Site Preparation Effects on 20 Year
Survival and Growth of Douglas-Fir
(Pseudotsuga menziesii) and on
Selected Soil Properties
Kathryn B. Piatek, SUNY College of Environmental Sciences and Forestry,
Syracuse, NY 13210; Constance A. Harrington and Dean S. DeBell (retired),
USDA Forest Service, Pacific Northwest Research Station, 3625 93rd Ave. SW,
Olympia, WA 98512-9193.
ABSTRACT: Long-term effects of site preparation on tree performance and soil properties are not well
known. Five site preparation treatments were evaluated to determine how they affected survival and growth of
Douglas-fir 3, 10, and 20 yr after planting, and soil bulk density, C, N, P, and organic matter concentrations at
0 to 20 cm soil depth 21 yr after planting. The site preparation treatments were imposed following logging of
three harvest units of old-growth forest on a volcanic soil in southwestern Washington; the units were logged to
leave 17, 38, and 53 ton/ha of woody residue. The site preparation treatments were hand-pile-and-burn,
machine-pile-and-burn, scarification, broadcast burn, and control. Mean survival ranged from 86% at age 3 to
70% at age 20, and average tree heights at 3, 10, and 20 yr were 0.6, 4.1, and 11.7 m. The scarification
treatment had the best growth; at age 20, its average tree was 21% taller, 26% larger in diameter, grid 82%
greater in volume than the control. The hand-pile-and-burn treatment did not differ from the control in tree
growth; the machine-pile-and-burn and broadcast burn treatments were intermediate in their growth response.
Average soil bulk density was 0.74 g/cm3, organic matter concentration was 118 g/kg, and C, N, and P
concentrations were 49, 1.6, and 0.7 g/kg with no significant treatment effects. Site preparation may have
benefited growth of the trees on these units by decreasing competition from invading and regrowing vegetation,
increasing nutrient availability, or increasing soil temperature. West. J. Appl. For. 18(1):44-51.
Key Words: Long-term site productivity, soil nutrients, coarse woody debris, prescribed fire.
The primary objective of site preparation is to create favorable
conditions for regenerating a forest stand. Site preparation
removes logging slash and competing residual vegetation and
alters the environment where seedlings are planted (Stewart
1988). Suppression of herbaceous and woody competitor species,
in particular, enhances the establishment and growth of shadeintolerant tree species by increasing the availability of light and
moisture to seedlings (Cole and Newton 1986, 1987). Removal
of slash in areas where low soil temperatures limit growth may
result in higher soil temperatures and growth increases
(Zabowski et al. 2000).
Site preparation usually results in some nutrient loss or
displacement because it reduces or redistributes the mass of
organic matter present on site (Miller et al. 1974). It has been
suggested that loss of nutrients by organic matter removal
may lead to a long-term decline of site productivity (Burger
NOTE:
44
Constance Harrington is the contact author and can be reached at
(360)753-7670; Fax: (360)956-2346; E-mail: charrington@fs.fed.us.
This article was written by U.S. Government employees and is
therefore in the public domain.
and Kluender1982, review by McColl and Powers 1984,
Neary et al. 1984, Morris and Lowery 1988, Fox et al. 1989,
Allen et al. 1990, Powers et al. 1990, Thornley and Cannell
1992, Munson et al. 1993). Forests regenerated after site
preparation, however, do not exhibit consistent responses,
and evidence of a decline in site productivity due to nutrient
loss is elusive.
Slash-burning in particular seems to have as many posi­
tive as negative effects on tree performance (Feller 1982).
Greater growth was reported in 5- to 15-yr-old Douglas-fir
after slash-burning, which also decreased competition from
salal (Vihnanek and Ballard 1988). Ten-year-old Douglas-fir
in the Coast Range of Oregon exhibited higher rates of
survival and greater height and diameter growth after site
preparation by broadcast burning, burning of slash piles,
aerial spraying and burning, or by aerial spraying alone, than
after manual spot-clearing or no site preparation (Stein
1995). Biomass of ponderosa pine seedlings was greater in
areas site prepared by burning than in untreated areas (McColl
and Powers 1984). One of the few long-term site preparation
studies reported no differences in Douglas-fir site index
Reprinted from Western Journal of Applied Forestry, Vol. 18, No. 1, January 2003. Not for further reproduction.
between burned and unburned plots 35 to 42 yr after treat­
ment in western Washington and Oregon (Miller 1989).
Because site preparation may differentially affect all or some
soil properties and other site characteristics that influence
tree growth, it is not surprising that reported responses have
been inconsistent. Moreover, it is not known if long-term tree
performance will be consistent with the short-term results
available for most studies.
Here we report on the effects of five site preparation
treatments on survival and 20 yr growth of Douglas-fir. The
study was installed in three harvest units that had different
levels of woody residue after logging. The original purpose
of the study was to determine which site preparation treat­
ment or treatments resulted in the best initial survival and
growth of planted Douglas-fir. The areas had substantial
woody residues remaining after harvest, and it was assumed
that some level or type of site preparation would be
necessary to ensure good seedling establishment. We
expanded the study to evaluate longer-term response of the
planted trees to site preparation and to determine if selected
soil characteristics were altered by treatment.
Methods
Study Site
The study was conducted on the lower slopes of Trout
Creek Hill in the Wind River Experimental Forest, Gifford
Pinchot National Forest, Skamania County, WA (45°50'N,
122°0'W). Soils are moderately deep, well-drained, and
derived from aeolian deposits of ash and pumice and
residual hard andesite and basalt. The soil mapping unit is
similar to the Stabler series (medial, amorphic, mesic Vitric
Hapludands). The organic layer and A horizon are densely
rooted. Surface soil is loamy, and subsoil texture ranges
from sandy loam to silty clay loam. Depth to bedrock is
greater than 1 m. Mean annual temperature measured at a site
5 km away and 150 m lower in elevation from the study area is
8.8°C, with average January and July temperatures of -0.1
and 17.6°C. Mean annual precipitation is 2,505 mm, with
dry summers and mean annual snowfall of 2,329 mm (PNW
Research Station 1987). In addition to the planted Douglasfir, the site is occupied by vegetation commonly found in
the mixed Pacific silver fir (Abies amabilis) and western
hemlock (Tsuga heterophylla) community types. In 1999,
salal (Gaultheria shallon), Oregon grape (Berberis nervosa),
bracken fern (Pteridium aquilinum), and bear grass
(Xerophyllum tenax) were the most common understory spe­
cies on the site, but Vaccinium spp. and many herbaceous
species were also evident. A few volunteer western hemlock,
Pacific silver fir, Douglas-fir, red alder (Alnus rubra), Pacific
dogwood (Cornus nuttallii), and vine maple (Acer circinatum)
were also present.
Table 1. Measured residue levels by diameter class for each
harvest unit (from Adams 1980b).
Residue diameter class (cm)
Unit
0.6-7.9
8-22.9
>23
Total
- - - - - - - - - - - - - - - - - ton/ha - - - - - - - - - - - - - - 1
11
6
0
17
2
18
11
9
38
3
17
17
19
53
Stand Description and Treatments
Three harvest units were established in an old-growth
(>450 yr old) forest of western hemlock, Douglas-fir
(Pseudotsuga menziesii), and Pacific silver fir. The units,
each 25 to 29 ha, were about 1,000 m apart along a common
road. Aspect on all units ranged from WSW to S, with a slope
gradient of 0 to 15%. The harvest units all had a mean
elevation of 550 m and were very similar in landform and
vegetation characteristics. Timber was removed by cablelogging during 1974 and 1975, and woody residues were
yarded during 1976 to result in different amounts of residue
remaining in each unit. Details on the logging and residue
removal are available in Adams (1980a, b). Unit 1 had the
least residue at 17 ton/ha, and unit 3 had the most at 53 ton/
ha (Table 1). Unwanted residue was yarded and piled at
landings. After harvest, the forest floor was composed of a
deep duff layer (up to 30 cm deep) and shallow, interlaced
roots of hemlock (Adams 1980b).
Each harvest unit was divided into 15 plots, 1.2 to 1.6 ha
each. Five site preparation treatments were applied to three
plots per harvest unit. Site preparation treatments were: (1)
control, or no further treatment; (2) hand-pile-and-burn, in
which logging residue was hand-piled and later burned; (3)
machine-pile-and-burn, in which residue was piled by a
bulldozer fitted with a brush blade and piles were burned
later; (4) scarification, in which mineral soil was exposed by a
small tractor with a blade; and (5) broadcast burn, in which
residue was ignited over the whole plot area. All of the site
preparation treatments were either being used in forest opera­
tions or were being considered for operational use at the time
the study was implemented. Except for the broadcast burn,
treatments were assigned to plots randomly. Broadcast burn
plots were placed next to each other to facilitate fire control
and to provide conditions more typical of those that result
when operational units are burned. As determined later from
photos taken immediately after treatment, the broadcast
burned areas were darkened by fire, and most of the small
woody debris was eliminated. The hand-pile-and-burn treat­
ment moved the small-diameter material that could be readily
lifted by one person; it resulted in many small piles through­
out the area. The scarification treatment had exposed mineral
soil, small-diameter woody material scattered over the plot
area, and medium-sized residue in small piles.
Site preparation treatments were accomplished during
the summer of 1977. In each plot, three subplots, 50 x 15
m, were established, and 2-yr-old Douglas-fir seedlings
were planted in the spring of 1978 by a research crew. The
rest of the area was planted by a contract planting crew.
Large bareroot, small bareroot, and container-grown
seedlings were each planted in two rows per subplot, for a
total of six rows of 20 seedlings each. One row of each
seedling type was planted by auger and the other was
planted by planting hoe. A 2.4 m spacing was maintained
throughout. Because large bareroot seedlings had the best
survival
and growth, this report deals with their
performance only. There were no growth differences
related to the planting method, and no distinction was
made in the analysis.
WJAF 18(1) 2003
45
Tree Measurements
Survival was calculated as the number of live trees in
the subplot (found in any measurement year), divided by 40
(the number of large bareroot seedlings planted per
subplot), and given as a percentage. All live trees, healthy
and damaged, were included in the survival calculation. Tree
heights were measured 3, 10, and 20 yr after planting, and
diameter at breast height (dbh) was measured 20 yr after
planting. Tree heights were measured with a height pole at
younger tree ages and, later, with laser ranging instruments.
Fewer observations were recorded for tree heights than for
survival because trees with dead, broken, or forked tops
were not measured for height.
Tree volume (at age 20) was calculated from height and
dbh using the equations of Bruce and DeMars (1974).
These equations slightly overestimate mean tree volume for
our study because the equations were based on diameter
taken at 1.37 m, while we measured at 1.3 m aboveground.
We did not calculate volume/ha because we did not think
that two measurement rows per subplot were a large
enough sample upon which to base estimates of volume
production on an area basis.
Soil Properties 21 Years After Planting
Soil bulk density, nutrient concentration, and organic
matter content were determined in 1999 in one randomly
selected plot of each treatment in each harvest unit. This
experimental design differed from the original in that we did
not sample all three replications of treatment plots within
units. We used 30 cm long and 10 cm diameter PVC tubes,
sharpened at one end, to collect two sets of mineral soil
samples from 0 to 20 cm depth at nine randomly located
locations in two subplots per plot. Forest floor was pushed
aside prior to taking of samples. One set of nine samples in
each subplot was used for bulk density determination. Tubes
were pounded into the soil with rubber mallets and carefully
extracted. The bottom of each tube was examined to make
sure that soil filled the tube to the edge. Cores with missing
soil were discarded and the sample retaken. Protruding soil
and roots were carefully cut flush with the edge. Individual
samples were transported to the laboratory, where they were
dried at 105°C until constant weight. A second set of nine
samples was collected in each subplot for nutrient analysis,
but without special care to ensure a flush tube bottom. The
nine samples collected for nutrient analyses were composited
in the field, mixed well, and transported to the laboratory
where they were sieved through a 2 mm mesh and air-dried.
Subsamples were analyzed for total C and N on a CHNAnalyzer (model 2400, Perkin Elmer Corp., Norwalk, CT)
and for total P (all forms) on an ICP spectrometer (model 61
E, Thermo Jarrell Ash Corp., Franklin, MA) at the University
of Washington Analytical Laboratory (Seattle, WA). In
addition, organic matter concentration was determined by
loss-on-ignition. Duplicate 3 g subsamples were ashed in a
muffle furnace at 450°C for 8 hr and weighed again after
cooling to determine ash content.
Analyses of Tree Growth and Soil Samples
Several subplots and whole plots were eliminated from
measurements because a large percentage of trees in those
46
WJAF 18(1) 2003
plots were killed or severely set back by heavy browsing or
repeated frost damage. Damage appeared to be associated
with areas of high elk usage or microtopography (e.g., frost
pockets) and was not related to treatment. Of the original 135
subplots, 94 (70%) were included in the following analyses
of survival and tree size.
Site preparation effects on tree survival, tree height, diam­
eter, and mean tree volume were evaluated at age 20 with
analysis of variance (ANOVA). Because the broadcast burn
treatment was not assigned randomly to plots within harvest
units as were other treatments, we did not include it in the
ANOVAs. We report the broadcast burn in the results and
include it in the discussion, however, as broadcast burning
used to be the most common site preparation treatment in the
region, and we thought many readers would like to see how it
performed in relation to the other treatments. The model
tested main effects of harvest units, site preparation, and an
interaction of unit with site preparation. Subplot values were
averaged prior to analysis. Replications of treatment plots
within units were used as error a for testing of the maintreatment effects. Differences were judged statistically sig­
nificant at P ≤ 0.05. Treatment means were compared with
Ryan-Einot-Gabriel-Welsch multiple range test (SAS Insti­
tute 1987). We chose to analyze only tree size and survival at
age 20. Data from multiple measurements of the same trees
are correlated, and analysis of data from all years would have
required the more complex repeated-measures analysis that
is more difficult to interpret. We present data from age 3 and
10 in graphs and tables.
Soil bulk density, C, N, P, and organic matter concentra­
tion were analyzed by ANOVA for the main effects of harvest
unit and site preparation treatment. There were no replica­
tions of treatment plots within units in this part of the study,
and subplot values were averaged prior to statistical analysis.
In contrast to the analyses of tree growth, the ANOVAs for
soil parameters included the broadcast burn treatment. We
made that decision because only one plot per site preparation
treatment per harvest unit was sampled for soil parameter;
thus, we thought the fact the three plots in the broadcast burn
treatment were grouped was not a major concern.
We also determined correlation coefficients between tree
size and survival. Correlations were calculated using indi­
vidual subplot values or using treatment or unit averages
depending on the comparison of interest.
Results
Survival
Survival of all units and treatments was 86% at age 3,
78% at age 10, and 70% at age 20. Site preparation treatments
did not significantly affect survival at age 20 (Table 2);
however, the effect of harvest unit was significant. Harvest
unit 3, which had the highest level of residue, had only 63%
survival at age 20 whereas unit 1, with the lowest residue,
had 77% survival (Table 3). The unit×site preparation
interaction was not significant.
Tree Height and Height Growth
Site preparation had a significant effect on tree height
Table 2). At all three ages, all the site preparation treatments
Table 2. Summary of ANOVAs for tree survival, tree height, diameter at breast height (dbh), and tree volume at age
20. Broadcast burn treatment excluded. Numbers in bold indicate effects significant at P ≤ 0.05.
Source of variation
Unit
Site preparation
Unit x site preparation
df
2
3
6
Survival
0.049
0.822
0.476
Height
0.124
0.001
0.636
Probability of > F
Dbh
0.044
0.000
0.785
Volume
0.039
0.000
0.862
advantage over trees in units 1 and 2, while between
ages 10 and 20, trees in unit 2 had a 0.6 m (8%) greater
height growth than units 1 and 3.
Tree Diameter and Volume
Dbh at age 20 differed by site preparation treatment
and by unit (Table 2). Mean diameter was significantly
greater in the scarification treatment than in the other
treatments; it was 2.0 cm (12%) larger than the average
of machine-pile-and-burn and broadcast burn treatments
and 3.7 cm (26%) larger than the average of control and
hand-pile-and-burn treatments (Table 5). Mean diameter
per subplot or per treatment was not significantly
correlated with survival at age 20. On average, diameter
in units 2 and 3 was 1.6 cm (11%) larger than in unit 1
(Table 3). Mean diameter per unit was negatively
correlated with survival at age 20 (r = -0.98, P = 0.13).
Site preparation had a significant effect on mean tree
volume at age 20 (Table 2). Scarification resulted in a
27% greater mean tree volume than broadcast burn or the
machine-pile-and-burn, and 82% greater than the control
(Table 5). The effect of harvest unit was significant; on
average, unit 3 produced 33% greater mean tree volumes
than unit 1 (Table 3).
were taller on average than the control (Table 4). At
age 20, tree heights in the scarification and machinepile-and-burn treatments were significantly greater than
in the control treatment. The broadcast burn treatment
had the tallest trees at ages 3 and 10, but that apparent
advantage was no longer present at age 20, when trees
in the scarification treatment were slightly taller
(Figure 1). Harvest units did not differ significantly in
tree height at age 20 (Table 2). The harvest unit x site
preparation interaction was not significant.
The site preparation treatments differed in their
effect on height growth, and the treatments changed
ranking over time (Figure 2). Between ages 3 and 10,
trees in the broadcast burn and scarification treatments
averaged 0.9 m (30%) greater height growth than trees
in the control. Between ages 10 and 20, trees in the
scarification treatment grew 0.4 m (5%) more than in
the broadcast burn treatment, and the scarification and
machine-pile-and-burn treatments grew 19 and 14%
more in height than the control (Table 4). Harvest units
also differed in height growth, and the ranking of units
changed over time (Table 3). Between ages 3 and 10,
trees in harvest unit 3 had a 0.6 m (19%) height growth
Table 3. Summary of harvest-unit means for the measured parameters. Broadcast burn treatment excluded. Means
followed by the same letter are not significantly different. Initially 5,400 trees were planted and measured (1,080 per
treatment), by age 20 approximately 3,400 trees (680 per treatment) were measured; the reduction in number of trees
was due to mortality and deletion of nontypical plots.
Survival (%)
Age 3
Age 10
Age 20
Height (m)
Age 3
Age 10
Age 20
Height growth
Ages 3-10 (m/7 yr)
Ages 10-20 (m/l0yr)
Dbh (cm) age 20
Tree volume (m3 /tree) age 20
Harvest unit 1
Harvest unit 2
Harvest unit 3 Mean standard error
90
83
77a
84
73
70 ab
85
75
63 b
2.3
3:3
3.6
0.6
3.8
11.1a
0.6
4.0
11.9 a
0.7
4.5
11.9 a
0.02
0.18
0.29
3.2
7.3
14.8 a
0.086 a
3.3
8.0
16.1 b
0.104 ab
3.8
7.5
16.7 b
0.114 b
0.13
0.14
0.42
0.007
Table 4. Average tree heights and height-growth increments in each site preparation treatment. Means follow ed by
the same letter are not significantly different.
Site preparation treatment
Control
Hand-pile-and-burn
Machine-pile-and-burn
Scarification
Broadcast burn*
Mean Standard error
Tree height at age
3
10
20
- - - - - - - - - - - - - - - - (m) - - - - - - - - - - - - - - - 0.55
3.6
10.5 a
0.60
4.0
11.3 ab
0.68
4.2
12.1 bc
0.68
4.5
12.7 c
0.77
4.7
12.6
0.03
0.21
0.33
Height growth
3-10 m/7yr
10-20 m/10yr
3.0
3.4
3.5
3.8
4.0
0.19
6.9
7.3
7.9
8.2
7.8
0.20
* Broadcast burn not included in ANOVA or mean standard error.
WJAF 18(1) 2003
47
Soil Bulk Density, C, N, P, and Organic Matter Concentration
Site preparation had no significant effect on the soil
properties analyzed in this study (Table 6). The average soil
bulk density was 0.75 (± 0.04) g/cm3, with a significant unit
effect. Unit 1, which had the greatest amount of woody
residue removed, had the highest bulk density. The average
organic matter concentration was 118 (± 3.6) g/kg. The
average carbon concentration was 49 (± 1.6) g/kg. The
average N and P concentrations were 1.6 (± 0.06) g/kg and
0.7 (± 0.02) g/kg. The average C:N ratio was 31:1 (± 1),
with a significant unit effect. Units 2 and 3 had lower C:N
ratios than unit 1.
One of the broadcast burn subplots had a high mean value
for bulk density (0.94 g/cm3); it was near a major landing and
may have been affected by the harvesting and loading
activities. If the value from the subplot with the high bulk
density value was discarded, the mean bulk density for the
other five broadcast burn subplots was 0.72 g/cm3. If we use
the mean based on five subplots broadcast burn treatment,
the three for the treatments that did not use heavy equipment
Table 5. Dbh and mean individual tree volume at age
20. Means followed by the same letter are not
significantly different.
Tree volume
Dbh (cm)
Site preparation treatment
(m3/tree)
14.4 a
Control
0.076 a
Hand-pile-and-burn
14.6 a
0.083 a
16.3 b
Machine-pile-and-burn
0.108 b
Scarification
18.2 c
0.138 c
Broadcast burn*
16.1
0.110 Mean standard error
0.56
0.01 * Broadcast burn not included in ANOVA.
48
WJAF 18(1) 2003
(control, hand-pile-and-burn, and broadcast burn) had
lower values for bulk density than the two treatments that
utilized heavy equipment (scarification and machine-pile­
and-burn).
Discussion
Survival
The site preparation treatments used in this study did not
influence survival of large bareroot Douglas-fir seedlings.
This suggests that factors influenced by site preparation—
such as amount of exposed mineral soil or competing
vegetation—were not at levels critical for seedling survival in
the control treatment or were not altered enough by treatment
to have an impact. Previous studies in the Pacific Northwest
have reported both significant (Stein 1995) and
nonsignificant effects of site preparation on survival of
Douglas-fir (Morris 1970, Miller 1989); results differed
depending on site factors, amount of competing vegetation,
populations of damaging animals, and weather conditions
during the establishment phase.
We did observe significantly lower survival at age 20
on Unit 3 compared to the other two units. If the unit
differences in survival were due to the amount of woody
residues left on site after harvest, we would have expected
the differences among units to have been greatest at age 3
and then to have leveled off; in contrast, we observed a
pattern of increasing differences in survival among units
over time. We suggest that this pattern could indicate
inherent variation among the units in a factor that would
become more important over time. For example, prior to
harvest of the old-growth stand, Unit 3 may have had a
higher rate of infection by Phellinus weirii
Table 6. Mean values for soil parameters by site preparation treatment and harvest unit. Broadcast burn
treatment included in analyses. Values based on 0-20 cm soil depth. C, N, and P values represent the total of
organic and inorganic forms. For bulk density, n = 270; for the other parameters, n = 30. Site preparation was not
significant for any parameter, harvest unit was significant only for bulk density and C to N ratio, and the
interactions were all nonsignificant.
Soil parameter
BD
(g/cm3)
Site preparation treatment
Control
Hand pile and burn
Machine pile and burn
OM
C
N
P
- - - - - - - - - - - - - -(g/kg) - - - - - - - - ­
C:N
C:P
29
30
31
71
72
70
N:P
0.71
0.73
0.76
122
114
112
56
46
49
1.9
1.5
1.7
0.8
0.6
0.7
Scarification
0.79
111
44
1.5
0.7
30
65
2.2
Broadcast burn
Harvest unit
1
2
3
0.76
130
49
1.4
0.7
34
74
2.1
0.79 a
0.75 b
0.70 c
118
113
122
52
47
48
1.4
1.7
1.8
0.7
0.8
0.7
37 a
29 b
28 b
79
61
72
2.2
2.1
2.6
than the other two units; if so, the higher infection rate would
have become more significant over time as root systems
expanded and came in contact with existing inoculum
sources.
The trees in our study plots were carefully planted by a
research crew. In operational plantings, greater differences in
survival associated with site preparation activities could
result if planters do not make an extra effort to plant
seedlings correctly in areas where slash and duff make
proper planting more difficult.
Growth
The site preparation treatments used in our study stimu­
lated tree growth. Trees in the scarification, machine-pile­
and-burn, and broadcast burn treatments grew faster than
those in the hand-pile-and-burn and control treatments, and
trees in the hand-pile-and-burn treatment grew better than
those in the control. Percent differences between the four
active treatments and the control in height growth increment
(Figure 2) strongly suggest that the mechanisms
responsible for increased growth in each treatment changed
through time. Differences in response among treatments
and growth periods can probably be attributed to their
short- and long-term influences on competing vegetation,
soil temperature, and nutrient availability. Each of these
factors is discussed below.
Site preparation, through fire or mechanical scouring,
can suppress the regrowth of residual vegetation, and
Douglas-fir can benefit from reduced competition mainly
because of increased light and moisture (Cole and Newton
1986, 1987). In the Coast Range, invading and residual
plants covered up to 80% of the area without site
preparation within the first growing season (Stein 1995).
Our units were planted two growing seasons after site
preparation; photographs taken at the time of treatment
show that some vegetation was present. Relative to treated
areas, seedlings in the control and in the hand-pile-and­
burn treatments may have been somewhat suppressed by
regrowing vegetation. For example, salal attains maximum
predicted cover 3 to 4 yr after clearcutting on unburned
2.4
2.4
2.3
areas, and 7 yr after clearcutting on burned areas (Knowe et
al. 1997). Although we did not assess vegetation, we
assume that the broadcast burn, machine-pile-and-burn, and
scarification treatments would have killed or reduced the
competitiveness of some vegetation and, thus, would have
provided a better environment for seedling growth.
Scarification normally reduces competition from woody
plants but creates a good seedbed for herbaceous plants.
Many of the herbaceous invaders are shade intolerant and
will die out as crown closure occurs; thus, their impact on
tree growth would not be expected to last long after crown
closure (note that in Figure 2 the scarification treatment
improved relative to the other treatments after crown
closure). Diameter growth is considered to be more
sensitive to competition than height growth (Brand 1991,
Wagner et al. 1999) and the effects of these three intensive
treatments at age 20 were greater for tree diameter than for
tree height. Therefore, the growth results are consistent
with our assumption that the intensive treatments reduced
competing vegetation.
In addition, the broadcast burn, machine-pile-and-burn,
and scarification treatments would have provided conditions
conducive to soil warming by reducing shading from woody
residue and, in the broadcast burn treatment, through a
darkened soil surface. Warmer soil may have been beneficial
in promoting root growth during spring and early summer.
Reduction of woody residue after timber harvest by burning
enhanced growth of Douglas-fir and ponderosa pine seed­
lings in the eastern Cascade Mountains of Washington; this
effect was attributed to more optimal growth temperatures
when slash was not shading the soil (Zabowski et al. 2000).
The broadcast burn, machine-pile-and-burn, and handpile-and-burn treatments all involved fire; however, they
differed in the spatial coverage and intensity of fire and thus
they may have had disparate short- and long-term effects on
nutrient availability and tree growth. The broadcast burn
treatment covered the whole plot and would have resulted in
an initial widespread burst of nutrients available from the
forest floor and fine woody materials. This initial burst would
have been short-lived, however, and the remaining largeWJAF 18(1) 2003
49
diameter, charred woody residue would not supply much
additional nutrient input. Thus, it is not surprising that the
trees in the broadcast burn treatment lost their growth advan­
tage (Figure 2). The machine-pile-and-burn treatment would
have resulted in more intense fires at the pile locations but the
machine piles impacted much less of the area than the
broadcast burn; forest floor and fine debris on the rest of
the area would have been available to decompose and
supply nutrients over time. The hand-pile-and-burn
treatment would have had many more but much smaller
piles than the machine-pile-and-burn treatment. The greater
relative growth on the hand-pile-and-burn treatment
compared to the control at tree ages 3 to 10 (Figure 2) may
have resulted from tree roots growing into the burn-pile
areas, which presumably were nutrient-enriched in a
manner comparable to the broad-cast burned areas.
Mechanical site preparation, such as that in the machinepile-and-burn or scarification treatments, may have also
increased nutrient availability, as many forest soils respond
to disturbance with an increase in N availability (Vitousek
and Matson 1985, Vitousek et al. 1992, Smethurst and
Nambiar 1995, Duchesne and Tellier 1997). Later on, de­
composition of residual forest floor and fine woody residue
will have contributed nutrients. For example, using the
exponential decay model for Abies presented by Harmon
and Sexton (1996), we estimate that 69% of the smallest
residue class (0.6-7.9 cm) would have decomposed after 20
yr. Assuming an average of 15 ton/ha at the beginning
(mean of the three harvest units), an initial decay class of 2 or
3, and an average N concentration of 1 g/kg (Harmon and
Sexton 1996), the decomposition would have supplied 10 kg
N/ha to the treatments with residual fine woody residues.
The broad-cast burn treatment, however, would have had
much less material in the small diameter residue class (as
most of this material was consumed during the fire) and,
thus, it would have had a smaller nutrient pool available for
tree growth. Nitrogen is likely to be limiting tree growth on
these units as nearby sites have been very responsive to
nitrogen fertilization (Miller and Reukema 1973, Miller and
Tarrant 1983).
Residue Levels
Leaving more woody residues after harvest may increase
the intensity and response or result in more intensive site
preparation activities. For example, the effects of mechanical
treatments such as piling or scarification would be expected
to be proportional to residue levels, as greater residue would
increase the number of times machinery entered and traveled
through the area. The effects of fire on tree growth and soil
characteristics depend on burning intensity and duration,
both of which are related to fuel (or residue) loading and
flammability (Morris 1970, Bigley and Henderson 1989,
Brockley et al. 1992, Agee 1993). We were not able to test
the effects of woody residues as our design did not have
replicates of each residue level. Little and Klock (1985)
reported that when preburn fuel conditions were similar,
more nitrogen was lost per hectare if more residue was left
on site; in their study, a greater loss of nitrogen by harvest
was more than compensated for by the lower losses during
burning that resulted from lower residue loadings. If units
with similar residue loadings had different preburn moisture
50
WJAF 18(1) 2003
contents, however, they ended up with different nutrient
losses due to fuel consumption (Little and Klock 1985).
Since our units were harvested and then yarded over several
years, they could have had different fuel conditions when
the site preparation treatments were implemented. If future
studies are to examine the effects of residue loadings or the
interactions between residue loadings and site preparation
treatments involving burning, care should be taken to
equalize preburn fuel moisture contents. In addition, the tree
residue levels used in this study did not differ much in the
amount of residue left in the smallest size class (Table 1); a
future study may want to create contrasting residue levels
for materials in this size class as the smallest diameter
materials would have the highest nutrient concentrations and
the shortest decay periods.
Soil Characteristics
Bulk density was fairly low throughout the study area;
low bulk density values are common for volcanic ash soils
(Nanzyo et al 1993). The ranking of harvest units by bulk
density was consistent with the rankings of residue
removed; that is, Unit 1, which had the greatest amount of
residue removed, also had the highest mean bulk density,
and Unit 3, which had the least amount of residue removed,
had the lowest mean BD. Although the site preparation
treatments did not result in statistically significant
differences in bulk density, the treatment means are
consistent with our expectation that treatments utilizing
heavy equipment have the potential to increase bulk
density. If there were initial treatment-induced differences
in bulk density, however, they apparently did not have a
detrimental effect on tree growth as the scarification
treatment had the highest bulk density and the best 20 yr
growth rates. The soils were also fairly high in organic
matter (>110 g kg-1 or 11%). Although not significantly
different from the other treatments, the scarification
treatment had the lowest organic matter level at age 21 and
had the tallest trees at age 20; thus, as we would have
expected, the organic matter levels measured were not
limiting tree growth. We do not know if differences among
treatments in soil nutrient characteristics were present
immediately following treatment; if they were present they
have apparently dissipated over time. The lack of treatment
differences in all measured soil parameters may indicate that
these site preparation treatments did not adversely impact
the inherent productivity potential of these sites. Insufficient
evidence is available to speculate on the effects of removals
of woody materials over an entire rotation or over multiple
rotations or on the impacts of such removals on other values
such as wildlife habitat.
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