Long-Term Change in Oak Woodlands and
Its Influence on a Forest Disease1
Ross Meentemeyer2 and Hall Cushman3
Abstract
Land-use practices to sustain expanding human populations have resulted in dramatic
alterations of forested ecosystems worldwide. One potential consequence of land-use change
is the spread of infectious disease. In this research, we investigated patterns of human-related
land-cover change between 1942 and 2000 and examined whether or not changes have led to
conditions that promote the establishment and spread of the invasive pathogen Phytophthora
ramorum, causal agent of the destructive forest disease Sudden Oak Death. We established
102 field sites randomly located within a 275 km2 region of Sonoma County, CA comprised
of several land cover types. Within a 150 m radius area around each site, we mapped finescale changes in land cover in a GIS using 1942 and 2000 imagery. We found that woodland
area increased by 25% from 1942 to 2000, while grassland and chaparral decreased in area by
34% and 49% respectively, and development increased by 1105%; mean size of woodland
patches increased by 51% and number of woodland patches decreased by 41%. To examine if
these changes influenced the establishment of P. ramorum, we sampled P. ramorum infection
level on host species and measured woody species abundance within each 150 radius area in
spring 2005. Multiple regression analyses showed that density of the host bay laurel (an
important carrier of the disease) was positively related to woodland cover change. Also, there
were a greater number of bay trees showing symptoms of P. ramorum in areas that have
increased in woodland. We hypothesize that these increases in woodland density and
expansion into grasslands and shrublands facilitates spread of P. ramorum by increasing
contagion of host and increasing shading and moisture levels. Understanding the influence of
land cover change on disease spread is critical for informing land management and preventing
intensification and spread of destructive pathogens.
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
University of North Carolina at Charlotte. e-mail: rkmeente@email.uncc.edu.
3
Sonoma State University. e-mail: cushman@sonoma.edu.
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