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i
Effects of Prescribed Fire on TimberCrowth and Yield" Richard E. Miller and Kenneth W. Seidel
Executive Summary
Forest practices are undertaken when they appear to have benefits that
, exceed immediate or long-term costs. Decisions to use or not use some
practices are often made without critical information because their effects
.
on tree growth are inadequately quantified.
In this chapter we show that the effect of prescribed burning on timber
yields has been measured at relatively few locations, and the results evi­
dently depend on local conditions. Consequently, decisions to burn or how
to burn must recognize that predictions of subsequent timber yield and costs
of production are uncertain; the effect can range from positive to negative.
Like most forest practices, slash burning has potential benefits and costs
with respect to timber yields in both the immediate and the long term.
Prescribed fires-and the wildfires they attempt to reduce-can affect trees
directly and also indirectly through factors that influence tree growth. When
growth of individual trees and stands is aggregated, forest and regional pro­
ductivities are affected. Site-to-site variation and a paucity.of rei iable infor­
mation about tree growth after prescribed burning in both eastside and
westside forests requires that interim decisions be made on judgment"and
experience. Uncertainty will continue untillQng-term data and reliable pre­
.
.
dictions are available.
Monitoring programs should be established to document response of
trees and site factors to prescribed slash burning and underburning. Data
from long-term monitoring plots could be used to evaluate current practices,
and to construct reliable predictive models that quantify effects of fire and
other forest practices on stand and forest productivity.
1990.
In:
Walstad, John D:; Radosevic
h,
Steven R.; Sand erg David
V., eds. Natural
and prescrlbed fl e 1n the Paci
fic Norhthwest
Fo ests
Corva111s, OR:
Oreg
on
State
:
Umvers1ty Press.
336 p.
Reproduced by USDA Forest Service
for official use.
177 178
Natural and Prescribed Fire in Pacific Northwest Forests Introduction
Forest practices are undertaken when' they ap­
pear to have benefits that exceed immediate or long-term costs. Effects of some practices on tree growth are 'poorly quantified, so current forest management decisions are often made without po­
tentially critical information. Predicting the net effect of most practices on forest productivity is difficult given the change in practices and environ­
mental cond
' itions over time; the different mix of practices that may be used, and the interactions among them; the difference in expertise with which practices are carried out; the variability of sites and stands; and the scarcity of appropriate, long-term sample plots where effects on tree growth can be isolated and measured. These varia­
tions and uncertainties provide ample room for divergent results and viewpoints of forest prac­
tices on timber growth and yield. Fire is commonly prescribed after timber har­
vest in the Pacific Northwest to reduce risk of
wildfire in logging slash and to prepare harvested
areas for regenerating a new crop (Chapters 6 and
8). Yet slash fires, like wildfires, present a risk to
nearby trees and other resources. Slash fires usu­
ally have high burning intensity and can affect
growth and species composition of the new forest
through fire's effects on residual seedlings and
shrubs, soil properties, microclimate, and subse­
quent plant establishment and succession.
East of the Cascade Range, low-severity fires
are increasingly prescribed for established stands
to reduce the fire hazard created by accumulations
of organic matter on the soil, or control the num­
ber or species composition of trees and other vege­
tation (Chapter 6). Although these controlled un­
derburns generally consume much less organic
matter than do'slash burns or the wildfires they
replace, they too can affect subsequent yield of
merchantable timber from the current stand and
perhaps from future stands. The lower inherent
productivity of eastside sites suggests a greater
susceptibility of eastside forests to nutrient and or­
ganic matter losses. This justifies concern about
the effects of repeated underburning on long-term
site productivity, especially in the absence of cor­
rective measures like fertilization.
In this chapter, we present evidence that the ef­
fects of prescribed burning on timber yields are
poorly quantified and depend on local site and
stand,conditions. Consequently, decisions to burn
or how to burn must recognize that net effects of prescribed fire on subsequent timber yield, tim­
ber value', and costs of timber production are uncer­
tain andapparently range from positive to negative. Growth & Yield After Slash Fires
in Westside Forests
Effects of slash burning on early survival and
growth of natural and planted seedlings were dis­
cussed in Chapter 6. Subsequent growth of estab­
lished stands is controlled strongly by inherent
productivity of the soil, which can be changed by
previous harvest and site preparation (Chapters
12, 13, and 14). Because tree growth frequently
can be increased by control of competing vegeta­
tion, by thinning, and by fertilization, such silvi­
cultural practices may offset impairments of inher­
ent site productivity that may occur on some sites
from physical impacts of machinery or nutrient
losses.
The westside experience
Valid comparisons of long-term growth and
yield on burned and unburned sites that are other­
wise identical would provide direct proof of slash
burning effects. Unfortunately, current data are
limited in amount, geographic distribution, relia­
bility, and period of measurement. Long-term data
are not available, even for one rotation of 50 or
more y ears. Moreover, omparisons at numerous
locations are necessary because of large variabili­
ty in stand growth and yield caused by soil and
climatic differences and by harvest and manage­
ment activities.
Morris ( 1958) started the most extensive, long­
term study on effects of slash burning in the Pacific
Northwest. Recent measurement of trees on the
burned and the adjacent unburned plot at 44 of his
original locations in the Cascade Range of western
Oregon and Washington provides growth compar­
isons over the longest period available to quantify
and explain the net effects of slash burning in this
area (Fig. 15-1). The stands on these 0.12- to 0.58acre plots were measured 35 to 42 years after
clearcutting.
Burning old-growth slash at these 44 locations
resulted in a mosaic of microsite conditions de­
pending on local fire severity. Small areas of un­
burned slash were observed in almost all burned
plots, as were severely burned patches with ab­
I
'.,
.�'
.:f:_
Effects of Prescribed Fire on Timber Growth and Yield
179 I
!
.
Figure 15-1. Burning logging slash after harvest can remove large quantities of above-ground organic matter.
(A) Unburned plot and (B) burned plot at Location 18 near Blue River, OR.
sence of the duff layer and reddening of the -soil
(Moms 1970). Unburned and severely burned
patches have been observed in other locations in
the Cascade Range (Tarrant 1956) and the Oregon
Coast Range (Dyrness et al. 1957). Young seed­
lings survived harvesting activities at some loca­
tions. This advance regeneration on unburned
plots was a potential component of the subsequent
stand. Few seedlings survived the slash fire on
burned plots. This elimination or reduction of ad­
vance regeneration is a direct effect of fire which
can affect subsequent species composition, stand
density, tree size, and stand volume at some loca­
tions.
Results and interpretations
Effects onsite quality. Height growth is a meas­
ure of site quality; trees grow taller on good quality
sites. A difference in height of same-age crop trees
on burned and unburned areas, therefore; can in­
dicate a change in site quality. To compare site
quality between areas where crop trees are not
equal-aged, however, foresters relate height/age
measurements to the height at a standard age (usu­
ally 25, 50, or 100 years). This height- is called site
index. For example, site indexso 110 indicates that
the largest trees in a stand average 110 feet tall
When their breast-high age is 50 years; site index
110 is about average site quality for coast Douglas-
fir. Younger trees on this quality land will average
54 feet at 20 years breast-high age and 95 feet at 40
years (King 1966).
Current analysis of the data from Moms-'s 44
test plots indicated that the average difference in
site quality between burned and unburned plots
was not statistically significant. Although 10 or
more percent differences in site index between the
two plots were measured at some locations, no
consistent pattern related to burping is evident
(Fig. 1 5-2). Moreover, no consistent difference in
site index existed between burned and unburned
plots at locations that had been planted instead of
naturally regenerated. Future analyses may pro­
vide bases for predicting where slash burning may
have the greatest impact. Clearly, these data must
be supplemented by additional site index and
growth data collected in planted stands because
planting is the conventional method of regenerat­
ing Douglas-fir forest.
Effects on species composition Twenty-eight
locations were regenerated naturally; 16 were
planted, but frequently with limited .s_uccess. Sub­
sequent species composition differed among the
44 locations and differed between the burned-and
unburned
plot at most locations. In general, Doug­
las-fir was more abundant in numbers on burned
plots, whereas western hemlock, western red­
cedar, and true fir species were more abundant on
unburned plots. Advance regeneration of these
. .
j
:
180
, Nafu,;al and Prescribed Fire in PacijicNorthwest Fores'ts
c
TI
140
;
......
­
-
....... 120
X
w
0
z
100
H
w
f(f)
Jr
I
I
I
80
ef
II
,J
I
"
II
Q
6
•
6 ,',
rII
pe
.
I
I
I
I
IIe
•
0
60
0
o
1.0
20
30
LOCATION
Burned
Unburned
50
40
I
60
NUMBER
Figure 15-2. Douglas-fir site index (5/50) on 42 plot-pairs (burned \'s. unburned) in 35-42-year-old stands. Locations are
numbered in ascending order from the most northerly near Enumclaw, WA, to the most southerly near Roseburg, OR.
(Source: unpublished manuscript on file at USDA Forest Service, Forestry Sciences Labo atory, Olympia, WA.)
I
more shade-tolerant conifers survived on un­
tion, in trees 1.6 and 7.6lnches diameter at breast
burned areas, whereas the greater extent of ex­
height (dbh) and larger for all species combined,
posed mineral soil on burned. areas initially fa­
on the burned and unburned plots at 44 locations is
vored Douglas-fir establishment (Morris 1970).
shown in Table 15-1. The average difference in cu­
Species composition by cubic volume (Fig. 15-3)
mulative growth (live, cut, dead volume) between,
corresponds to these differences in tree numbers.
burned and unburned plots was not statistically
Effects on volume growth. Tree size and num­
significant either for the total stand (1.6 inches dbh
ber differed on burned and unburned plots de­
and larger) or for the near-merchantable stand (7.6
pending on the cumulative effects of initial har­ . inches dbh and larger). Douglas-fir volume pro­
vest, slash fire (or lack of it), and subsequent - duction, however, averaged greater on burned
plots and, conversely, that ()f other conifers aver­
silvicultural activities. At each location, the same
aged greater on unburned plots (Fig. 15-3).
activities were applied to both plots; however the
At seven locations, cumulative volume growth'
same activities did not occur at all locations. For
on a burned and planted plot could be compared to
example, both plots at some locations were
that on an adjacent burned and naturally regener­
thinned to concentrate growth on fewer trees, but
thinning was not done at all locations.
ated plot (Fig. 15-4). Volume production on the
For the combined species (including hard­
planted plots averaged 892 ft3 per acre or 34 per-"
woods), mean annual volume production since
cent more than on naturally regenerated plots. No
h.arvest of the previous stand was greater on
long-term data are available from this or other
burned plots at some locations and on unburned at
studies to compare validly the performance of
others '(Fig. 15-4). Average total v?lume producplanted stands on burned vs. unburned areas.
Effects of Pre'scribed Fire on Timber Growth and Yield
"
3000
Burned
.
Unburned
2,569
2',501
2400
C,)
0
'r<l
......
w
::::J
-.J
0
>
Oo'uglas-fir
c=J Other Conifers
_ Hardwood
'"
Burned
1,967
181 i
Unburned
1,940
1800
1200-
600
O L-������L---�
1.6+
7.6+
MINIMUM TREE DIAMETER (inches)
Figure 15-3. Average cumulative gross volume production on burned and unburned plots since harvest, by species and
minimum tree size.
C,)
a
180 f')'
+-
­
150
Iz
w
::::! 120
W
0::
u
z
90
H
-1
<!
:::J
Z
Z
<!
60
30
z
<!
w
::::!
0cI °(1
o
0
0
o
ec
ro
6
10
0
oI
o
I
$
o
.
0
r!
:I /
I
I
°
,
Burned
Un burned
Burned and Planted
30
40
:
1
1
50
>
60
LOCATION NUMBER
Figure 15-4. Average annual gross volume growth in the 35-12 years since harvest or slash burning; trees 1.6 inches and
larger in diameter at breast height of all species on 44 plot-pairs (burned vs. unburned). Locations are numbered in
ascending order from the most northerly near Enumclaw, WA, to the most southerly near Roseburg, OR. At seven
locations, a third plot was established next to the existing burned plot to compare growth o.n the burned, planted plot vs.
that on the burned, naturally regenerated plot. (Source: unpublished manuscript on file at USDA Forest Service, Forestry
Sciences Laboratory, Olympia WA)
182
Natur lcind Prescribed Fire in Pacific Northwest Forests Interpretation s. Current analyses indicate that
the net effect of slash burning on site 'index and
stand productivity varied among these 44 loca­
tions in the western Cascade Range. Slash burning
affected future stand productivity directly, by kill­
ing advance regeneration" and indirectly, by re­
ducing or enhancing competing vegetation, creat­
ing or destroying seedbeds and changing microcli.,
mate. Consequently, the effects of lash burning
on growth and yield can be positive, negative, or
neutral, depending on location and tree species of
interest. Gains in coast Douglas-fir yield can be
expected at most locations because fire creates ad­
ditional mineral soil seedbeds and reduces cover
of shrub species (except Ceanothus spp.). Ad­
vance regeneration of other conifers is also de­
stroyed or damaged by slashfues and this also in­
creases the Douglas-fir component. Conversely,
reduced growth can be expected where forest re­
generation is strongly dependent on advance re­
generation and where fire stimulates cover of Cea­
nothus spp. and this is not controlled in other
ways. We have no direct evidence that slash fires
reduce tree growth by affecting inherent physical,
chemical, or biological properties of soils in the
western Cascade Range. Pending analyses will at-'
tempt to define characteristics of locations that
show the greatest and least effects of burning. Da­
ta from a planted versus naturally regenerated plot
at seven of these locations indicate substantial
benefits of planting on burned areas to avoid de­
lays in natural regeneration and to gain uniform
stocking.
Long-term implications and projections
Like most forest practices, slash burning has
potential benefits and costs with respect to timber
production in both the immediate and the long
term. Prescribed fire-and the wildfires they at­
tempt to reduce-can affect many factors that iIi
tum affect stand growth and, when growth of
many stands is aggregated, forest and regional
productivity. Both slash fires and wildfires kill or
damage standing timber and they, or measures to
control them, may damage soil or other resources.
Projection s by computer models. Because only
short-term tree response information is available
to indicate treatment effects, other means for mak­
ing reasonable predictions are needed. Current
sho,rt-term data on yields of coast Douglas-fir after
slash burning can be extrapolated by existing em-
Table 15-1. Average total volume production on burned
and unb,urned plots at 44 locations35 to 42 }'ears after
,clearcutting.
Cumulative
Gross Volume
Stand Component
Treatment
Trees 1.6 in. dbh
and larger
Burned
Unburned
Difference ± S.E.
Burned
Unburned
Difference ± S.E.
Trees 7.6 in. dbh
and larger
(jt3Iacre)
2,568 2,501 67 :: 197 (3%) 1.967 1,940 27::: 175 (1%) pirical growth and y ield models, such as DFSIM
(Curtis et aI. 1981) or by the biologically based
model, FORCYTE (Kimmins and Scoullar 1984).
The accuracy of these projections is unknown,
however, because no longer-term data are avail­
able to check against them. Nonetheless, such
projections of yields from burned versus unburned
plot data at 35 to 40 years to the end of an assumed
80- to 100-year rotation should at least provide an
indication offuture trends in yields.
We used DFSIM to project current stand statis­
tics of burned and unburned plots. Of the original
44 pairs, 17 pairs contained 80 or more percent of
the current basal area in Douglas-fir and were thus
suitable for DFSIM projections. In this subset of
locations, current volume of combined species av
eraged 20 percent gre ter on burned plots (Fig. 15­
5); this difference was statistically significant for
the total-stand (trees 1.6 inches dbh and larger;
p == 0.10) and for trees 7.6 inches dbh and larger
(p == 0.06). Future volume in these alternative size
classes was estimated for stand age 60, 80, and 100
years (Fig. 15-5). These simulations indicated that
the current difference in volume (and number of
trees) of all species between burned and unburned
plots would gradually diminish in future decades.
By stand age 100, projected volume on burned
plots averaged only 5 percent more than on un­
burned plots. Average tree size (diameter and vol­
ume) was about the same for burned and unburned
plots in both current and projected stands. Thus,
the greater number of trees on burned plots accounted for the greater observed and projected
volume. Depending on merchantability standards,
these estimated gains in volume yield on burned
areas might be recovered by intermed iate ·thinnings and final harvests.
:
, :. ;
r; i
'.1·:
�:t
'.f..:_t.:
:,:,"
'.:t
J
-4
, ..
'"
'.:_:1=
..
' ;0'.
'
J
.
,, :: .. ;:.
.
l
.
-
Effects of Prescribed Fire Oil Timber Growth and Yield
No FORCYTE simulation was attempted b,e­
cause the current model (Kimmins and Scoullar'
1984) is not calibrated for use in western Washing­
ton and Oregon, and because some of the site-de­
scriptive data necessary to make projections spe­
cific to each pair of burned-unburned plots were
not available. Because FORCYTE yield projec­
tions are strongly conditioned by estimated
amounts of nitrogen supply and demand, we antic­
ipate that major losses of nitrogen from slash burn­
ing would result in FORCYTE projections Of re­
duced future yields on burned plots, at least on
poor quruity sites with relatively
small amounts of
.
nitrogen.
Growth & Yield After Prescribed
Underburning in Eastside Forests
l'vfajor objectives of underburning
Prescribed underburning has been used to re­
duce fire hazard in mature stands; to reduce com­
petition from understory shrubs in open.stands; to
improve quality of forage, particularly bit­
terbrush; and to reduce numbers of trees in young,
overstocked coniferous stands. If successfully ac­
complished, weeding- and thinning-by-fire sup­
presses understory shrubs and kills surplus trees;
such fires could increase site resources available
for crop trees and increase merchantable yields.
Because of greater risk of crown fire, however,
application of underburning in overstocked stands
is less controllable and results are less predictable
18 15
alii
c:::=J
1.6+ inches 7.6+ inches w ;;: 9
:!
·­
c
>
3
40
60
STAND AGE
80
100
(years)
Figure 15-5. Average current volume of 17 Douglas-fir
stands at average age 40 years and their simulated vol­
ume through stand age 100 years, by treatment and size
class.
183
thanunderburning in open or previously thinned
stands.
,
Reduction of forest residues and competing
vegetation are the two major objectives of most
underburning in the United States. Underburning
has been a standard forestry practice in the South­
east and Southwest for many decades. East of the
Cascade Crest in the Pacific Northwest, under­
burning has progressed in the last decade from ex­
perimental to operational. In westside forests, un­
derburning has bee·n applied experimentally in
previously thinned stands of Douglas-fir to reduce
fire hazard from slash (Sandberg 1980). No evalua­
tion of the effects of such underburning on Doug­
las-fir growth has been published; therefore, fur­
ther discussi on i s limited to the e a s tside
.experience.
At issue aretfie potential trade-offs between im­
mediate and long-term benefits and costs of under­
burning. Potential benefits include reduced fuel
and thus hazard of wildfire, increased forage pro­
duction and thus improved animal habitat, mainte­
nance of fire-adapted species, and reduced density
in overstocked stands and thus increased growth
of crop trees. Besides the treatnient cost, potential
costs include damage from escaped fire and from
smoke, consequences of nutrient losses, and net
effects on short- and long-term tree growth and
timber yields.
-
Data available in the South and Southwest
I
Only limited and relatively short:!term data are
available for quantifying the effects of prescribed
underburning on growth and yield of forests; infor­
mation is available from the South (Clason 1978,
Waldrop and Van Lear 1984, Cain 1985, Wade and
Johansen 1986, Boyer 1987, Waldrop et al. 1987)
and the Southwest (Lindenmuth 1962). Under­
burning can be extremely variable in its severity.
Consequently, the direct effects of underburning
on trees and indirect effects on site factors that
influence tree growth produce conftictiJ:?g informa­
tion and results.
Before summarizing current inforination for
eastside forests, general results oCprescribed
burning and their interpretation are presented.
Direct effects of underburning on trees
As discussed in Chapter 4, tree size, stand con­
dition, and fire severity (largely determined by fu­
el, weather, soil moisture) generally determine the
184
Natural and Prescribed Fire in Pacific-Northwest Forests extent to which the bole cambium is damaged;
needles and buds are scorched or killed; roots are
scorched or killed; and trees are killed directly by
fire or later by insects or disease. Young, short,
and shallow-rooted trees are most vulnerable to
direct damage by fire. At least short-term reduc­
tions in growth, if not outright tree mortality, are
likely from damage. If damage and mortality are
restricted to noncrop trees, then improved growth
and yield of underburned stands can be expected'
-at least in the short term.
Experiences in the Southeast (Ferguson 1955,
Hodgkins and W hipple 1963, Waldrop and Van
Lear 1984, Cain 1985, Wade and Johansen 1986)
and Southwest (Lindenmuth 1962) and in Idaho
from wildfires (Lynch 1959) indicate that tree dam­
age and potential reductions in tree growth and
yield can be minimized by reducing burn severity
and avoiding crown fires. Yet this presents a di­
lemma because the greater the fire severity, the
greater percentage of fuel is consumed and the
larger the percentage of released trees. Cooler
burns are more likely with-winter (cool, wet condi­
tions) than with summer burns, on level than on
steep topography, with backing fires than with
head fires, in open stands than in dense stands, and
with light rather than heavy accumulations of for­
est resid'ues and duff (Lindenmuth 1962). Wade
and Johansen (1986) suggest numerous ways to
avoid fire damage to trees during prescribed
burns. See Chapter 5 in this book for a description
of underbuming techniques.
Indirect effects of underburning on trees
Underburning also can affect tree gro\\1h by af­
fecting factors that influence tree growth; these in­
direct effects can increase or decrease tree growth
and timber yields. For example, fire temporarily
increases amounts of available nutrients for trees,_
vegetation, and microorganisms (KJemmedson
1976, Ryan and Covington 1986). However, ash
losses in convective air movement and volatiliza­
tion of nitrogen, sulfur, and phosphorus during
burning reduce the total amount of nutrient ele­
ments in the ecosystem unless they are subse­
quently replaced by nitrogen-fixing organisms, at­
mospheric deposition, mineral weathering (of
sulfur and phosphorus), or fertilization. The fre­
quent response of conifer stands to nitrogen fertil­
ization in the Pacific Northwest suggests that this
nutrient commonly limits growth (Miller et al.
1986, Powers 6t al. 1988); ther for'e, i is especially prudent to conserve-nitrogen or attempt to replace losses by fertilization. Published (Cochran 1978) and unpublished data from fertilization trials in eastside lodgepole pine and ponderosa pine stands' suggest that sulfur losses should be also avoided, especially on soils deri ed from pumice. The eastside experience
Underburning to thin overstocked coniferous stands. Extensive acreage of naturally regenerat­
ed, overstocked stands exists in the western Unit­
ed States. These stands usually occur on poor quality land where returns from silvicultural in­
vestments are marginal. Although costs of pre­
ri_bed underburning can be much less than thin­
ning with power saws, the comparative effects of these two thinning tools on tree growth have not been investigated. Risk of stand damage is clearly greater with underburning than with mechanical thinning. Thinning with fire is feasible only at early stand ages and with low-severity fires (personal communication from the late George Fahnestock, forest fire consultant, Seattle, W A). Several inves­
tigators have described general effects of under­
burning pine thickets in the western United States, but growth data are only available from two stands in northeast Washington. Underburnin g open or previously thinned
stands. Landsberg et aI. (1984) and J.D. Lands­
berg and P.H. Cochran (unpublished manuscript
[Ponderosa pine tree growth after prescribed un­
derburningJ on file at USDA Forest Service, Pacif­
ic Northwest Research Station, Bend, OR) pro­
vide growth data for underburning at a location in
central Oregon (Table 15-2). Lindenmuth (1962)
provides descriptive information from a systemat­
ic survey of prescrilJed fire on about 27, 000 acres
intensively burned in Arizona. The information
describes burning severity, fuel consumption, and
percentage of potential rop trees that were re­
leased, damaged, or killed in a wide variety of ini­
tial stand conditions.
'
Results and interpretation
.
-/{f
'
:'''
Underburning to thin overstocked coniferous, ·fi
stands. Effects of underburning to thin two over­
stocked, ponderosa pine stands are summarized in Table 15-2. EvaluatIOns made 6-15 years after the ::'$
fire showed that the total number of trees per acre
was reduced by 67 percent at Coyote Creek and 86 ".��
:
, ].:
:....�:
<�.r
....: -
'
'
. Effects of Prescribed Fire on Tiinber GrOlvth and Yield
percent at Pe-EU, while the diameter and height
growth of predesigriated crop trees was increased
at Coyote Creek, but not at Pe-Ell (Wooldridge
and Weaver 1965). The analyses at Coyote Creek
indicated that fire-induced reductions in compet­
ing trees explained most but not all of the increases
in crop tree growth; increased availability of water
and nutrients also may have contributed to in­
creased growth at this location. Although crown
scorch and fire scars adversely affected height
growth, negative effects on diameter growth were
offset apparently by a concurrent reduction in
competition (Morris and Mowat 1958). Unfortu­
nately, effects on per acre growth and yield were
not reported for either study area.
Underburning open or previously thinned
stands. Landsberg et al. (1984) and Landsberg and
Cochran (1980 and unpublished manuscript on file
at USDA Forest Service, Pacific Northwest Re­
search Station, Bend, OR) describe stand growth
after moderate and after severe fuel consumption
185
in a 45-year-old ponderosa pine stand near -Bend,
Oregon (Table 15-2). The stand had been thinned
about 20 years before the 'burn. Initial effects of
the more severe underburning were as follows:.
88 percent of the duff layer was consumed; ,
"most of the fine feeding roots located near the
surface of the soil were destroyed";
needle weight and nitrogen content were re­
duced by crown scorch; and ..
4 percent of the initial trees were killed.
•
•
•
.
•
Effects of the moderate burn were much less
severe. In t,h 4 years after the severe burn, needle
mass and nitrogen content declined to even lower
levels, and in the 8 years after burning height, di­
ameter, and volume growth were significantly re­
duced. Landsberg et al. (1984) concluded: "Pre­
scribed burning needs further evaluation in larger
studies conducted over a longer time in a variety of
ponderosa pine communities to determine long­
term effects on tree growth" (p. 12).
Table 75-2. Results of underburning experiments in ponderosa pine, per acre basis.
Bend, Oreganb
Northeast Washington'
Coyote Creek"
Pe-Elld
Moderate burn
Severe burn
2,550
95
20
2,032
140
mixed
24D
240
45
240
240
45
Initial stand
Total stems
Crop stems
Age(years)
Fire characteristics
: '
Fuel reduction
Wood(%)
Duff(%) 86
35
49
69
88
4
20
46 Scorch(%)
Needle loss(%)
Pastfire data
After fire (yrs)
1-7
7-15
1-6
1-8
1-8
Surviving stems
Total
Crop
830
83
820
80
294
237
231
+37
+ 7"
+27
+23
o
-12
-10
-14
-16
-20
-22
Crop tree growt h
Burned vs. unburned % change
Diameter
Height
Volume
-14
Sources and notes;
.
'Objective: to reduce stocking. bObjective: to reduce fire hazard. Source: Landsberg aDd Cochran, unpublished manuscript. cMorns and Mowat 1958 (years 1-7); Weaver 1967 (years 7-15). dWooldridge and Weaver 1965. <Height growth was reduced in 20% of surviving crop trees that were fire scarred. '.. ;:
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186
Natural and Prescribed Fire in Pacific Northwest Forests Long-term implications for overstocked.
.
and open stands
The limiteo data available on the effects of pre­
scribed underburning in eastside forests precludes
meaningful estimation or even speculation about
effects on growth and yield.
A general consensus exists among foresters and fire ecologists that prescribed fire is generally nec­
essary to reduce risk of wildfires that are frequent in areas of low rainfall. In discussing the extensive mixed-conifer-pinegrass commu
· nity in eastern
Oregon, Hall (1976) concluded that successful fire .
prevention and control have created an increased
hazard in a known fire environment. This has
chahged a naturally fire-resistant community to a
fire-susceptible one. "We may not have a choice
about burning-only a choice of how to burn; pre­
scribed fire or wildfire" (p. 168-9). A similar con­
clusion was made for ponderosa pine forests of
Central Arizona (Biswell et al. 1973). The periodic
outbreak of extensive wildfires in western states
fu her supports the apparent neGessity of pre:­
scnbed fire to protect forests in specific environ­
ments. See also chapters 3 and 4 of this book.
Lindenmuth concluded:
p
Intentional burning of onderosa pine timber­
lands to treat fuels and timber stands presents
many unsolved complex problems. More control
over the intensity of fire will be necessary for con­
sistently accomplishing specific objectives. More
knowledge of desired fire intensities and practical
techniques for controlling fire intensity is urgen tly
needed (1962, p. 810).
Since that time, techniques for controlling intensi­
ty and area of prescribed burning have improved
(Chapter 5). Yet the paucity of data to quantify
effects of prescribed underburning on forest
growth justifies concern and action to secure more
information about short- and long-term effects.
Conclusion
Biologically sound forest management requires
that factors controlling tree growth be maintained
in suitable quantities and balance for sustained,
high levels of forest productivity. In the Pacific
Northwest-as in most ·forest regions-reliable
and long-term quantitative data about the relative
contribution of inherent site factors and or-man­
agement practic es to long-term stand productivity
are not available to either support or reject most
current practices. This pauc ty of reliable informa­
tion requires that i!!terim decisions be made on
judgment and experience. Uncertainty will contin­
ue to prevail until long-term data or reliable pre­
dictions are available.
Forest productivity is determined by many fac­
tors including soil, climate, species, management
practices, disease, insects, time, and the interac­
tions among these factors. Maintenance of or even
increase,s in timber production may be possible by
substituting intensive s.ilvicultural practices for
some losses of inherent site productivity caused
by prescribed burning or other practices. In the
final analysis, however, the comparative biologi­
cal and economic benefits and costs of soil conser­
vation versus replenishment or substitutions must
be- evaluated. Reliable economic analyses require
direct, quantitative evidence, including measured
or reliably predicted tree growth or yield.
Long-term effects of prescribed fire on factors
of site productivity are not easily predicted be­
cause sufficient investigations have not been
ma ? e. The research is complex because pre­
scnbed fire varies in severity, and sites differ in
their tolerance to initial effects and in their capaci­
ty to recover over time. Apparent negative effects
on soil properties do not always result in reduced
tree growth. Forest growth is a complex process
necessitating well-designed
_ experiments to isolate
the effects of treatment. Currently, we can state
with confidence only that predicting conse­
quences is uncertain; that tree response informa­
tion is needed to improve predictions; and that,
therefore, forestry techniques or practices should
be prescribed prudently and specifically to fit the
.
local situation.
Monitoring programs should be established to
document the response of trees and site factors to
prescribed slash burning and underburning. Long­
term plots must be established, and reliable pre­
dictive models must be constructed to evaluate
forest practices.
-
-'
Effects of Prescribed Fire on Timber Growth and Yield
" Literature Cited & Key References
Biswell, H.H., H.R. Kallander, R. Komarek, R.J.
Vogl, and H. Weaver. 1973. Ponderosa fire m ag ­
ment a task force evaluation of controlled burnmg m
pond rosa pine forests of central Arizona. Tall Tim­
bers Res. Sta. Tallahassee, FL. Misc. Pub. 2. 49 p.
Boyer, W.D. 1987. Volume gr0v.:th l ss: A hidden os
of periodic prescribed burnmg m longleaf pine .
South. J. Appl. For. 11:154-157.
Cain, M.D. 1985. Prescribed winter burn can reduce
the growth of nine-year-old loblolly pmes. USDA
For. Serv., South. For. Exp. Sta., New Orleans,
LA. Res. Note SO-312. 4 p.
Clason, T.R. 1978. Removal of hardwood vegetat on
increases growth and yield of a young loblolly pine
stand. South. J. Appl. For. 2:96-97.
Cochran, P.H. 1978. Response of a pole-size ponderosa
pine stand to nitrogen, phosphorus, and sulfur.
USDA For. Serv., Pac. Northwest For. Rge. Exp.
"
Sta., Portland, OR. Res. Note PNW -319. 8 p.
Curtis, R.O., G.W. Clendenen, and D.J. Demars. 1981.
" A new stand simulator for coast Douglas-fir: DFSL\f
user's guide. USDA, For. Serv., Pac. Northwest
For. Rge. Exp. Sta., Portland, OR. Gen. Tech. Rep.
PNW-128. 79 p.
Dyrness, C.T., C.T. Youngberg, and R.H Ruth. 195 .
:
Some effects of logging and slash burnmg on phYSI­
cal soil properties in the Corvallis watershed. USDA
For. Serv., Pac. Northwest For. Rge. Exp. Sta.,
Portland, OR. Res. Pap. 19. 15 p.
Ferguson, E.R. 1955. Fire-scorched trees-will they
live or die? p. 102-113. In Proc., 4th Annu. Forest
Symp., Louisiana State Univ., Baton Rouge, LA.
Hall, F.C. 1976. Fire and vegetation ill the Blue Moun­
tains- implications for land managers. Tall Timbers
Fire Eco!. Conf. Proc. 15:155-170.
Hodgkins, E.J., and S.D. Whipple. 1963. Changes in
stand structure following prescribed burning in a
loblolly-shortleaf pine forest. J. For. 61:498-502.
Kimmins, J.P., and K.A. Scoullar. 1984. FORCYTE­
11: A flexible modelling framework with which to
analyze the long-term consequences for yield, eco­
nomic returns and energy efficiency of alternative
forest and agro-forest crop production strategies, p.
1-5. In Proc., 5th Canadian Bioenergy R& D Semi­
nar. Nat. Res. Counc., Ottawa, Canada.
King, J.E. 1966. Site index curves for Douglas-fir in the
Pacific Northwest. Forestry Research Center, Wey­
erhaeuser Co., Centralia, W A. Weyerhaeuser For.
Pap. 8. 49 p.
;
References marked by an asterisk are recommended for generallnfor­
mation.
,·,!t:·
'. '!./-
187 Klemmedson, J.O. 1976. Effect f thinning and" slash
•.
burning on nitrogen and carbon in ecosystems of
" young dense ponderosa pine. For. Sci. 22:45-53.
Landsberg, J.D., and P.H. Cochran. 1980. Prescribed
burning effects on foliar nitrogen content in pondero­
sa pine. Fire For. Meteorol. Conf. Proe. 6:209-213.
*Landsberg, J.D., P.H. Cochran, M.M. Finck, and R.E.
Martin. 1984. Foliar nitrogen content and tree
growth after prescribed fire in ponderosa pine.
USDA For. Serv., Pac. Northwest Res. Sta., Port­
land, OR. Res. Note PNW-412. 15 p.
*Lindenmuth, A.W., Jr. 1962. Effects on fuels and trees
of a large intentional burn in ponderosa pine. J. For.
60:804-810.
Lynch, D.W. 1959. Effects of a wildfire on mortality
and growth of young ponderosa pine trees. USDA
For. Serv., Intermt. For. Rge. Exp. Sta., Ogden,
UT. Res. Note 66. 8 p.
Miller, R.E., P.R. Barker, C.E. Peterson, and S.R.
Webster. 1986. Using nitrogen fertilizers in manage­
ment of coast Douglas-fir: Regional trends of re­
sponse, p. 290-303. In Douglas-fir: Stand Manage­
.
.
ment for the Future. Umv. Washmgton, Seattle,
WA. 388 p.
Morris, W.G. 1958. Influence of slash burning on :egen­
eration, other plant cover, and fire hazard m the
Douglas-fir Region. USDA For. Serv., Pac. North­
west-For. Rge Exp. Sta.", Portland, OR. Res. Pap.
PNW-29. 49 p.
*Morris, W.G. 1970. Effects of slash burning in overma­
ture stands of the Douglas-fir region. For. Sci.
16:258-270.
Morris, W.G., and E.L. Mowat. 1958. Some effects of
thinning a ponderosa pine thicket with a prescribed
fire. J. For. 56:203-209.
Powers, R.F., S.R. Webster, and P.H. Cochran. 1988.
.
Estimating the response of ponderosa pine forests to
fertilization, p. 219-225. In Proe., Symp. on Future
Forests of the Mountain West, Sept. 29-0ct. 3, 1986.
Uill v. of Montana, Missoula, MT. USDA For. Serv."
Intermt. Res. Sta., Ogden, UT.
Ryan, M.G., and W.W. Covingto . 1986 Effect of a
:
.
.
prescnoed burn in pondero a pme o? morganlc nI­
trogen concentrations of mineral soil. USDA For.
Se;"., Rocky Mt. For. Rge. Exp. Sta., Fort Collins,
CO. Res. Note RM-464. 5 p.
Sandberg, D. 1980. Duff reduction by prescribed un­
derburnirlg in Douglas-fir. USDA For. Serv., Pac.
Northwest For. Rge. Exp. Sta., Portland, OR. Res.
Paper PNW-272. 19 p.
Tarrant, R.F. 1956. Effects of slash burning on some
soils of the Douglas-fir region. Soil Sci. Soc. Amer.
Proe. 20:408-411.
"
:
_
188
Natural and Prescribe'd Fire.in Pacific Northwest Forests Wade, D.D., and R.W. Johansen. 1986. Effects of fire
on southern pine: Observations and recommenda­
tions. USDA For. Serv., Southeastern For. Exp.
Sta., Asheville, NC. Gen. Tech. Rep. SE-41. 14 p.
Waldrop, T..A., and D.H. Van Lear. 1984. Effect of
crown scorch on survival and growth of young
loblolly pine. South. J. Appl. For. &:35-40.
*Waldrop, T.A., D.H. Van Lear, F.T. Lloyd, and W.R.
Harms. 1987. Long-tenn studies of prescribed burn­
ing in loblolly pine forests of the southeastern coastal
plain., USDA For. Serv., Southeastern Res. Sta.,
Asheville, NC. Gen. Tech. Rep. SE-45. 23 p.
*Weaver, H. 1967. Some effects of prescribed burning
on the Coyote Creek test area, Colville Indian Reser­
vation. J. For. 65:552-558.
Wooldridge, D.D., and H. Weaver. 1965. Some effects
of thinning a ponderosa pine thicket with a pre­
scribed fire, II. J. For. 63:92-95.
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