7
Effects of Slash Burning on Subsequent
Site Index and Volume in
Midrotation-aged Stands
Richard E. Miller
ABSTRACT Stands were measured at forty-four loc:1tions to compare stand develop­
ment on a sl h burned plot and an unburned one at each location. The paired plots were
established thirty-five to forty-two years earlier after clearcut harvest and fall burning of
old-growth slash in the Cascade Range of western Washington and Oregon. Although
some pairs showed a large difference insite index, no consistent partern related to burning
was apparent; the average difference in site index between the plot pairs was statistically
nonsignificant. The average difference in cumulative, gross volume growth since old­
growth harvest was also nonsignific:1nt; yet volume growth was clearly greater on the
burned plot at some locations but on the unburned plot at others. Differences in species
composition of these midrotation-aged stands were statistically significant. Number and
volume of Douglas-fir and hardwoods were greater on burned plots, but number and
volume of other conifers (hemlock, true fir, redced.ar) were greater on unburned plots.
Furure analyses of these data will attempt to improve predictions and reduce uncertainty.
Additional field trials are needed; current datafor quanrifying the effects of slash burning
on stand development in the Douglas-fir region are short term and restricted ingeographic
distribution. Ma nagers should recognize that silvicultural practices such as planting,
vegetation management, spacing, and fertilizatio n can moderate or compensate for
undesirable effects of slash fires a.t some locations.
Broadcast burning of logging slash C3.!l affect establishment (see Bigley and
Henderson, r.his volume) and development of forest stands. This paper considers
the effects of broadcast burning on subsequent stand productivity and is based
on plots at forty-four locations in western Washington and Oregon. These
locations had been clearcut thirty-five to forty-two y
earlier. From this
evidence, preliminary, regionwide conclusions are drawn about the effects of
slash burning on site index, stand volume growth, and species composition.
Some implications of these preliminary findings to forest management are also
discussed. Summaries and statistical analyses of these data are not complete so
additional, more detailed infonnation and conclusions will be published in the
fueure.
69
Miller
Some foresters are concerned about the effects of forest practices such as
slash burning on long-term productivity of forests and on costs of timber
production. Quantitative data needed to support or allay these concerns are
available for North America only for periods less than a single rotation. The
goal of sustained timber yields implies, however, that consideration be given to
production and costs in multiple rotations. In this paper, "long-term" denotes
effects that exist or accumulate during three or more rotations. By using this
definition, one must conclude that no long-term growth data are available to
guide forest management in the Pacific Northwest-and elsewhere in North
America.
What is suitable evidence of slash burning effects? Evidence furnishes proof,
yet not all evidence has equal value; for example, direct evidence has greater
value than circumstantial evidence. A difference in stand growth on carefully
matched areas, either burned or unburned, is direct evidence of effects or lack
of effects of slash burning on future yields at that location. Such evidence
becomes increasingly convincing as the observation period approaches one or
more rotations. Data about seedling stocking and growth are also direct
evidence, but are less reliable indicators of future timber production, because
initial effects of a disturbance may not represent the long-term impact. Indirect
evidence, such as measured losses of nitrogen after slash burning, can also
indicate a potential for reduced forest growth, because these losses may ag­
gravate a well-demonstrated deficiency of nitrogen in most Northwest forests.
In some situations, however, ftre may temporarily improve nitrogen nutrition
by increasing the amount of available nitrogen. Thus foresters must retain a
critical perspective about evidence of factors affecting site productivity.
To what situation does the evidence apply? Growth response measured at a
given site is a sample of response by a larger population; this larger population
could be an individual stand or all stands in a general area like the Douglas-frr
region. With adequate sampling, one can predict the average response of a larger
population or some subpopulations to a treatment such as slash burning. Predic­
tions or inferences derived from samples apply only to the sampled population,
however. Thus the scope of the sampling defmes the geographic or descriptive
boundaries f r predictions, inferences, and conclusions.
Evidence from Midrotation-aged Stands
The Data Base
Morris (1958, 1970) started an extensive study of the effects of burning
old-growth slash on frre hazard and on conifer regeneration in the Douglas-frr
region. He installed a burned and an unburned plot at more than sixty locations
in western Washington and Oregon. At each location, these adjacent plots were
similar in size (0.12 to 0.58 acres) and were matched for comparability in slope,
aspect, soil, and logging disturbance. Both plots were established at the same
Effects on Subsequent Site Index
time, up to one growing season after burning and within two growing seasons
after logging. All burns were conducted in fall, although with a variety of fuel
and moisture conditions. Line-intercept transects and photographs documented
postburn conditions on the burned plot and the unburned one at each location.
Repeated photographs, measurements over these transects, and seedling surveys
documented trends in fuel, vegetation, and stand development In 1986 and
1987, trees in forty-four remaining plot pairs were measured (Miller et al.,
unpublished manuscript). With one exception, all locations sampled the west
side of the Cascade Range (Figure 1). Fifteen of the pairs had been planted at
least once to Douglas-fir and one pair planted to ponderosa pine; planting
success was variable. The remaining twenty-eight pairs regenerated naturally,
and usually to Douglas-ftr. About half of the forty-four pairs had been precom­
mercially thinned, and a few had been fertilized with nitrogen. Measurement of
these midrotation-aged stands provides the longest term and widest geographic
scope availabie for quantifying the net effects of slash burning on forest
composition and growth in the Douglas-frr region.
Statistical Analyses
The original experimental design (Morris 1958) was randomized block,
unreplicated at any location. Consequently, no statistically supported statement
can be made about the response at any single location. Response variables were
expressed as a difference· at each location-for example, growth on the burned
plot minus growth on the unburned plot. The statistic, t, was used to test the null
hypothesis that the mean difference in site index or in growth among the
locations was zero. The significance of all tests of these null hypotheses was
judged at p s; 0.10. SPSS programs (Norusis 1986) were used for statistical
analyses.
Slash Burning Effects on Site Index
Throughout this Cascade region, does Douglas-fir site index (King 1966)
differ between burned and unburned plots? No. As shown in Table 1, the average
difference in site index (1.6 foot) between the forty-two plot pairs that
regenerated to Douglas-fir was statistically nonsignificant (p < 0.21).
Table 1. Fifty-year site index (in feet) of plot pairs that regenerated to Douglas-fir.
Treatment
Item
Burned
Unburned
Mean Difference
Mean
106.5
104.8
1.6
2.4
2.1
1.3
SE±
Miller
Oregon
X single pair of plots
4X number of pairs of plots
Figure
1.
Locations of forty-four pairs of burned and unburned slash plots in the Douglas-fir region of western Oregon and Washington. Modified from Morris
(1970). Effects on Subsequent Site Index
x aurn&d
0 Unburned
140
130
'/,
120 o;
-!
"
.,
..,
..5
"
()3
1
110 '-'
100
90
80
70
60
0
iil
--------------s<"J
.t'::;
4o
3.:
30
25
20
15
1'o
5
---),,
55
Location Number
Figure 2. Site index (flfty-year base) of Douglas-fir on a burned plot and an unburned
one at forty-two locations, thirty-five to forty-two years after old-growth harvest. Loca­
tions are numbered in ascending order from the most northerly near Enumclaw,
Washington, to the most southerly, number 49, near Myrtle Creek, Oregon. Number 56
is the only location in the Coast Range.
Site index appears greater on the burned plot at some locations, but less at
others (Figure 2). Seven plot pairs showed more than a 10% difference in site
index (locations 8, 30, 31, 40, 45, 49, 56), but no consistent pattern related to
burning was detected by the current analyses. A similar absence of consistent
effect of slash burning on Douglas-fir site index was shown by separate analyses
of the fifteen plot pairs planted with this species (p < 0.41) and twenty-seven
pairs that naturally regenerated to Douglas-frr (p < 0.37).
Slash Burning Effects on Cumulative or Gross Volume Growth
For this Cascade region, does total stem volume production by all species
through thirty-five to forty-two years differ on burned and unburned plots? For
trees 1.6 inches dbh and larger, the average difference in cumulative production,
85 ft3/acre in favor of burning, was not statistically significant (p < 0.67). As
shown in Table 2, the large standard error of this difference indicates the
variability of results among these forty-four locations. Mean annual volume
increment (MAl) since harvest or burning appears greater on the burned plot at
some locations, but less at others (Figure 3). This variability in results explains
the large standard error and thus uncertainty about an average effect.
For trees 7.6 inches dbh and larger, average difference (38 ft3/acre) in
cumulative volume growth was also nonsignificant (p < 0.83); the large standard
error of this difference, as shown in Table 3, again indicates that inconsistent
results among the forty-four locations explain a lack of real, general difference
between burned and unburned plots.
Miller
• -;:r·,
,.J'•'
x
eumed
C Unourned
0
·sc
:
1
e
0
.!<
("'):.
<
:zc
:'c.
lr
:::;
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iji
I i
i
6C
30
1
I
i
:J I - .l;j
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i
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-' ,
i
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Fllanled
:.:
-
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x
l
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Location Number
Figure 3.
Mean annual gross volume growth in thiny-five to forty-two years since
old-growth harvest or slash burning, trees 1.6 inches dbh and larger of all species. The
forty-four are numbered in ascending order from the most northerly near Enumclaw,
Washington, to the most southerly, number 49, near Myrtle Creek, Oregon. Number 56
is the only location in the Coast Range. At seven locations, a third plot was recently
established in the plantation surrounding the original burned plot that
regenerated.
was
naturally
3
Table 2. Total stem volume growth of trees 1.6 inches dbh and larger (ft /acre).
Treatment
Item
Mean
SE±
Burned
Unburned
2.628
2,543
85
217
200
198
Difference
3
Table 3. Total stem volume growth of trees 7.6 inches dbh and larger (ft /acre).
Treatment
Item
Mean
SE±
Burned
Unburned
Difference
1,996
1.958
38
205
171
74
Effects on Subsequent Site Index
Slash Burning Effects on Species Composition
Throughout this Cascade region, does species composition differ between
burned and unburned plots? Yes. Differences in species composition by both
numbers and volume were statistically significant. In the thirty-five to forty-two
years after harvest of the previous stand, cumulative numbers (p < 0.00 and 0.02)
and volume growth (p < 0.03 and 0.10), respectively, of Douglas-fir and
hardwoods by trees 1.6 inches dbh and larger on burned plots exceeded that on
unburned plots. Conversely, cumulative numbers (p < 0.00) and volwne growth
(p < 0.00) of other conifers (western hemlock, true fir, and western redcedar)
on unburned plots exceeded those on burned plots (Figure 4).
.3000
8
2500
2000
'[
"'
..,-
s.
.,
E
1500
[]
0
IJI
UB
JIJ···--·-·
!
I
Oouglts•llr
Other conUtrs
HardwoodS
8
us
I
I
I
II
I
1000
500
0
1.6+
7.6+
Minimum Tree Diameter (Inches)
Figure 4. Average cumulative gross volume production on burned and unburned plots
at forty-four locations and thirty-five to forty-two years since old-growth harvest, by
species and minimum tree size.
The generally greater numbers and volume of Douglas-fir on burned plots
can probably be explained by the greater extent of exposed mineral soil on
burned areas that favor natural regeneration of this species (Morris 1970).
Conversely, the greater numbers and volume of other conifers on unburned plots
could be explained by a larger amount of organic seedbed, which favors
hemlock establishment (Minore 1979), and by the greater likelihood that ad­
vance regeneration of shade-tolerant conifers survived on unburned plots. These
results confmn conventional wisdom that these tolerant species generally are
more numerous on unburned sites.
Miller
Performance of Planted and Natutal Regeneration
At fl.fteen of the forty-four locations, plot pairs were planted to Douglas-flr;
at the remaining twenty-nine, twenty-eight plot pairs regenerated naturally and
one was planted to ponderosa pine. Initial survival of planted trees differed
among these locations. Unfonunately, the ultimate fate of planted trees was not
recorded, so it is not possible to determine whether survival differed between
burned and W1burned areas. Seedling stocking data that identifled planted and
natural seedlings indicate, however, that natural regeneration contributed
strongly to volume growth of the "plantations."
Valid comparison of volume production by these "planted" stands as opposed
to natural ones is not possible, because the fifteen planted Douglas-fir stands
were at different locations than the twenty-eight natural stands. Stands of both
•
ori ns showed no consistent difference between burned and W1burned plots in
total volume production by all species. Both planted and volunteer stands had
more trees and volume of Douglas-fir and hardwoods on burned plots and,
conversely, fewer trees and less volume of other conifers.
Table 4. Average characteristics of planted and volunteer stands on burned portions at
seven locations in the Cascade Range, per acre basis for trees 1.6 inches dbh
and larger, except where noted.
stand Origin
Item
Planted
Breast height age (yrs)
Establishment period (yrs)
Natural
Difference
p<
27.1
25.9
1.3 = 0.5
0.04
9.0
10.3
-1.3 =0.5
0.04
Cumulative stems (no.):
All species
438
453
-15 = 30
0.63
Douglas-fir
377
352
26::: 27
0.04
0.01
Dbh (inches):
All species
7.9
7.0
0.9 = 0.2
Douglas-fir
8.2
7.6
0.7 = 0.3
0.04
71.0
68.3
2.7 =3.6
0.10
Douglas-fir
112.7
111.7
3
Cumulative volume (ft ) trees 1.6+ inches:
1.0=2.2
0.67
FL!o (ft)
50-year site index (ft)
All species
3,639
2.736
903 = 186
0.01
Douglas-fir
3,527
2,632
895 = 173
0.01
All species
2,977
2,118
859=167
0.01
Douglas-fir
2,948
2,131
818=170
0.01
3
Cumulative volume (ft ) trees 7 .6+ inches:
Effects on Subsequent Site Index
At only seven locations was a pure comparison possible between production
on planted and natural stands. This comparison was available for burned
portions but not for unburned. According to the planting records and local
foresters, these seven plantations were of local seed source and successfully
established. When measured in 1986, these fourteen stands averaged 36.1 years
after burning. Breast height (4.5 feet) age of the forty largest trees per acre
averaged 27.1 years on planted plots, indicating a 9 .0-year establishment period
(Table 4). The natural stands averaged 1.3 additional years to attain breast
height, and this difference was statistically significant (p < 0.04). Cumulative
number of stems of all species and of Douglas-fir averaged about the same for
both planted and volunteer stands (Table 4), suggesting that volunteer Douglas­
fir contributed strongly to both planted and nonplanted stands. This also sug­
gests, conversely, that planted Douglas-fir contributed minimally. Current
average dbh was greater in planted stands for all species (p < 0.00) and for
Douglas-frr (p < 0.04). Height of the forty largest diameter stems per acre also
averaged greater in the planted stands (p < 0.10). Cumulative volume production
by all species since burning averaged 33% and 41% greater in planted stands
for trees 1.6 and 7.6 inches dbh and larger, respectively (p < 0.00). Douglas-fir
volume production was also greater in the planted stands (Table 4). Differences
in MAl at individual locations are shown in Figure 3. These data indicate larger
tree size and volume production on burned areas after planting supplemented
by natural regeneration than after natural regeneration alone. Similar com­
parisons for unburned areas were not available. The gains seemed to be derived
in part from more uniform stocking in planted stands, because both planted and
natural stands had similar numbers of trees .
Discussion
Slash fires can affect several factors contributing to stand establishment,
development, fmal volume production, and, ultimately, costs of timber produc­
tion. Successful stand establishment, for example, requires an adequate number
of well-distributed seedlings maintaining dominance over competing vegeta­
tion. Slash burning can influence stand establishment by affecting advance
regeneration, seedbed conditions, and vegetative competition. The effective use
of planting, vegetation management, spacing, and fertilization can moderate or
compensate for potentially negative effects of slash burning at some locations.
Delayed establishment of both naturally regenerated and planted stands
clearly affected volume production. On the eighty-eight plots at these forty-four
locations, the forty largest crop trees per acre attained breast height six to
twenty-four years after harvest or burning. This establishment period averaged
twelve years and was not related consistently to slash burning. Individual trees
sampled at twenty-six locations required only the average four to ten years to
o:-" from ground line to breast height (King 1966, McArdle et al. 1961). This
Indicates that delayed or inadequate seedfall and loss of new germinants
Miller
contributed to delayed establishment and later to reduced volume production on
both burned and unburned areas.
Will genetics and handling of current planting stock lead to similar responses
on current burned areas? How does vegetation management affect the com­
parison? Few of these planted or natural stands received vegetation manage­
ment other than that provided by slash burning. The results for these 1947-52
plantings, therefore, may not correspond to those obtained with current planting
and other silvicultural practices. In short, the applicability of these data-the
best currently available-to current management remains unknown and re­
quires investigation.
Further analyses of these data may lead to more site-specific conclusions
about slash burning and tree growth. The importance of burn severity, site
quality, and vegetation succession needs careful evaluation. Sites undoubtedly
vary in resistance to and recovery from organic matter displacement and loss
from slash burning. Steep slopes, for example, are particularly vulnerable to
disturbance from harvesting and ftre, because they commonly have soils with
small amounts of organic matter and large amounts of gravel. Such soils are
potentially most affected by organic matter losses. Because absolute and per­
centage volume gains from nitrogen fertilization of Douglas-frr are inversely
related to site index (Miller et al. 1986), it seems reasonable to expect that
growth on poor quality sites would, conversely, be more strongly reduced by
losses of nitrogen via slash burning.
Conclusions, Implications, and Recommendations
Current data for quantifying the effects of slash burning on stand develop­
ment and growth in the Douglas-frr region are short term and restricted in
geographic distribution or scope. Additional and stronger quantitative data are
needed to guide slash burning and silvicultural practices in this region. The data
from midrotation stands on the Morris plots provide the longest-term and most
widely distributed sample available. Measurement should be continued on these
trials; yet additional well-designed comparisons are needed. Recent experimen­
tal areas created by frre researchers provide an opportunity for comparing forest
establishment and development after known burning severity (Otunar 1983,
Otttnar and Sandberg 1981).
Data from these midrotation-aged stands suggest that the consequences of
slash burning on volume production differed greatly among these forty-four
locations. In the absence of replicated plots at any location, however, no
statistically supported statement is possible about the effects of slash burning at
a single location. Apparent differences between burned and unburned plots in
site index and total volume production by all species were small at most
locations, but large and inconsistent at others. This variation means that general
recommendations about the effects of slash burning on stand productivity
should not be applied across
all
sites in the Cascade Range of western
Effects on Subsequent Site Index
washington and Oregon. Slash burning did have some general effects on species
composition. Volume production of Douglas-fir was generally increased and
that of other conifers decreased. Further analyses of data from these and
additional tri!lls are necessary to develop site-specific guidelines.
Current analyses of data from these midrotation-aged stands provide direct
evidence that decisions about whether and how to bum slash at some locations
should consider more than primary needs to reduce risk of wildfire, create
planting spots, and reduce planting costs. Aboveground organic matter that is
commonly an obstacle after harvest is also a potential source of humus and
nitrogen for the soil. The general response of Northwest stands to nitrogen
fertilization is direct evidence of the importance of conserving that nutrient. To
attain the goal of maximizing net returns from forests, managers should quantify
the advantages and disadvantages of alternative methods to reduce risk of fire
and to establish successful plantations. Sustainable, minimal-cost wood produc­
tion requires balance between soil conservation and soil replenishment through
fertilization.
Acknowledgments
The U.S. Department of Energy, BioEnergy Program, contributed a large portion of
the funding for this work. The Bureau of Land Management and the USDA Forest
Service, National Forest Administration, contributed funds and field assistance. The
Washington State Department ofNatural Resources provided data processing assistance.
Useful review comments were provided by Harry W. Anderson, Richard E. Bigley,
Donald W. Connett, Dean S. DeBell, Stanley P. Gessel, David L. Handley, Timothy A.
Max, and Diane White.
Literature Cited
King, J. 1966. Site index curves for Douglas-fir in the Pacific Northwest Weyerha euser
For. Pap. 8. Weyerhaeuser For. Res. Center, Centralia, Washington. 49 p.
McArdle, R. E., W. H. Meyer, and D. Bruce. 1961. The yield of Douglas-fir in the
Pacific Northwest USDA Tech. Bull. 201. 72 p.
Miller, R. E. , P. R. Barker, C. E. Peterson, and S. R. Webster. 1986. Using nitrogen
fertilizers in management of coast Douglas-fir. I. Regional trends of response. In
Douglas-fir. Stand management for the future, pp. 290-303. Institute of Forest
Resources Contrib. 55. College of Forest Resources, University of Washington,
Seattle.
Miller, R. E., R. E. Bigley, and S. N. Uttle. The effect of burning Douglas-frr logging
slash on subsequent stand development and site productivity. Unpublished
manuscript on file at the Forestry Sciences Laboratory,· Olympia, Washington.
Miller
Minore. D.
1979. Comparative autoecologi cal characteristics of northwestern trees
species: A literarure rev-iew. USDA For. Serv. Gen. Tech. Rep. PN\V-87. Pac.
Northwest For. and Range Exp. Stn., Portland, Oregon. 72 p.
Morris, W. G. 1958. Influence of slash burning on regeneration, other plant cover, and
fire hazard in the Douglas-fir re gio n. USDA For. Serv. Res. Pap. 29. Pac.
Northwest For. and Range Exp. Stn., Portland, Oregon. 49 p.
. 1970. Effects o f slash burning in overrnature stands of the Douglas-fir region.
For. Sci. 16:258-270.
--
Norusis, M. J. 1986. SPSS/PC for the ffiM PC/Kf/AT. SPSS, Inc .• Chicago.
Ottmar, R. 198 3 . Flaming-stage duff consumption. Srudy pl an on file at the Forestry
Sciences Laboratory, Olympia. Washington.
Ottmar, R., and D. V. Sandberg. 1981. Flaming-stage fuel consumption. Study plan on
file at the Forestry Sciences Laboratory. Olympia, Washington.
In: The Burning Decision: Regional Perspectives on
Slash. Hanley, Donald P.
Kammenga, Jerry J.; Oliver,
Chadwick D. eds. College of Forest Resources ,
University of Washington, Seattle, Washinqton.
Reproduced bv USDA Forest Service For Official Use About this file: This file was created by scanning the
printed publication. Misscans identified by the software
were corrected; however, some mistakes may remain.