational experience in red alder tree growing. The 

intent is to provide a practical guide to assist landowners in 

growing red alder plantations successfully.

As with any tree species, the factors to successful 

red alder plantations are: (1) site selection; (2) quality 

planting stock; (3) correct site preparation; (4) proper out 

plant timing; and (5) planting quality. The exception for 

red alder is that the species is generally less forgiving of 

sub-optimums for these factors than other commercial tree 

species grown in the Pacific Northwest.

Site Selection

Site selection for red alder requires evaluation of site 

characteristics relative to the risk of plantation failure from 

environmental factors, red alder productivity potential, 

and the need for herbaceous weed control. The objective 

is to exclude from consideration planting units that will 

have obviously low red alder productivity potential and/

or obviously high risk of plantation failure due to poor 

soil drainage, frost, drought or inability to control weed 

competition. 

Risk of Plantation Failure

Some characteristics of the better sites for red alder 

management are (Harrington 1986, Harrington et al. 1994):

Elevation – Less than 300 m;

Physiographic position – Flood plain, terrace, bench, 

lower slopes;

Aspect – West to east slopes;

Slope – 10-30%;

Soil drainage –Well drained;

Soil texture – Silt loam, silty clay loam, clay loam, silty, 

clay;

Soil depth – Greater than 75 cm;

Depth to summer water table – 2- to 3-meters; and

Parent material – Sedimentary, sedimentary/volcanic, 

and volcanic.

Planting units with a combination of the following site 

characteristics are probably undesirable candidates for red 

alder production (site productivity will be low and the risk 

of plantation failure will be high):

• Bog or marsh areas;

• Upper slope positions;

• Ridge tops;

• S to SW exposures;

• Frost pockets (within site & “macro” cold air 

drainage);

• Exposed and windy sites;

• Droughty sites - sandy, excessively drained, soils;

• Poorly drained soils;

• Depth to summer water table less than 1-meter or 

greater than 4-meters.

• Highly weathered/leached soils.

• Elevation greater than 450 meters.

• Sites with expected very high levels of weed 

competition that can not be adequately controlled 

to low levels—i.e. salal (Gaultheria shallon).

Foresters need to use their judgment when determining 

the suitability of a site for growing red alder. Any 

single characteristic that would result in a high risk of 

plantation failure may be sufficient to exclude a unit from 

consideration for red alder production (e.g., a severe 

frost pocket). If a unit has an “undesirable” characteristic 

that does not increase the risk of mortality, it may still 

be acceptable for red alder production depending on the 

combination of other factors (e.g., S to SW Exposure). A 

site with multiple “undesirable” characteristics is probably 

unacceptable as a red alder management unit (e.g., S to SW 

exposure and excessively drained soils).
89

Select sites with a low risk of regeneration failure. Poor 

soil drainage, frost, drought and difficult to control weed 

communities can hinder successful plantation establishment 

considerably.

Although naturally occurring red alder can tolerate 

poorly drained soils, careful examination shows that it 

occupies only the better drained micro-sites within the 

irregular topography of a wet site. The establishment of 

a well-stocked plantation is significantly hindered on wet 

sites because suitable micro-sites occur infrequently and are 

poorly distributed.

Newly planted seedlings are adversely affected by poor 

drainage. Where saturated soils persist into the growing 

season, poor drainage induces seedling mortality and also 

severely restricts root growth of those seedlings that survive 

periodic soil saturation. The diminished root system can 

predispose newly planted seedlings to later summer drought 

stress. Given the heavy herbaceous weed communities 

that can develop on these sites and limited site preparation 

options, drought stress effects can be compounded, resulting 

in considerable seedling mortality. Red alder is a riparian 

and upland site species—not a wet-site species.

Areas of high frost hazard should not be regenerated 

to red alder. These sites are associated with topographic 

features having a high probability of cold air drainage from 

higher elevations in the spring and fall seasons (fig. 1). 

Vegetation condition and topographic features can be used 

to assess the likelihood that a site is in a cold-air drainage. 

Red alder and other naturally occurring woody vegetation 

can show evidence of previous frost events. Areas where fog 

hangs for long periods indicate potential for a frost pocket. 

Valleys exhibiting a gentle gradient from high elevation 

areas or topographic features that form blockages tend to 

slow the flow of cold air and be more frost-prone. Frost 

risk is increased if there is a cold-air dam such as a ridge of 

mature trees downhill or downwind (a planting unit that is 

surrounded by timber can create a frost pocket).

There is evidence that soils with a high summer water 

table may be more prone to frost risk. These soils provide 

a good supply of late-growing season soil moisture—

prolonging red alder growth well into the early fall. Alder 

in this de-hardened state are very vulnerable to early frost. 

This type of an effect is usually very clustered within a 

plantation. Exposed sites in the Cascade foothills, that are 

prone to east-winds in the winter, also have a high risk of 

freeze related mortality and damage. The effects of cold 

associated with normal seasonal weather can be mitigated a 

considerable amount through site selection. However, young 

red alder plantations will always be vulnerable to the effects 

of arctic outbreaks in the early-autumn because red alder 

tends to grow late into the growing season and is usually not 

hardened-off by early-autumn.

Summer drought and heat stress contribute significantly 

to reduced performance of newly planted red alder 

seedlings. Regeneration difficulties have been particularly 

noted on droughty sites typified by south-southwest aspects, 

steep slopes, and coarser textured soils. Units with heavy 

textured soil that have been seriously compacted from 

ground-based logging tend to have excessive alder mortality 

on logging trails—root system development seems to be 

seriously impeded and moisture stress results as the crown 

foliage mass continues to develop.

There may be portions of a planting unit that is 

unsuitable for red alder—frost, wind, or drought prone. 

If these micro-sites are large enough to be identified and 

flagged-out, Douglas-fir (Pseudotsuga menziesii (Mirb.) 

Franco) or some other species should be planted. An 

example of this would be units in the Cascade foothills. The 

mid- and lower-slopes can be ideal for red alder and the 

ridge-top slope position unsuitable. Steep southwest slopes 

in a unit with predominantly moderate slope and favorable 

aspect are another example.

Red Alder Site Productivity

Site productivity for red alder is an important 

consideration for commercial production. Site index based 

on the heights of an appropriate sample of trees near index 

age (i.e., base-age 20 or 50-years for red alder) is a common 

practice for making this determination. Most often site index 

can not be estimated directly from tree height measurements 

because the tree species of interest is not presently growing 

in even-aged stands, of the correct age, in the vicinity of 

Figure 1—Shown is an example of how topographic conditions can be 

evaluated to judge the risk of cold damage—“Dw” has a high risk of 

frost—cold air accumulation; “Ll” on mid- to upper slope positions has 

enough “air” drainage to prevent serious frost. Note photograph plantation 

failure from cold damage on “Dw” and vigorous, well-stocked plantation 

just upslope on soil “Ll”.
90

sites to be regenerated. A useful estimate of red alder site 

productivity (base age 50-years, Worthington 1960) can be 

obtained in the absence of site trees to measure following 

the procedures given in, “A Method of Site Quality 

Evaluation for Red Alder” (Harrington 1986).

The evaluation process requires that the user score 

soil-site factors that pertain to geographic/physiographic 

characteristics, soil moisture/aeration, and soil fertility/

physical properties. Applying the site quality evaluation 

method requires the soil survey/soil series profile 

descriptions, long-term meteorological data, local 

knowledge and on-site observation. 

Soil survey and soil series profile descriptions provide 

information on the following: 1) elevation; 2) physiographic 

position; 3) aspect and slope; 4) soil texture; 5) soil depth; 

6) soil rock and gravel content; 7) soil parent material; 8) 

soil pH; 9) soil organic matter, and 10) soil bulk density. 

Published long-term meteorological data from the recording 

station closest to the planting unit is used to determine 

growing season precipitation (April 1 through September 

30). Local knowledge is used to assess special hazards (frost 

pockets, exposure, wind, etc.), internal soil drainage, and 

approximate depth to the summer water table.

On-site observation is required to: 1) confirm the soilsite factors taken from the soil survey; 2) determine site 

specific special hazards; 3) verify internal drainage and 

summer water table depth assumptions; 4) evaluate the 

character of red alder growing in the vicinity of the unit 

(frost or heat damage, health, stem form, etc.).

Experience has shown that this approach classifies sites 

accurately as to good, intermediate or poor sites for growing 

red alder. The accuracy of the method can be increased by 

giving more of a deduction in site-points for droughty sites, 

exposed windy sites, frost pockets, and when pH in the 

surface soil is 4.0 or less.

Lands selected for growing red alder should be good 

sites and have a very low risk of poor establishment success. 

Generally, the best sites biologically for growing red alder 

are also some of the best sites for growing Douglas-fir.

Planting Stock

All stock types (containerized plug, bare-root, plug+1/2 

transplant) have been used successfully to establish red 

alder plantations (Radwan et al. 1992, Dobkowski et al. 

1994). Research trials have shown that survival and early 

growth of bare-root seedlings is predicted by the following 

morphological characteristics and other factors: (1) basal 

caliper (measured 25mm above the root collar); (2) height; 

(3) height to the lowest healthy bud or lateral branch; (4) 

percent of the stem with healthy buds or branch laterals; 

(5) total vegetation ground cover in years 1 and 2; and (6) 

root system fullness. Root System fullness for bare-root 

seedlings is classified as follows:

Good (fig. 2)—An abundance of fibrous, flexible, 

heavily branched roots, and an absence of heavy, stringy, 

non-branched roots; root mass is dense enough to indicate a 

good balance with the shoot; abundant Frankia nodulation 

present.

Fair (fig. 3)—One or several heavy, woody, nonbranched roots, but a portion of the root system is fibrous 

and well-branched, shoot appears proportionately heavier 

than the root system (i.e., shoot and roots are not “well 

balanced”); Frankia nodulation present.

Poor (fig. 4)—Root systems are predominately 

composed of heavy, woody, non-branched roots, or if there 

are fibrous roots, they are too few to support the tree; the 

root system is clearly out of balance with the shoot. Frankia

nodulation is rare.

Basal caliper appears to be the most significant factor 

for seedling survival and growth—presumably because 

more stored carbohydrates are available for new root growth 

early in the growing season. This allows the tree to develop 

an active root system before the demands from rapid leaf 

growth begins.

Figure 5 gives an example from one experiment 

showing the relationship between the probability of 

achieving an age-1 plantation-height target and seedling 

root system, basal caliper and height (A. Dobkowski, 

unpublished data 2004). This result from a logistic 

regression analysis indicates that seedlings with a good root 

system, basal caliper of 6 mm or more, and height of 60 cm 

has a 0.80 probability of achieving the age-1 height target. 

Results from this and other field experiments, coupled 

with operational experience, suggests the following 

specifications for bare-root seedlings (Ahrens 1994, 

Dobkowski et al. 1994, Molina et al. 1994, Kendall et al. 

2003): 

• Height—range 45-100 cm—with approximately 

70% of the seedlings between 60 and 70 cm;

• Basal caliper (measured 25 mm above the 

root collar)—5 mm minimum caliper—with 

approximately 70% of the seedlings between 6 and 

9 mm;

• Root systems characterized by an abundance of 

fibrous, flexible, heavily branched roots, and an 

absence of heavy, stringy, unbranched roots, root 

mass dense enough to indicate a good balance with 

the shoot; many Frankia nodules;

• Healthy branches or buds along the full-length of 

the stem; and

• Free from disease and top-damage.
91

Bare-root seedlings with these attributes, when planted 

on good sites and with proper site preparation, will achieve 

survival and height-age targets most of the time. Generally, 

large seedlings (large caliper, balanced height and caliper, 

good root systems and Frankia nodulation) are preferred—

even when taking into consideration that the logistics of 

handling and planting large stock is more problematic and 

can incur more cost. Smaller stock types and seedlings 

generally have a much greater risk of poor performance.

Weed Control

Heavy first and second-year weed competition, 

particularly grass and herbaceous plant life-forms, has been 

shown to be detrimental to red alder survival and growth 

(Figueroa 1988, Dobkowski et al. 1994). Weed competition 

thresholds based on first-year cumulative ground cover of 

grass/sedge, forbs ferns, and woody plants:

• Minimal Effects – Below 20%; 

• Growth Impacts – 30% to 80%; and

• Survival Impacts – Greater than 90%.

Weed competition occurring late in the first growing 

season, and in the second growing season, can have serious 

impacts on rapid tree growth. Effective control of weed 

competition can often be the difference between plantation 

success and failure (fig. 6).

Figure 7 (A. Dobkowski, unpublished data 2004) 

shows the probability of achieving plantation height age 

targets by seedling basal stem diameter and height at time 

of planting with heavy (80%+ cumulative vegetation 

Figure 2—Shown are bare-root seedlings (1+0)—height and basal caliper 

suitable for out-planting—“Good” root systems.

Figure 3—Shown are bare-root seedlings (1+0)—height and basal caliper 

suitable for out-planting—“Fair” root systems.

Figure 4—Shown are bare-root seedlings (1+0)—height and basal caliper 

suitable for out-planting—“Poor” root systems.

Figure 5—Shown is the probability of bare-root seedlings, with poor or 

good root systems, achieving 1-meter in height one growing season after 

planting.

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

3 4 5 6 7 8 9 10 11 12 3 4 5 6 7 8 9 10 11 12

 

30 CM 45 CM 60 CM

Poor root system Good root system

Seedling stem diameter (mm) and root system

Probability
92

ground cover) and very-low weed competition (10%-15% 

cumulative vegetation ground cover). This result indicates 

that even seedlings with larger basal caliper (7 to 9 mm) do 

not perform satisfactorily under heavy weed competition. 

Conversely, most seedlings large enough for operational 

out-planting (basal caliper greater than 5 mm and height 

of greater than 45 cm) perform well under low weed 

competition.

Weed control prescriptions need to consider weed 

communities that existed in the under-story of the harvested 

stand as well as weed invasion by forbs, grasses, and woody 

shrubs into newly harvested areas. There are currently a 

very limited number of herbicides for site preparation and 

the release of planted red alder from weed competition. For 

practical purposes, all broadcast herbicide control measures 

must be taken prior to planting. Atrazine 90 WSPTM (active 

ingredient, Atrazine; Helena Chemical Company 2005; 

Washington State only), Accord ConcentrateTM (active 

ingredient, glyphosate; Dow AgroSciences 2005), and 

EscortTM (active ingredient, Metsulfuron methyl; DuPont 

2005) are registered/labeled for use in red alder site 

preparation. Atrazine 90 WSP can be broadcast over the 

top of dormant red alder as a release treatment. Accord is 

labeled for the release of hardwoods from weed competition, 

but only as a directed spray—not as a broadcast application.

Cumulative vegetation ground cover in the first 

growing of less than 10-15% is desirable for rapid stand 

establishment and growth and to maintain ground cover in 

the second growing season below competition thresholds for 

red alder survival and growth. If ground cover of herbaceous 

weeds is expected to exceed 30-40% in year two, it is 

advisable to use herbicides in late-winter/early-spring, when 

red alder are still dormant and weeds are active. A broadcast 

application of Atrazine 90 WSPTM in late-winter and/or a 

directed spray of Accord ConcentrateTM applied in earlySpring can be effective as a release treatment.

Anecdotal evidence suggests soil scarification that 

exposes mineral soil (when combined with weed control) 

can increase survival and growth when heavy slash/forest 

floor is present.

Planting

Out-plant Timing

A planting date should be selected to balance the risks 

of freeze damage and drought stress. The spring planting 

period begins when the probability of a killing frost is low 

and ends before there is an appreciable seasonal drying of 

the soil. 

The recommended planting window for elevations 

less than 300 m, is mid-March to mid-April. Planting in 

November through February can result in serious freeze 

damage (top-kill and diminished root growth potential). 

Planting in late April to mid-May may not allow enough 

time for an adequate root system to develop before the onset 

of summer drought stress. It is advisable to begin planting 

in early March (at sites with minimal risk of spring frost) 

rather than planting into late April or early May.

Other considerations at time of planting

Red alder seedlings should be lifted from the nursery 

beds or styro-blocks/plugs in early January, freezer stored 

(-2°C) until the spring planting time is reached. Seedlings 

should be defrosted slowly under cover at approximately 

4 to 5°C. Once the seedlings have thawed they should be 

cooler stored (2°C) until planted. It is advisable to keep 

seedlings frozen until just before the planting date—

minimizing cooler storage to a week or less. Because red 

alder can de-harden very rapidly when removed from cold 

storage, seedlings stored in the field on the day of planting 

need to be protected from heat to prevent premature dehardening. On-site daily storage under an insulated truck 

Figure 6—This photograph shows an example of the effects of through 

weed control on first year field performance for red alder (pre-plant 

application Atrazine and Glyphosate on the left vs. no weed control on 

the right). Meter-stick shown for reference.

Check regulations before applying these or any 

herbicides—read and follow herbicide labels very 

carefully when applying herbicides. Figure 7—The probability of bare-root seedlings with “good” root systems 

achieving 1-meter height, one growing season after planting, given high 

and low levels of weed competition.

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

3 4 5 6 7 8 9 10 11 12 3 4 5 6 7 8 9 10 11 12

 

 

30 CM 45 CM 60 CM

Heavy weed competition Low weed competition

Seedling stem diameter (mm)
93

canopy or in the shade of standing timber covered with 

a heat shield (i.e., MylarTM) seedling protection tarp is 

recommended. Take only the number of seedlings that can 

be planted in one-day from cold storage to the field.

Red alder seedlings are brittle and prone to breakage. 

Planting crews accustomed to handling conifers need to be 

cognizant that alder seedlings require more care. Careless 

loading of seedlings into planting bags can result in 

considerable breakage to roots and stems. Care needs to be 

taken when closing the planting hole to assure that the stem 

is not wounded by the planter’s boot.

To partially offset the effects of heat and drought 

on newly planted seedlings, deep planting (ground 

level approximately 25-50 mm above the root collar) is 

recommended. Minimizing the scalping of forest floor 

during the planting process can reduce heat girdling; 

exposed mineral soil at the base of the stem from scalping 

acts as a heat-sink, and the thin bark of alder is readily 

damaged. Similarly alder planted directly against logging 

slash or stumps is easily damaged by sun-scald and/or 

mechanical abrasion—make certain that the planting spot is 

clear of surface slash for approximately a 30-cm diameter 

around the stem.

Conclusion

It is essential that “Best Management Practices” for 

site selection, plantation establishment and silviculture be 

followed in order to achieve successful red alder plantations. 

Achieving uniform stocking and rapid early growth, so 

that the stand “captures the site” within the first 3-years, 

is critical. It is essential that early intensive silviculture be 

practiced in order to assure rapid site occupancy. Rapid site 

occupancy by the crop promotes full stocking, rapid growth 

and good stem quality.

Red alder can be difficult to grow successfully in 

plantations – generally the species is “less forgiving” than 

conifers when planted on the “wrong” sites, and/or with 

improper silvicultural practices. For example; planting 

frost-prone sites or ineffective herbaceous weed control may 

reduce growth of Douglas-fir, but can result in extensive 

mortality and very poor growth of red alder. Effective 

weed control is more difficult for red alder because of the 

limited number of herbicides available for use on red alder, 

particularly for release treatments following planting. Also, 

the planting window is much narrower for red alder than 

for Douglas-fir (March 15–April 15 for red alder, vs. earlywinter to early-spring for Douglas-fir). The consequence 

of planting red alder too early or too late can be excessive 

mortality. 

Though the risks of failure and difficulty of growing red 

alder may be higher than growing conifers, we have learned 

how to mitigate the risks (through site selection, nursery 

practices and silviculture). The uncertainties of growing red 

alder must be weighed against the demand for red alder in 

commercial applications in the world-wide lumber market. 

Plantation-grown red alder can produce quality saw-logs in 

a relative short rotation of 25 to 30 years (compared to 60-

years plus for other important North American hardwoods). 

Some landowners think that red alder will continue to 

have increasing value potential because of its desirable 

commercial wood properties and relatively short rotation for 

saw-logs.

Literature Cited 

Ahrens, G.R. 1994. Seedling quality and nursery practices 

for red alder. Red alder plantation establishment. In: The 

biology and management of red alder. D.E. Hibbs, D.S. 

DeBell, and R.F. Tarrant (editors). Oregon State Univ. 

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screening trial in a newly established red alder plantation 

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1988 Western Society of Weed Sci. Mtg. Weed Science 

41:108-124.

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95

1Leslie C. Brodie is a forester and 2Constance A. Harrington is a research forester, Forestry Sciences Laboratory, 3625 93rd Ave SW, Olympia, 

WA, 98512–9193, email: lbrodie@fs.fed.us

— Red Alder: A State of Knowledge —

Response of Young 

Red Alder to Pruning

Leslie C. Brodie1 and Constance A. Harrington2

Abstract

Red alder (Alnus rubra, Bong.) is the most common 

hardwood in Washington, Oregon and British Columbia. It 

is used for a variety of products including firewood, pulp, 

and solid wood products such as furniture, cabinets and 

musical instruments. Pruning may be a viable management 

technique for increasing clear wood and, thus, value in 

managed stands but little information has been available. 

To determine the biological effects of pruning red alder, 

we selected 530 trees in 3-, 6- and 10-year-old plantations. 

Sample trees were from plots that had different previous 

silvicultural treatments, providing a range of growth rates. 

Pruning removed one third of the live crown, was performed 

on seven dates throughout the year, and included both live 

and dead branches as well as a sub-sample of intentionally 

damaged collars around dead branches. The rate of branch 

occlusion (healing) was well correlated with tree growth 

at breast height and with distance from the base of the live 

crown. Live branches occluded more rapidly than dead 

branches and dead branches occluded more rapidly if the 

branch collar was intentionally wounded during pruning. 

The number of epicormic branches induced by pruning was 

minimal, but increased with tree age and where trees were 

growing in an open condition. No stem breakage or sunscald 

was observed as a result of treatment. Six years following 

pruning, 91% of pruned branches less than 1 cm in diameter 

and 80% of branches 2–3 cm in diameter had completely 

occluded. Those that had not occluded by that time were on 

trees with low growth rates. Time of year of treatment had 

little effect on tree growth rates, occlusion rate, epicormic 

branch formation, and damage. Pruning young trees did 

not result in any damage or loss of growth. To maximize 

the amount of clear wood it would be best to prune as soon 

as logistically possible. Thus, if economic incentives are 

present for clear wood, landowners and managers may want 

to consider pruning young trees, taking into account the 

possible need for multiple lifts.

Keywords: red alder, Alnus rubra, pruning, branch 

occlusion, silviculture

This paper was published in: Deal, R.L. and C.A. Harrington, eds. 2006. Red alder—a state of knowledge. General Technical 

Report PNW-GTR-669. Portland, OR: U.S. Department of Agriculture, Pacific Northwest Research Station. 150 p.
96

Introduction

It is well known that initial density affects diameter 

growth. We can produce larger diameter trees more quickly 

if they are grown at wider spacings than if they are grown at 

narrower spacings. Associated with wide spacings, however, 

is the production and retention of large diameter branches, 

especially on the lower bole of the tree. These branches 

reduce the log grade and consequently reduce the products 

(and values) for which the log can be used. Some foresters 

have speculated that the lowest-cost option to produce 

quality wood would be to grow the trees at spacings close 

enough to cause natural death and abscission of the lower 

branches. However, this strategy assumes that dead branches 

will be shed soon after the abscission process is complete 

and that stem breakage will occur at a point close to the tree 

bole. It will also either increase the time required to produce 

trees of a specified size or require thinning after branches 

have naturally abscised. An alternative strategy would be to 

plant at wide initial spacings and plan to artificially prune 

the trees. While pruning can be an expensive process, the 

costs may be justified if increases in log quality can be 

transformed into increases in log value.

Pruning red alder has not previously been examined in 

a comprehensive manner. The only published report is that 

of Berntsen (1961) where 21-year-old trees were pruned, 

then examined 22 years later to see what the effects were 

on wood quality. Bernsten concluded that pruning did 

increase the amount of clear wood produced, but warned 

that epicormic branches could result in defects such as knots 

or blemishes. No other information is available specific to 

alder, however, much information is available in the forestry 

or horticultural literature on the effects of pruning on plant 

growth and on methods for pruning (O’Hara 1989). In 

addition, there is substantial information in the botanical 

literature on the physiological responses of plants to 

wounding (i.e. the removal of live branches).

Studies on the time of year of pruning are generally 

limited to only a few dates and do not include information 

on the phenological status of the trees. In addition, 

the results differ by species and the response variable 

considered. For example, Krinard (1976) concluded that 

summer pruning of eastern cottonwood was better than 

spring pruning because it resulted in fewer and shorter 

epicormic branches. McQuikin (1950) recommended 

pruning eastern hardwoods (except red maple) in late 

winter or early spring to achieve fastest wood occlusion; 

he recommended pruning red maple in late spring or early 

summer to reduce excessive bleeding (sap exudation). 

Neely (1970) reported May-pruned eastern hardwoods 

healed more quickly than July- or March-pruned trees. 

October-pruned trees healed much more slowly than trees 

pruned at the other three dates. Childs and Wright (1956) 

reported higher incidence of infection in Douglas-fir springpruned wounds than fall-pruned wounds. Shigo (1976) 

reported different decay mechanisms invaded eastern 

hardwoods depended on the time of the wounding. Leben 

(1985) concluded that spring wounding of red maple 

resulted in less dieback around the wounds and less decay 

than fall wounding. Puritch and Mullick (1975) reported 

water stress substantially slowed formation of an impervious 

layer in grand fir seedlings. Biggs and Cline (1986) reported 

irrigation to speed up formation of the impervious layer at 

some sampling dates. Given the range of responses reported 

in the literature, we could not predict the time of year or 

growth conditions that would be optimum to prune red alder.

Most pruning studies have dealt with the removal 

of live branches, but a few have also examined dead 

branches. Shigo (1986) indicates that branch collars 

should not be damaged when live branches are pruned, but 

recommendations are contradictory on the treatment of dead 

branches. It appears that injuring the branch collar when 

removing the dead branches will greatly speed up filling in 

the cavity at the base of the branch, however, Shigo et al. 

(1979) suggests that this will increase the amount of decay 

associated with the dead branch. Casual observations have 

noted that occlusion rates over dead branches on red alder 

can be slow.

We designed a study to determine the biological effects 

of pruning young red alder. The study focused on four 

questions. Are there any negative effects, such as sunscald, 

sap exudation, or production of epicormic branches, 

associated with pruning young red alder? If there are 

negative effects, do they vary by time of year of pruning? 

What effects do factors such as tree age, growth rate or 

height on the bole have on the rate of occlusion? Are there 

positive or negative effects of wounding the collar area 

around dead branches? A second study (DeBell et al. 2006) 

quantified internal decay rates associated with a subset of 

treatments.

Materials and Methods

Alder plantations established at the Washington State 

Department of Natural Resources’ Meridian Seed Orchard 

near Lacey, Washington and at the Centralia Mining 

Company—now owned by TransAlta Corporation—near 

Bucoda Washington, were selected to represent a variety 

of age groups and silvicultural regimes. The Meridian 

Seed Orchard included both thinned and non-thinned 

areas (at 1- and 2-m spacings respectively) from 3-yearold plantations established in 1989, and areas that had 

been previously irrigated and non-irrigated in 6-year-old 

plantations established in 1986. Trees selected at TransAlta 

were in 10-year-old plantations that had been established 

in 1982; these trees had been planted as part of a spacing 

trial and included a range of spacings from 2 to 4 m. Initial 

diameter at 1.3 m and total height are given in table 1. 

Mean annual precipitation at both locations is 1245 mm 

with 150 mm falling as rain from June 1 through September 

30. Mean annual, mean January minimum, and mean 
97

August maximum temperatures are 10.1, 0.1 and 25.1 °C, 

respectively, at the Lacey site and 10.6, 0.6, and 25.3 °C at 

the Bucoda site (Daly, 1994. http://www.ocs.orst.edu/prism).

The time-of-year portion of the study examined trees 

pruned on seven separate dates selected to cover the range 

in tree phenology. The dates (and general phenological 

condition) were: May 1 (budbreak and leaf expansion), June 

15 (active height and diameter growth), August 1 (active 

diameter growth, height growth primarily over), September 

15 (diameter growth continuing but some leaf abscission 

beginning), November 1 (leaf abscission almost complete), 

January 1 (deep dormancy, cold temperatures), and March 1 

(trees still dormant but temperatures moderating). The first 

five dates were in 1992 and the last two were in 1993. 

On each treatment date 20 trees from the 3-year-old 

plantation (10 thinned and 10 not thinned); 20, 6-year-old 

trees (10 irrigated and 10 not irrigated); and 20, 10-year-old 

trees were pruned to examine the effects of time of year, 

growth rate, and bole height on response. Pruning was 

done with a handsaw and the pruners were instructed to not 

wound the branch collars of either dead or live branches. 

Furthermore, at the May 1 pruning date, 40 additional 

10-year-old trees and 60 additional 6-year-old trees were 

pruned at the same two sites. This second group of trees had 

the live branches pruned in the same manner as described 

above, however, half of all trees at each site had dead 

braches pruned by removing the entire branch collar (i.e., 

the swelling around the base of the branch, particularly on 

the lower side). Removing the entire branch collar (cutting 

the branch flush with the stem) considerably increased the 

size of the wound created for all but the smallest branches 

which had minimal branch collars. 

Prior to pruning, all trees to be pruned were marked 

at 0.3 m on the bole and then every 1 m up the bole until 

pruning height was reached. Trees in the time-of-year 

portion of the study were pruned up to a point representing 

one-third of the existing live crown. For most trees this was 

approximately a height of 3.3–5.3 m, although 1 tree was 

pruned up to 7.3 m. All trees in the damaged-branch-collar 

portion of the study were pruned to 3.3 m. Each pruned 

branch was tallied by stem section from which it was 

removed (0–0.3 m, 0.3–1.3 m, 1.3–2.3 m, etc), whether it 

was alive or dead, and by 1-cm diameter class. Epicormic 

branches present before pruning were also tallied by stem 

section and then removed (usually with hand clippers).

After the 1992 growing season, all study trees were 

measured for stem diameter at 1.3 m, total height, and height 

to live crown. Diameter at 1.3 m, total height, and number 

and length of new epicormic branches was remeasured after 

each growing season from 1993 to 1997. Percent occlusion 

of all pruning wounds was also measured after each growing 

season from 1993 to 1997 by visually estimating the percent 

of the wound area that had been covered by callus (tissue 

formed over and around a wound). Each tree in the 10-yearold plantation was coded for proximity and direction of 

adjacent openings that might induce epicormic branching. 

These codes were not based strictly on spacing of adjacent 

trees, but also if the opening was sufficiently large to cause 

a discontinuity in crown closure and allow light to fall upon 

the bole of the edge tree. Form was assessed after the 1994–

1997 growing seasons by measuring stem diameter at 0.3, 

1.3, 2.3, and 3.3 m heights on the pruned study trees and on 

unpruned trees in the same plantations with 1992 diameters 

similar to those of the pruned trees.

Results and Discussion

Damage to pruned trees

Following pruning, no problems were encountered 

with sap exudation (bleeding), sunscald, stem lesions, top 

breakage, excessive production of epicormic branches, 

or damage by red-breasted sapsuckers on trees from any 

pruning date. Top breakage and sapsucker damage have 

been observed on red alder trees operationally pruned when 

50% or more of the live crown was removed (C. Harrington, 

personal observation) but removal of 33% of the live crown 

in this study did not predispose the trees to damage.

The frequency of epicormic branches increased 

immediately after pruning (table 2). However, by 1997, the 

3-year and 6-year old trees had less than 1 epicormic branch 

per tree and most of these branches were short and small in 

diameter. There was a reduction in the percent of trees with 

epicormic branches in all the plantations throughout the 

study period. The mean number of epicormic branches per 

tree remained relatively the same in the 6- and 10-year-old 

Mean d.b.h. Mean ht D.b.h. growth 1991-1997 (cm)

AGE Treatment 1991 (cm) 1991 (m) Min Max Mean

3 Thinned 4.8 4.7 2.1 12.5 7.5

Not thinned 4.1 4.1 0.5 6.5 3.3

6 Irrigated 11.6 11.6 1.2 10.6 5.0

Not irrigated 9.5 9.5 0.8 12.0 4.6

10 n/a 13.3 10.5 1.4 10.3 4.6

Table 1—Initial d.b.h. and height, and mean diameter growth from 1991 to 1997.
98

plantations and decreased by 37% for trees from the 3-yearold plantation. No relationship was found between the time 

of year that pruning took place and the number of epicormic 

branches produced.

Epicormic branches were more likely to be present in a 

more open environment. At the time of pruning, 56% of the 

trees in the thinned portion of the 3-year-old plantation—

the more open environment—had epicormic branches, 

compared to only 27% in the non-thinned portion (although 

both portions had a mean of less 2 small epicormics per 

tree). By the end of 1994, the crowns had closed in the 

thinned portion and the number of trees with epicormic 

branches and the mean number of epicormic branches per 

tree in the thinned and not thinned portions of the plantation 

were not significantly different. Although most areas in the 

10-year-old plantation had a closed canopy, there were some 

openings sufficiently large enough to cause discontinuities 

in crown closure throughout the study period. Trees that 

had an opening directly adjacent to them (n=50) generally 

had more epicormic branches per tree than trees with no 

opening present within 2 rows (n=58), unless the opening 

was to the north side of the tree (fig. 1, a). If an opening was 

present 1 row away (n=41), it did not increase the number 

of epicormics formed unless the opening was in a southerly 

direction (fig. 1, b). 

Berntsen (1961) suggested that production of epicormic 

branches could be a problem when pruning red alder; 

however, closer examination of the data from his study 

(Rapraeger 1949 report on Cascade Head pruning and 

thinning study, data on file, Olympia Forestry Sciences 

Laboratory) indicated that the stem sections he examined 

came from a 21-year-old red alder stand that was heavily 

thinned (two-thirds of the stems removed) and the large 

overstory conifers were girdled at the same time as pruning. 

Thus, the red alders were exposed to a major change in their 

light environment at the same time as pruning. Even so, the 

trees at Cascade Head had fairly low numbers of epicormic 

branches 10 years after these treatments were imposed 

(14 per tree on the 5-m section that had been pruned) and 

most (75%) were less than 0.1 cm in diameter. Thus, both 

the Cascade Head study and our observations confirm 

that although red alder does produce epicormic branches 

following pruning, it is not particularly prone to doing so. 

The results from the Cascade Head study are also consistent 

with our observation that epicormic branches are more 

likely to form and persist on older trees.

Growth Rate and Stem Form 

of Individual Trees

Stem form appeared to be very slightly altered by 

pruning in all the plantations; however, the differences were 

not significant (p<0.05). The differences in form were most 

pronounced in the 10-year-olds and although differences 

between stem form in pruned trees in comparison to trees 

not pruned was greater in year 5 than in year 2 (fig. 2), it 

was still not statistically significant. From year 2 (1994) 

to year 5 (1997), the mean diameter of the bole at heights 

closer to the base of the live crown, once pruned, increased 

at a rate greater than the rates at sections lower down on the 

bole. If this trend continues, it appears the boles of pruned 

trees will become more cylindrical. 

The rate of branch occlusion at various heights up 

the bole also indicates greater rates of diameter growth 

closer to the base of the live crown. For example, after 3 

growing seasons (trees pruned in May, 1992), mean percent 

occlusion of live branches 2 to 3 cm in diameter pruned in 

sections at 1-m intervals (starting at 0.3) up the bole of the 

trees were 51%, 56%, 60%, 61%, 77%, and 95% (ending at 

5.3 m height), respectively. Separated into tree age groups, 

all 3 ages showed the same trends, although it was more 

difficult to discern in the 3-year-olds due to smaller branch 

sizes, reduced number of sections, and more rapid percent 

diameter growth.

Age % Trees with 

epicormic branches

Mean number of epicormic 

branches per tree

% Epicormic branches 

with a length >=15 cm

1992-1993* 1994 1997 1992-1993* 1994 1997 1994 1997

3 41 69 29 1.2 1.9 0.7 0 36

6 5 27 19 0.1 0.8 0.5 3 27

10 33 91 89 0.8 7.4 7.3 65 80

*At time of pruning

Table 2—Frequency, mean number and size of epicormic branches at the time of pruning (1992 and 1993) 

and after the 1994 and the 1997 growing seasons.
99

Factors affecting branch occlusion

There were substantial statistical differences in the 

degree of branch occlusion by initial branch size, tree size, 

tree age, and overall growth rate of the tree. For example, 

2 years after pruning, mean percent occlusion of branch 

wounds in the 3-year-old and 6-year-old plantations ranged 

from only 67% (non-irrigated portion of the 6-year-old 

plantation) to 94% (thinned portion of the 3-year-old 

plantation) (fig. 3). Silvicultural factors that affected the rate 

of diameter growth, such as thinning or irrigation, also had 

an effect upon the rate of occlusion. Differences in rates of 

occlusion between the different tree ages can be attributed to 

both the initial differences in branch size by plantation and 

to tree growth rates. For example, branches 1 cm in diameter 

or smaller were 97%, 74%, and 45% of the branches pruned 

in the 3-, 6-, and 10-year-old plantations respectively.

Mean percent occlusion was also closely related to both 

branch size and basal area growth (fig. 4). Stem diameter 

growth varied significantly by age and treatment, as well as 

by initial tree size (table 1). Diameter growth as a percent 

of initial diameter prior to pruning (1991) to the end of the 

1994 growing season was 49% for the 3-year-olds, 21% for 

the 6-year-olds, and 17% for the 10-year-olds. These age 

trends in growth rate as a percentage of initial diameters 

are also reflected in the mean percent occlusion by 2-cm 

diameter growth classes. For example, live branches 2 to 

3 cm in diameter pruned in the 3-year-old plantation had a 

mean percent occlusion of 65%, 94%, and 100% for a d.b.h. 

(diameter at 1.3 m) growth of 0 to 2 cm, 2 to 4 cm, and 4 to 

6 cm, respectively. For the same branch diameter and the 

same dbh growth classes, the mean percent occlusion was 

59%, 70%, and 94% for the 6-year-old plantation and 31%, 

46% and 51% for the 10-year-old plantation. So, for any 

given amount of d.b.h. growth, the younger trees grew more 

in respect to their initial diameter and occluded wounds at a 

faster rate.

As expected, at the time of the first occlusion 

assessment at the end of the 1994 growing season, the 

branches that had been pruned at the earlier pruning dates 

showed greater occlusion. For example, the percent of live 

2 to 3 cm-branches that had reached 90% occlusion or 

greater was 16% for the May 1, 1992 pruning date, 14% 

for the June 15, 1992 pruning date, 2% for the August 1, 

1992 pruning date, and 1% for the 4 subsequent pruning 

dates (September 15, 1992; November 1, 1992; January 1, 

1993 and March 1, 1993). By the end of the 1995 growing 

season, the early spring treatments (May ‘92 and March ’93) 

exhibited slightly slower than expected occlusion rates, but 

branches of all size classes for all the pruning dates reached 

90% or greater mean occlusion by the end of the 1997 

growing season (fig. 5).

Occlusion of live vs. dead branches and 

effect of branch collar treatment

We compared the rates of occlusion for pruning 

live branches without wounding the branch collar, dead 

branches pruned without wounding the branch collar, and 

dead branches pruned by removing or damaging the branch 

collar. For each of the smallest branch size classes (the 

size classes where n>100), differences among the three 

categories were small at the time of the first assessment 

in 1994 and even smaller in 1995, but two consistent and 

statistically significant patterns were present. First, the 

mean rate of occlusion was greater for live branches than 

for dead branches that were pruned without wounding the 

branch collar (fig. 6). Second, even though damaging or 

removing the branch collar during pruning increased the 

Figure 1—Mean number of epicormic branches in 1994 present on 10-yearold trees that a) had an adjacent opening and b) had an opening 1 row away. 

Trees with no opening within 2 rows had a mean of 1.2 epicormics per tree 

and are represented by the smaller inner circle.

Figure 2—Stem diameter at 0.3, 1.3, 2.3 and 3.3 m height within the 10-

year-old plantation on both pruned and non-pruned trees in years 2 and 5.

Diameter (cm)

14 16 18 20 22

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Not Pruned Year 2

Pruned Year 2

Not Pruned Year 5

Pruned Year 5

Height (m)

N

E

S

W

1.2

2.2

N

S

W E

1.2

2.4

a) Opening adjacent b) Opening 1 row away
100

size of the wound to be occluded, wounds associated with 

damaged branch collars occluded more quickly than those 

associated with undamaged branch collars. Although this 

difference in occlusion rates between injured and uninjured 

branch collars is unlikely to be of practical importance, it 

indicates that pruners do not need to be especially careful 

to avoid injuring branch collars. Additional examination of 

the wood around these wounds also indicated no difference 

in the amount of internal decay associated with damaged or 

undamaged branch collars or the time it took to form clear 

wood (DeBell et al. 2006).

Red alder wood decays rapidly after the death of the 

tree; thus, in past years some foresters have speculated 

that alder might not be a good candidate for thinning 

or pruning because of the potential damage associated 

with these activities. More recent evidence, however, 

has shown that living red alder trees are quite effective 

in compartmentalizing decay (Allan 1993). Allen’s study 

on Vancouver Island showed decay volumes to be small 

and most decay associated with natural breakage did not 

spread outward beyond the location on the bole where 

damage was sustained. Additional assessments of decay 

1994 1995 1996 1997

0

20

40

60

80

100

Not thinned

Thinned

1994 1995 1996 1997

Not irrigated

Irrigated

3-year-old trees 6-year-old trees

% Occlusion

Branch diameter (cm)

0.5 1.5 2.5 3.5 4.5 5.5 6.5

0

20

40

60

80

100

BA growth (cm2)

0 100 200 300

1994 mean percent occlusion

Figure 3—Mean percent occlusion of all pruned branches in the 3- and 6-year old plantations 1 to 5 years following pruning (dependant 

upon time of pruning).

Figure 4—Relationship between percent occlusion of branches pruned in 3-, 6-, and 10-year-old red alder stands by diameter of pruned 

branch and basal area growth measured at 1.3 m.
101

1994 1995 1996 1997

0

10

20

30

40

50

60

70

80

90

100

May '92

June '92

August '92

September '92

November '92

January '93

March '93

Mean percent occlusion

Branch diameter

0-1 cm 1-2 cm 2-3 cm

0

20

40

60

80

100

Dead branch - no damage to collar

Dead branch - damage to collar

Live branch - no damage to collar

Branch diameter

0-1 cm 1-2 cm 2-3 cm

1994 1995

% Occlusion 

Figure 6—Comparison of occlusion rates by branch size classes and year for live branches pruned without damaging the branch collar and 

dead branches pruned with and without damaging the branch collar during pruning.

associated with pruning were made in the Cascade Head 

pruning study (C.W. Jacobs memo dated Feb 28, 1947 on 

rot found in alder stands, on file Olympia Forestry Sciences 

Laboratory). The wood in and near 25 knots on 6 pruned 

trees was assessed for signs of decay 10 years after pruning 

by sampling with an increment borer. On 24 of the 25 knots, 

the decay was found to be confined to the knot area; on 

the 25th knot, the rot extended to the pith and about 25 cm 

below the branch, but did not extend into the wood formed 

after pruning. The companion study to our young-stand 

pruning (DeBell et al. 2006) also found minimal amounts 

of rot associated with the branch scars. Thus, there is no 

evidence that decay will be a problem after pruning young 

red alder.

Figure 5—Mean percent occlusion for all branches sizes at all pruning dates at 4 successive assessment years.
102

Conclusions

Pruning at plantation ages 3, 6, or 10, did not have 

negative effects on the incidence of decay or defects within 

the tree bole, production of significant numbers of epicormic 

branches, or growth of the tree. Contrary to reports for 

several other species, the results were consistent across all 

seven pruning dates; thus, time of year of pruning had little 

or no effect. Differences in rates of occlusion could largely 

be accounted for by the differences in growth rate due to 

tree age or other management activities (such as thinning 

or irrigation). Although our study examined the effects of 

wounding the branch collars only on dead branches, we 

suspect that incidental damage during the pruning operation 

to branch collars of live branches should not be a source of 

concern. However, because there was no practical benefit 

from removing the branch collar, and it would take more 

time and effort to do so, we would not recommend it as a 

standard practice.

Because both small and live branches occlude more 

quickly than large or dead branches, pruning at an earlier 

age would not only result in a smaller knotty core, but 

would produce straight-grained clear wood more quickly 

after pruning. In addition, if epicormic branches are a 

concern, pruning young stands appears to result in the 

lowest number of epicormic branches. Epicormic branch 

production was also most closely tied to the presence of 

openings on the south side of the tree, thus, trees north of 

stand boundaries or gaps are the poorest candidates for 

pruning. From a biological standpoint, there appear to be 

no obstacles and several benefits to pruning red alder at an 

early age; thus, the decision of timing of pruning should be 

based on economic factors.

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