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TRANSCRIPT
New Forests 21: 71-87,2001. ,. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.
Growth and early stand development of intensively cultured red alder plantings
PETER D. HURD and DEAN S. DEBELL Pacific Northwest Research Station, 3625 93rd Ave. Sw, Olympia, WA 98512-9193 USA
Accepted 8 November 2000
Abstract. (0.5-, 1.0-
This study evaluates the performance of Alnus rubra in three square spacings
and 2.0(0 300
-m), two irrigation
). regimes (low and high), and two pre-planting fertilizer
treatments and kg P ha -1 Initial survival and growth were excellent, and differences
among various cultural treatments were apparent by the end of the second growing season. At
age ten, mean tree sizes in specific regimes ranged from 4.8 cm to 11.5 7.7 13.1
cm in diameter and
m to m in height, with largest trees produced in regimes with wide spacing and high
irrigation. Beneficial effects of fertilizer were minimal and were limited primarily to enhanced
early survival during the first two years in the closest spacing. Growth of the plantings was
greater than that estimated for fully stocked, natural stands of the same age and site index (or
height). Data from our study provided general confirmation of the level and slope of the tree
size-stand density lines currently used in density management guidelines for alder, except that
mortality in the densest spacing occurred at diameters smaller than those assumed to indicate
the threshold for inter-tree competition. This difference, however, was lessened by irrigation.
Key words: Alnlls rubra, fertilization, irrigation, self-thinning, spacing, stand density
Introduction
Red alder (Alnus rubra Bong.) grows naturally from central California to southeastem Alaska and is the most abundant hardwood tree species in westem Oregon, Washington, and British Columbia. It grows in pure stands and in mixed stands with Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco varmenziesii), westem hemlock (Tsuga heterophylla (Raf.) Sarg.), westem redcedar (Thuja plicata Donn.), Sitka spruce (Picea sitchensis (Bong.) Carr.), grand fir (Abies grandis (Dougl.) Lindl.), black cottonwood (Populus
trichocarpa Torr. & Gray), and bigleaf maple (AceI' macrophyUum Pursh) (Harrington 1990). Once regarded as a weed species by forest managers, alder is now appreciated for its unique ecological attributes (e.g., N2-fixation and immunity to Phellinus root rot) and its contribution to the forest products
The U.S. Government's right to retain a non-exclusive, royalty free licence in and to any
copyright is acknowledged.
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72
economy of the region (Tarrant et al. 1994). Several operational alder plantations have been established, the biology and management of the species have been summarized (Hibbs et al. 1994), and a university-industry-agency cooperative (The Hardwood Silviculture Cooperative) exists to foster silvicultural research (Hibbs 1999). Nevertheless, research data and experience concerning various silvicultural options remain limited. This paper describes a trial undertaken to evaluate several short-rotation, intensive culture practices for producing wood, fiber, or biomass in red alder plantations. We report the influence of spacing, irrigation, and phosphorus fertilizer treatments and their interactions on patterns of survival, growth in height and diameter, mortality, and accumulation of basal area over a lO-year period. We compare performance of the planted stands with characteristics of unmanaged but fully stocked, naturally regenerated stands of the same age and site index (Worthington et al. 1960). In addition, we examine stand trajectories (depicted as mean diameter plotted over stand density) in relation to a stand density diagram and currently used density management guidelines (Puettmann et al. 1993).
Methods
Study area
The study was established in 1986 in cooperation with the Washington State Department of Natural Resources (WDNR) at the WDNR Meridian Seed Orchard, 12 km east of Olympia (47°00' N, 122°45' W). Climate is mild with an average growing season of 190 frost-free days (US Dept. of Commerce 1961). Based on data collected from a weather station at the site from 1986 to 1992, precipitation averaged 112 em yet with only 15 em falling from May 1 through September 30. The average July temperature was 17 °C.
The site, previously farmed for strawberry and hay crops, was prepared for planting by plowing and disking in the winter of 1985-1986. The soil is Nisqually loamy fine sand (a sandy, mixed, mesic Pachic Xerumbrept); it is a deep, somewhat excessively drained, medium acid (pH 5.6) soil formed in glacial outwashes (Pringle 1990) and would not be considered suitable for alder growth without irrigation. Slope is 0-1 %; elevation is 50 ill. Adjacent unmanaged land is occupied either by prairie vegetation or Douglas-fir mixed with several species of hardwood trees and shrubs.
Experimental design
The experimental design was a split-split-plot replicated on three adjacent blocks. Two irrigation rates (high and low) formed the main plots that were
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73
split to provide a test of pre-planting fertilizer application (none and a onetime application of triple superphosphate at 300 kg P/ha). The second split involved three square spacing (0.5-, 1.0- and 2.0-m) treatments. Each plot was planted in spring 1986 with container-grown seedlings from each of 19 openpollinated alder families. The size of the treatment plots varied with spacing; all plots were large enough to provide 100 interior measurement trees and a buffer of eight, four, and three rows of similarly spaced trees for the 0.5-, 1.0-, and 2.0-m spacing treatments, respectively. The intent was to provide a buffer at least one-half as wide as the estimated height of measurement trees at the end of the study. All plots were irrigated by a drip system. The irrigation lines were laid down 2.0 m apart with emitters spaced at 1.0-m intervals in each line. During the first year, 25 cm of water was applied in addition to rainfall. Thereafter, the high-inigation treatment received 40 to 50 cm of supplemental water during each growing season. The low-irrigation treatment received no supplemental water during the second year and minimal applications «8 cm per growing season) in subsequent years. The high-irrigation treatment was intended to increase tree growth and accelerate the rate of stand development on the dry site, while the intent of the minimal application was to prevent mortality caused by water stress alone. Phosphorus fertilization was chosen because response to P fertilizer had been observed in other studies (Hughes et al. 1968; Radwan 1987; Radwan and DeBell 1994). Prior to planting, the fertilizer was applied with a spreader and disked into the surface soil. All plots were kept weed-free by tilling, hoeing, and selective application of herbicides.
Data collection and analyses
Total height of all trees was measured annually from plantation age 1 to plantation age 6 and a sub-sample of trees in each plot was measured at ages 7, 8, and 10. Breast-high diameter (dbh) was measured on all trees annually from age 2 to age 8 and at age 10. A regression equation to predict height from dbh was developed for each treatment combination; predicted heights then supplemented the measured height samples for the 7-, 8-, and lO-year measurements.
Treatment means for survival, tree size, and basal area were plotted to illustrate trends in tree and stand development over time as well as the nature of significant interactions. Effects of irrigation, fertilization, and spacing treatments on survival, tree size, and basal area per hectare at plantation age 10 were tested with ANOVA for a split-split-plot experiment by using SAS's GLM procedure (SAS Institute, Inc. 1988). Sizes of the 2000 largest and 800 largest trees per hectare as well as overall means were tested. Treatment effects were judged significant at P ::::: 0.05.
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74
Stand characteristics at age 10 were compared with estimates for fully stocked natural stands given in published yield tables (Worthington et al. 1960).
Stand trajectories were examined by plotting quadratic mean diameter for each treatment combination at each age over stand density (number of trees per hectare), and results were compared with a density management diagram and guidelines published by Puettmann et al. (1993).
Results and discussion
Establishment and subsequent trends in survival and growth
Establishment
Initial survival averaged 97% at the end of the first growing season. Survival in the two wider spacings was very high (97 to 99%), but the O.S-m spacing had somewhat lower initial survival (91 to 96%). High levels of irrigation and fertilization improved survival in the O.S-m spacing treatment during the first two growing seasons (Table 1). At the end of the second growing season, survival in the O.S-m spaced plots averaged 3% higher in the high-irrigation plots than in the low-irrigation plots, and fertilized plots averaged 4% higher survival than the unfertilized plots. Fertilizer provided negligible gain (1 %) in high-irrigation plots but increased survival by 7% in the low-irrigation plots. High-irrigation provided no benefits to survival in fertilized plots yet increased survival of unfertilized plots by 6% over the low-irrigated unfertilized plots. We suspect water as the primary factor limiting survival in these densely planted plots. High-irrigation directly affected water availability and we suspect that application of phosphorus fertilizer enhanced root growth (Pritchett 1979; Radwan 1987) and thereby indirectly increased amounts of water accessed and absorbed by the seedlings. Differences in survival in the two wider spacings (1.0-m and 2.0-m) during the first two years were negligible and unrelated to irrigation and fertilization.
Subsequent trends in survival
In general the high-irrigation treatment over time decreased the number of trees (i.e., percent survival) which is a natural outcome of enhanced growth and increased competition-related mortality (Figure 1). The effect of fertilization on survival was also generally negative, though markedly less than that of irrigation and with less of an effect in wider spacings (Tables 1 and 2). Such reductions in survival associated with fertilization were probably associated with improved growth and increased competition (cf. Diameter, height, and survival in Table 2).
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40000 �--------------------------------�
+--'--�--'---'--'---'--'--'�-'--4
75
Table 1. Initial average percent survival and standard deviations of O.5-m
spacing plots as related to in'igation and fertilization.
Age Low-irrigation High-irrigation
(years) Unfertilized Fertilized Unfertilized Fertilized
91± 3 96± 6 94±6 96±2
2 86± 6 93 ± 8 92±4 93 ±5
3 67 ±6 67 ± 12 51 ± 2 51 ± 5
4 53 ±7 55 ±7 45 ±2 40±5
-- O.5-m flow-irrigation0--··0 O.5-m / high-irrigation ---- 1.0-m flow-irrigation
30000 0-··0 1.0-m f high-irrigation
2.0-m flow-irrigationQ) ",- • • />. 2.0-m f high-irrigation ..c:Oi 20000 a.
o o 2 3 4 5 6 7 8 9 10
age (years)
Figure 1. Number of trees surviving as related to spacing, irrigation and age.
Closer spacings also increased competition-related mortality and resulted in decreased survival throughout the study. Survival at the end of the second growing season in the O.5-m spacing treatment was 7% and 8% lower than in the l.O-m and 2.0-m spacing treatments respectively. This trend of decreased survival with closer spacing, high-irrigation, and fertilizer application continued, and generally intensified, through age 10 (Figure 1 and Table 2).
Patterns of height, diamete7; and basal area growth
The annual increments for diameter and height increased during the first three years and tended to decline gradually in subsequent years as age, size and inter-tree competition increased (Figures 2a and 2b). After the second year, annual increments for diameter growth increased with wider spacing in both
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Table 2. Tenth-year averages and standard deviations of survival and tree and stand characteristics as related to spacing, irrigation and fertilizer
treatments.
Mean tree size
Quadratic mean breast- Basal area
Spacing % Survival high diameter (cm) Height (m) (m2 ha-1)
(m) Irrigation Unfertilized Fertilized Unfertilized Fertilized Unfertilized Fertilized Unfertilized Fertilized
0.5 Low 26±7 23 ± 8 4.8 ± 1.4 5.4 ± 1.0 7.7 ± 2.1 8.2 ± 1.2 18.2 ± 7.6 20.0 ±4.1
High 11 ±2 8±4 8.6 ± 0.2 9.1 ± 0.3 12.4 ± 1.0 12.8 ± 0.3 26.4 ± 4.7 20.8 ± 9.9
1.0 Low 67±9 60±3 6.1 ± 1.1 7.0 ± 0.7 8.2 ± 0.8 9.0 ± 0.6 19.0 ± 3.7 23.1 ± 4.6
High 34±2 35 ±3 10.2 ± 0.4 10.1 ± 0.4 13.1 ± 0.2 13.1 ± 0.2 27.6 ± 2.4 27.2 ± 0.9
2.0 Low 95 ± 1 90±3 8.8 ± 1.5 9.3 ± 0.4 9.0 ± 1.2 9.4 ± 0.2 14.4 ± 4.8 14.7 ± 1.3
High 81 ±2 78 ±7 11.4 ± 0.4 11.5 ± 0.2 12.5 ± 0.1 13.1 ± 0.1 20.4 ± 2.5 19.6 ± 1.6
5
I =
I
,
.1
age (years) ;-2
14
77
a.
E .J:: 0'-0> '-
$<!lE
'0
12 10
8
6
4 2 0
low-irrigation high-irrigation_ 4 IIlIIlD 5
8 ..... 9-10
b,
12 .J::
10
8
0> - 6
'iii.J::
40> .J::
2 0 0.5-m 1.0-m 2.0-m O.5-m 1.0-m 2.0-m
low-irrigation high-Irrigation
Figure 2. lO-year growth as related to spacing, irrigation and age: (a) quadratic mean
diameter; (b) height.
irrigation treatments. This trend is presumed to be associated with increased room for crown expansion and associated leaf area as spacing widened.
Height growth was initially greater in the l .O-m spacing in both highand low-irrigation environments. By age 5, however, best growth in lowirrigated plots occurred in the widest spacing whereas in the high-irrigation plots, subsequent growth was similar in the l.0- and 2.0-m spacings. As a result, the l.O-m spacing in high irrigation plots remained best in cumulative height growth through age 10. Growth-enhancing effects of close spacings
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78
on trees - particularly on height growth - are common early in the life of a stand but they have yet to be explained physiologically and they are usually reversed at older ages (DeBell and Giordano 1994). Similar results have been
observed in poplars (DeBell et al. 1997), loblolly pine (Adams et al. 1973), and Douglas-fir (Scott et al. 1998).
Our data suggest that the initial beneficial effects of high or intermediate densities may be more pronounced and of longer duration in more favorable (i.e., high irrigation) than less favorable (i.e, low irrigation) growing environ
ments. Such a relationship is also consistent with a comparison of results from two Douglas-fir spacing trials: (1) on a low quality site, trees were taller at age 5 in higher density than in lower density plantings but differences disappeared by age 10 (Isaac 1937), and (2) on a high quality site, trees were taller in intermediate densities through age 20 (Reukema and Smith 1987).
Our results indicate that subsequent beneficial effects of wider spacing on height growth of red alder in later years is greater under more adverse growing conditions (low irrigation vs. high irrigation). This observation also is consistent with a comparison of long term results in the Douglas-fir trials on low quality (Reukema 1979) and high quality sites (Reukema and Smith 1987).
Basal area per hectare was greater with high irrigation than with low irrigation throughout the study (Figure 3). Within a given irrigation treatment, basal area increased with higher density through age six, after which basal area was greatest in the 1.0-m spacing and least in the 2.0-m spacing. Basal area growth tended to accelerate through year seven in most treatments. Exceptions were in the high-irrigated, 0.5-m and 1.0-m spacing treatments which had the greatest growth during the first three years, had a temporary depression in growth rate at age 5 and 6, respectively, and then resumed the former growth rate in year 7. Beyond these periods, basal area growth decreased due to increased competition and associated mortality. Cumulative basal area in the high-irrigated, 0.5-m spacing treatment decreased between the eighth and tenth growing seasons, and basal area growth was substantially reduced in the low-irrigated, 0.5-m spacing treatment during the same period, as compared with other treatments. Such losses in cumulative basal area and the reduced basal area growth from age 8 to age 10 probably are associated with high levels of competition-related mortality; numbers of trees dying in the 0.5-m spacing averaged 2.7 times the number of trees dying in the1.0-m spacing under the same irrigation treatment.
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/ O �------.--------.-------,------�
.' ;..
79
30 --- O.5-m !Iow-irrigatlon 0-.-0 0.5-m ! high-irrigation ..'---.--- 1.0-m !Iow-irrigation ---.25 0-" -. .---. .D-·IJ 1.0-m ! high-irrigation -.
A---4 2.0-m !Iow-irrigation .,,.JY- ;:::::: . A-- . b, 2.0-m ! high-irrigation,/ - :
20 .
Co .,,-fi.cN5 .;::/ ' co 15
co IJ 'iii allIf)co.0 10 ://IJ
// 5
2 4 6 8 10
age (years)
Figure 3. Basal area as related to spacing, irrigation and age.
Tree and stand characteristics at age 10
Survival
Irrigation, fertilization, spacing, and the interaction of irrigation and spacing significantly affected survival at age 10 (Tables 2 and 3). Spacing impacted survival the most with the greatest survival (86%) occurring in the 2.0m spacing treatment and the lowest survival (17%) occurring in the 0.5-m spacing treatment. Survival was, on average, 19% lower in the high-irrigated than in the low-inigated treatments, and 3% lower in the fertilized than in the unfertilized treatments. Irrigation differentially affected survival in each of the spacings. The greatest differences between the high- and low-irrigated treatments were in the 1.0-m spacing (29%), whereas differences were only 14% and 13% in the 0.5-m and 2.0-m spacing treatments, respectively. This apparent curvilinear relationship is largely attributable to the use of percent of original number of planted trees as the parameter of survival. If the reduction in survival associated with high irrigation was assessed in terms of absolute numbers of trees or as a percent of the number surviving with low irrigation, the reduction would decline more or less linearly with increased spacing.
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80
(0/0)
0.64
Table 3. Significance of ANOVA model components on lO-year survival, breast-high
diameter, height, and basal area.
Probability of> F-value
Quadratic mean breast-
high diameter (cm) Height(m)
Source of Survival All Largest Largest All Largest Largest Basal area
2000 trees 800 trees trees 2000 trees 800 trees (m2 ha-i)variation Df trees
0.03 0.03 0.02 0.02 0.02 0.05
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0,02 0.03lITigation
0.28 0.10 0.24 0.70 0.910.02 0.06Fertilization
Irrigation x fertilization 1 0.13 0.17 0.08 0.16 0.62 0.84 0.88 0.11 Spacing 2 0.00 0.00 0.00 0.00 0.03 0.02 0.32 0.00 Irrigation x spacing 2 0.00 0.01 0.02 0.26 0.16 0.30 0.66 0.84 Fertilization x spacing 2 0.96 0.80 0.71 0.58 0.97 0.80 0.34 0.53 lITigation x fertilization 2 0.60 0.56 0.62 0.61 0.66 0.82 0.76 0.65
x spacing
Note: Irrigation tested using irrigation x block as the elmr term (df = 2); fertilization and
il1'igation x fertilization tested using block x fertilization(irrigation) as the error telID (df
= 4); the split-split-plot error term had 16 df.
Diameter
Quadratic mean breast-high diameter at the end of the tenth year varied significantly with irrigation, spacing, and the interaction of irrigation and spacing (Tables 2 and 3). Diameters increased with higher irrigation and wider spacing. Mean breast-high diameter increased 47% (+3.3 cm) with higher irrigation and 46% (+3.3 cm) as spacing increased from 0.5-m to 2.0m. The irrigation and spacing interaction was also significant; the greatest increase in mean breast-high diameter due to high-irrigation was 75% (+3.8 cm) in the 0.5-m spacing treatment with increases of 55% (+3.6 cm) and 27% (+2.4 cm) occurring in the l.O-m and 2.0-m spacing treatments, respectively. Even though fertilization was non-significant (p = 0.06,) there tended to be larger diameters in fertilized plots in the low irrigation treatment and in the closest spacing; i.e., the low- and high-irrigation, 0.5-m spacing treatments and the low-irrigation, 1.0-m and 2.0-m spacing treatments. With fertilization, diameters were 6% (+0.5 cm) greater in the low-irrigated, 2.0-m spacing treatment and 15% (+0.9 cm) greater in the low-irrigated, l.O-m spacing treatment.
Diameter, largest 2000 and 800 trees per hectare
Quadratic mean diameters of the largest (in diameter) 2000 trees per hectare responded to the cultural treatments similarly to all trees or mean trees as discussed above (Tables 2 and 3). This comparison considers equal numbers of trees per hectare in all treatments (i.e., approximately 2000 trees per hectare survived in the high irrigation treatment of the widest spacing; all other treatments had more than 2000 trees). The mean diameters of the largest
81
800 trees per hectare also responded to treatments in the same fashion as did all trees. This stand component was selected because it represented the fewest trees that could be examined with what we considered a minimum acceptable sample from each plot (i.e., 2 trees in each plot planted at 0.5-m spacing).
Height
Heights were significantly greater in the high-irrigation treatment and, on average, at wider spacing (Tables 2 and 3). Mean height increased 49% (+4.2 m) due to higher irrigation and 7% (+0.7 m) as spacing increased from 0.5-m to 2.0-m. Mean height increased more or less linearly with increasing spacing in the low-irrigation treatments. It was curvilinearly related to spacing in the high irrigated treatment, however. Mean height increased 4% (+0.5 m) as spacing increased from 0.5-m to 1.0-m, but decreased 2% (-0.3 m) as spacing increased from l.O-m to 2.0-m. Mean height was slightly greater with fertilization though differences were not significant (p = 0.10); the greatest difference was 10% (+0.8 m) in the low-irrigated, l .O-m spacing treatment; as with diameter, the greatest benefits from fertilizer application occur in the low-irrigation treatments and in the closest spacing.
Height, largest 2000 and 800 trees per hectare
Heights of the largest (in diameter) 2000 trees per hectare were significantly related to spacing and irrigation in much the same manner discussed above for all trees (Table 4). Heights of the 800 largest trees were significantly related to irrigation (p = 0.02) but not spacing (p = 0.32). Mean height of these trees is probably the best approximation we have of site productivity: 10m at age 10 for low-irrigation and 14 m at age 10 for high-irrigation. These values suggest a SI20 (Harrington and Curtis 1986) of 17 m (or average site) for the low-irrigation plots and a SI20 of 24 m (or high site) for the high-irrigation plots.
Basal area
Basal area at the end of the tenth year varied significantly with irrigation and spacing (Tables 2 and 3). High-irrigation increased basal area by 30% (+5.4 m2 ha-1) on average. Basal area was curvilinearly related to spacing and increased 13% (+2.8 m2 ha-1) as spacing widened from 0.5-m to 1.0m and decreased 29% (-6.9 m2 ha-1)as spacing increased from 1.0-m to 2.0-m. Overall, fertilization had no effect on basal area (p = 0.91); however, basal area tended to increase with fertilizer application in the low-irrigated treatments and to decrease with fertilization in the high-irrigated treatments (fertilizer x irrigationp = 0.11).
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82
Table 4. Diameter and height averages and standard deviations at age 10 as related to 1spacing and irrigation treatments of all, 2000 largest, and 800 largest trees ha - .
Quadratic mean breast-high diameter (cm) Height (m)
Spacing All Largest Largest All Largest Largest(m) Irrigation trees 2000 trees 800 trees trees 2000 trees 800 trees
0.5 Low 5.1 ± 1.2 7.2 ± 1.1 7.9 ± 1.2 7.9 ± 1.6 9.2 ± 1.4 9.7 ± 1.3 High 8.8 ± 0.4 10.2 ± 0.6 11.4 ± 0.8 12.5 ± 0.7 13.0 ± 0.5 13.3 ± 0.4
1.0 Low 6.5 ± 0.9 9.1 ± 1.2 9.3 ± 1.2 8.6 ± 0.7 9.8 ± 0.9 9.9 ± 0.8 High 10.1 ± 0.4 12.9 ± 0.7 13.3 ± 0.8 13.1 ± 0.2 14.0 ± 0.4 14.0 ± 0.4
2.0 Low 9.1 ± 1.0 9.4 ± 1.0 11.0 ± 1.2 9.2 ± 0.8 9.5 ± l.3 9.9 ± 1.2 High 11.5 ± 0.3 11.5 ± 0.4 13.9 ± 0.5 12.8 ± 0.3 12.8 ± 0.5 13.6 ± 0.5
Comparison with natural stands at age 10
The most relevant unmanaged natural stand data available for comparison with our results are those published in the normal yield tables for red alder (Worthington et al. 1960). The latter data were derived from plots centered in the most uniform part of pure fully stocked red alder stands (> 80% red alder by basal area with a closed canopy) and result from a combined multiple regression analyses of characteristics of more than 400 plots.
Compared to trees in 1O-year-old natural stands growing in height at the same rate (i.e., same site index), the planted trees in the l .O-m spacing of the high irrigation treatment (i.e., the "high site"), had identical mean diameters and basal areas that were 46% (+8.6 m2 ha-1) greater. Diameter and basal area of trees in the low irrigation treatment ("the average site"), were 8% (+0.5 em) and 24% (+4.1 m2 ha-1) greater, respectively, than trees in a 1O-year-old natural stand. In addition, basal area of the 2.0-m, high-irrigation treatment at age 10 was slightly greater (20.0 m2 ha-1 vs. 18.8 m2 ha-1) than that of 1O-year-old natural stands even though there were more trees in the natural stand. The reason for this is that the average diameter of the study trees was about 13% greater. The 2.0-m, low-irrigation treatment (the "average site") had lower basal area than a 1O-year-old natural stand (14.6 m2 ha-1 vs. 17.0 m2 ha-1), but average diameters were nearly 50% (+3.0 em) larger.
Thus, it is evident that planted stands will produce wood more rapidly than uniform natural stands, even when we assume that the natural stand and the plantation have similar height growth (i.e., same site index). Most likely, however, initial height growth and the apparent site index will be higher in plantations and will provide even greater gains in stand productivity.
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:::l 0"
83
10E ..c.0"0
c:roQ)
E0 ...."0ro
1000 10000 100000
trees per hectare
-- O.5-m I low-irrigation 0-.. 0 O.5-m I high-irrigation
-- 1.0-m I low-irrigation _ ..Q 1.0-m I high-irrigation
/1>.-----10. 2.0-m I low-irrigation A,-..A 2.0-m I high-irrigation
Figure 4. Stand trajectories in relation to density management diagram for red alder
(Puettmann et al. 1993).
Density management
The lO-year data from this study provide further support for the density management guide developed for red alder (Puettmann et al. 1993). This guide was based on data from several alder spacing and thinning studies including the first five years of this study (Figure 4). The biological maximum line for alder (2540 trees per hectare at 15 cm breast-high diameter) defines the potential maximum diameter attainable for any given density and is the basis for the relative density measures. The average maximum line is the average relative density that stands approach as trees grow and mortality reduces their numbers, and is sometimes referred to as the self-thinning asymptote. Alder stands can be expected to have appreciable densitydependent mortality (i.e., 20% of initial tree numbers) at a relative density of 45% which is considered to be the operating maximum. The suggested management zone is 25%-45% relative density; this assures full-site occupancy while maximizing growth and minimizing mortality.
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86
tions more closely than did the low-irrigation, O.5-m spacing treatments. Such anomalies associated with the O.5-m spacing provide some information on early growth patterns in very dense stands and may stimulate research and
yield insight into the limits of productivity. For the present, the existing guidelines for red alder (Puettmann et al. 1993) can be used to manage stand density within the range of spacings typical of operational plantations.
Acknowledgments
This research was supported in part by the Short Rotation Woody Crops Program (now Biofuels Feedstock Development Program) of the U. S. Department of Energy under interagency agreement No. DE-A10531OR20914. We thank Karl Buermeyer, Constance Harrington, David Hibbs, and David Marshall for constructive reviews of this manuscript.
References
Adams, W.T., Roberds, lH. and Zobel, B.J. 1973. Intergenotypic interactions among families
of loblolly pine. Theoretical and Applied Genetics 43(7): 319-322.
DeBell, D.S. and Giordano, P.A. 1994. Growth patterns of red alder, pp. 116-130. In: Hibbs,
D.E., DeBell, D.S. and TalTant, RF. (Eds.) The Biology and Management of Red Alder.
Oregon State Univ. Press, Corvallis.
DeBell, D.S., Harms, W.R and Whitesell, C.D. 1989. Stockability: A major factor in
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