Species Characteristics and Stand
Structure of Quercus garryana and Q.
pyrenaica Woodlands in the Mediterranean
Regions of California and Spain1
Michael G. Barbour, 2 Stephen Barnhart, 3 Emin Ugurlu, 4 and
Daniel Sanchez–Mata 5
Abstract
Quercus garryana in western North America has a similar lack of recent regeneration as does
Q. pyrenaica in Spain. Both species are deciduous white oaks whose ranges extend south to
Mediterranean-type climates from more northerly temperate regions. Although the degree of
genetic similarity and phylogenetic relatedness between the species is unknown, their degree
of ecological convergence is remarkable in terms of geographical and elevational range, and
in population characters such as density, canopy cover, basal area, frequency, and
physiognomy. In Spain, both young stands (oldest individuals up to 90 years of age) and oldgrowth stands (individuals up to 368 years of age) were sampled. In California, two young
stands and four intermediate-age stands (oldest individuals about 200 years of age) were
sampled. The six California stands consistently had zero regeneration for the past 25 to 50
years and lower regeneration than expected for the past 50 to 100 years. Seven young Q.
pyrenaica stands consistently showed regeneration below expectation during the past 30
years, and four old-growth stands exhibited lower regeneration than expected for the last 75
years. There was little evidence of high episodic past regeneration in any of the stands that
would suggest the current period of low regeneration is a recurring pattern.
Keywords: Age structure, age-dbh relationship, California, Coast, convergence, forest
ecology, Garry oak, melojo, montane vegetation, regeneration, physiognomy, succession.
Introduction
Poor regeneration among four species of white oaks in California has been described
by a number of range ecologists since the 1960s (e.g., White 1966, Griffin 1971).
More recent summaries by Allen-Diaz and Holzman (1991), Allen-Diaz and others
(2007), Bartolome and others (1987), Bolsinger (1988), Muick and Bartolome
(1987), Pillsbury and others (1997), and Waddell and Barrett (2005) concluded that
poor regeneration (specifically the transition from seedling to sapling stages) has
characterized the majority of Quercus douglasii, Q. engelmannii, Q. garryana, and
Q. lobata stands for at least the past five decades.
1
An abbreviated version of this paper was presented at the Sixth California Oak Symposium: Today’s
Challenges, Tomorrow’s Opportunities, October 9-12, 2006, Rohnert Park, California.
2
Professor, Plant Sciences Department, University of California, Davis, CA 95616. e-mail:
mgbarbour@ucdavis.edu.
3
Professor, Biological Sciences Department, Santa Rosa Junior College, Santa Rosa, California 95401.
e-mail: sbarnhart@santarosa.edu.
4
Dr., Biological Sciences Department, Celal Bayar University, Manisa, 45100, Turkey. e-mail:
emin.ugurlu@bayar.edu.tr.
5
Professor, Biologia Vegetal II, Universidad Complutense, Madrid E-2804, Spain. e-mail:
dsmata@farm.ucm.es.
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GENERAL TECHNICAL REPORT PSW-GTR-217
Among the hypotheses that have been offered as an explanation for poor
regeneration among these oaks are: competition between oak seedlings and exotic
annual forbs and grasses; browsing of seedlings by deer and burrowing rodents;
browsing of seedlings by domesticated livestock; absence of surface fires;
compaction of soil by livestock, preventing seedling establishment; absence of
sufficient shade for seedling survival; and invasion by other trees (Pseudotsuga
menziesii in the case of Q. garryana woodlands; see Barnhart and others (1987,
1991). At this time, some supporting evidence exists for each hypothesis, but no
overwhelming evidence for any one hypothesis.
Our objective in this paper was two-fold: to quantify the degree to which
regeneration in stands of Q. garryana Dougl. ex Hook. (Garry oak or Oregon white
oak) departs from values expected in a self-sustaining stand; and to see if a similar
departure characterizes an ecologically similar taxon — Q. pyrenaica Willd.
(melojo), which grows in Spain in a similar Mediterranean-type climate.
Garry oak is a white oak (i.e., it is in the subgenus and section Quercus). It is
winter-deciduous, but tardily so (marcescent). Its distribution is mainly within the
coast ranges, extending from San Francisco Bay north to Vancouver Island, covering
15 degrees of latitude (Burns and Honkala 1990). Within this span, the species occurs
from sea level to 1,200 m elevation, but most commonly > 450 m. Mean annual
temperature ranges from 8 to 18° C, and individuals are capable of tolerating hard
frosts to -34° C. Although annual precipitation ranges from 17 to 263 cm, this species
is considered to be drought-tolerant. Q. garryana is rated as intermediate in shade
tolerance, although it is capable of reproducing in its own shade. Trees grow on
Alfisols, Inceptisols, Mollisols, and Ultisols, all of moderate to strong acidity (pH
4.8-5.9). The species’ range covers approximately 600,000 ha, one-third of which lies
within the state of California (Bolsinger 1988).
The elevation, climate, and associated species place Garry oak as an element of
the mixed evergreen forest, defined by Sawyer and others (1988) and Barbour and
Minnich (2000) as an ecotonal forest sandwiched between broadleaf-dominated
foothill woodland below and conifer-dominated montane vegetation above. Other
broadleaf tree species that characterize this forest include Acer macrophyllum,
Quercus chrysolepis, and Umbellularia californica; conifers most often include
Pseudotsuga menziesii. Stands may have a nearly closed canopy, either consisting of
Q. garryana alone or mixed with conifer and broad-leaved evergreens in two distinct
height classes, with few shrubs and herbs beneath. Alternatively, stands may have
scattered trees with open canopies and a rich herbaceous understory. Mature trees
reach 37 m tall and >200 cm dbh, attaining ages >300 years old. Sawyer and KeelerWolf (1995) and Sawyer and others (2007) have formally defined and characterized a
Garry oak alliance that includes the geographic and compositional variations
described above, but they have not yet described all of the associations.
Quercus pyrenaica Willd. (melojo) is a marcescent white oak tree species
widely distributed throughout the Iberian Peninsula. Its northern limit extends to
western France and its southern limit to the Rif Mountains of Morocco (Costa and
others 1998, Rivas-Martínez and Sáenz 1991). It is almost exclusively restricted to
siliceous substrates at elevations of 400 to 1,800 m (2,000 m in the Rif Mountains),
receiving 400 to 1,200+ mm annual precipitation in meso- and supra-Mediterranean
thermotype territories (a temperate bioclimate with a sub-Mediterranean character in
some northern areas; the same bioclimate applies to the range of Q. garryana)
474
Species Characteristics and Stand Structure of Quercus garryana and Q. pyrenaica Woodlands in
the Mediterranean Regions of California and Spain—Barbour
(Gavilán 1994, Gavilán and Fernández-Gonzalez 1997, Gavilán and others 1998,
Rivas-Martínez and others 2002).
Q. pyrenaica typically forms a monospecific closed-canopy forest with low
cover by understory shrubs and herbs. Its physiognomy is similar to that of mixed
evergreen forest in California. Below the zone of melojo forest is an open woodland
of Q. rotundifolia; with increasing elevation, melojo first shares dominance with,
then yields to Pinus sylvestris, which thoroughly dominates a montane conifer forest.
Like Garry oak, melojo occupies an ecotonal position between low-elevation
broadleaf woodland and upper-elevation montane conifer forest.
Methods
In California, we selected a total of six Garry oak stands to sample, all within a 100km radius of the city of Santa Rosa: two each at Annadel State Park, Warm Springs,
and Pepperwood Preserve. The state park is immediately southeast of Santa Rosa and
encompasses nearly 2,000 ha. Warm Springs is north of Santa Rosa and west of
Healdsburg. It is a large public-access watershed around the Lake Sonoma Reservoir,
managed by the Army Corps of Engineers. Pepperwood Preserve is a privately held
reserve of approximately 1,200 ha northeast of Santa Rosa that is owned by the
Pepperwood Foundation for the purposes of research and education. All three areas
are dominated by volcanic substrates. We chose homogeneous stands uniformly
dominated by Garry oak and located in areas protected from grazing and logging for
at least the past 50 years. The stands varied in area from 1 to 20 ha. Elevation, slope,
and aspect were measured for each, using a topographic map, inclinometer, and
compass (table 1).
Table 1—Locational data for Quercus garryana stands sampled in this study. All are mid- to
late-seral stands located in Sonoma County, centering on 38°30’ N latitude x 123° W
longitude. Area (ha) is the stand’s estimated extent, elevation is in meters, aspect is in
degrees, and slope is in degrees.
Location name/stand
Pepperwood/1
Pepperwood/2
Annadel State Park/1
Annadel State Park/2
Warm Springs/1
Warm Springs/2
Area (ha)
10
110
15
1
1
1
Elev (m)
240
240
215
140
190
210
Aspect (°)
345 (NNW)
350 (NNW)
360 (N)
280 (NW)
350 (NNW)
315 (NW)
Slope (°)
40
40
23
10
20
37
Within each stand we randomly located three circular plots, each 100-squaremeters in size (radius = 5.6 m). In each plot, we measured the diameter breast height
(dbh) of all Garry oaks, including standing dead individuals. In addition, all Garry
oak saplings (defined as being < 1 cm dbh and > 20 cm tall) were counted, as were
all Garry oak seedlings (defined as being < 20 cm tall). Then the entire stand was
walked and ocular estimates of cover were made for each species of tree and shrub,
for all trees (as a class), for all perennial bunch grasses (as a class), and for all annual
herbs (as a class). Several trees, judged to be of average overstory height, were
measured for both height and dbh. One or more individuals judged to be of maximum
height and dbh were also measured. We used dbh and ring count data accumulated by
Anderson and Pasquinelli (1984), who cored 27 Garry oak trees of various dbh (8 to
72 cm) in four locations within Sonoma County. We constructed our own linear
475
GENERAL TECHNICAL REPORT PSW-GTR-217
regression of age as a function of dbh, from which we could assign ages to all trees
we measured for dbh. Their locations were Warm Springs, a property owned by the
California Academy of Sciences (8 km east of Windsor), Annadel State Park, and
Sonoma Valley (7 km south of Kenwood).
In Spain, we sampled seven young stands, one intermediate stand, and four oldgrowth stands. The young stands were located in the Guadarrama Mountains portion
of the Central System Range, 100 km north of Madrid. These showed no cut stumps
nor evidence of disturbance by domesticated livestock or off-road travel by
motorized vehicles. Locational information for the Spanish stands appears in Table 2.
Based on the absence of any trees >100 years old, we surmised that all these stands
had regenerated following some nearly synchronous catastrophic disturbance, such as
clear-cutting followed by stump removal.
Table 2—Locational summary for Q. pyrenaica stands sampled in this study. Latitude and
longitude are given in degrees N and W, respectively; elevation is in meters, slope in degrees,
and aspect (face) also in degrees. Area (ha) is the stand’s estimated extent, and category
refers to seral stage.
Stand
Region
1
Guadarrama
2
Guadarrama
3
Guadarrama
4
Guadarrama
5
Guadarrama
6
Guadarrama
7
Guadarrama
8
San Vicente
9
Ancares
10
Cantabrica
11
Vitoria
12
Mendilerroa
Lat x Long
o
41 06 x
3o38
40o50 x
3o48’
40o35’ x
4o10’
40o53’ x
3o50’
40o58’ x
3o48’
40o56’ x
3o52’
41o03’ x
3o38’
39°30’ x
4o30’
42o50’ x
6o50’
43o9’ x
4o31’
42o40’ x
2o30’
42o56 x
2o24’
Elev
Slope
1380
14
1360
20
1100
Aspect
45 (NE)
Area
Category
10
early
14
early
20
235
(WSW)
20 (N)
4
early
1420
19
350 (N)
50
early
1360
25
75 (ESE)
50
early
1360
20
160 (SSE)
4
early
1380
16
30
early
1180
6
55
early-mid
1270
32
1095
26
750
7
60
(ENE)
40
(NNE)
200
(SSW)
205
(SSW)
160 (SSE)
710
6
175 (S)
3
late
2
late
>1000
2
late
late
For each selected stand, the elevation, slope, and aspect were noted, using an
inclinometer, altimeter, and compass. The area of the stand was estimated. A record
of the vegetation was taken in an area of 1,000 m2 subjectively judged to be
representative of the stand. All taxa present were estimated for cover/abundance. A
100-m long transect tape was randomly anchored and then laid out straight through
the stand. Along it, a tally of all trees >1 cm dbh was kept for a belt 4-m wide by
100-m long, the center of the belt being marked by the transect tape. Every tree
encountered was measured for its dbh. Dead trees (snags) were noted as dead and
measured for dbh. The 400-m2 sample area was chosen because it was large enough
476
Species Characteristics and Stand Structure of Quercus garryana and Q. pyrenaica Woodlands in
the Mediterranean Regions of California and Spain—Barbour
to enclose the stand. The collective cover of tree canopies and tree gaps along the
tape was then estimated. Several trees, judged to be of average canopy height, were
measured for their height by using an inclinometer. Two to three saplings of breast
height were cut down at their base and one 2-cm-thick section of the base and another
at breast height were taken back to the laboratory for drying, sanding, and ring
counting. Approximately six uninjured and unsupressed trees with a range of
diameters were subjectively chosen and cored at breast height, and the cores taken
back to the laboratory for drying, sanding, and ring counting.
Two regions of northern Spain were visited to find old-growth stands: Two
locales were chosen in the Montes de León region; and two other locales were chosen
in Pais Vasco region. These four stands are in the Eurosiberian biogeographical
region, having an oceanic temperate macrobioclimate with a submediterranean
character (Rivas-Martínez and others 2002). No old-growth stands could be located
in the Mediterranean-climate part of Spain.
The density of trees was much lower than in young stands, and for that reason
the belt transect was increased in size to 8 x 100 m.
Results and Discussion
Quercus garryana
Our regression of tree age as a function of dbh—based on Anderson and Pasquinelli’s
(1984) data—generated a very strong linear relationship (fig. 1), years of age =
[(6.22) x (dbh in cm)] — 13.43 with an R2 value of 0.91 and a P = 0.001 that the
slope departed from zero. Based on our formula, the largest tree we encountered
(72cm dbh) would have had a predicted age of 434 years, not counting the additional
number of years it would have taken to reach breast height.
Figure 1—Regression of Q. garryana age against diameter breast height for 27 trees
cored by Anderson and Pasquinelli (1984).
477
GENERAL TECHNICAL REPORT PSW-GTR-217
When we transformed our dbh data into years of age and expressed tree density
per hectare, all three sites exhibited poor regeneration for as long as the past 50 to
100 years and zero regeneration for the past 25 to 50 years (fig. 2). Seedlings and
saplings were completely absent from our plots, but an earlier study at Annadel State
Park by Barnhart and others (1991) tabulated 178 seedlings per hectare. However,
they were sampling woodland vegetation, in which herbs covered much of the
ground. Three-fourths of the seedlings were found in understories dominated by
perennial bunch grasses, mainly Festuca californica; the rest were found among
invasive annuals. None of our six stands had an understory dominated by bunch
grasses.
478
Species Characteristics and Stand Structure of Quercus garryana and Q. pyrenaica Woodlands in
the Mediterranean Regions of California and Spain—Barbour
Figure 2—Stand age structure for Quercus garryana stands. (Top) Age structure for
two Warm Springs stands combined. N = 70. (Middle) Age structure for two stands
combined at Annadel State Park. N = 47. (Bottom) Age structure for the two younger
Pepperwood stands combined. N = 64.
479
GENERAL TECHNICAL REPORT PSW-GTR-217
Tree density (table 3; defined as individuals > 1 cm dbh) was widely variable
among the three sites: 600 to 1,100 per hectare at Annadel State Park and Warm
Springs, and 1,500 to 1,600 at the younger Pepperwood stands. Standing dead trees
were highest at Pepperwood, 12 percent of all (live + dead) trees, compared to 8 to 10
percent in the older stands at Annadel and Warm Springs. Only Quercus garryana
was in the canopy at Pepperwood, whereas the other two forests included small
numbers of Quercus agrifolia, Pseudotsuga menziesii, and Unbellularia californica.
The Pepperwood forest had very little herbaceous cover, whereas the other two
forests had high herb cover, typically > 70 percent.
Table 3—Stand data for Quercus garryana. Mean tree canopy height is in meters; tree
canopy cover is in percent; live tree density is per hectare; dead standing trees are shown in
absolute numbers per hectare before the slash and as a percent of all trees, dead + live, after
the slash. The last two columns summarize mean tree diameter breast height (cm) and basal
area (square meters per hectare).
Location and
DBH
Ht.
Canopy
Live
Dead / %
BA
stand
mean
Pepperwood, 1
23 / 13
6.4
11
53
1650
7
Pepperwood, 2
0
/
0
4.7
11
100
1550
6
Annadel, 1
0/0
22.9
9
73
635
22
Annadel, 2
133 / 19
22.6
11
88
802
19
Warm Springs, 1
133 / 11
38.5
9
75
1069
22
Warm Springs, 2
65 / 5
61.3
9
83
1136
26
Quercus pyrenaica
Vegetation in seven early-successional stands had high densities of small-diameter
trees (table 4; 1600-11,500 per ha), with pronounced mortality, averaging 16 percent
of all trees (dead plus live). Common associates in these stands include the shrubs
Genista florida subspecies florida, Crataegus monogyna, and Rubus ulmifolius, and
the perennial herbs Pteridium aquilinum, Poa nemoralis, Holcus mollis, Arenaria
montana, Clinopodium arundanum, and Luzula forsteri.
Table 4—Stand data for Q. pyrenaica. Stands 1-7 are young stands; stand 8 is intermediate,
and stands 9-12 are old-growth. Height mean/max is in meters. Cover is in percent (for tree +
shrub + herb canopies). Mean diameter breast height (dbh) is in centimeters. Density is
number of live Q. pyrenaica trees >1 cm dbh per hectare/percent of all trees that were dead.
Basal area is in square meters per hectare.
Stand
1
2
3
4
5
6
7
8
9
10
11
12
480
Height
10 / 15
11 / 14
7 / 11
9 / 12
12 / 14
13 / 17
11 / 13
14 / 19
16 / 20
17 / 20
20 / 23
15 / 17
Cover
96 / 2 / 5
97 / 1 / 1
75 / 1 / 2
93 / 0 / 1
93 / 1 / 2
93 / 1 / 2
96 / 1 / 2
80 / 1 / 1
64 / 20 / 3
100 / 1 / 1
95 / 2 / 2
69 / 13 / 1
Mean dbh
10
9
12
10
9
11
10
16
27
37
26
21
Density/dead
5720 / 21
11,200 / 2
4600 / 13
10,000 / 22
9250 / 18
6000 / 23
11,500 / 18
1600 / 12
513 / 0
500 / 0
475 / 3
563 / 0
Basal area
4.5
7.1
5.2
7.8
5.8
5.7
9.0
24.5
29.2
53.4
25.0
19.4
Species Characteristics and Stand Structure of Quercus garryana and Q. pyrenaica Woodlands in
the Mediterranean Regions of California and Spain—Barbour
Although we were unable to find successional young stands of Q. garryana in
California, we found many young stands of Q. pyrenaica (finding mature stands was
difficult) and decided to summarize their composition and structure separately. For
the purposes of this study, we defined young stands as having a mean dbh <12 cm,
mean tree height <14 m, density per hectare >1,500, and an absence of trees >100
years old. We defined old-growth stands as having a mean dbh >20 cm, mean tree
height >14 m, a density per hectare <600, and the presence of many trees >200 years
old. We propose that these young stands were initiated following some standreplacing disturbance such as clear-cutting or fire.
Stand 8, in the Montes de Toledo, appeared to be a mid-seral forest, given the
intermediate dbh, height, and density of trees. Unfortunately, the cores from trees at
this site have been lost, so we have no age data for that stand.
The late-successional stands had densities only 1/3 to 1/2 of young stands
(averaging about 500 trees per ha) and practically no mortality, possibly indicating
that self-thinning had ended some time in the past. Stumps were absent, indicating
that snags had not been harvested for the past several decades. Somewhat
surprisingly, although the mean canopy height of old-growth trees was significantly
(P < 0.01) taller than that of young stands, the difference was modest: 16.0 vs. 11.0
m. Most allocation of photosynthate over time, therefore, must be channeled to the
root system and/or stem girth.
When all 45 cores from stands 1-7 had been counted and graphed against dbh, a
linear regression line having the formula age in years = [(1.41) x (dbh in cm)] +
21.31 was the result. It has a slope significantly different from zero (P < 0.001) and
an R2 of 0.69. When the 1,152 dbh records taken from belt transects in stands 1-7
were converted to ring age classes (fig. 3), the age structure of the combined seven
stands showed: (1) almost 2/3 of all trees were between 30 and 39 years of age, (2)
almost zero regeneration characterized the most recent 30 years, and (3) the oldest
tree encountered was just over 90 years old—possibly a remnant from a previous
forest harvested half a century ago or one that had stood alone in a cultivated field
that was abandoned.
Figure 3–Age structure for seven young stands of Quercus pyrenaica
combined. N = 1152.
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GENERAL TECHNICAL REPORT PSW-GTR-217
Twenty trees in the four old-growth stands were cored. For those trees with a
diameter too large for our increment corer to reach the center, the missing number of
rings was estimated as follows. The rings along the length of the extracted core were
counted and divided by the length of the core (excluding bark) to obtain an average
number of rings per centimeter. The missing centimeters of core were estimated by
subtracting the extracted core’s length from the radius of the tree (again, excluding
bark). The predicted number of missing rings was calculated by multiplying the
missing core length times the average number of rings per centimeter, and the
missing rings were then added to the counted rings to obtain an estimate of tree age.
The regression line for dbh against ring count fits the formula, age in years =
(3.56) (dbh in cm) – 22.20, with an R2 of 0.81. The slope of increasing age with girth
(3.56) is steeper than that for young stands (1.41), which is in agreement with our
conclusion earlier that the allocation of photosynthate in older melojo trees goes
more towards increasing trunk girth than to increasing height.
When all 164 diameters, from all four stands, were converted to age, the
combined age structure showed an absence of regeneration for the past 5 years and
depressed regeneration 6 to 75 years ago, a peak of trees in the 76 to 100 year-old
cohort, and a smoothly declining abundance of older trees, the oldest being (an
estimated) 351 to 375 years old (fig. 4). Maximum dbh was 93 cm, three to five times
the maximum for early-successional stands.
Figure 4—Age structure for four old-growth stands of Quercus pyrenaica
combined. N = 165.
Thus, both young and mature stands of Q. pyrenaica show depressed
regeneration for the past 75 years. Is this depression a result of human
(mis)management, which can be corrected, or does it merely reflect a natural cycle
where pulses of establishment are separated by long periods of regeneration failure?
And if the latter, what causes the pulses: unusual weather, fluctuations of herbivore
populations, masting behavior? If we suspect natural cycles on the order of one per
century, then figure 3 does not provide much support, for it fails to show obvious
second or third peaks for cohorts 200 or 300 years old. (Such peaks would be lower
482
Species Characteristics and Stand Structure of Quercus garryana and Q. pyrenaica Woodlands in
the Mediterranean Regions of California and Spain—Barbour
than the 100-year-old peak, because of more accumulated mortality, but there is
actually an absence of individuals in those cohorts.)
The distributional and physiognomic similarities of young and old Garry oak
and melojo oak stands are obvious and striking. Q. pyrenaica and Q. garryana appear
to be convergent not only in habitat, tree morphology, phenology, community
architecture, and their ecotonal nature, but also in their unexplained regeneration
failure for the better part of the past century.
Acknowledgements
The authors give sincere thanks for guidance during field trips in Spain from
Professors Felix Llamas and Javier Loidi and from Dr. Maria Pilar Rodriguez-Rojo.
We also thank Professors Rosario G. Gavilan and Salvador Rivas-Martinez for
advice and comments on early drafts.
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