SOME RESULTS AND DESIGN CONSIDERATIONS FROM
A LONG-TERM STUDY OF SLASH BURNING EFFECTS
R.E. MILLER
USDA Forest Service
Pacific Northwest Research Station
Olympia, WA 98502, USA
J.W. HAZARD
Statistical Consulting Services
Bend, OR 97701, USA
R.E. BIGLEY
USDA Forest Service
Pacific Northwest Research Station
Olympia, WA 98502, USA
T.A. MAX
USDA Forest Service
Pacific Northwest Research Station
Portland, OR 97208, USA
ABSTRACT
Original. data and recent measurements from a field study of slash burning
installed by w.G. Morris in 1946-1952 were used to assess long-term effects
of burning on forest ecosystems in the Cascade Range of western Washington
and Oregon. This paper (1) describes the controlled and non-controlled
features of this 40-year-old study; (2) describes some objectives of, and
inferences from, the statistical analyses of the original and recently collected
data; (3) discusses strengths and weaknesses of the Morris plots as an
example of a retrospective study superimposed on an older study with
randomized block design. Retrospective studies can provide useful, timely, but
Cite
as:
MILLER, R.E., HAZARD, J. W., BIGLEY, R.E., and MAX, T.A. 1989: Some results and design
considerations from a long-term study of slash burning effects. Pp. 63-78 In: W.J. Dyck and C.A.
Mees (Ed.).
Research Strategies for Long-term Site Productivity.
Proceedings, IENBE A3
Workshop, Seattle, WA, August, 1988. IENBE A3 Report No. 8. Forest Research Institute, New
Zealand, B ulletin 152.
63
'\
RETROSPECTIVE RESEARCH temporary answers to current questions from land managers, modellers,
classifiers, and designers of new field experiments. The location, type, and
reliability of initial observations and measurements, however, frequently limit the
interpretation of current results and the scope of inference.
INTRODUCTION
One purpose of forest research is to provide quantitative predictions about response
of ecosystems to forest practices. These predictions may be used by land managers
as best-available estimates. If, however, these predictions are to contribute to scientific
knowledge, they should be statistically reliable. Predictions of long-term response are
especially needed, because of concerns about sustaining forest productivity in future
rotations. Ideally, both long- and short-term predictions should be s pecific for well­
defined or clearly delineated management strata, forest zones, or sites (target
populations). The manager would thus know that action X at condition Y should have
result Z (p < O.xx). In the interim, and more realistically, most current predictions are
based on short- term information and are best guesses to support or modify current
forest management prescriptions. To improve these predictive models, additional and
more reliable data are needed. Because the search for ideal, complex predictive
models may be an unattainable quest, modelling to guide forest management should
focus on providing useful, reasonably accurate predictions at reasonable cost.
Quantitative data for reliable predictions about tree response are best obtained from
appropriately designed investigations in which one or more treatments are applied to
a representative sample of the management strata of interest and the effects of these
treatments on tree growth are measured accurately. In its simplest application as
empirical trials, this a pproach can provide a reliable statement that a response
occurred, but little about why it happened. Predictions can also be generated from
empirical trial data by using soil, site, and other variables that are casually or causally
related to the measured response. Some researchers believe that additional,
concurrent measurement of treatment effects on individual biological processes that
affect tree growth is the only way to produce reliable predictions. This approach
considers that knowing biological ·cause· or "Why· of response is the ultimate basis
for predictions about tree response. Clearly, this explanatory ·why" can be obtained
at different levels of biology -- for example, a gross process, like change in leaf
biomass, through to the biochemistry of photosynthesis. Reliable experimental design
and sampling, however, remain prerequisites to successful application of this method
to produce reliable predictions.
Tree and process data collected from well-designed field trials can be subjected to
statistical analYSis, a discipline designed to develop objective criteria to aid decision­
making in the face of uncertainty (Warren 1986). Statistical analysis cannot eliminate
uncertainty, but can quantify it. Unfortunately, application of experimental design is a
recent development in forestry, so well-designed investigations are few in number and
seldom older than a few decades.
As an alternative to designed, controlled, or manipulative (Hurlbert 1984) experiments,
longer-term data or information can sometimes be obtained by measuring, observing,
or describing stands that have documented histories of past management activities.
Such non-controlled studies are considered observational (Cochran 1983), retrospective
(Schlesselman 1982; Sartwell 1974), case-control (Sartwell 1974; Hill 1977; Feinstein
1988), and mensurative (Hurlbert 1984). Non-controlled studies can provide estimates
of tree response to past practices and climate. They provide little opportunity, however,
64
LONG-TERM STUDY OF SLASH BURNING EFFECTS to determine change in biological processes that caused the observed response
because process information rarely exists for initial measurements. A key requirement
for valid conclusions from non-controlled studies is that the initial conditions were
comparable for all treatments. Such retrospective studies can also use data from
earlier field trials which may have been designed for a different objective. The design
in older trials may be more elementary because less was known about the problem or
population under study. The older the study, the less chance it will have a formal
experimental design. The investigator should seek to extract as much information from
these previously treated areas as is pertinent to current objectives, but not misuse the
data.
An opportunity to use an earlier field trial was provided by W.G. Morris of the Pacific
Northwest Forest and Range Experiment Station (United States Forest Service). In
1947, Morris initiated an extensive study of the effects of old-growth slash on fire
hazard and conifer regeneration. Data from the Morris plots provide longer-term
answers to the original and to new questions about the effects of slash burning on
forest ecosystems. In this paper, we will describe the controlled and non-controlled
features of this 40-year-old study; describe some objectives of, and inferences from, the
statistical analyses of the original and recently collected data; and, discuss some
strengths and weaknesses of the Morris plots as an example of a retrospective study
with some controlled-study components.
METHODS
X
single pair of
plots
4X number 01 pairs 01 plots
Figure 1:
Location of 44 pairs of burned and
unburned plots in western Oregon
and Washington.
Morris (1947; study plan on file at Forestry
Sciences Lab., Olympia, WA 98502) started a
study of the effects of burning old-growth
slash on fire hazard and conifer regeneration.
He installed a burned and an unburned plot
at about 75 locations in western Washington
and Oregon (Figure 1). Morris visited more
than 120 clearcuts before slash burning and
tentatively selected them as study areas if:
1) 0.5 ha or more of .uniform conditions were
available to accommodate two plots; and 2)
he believed that fire could be kept out of one
tentative plot area when slash in the
remainder of the clearcut was burned.
Wherever the operational slash burn was
successful, Morris installed two plots, burned
and unburned. Slash burning was conducted
in fall after rains had moistened forest floors,
although presumably with a variety of fuel
Line-intercept
and moisture conditions.
transects and photographs documented post­
burn conditions on the burned and the
unburned plot at each location. Repeated
photographs and measurements over these
transects, along with vegetation and seedling
surveys, documented trends in fuel, vege­
tation, and stand development.
Original
results of this investigation were first
. published 10 years after study establishment
(Morris 1958, 1970).
65 RETROSPECTIVE RESEARCH In 1986 and 1987, plots at 44 of Morris's locations were relocated and resampled
(Figure 1). Plot- pairs were similar in size (0 05 to 0.24 ha), slope, aspect, soil, and
logging disturbance. Both plots had been established at the same time, 0 to one
growing season after burning and within two growing seasons after logging. Each
plot- pair had the same management history; pairs with unequal management history
were eliminated from these analyses. Fifteen of the pairs had been planted
operationally at least once in Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and
one pair had been planted in ponderosa pine (Pinus ponderosa Dougl. ex Laws.);
planting success was generally poor. The remaining 28 pairs regenerated naturally and
usually in Douglas-fir. About half of the 44 pairs had been precommercially thinned
and a few had been fertilized with nitrogen.
.
STATISTICAL ANALYSES
The Morris plots are a combination of retrospective and controlled research (Table 1).
Treatment occurred in the past and the results are currently available; yet, past
treatment to each plot was controlled. The specific questions addressed and the
inferences from the statistical analyses differed for these two types of research.
Type
------------------------ Time and Action -----------------------Current
Future
Past
(Retrospective)
(Prospective)
Controlled
(calibrate)
treat
results
Retrospective
treat
results
application
Mixed
treat
results
re-treat
application
results
As a Controlled Study
The original experimental design (Morris 1958) was a randomized block, with only one
replicate at each location. This lack of treatment replication precludes any test of
statistical significance between the burned and unburned plot data at any single
location. A paired !-test of the data for all locations is appropriate for the original
Morris experiment. Response variables were expressed as a difference at each
location; for example, growth on the burned plot minus growth on the unburned plot.
The null hypothesis tested was that the mean difference among the locations was zero.
The significance of all tests of these null hypotheses reported here was judged at p
.s 0.10. SPSS programs (Norusis 1986) were used for statistical analyses.
Questions that were posed included the following:
1. Does establishment period (years from harvest or burning required to reach breast
height) differ on burned and unburned plots?
66 LONG-TERM STUDY OF SLASH BURNING EFFECTS 2. Does average site index (height of 100 largest Douglas-fir trees/ha) differ on
burned and unburned plots at stand age 35-40 years?
3. Does cumulative, gross (live + cut + mortality) volume production since harvest
or burning for two size categories (trees 4.1 cm and larger or 19.4 cm and larger)
differ on burned and unburned plots through stand age 35-40 years?
The scope of inference for these questions is the Cascade Range in Oregon and
Washington (Figure 1).
As a Retrospective Study
Retrospective studies have value when the population treated and sampled is related
to a current research o bjective or forest management issue. For example, in the Pacific
Northwest, the benefits and costs of slash burning are being re-evaluated. This is
prompted by increasingly stringent regulations to reduce atmospheric pollution and
by concerns about the high cost of slash burning and about possible effects on long­
term site productivity. Forest managers also want more site-specific recommendations
for slash burning so they can apply the technique beneficially.
These current questions differ from those originally posed by Morris: What are the
general effects of burning old-growth slash on fire hazard and natural restocking after
logging, and on brush and herbaceous cover? The original study objectives were
stated as identifying simple differences between two treatments at a regional level or
scope; yet these o bjectives clearly had underlying management implications. Current
concerns about slash burning also relate to this accumulated data base. For example,
these data can help answer, retrospectively, the following questions:
1. Do the effects of burning differ within the two forest zones represented by these
data?
2. Do the effects of burning differ among the seven plant associations (Table 2)
represented by these data?
$. Can models be constructed to predict reliably differences in mean annual volume
growth between burned or unburned plots through stand age 35 after clearcutting?
4. To what extent are differences in establishment period, site index (SI), and volume
production between burned and unburned plots related to initial site conditions
(SI, amount of solar radiation, and amount of advance regeneration), and to post­
burning conditions such as severity of burning, exposed mineral soil, and
percentage vegetative cover?
To answer questions 3 (prediction) and 4 (description), dependent (response) variables
were related to "independent" variables that were either continuous or dummy
(classification) variables. Regression analyses and paired t-tests (questions 1 and 2)
were made on the same data set partitioned according to the s pecific question (1
through 4). Thus, the individual analyses are not independent. Consequently,
statistical tests are not inde pendent and the computed levels of significance may be
biased. The extent of this bias is unknown and, thus, inferences are conditional.
Such information is primarily useful for generating hypotheses; its usefulness for making
inferences about cause and effect relationships is severely limited.
67 RETROSPECTIVE RESEARCH
Name
Code*
No.t
Scientific Name
Locations
TSHE:
POMU­
BENE-GASH
RHMA-OXOR
RHMA-BENE
RHMA-GASH
1
2
3
4
5
Tsuga heterophylla
Polystichum munitum
Berberis nervosa - Gaultheria shallon
Rhododendron macrophylum - Oxalis oregana
Rhododendron macrophylum - Berberis nervosa
Rhododendron macrophylum - Gaultheria shallon
31
(6)
(6)
(3)
(7)
(8)
A BAM:
RHMA-BENE
VAME-CLUN
7
8
Abies amabilis
Rhododendron macrophylum - Berberis nervosa
Vaccinium membranaceum - Clintonia uniflora
13
(7)
(6)
*
Code derived from first and second letter of genus and species.
Code numbers as used in Figures 2, 3, 4, and 5.
RESULTS AND DISCUSSION
Results of some data analyses are presented and discussed in this section.
Stand Establishment
Slash burning eliminated most advance regeneration and exposed more mineral soil
(Figure 2). Both actions helped lower the initial percentage of stocked quadrats and
shifted s pecies composition to more Douglas-fir and fewer other conifers. Mineral soil,
for example, enhances establishment of Douglas-fir from seed. The time required for
the 1 00 largest trees/ha to reach breast height averaged 1 2 years for both planted
and naturally regenerated stands; establishment period was not related to burning.
Delayed or inadequate seedfall, coupled with loss of new germinants and planted
seedlings, contributed to delayed establishment and, hence, to reduced rates of volume
production.
In the 35 to 42 years after harvest of the previous stand, total numbers of trees (all
species) were greater on unburned plots in the higher elevation, Pacific Silver Fir Zone
(PSF; Abies amabi/is Dougl. ex Forbes). Conversely plant associations 1 -5 in the
western hemlock zone (WH; Tsuga heterophyl/a (Raf.) Sarg.) (fable 2) had fewer trees
on unburned plots. Cascade-wide, cumulative numbers of Douglas-fir and of
hardwoods (4.1 cm d.b.h. and larger ) on burned plots exceeded that on unburned
plots (Figure 3). Conversely, cumulative numbers and volume growth of other conifers
including western hemlock, Pacific silver fir (and some noble fir, Abies procera Rehd.),
and western redcedar (Thuja plicata Donn ex D. Don) on unburned plots exceeded
those on burned plots. These results confirm conventional wisdom that these shade­
tolerant conifers generally are more numerous on unburned sites because of their pre­
harvest presence and, with western hemlock, the greater extent of organic matter for
seedbeds.
68 LONG-TERM STUDY OF SLASH BURNING EFFECTS Exposed Minerai So l i (%.)
---------------------------------------------'
6 0 r-
_ Burned
E'22l Unburned
50
40
30
20
10
o
1
2
3
4
5
8
7
Plant Association
Figure 2:
Percentage of exposed mineral soil on burned and unburned plots in year 1, by designated plant
associations in the Cascade Range; the plant associations are identified in Table 2.
Cumu lativ9 N urn bars (no.lha)
2000 r-------��---,
B
EEEE AH
B
E2Zl OC
_ OF
1500 1000
500
o
1
2
3
4
5
7
8
Plant Association
Figure 3: Cumulative numbers of trees on burned and unburned plots 35 to 41 years after harvest, by
designated plant associations in the Cascade Range; the plant associations are identified in Table
all hardwoods; OC
other conifers; DF
Douglas-fir.
2. AH
=
=
=
Site Productivity
We used Douglas-fir site index (King 1966; Figure 4) and production of stem volume
since harvest or burning in trees 4.1 cm d.b.h. and larger (Figure 5) as measures of
site productivity and found no evidence that site productivity in the ensuing 35-41
years was reduced by first-time slash burning. Both numbers and volume of individual
69 RETROSPECTIVE RESEARCH s pecies were changed, however, by the multiple effects of slash burning. Burning
shifted species composition to Douglas-fir and hardwoods and, conversely, from other
conifer s pecies. Some volume production was lost on burned areas when burning was
delayed for one or more growing seasons. On both burned and unburned plots, an
extended establishment period for crop trees to reach breast height (fable 3) clearly
reduced volume production.
Site Index em)
40 .-------�
_ Burned
30
20
10
a
2
1
3
4
5
8
7
Plant Association
Figure 4: Site index, 50-year-base age, on burned and unburned plots 35 to 41 years after harvest, by
designated plant associations in the Cascade Range; the plant associations are identified in Table
2.
MAl 8tem volume (m3..ha)
9 r-------�
us
8
EHH AH
ELZl oc
7
_ OF
6
5
4
3
2
o
1
2
3
4
5
8
7
Plant Association
Figure 5: Mean annual increment since harvest or burning in trees 4.1 cm d.b.h. and larger on burned and
unburned plots, by designated plant associations In the Cascade Range; the plant associations
all hardwoods; OC
other conifers; DF
Douglas-fir.
are identified in Table 2. AH
=
70 =
=
LONG-TERM STUDY OF SLASH BURNING EFFECTS Item
Treatment Burned Unburned Difference (SE)
p
<
Establishment (yr) *
11.9
12.3
-0.4
±
0.4
0.96
50-yr site index (m)
32.5
32.0
0.5
±
0.4
0.21
4.9 4.1
0.5
0.3
4.7
3.2
1.2
0.2
0.2
0.9
-0.7
0.1
±
0.4
0.4
0.1
0.1
0.52
0.02
0.00
0.06
3.7
3.3
0.3
0.1
3. 6
2.6
0.9
0.1
0.1
0.7
-0.6
0.0
0.3
0.3
0.1
0.0
0.70
0.05 0.00
0.51
Cum. gross growth (m3/ha/yr) t
- (Trees 4.1 cm and larger):
All species
Douglas-fir
Other conifers
Hardwoods
±
±
±
- (Trees 19.3 cm and larger):
All s pecies Douglas-fir Other conifers
Hardwoods
*
±
±
±
±
Equals years needed by the 100 largest trees/ha to attain breast height. Equals bole volume of live, dead, cut trees since harvest or burning. In naturally regenerated stands, the time required after disturbance for crop trees to
reach breast height has two components: (1) time required for successful germination
and survival of germinants; and (2) time required for seedlings to attain 1.3 m. As
expressed in Table 3, mean annual increment (m.a.i.) is not adjusted for these periods.
Alternatively, m.a.i. can be calculated as cumulative volume growth divided by estimated
stand age (from seed germination). Because the divisor, estimated stand age, was less
than years since disturbance, stand m.a.i. averaged 10% more than site m.a.i. for both
burned and unburned plots (Cascade-wide). Stand m.a.i. is probably the better
expression of stand production and the closer estimate of potential site productivity.
The results from paired t-tests, however, were similar: stand m.a.i. of combined species
did not differ on burned and unburned plots grouped by plant associations, forest
zone, or Cascade-wide; s pecies composition was shifted to Douglas-fir by burning.
In summary, slash burning directly and indirectly affected many factors that influence
stand establishment, development, and volume production. The latter is a direct
estimate of site productivity based on the species and management at these locations.
This estimate or index of site productivity indicated that first-time slash burning did not
reduce site productivity.
71 RETROSPECTIVE RESEARCH DESIGN CONSIDERATIONS
Scope of Inference
The geographic distribution, experimental situation, and forest type sampled define
the scopes of inference of this experiment. Inferences about sampled populations are
. statistically reliable if samples are randomly chosen and properly analyzed. The
researcher then extrapolates these inferences from the sampled population to the
target population. It is reasonable to expect that results from a controlled or
retrospective study will apply to a similar geographic area; it is less reasonable,
however, to expect that the results from one forest type, rotation, or practice will a p ply
to other types, rotations, or practices. In short, valid extrapolation of inferences
requires that the sampled and target populations be coincident in time and s pace.
The potential scopes of inference from the Morris plot data are as follows:
Description
Geographic Western slope of Cascade Range between latitudes 47°
04' and 43° 07'
Forest Zone Western hemlock and Pacific silver fir
Practice Burning slash after clearcut harvest of 200-400 year-old
stands; silviculture of some succeeding stands included
planting, early thinning, and fertilization.
Data from all 44 locations were the basis for inferences pertinent to the entire
geographic area. Data from specific strata or subpopulations within the Cascades
Range were also analyzed. These analyses included burned v. unburned within each
forest zone, within naturally regenerated stands and within planted stands. These
subanalyses did not apply to Morris's original objectives but provide insight about
specific subpopulations of his original population. Consequently, inferences based on
these retrospective subanalyses are conditional.
Inferences and Target Populations
Current statistical theory provides a basis for deriving valid inferences from data
collected from a formally sampled population (Table 4). The sampled population in
a field experiment is the population of all possible repetitions of the experiment
(Cochran 1 983). Valid inferences require random or systematic sampling with a
random start; non-random sampling (purposive) may be biased; consequently,
inferences may be invalid and the researcher does not have the assurance of
statistical theory. Using past experience or additional knowledge, the researcher may
choose to extrapolate statistically valid inferences to the target population represented
by the sampled population. Researchers should explicitly state, however, the risks
and limitations of a pplying their research findings, especially when based on
retrospective investigations.
Was the sampling random in the Morris investigation? Yes and no. Was it biased?
Probably not as discussed below, but we can not know for sure.
72 LONG-TERM STUDY OF SLASH BURNING EFFECTS Population
Item
Sampled
Sampling
random
systematic
purposive
Inference
valid
conditional
invalid
Extrapolation
*
Target via judgment
Valid inferences are based on random sampling of a sampled population and extrapolated
to a target population.
Selection of locations;
Morris visited all available locations that met his selection criteria; he installed temporary
plots at 120 or more locations before slash burning. Inadequate burning on the
designated burned plot or accidental burning of the control plot reduced permanent
plot-pairs to about 75, of which 62 were maintained through 1962. This was non­
random sampling from a geographic area that happened to include two forest zones
as recognized retrospectively. The statistical analyses of these data, however, assume
these locations were a random selection from a well-defined po pulation.
The samples were restricted to land administered by the U.S. Forest Service, implying
a silvicultural system that could differ from that on other ownerships within these forest
zones. Similarly, in agricultural field research, trial locations are 'seldom drawn strictly
at random, since both willingness and some skill are required of the farmer; but the
objective is to sample the range of conditions that occur in farmers' fields. These trials
may be continued for several years to sample climatic variation' (Cochran 1983, p. 5).
Selection of treatments;
Usually, the unburned plot was in that portion that could be protected from fire when
the remainder of the clearcut was burned. In most cases, topography, skid trails, and
orientation of logs probably determined the unburned plot location; we suspect that
Morris used a random flip of a coin to randomize treatment at 10-20% of the locations.
Again, the statistical analyses assume a random assignment of treatment to all pairs.
Selection of five transects within each plot:
Each transect line was located randomly within a 20-percent segment of the plot.
Thus, sampling of conditions on each plot was random.
73
RETROSPECTIVE RESEARCH Selection of 25 subplots for vegetative cover and conifer stocking:
This was a systematic sampling with random starts.
representative and random sample of each plot.
This sampling provided a
Causal and Casual Relations (Implications)
A large portion of forestry research should attempt to determine (1) ·causes of
observed effects· and (2) changes in biological or physical processes that determine
tree growth and stand productivity. Controlled experimentation is a systematic way
to accomplish this. Alternatively, retrospective studies may be expected to provide
(1) useful descriptive information, (2) weak causal implications for limited geographic
areas and specific situations or practices, (3) an appreciation for the relative importance
of factors that were not designed into the original study, and (4) hypotheses to be
tested in subsequent research. Thus, retrospective studies can provide temporary
answers to current questions and can provide information to improve experimental
designs of future confirmatory investigations.
Experience has shown that many potential sources of variation contribute to plant
response in field experiments. In this investigation, large differences in volume
production on the matched, burned v. unburned plot at most locations demonstrate this
(Figure 6). Cochran (1983) discussed some research techniques for reducing or
removing the influence of non-treatment sources of variation (Table 5). Some of these
techniques were used by Morris when these plots were installed. For example, Morris
(1) placed his plots on areas of uniform post-harvest conditions within each clearcut;
(2) matched conditions for the two plots within these uniform portions; (3) re plicated
the pairs at numerous locations and in 5 years of establishment.
3
MAl (m Iha)
14 .-----,
X
12
'0
I
10
8
6
4
2
6
3
Unburned
1I
1
e§
Burned
1
*d
o ������
1
11
35
47
21
Location Number
Figure 6: Variation in mean annual volume increment by combined species on burned and unburned plots
by location, trees 4.1 cm d.b.h. and larger.
74 LONG-TERM STUDY OF SLASH BURNING EFFECTS Control
Average-out
Residual
uniformity
match/block
stat. analysis:
Covariance
Mult. Regression
randomize
replicate
subsample
keep unbiased
(random)
Other techniques to control non-treatment sources of variation were used in multiple
regression analyses of these data. These analyses have two purposes: (1) to provide
simple predictive equations for guiding current field decisions about slash burning
effects on vegetation succession, natural regeneration, species composition, and
volume growth, and (2) to clarify casual, and perhaps suggest causal, relations among
dependent and independent (descriptor) variables which must be further tested in
subsequent designed studies.
POTENTIAL STRENGTHS AND LIM ITATIONS OF THE MORRIS PLOTS
As a retrospective study with a designed-study component, the Morris plots display
some characteristic strengths and limitations.
Potential Strengths
*
Provide timely answers to current questions:
The Morris plots can provide timely answers to questions about cumulative, net effects
after about 40 years from clearcut harvest and slash burning. Thus, the researcher can
avoid the waiting inherent to new investigations specifically designed to answer current
questions -- which probably will not remain current!
Measurement of these
mid-rotation-aged stands provides the longest term and widest geographic scope
available in the coast Douglas-fir type for quantifying the net or composite effects of
slash burning on forest composition and growth.
*
Capitalize on previous research and costs:
The Morris plots provide an opportunity to obtain a preliminary or temporary answer
at a relatively low cost. The costs of imposing treatments and recording early results
have been paid. Risk of accidental loss of experimental areas are avoided; losses have
already occurred in retrospective studies! Lost plots, however, may introduce biased
data because the remaining plots may be less representative of the target population.
This retrospective study provided knowledge that is a basis for future research.
Clearly, the more known a bout a problem, the better research can be focused.
75 RETROSPECTIVE RESEARCH *
Complete previous research:
Retrospective studies can provide an opportunity to complete, u pdate, or extend earlier
studies. It is seldom possible for an originating researcher or even research
organization to follow field studies to a planned completion date. A commendable
desire for knowing effects over decades and centuries often conflicts with the desire
or need to produce rapid answers and proceed to new investigations. Long-term plots
need essential protection and maintenance. Consequently, many research studies are
prematurely abandoned, consciously or by oversight.
Potential Limitations
*
Past practice may be unequal to current or future practices:
Land managers are interested in the future effects (1 ) of current practices and (2) of
past practices that are a 'given' or an uncontrollable starting situation for their
custodianship. Retrospective studies can only quantify or describe the effects resulting
from past practices applied to past forest situations and climates; the results may not
correspond to those from current practices. For example, to what extent is the effect
of past burning of old-growth logging slash after low utilization standards (for example,
the Morris plots) pertinent to the effects of burning slash after second-growth, natural
stands or plantations harvested to more complete utilization standards? Are the results
from the Morris plots obsolete? Some people consider harvesting and slash burning
practices in old-growth timber as worse-case situations with limited application to
current forest management. Others believe that changes in burning prescriptions ­
how, when, what - from rotation to rotation will change the magnitude of the effects but
not the principles.
*
Data or information may be missing, limited, or spurious:
Growth of forest stands can be affected by many interacting factors. For example,
slash burning affects (1 ) amount of advance regeneration that survives to the next
stand, (2) amount and quality of seedbeds, and (3) amo.unt and composition of
competing vegetation. About half of the Morris plot locations were operationally
thinned. Data about stump size and numbers collected retrospectively in 1 975 and
1 986-87 indicated that both plots at these locations were thinned to the same
standards and prethinning differences were only slightly modified. In such a widely
distributed sample of plots, few growth factors are controlled. Thus, it is essential or
desirable to quantify the influence of some uncontrolled factors common to all
locations. With such quantification, these factors can be introduced as explanatory
variables in statistical analyses such as multiple regression, provided the independent
variable has a reasonable range of variation. If a reasonable range and distribution is
not present in the sample data, statistical nonsignificance can result even when
dependent and independent variables are truly related.
CONCLUSIONS
1.
76
The Morris plots began as a controlled experiment. The original design was a
randomized block, but treatments (burned, unburned) were seldom assigned at
random. Initial site conditions (soil, soil disturbance, slash amounts or distribution)
·
/
LONG-TERM STUDY OF SLASH BURNING EFFECTS probably differed to a minor degree, but these small differences favoured the
burned plot at some locations and the unburned at others. At the 44 locations
that we recovered in 1 986-87, soil and topography of the two plots at each
location a ppeared well-matched.
2. The data suggest that slash burning affected several factors favouring the number
and volume of Douglas-fir and hardwood trees and disfavouring number and
volume of other conifers. Volume production by combined species, however, was
not affected by slash burning. These results have direct application to similarly
managed stands. Different findings would be expected, however, if planting or
weed control had been more successful. For example, successful planting and
weed control would offset the effects of greater cover of competing vegetation and
more advanced regeneration measured on unburned plots. Such dissimilarities
in management should be considered when the results from the Morris plots are
extrapolated to a more general population in the Cascade Range or elsewhere.
3. The lack of random sampling (of locations and treatment within locations)
constrains the validity of inferences from our statistical analyses. Because of
large, well-distributed sample size, however, the Cascade-wide analyses appear
to be representative of this geographic area (the target population).
4. Regrouping of the locations into two forest zones and seven plant associations
was retrospective. Consequently, the results of paired t-tests are conditional. The
results do not provide a valid test of an initial hypothesis but suggest new
hypotheses to test in further controlled experimentation. For example, the two
forest zones and some plant associations appear to differ; future experiments
should test these as potential strata for management guidelines.
5. Retrospective studies can sometimes provide useful, quick, but temporary answers
to current questions from land managers, modellers, classifiers, and designers of
new field experiments. The location, type, and reliability of initial observations
and measurements frequently limit the interpretation of the results and scope of
inference.
6. Researchers should explicitly state the risks and limitations of applying their
research findings, especially when based on retrospective investigation.
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 U.S. Forest Service,
National Forest Administration, contributed funds and field assistance. The Washington
State Department of Natural Resources provided data processing assistance. Useful
review comments were provided by Dean S. DeBell and three anonymous reviewers.
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