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COMPARISONS OF TREE HEIGHT
GROWTH ON BROADCAST-BURNED,
BULLDOZER-PILED, AND NONPREPARED SITES 15 TO 25YEARS
AFTER CLEARCUT LOGGING
Bob Bosworth
Dan Studer
been the most frequent treatments. Some stands have
had no site preparation treatments.
The district is continually juggling its program, considering the limited times suitable for broadcast burning and
the total acres to be treated for planting each year. Typically, debris on slopes less than 35 percent is piled, while
steeper slopes are burned. In years of larger programs,
bulldozers are moved onto even steeper slopes to balance
the program and get all sites prepared in a timely manner. With increasing concerns over air quality, there is
more pressure to move away from broadcast burning and
to piling and burning to reduce impacts on airsheds.
As our knowledge base on the complex interactions of
our forest ecosystems expands, the questions of short- and
long-term impacts on growth related to site treatments
are repeatedly asked. Bonners Ferry District personnel
have been intensively monitoring progress of regenerating
stands for many years. We wondered if the data from
monitoring might be able to shed some light on the effect
of treatment on regeneration performance. We were particularly interested in looking at performance of stands
15-25 years after harvest.
This paper reports the extension of a study that was
presented in the Prescribed Fire in the Intermountain
Region Symposium Proceedings (Bosworth 1989). The
1989 paper discussed differences in height growth found
on clearcuts that were logged in the years 1960 to 1965.
Since then the study has been extended to include clearcuts that were logged in 1966, 1967, and 1968.
ABSTRACT,·
Tree height growth was compared on clearcuts that were
broadcast burned, bulldozer piled, or had no preparation
for regeneration in the Bonners Ferry Ranger District,
northern Idaho. Differences in heights and growth are
related to treatment. Height growth on burned sites exceeded that on piled sites in 8 of9 years. Questions arise
as to effects on site productivity of piling. Reasons for differences include nutrients available to regeneration related
to site preparation method, and compaction effects from
piling.
INTRODUCTION
The Bonners Ferry Ranger District of the Idaho Panhandle National Forests lies in northern Idaho. Generally, it is bounded by Canada to the north, Montana to
the east, the crest of the Selkirk Mountains to the west,
and extends south to the BoundarylBonner County line.
Elevations range from 2,500 to over 6,000 feet. Habitat
types range from those of the relatively warm, dry
Douglas-fir (Pseudotsuga menziesii) series to those of
the cool, moist subalpine fir (Abies lasiocarpa) series
(Daubenmire and Daubenmire 1968).
Subsoils in the district are dominated by glacial tills
from both continental and alpine glacial action. Some
are subsoils formed from glacial-fluvial outwashes. Most
of the surface soils are volcanic ash in origin. This ash
cap is a very good medium for growing trees (Ford 1985).
Clearcutting as a regeneration system has been practiced since as early as 1916 (Roe and DeJarnette 1965)
in the district. However, use of the clearcut system became increasingly routine from the late 1950's to the mid
1970's. Most of this cutting has been in overmature to
decadent timber stands where large quantities of slash
and debris dictate some sort of site preparation to achieve
adequate regeneration and to meet fire hazard reduction
objectives. Broadcast burning and bulldozer piling have
METHODS
District personnel monitor the progress of stands after
harvest with a regeneration status survey. On a plot,
regeneration is tallied by species, height to the nearest
foot, and last year's leader growth to the nearest inch,
and trees are classified "crop" or "excess." Relatively high
numbers of plots are surveyed, since one of our main concerns early in the rotation is poor distribution of regeneration. The high number of plots monitored was important
to this study, as this gave us many trees to assess. Plots
are systematically distributed throughout the clearcuts
on a grid basis.
The stand database was queried for all clearcuts harvested during the period of interest. Stands were selected
Paper presented at the Symposium on Management and Productivity
of Western-Montane Forest Soils, Boise, ID, April 10-12, 1990.
Bob Bosworth is Fire Management/Silviculture StafTOfficer, and Dan
Studer is Forest Technician, Silviculture, Bonners Ferry Ranger District,
Idaho Panhandle National Forests, Bonners Ferry, ID 83805.
197
for analysis if both tree height and leader growth estimates were recorded for the plots. A total of 104 stands
had adequate data from 2,752 plots.
Stands were stratified by site preparation method; 58
were treated by broadcast burning, 38 by bulldozer piling,
and eight had no site preparation.
Each regeneration status plot in a selected stand was
analyzed. The dominant tree was selected, and species,
total height, and last year's leader growth of that tree
were recorded. Since the stands were on differing inventory schedules, and year oflast examination varied, it was
necessary to normalize the data for analysis. For phase 1,
1960-65, of the study trees on plots examined in years
prior to 1982 were "grown" to 1982 by adding appropriate
numbers of annual leader growths to the total height
estimate at time of exam. For phase 2, 1966-68, a similar
process was applied for trees on plots examined prior to
1986. For comparison between the two phases of this
study, years 1963/1966; 1964/1967; and 1965/1968 had
been c1earcut for the same number of years at time of
analysis.
A mean total height (1982 basis, phase 1 and 1986 basis, phase 2) and mean leader growth were computed for
each site preparation treatment for each year of c1earcut
origin. Analysis of variance was then used to compare
treatments to see if differences in the means were related
to treatment (Freese 1967).
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61
62
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64
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Figure 1-Mean growth of crop trees; comparison of
site preparation methods.
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RESULTS AND DISCUSSION
20 ............_..................................................................................... ............... .
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In phase 1, we determined that, in general, bulldozer
piling for site preparation had a negative impact on both
height growth and total heights of trees, when compared
with broadcast burning and no site preparation. From
the analysis of phase 2 data, it is apparent that these
same trends are still evident, although there is some evidence that the differences between treatments are not as
great in the newer stands.
Figure 1 shows average leader growth by year of harvest. In 8 years out of9, growth on burned sites exceeded
growth on bulldozer-piled sites. Growth on burned sites
also exceeded that on nonprepared sites in 7 years out
of9. These differences are related to treatment and
are significant at the 0.01 level in all years except 1962.
In 1962 there is no significant difference in growth between burned and piled sites, but the differences between
grow- th on nonprepared sites are significant when compared with the other treatments.
Where leader growth differences were significant,
growth on burned sites was from 1.6 to 5.2 inches per
tree better than on piled sites, and from 1. 7 to 2.0 inches
better than on nonprepared sites in years where burned
areas had the best growth. When growths were best on
non prepared sites (1964, 1967) they exceeded growths
on burned sites by 0.6 and 1.4 inches per tree.
When average total heights are compared, in 7of9
years, trees on burned sites are taller than trees on
bulldozer-piled sites (fig. 2). The exceptions are years
1962 and 1966. In the 5 years when all three treatments
occurred, burned sites had the tallest trees twice, nonprepared sites had the tallest trees twice, and bulldozerpiled sites had the tallest trees once. Analysis of variance
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60
61
62
63
64
65
66
67
...
68
YEAR OF HARVEST
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BULLDOZER.
NO
PILED
PREPARATION
Figure 2-Mean height of crop trees; comparison of
site preparation methods.
shows that the differences between treatments for all
9 years is significant at the 0.01 level.
In the 7 years where average heights on burned sites
exceeded the heights on piled sites, the differences in
means ranged from 0.9 foot in 1968 to 5.7 feet in 1960,
with an average 2.9-foot height advantage on burned
sites. In the 2 years when piled sites had the taller trees,
the differences were 5.3 and 1.3 feet.
Caution is advised in drawing conclusions about the
total height differences expressed by the nonprepared
198
site data. Examination of the trees included in these
plots showed that they include some very old (70-year)
advanced regeneration, and this factor is accentuated by
the fact that in years where the differences are greatest,
there is a very small sample. This advanced regeneration
was not removed from the study because it is possible that
some old advanced regeneration may also have survived
piling or broadcast burning and may be in those plots
also. Their effect is less in the pile or bum plots because
there are generally many more trees in this part of the
analysis, and a few trees do not affect the average as
much.
The implications of the differences in height growth
displayed in figure 1 in favor of burned sites over bulldozer piled sites, combined with the differences in total
height displayed in figure 2, generally in favor of burned
sites, cannot be ignored. A total of 94 stands comparing
2,496 trees are represented. Smith (1962) stated, "Within
a stand, few trees ever recover a dominant position after
they have fallen behind in the race for the sky." The trees
in this study on the bulldozer-piled sites have not lost
their dominant position in the stand, but even so, on the
average, they are falling behind in height growth when
compared with trees on burned sites. Based just on differences in total height, with no allowance for differences in
diameter growth, if this trend continues for the remaining
four-fifths of the rotation, the trees on the burned sites
will average 10 to 22 percent greater sawlog volume than
those on piled sites. It is reasonable to expect that iffactors are reducing height growth of trees, they would also
be adversely affecting diameter growth, thus producing
even greater volume differences between treatments.
There are several reasons that these differences in
height growth are being observed. One possibility is the
flush of nutrients available for plant growth following
broadcast burning (Holdorf 1982). There are also at least
two possibilities for the reduced growth on piled sites.
Compaction of soils has been shown to reduce height
growth (Froehlich 1978). Even with one or two passes
of a tractor over the soil while piling, some increase in
soil bulk density occurs, particularly if the soils are wet.
Another, and perhaps more important, reason may be
related to nitrogen fixation. Nitrogen-fixing activity is
normally between 5 and 10 times greater in organic versus soil horizons (Jurgensen and others 1980). It is not
uncommon for much, if not all, of the litter and humus
layers to be shoved into the piles during bulldozer site
preparation. This is especially true when high volumes
oflogging debris exist. In that case, it is very difficult
to keep from scraping all the organic matter off the site.
This would greatly reduce the soil nitrogen-fixing process
over much of the area piled.
It is likely that the differences in height growth demonstrated in this study are the result of a combination of
these factors. The design of this study was not sufficient
to ferret out precisely what the causes of the differences
were.
Results of the phase 2 analysis are encouraging from
several aspects. First, differences in height growth on
burned and piled sites are not as great as they were in
phase 1. This indicates that management decisions to
use brush blades versus straight bulldozer blades, and
to not pile slash as thoroughly, were in fact good decisions
and are resulting in less impacts to the site.
The second encouragement from phase 2 is the indication that newer work is producing better trees than older
work. Phase 1 trees-year 1963 are the same age as phase
2-year 1966; year 1964 the same age as 1967; and year
1965 the same age as year 1968. In figure 2, it is apparent that newer work is producing taller trees at younger
ages in both the burning and the piling treatments. Some
factors that may be causing these improved tree heights
may be: (1) improving coordination to get prompt planting
accomplished following site prepartation; (2) improving
planting stock handling and storage prior to planting;
(3) improving attention to detail of properly planting each
tree; (4) decreasing site preparation effects (less intense
broadcast burns and less site disturbance during piling
operations); and (5) earlier reaction to scattered mortality
in plantations by interplanting.
CONCLUSIONS
These data indicate that trees on broadcast-burned
sites will have a height growth advantage over trees
growing on bulldozer-piled sites. This advantage will be
evident for at least 20 years. If the manager has a choice
of methods, burning should be chosen over piling.
However, in situations where there is a big program,
the manager needs the flexibility to use both methods in
order to accomplish timely reforestation. Consequently,
when piling must be used, it is important that everyone
involved in the process be aware of the potential for longterm growth losses on those sites. The logger, the contract administrator, and the bulldozer operator need to
know the effects of excessive scraping of the organic layers of the soil and also the potential for compaction of the
site from "overworking" the area with a tractor.
In addition, silviculturists and fuel managers must take
a critical look at some stands to determine if site preparation is in fact needed. Unnecessary working of a site by
heavy equipment may cause hidden long-term damage.
REFERENCES
Bosworth, Bob. 1989. Height growth on burned, piled and
nonprepped clearcuts, 17 to 22 years after harvest. In:
Baumgartner, David M.; Brewer, David W.; Zamora,
Benjamin A.; [and others], eds. Prescribed fire in the
intermountain region: forest site preparation and range
improvement; 1986 March 3-5; Spokane, WA. Pullman,
WA: Washington State University, Conferences and
Institutes: 65-67.
Daubenmire, R.; Daubenmire, Jean B. 1968. Forest vegetation of eastern Washington and northern Idaho. Tech.
Bull. 60. Pullman, WA: Washington Argricultural Experiment Station, College of Agriculture, Washington
State University. 104 p.
199
Ford, Gary L. 1985. Preliminary report, soil survey of the
Bonners Ferry Ranger District. Coeur d'Alene, ID: U.S.
Department of Agriculture, Forest Service, Idaho Panhandle National Forests. 139 p.
Freese, Frank. 1967. Elementary statistical methods for
foresters. Agric. Handb. 317. Washington, DC: U.S.
Department of Agriculture, Forest Service. 87 p.
Froehlich, Henry A. 1978. The effects of soil compaction
by logging on forest productivity. Final report to Bureau of Land Management, Portland, OR.
Holdorf, Herbert. 1982. Effects of site preparation and
fuel management practices on soil productivity. In:
Baumgartner, David M., ed. Site preparation and fuels
management on steep terrain symposium proceedings;
1982 February 15-17; Spokane, WA. Pullman, WA:
Washington State University Cooperative Extension:
63-65.
Jurgensen, M. F.; Arno, S. R.; Harvey, A. E.; [and others].
1979. Symbiotic and nonsymbiotic nitrogen fIXation in
Northern Rocky Mountain forest ecosystems. In: Symbiotic nitrogen fixation in the management of temperate
forests workshop: proceedings; 1979 April 2-5; Corvallis,
OR: National Science Foundation: 294-308.
Roe, A. L.; DeJarnette, G. M. 1965. Results of regeneration cutting in a spruce-subalpine fir stand. Res. Pap.
INT-17. Ogden, UT: U.S. Department of Agriculture,
Forest Service, Intermountain Forest and Range Experiment Station. 14 p.
Smith, David Martyn. 1962. The practice of silviculture.
7th ed. New York: John Wiley and Sons. 578 p.
Q. (from JeffCollins)-Your graph indicates that
height growth is equal or greater on nonprepared sites
compared to bulldozer piling. Would you consider not
piling if you could not burn?
A.-Yes, I would consider no treatment, if neither site
preparation or hazard reduction are needed to meet objectives. Caution is warranted in placing strong interpretation on the nonprepped sites, due to the very small number of trees in those samples and the fact that inspection
of those data shows that many of those trees are very old
(70 year) advance regeneration.
Q. We heard earlier that tree height may not be a good
indicator of fertility response in conifers. How might this
observation affect the outcome of your study results?
A.-In trees the age that were studied, growth as measured in leader growth is simple to estimate. This leader
growth indicates the sum of all factors that are affecting
the tree. One uniqueness of this study is that it involves
many trees, in many stands, on many sites, from several
years of treatment. These trees are demonstrating differences that are related to the different treatments. As I
stated in my conclusions, we do not know precisely why
these differences in growth are demonstrated, but we do
know that it is related to piled versus burned sites. The
reduction in height growth on piled sites, regardless of
the reason, cannot be ignored. It does seem very probable
that nutrient availability is a part of the differences
shown, given our current understanding of the importance of organic material to forest soil nutrients and
knowing the extent of scraping off of organic material
in the bulldozer piling that was typical during the 1960's.
Speakers answered questions from the audience after
their presentations. Following are the questions and
answers on this topic:
200