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:
[email protected]. 3 Professor, Department of Environmental Science, Policy and Management, UC Berkeley, Berkeley, CA, 94720. e-mail:
[email protected]. 4 Associate Professor and UCCE Specialist, Department of Environmental Science, Policy and Management, UC Berkeley, Berkeley, CA, 94720.
617
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
618
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).
619
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 (
