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B. •.
Watershed-Management
Aspects of
Thinned Young Lodgepole Pine Stands
THE CENTRALSECTIONof the Rocky
duction
in water
losses from
School of Engineering,
(JoodelP
Johns Hopkins
University. Baltimore, Md.
trees.
now
covered
with
a dense second
Increasing water production growth of lodgepole pine. The
through forest management is the blockswere chosento samplea wide
subject of a series of experiments range of slope, aspect, and stand
being conducted on the Fraser Ex- density. Slopesvary from 2 to 25
perimental Forest in north-central percent and aspectsrange through
Colorado. One completed study, all cardinal directions. Stand denactual source of most of this water
conducted on a plot basis, has sities varied from over 8,500 stems
is located above 8,000 feet in eleva- shown that the harvest cutting of per acre on one block to less than
tion. Here snows are heavy and mature lodgepolepine does,in fact, 2,100 on another. Basal areas differed less; the maximum basal area
water yields run as high as 24 increase the water available for
stream flow (4). Another study is being 116.3 square feet per acre;
inches per year.
This same high mountain region now under way that will make use the minimum, 46.3; and the averof
actual
stream-flow
measureage, 89.4. Dominant trees averaged
is also one of low erosion hazard.
ments
to
test
the
effects
of
timber
•
23
feet in height.
Most of the precipitation is snow
The soils on all blocks are deharvesting
both
on
quantity
and
which,in melting, releaseswater at
• ?ate readily absorbedby the soil. quality of water yields. The study veloped from stratified coarse-texSummer rains are frequent, but reported here is an extensionof the tured glacial outwash, consisting
usually very light. The erosion research on harvest cutting to in- of schist, gneiss, and granite. In
clude the results of thinning young two blocks, a clay layer exists at•
hazard is limited
by the dense
depths of 16 to 24 inches; in the
vegetation that clothes the moun- stands of lodgepole pine.
remaining blocksthis layer was not
tain slopesand ridge tops; trees to
Description of the Area
found. Volume weights averaged
elevations of about 11,000 feet,
The
Fraser
Experimental
Forest
1.04 for the surface 3 inches of all
grasses and tundra vegetation on
encompasses
an
area
of
23,000
blocks,
but increased sharply to
higher areas. The principal forest types are lodgepole pine and acres and ranges in elevation from 1.61 at the 16-inch depth. Depth
E n g e 1m a n n spruce-alpine fir. 8,800 to nearly 13,000 feet. An- of outwash material was in excess
Young standsof lodgepolepine are nual precipitation averages about of 48 inches throughout the study
extremely dense and this density is 24 inches, with the proportion of area.
snowfall varying from two-thirds
Each of the •sixblockswas dimaintained
in overmature
stands
by the advance growth of pine or to three-quarters of the total. Snow vided into three •-acre plots and
accumulation starts. in October or random drawings made to deterspruce and fir. The spruce-fir type
early November and continues mine the thinning treatment to be
has a dense cover in both the overwithout
melt until late March or
applied to each plot. One plot of
story and understory, the latter
early
April.
Prolonged
by
heavy
each block was left untouched; the
composedof spruce and fir reproduction and shrub growth. A com- spring snowfall, a snow cover usu- other two received different types
plete litter cover' prevails in both ally persistsuntil late May or early of thinnings.
June.
June is a month of little
forest types, although it is deeper
Silvicultural
Treatments
precipitation,
but convection rain
nnder the spruce-fir.
Mountains
v6thin
Colorado
and
Wyoming is an important wateryielding area. Stream flow which
originates in the mountains supplies water for irrigation, power
generation, and domestic use. The
The combination of high water
yields and low erosion hazard on
the same area is fortunate.
means that
This
the mountains, pre-
dominantly valuable for their water yields, can also be used for the
commercial production of timber.
The favorable
combination
cates that the actual value
indiof an
area for water production may be
increasedby somereduction in tree
canopy and an accompanying reXFormerly of the Division of Forest
Influences, Rocky Mountain Forest and
Range Experiment Station, maintained
by ILS. Department oœAgriculture, Forest Service, in cooperation with Colorado
A & hi College, Fort Collins, Colo
showersbecome'frequent in July
and persist throughout the summer. In 8 years of record at the
Fraser Experimental Forest at ele-
The silvicultural thinning of sec-
ond-growth lodgepole pine stands
has been extensively practiced in
the Rocky Mountain region and has
vation of 9,000 feet, the heaviest proved beneficialto timber growth.
storm produced1.75 inchesof rain- The present experiment was spefall; the second heaviest. 0.75 cifically designed to measure the
inches.
effect of such treatments on hy-
The present study was conducted
on six blocks of 2¾• acres each located within the main valley of St.
Louis Creek, the main drainage of
the Experimental Forest, at an
average elevation of about 9,000
feet. Logging of the original forest of lodgepole pine and sprucefir in this valley around 1910 was
followed by fire and a large area is
drologic factors related to stream
374
flow.
Two different
methods of thin-
ning were applied to the young
pine stands. One, called "singletree thinning," consistedsimply of
reserving the better trees that were
spaced about 8.5 feet apart over
the plot. This left a total of approximately 600 trees per acre. All
1952
375
USFS
}via. t.--A
winter
plot thinned by the single tree method showing the
snow
other trees were cut (Fig. 1). The
same type of thinning' had been
or elevation
1). Expressed in square-foot basal
area, the percentage reductions
were 51.2 for the crop-tree and 59.7
for the single-tree thinning.
stand is removed
and ]ess time is needed for the selection of trees to be reserved. Fur-
thermore, it leavesa pattern of forest cover which approaches that
recommended by Church and others (1, 2) as the ideal for the maximum accumulation and storage of
and
its
silvieultural
value
may be as great as that of the sino<le-treethinning. The appearance
of the crop-tree plots is indicated
by Fig'. 2. In this type of thinning
as well as the single-tree type, all
trees were felled parallel to each
other and large branches were
lopped to reduce the depth of slash
and hasten decay. Treatments were
completed on all plots in the summet
densities
in
stems
and
basal area per acre were reduced
by more than half their original
values by both treatments (Table
for-
that would affect conclusions drawn
•rom post-treatment data.
Mea-
surements
were
after
taken in
(Fig. 1).
Spriug
trcatmenl
1947, 1948. and
snowfall
occurs
1949
xvhile
snow-melt
is intermittently
prom'essing. Snow on the ground at
any time during' this period represents the winter accumulation plus
the prior spring snowfall minus the
prior snow-melt. Snowfall during
this period can be measured by
precipitation
gages. The results
are then free of snow-melt effects.
?4tandard
8-ineh-diametm'
rain
sirable beemine snow-melt rarely
occurs during' the winter months.
2now falling' during this period aetunrelates withoul loss (except by
sublimation) until late Mm'cb or
om'ly April. The smu of xvinter
snowfall can be mcasm-ed by
surveys taken just prior to melt in
gages were used for this purpose
with periodic movement to new
early spring. Samples at 16 randomly located points on each
acre plot comprised these surveys.
Intensive study of snow-melt
rates was made only in 1949; but
random locations for greater sam-
pling efficiency (3). One gage per
plot was used in 1947 and 1948;
two ,-agesper plot in 1949 (Fig 3).
Snow-melt
observations
in
snow
accumulation
mation.
associated
with inherent differencesin aspect
I'ABLd• I.
made in 1947 and 1948
,_o'ave
additional qualitative infor-
Measure:cents were started i year
b,efore the thinning treatments in
order to detect any ploi variatious
The
Nmnber of stems
Treatn,ent
Pretreatment
Posttreatn, ent
None __
Crop-tree _
Single tree
Average _ _
4,413
4,275
4,025
4,238
4,413
2,072
649
.....
1949
measurements
were made by means of weekly
readings of the gages used to measure current snowfall and weekly
STAND [)F,NSlq•IES AS Arl•gt.'q'rl) BY THIN•NG
of 1945.
Stand
of the undisturbed
('st stands. These data, statistically
analyzed, showed no differences
applied by the Civilian Conservation Corps over thousands of acres
in the Rocky Mountain region.
Hydrologic Measurements
The other type of thinning, desThe
hydrologic factors measured
ignated "crop-tree," was done by
nero tho.qe of snowfall,
rate
antting openings 8 feet in radius
snow-molt, rainfall. and soil-moisaround each of 100 trees per acre.
tllre Josses. These Wel'e 1110asln'l.d
Selection of crop trees was based
on their potential to deveh)p into on all plots in the same manner and
desirable specimens in the mature to the same intensity so that direct
stand; secondary consideration was .omparisons could be made of the
given to providing a uniform spac- thinnin._o:treatment effects.
Snowfall
in• of 21 feet. All trees outside
the cleared circle surrounding each
The
measurement
of snowfall
crop-tree were left standing. This was divided into two parts and
method of thinning' is less expen- procedures; snow smwcys and
sive than the sin•'le-tree since less precipitation gagcs. This was de-
snow
447363
nates one of the crop •rees.
accumulation.
of the ori•inal
Photo
Fro. 2•--A p]o• thinned by the crop tree method. Arrow dcsig
Tg•HT8,
ALL DATA
Basal area • square feet
Percent
reduction
0.0
51.5
83.9
eretreatment
89.6
88.8
89.8
89.4
Posttreatment
89.6
43.3
36.2
......
Percent
reduction
0.0
51.2
59.7
376
JovR•aL
snow surveys to determine the snow
remaining on the ground. The
snow-melt occurring in any period
was computed by adding current
snowfall to that already accumu-
lated at the beginning of the period and subtracting the snow remaining in storage at tim end of
tbe period.
rainfall
determinations
was insufficient
their
orifices
to measure
net
storm
to
level.
new
random
rainfall
Rainfall
BY
THIIffNIIqG
Thinning treatments
Cro&
Single
Year
/'gone
•947
trec
tree
.... 10.3• - 11.49
1948
1949
9.70
_ 10.00
Average _
10.03
Thinaiag effect in
of
water
12.34
11.74
11.94
12.2S
12.41
11.72
12.34
1.•;9
_.31
23.o
'[nABI•E :].--•PI•ING
T•EATMEN•S,
•N(D,V ]•E.L.T. IN ] N('I[E.'q
R•COgD OF 1 YI,]AI(
Avel'•lg('
Total
Tr• attar,at
r['l'l'•l[lllPllt
111elt
t
None
5.53
e•ect
mtqt
ptq' ally
0.26
• h'op tree
7.54
2.o l
_.3.
8iaglc tree
7.Sg
'' q)uring first 21 days of melt period
above the level
of the slash; th• fraction of rainfall intercepted by slash also had
to be considered. Two cylindrical
gages per plot. moved after each
measured
AND
were
raiufall by standard gages placed
with
WINTIgl•
Thinning effect ia percent 16.•
complicated by the thick mantle
debris titat covered the ground in
the treated plots after the cuttings
were completed; debris that obviously might intercept an impel
taut fraction of the gross rainfall.
It
AFFECTED
inches
Rainfall
Net
rJ•ABLE 2.--I•ET
•IffO;VFALL• IN INCHES OP WATER, AS
TABL15 4.
1N•tlES•
Anov•
locations.
8t:5iME:•
SnASH
•AINFA[.I,.
IN
LEVg'L
Thining iron;meats
('rop
•ingh.
above
under
the slash
the slash was
•Ek'
AS APFFA'TI,)D IIY q'IlINNI•G
Year
None
tree
tree
•aged by two shallow troughs
equipped with screen baffles to
i946
2.90
3.3o
1947
4.47
5.o4
5.16
eliminate splash. These two gages
Average
3.68
[.17
4.33
were
Thinning effect (inches) 0.49
Thinning effect (percent) 13.3
0.65
17.7
also moved
after
each storm
and were slipped beneath any slash
thal wa.• present at the rando,,,
.•ampling point (Fig. 4). Measuremeats by both methods were made
in 1946 and 1947. 1 and 2 years
after the thinnings.
This period approximated the
mml low point in soil moisture
caused by the excess of evaporatiehal and transpirational losses
over summer rainfall. ,qampling
Soil Moisture
Losses
was done 1)3' means of a 1-inch
Soil-moisture
losses as affected
geetome which extrm.ted a ('ore of
by the thinning treatments were soil. Samples were taken to a 48determined by means of extensiw• inch depth, bu! were separated
samplino,' done in mid-•qeptember. segmentsof 13-18inches aml 18-48
or
FOgESTR,-
inches in order to determine if the
treatment
effect
varied
between
these two depths. Sixteen annual
samples were taken from each plot
at both depths. during the period
of study.
Results
Snowfall
The thmnin,,,' treatments had a
marked
effect
on
the
snowfall
reachlug' the •round within the experimental plots. Over the 3 years
of the study. the average water
equivalent inorease in net snowfall
due to the er•q•-tree thinning was
1.69 im.he•; while the iner•ase
caused by the single-tree treatmere
was 2.31 inches (Table 2). Artc
z(mal comparisons indicated Hint
these ira-teases were real and not
attributable
to chance
However.
the sizable difference between the
effect of the crop-tree thinning and
that of the single-tree could have
been by chance alone.
Of the total
increases
in snow-
fall, the g'ains • net winter accumulation accounted for most. Croptree and sina'le-tree thinning increased the winter accumulation by
an averaa'ewater equivalent of 1.31
•ehes and 1.68 inches, respectively.
Snow-melt
()bservations
1948
indicated
made
in
•at
1947
and
the th•ning'
treatments produced acceleration
in the rate of •ow-melt.
The inten-
sive quantitative measurementsof
1949 bore out this conclusion (Table 3). During the first 3 weeks of
ihe melt period when the snow •ovm' was still complete or nearly so.
:%½-
U,•I",•
Photo
463,522
Fro. 3.--Gage for measuring spring snowfall located in a plot
thinm.d by the crop-tree method.
l•q Photo
Fro. 4.•ylindrical
and trough gages in place on plot thinned
by single-tree method.
MAY 1952
377
the melt under the crop-tree thinnine amounted to a total of 2.01
inches of water
more than
did that
on the unthinned plots. The singletree thinning' increased snow-melt
by a total of 2.35 inches of water
in the same period. Beyond the
3-week period the snow cover became spotty (Fig. 5) with lesssnow
area under
the treated
under
untreated
the
stands
and
than
further
comparison could not be made because of the unequal snow surface
areas subject to the snow-melt factors.
Rainfall
The partial removal of the forest
canopy by the thinnina' treatments
allowed more rainfall to penetrate
beneath the crowns and to the level
of the slash. The extent of these
increases is shown in Table 4. For
the 2 years of measurement, the
average increase caused by the
USES
Fie. 5.--Snow
It is concluded from
sults and the information
these reobtained
face
moisture
crop-tree thinning was 0.49 inches
or 13.3 percentof the summerrain- on rainfall reaching the soil surfall
received
under
the untreated
that
the
losses
of
plots. The single-treethinning pro- from the sell through evaporation
and transpiration were unaffected
duced an increase of 0.65 inches
or 17.7 percent. Statistical tests by the thinning treatments. The
showed these increases
and not due to chance.
to be real
Different results were obtained,
however, from the measurements
by trough gages of the rainfall
penetratinE the slash and actually
reaching the soil surface. These
measurements, taken •n 1946 and
1947 when many needles still remained on the felled trees, showed
no real influence of the thinning
treatments.
The felled
trees while
still holding their needles were ap-
parently about as effectivein intercepting rainfall as they were when
standing.
Soil
soil-moisture
content in late summer is the total of summer rainfall
on the soil, less the evaporationtranspiration losses. As sho•vn by
the trough gages, the amount of
rainfall reaching the soil was similar
both
the
treated
and
un-
treated plots.
Summary and Discussion
4633•0
ing the ground. It
however, that as the
grates, the rainfall
soil will increase. It
is probable,
slash disintereaching the
is also prob-
able that if the felled trees
been removed
for utilization
had
as
poles or small posts, some immediate increase in net rainfall
have
The
been
would
realized.
unmeasured
effect ot' the
was that on the sub-
treatments
limation and evaporation lossesof
water
from
the snow.
Because the
thinnings allowed more wind and
sunlight to reach the snow surface,
they very likely increased the sublimation lossesduring cold weather
This study has again demon- and the evaporation lossesduring
strated that the thinning of dense periods of melt. There is no known
forest stands will appreciably in- technique for accurately measurcrease the net precipitation re- ing these vaporization losseson an
ceived on the forest floor.
For
the
particular environment and type
of
Moisture
in
Photo
cover under crop-tree thinning late in spring melt period.
stand
studied,
the
treatments
areal basis so the influence
on them
of the thinning treatments could
not be evaluated. This deficiency
•.osses
applied caused an average increase does not weaken the conclusions reThe intensive sampling of soil of 20 percent in net snowfall and •ardin.o winter snow accumulations
15.5 percent in summer rainfall.
since the snow measured in early
moisture in each of 3 years after
the thinning revealed no effect by
Partially offsetting these appar- spring is the total snowfall less
benevaporization losses. The extent of
the treatments. Mid-September ent watcrshed-mana_•ement
measurements
showed no real differences/:etween
the moisture content of the soil under the treated
plots and that under the untreated.
Both
the 0-18-inch
and
the
18-48-
inch depth of soilsshowedthis lack
of difference in the year immediately following thinning as well
as 4 years after cutting.
fits were
two
other
influences
on
these losses is unknown and prob-
precipitation: one was measured, ably can only be definitely estabthe other was unmeasured
and can
lished by watershed studies which
only be discussed. The measured measure in terms of stream flow
effect was that the felled trees inthe integration of all the hydrologic
tercepted an appreciable fraction factors.
of the summer rainfall; sufficient,
The evaporation and transpirain fact, to reduce to zero the effect tion losses of moisture from the
of the thinnings on rainfall reach- soil were found to be unaffected
378
JOLENAL OP FORESTRY
by the thinning treatments. This
quantity
result is in support of the conclusion reachedin 'the earlier study
of harvest cuttings in mature
lodgepolepine (4). In that study
it was apparent that the effectsof
yields.•Vith respectto soil erosion,
and manner of
water
soil-moisture losses. It is concluded
that where climatic and forest con-
there has been no visible increase
ditions are similar to thosestudied,
sincethe thinning operations.The the thinning of dense pine stands
soil is still stableunder the present should appreciably increase water
rainfall and snow-melt conditions
yields A more completeand definithe cutting on autumn soil mois- to whichthe plots are subjected.
tive answer must await watershed
ture were produced not by decreasing the soil-moisture losses but
rather by increasing the rainfall
Conclusions
studies
where
the
answer
can be
obtaineddirectly in termsof stream
The thinning of dense young flow, and vaporization lossesfrom
reaching the soil. In the present lodgepole pine stands .is an ac- the snow surface can be more accustudy, the net rainfall was unaf- cepted, silvicultural method of
fected by the treatments and no stimulatinggrowth and shortening rately accounted for. Accelerated
erosion has not resulted from the
influence was found on the soil the rotation required for saw timmoisture of late summer.
ber or pole production. Concern- thinning operationsand it appears
The measured increases in snowing areas such as those in the high that the quality of stream flow
would not suffer from similar thinmelt rates could be detrimental to
mountains of Colorado and •Vyothe stream flow from some waterming where the primary value of nings where the climate and vegesheds and under some weather conthe land lies in its high yield of tation are such that the erosion
ditions.However,the •pecificsitua- quality water, the question arises hazard is low.
tion includingthe opportunityfor as to whether thesethinning operaLiterature
Cited
flood storage would determine the tions are beneficial or harmful to
1. CHUrCh, J. E. 1933. Snow surveydegreeof harm (or benefit) in such water yields.
ing; its principles and possibilities.
increasedmelt rates. Thinnings Indicative answersto thesequesGeog. Rev. 23:529-563.
which reducedbasal area by an tions covering a period of 4 years 2. CONNA•Gn•roN,C. A. 1935. The accumulation and rate of melting of snow
average 55 percent causedan addi- following thinning operationswere
as influenced by vegetation. Jour.
tional 2.2 inches of total snow-melt obtained from a study made on a
1%restry 33:564-569.
in a 21-dayperiod. Per day, this typical area along the Continental 3. WmM, 1:[. G. 1946. The design and
analysis of methods for sampling
amounted to an average 0.10 inch Divide where the erosion hazard is
micro-climatic
factors.
Jour. Amer.
but the maximumvalue on a par- low and the forest growth is dense.
Statis.
Assoc. 41:221-232.
ticular day was i•robably well in On experimental plots it was found 4.
and F,. G. DuNrOra).
1948.
Effect of timber cutting on water
excessof this figure.
that the thinning of dense young
available
for stream flow from
a
The above are the results with
lodgepolepine standsincreasednet
lodgepole pine forest. •J. S. Dept.
regard to the treatment effects on precipitation while not increasing
Agric. Tech. Bul. 968.
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