Using Vegetation Type Map Data to Increase
Our Understanding of Long-Term
Ecological Change in the Woodlands
Surrounding San Francisco Bay1
Tim Doherty,2 Barbara Allen-Diaz,3 and Maggi Kelly4
Abstract
In the 1920s and 1930s A.E. Wieslander, a silviculturist with the U.S. Forest Service
California (now Pacific Southwest) Forest and Range Experiment Station, and his crew
surveyed much of the California landscape. The data they collected, known as the Wieslander
Vegetation Type Mapping collection (VTM), contains vegetation data, detailed vegetation
type maps, and an extensive photograph collection. We examined VTM plots in the Quercus
agrifolia-Umbellularia californica woodlands surrounding San Francisco Bay in order to
examine the relationships between current and historical stand conditions. We selected 12
VTM plots to resample across a gradient of Sudden Oak Death (SOD) infection. Our initial
findings from re-located plots show no significant increase in basal area between sampling
dates for any tree species. We identified two distinct plant communities that were independent
of sampling date; two of the 12 plots had been converted through management actions. This
research required significant coordination and access to private and public lands for resampling purposes.
Keywords: Quercus agrifolia, Umbellularia californica, Wieslander VTM plots.
Introduction
In the first half of the 20th century, A.E. Wieslander and his crew surveyed much of
the California landscape. The data they collected, known as the Wieslander
Vegetation Type Mapping collection (VTM), contains vegetation data collected from
more than 18,000 plots, roughly 330 colored type maps and more than 3,000 black
and white photos. The collection is now available online at http://vtm.berkeley.edu.
The digitization and georeferencing process was funded by the USDA Forest Service
(Pacific Northwest and Pacific Southwest Research Stations), the USDA Cooperative
State Research, Education and Extension Service, University of California Digital
Library Project, and the College of Natural Resources at UC Berkeley. The
georeferencing process entails assigning map coordinate data to historical VTM plots
and topo maps so that they can be used in a geographic information system—GIS
(Kelly and others 2005). The VTM collection serves as a valuable public resource to
increase our understanding of long-term ecological change within California’s
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
Graduate Student, Department of Environmental Science, Policy and Management, UC Berkeley,
Berkeley, CA, 94720. e-mail: tdoherty@nature.berkeley.edu.
3
Professor, Department of Environmental Science, Policy and Management, UC Berkeley, Berkeley,
CA, 94720. e-mail: ballen@nature.berkeley.edu.
4
Associate Professor and UCCE Specialist, Department of Environmental Science, Policy and
Management, UC Berkeley, Berkeley, CA, 94720.
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GENERAL TECHNICAL REPORT PSW-GTR-217
dynamic vegetation communities. We are particularly interested in understanding
decades-long change in the woodlands surrounding San Francisco Bay, which is
dominated by coast live oak (Quercus agrifolia) and California bay (Umbellularia
californica) (fig. 1). Using the VTM data provided a unique opportunity to examine
change in species composition, stand structure, and community stability over a period
of 70 years.
Figure 1—General locations of VTM plots selected for resampling.
The species composition and management of California’s diverse oak
woodlands have been subject to varied amounts of change since the arrival of
European settlers. Our study area is distinct from other oak savannas and woodlands
of California because of its co-dominance of coast live oak and California bay, and
understory of shrubs (Allen 1990). Many native grass and forb species found in the
understory of California’s oak savanna have largely been replaced by introduced
Mediterranean grasses and forbs across broad spatial scales within the last 250 years
(McClaran and Bartolome 1988). The highly invaded oak savannas are generally
considered stable communities now, and are not easily converted back to historic
community types (Griffin 1977). Grasses are found in the coast live oak and
California bay woodland, but this community is still dominated by woody shrubs
such as toyon (Heteromeles arbutifolia), oceanspray (Holodiscus discolor), and
poison oak (Toxicodendron diversilobum).
One of the most recently detected invasive species in California’s oak
woodlands is the pathogen Phytophthora ramorum, which causes SOD. Presumably
none of the 12 plots sampled in 1933 were infected with P. ramorum. The symptoms
of SOD were first reported in Marin County in 1994 (Svirha 1999). P. ramorum has
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Using Vegetation Type Map Data to Increase Our Understanding of Long-Term Ecological Change
in the Woodlands Surrounding San Francisco Bay—Doherty
led to widespread mortality in oaks and tanoaks across 14 counties in California and
one county in Oregon (Rizzo and Garbelotto 2003). California bay is a foliar host and
thus serves an important role in the spread of P. ramorum. Foliar hosts are shrub and
tree species that do not die, even though P. ramorum may be present on leaves and
branches. At the landscape level, the density of California bay is a predictor of oak
mortality (Kelly and Meentemeyer 2002) presumably because infected California bay
leaves produce large numbers of sporangia (Davidson and others 2005). Past research
by McBride (1974) showed California bay to be the climax species in the coast live
oak and California bay woodlands of the East Bay in the absence of disturbance such
as livestock grazing. The ecological implications of California bay acting as the
climax species and as a foliar host are important to consider in increasing our
understanding of SOD disease dynamics. A future research question of ours is to
determine if there are certain characteristics of historical stands that are associated
with the current presence of P. ramorum.
In this study we: 1) evaluated the changes in basal area of primary tree species,
and 2) determined tree and understory species composition change since the 1930s.
Our ultimate goal is to increase our understanding of the stability of the coast live oak
and California bay community as well as successional dynamics within these diverse
and biologically productive woodlands.
Methods
Beginning in winter 2005, we randomly selected 12 VTM plots from a gradient of
SOD infection based on previous research (Brown and Allen-Diaz 2006) in the coast
live oak and California bay woodlands surrounding San Francisco Bay. Plot selection
criteria were based on presence of coast live oak and California bay within the Coast
Ranges of Alameda, Contra Costa, Marin, Sonoma, and Napa counties. The majority
of our plots were located on north to northwest ridges between 152 and 487 m (500
and 1,600 ft). Long-term average annual precipitation ranged from 68 to 96 cm (27 to
38 inches). The plots were located across a broad spectrum of land uses, including
the edge of rapidly urbanizing cities to ranches, vineyards, and wildlands and were
located on both private and public land. There was no evidence of fire scarring on
any of the trees measured within our plots, and only one plot was grazed by livestock.
Although the VTM plots were not permanently marked, we were confident that
we sampled within 20 to 100 meters of the original plot location, based on modern
GPS coordinates created through the georeferencing process (Kelly and others 2005).
Plot relocation efforts were aided by original topographic maps and environmental
variables, such as slope, elevation, and aspect. In addition, tree species and respective
size classes improved our plot relocation efforts.
Vegetation and environmental data was recorded using the original Manual of
Field Instructions for Vegetation Type Map of California protocol (see
http://vtm.berkeley.edu). All original field data collection was done in English units;
we provide the English units in parentheses. A rectangular plot, 40 m x 20 m (132 by
66 ft, 2 chains by 1 chain) was placed with the long axes parallel with the slope. The
diameter at breast height (DBH) at 1.3 m (4.5 ft) of each tree > 10 cm (4 inches) was
recorded and classified into four categories: 10 to 27.9 cm (4 to 10 inches), 28 to 58.4
cm (11 to 23 inches), 58.5 to 91.4 cm (24 to 36 inches), and 91.4 cm+ (36 inches+).
Within this larger plot, a smaller plot, 40 m x 10 m, (132 x 33 ft), was arranged, and
dominant understory cover was recorded for each cell, 2 m x 2 m (6 x 6 ft).
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GENERAL TECHNICAL REPORT PSW-GTR-217
TWINSPAN (Hill 1979) analysis (n = 24) was used to determine presence and
stability of coast live oak and California bay communities. Understory plants were
collected in units of percent cover, while overstory trees were categorized by DBH
category and converted to basal area based on the mid-point of the diameter class.
Thus basal area is the metric used in the TWINSPAN matrix for trees (in m2/ha). The
data matrix was carefully examined to ensure that default cut levels of 0, 2, 5, 10, 20
were appropriate and spanned all the percent cover and DBH values contained in the
species matrix. This analysis procedure follows from Allen and others (1991).
Student’s t-tests were used to determine significant differences among species in
community types and between sampling dates (SAS Institute 2002). A p-value of
0.05 was used as the cut-off for significance between species for each group.
Results
Thirty-six species were found in the sample plots in 2006, with very little change in
species composition between the years (table 1). Grass is only identified to family in
some of the original VTM plots, thus we are not able to evaluate the change in
Poaceae between decades.
Two plant communities were identified using TWINSPAN. Group I (n = 12) was
characterized by a canopy of coast live oak and California bay, and an understory
dominated by Holodiscus discolor. Group II (n = 9) is characterized by a coast live
oak and California bay overstory with a Heteromeles arbutifolia, California bay
sapling, and grass understory. Only one plot in Alameda County shifted species
composition from Group I to II, and that was based on the decrease of Holodiscus
discolor and Montia perfoliata since 1933. Three plots were distinguished from the
rest by the dominance of grass in the understory. One pair of plots was converted
from coast live oak and California bay laurel-grass (1933) to just two oak trees with
grass understory in 2006. Another plot was converted from Group I to grass, with no
tree overstory remaining in 2006. The basal area of coast live oak was significantly
higher in Group I, while the percent cover of California bay saplings (<10 cm DBH)
was lower. Holodiscus discolor cover was significantly higher in Group I (table 2).
There was no significant difference in tree basal area between 1933 and 2006 among
tree species (table 3). However, grass cover in the understory changed in constancy
from 33 percent of the plots in 1933 to 58 percent of the plots in 2006.
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Using Vegetation Type Map Data to Increase Our Understanding of Long-Term Ecological Change
in the Woodlands Surrounding San Francisco Bay—Doherty
Table 1—List of tree and understory species found in 1933 and 2006 with constancy values (#
of plots species occurred in / total # of plots; n = 12).
1933 tree species list
Quercus agrifolia
Umbellularia californica
Arbutus menziesii
Quercus kelloggii
Quercus lobata
Acer macrophyllum
Constancy
Value
100%
100%
75%
66%
25%
16%
Pseudotsuga menziesii
Aesculus californica
Quercus garryana
Quercus morehus
Sequoia sempervirens
16%
8%
8%
8%
8%
1933 understory species
list
Toxicodendron diversiloba
Symphoricarpos albus
Umbellularia californica
Heteromeles arbutifolia
Holodiscus discolor
Rhamnus californica
Corylus rostrata
Grass
Rosa californica
Arbutus menziesii
Quercus agrifolia
Rubus vitifolius
Quercus kelloggii
Acer macrophyllum
Aesculus californica
Ceanothus sorediatus
Montia perfoliata
Polystichum munitum
Pteris aqulina spp
Quercus chrysolepsis
Quercus morehus
Sequoia sempervirens
100%
58%
58%
50%
50%
41%
33%
33%
25%
25%
16%
16%
16%
8%
8%
8%
8%
8%
8%
8%
8%
8%
2006 tree species list
Quercus agrifolia
Umbellularia californica
Arbutus menziesii
Quercus kelloggii
Quercus lobata
Acer macrophyllum
Heteromeles arbutifolia
Pseudotsuga menziesii
Aesculus californica
Quercus garryana
Quercus morehus
Constancy
Value
83%
83%
50%
50%
25%
8%
16%
16%
8%
8%
8%
2006 understory species
list
Toxicodendron diversiloba
Symphoricarpus albus
Umbellularia californica
Heteromeles arbutifolia
Holodiscus discolor
Rhamnus californica
Corylus rostrata
Grass
Rosa californica
Arbutus menziesii
Quercus agrifolia
41%
8%
50%
41%
41%
8%
8%
58%
8%
16%
25%
Quercus kelloggii
16%
Ceanothus sorediatus
8%
Polystichum munitum
Pteris aquilina sp
25%
8%
Quercus morehus
8%
Mimulus aurantiacus
Arctostaphylos sp.
Baccharis pilularis
Carduus pycnocephala
Satureja douglassii
Adenostoma fasciculatum
Pseudotsuga menziesii
Stachys adjugoides
Vaccinium ovatum
25%
16%
16%
16%
16%
8%
8%
8%
8%
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GENERAL TECHNICAL REPORT PSW-GTR-217
Table 2—Comparison of mean basal area (m2/ha) and percent
occurring in Groups I and II (with standard error).
Basal area
Quercus Umbellularia
Umbellularia
agrifolia* californica
all tree
californica*
cover of selected species
% cover
Holodiscus
all
discolor* understory
Group I
14.54
(2.28)
5.36
(1.07)
32.43
(4.54)
4.41
(1.55)
16.58
(5.46)
60.58
(7.34)
Group II
6.19
(1.56)
3.44
(0.82)
36.09
(4.15)
11.44
(2.74)
1.11
(0.99)
63.33
(5.94)
* = significant at p = 0.05
Table 3—Comparison of basal area (m2/ha) and percent cover for selected species between
two sampling points in time (with standard error).
Quercus
agrifolia
Basal area
Quercus
kelloggii
Umbellularia
californica
% cover
Umbellularia
californica
2006
8.56
(2.34)
4.33
(1.89)
6.6
(1.75)
6.5
(2.43)
1933
9.42
(2.2)
5.45
(3.11)
3.37
(0.82)
5.33
(1.77)
Discussion
Our limited sample thus far suggests that coast live oak and California bay
woodlands in the Bay Area have been relatively stable over the last 70 years. In the
overstory tree component, little change in basal area has occurred which is to be
expected for large, long lived mature trees. Two plots had their overstory trees
removed by management. One plot had a species shift from an overstory of coast live
oaks with redwoods to California bay and Douglas-fir. Understory species
composition has changed somewhat which is expected with shorter-lived shrub and
herbaceous components. An unfortunate artifact of this historic data set is the lack of
identification of the grass component to species. Detecting species composition
change in the understory is also exacerbated by the original data collection method;
only a single dominant understory species was recorded in each 2 x 2 m quadrat. This
limits our ability to evaluate changes in biodiversity or invasive species in a plot-byplot comparison, but does not de-value the collection for larger-scale analyses of
plant community dynamics.
The coast live oak and California bay woodlands exist within the wildland urban
interface where urbanization, woodland fragmentation, and introduced species will
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Using Vegetation Type Map Data to Increase Our Understanding of Long-Term Ecological Change
in the Woodlands Surrounding San Francisco Bay—Doherty
shape the composition, structure and animal communities. P. ramorum is one such
introduced species and its affect on the successional dynamics between coast live oak
and California bay are uncertain (Brown and Allen-Diaz 2006). There is evidence
from this study of a modest increase in California bay saplings and mature California
bay basal area since the initial sampling 70 years ago. Although it remains to be seen
how California bay will respond to stand structural changes with P. ramorum
infection, the increase in the sapling class hints at a successional trend toward
California bay dominance. Historic stand characteristics that indicate resistance to P.
ramorum infection will be equally important in understanding SOD disease
dynamics.
Resampling VTM plots on both public and private land is an important tool in
understanding long-term plant community change in California. However, it is
important to be aware of difficulties surrounding the exact re-location of plots as well
as the difficulties of sampling on private property. Re-location error based on
digitized GPS coordinates can be large (Kelly and others 2005), and field samplers
must use maps, topography, aspect, and mature tree stand characteristics to narrow
the location of the plot. Acquiring permission to sample VTM plots on private (as
well as public) lands requires finding the plot locations through the county assessor’s
office in order to link plots to individual land owners. We sent a letter requesting
landowners’ permission to allow us to sample on their property with a brief
description of our project, and simultaneously UC Cooperative Extension advisors
and local professionals were very helpful in assisting in contacts with individual
landowners (Hilty and Merenlender 2003). Still, this process takes many, many
months and resulted in approximately an 80 percent positive response.
California’s population has grown from less than 7 million in the 1940s to 37
million people today, while in the Bay Area alone, ~6 million more people have
come to live and work in the area’s diverse ecosystems in the last 70 years (Gregor
1963). This influx of human residents has changed land use patterns, altered
disturbance regimes, fragmented ecosystems and introduced countless new species
into the region. The VTM collection is a plot-rich data set that offers the opportunity
for examining plant community dynamics across the last 70-80 years. The collection
may provide insight into the stand characteristics that resist the deadly effects of P.
ramorum or other future invasions, while providing a valuable baseline of historic
plant community data.
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