Stump Sprouting of Blue Oaks 19 Years
After Harvest1
Doug McCreary,2 Bill Tietje,2 and Bill Frost3
Abstract
In 1987, a study was initiated to determine how the sprouting of harvested blue oak (Quercus
douglasii) trees was affected by the season the trees were cut down, the height of the residual
stumps, and whether the stumps were protected by fencing. Five sites throughout the range of
this species were selected, and at each site 192 trees were harvested. After 19 years, 31
percent of the stumps had viable sprouts. The greatest differences among treatments were
between stumps protected with fencing and those unprotected from browsing animals, with
five times as many protected stumps surviving, compared to unprotected ones. Twice as many
90-cm-tall stumps than basal-cut stumps had surviving sprouts, and these sprouts were both
taller and had larger diameters. Differences among harvest dates were relatively small, but
there were significant differences among sites for most response variables. However, there
were no clear site attributes to explain these differences. These results indicate that if the
cutting of trees in densely stocked blue oak stands is necessary, it may be possible to alter
stand age structure and promote the establishment of young sprout-origin trees. The
replacement of even-aged stands with stands of varying ages may help mitigate the negative
impacts of inadequate natural regeneration.
Keywords: Browsing, Quercus douglasii, regeneration, stumps, thinning.
Introduction
Purpose of Original Study
For nearly a century there has been concern that several native California oak species
are not regenerating adequately to sustain populations (Jepson 1910). One of the
species identified as having poor regeneration is blue oak (Quercus douglasii), a
member of the white oak sub-genera endemic to the state (Bolsinger 1988, Muick
and Bartolome 1987, Swiecki and others 1997). This species has a wide distribution,
extending from the Siskiyous in the north to the Tehachapis in the south. It is
commonly described as forming a “bathtub ring” around the Central Valley, with
extensive stands along the lower elevation foothills of the east side of the coast range,
the west side of the Sierra Nevada, and the foothills of the transverse ranges.
Because natural regeneration of blue oak is not always adequate to replace
mortality (Bolsinger 1988, Muick and Bartolome 1987, Swiecki and others 1997), we
wanted to determine whether sprouting could successfully help reestablish a new
cohort of trees in cases where trees were going to be harvested to meet other
management objectives (firewood harvesting, thinning densely stocked stands, etc.).
If it could, then tree harvesting might result in multi-aged stands, partially mitigating
concerns about poor natural regeneration and helping to promote stand longevity.
1
An abbreviated version of the paper was presented at the Sixth California Oak Symposium: Today’s
Challenges, Tomorrow’s Opportunities. October 9-12, 2006, Rohnert Park, California
2
Natural Resources Specialists, University of California Cooperative Extension (UCCE).
33
Natural Resources Advisor, University of California Cooperative Extension (UCCE.).
333
GENERAL TECHNICAL REPORT PSW-GTR-217
One of the principal objectives of this project was therefore to identify variables that
influence sprouting. The variables examined in this study were stand location, season
of harvest, stump height, and protection of the sprouts from browsing animals.
Methods of the Original Study
In 1987, we identified five densely stocked blue oak stands throughout its range in
California to determine sprouting response in a wide range of environments. The
stand characteristics at each of these sites are shown in table 1. The five harvest sites
ranged from Glenn County in the north to San Luis Obispo County in the south.
Three were located in the foothills of the coast range, and two in foothills of the
Sierra Nevada. Four plots were established at each site. Each of these plots contained
at least 96 trees with a diameter breast height (DBH) of 7 cm or larger. Plots ranged
in size from 0.15 to 0.30 ha. Two plots per site were randomly chosen to be
“protected,” and two were left “unprotected.” Protection consisted of erecting a 1.8m-tall fence around the plot perimeter, effectively preventing access by deer, cattle,
and sheep and eliminating browsing of any new sprouts that emerged following
harvest. While the unfenced plots at all sites were grazed by domestic livestock,
season of use and stocking rate were not recorded.
Table 1—Site characteristics of the blue oak harvest sites1
Site
Mendocino
Glenn
Butte
Amador
San Luis
Obispo
Density
(stems/ha)
483
518
464
470
Basal
area
(m2/ha)
19
9
15
12
Average
DBH
(cm)
19.8
13.7
18.5
15.0
Annual
precip.
(cm)
93
55
52
46
Soil
depth
(cm)
60
70
45
100
485
11
14.7
55
42
Soil
texture
Silt loam
Clay loam
Clay loam
Grav.
loam
Grav.
loam
1
Precipitation data are from the nearest NOAA weather station. Soils information is from U.S. Soil
Conservation Service reports for Glenn, Butte, Amador, San Luis Obispo, and Mendocino Counties.
Within each plot, all trees were tagged with sequential numbers from 1 to 96.
Half of the trees within each plot were randomly selected for harvest. Of these 48
harvest trees per plot, 12 were chosen randomly for harvest in each of the four
seasons (summer, fall, winter and spring). Half of the 48 trees were selected to be cut
90 cm above the ground and half for cutting at the base of the tree.
All plots were evaluated in 1988, 1989, 1997, and 2006. Each harvested stump
was evaluated for presence or absence of living sprouts, number of sprouts, and
length of the tallest sprout. During the last two evaluation dates, the diameter of the
largest living sprout per stump was also recorded. Prior to the 2006 assessments,
sprouts were also assessed for browsing damage.
Results from the earlier evaluations for this study were presented in 1990 at
Symposium on Oak Woodlands and Hardwood Rangeland Management at Davis in
1990 (McCreary and others 1991), and in 2001 at the Fifth Symposium on Oak
Woodlands (McCreary and others 2002). The 1990 results indicated that harvest date
had relatively little influence on sprouting, while stump height had a large effect,
334
Stump Sprouting of Blue Oaks 19 Years After Harvest—McCreary
with a far greater percentage of taller stumps sprouting. Similarly, there were
significant differences in percent sprouting and average sprout growth among sites,
with the most favorable site having sprouts more than twice as long as sprouts from
the least favorable site. Fencing had relatively little effect on the incidence of
sprouting, but greatly influenced the lengths of longest sprouts, with fenced plots
having longer lengths. Incidence of sprouting was negatively correlated with DBH.
In the 2002 paper, we reported that site and stump height still had significant
impacts on sprouting response 10 years after harvest. However, unlike the
evaluations in 1988 and 1989, protection of the sprouts was the most influential
variable with nearly four times as many protected stumps having living sprouts (54
percent) as unprotected stumps (14 percent).
Methods of the 2006 Evaluation
In summer 2006, all plots were revisited and all stumps that had living sprouts were
measured. After 19 years, not all the original stumps that failed to sprout or whose
sprouts had died could be located, due to decay and deterioration. We are confident
we located all stumps with living sprouts because they were easy to see and find,
especially with the assistance of plot maps that showed the location of all of the
original stumps. Each stump with living sprouts was assessed for the number of
sprouts emerging from the stump, the length of the longest living sprout, and the
diameter of the largest living sprout at its base. The length of the longest (tallest)
sprout was the distance from the base of the stump to the tallest point in the sprout
clump. Except for sprout diameter, these same variables had also been recorded
during each of the three previous assessments.
As it was for the earlier assessments, data was analyzed using analysis of
variance for a doubly nested randomized block design, with sites as the main plots,
fences as the sub-plots, and combinations of harvest dates and stump heights as the
sub-sub plots. However, due to missing data resulting from sprout mortality, by 2006
(as well as for the 1997 data) a simplified model was required to analyze all
variables. Because few differences were found for harvest dates when this study was
evaluated in earlier assessments, the simplified model we used combined responses
among harvest dates.
Results
Survival (the percentage of stumps with living sprouts)
Although sprout survival increased slightly during the first two years of the study
(table 2), between 1989 and 1997 it dropped markedly. Between 1997 and 2006 there
was a continued reduction in survival, but at a far diminished rate. Average survival
for the entire study dropped from 34 percent at time of the last assessment in 1997, to
31 percent in 2006 (table 2). The greatest reduction in survival occurred at the San
Luis Obispo site, dropping from 38 percent in 1997 to 32 percent in 2006.
However, the reduction in survival for all sites combined was not equally
apportioned among treatments, with the largest reduction in survival for stumps in
unfenced plots. In 1989, average survival of unfenced stumps was 59 percent. By the
1997 assessment, this had dropped to 14 percent. By 2006 it had gone down further,
to 11 percent, a reduction of more than one-fifth (table 4). In contrast, average
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GENERAL TECHNICAL REPORT PSW-GTR-217
survival of fenced stumps was 67 percent in 1989, 54 percent eight years later, and 51
percent in 2006. There were also significant differences between the survival of basal
stumps and those cut at 90-cm, with the taller stumps having more than twice the
average survival of those cut at ground level (42 percent vs. 20 percent) (table 5).
This was consistent with results from measurements taken nine years earlier.
Table 2—Average survival (percent) for the five sites for each of the 4 measurement periods 1.
Site
Mendocino
Glenn
Butte
Amador
San Luis
Obispo
Total
1988
62 a
34 b
63 a
59 a
73 a
1989
72 a
37 b
65 a
65 a
76 a
1997
29 a
28 a
44 a
32 a
38 a
2006
26 a
28 a
40 a
29 a
32 a
58
63
34
31
1
Values within a column are significantly different (P< 0.05) if they are followed
by a different letter (Fisher’s protected LSD test).
Table 3—Average response of different sites 19 years after harvest 1.
Site
Mendocino
Glenn
Butte
Amador
San Luis
Obispo
Sprout
survival (%)
26 a
28 a
40 a
29 a
32 a
Number of
sprouts
15.0 a
7.8 b
3.0 c
3.9 c
8.2 b
Sprout
length (m)
1.9 a
2.3 ab
4.1 c
3.4 bc
2.7 ab
Sprout
diameter (cm)
2.8 a
4.6 ab
7.7 c
6.7 bc
3.8 ab
1
Values within a column are significantly different (P< 0.05) if they are followed
by a different letter (Fisher’s protected LSD test).
Table 4—Average response of different fencing treatments 19 years after harvest 1.
Fencing
Treatment
Unfenced
Fenced
Sprout
survival (%)
11 a
51 b
Number of
sprouts
9.5 a
5.7 b
Sprout
length (m)
2.1 a
3.6 b
Sprout
diameter (cm)
3.7 a
6.5 b
1
Values within a column are significantly different (P< 0.05) if they are followed
by a different letter (Fisher’s protected LSD test).
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Stump Sprouting of Blue Oaks 19 Years After Harvest—McCreary
Table 5 —Average responses of different stump heights 19 years after harvest 1.
Stump height
Basal
90 cm
Sprout
survival (%)
20 a
42 b
Number
of sprouts
6.1 a
9.1 b
Sprout
length (m)
2.3 a
3.5 b
Sprout
diameter (cm)
4.4 a
5.8 b
1
Values within a column are significantly different (P< 0.05) if they are followed
by a different letter (Fisher’s protected LSD test).
Number of Sprouts
The average number of living sprouts per living stump declined steadily over time, a
trend that continued into 2006. The average number of sprouts per living stump was
30.6 in 1989, 8.4 in 1997, and 7.6 in 2006. There were significantly more sprouts per
stump in unfenced plots (table 4), and taller stumps had significantly more sprouts
than basal stumps (table 5). There was also a significantly larger number of sprouts
on stumps at the Mendocino site than at the other four sites (table 3).
Sprout Length
The lengths of the longest sprouts were significantly greater in fenced than in
unfenced plots (table 4); lengths were also greater for taller stumps (table 5). There
were significant differences in sprout length among harvest sites, with those in
Mendocino being significantly shorter than those in Butte or Amador counties (table
3).
Sprout Diameter
Sprout diameter and sprout length followed a similar pattern. There were
significantly larger diameter sprouts for taller stumps (table 5) and for stumps within
fenced areas (table 4). As was the case for sprout length, sprouts in Mendocino were
significantly thinner than those in Butte or Amador counties (table 3).
Interactions
There were several interactions among response variables that warrant comment. For
survival, there was a significant interaction between fencing and stump height. Even
though the taller stumps had greater survival in both fenced and unfenced plots, the
magnitude of the difference was much greater in the fenced plots.
There were also significant interactions for height and diameter. For height,
there was a significant interaction between site and stump height and between site
and fencing treatment. At all sites, taller stumps grew more than shorter stumps and
fenced stumps grew more than unfenced stumps. However the magnitude of these
differences varied considerably among sites, ranging from only being slightly greater,
to being more than double (the heights were more than twice as tall).
The interactions for diameter followed a similar pattern. At all sites except one
(Glenn), shorter stumps had smaller diameters than taller stumps. At the Glenn site,
however, the shorter stumps had slightly larger average diameters, though these
differences were not significant. At all sites except one (Amador), fenced stumps had
larger diameters than unfenced stumps. At this site, however, the diameters of stumps
in fenced plots and unfenced plots were very similar, but were slightly larger—
though not significantly so—in unfenced plots.
337
GENERAL TECHNICAL REPORT PSW-GTR-217
Finally, there was also a significant interaction between fencing and stump
height for diameter. Taller stumps had greater diameters both within and outside
fences, but these differences were greater in the unfenced plots.
All in all, the significant interactions indicate that the responses to stump height
and fencing at the different sites were quite similar—with taller stumps and fenced
stumps performing better than shorter or unfenced stumps—but that the degree of
difference between these treatments can and does vary somewhat by site.
Discussion
Sprouting is common for many species of hardwoods and it is likely that many of the
oak trees that are alive in California today originated from sprouts that grew from
stumps after the top of the tree was killed by fire or felling. For example, studies
reveal that harvested stands or stands destroyed by fire were replaced by multiplestem trees of similar age, presumably from sprouting (McClaran and Bartolome
1989, Mensing 1992). Sprouting is apparently an adaptive strategy that allows trees
to remain alive and reoccupy a site following disturbance.
The ability of oaks to sprout from their base following death of the aboveground
portion of the tree varies by species (Longhurst 1956), size of the individual tree
(Johnson 1977), and environmental conditions at the site (Hannah 1987). Generally,
sprouting is greater for evergreen or live oaks than for deciduous oaks, for smaller
diameter stumps, and for trees growing in moister environments with more light.
While blue oak is commonly thought of as a weak sprouter compared to tanoak and
California black oak (McDonald 1990), the original assessment of the 960 trees
harvested in this study found that a relatively large proportion (63 percent) had living
sprouts two years after harvest (McCreary and others 1991). In general, the smaller
stumps tended to sprout more, but we detected no differences in sprouting among the
four seasons of harvest, in contrast to Longhurst (1956) who reported higher
sprouting for blue oaks harvested in winter. Standiford and others (1996) also
reported relatively high sprouting for blue oak in the northern Sacramento Valley,
with 54 percent of the trees having sprouted, even though many stumps had
originally been treated with herbicides to prevent re-growth.
One of the initial goals of this study was to identify variables that influence
sprouting so that when harvesting is done as part of other management objectives, it
may be possible to promote sprouting in the resultant stumps. Encouraging sprouting
could then be used to promote stand longevity and even if natural regeneration was
insufficient to replace mortality under natural circumstances, lengthening the life of
stands could allow more time for seedling recruitment to occur. However, because
there would be more ladder fuels, such stands may be more susceptible to
catastrophic fire and they would remain vulnerable to anthropogenic pressures
including development, agricultural conversions, and firewood harvesting. It is also
worth mentioning that many blue oak stands are regenerating adequately and sprout
regeneration would not be necessary or encouraged.
It was further speculated that opening the stands up through thinning, and
fencing out grazing and browsing animals, might also promote natural regeneration,
though these effects were not officially evaluated in the study. However, previous
research has demonstrated that thinning of blue oaks can result in increased acorn
production (Standiford and others 1999) that could lead to greater regeneration of
338
Stump Sprouting of Blue Oaks 19 Years After Harvest—McCreary
seedlings. We also did observe many vigorous naturally regenerating oak seedlings in
fenced plots, especially at the plot in Mendocino County. In contrast, few seedlings
were observed in unfenced plots.
The data from this study suggest that sprouting can be used to alter age structure
in stands since, after 19 years, a sizeable number of the harvested trees have living,
vigorous sprouts. However, it is also clear that sprouting is influenced by a number of
variables, especially protection from browsing animals using fencing. In most
locations, nearly two decades of exposure to browsing animals resulted in high sprout
mortality and greatly reduced sprout growth. This effect was particularly evident at
the plot in Mendocino County where high populations of deer from a resident herd,
and additional pressures from a flock of sheep, resulted in the sprouts on all but one
stump being killed in the unfenced plots. In addition to causing increased mortality at
all sites, browsing in unfenced plots reduced sprout size, but tended to increase the
number of sprouts per stump. Repeated browsing apparently caused the stumps to
continue producing more young sprouts and interrupted the normal thinning response
observed inside the fenced plots. The data also demonstrated that if the sprouts
survive for a decade, there is a high likelihood they will remain alive for at least 19
years.
While there were differences in sprout growth among sites, it is not clear what
site variables were likely responsible. The site with the highest average annual
precipitation (Mendocino) had the smallest average sprout length and diameter.
Perhaps growth was influenced by environmental conditions associated with different
elevations, since the sites with greatest height and diameter growth (Butte and
Amador) were at the two lowest elevations. It is also possible that stump diameter
adversely affected sprout response since the stumps at the Mendocino site had the
largest average diameters.
Another concern we had after the study was in place for only two years was that
the tall stumps from which sprouts were growing might decay more rapidly, causing
the sprouts to die. After 19 years there is no evidence that this is a serious problem,
and fewer 90-cm stumps had sprout mortality between 1997 and 2006 than did basalcut stumps. By 2006, the average percent sprouting for 90-cm stumps was more than
double that for basal-cut stumps, and the average height and diameter of sprouts from
taller stumps were significantly greater than for sprouts growing from basal-cut
stumps. It thus appears that retaining taller stumps during harvest promotes greater
sprouting success.
Finally, it should be mentioned that there are other potential drawbacks to
relying exclusively on stump sprouting to regenerate harvested stands. While a
substantial number of harvested trees in this study did sprout, we would expect the
percentage to go down with each harvest cycle. That is, trees likely cannot be
repeatedly cut down and continue to sprout. Also, sprout-origin trees are genetically
identical to the harvested trees so there is no additional genetic diversity associated
with sexual reproduction as there would be for acorn-produced trees. In forested
stands where the environment is changing over time, as many predict from global
climate change models, replacing trees from sprouting—rather than from acorns—
may make it less likely that species will adapt to these changes.
339
GENERAL TECHNICAL REPORT PSW-GTR-217
Conclusions
Results from this study indicate that even after 19 years, over 30 percent of harvested
trees have viable sprouts. Whereas there was considerable attrition in sprout survival
between assessments in year 2 and year 10, sprout survival of all originally harvested
trees only went down by 3 percent in the subsequent 9 years. The majority of stump
death during this interval appeared to be caused by repeated browsing damage since
the increase in mortality outside fenced exclosures was approximately four times that
inside. In areas where browsing pressures are intense, either from livestock or deer, it
is critical to protect sprouts.
Since the study was established in 1987, season of harvest has had relatively
little impact on sprouting. However, there was considerable variation in survival of
sprouts, as well as sprout growth, among harvest sites. Sprouts from stumps cut at 90
cm continued to perform better than those cut at ground level, and there was no
indication that these stumps deteriorated rapidly or that their sprouts died. All in all,
this study indicates that if blue oak stands are thinned for other management
objectives, sprouting can help these stands replace themselves. Sprouting can alter
stand structure and promote the establishment of a new, younger, cohort of trees. In
this manner, it may prolong the life of stands, even when there is poor natural
regeneration.
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