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6
Effects of Slash Burning on
Plant Succession and Timber
Stand Establishment
R. E. Bigley and J. A. Henderson
ABSTRACI' Conifer establishment and growth on burned and unburned areas vary
·
considerably by site. This paper deals with the influence of bum severity on the
environment, vegetation response after burning, conifer establishment and early growth,
and documentation of variations in effects of burning on plant succession. Controlled
burning of logging slash in the Pacific Northwest strongly influences the initial environ­
mental conditions in which vegetation must develop and the possible plants that may
establish. Knowledge of the subject is limited both by the few sites where direct
comparisons of burned and unburned plots have been made and the short duration of
most observations, but enough is known about several widespread species to predict their
response to prescribed f1re. The literature suggests that conifers planted on burned areas
in coastal Washington and Oregon gain an advantage from reductions in competing
vegetation but may suffer from increased animal damage. The conclusion is reached that
information on the effects of burning on vegetation could be better utilized if respose
data were stratified by forest zones and plant associations (or groups of associations).
Controlled burning of logging slash is a major influence on initial plant succes­
sion and early stand development of Pacific Northwest forests. The extent of
our knowledge, however, is limited both by the relatively few sites where direct
comparisons have been made on paired burned and unburned areas arid the shon
duration of most observations. Enough is known about the response of some
species to prescribed frre to give general predictions of their response. Variation
among sites and variation in the influence of burning on vegetation and the
environment will continue to complicate the transfer of information about slash
burning effects to potential users.
Succession is not only a sequence, as described by a series of plant species
and abundances, but a process in which the environment and thus ihe suitability
of the site for different species change. The main effect of slash burning is to
modify the environment and influence species survival, which in tum influences
processes such as initial plant establishment and growth. The effect is ultimately
reflected in differences in vegetation on slash burned and unburned areas. This
Bigley and Henderson
paper focuses on those changes that take place in the first two decades of stand
growth, with emphasis on areas west of the Cascade crest in the Pacific
Northwest This period includes the most rapid changes in vegetation and
typically the establishment of a conifer canopy. Once closed, a canopy has
considerable influence on slowing the rate of vegetation change.
We will fmt examine the major effects that burning has on the environment
in which vegetation develops. Then we will discuss the general state of
knowledge about burning effects on herbaceous and woody plants, and, more
specifically, conifer regeneration and early growth. We will conclude with a
few ideas on how to utilize existing information dealing with burning effects on
succession.
Influence of Burn Severity on the Environment
The term "burn severity" as used in this paper refers to the postfll'e surface
conditions as described by Morris (1958). A burn of low severity would leave
the forest floor intact, consuming only the litter layer. A severe burn would
remove the forest floor, and usually discolor the surface of the mineral soil.
Slash burning rarely affects an area evenly. Describing a site as "burned"
may give little information as to the extent of impact on site or vegetation. The
variable influence of slash burning has made the detailed interpretation of
treatments difficult over entire slash burned units. The severity and distribution
of burning effects are related to the moisture content, amount, and distribution
of fuels, but can vary with ignition patterns, topography, and weather conditions.
Spring bums are generally lower in severity because of higher fuel moistures
and result in less reduction of forest floor and exposure of mineral soil than
summer and autumn burns (Hawkes 1986, Little et al. 1986).
In studying the effects of slash burning on fire hazard and revegetation,
Morris (1958, 1970) conducted extensive line intersect sampling of ground
conditions after broadcast slash burning. These records allowed the reconstruc­
tion of the distribution of burn severity over a 0.25 acre plot shown in Figure 1.
Intervals between line intercept transects are averaged, resulting in an over­
simplification of the actual pattern. The pattern and size of affected areas vary
considerably over the plot, creating a wide range of microsites for new plant
establishment Where unburned patches of vegetation are adjacent to moderate­
ly or severely burned areas, existing shrub canopies or below-ground growth
could easily expand and occupy the burned space. Both the size of a burned area
and the condition of the neighboring area may influence the type and rate of
regrowth at a given location.
Stratification of burning effects by severity classes provides good informa­
tion for specific small locations, but little guidance on stand-level effects. This
is because of the tremendous variation possible in the proportion and distribution
of areas of different bum severity and the response at different sites. Several
studies provide examples of how plant response differs in areas of different bum
Effects on Plant Succession
.Severe
Figure
1.
.Light
.Moderate
Bum severity distribution on
a
0.25
acre
Unburned
plot, broadcast-burned
area
near
Lowell, Oregon.
severity. Morgan and Neuenschwander (1988) reported differences in shrub
cover response on 172 square foot plots having at least 90% of bums of low or
high severity on a speCific site type in Idaho. They concluded that shrub
community differences following low and severe bums were greatest in the first
three years after burning; thereafter, composition became more similar with
time. Regrowth from species with or without rhizomes was greater on the low
than on the high severity bum locations, but shrub seedlings were more prolific
on areas of severe bum. Both the composition and frequency of occurrence of
species differed between the areas of bum severity. Halpern (1987) found that
the duration of herb and shrub dominance differed with site type and bum
severity. On one site, the duration of the herb layer dominance decreased with
increasing bum severity because both redstem (Ceanothus sanguineus) and
evergreen ceanothus (Ceanothus velutinus) showed greater development on
burned sites and shaded the herbaceous plants.
Slash burning has many possible effects on the characteristics of the soil,
ecosystem processes, and the microclimate of a site which may also influence
the vegetation that reestablishes. Several works deal with these topics in detail
(Neal et al. 1965, Feller1982, Ahlgren and Ahlgren 1960, Boyer and Dell1980,
Barnett 1984, de Montigny and Auclair 1982, Grier 1972).
Changes to the soil surface are particularly important to plant establishment
and initial growth on burned sites. To some extent burning increases overland
flow of surface water by the removal of forest floor and live vegetation, and
increases the removal of seed and young plants by erosion. Drying of the soil
surface as a result of the loss of organic matter and increasing surface tempera­
Bigley and Henderson
tures in summer may prevent seed gennination and survival of seedlings. Boyer
and Dell (1980) noted that soils under some plant associations are more prone
to develop water repellent surfaces than others after burning because of
volatilization of organic compounds during burning.
Ambient air and soil surface temperatures generally display wider extremes
following slash burning (Lafferty1980) by the reduction of thennal cover ( both
forest floor and vegetation) and changes in surface albedo. In the winter, surface
soil temperatures can be lower on burned than unburned sites, resulting in frost
heaving of seedlings' roots or foliage damage on burned areas.
Burning generally causes temporary increases in available nutrients. Isaac
( 1940) speculated that increased nitrogen availability on burned sites may result
in development of conifer foliage in advance of roots, leaving them susceptible
to drought; however, this increased nutrient availability is generally seen as a
benefit to early growth of reestablishing vegetation. Failure to reestablish the
next conifer crop promptly after burning could influence succession for a
considerable time if nutrients are lost from the site or allocated to competing
vegetation.
Vegetation Response after Burning
Modes of Plant Regrowth
The most visible influence of burning on succession is the removal of part
of the existing vegetation. Plants can survive burning both as seed and as
aboveground and underground structures. Survival of established plants through
a bum is related to the depth and type of root system (McLean 1969). Plants
with shallow roots are susceptible even to light bums. Shrubs with rhizomes
within the mineral soil are rarely seriously damaged by burning. Miller (1977)
found that the number of blue huckleberry
(Vaccinium globulare) sprouts was
affected by the depth of heat penetration. Low severity spring burns resulted in
multiple new shoots arising from a single old branch through stimulation of buds
along the rhizomes. In this case, stem density could increase 80 to 120%.
Autumn burns were more likely to reduce stem density because of greater heat
penetration. The season of burning can be important for the vigor of regrowth,
because carbohydrate reserves in underground tissues are low in the summer
and highest in the autumn (Volland and Dell1981).
It is well known that delayed seed gennination can result in large reserves
of buried seed within the forest floor and mineral soil. Although the numbers of
buried seed may vary greatly, the distribution of seed in an undisturbed profile
decreases sharply with depth (Figure 2 ). The number of seeds and the species
composition may vary with the longevity of the seed and resistance to frre. In
some cases seeds can persist in the soil for many decades (Roberts1981).
Pratt et al. (1984) studied a ponderosa pine community in eastern Washington
and found viable seed of flfty-seven species in the surface soil layers, twenty-
Effects on Plant Succession
Seeds/tt2
0
40
20
LFH
0
.8
(/)
Q)
-
..c
CJ
c:
1.6
2.4
4.0
0
-
..c
Q.
Q)
c
Ceanothus ve/utinus
200
100
LFH
0
.8
1.6
2.4
Stellaria media
4.0
Figure 2.. Distribution and number of buried viable seed per square foot of two species.
Source: Weatherspoon (1985), Pratt et al. (1984).
one of which were not represented in the aboveground vegetation. In Oregon
and Washington, Ceanothus species provide the best known example of burning
as a stimulation to germinate seed that has been buried for long periods
(Gratkowski 1961). Orme and Leege (1976) found that seedling emergence,
survival, and growth ofredstem ceanothus (C. sanguineus) was greater after
fall
than spring bums.
Burning can remove some buried seed by heating to lethal temperatures.
Below that zone, buried seed may benefit from heat scarification; seed that lies
still deeper may remain viable and dormant. Recruitment of new seedling s may
also be influenced by the remaining aboveground vegetation. Shading can
inhibit germination of most species, but to a lesser extent for species charac­
teristic of later successional stages (Pratt et al. 1984).
Determinants of Vegetation Development
The course of succession after a disturbance such as slash burning has two
general detenninants: the potential plant species available and the specific
environment. both of which include chance. The species composition of the site
before burning exerts a strong influence on subsequent vegetation development
(Dymess 1973; Halpern, in press; Bigley, in preparatioQ). Besides the initial
aboveground vegetation, the influx of seed is very important, especially on
burned sites. Many wind-dispersed weedy species are typically found on burned
sites. Kellman (1974) found seed input for the fli'St three years after clearcutting
on a site in coastal British Columbia to be 60, 55, and 23 per square foot,
and Henderson
respectively, for years one, two, and three. Vegetation on unburned sites is
usually composed of a greater proportion of remnant and persistent vegetation.
Environmental characteristics of the site, including historic factors such as
burning impacts, comprise the second determinant of succession. The specific
characteristics of the site can affect the rate of succession and prolong or reduce
the influence of burning effects (Bigley, in preparation). At high elevations
where colonization and growth rates are slower, even low severity bums can
reduce plant cover for decades, bycreating poor conditions for reestablishment.
Because of the potential for many plant species tocontributeto a community,
and the various impacts of burning combined with chance occurrences, it is not
surprising that plant succession can take different paths on burned sites com­
pared with unburned ones (Connell and Slatyer 1977; Cattelino et al. 1979;
Halpern, accepted; Bigley, in preparation). Amo et al. (1985) have provided a
good example of how chance history of a site and burning interact to influence
early plant succession (Figure 3). In this example, representations of plant
T
S
R UCT URAL
NO.
Com­
mun­
ity
1
SH RU
·
B
HER B
SAPUNG
STA GE
S
POLE
MATURE
FORE
T
S
OD
L
GROWTH
FORE
S
T
VAGL
A
V GL
�������rr�
types
U
bu
n r
e
nd
u
Br
e
nd
B
ur
n
ed with C EVEseeds
Figure 3. Idealized multiple pathway of succession in a Douglas-fir (Pseudotsuga
menziesii, PSME) and blue huckleberry community (Vaccinium globulare, VAGL)
showing the effects of burning depending on stand history. Species abbreviations:
evergreen ceanothus (Ceanothus velutinus, CEVE), lodgepole pine (Pinus contorta,
PICO), pinegrass (CalQITUlgrostisrubescens, CARU). Source: Amo et al.(1985).
dominance are shown for unburned sites and for sites burned with and without
ceanothus seed in the soil. Removal of the shrub community with severe burning
(communities 2 and 3) resulted in two possible paths· of community develop­
ment, depending on the presence of ceanothus seed.
Effects on Plant Succession
Broadcast Burning
Information on early succession after slash burning generally comes from
short-term observation of often poorly characterized sites. Chronosequence
approaches are good for deriving long-term trends; however, it is often impos­
sible to determine
if differences in the treatments and conditions that occurred
(1982)
on the different sites gave rise to the communities observed. Franklin
emphasized that there is no substitute for observation over time in the study of
plant succession. Few studies have reported observations from matched burned
and unburned areas for an extended period.
There is a scattered and disjointed data base for the response of vegetation
to prescribed slash burning west of the Cascade crest in the Pacific Northwest
(Table 1). Most reports consist of observations, general!y from three to ten years,
and provide only general trends in plant cover.
Vegetation development records from repeated sampling of permanent plots
over twenty-five years come from only two plot networks. The most extensive
are those established by Moms from 1946 to 1951 and consist of roughly 0.25
acre plots in burned and unburned pairs originally at sixty-two locations in
Washington just north of Mount Rainier to south of Roseburg, Oregon. As of
1988, forty-nine pairs remained intact for continued sampling. The second series
of small permanent succession plots, with observations through twenty-five
years, was established by C. T. Dyrness in 1962 on two experimental watersheds
in the H. J. Andrews Experimental Forest, Oregon.
A common feature of studies listed in Table 1 is combination of data from
sites that are not similar. This practice results in large variations and the inability
to compare results with studies having a different combination of sites. To
illustrate how different some plant responses can be on different sites, Figure 4
shows plant cover on two plant associations on the Lowell area of Oregon. Both
areas burned at similar severities, but species response was different on the two
plant associations. On the western hemlock and sword fern site, thimbleberry
increased on the burned plot, probably as a response
to
growing space made
available by the removal of other species such as vine maple, mineral soil
exposed by burning, and perhaps the increase of available nutrients; salal was
completely removed from the burned area for many years after the frre. On the
rhododendron site, thimbleberry grew poorly on the burned plot, as did most
species on this site (where summer moisture stress can be substantial).
Rhododendron regrowth was delayed for several years by burning. Valuable
information could be lost if the results from these two sites were combined.
Since the studies in Table
1 cover a wide range of sites and bum severity,
general conclusions cannot easily be made; however, a pattern in early succes­
sion is commonly reported which shows that both remnant vegetation and
opportunistic invaders inhabit burned and unburned sites. Burning reduces the
predisturbance flora early in the sere and shifts to a greater preponderance of
invading species (e.g., West and Chilcote 1968). Shrub regrowth is usually
slowed on burned areas compared with unbwned ones.
Bigley and Henderson
Table 1. Summary of srudies comparing vegetation on bmned and unburned areas in
western Washington, Oregon. and British Columbia.
Authors
lngTam
( 1931)
General
Location
Number
of Study
Areas
Wind River
Experimental
Forest near
Carson.
Washington
Years of
Postbuming
Observation
10
Reid et al.
( 1938)
Eight observations over
ten years on small
replicated burned and
unburned plots
Shows effects of rebuming and grazing
Added statistical analysis and discussion
from Ingram 1931
Kienholz
(1928)
Southern Puget
Sound basin,
Washington
Isaac
Western
Washington
and Oregon
(1940)
Comments
on Data
Reported
7
3
Small plots stratified by
bum severity, slope,
and aspect
IS
8
Permanent plots examined annually, not
paired
Data confounded by
combining results
from burned and
unburned plots
General trends of speci.fic species reponed
Yerkes
(1960)
H. J. Andrews
Experimental Forest, Oregon Feller
UBC Research
Forest near Maple Ridge, B.C.
(pers.
comm.)
Morris
(1958)
Western
Washington
and Oregon
14
s
2
1 and 7
62
7
Areas ranging from
1,850 to 3,800 feet
in elevation combined
in different groups
from year to year for
average species
occurrences
Detailed data not presented for unburned
plots
Data available for
individual species
Good description of
srudy areas
Several observations of
small plots on paired
treatment areas
Effects on Plant Succession
Table 1. Continued.
Authors
General
Location
Number
of Study
Areas
·
Years of
Postbuming
Observation
Comments
on Data
Reported
62
11-16
Morris 1958 updated
Overall averages for
Cascades and coastal
areas presented
Oakridge,
Oregon
2
13
Photographs from plots
established by Morris
to year 13
Steen
(1966)
Oakridge,
Oregon
13
11-16
More detailed treat­
ment of a subset of
the plots treated by
Morris (1970)
Kraemer
(1977)
Western
Washington
and Oregon
33
25
Detailed update of
some of the areas
covered by Morris
(1970)
Individual species
Morris
(1970)
Western
Washington
and Oregon
Steen
(1965)
discussed
Bigley
Western
Washington
and Oregon
Dymess
(1965)
H. J. Andrews
(in prep.)
49
37-40
2
Comparison of pre- and
postlogging and post­
bmning vegetation
Experimental
Forest, Oregon
Dymess
(1973)
H. J. Andrews
Halpern
(1987)
H. J. Andrews
Halpern
H. J. Andrews
2
5
Experimental
Forest, Oregon
Experimental
Forest, Oregon
(pers. comm.) Experimental
Forest, Oregon
Detailed update of
some of the areas
covered by Morris
(1970) and Kraemer
(1977)
Focus on vegetation
development between
plant communities
Annual observations on
smJill pennanent plots
Irtdividual species data
by year
2
17-21
Update of Dymess
(1973)
Detailed analysis of
bmn severity and
plant communities
Update of previous
Bigley and Henderson
24 -.------.
12.0-r------,
21
Percent
cover
9.0
18
Percent
cover
15
12
9
T/lhnbleberry
6.0
3.0
Sala!
Percent
cover
13
Unburned
26
36
Burned Figure 4. Cover of selected species on burned and Wlbumed plots on two different plant
associations: Left: western hemlock-sword fern association. Right: western hemlock­
rhododendron association.
Conifer Establishment and Early Growth
Advance Conifer Regeneration
Advance regeneration makes a major contribution to stocking on unburned
areas in many regions (Heavilin 1977, Isaac 1943) but is largely removed from
burned areas. Natural conifer regeneration differs substantially by plant associa­
tion (Shearer 1985) and forest zone. The potential importance of advance
regeneration depends on its abundance, species composition, and the prospects
for successful artificial regeneration.
The three major vegetation zones in the Cascades (Figure 5) show substantial
differences in advance regeneration. The number of seedlings (between 6 inches
in height and 1 inch dbh) was found to be 1,320, 24,834, and 43,380 per acre,
respectively, for the western hemlock, silver fir, and mountain hemlock zones
(Figure 6). The number and species composition of saplings in each zone reflect
the potential crop composition. We are unaware of any data on the comparative
survival of advance regeneration in the different forest zones of the· Cascades.
Where artificial regeneration can be difficult (e.g., mountain hemlock zone and
some areas of the silver fir zone), loss of advance regeneration can significantly
reduce stocking for many years (Bigley and Miller, in preparation). On other
sites, burning is an effective means to control species composition by the
removal of advance regeneration.
Effects on Plant Succession
6000
5000
Q)
-
Q)
:!::.
c:
0
·.;:
co
>
Q)
[jj
N
s
4000
PACIFIC SILVER
3000
FIR ZONE
2000
WESTERN HEMLOCK ZONE
1000
0
Figure 5. Relative position of the major forest zones west of the Cascade crest in relation
to aspect and elevation.
100000
WESTERN HEMLOCK SILVER FIR ZONE
ZONE
MTN HEMLOCK ZONE
10000
Q)
;;;
a.
VI
Q)
1000
100
10
2
I
3
iI
4
5
2
3
4
I
5
2
3
4
5
Size Class
E] PSME
. TSHE
m THPL
ABAM
0 TSME
Figure 6. Number and species distribution of tree saplings in the major forest zones.
Abbreviations: Douglas-fir (Pseudotsuga menziesii, PSME), western hemlock (Tsuga
heter ophylla, TSHE), western redcedar (l'huja plicata, THPL), Pacific silver fir (Abies
amabilis, ABAM), mountain hemlock ([suga mertensiana, TSME).
and Henderson
Conifer Establishment and Stocking
The feasibility of conifer regeneration within a reasonable time is a major
factor in deciding whether to burn. Planting costs may not differ between burned
and unburned sites if some yarding of unmerchantable materials is done (Lance
Raff, USDA Forest Service, Darrington, pers. comm.). However, seedling
distribution, planting quality, and early growth rates also must be considered.
Much of the existing infonnation about conifer establishment on burned sites
focuses on natural regeneration. Most dat3. on planted seedling success and
growth rates on burned and unburned sites (such as stocking inventory results)
never fmd their way to the published literature. The data that are published
reflect the same variation and contradictions seen from site to site in understory
succession resulting from differences in burn severity and the site environments.
In the first half of this century, a considerable effort was made to determine
the effects of slash burning on establishment of natural conifer regeneration.
Conclusions consistently emphasized the importance of the timing of the burn
and dispersal of the conifer seed crop (Allen 1946, Silen 1952). Research into
specific conditions comparing germinant survival on burned and unburned sites
has found the blackened surface of burned plots hazardous to germinants
(Kienholz 1928, Hermann and Chilcote 1965). Lavender et al. (1956) and
McCulloch ( 1944) emphasized a balance between moderate burn severity and
slash loading to ensure regeneration success.
In studies of burned areas, tree seedling survival, stocking, and growth
results, data are often stratified by burn severity class or simply presented as an
average response. It is difficult to compare the results of studies that report either
the general or microsite specific responses and provide no way to estimate the
other measure. Stratification of burned areas provides a good idea of the relative
performance of seedlings on those specific sites.
The conclusions of different studies on tree seedling response on burned and
unburned areas are often contradictory. Several authors have found poor sur­
vival on areas exposed to extreme burning (Isaac and Hopkins 1 937, Austin and
B aisinger 1955, Baker 1968). However, Miller and Breuer ( 1 984) reported that
survival was the same on burned and unburned areas with little brush competi­
ings
tion after three years. Burning can also increase the survival of planted s
(Vyse and Muraro 1973). Each study needs to be examined to detennine where
and under what situations the conclusions can be applied.
Combining of data from many widely distributed s tes can provide very
general but useful regional trends. Morris (1970) gave general trends in primari­
ly naturally regenerated conifer stocking on sixty-two paired burned and un­
burned plots. Burned plots in the Oregon Coast Range regenerated much quicker
than plots on the west slope of the Cascades. Although by year 12 there were
no differences between the burned and unburned coastal plots, the Cascade plots
found signs of having greater stocking on the unburned plots. On closer analysis,
Bigley (in preparation) found considerable differences in stocking rates between
different plant associations on Morris's plots.
Reports from unconfounded, paired, burned and unburned plot studies are
58
Effects on Plant Succession
rare. Isaac (1943) found that regeneration ofDouglas-frr occurred faster and in
gr ter numbers on unburned plots compared with paired burned plots where
other conditions were comparable. Munger and Matthews (1941) presented
results of stocking surveys at six burned and unburned locations on coastal
Washington and Oregon. At the end of seven years the unburned areas had over
five times as many seedlings as the burned areas. They also cited other data from
the same area that suppon this trend, but data were taken from unpaired units.
Gockerell (1966) reported results of a 4,130 acre survey (by the Washington
State Department of Natural Resources) of planted clearcuts in northwestern
Table 2. Survival and distribution of 1- to 4-year-old planted seedlings in burned and
unburned areas of 4,130 acres of planted clearcuts, 1958-63 (Gockerell
1966).
Unburned Areas
Burned Areas
Planted survival (%)
57
55
Planted stocking (%)
38
42
Planted and natural stocking(%)
79
69
Animal damage(%)
28
78
Planted Douglas-flr, average height(ft)
4.1
3.4
Washington around 1960. Survival and distribution of 1- to 4- year-old planted
seedlings were about the same on the burned and unburned areas; however, if
natural regeneration was included, stocking was greater on the unburned areas
(Table 2). Although not from paired areas, this srudy is valuable because of the
extensive area surveyed.
Early Growth of Conifers on Slash Burns
Little published information exists on direct comparisons of tree growth on
matched burned and unburned areas (Table 3). Available data typically cover
only the frrst few years of growth and may misrepresent the longer term growth
trends. Beese (MacMillan Bloedel Ltd., pers. comm.) compared growth rates of
planted Douglas-frr on burned and unburned areas and found the first year
growth gave little indication of the second year results. Seedlings planted in
intact forest floor grew best the frrst year, but seedlings on mineral soil humus
grew better the second.
Slash burning has a major influence on early growth by the reduction of
competing vegetation, particularly shrubs. This reduction in competition may
explain many of the differences observed in initial conifer growth on matched
burned and unburned sites. Vihnanek and Ballard (1988) examined stocking,
growth, and foliar nutrient levels of planted Douglas-frr on burned and un­
burned, salal dominated sites on the east side of Vancouver Island. Burning
Bialey and Henderson
Table 3. Summary of studies comparing tree growth on burned and lll1bu."11ed areas in
western Washington, Oregon, British Columbia. and northern California.
Number
General
Authors
Years of
Effect of
of Study Postbuming Burning on
Areas
Location
2
Cascades
Tarrant
and Wright
Observation
1 and 2
Growth
None
Oregon
( 1956)
coast
N aturally regenerated
Douglas-fir height
Root lengths presented
(1955)
Ruth
Comments*
5
None
Douglas-fir height growth
Heavy brush competition on paired burned
and unburned
Madison
(1959)
2
Oregon
coast
8
None
areas
Sitka spruce height
growth
Compares sites with
different aspects
Knight
( 1961)
Vancouver
Island, B.C.
1
2-3
Negative
Douglas-fir diameter
and height growth
Limited salal competition
deMon-
Vancouver
tigny
Island, B.C.
1
4
Negative
Chronosequence study
Douglas-fir height and
(1985)
growth de..'"Teased
with increasing
bum severiry
Heavilin
KlamathN.F.,
(1977)
California
3
7
Negative
Douglas-fir height growth
Advance regeneration
may have contributed
to the taller trees on
the unburned areas
Gockerell
( 1966)
Olympic
Peninsula,
Many
1-4
Negative
3
Positive
Douglas-fir height growth
Washington
Miller and
Oregon
Breuer
Douglas-fir height growth
Survey results
\
( 1984)
Stein
Oregon
(1986)
coast
6
5
Positive
Douglas-fir height growth
Paired plots replicated
at a location
Vihnanek
Vancouver
and B allard Island,
( 1988)
20
5-15
Variable
Douglas-fir height growth
Each location with paired
burned and unburned
B.C.
*If not specifically stated. trees were planted; although investigators mention difficulty
in difierentiating between planted and naturally regenerated Douglas-fir if the areas had
been planted more thm a few years.
60
Effects on Plant Succession
9A
o��r-��� ����
5 6 7 8 910
8 910111 2131415
���--���
8
91011 1 2 131415
Stand Age lyearsl
Figure
7. Generalized relationships of Douglas-flr height growth on sites on Van­
couver Island. Source: Vihnanek and Ballard (1988).
clearly reduced competing salal cover, increased Douglas-flr height and
diameter growth, and improved the status of several foliar nutrients compared
with conifers on the unburned portions in the 5- to 15-year-old stands measured.
Average tree diameters on eighteen of the twenty sites were greater on burned
plots. Height growth was characterized in three general curves (Figure 7). The
fl.l'st curve (A) shows similar growth rates on burned and unburned areas; the
others show greater growth on the burned areas. The extreme case (curve C)
shows growth between burned and unburned conditions as increasingly diver­
gent. Although burning may result in initially superior growth, there are cases
where growth trends on burned and unburned sites have crossed after seven to
nine years (Braathe 1973). This is about the same time that rapid rates of
mineralization promoted by burning would be expected to decline (de Montigny
and Auclair 1982).
In contrast, several studies have reported that initial height growth is greater
on unburned sites than on burned ones. Height growth trends can be deceptive,
however, because animal damage is usually greater on burned sites (e.g.,
Gockerell 1966, Stein 1986) and may be a source of error that discriminates
against "growth" on burned sites in much of the literature. Stein (1986) found
Table 4. Effect of use of vexar rubes on plant survival and growth on burned and un­
burned sites(Stein 1986).
Height(ft)
Survival(%)
Use of Vexar Tubes
Burned
Unburned
. Burned
Unburned
Us ed
88
83
5.6
5.0
Not used
59
46
4.6
3.8
Bigley and Henderson
that protection from animals by using vexar tubes increased height growth of
Douglas-fir on both burned and unburned sites (Table
As with
vegetation response to burning,
tree
4).
growth response varies from
location to location and appears influenced by bum severity. Stem diameter
growth close! y reflects the growth of the root system and thus the establishment
of a seedling, although few studies report diameter. Few reports provide enough
information on the study site environment, plant competition, and occurrence
of animal damage to transfer results to other areas with assurity.
Documenting Variations in Effects on Plant Succession
Differences in site type (e.g., plant association and soil type) and severity of
a bum strongly influence the establishment and growth of both conifers and
nonconifer vegetation on burned areas. Many of the contradictions evident in
the literature result from lack of recognition of the importance of these factors.
We suggest that information on the effects of burning on vegetation could be
better utilized if response data were stratified by forest zones and plant associa­
tions (or groups of associations).
Considerable information exists in the form of observations and experience
of local land managers, but is not available or is not being used by those who
should have access to it The following is an idea how this valuable locally based
information could perhaps be better utilized to support local problem solving
Wildlife
Re eneralion
WET
Burn as needed
l£d
Moisture
Avoid hot burns
Consider not burning
0
Figure 8. An idealized framework to compile and transmit information on burning
effects on different forest ecosystems.
Effects on Plant Succession
and planning.
A general framework to compile and ttansmit infonnation on
burning effects is shown in Figure 8. Infonnation is grouped on a site-specific
and objective-specific basis. To allow site specificity, observations are arran ged
by forest zone and plant assoc iation. The ordination of plant communities in this
example was generally based on a diagram of the western hemlock zone on the
Gifford Pinchot National Forest (Topik et al.
1986).
The areas of similar re­
sponse or recommendations are defined based on the expected need and effects
that broadcast slash burning will have for a particular objective. In the example
shown for vegetation management, maximum tolerance in burning is given in
the moister sites where severe bums may be required to control competing
vegetation. In mesic and drier sites, for the area covered by this diagram,
ceanothus seed may be buried in the soil. Therefore, this suggests severe bums
should be avoided; spring bums would reduce and delay the gennination and
establishment of ceanothus seedlings. In other areas of the ordination, a no-bum
recommendation is appropriate to maintain remnant vegetation that can aid in
conifer establishment. Such vegetation can act as the target of browsing or may
ameliorate the microclimate around planted seedlings.
Because response to burning can have a different value for different. objec­
tives, any treatment recommendations should be kept separate by objective.
Such a system would enable land managers to weigh the options for each objec­
tive and develop a prescription. This procedure would not only provide a place
for managers to record their site· and objective-specific observations and recom·
mendations but also show where sound data exist or where speculation is high.
The relation between treatment and plant community development is
generally vague in the literature as a whole, because sites studied may cover
many types, and results are often combined and thus confounded. Integration
of existing data into patterns by site type and overall bum severity would prove
useful.
Acknowledgments
W,e gratefully acknowledge those who provided unpublished data for use in this paper:
Bill Beese, Louise de Montigny, Michael Feller, Charlie Halpern, and Lance Raff. Useful
review comments on this manuscript were provided by Dean DeBell, Charlie Halpern,
Donald Hanley, Jerry Kamrnenga, Susan Little, Richard Miller, and Chadwick Oliver.
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