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Coast Redwood Responses to Pruning
Kevin L. O'Hara 1
Abstract
A large-scale pruning study was established in the winter of 1999 to2000 at seven different
sites on Green Diamond Resource Company forestlands in Humboldt County. The objective
of this study was to determine the effects of pruning on increment, epicormic sprouting, stem
taper, heartwood formation, and bear damage on these young trees. Pruning treatments varied
pruning severity and were usually applied in conjunction with thinning treatments. Trees were
assessed six years after pruning. Basal area increment decreased with increasing pruning
severity but results were inconsistent from one study site to another. Height increment was
unaffected by pruning. Six-year volume increment results resembled those for basal area
increment: heavier pruning sometimes resulted in lower increment. Additionally, the negative
effects of pruning on tree increment were probably short-lived in redwood because of the fast
growth rates of this species. The number of epicormic sprouts was generally unaffected by
pruning severity with the notable exception of the most severe pruning treatments. By year
six, the number of sprouts was no different in the unpruned treatments than in most pruned
treatments. The exception was the severe crown removal that left only approximately 15
percent residual live crown length. Epicormic sprouting does not appear to be a deterrent to
pruning in redwood. Tree stem taper was also unaffected by pruning severity. Heartwood
formation was expected to increase with pruning severity. However, no effects were evident
in these data. Apparently, the greater heartwood expected in more severely pruned treatments
was obscured by the same factors that minimized treatment effects on increment: in the six
years following treatment, the pruned trees had rebuilt crown foliage and required similar
sapwood for water transport as unpruned trees. Bear damage was observed at four of the
seven study sites and was severe in several plots. However, no trends were evident relative to
pruning treatment. In summary, pruning that leaves residual crown lengths of 40 to 60 percent
should result in minimal levels of epicormic sprouting and no effects on tree increment.
Key words: Sequoia sempervirens, forest pruning, timber stand improvement, silviculture,
wood quality
Introduction
Forest pruning is a common means to enhance wood quality in young forest
stands. By removing lower branches, wood is formed over the branch wound that is
clear and usually straight-grained. This clearwood is generally expected to produce
higher grade wood products and earn a premium. Pruning may also improve stem
taper from pruned logs and enhance heartwood development. In some species, such
as coast redwood (Sequoia sempervirens), heartwood has desirable appearance and
decay resistance properties which make it more valuable than sapwood.
1
Professor of Silviculture, University of California Berkeley, 137 Mulford Hall #3114, Berkeley, CA
94720-3114. ( kohara@berkeley.edu).
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GENERAL TECHNICAL REPORT PSW-GTR-238
Although the effects of pruning forest trees are generally positive, it can lead to
excessive epicormic branch production in some species, including coast redwood.
Epicormic sprouts respond to increased light or temperature on the pruned stem
resulting in a profusion of branches. These can negate the value of the pruning
treatment by producing wood quality defects in the clear wood produced in the outer
tree boles.
No previous study has provided an analysis of the effects of pruning on coast
redwood. This paper describes a six-year study to look at the effect of pruning on tree
increment, stem taper, epicormic sprouting, heartwood development, and bear
damage.
Project description
Pruning plots were established during the winter of 1999 to 2000 in seven stands
(study sites) that received pruning treatments in the fall of 1999. All seven study sites
were located in Humboldt County, California on Green Diamond land. These plots
were designed to measure the effects of pruning on individual trees. Hence plots were
assemblages of approximately 30 trees per treatment rather than being of fixed area.
All study sites included control plots and some included more than one pruning
treatment. Of the seven study sites, all but two were designed to represent standlevel, operational pruning treatments. The other two study sites – 299 Cutoff and Mline – provided variable pruning treatments over small areas, but not on an
operational scale. At time of initial measurement, trees were tagged, and marked at
dbh. Measurements taken at time of plot establishment included dbh, height, height to
live crown after pruning, height to base of live crown on unpruned trees, counts of
epicormic sprouts, and some upper stem diameter measurements (either at 2.7 or 5.5
m height). Additionally, trees of common stump origin were noted. Tree
characteristics at first measurement are presented in table 1.
The plot remeasurements and data analysis in 2001/02 and 2006 included studies
of epicormic sprouting, increment, heartwood formation and stem taper. Analysis of
variance was used to compare treatment means within study sites. When ANOVA
indicated significant differences between treatment means, Tukey’s HSD multiple
range tests were used to identify significantly different means.
Growth response studies
Trees were remeasured in 2001 and 2002 and spring 2006 for height and dbh.
Nearly all trees were also measured for diameter at tree height of 2.7 m (9 ft). The
original plot measurements in 1999 to 2000 included height and dbh measurements,
but not all trees were measured for the 2.7 m upper stem diameter measurement.
Cubic volumes were estimated assuming the tree base to bh was a cylinder, bh to 2.7
m a cone frustum, and above 2.7 m as a cone for smaller trees. Equations from
Krumland et al. (1977) were used for larger trees. Relative growth was calculated as
the 6-year volume increment divided by volume in 1999 to 2000.
Results resemble those from pruning studies with other species except for an
apparent lack of sensitivity to more severe pruning treatments in redwood than has
been observed in other pruning studies (table 2). In other species, a strong decrease in
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Coast Redwood Responses to Pruning
increment with increasing pruning severity is usually found (O’Hara et al. 1995). In
terms of plot comparisons, there were abnormalities in both the 299 Cutoff and MLine study sites. At 299 Cutoff, the control plot had much greater increment than
expected. At M-Line, the 15 percent residual treatment had greater increment than
expected.
Table 1—Average pruning heights, initial tree heights and diameters by plot and study site
(pruning heights for control treatments are live crown heights at beginning of study) Average
residual crown lengths show actual average crown lengths following pruning.
Basal area increment/tree was largely unaffected by pruning intensity. Many
study sites experienced increased basal area increment following pruning as
compared to controls (Maple Creek, Fortuna, and to a lesser extent M-150).
However, at 299 Cutoff, the control basal area increment/tree was considerably
greater than the pruned treatments. At M-line, no trend was evident and the 15
percent residual crown treatment had virtually the same basal area increment as the
control. This suggests a combination of site irregularities across plots and the
possible influence of more productive clones on some plots. For example, it is
possible that one plot could be dominated by the sprouts of one stump that is very
productive, whereas the next plot is dominated by a less productive sprout clump. An
additional factor was pretreatment tree size variability and its affect on growth.
Growth was therefore also expressed as “relative growth” by dividing 6-year basal
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GENERAL TECHNICAL REPORT PSW-GTR-238
area increment per tree by pretreatment basal area. This analysis produced
substantially different results than the absolute basal area increment/tree analysis. For
the 299 Cutoff study site, the highest relative growth was in the 30 percent residual
treatment whereas at M-line it was the 60 percent residual.
There were few significant differences in height increment among study plots.
This result was expected as only very severe pruning treatments generally affect
height increment. Significant differences were found at other study sites between
control and pruned height increment, but these may have been the result of bear
damage preferentially affecting taller trees thereby reducing average increment.
Cubic volume increment followed similar trends as basal area and height
increment (fig. 1, table 2). Statistical significance was only found at Mitsui and these
results were influenced by bear damage. Relative volume increment (volume
increment per unit of pretreatment volume) was statistically significant for some
treatments/study sites. For example, at 299 Cutoff, the 30 percent residual treatment
was significantly greater than the 15 percent and 35 percent treatment. This suggests
an affect related to clonal patterns within measurement plots rather than a positive
effect of that particular pruning treatment.
In summary, pruning apparently has relatively minor effects on increment in
coast redwood as represented by basal area increment, height increment, and volume
increment. There is some evidence of reduced increment with the most severe
pruning treatments, but for the operational range in pruning severity (> 35 percent
residual live crown) there is no effect 6 years after pruning.
Figure 1—Cubic volume response after six years at 299 Cutoff.
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Coast Redwood Responses to Pruning
Table 2—Six-year cubic volume increment and relative growth rates. Relative growth rates
were volume increment divided by volume at time of pruning. Means followed by common
letters within a study site were not significantly different (α=.05). Standard errors (SE) are
given in parentheses following means.
Epicormic sprout studies
Approximately 10 trees per plot were randomly selected in 2002 for epicormic
sprout sampling. For pruned trees, the pruned section of the bole was divided into six
sections and one section was randomly selected for sampling and marked with paint.
For example, a tree pruned to 3 m would have six 0.5 m sections and one of these
sections would be randomly selected for measurement. In these sections, all sprouts
were counted and the highest and lowest sprouts in the section were measured for
length and caliper (diameter at base). The estimated number of sprouts per tree was
the section count multiplied by six. Unpruned trees were sampled in a similar way:
the section of bole, equal to 5.5 m or the highest level pruned in adjacent pruned
plots, was divided into six sections and one section was randomly selected for
measurement. On control plot trees, only sprouts that originated at the time of the
pruning or after the pruning were counted. This may be a potential source of error as
sprout age was difficult to determine. Caliper measurements on small sprouts may
also be biased because the presence of scale-like leaves at the base of these sprouts
may have exaggerated their diameter. The presence of leaves on larger sprouts had
little effect on caliper measurements. This sampling procedure was followed in both
2002 and 2006. The same stem section was therefore sampled at each measurement.
Because of sprout mortality between 2002 and 2006, however, the same sprouts may
not have been measured in both years.
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GENERAL TECHNICAL REPORT PSW-GTR-238
The estimated number of sprouts per tree was unaffected by pruning severity
except at most severe pruning (table 2). Numbers of sprouts declined from 2002 to
2006. Figure 2 shows the relationship between estimated number of sprouts and
residual crown length for 2006 at 299 Cutoff. A flat trend in number of sprouts is
apparent until a sharp increase with the 15 percent pruning. After 6 years, there were
no significant differences in numbers of sprouts in any of the study sites except the
most severe treatments at the 299 Cutoff and M-Line. However, variance was large in
these samples because some trees appeared to be much more prone to sprouting than
other trees in the same treatment. There may also have been a clonal component to
these results where one clone dominated a plot and had a disparate influence on the
plot average. In a separate analysis of these data, O’Hara and Berrill (2009) showed a
clonal effect to sprouting where some clones were more likely to produce epicormic
sprouts than others.
Figure 2—Estimated number of sprouts per tree in 2006 for treatments at the 299 Cutoff study
site. The "5.5 m or 35 percent" residual crown length treatment was pruned to 5.5 m (18 ft) lift
or no less than 35 percent residual live crown.
Larger sprout length and diameter is indicative of the size of the resulting wood
quality defect, and probably indicative of sprout persistence over time. Larger sprouts
were found on the most severely pruned trees which suggested these sprouts are a
more important component of photosynthesis needs and are therefore likely to
persist. This result is consistent with other species including giant sequoia (O’Hara et
al. 2008). In this study, more severe pruning treatments resulted in a large increase in
average sprout size in most study sites, and particularly in the 299 Cutoff and M-Line
study sites that included severely pruned trees. For example, at 299 Cutoff, the
average sprout caliper in the 15 percent residual treatment was five times the average
in the 60 percent residual treatment. Because of the decline in number of sprouts
from 2002 to 2006 (table 3), the sample sizes for sprout measurements also declined
during this period. This contributed to the high variation in sprout sizes, the lack of
consistent trends in sprout size with increasing pruning severity, and the lack of
statistical significance when means were quite different.
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Coast Redwood Responses to Pruning
It should be noted that trees in unpruned plots also produced epicormic sprouts.
Some of these plots received precommercial thinning treatments at the time of
pruning while others were thinned up to three years before pruning. This indicates
that some background level of sprouting is present on young redwood trees. In
Table 3—Epicormic sprout data for pruned and unpruned plots in 2002 and 2006. The
Carlotta plot D was not measured for epicormic sprouts because of extensive bear damage.
Common lower case letters within study sites denote means that were not significantly
different. Standard errors (SE) are given in parentheses following means.
unpruned trees, this sprouting is not a major consideration since clearwood is not an
expectation from these plantations. In pruned trees, development and persistence of
sprouts, whether background level or not, is likely to cause wood quality defects and
reduce potential returns from a pruning investment. However, the results in 2006
indicate no difference in level of sprouting or persistence of sprouts between the
unpruned and all but the most severely pruned trees
Stem taper
A measure of stem taper was calculated by dividing tree diameter at 2.7 m (9 ft)
by tree diameter at bh (1.37 m). Note that some plots were not measured at 2.7 m, or
experienced so much bear damage by 2006 that samples were too small for analysis.
Pruning generally reduces stem taper through reduced radial increment at the base of
tree and only a minor effect further up the stem (O’Hara 1991). Results from this
study showed virtually no affect of pruning on stem taper. 299 Cutoff showed
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GENERAL TECHNICAL REPORT PSW-GTR-238
increased taper in the control but M-Line had the greatest taper in the most severe
pruning treatment. Only the results at 299 Cutoff were statistically significant.
These results suggest that pruning has no effect on stem form as represented by
the measure of stem taper used here. An upper stem diameter measurement higher on
the stem may have yielded a stem form measure more sensitive to pruning. These
results were also affected by the rapid growth rates and recovery of trees following
pruning that may have obscured affects of pruning on stem taper. Additionally, all
plots were thinned at about the same time as the pruning treatments. Since thinning
generally increases stem taper, these two opposite effects were both affecting stem
form of study trees.
Heartwood formation
Sample trees were cored at time of establishment of this study, and again in 2006.
These trees were randomly selected. Cores were removed at bh and were
immediately measured for sapwood thickness based on the visual translucence of the
sapwood. Sampling was limited to the same 10 trees/plot sampled for epicormic
sprouts.
Reducing the size of the crown reduces the transpiration requirements of the tree
and also the need for sapwood conducting tissues. However, contrary to expectations,
pruning had no effect on heartwood formation in this study.
Possible explanations for the lack of a positive heartwood response in these data
include the small sample sizes. The only post-pruning significant response was at
Maple Creek where sample sizes were also highest. However, when ignoring
statistical significance, these results do not reveal the expected trends of increasing
percent heartwood with increasing pruning severity. At both 299 Cutoff and M-Line
the control treatments were among the highest in percent heartwood and heartwood
expansion. A more likely explanation is the rapid growth of these trees over the 6
year period has allowed them to rebuild crowns (through both height growth and
epicormic sprouting) therefore requiring comparable sapwood conducting tissues
regardless of treatment. Affecting heartwood at final harvest would apparently
require repeated pruning or more severe pruning to significantly reduce crown size at
end-of-rotation.
Bear damage
Bear damage was noted in plots at both the 2002 and 2006 remeasurements. The
circumference of the damage around the base the tree was estimated to the nearest 10
percent. Bear damage was found in four of the seven study sites: M-Line, Carlotta,
M-150, and Mitsui. The worst damage was at Carlotta and Mitsui, but concentrated
in several plots. In the case of the Mitsui study site, the plots were considerable
distance from each other. There were also no patterns evident with regard to pruning
treatment. At Carlotta, two of the pruning treatments experienced damage to greater
than 50 percent of the trees whereas the control and another pruning treatment
experienced relatively light damage. At M-150, the percent of damaged trees was
constant across all treatments. The percent of bole circumference damaged is of
limited value because of healing that might have taken place since the damage
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Coast Redwood Responses to Pruning
occurred that reduced the visible damage and the percent values were just visual
estimates. Future studies to ascertain the effects of silvicultural treatments on bear
damage should involve much larger treatment areas and larger measurement plots
than those used in this study.
Summary and recommendations
The response of coast redwood to pruning varies from typical responses of
conifers. The typical decreasing increment with increasing pruning severity pattern
was not observed in these study plots despite the inclusion of very severe pruning
treatments. Instead, redwood growth is less sensitive to pruning and reductions in
crown size than other conifers.
Epicormic sprouting occurred following pruning and resembled patterns for other
species. These included an initial sprouting response after pruning but a decline in
sprouting over time. Six years after pruning, the number of sprouts in the moderate
pruning treatments had declined to levels that were comparable to the unpruned
controls. Pruning at moderate levels (leaving 40 to 60 percent live crown) had no
effect on epicormic sprouting.
Pruning did not affect stem taper based on the ratio of diameter at 2.7 m (9 ft)
and diameter at bh. A taper measure based on a higher stem diameter measure may
have produced results that were consistent with other pruned conifers. Additionally,
all plots were also thinned: pruned trees were therefore also reacting to the effects of
thinning which generally increases taper. Similarly, there was no effect of pruning on
heartwood development. The expectation was an expansion of heartwood because of
reduced sapwood requirements for smaller crowns in pruned trees. The rapid
recovery of foliage after pruning that contributed to the rapid growth recovery
apparently required sufficient sapwood conducting tissues and negated the effect of
crown reduction on heartwood development.
Several pruning plots have experienced major damage due to bear girdling. There
was no apparent pattern to this damage with regard to pruning treatment. However,
these small plots were too small to assess bear preferences for pruning treatments.
Without the threat of excessive bear damage to pruned trees, pruning would
appear to be a viable treatment to enhance wood quality in coast redwood. The longterm effect of pruning on epicormic sprouting is minimal with moderate pruning
regimes. Pruning has little effect on redwood volume or height increment making this
species one of the least sensitive conifer species to pruning.
References
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O'Hara, K.L. 1991. A biological justification for pruning in coastal Douglas-fir stands.
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O'Hara, K.L.; Parent, D.R.; Hagle, S.K. 1995. Pruning eastern Cascade and northern
Rocky Mountain species: biological opportunities. In: Hanley, D.P.; Oliver, C.D.;
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GENERAL TECHNICAL REPORT PSW-GTR-238
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