10. Plant Diversity in a Patchy Landscape
149
With my collaborators, I have examined patterns of plant diversity, individual
species distributions, population persistence, and reproductive success in a landscape consisting of serpentine and nonserpentine soils in northern California. Outcrops of serpentine are associated with fault zones, and may range in size from a
few m 2 to many km 2 . They support a distinctive flora because their high Mg++:Ca++
ratio excludes most species from the surrounding community, including the Mediterranean alien species that now dominate most of lowland California. Their flora
includes a substantial number of strict serpentine endemics, many of which are
listed as sensitive or rare taxa because of their narrow geographic distributions.
The flora on serpentine also includes many non-endemic native species that are
rare on nonserpentine because of competition from Mediterranean aliens (Kruckeberg
1984; Brooks 1987; Huenneke et al. 1990; Skinner and Pavlik 1994). For these
reasons, the plant community on island-like outcrops of serpentine in my study
region is both a good model system for studying patchiness, and of conservation
interest in its own right.
Our work is being conducted in Lake, Napa, and Sonoma Counties, California,
USA. The flora of this area is described by Barbour and Major (1977), Kruckeberg
(1984) and Sawyer and Keeler-Wolf (1995). Most serpentine soils in this region
are poorly developed, and seldom support grassland. Instead they support chaparral containing both serpentine endemics (e.g. Quercus durata, Fagaceae; Arctostaphylos viscida, Ericaceae; Ceanothus jepsoni, Rhamnaceae), and non-endemics
(e.g. Pinus sabiniana, Pinaceae; Adenostoma fasciculatum, Rosaceae; Heteromeles
arbutifolia, Rosaceae; Umbellularia californica, Lauraceae). Herbs are sparse, and
occur mainly on rocky slopes interspersed with the chaparral. Non-serpentine soils
in this region are mainly derived from sedimentary rocks; to the eastern (inland)
side of the study area, the vegetation is predominantly blue oak (Quercus douglassii)
woodland; toward the west, coastal mixed evergreen forest (Quercus agrifolia;
Pseudotsuga menziesii, Pinaceae; Arbutus menziesii, Ericaceae).
2 Patterns of Diversity on Small and Large
Serpentine Outcrops
In the initial phase of this study (Harrison 1997, 1999), I compared patterns of
species richness (henceforth "diversity") of herbaceous plants in two settings: 24
small (mostly < 1 ha) and isolated (> 1 km from large outcrops) patches of serpentine, and 24 similarly spaced and identically sampled sites within very large (> 5
km2) serpentine areas. Comparing patchy and continuous sites within a single type
of habitat and community allowed me to analyze the effects of patchiness per se
with respect to community structure, while avoiding the confounding effects that
would arise in comparing two different communities with different evolutionary
histories.
Using geologic maps, I located 24 small serpentine outcrops to which I was able
to gain access. The patches chosen were found in 4 clusters of 5-7, within which
149
With my collaborators, I have examined patterns of plant diversity, individual
species distributions, population persistence, and reproductive success in a landscape consisting of serpentine and nonserpentine soils in northern California. Outcrops of serpentine are associated with fault zones, and may range in size from a
few m 2 to many km 2 . They support a distinctive flora because their high Mg++:Ca++
ratio excludes most species from the surrounding community, including the Mediterranean alien species that now dominate most of lowland California. Their flora
includes a substantial number of strict serpentine endemics, many of which are
listed as sensitive or rare taxa because of their narrow geographic distributions.
The flora on serpentine also includes many non-endemic native species that are
rare on nonserpentine because of competition from Mediterranean aliens (Kruckeberg
1984; Brooks 1987; Huenneke et al. 1990; Skinner and Pavlik 1994). For these
reasons, the plant community on island-like outcrops of serpentine in my study
region is both a good model system for studying patchiness, and of conservation
interest in its own right.
Our work is being conducted in Lake, Napa, and Sonoma Counties, California,
USA. The flora of this area is described by Barbour and Major (1977), Kruckeberg
(1984) and Sawyer and Keeler-Wolf (1995). Most serpentine soils in this region
are poorly developed, and seldom support grassland. Instead they support chaparral containing both serpentine endemics (e.g. Quercus durata, Fagaceae; Arctostaphylos viscida, Ericaceae; Ceanothus jepsoni, Rhamnaceae), and non-endemics
(e.g. Pinus sabiniana, Pinaceae; Adenostoma fasciculatum, Rosaceae; Heteromeles
arbutifolia, Rosaceae; Umbellularia californica, Lauraceae). Herbs are sparse, and
occur mainly on rocky slopes interspersed with the chaparral. Non-serpentine soils
in this region are mainly derived from sedimentary rocks; to the eastern (inland)
side of the study area, the vegetation is predominantly blue oak (Quercus douglassii)
woodland; toward the west, coastal mixed evergreen forest (Quercus agrifolia;
Pseudotsuga menziesii, Pinaceae; Arbutus menziesii, Ericaceae).
2 Patterns of Diversity on Small and Large
Serpentine Outcrops
In the initial phase of this study (Harrison 1997, 1999), I compared patterns of
species richness (henceforth "diversity") of herbaceous plants in two settings: 24
small (mostly < 1 ha) and isolated (> 1 km from large outcrops) patches of serpentine, and 24 similarly spaced and identically sampled sites within very large (> 5
km2) serpentine areas. Comparing patchy and continuous sites within a single type
of habitat and community allowed me to analyze the effects of patchiness per se
with respect to community structure, while avoiding the confounding effects that
would arise in comparing two different communities with different evolutionary
histories.
Using geologic maps, I located 24 small serpentine outcrops to which I was able
to gain access. The patches chosen were found in 4 clusters of 5-7, within which