The Modoc Plateau Region
Authors: Dennis Odion, David Stoms, and Frank Davis
Plant Community Types
The Modoc Plateau Region (Figure MOD-1) encompasses 22,858 km²
of the volcanic tablelands east of the Cascade and northern Sierra
Nevada ranges in Modoc, Lassen, and Siskiyou counties. The Modoc
Plateau is part of the larger Columbia Plateau which extends into
eastern Oregon and Washington, southern Idaho, northern Nevada and
western Wyoming. Whereas the Cascade and Sierra Nevada regions are
mostly vegetated by montane pine and fir forests, the arid Modoc
Plateau primarily supports Great Basin (cold desert) vegetation
dominated by low sagebrush, sagebrush, bunchgrass and cheatgrass.
Western junipers protrude a few meters above the shrubs and range
in density from widely scattered to closed canopies. The Warner
Mountains form a distinct floristic subregion.
Figure MOD-1. Shaded
relief image of the Modoc Plateau Region.
Plateau is in the rainshadow of the Cascade-Sierra ranges (Schoenherr
1992). Precipitation ranges from about 10-12" at low elevations
to perhaps three times as much at the higher elevations at the western
edge of the region. As a cold desert, most of the precipitation
occurs as snow. Temperatures are too low for plant growth much of
the year, including when most precipitation falls. Drought and warm
temperatures are limiting to plant growth in summer. Thus, plant
productivity is low, and vegetation relatively sparse for these
(Artemisia arbuscula), big sagebrush (A. tridentata),
rabbitbrush (Chrysothamnus spp.), and bitterbrush (Purshia
tridentata) are the dominant shrubs of the region in that order
of abundance. Higher elevations (i.e. 1500-2000 m) support conifer
forests. White fir (Abies concolor) strongly dominates the
moister slopes, forming dense stands. Drier slopes are characterized
by open stands of Ponderosa (Pinus ponderosa) and Jeffrey
pine (P. jeffreyi) and Western juniper (Juniperus occidentalis).
Western juniper and Mountain mahogany (Cercocarpus ledifolius)
dominate on dry rocky sites in both mixed conifer and sagebrush
zones. At the lowest elevations, pluvial lakebeds form habitat for
silver sagebrush (Artemisia cana) where soils are somewhat
salt affected. Shadscale (Atriplex confertifolia) and greasewood
(Sarcobatus vermiculatus) are found on the more strongly
salt affected soils around Honey Lake and Surprise Valley. A wide
variety of perennial bunchgrasses and cheatgrass (Bromus tectorum)
are found in the understory of shrub and more open conifer vegetation,
usually accounting for 20-30% of foliar cover.
(Pinus monophylla), common in the other Great Basin portion
of California (East of the Sierra Nevada Region in Hickman 1993),
is entirely absent in the Modoc Plateau; Utah juniper (Juniperus
osteosperma) is uncommon. The Bristlecone Pine community found
on many of the higher ranges to the south and east does not occur
in the seemingly suitable timberline habitat of the Warner Mountains
which, like the northern Sierra Nevada, support whitebark pine (Pinus
albicaulis). Red fir (Abies magnifica), which occurs
above white fir in the Sierra Nevada, is absent from all but the
extreme southwestern edge of the region, and white fir occupies
a wider elevational range. Sugar pine (Pinus lambertiana)
and Douglas fir (Pseudotsuga menziesii) are other important
conifers limited to the southwest edge of the region, as are foothill
woodland dominants such as Oregon oak (Quercus garryana),
Foothill pine (Pinus sabiniana), and chaparral shrubs (Ceanothus
cuneatus and Cercocarpus betuloides). Mixed conifer species
such as black oak (Quercus kelloggii) and incense cedar (Calocedrus
decurrens) are uncommon, but scattered geographically over much
of the region (Griffin and Critchfield 1972).
As the region
has few settlements, the greatest impacts on the biodiversity of
the region have been logging, grazing, cultivation, and fire suppression.
Grazing (mostly cattle) occurs throughout the region, including
designated wilderness. Cattle have significantly altered the composition
of many plant communities. Native grasslands quickly declined in
relative abundance relative to sagebrush following the introduction
of livestock in the 1800's, leading to the extinction in California
of the sharp-tailed grouse (Jones & Stokes 1987). In addition,
considerable area of bottomland has been converted to irrigated
pasture through stream diversion or is cropped for hay (Pease 1965).
Juniper woodlands appear to be invading the sagebrush communities
in response to cattle grazing and fire suppression. Cheatgrass,
an exotic annual, has spread to dominate all non-salt affected upland
communities below subalpine zones. Peppergrass (Lepidium latipes)
is a new pernicious weed that is just now invading the region.
Figure MOD-2. Management
status of lands in the Modoc Plateau Region. See text for definitions
of management levels.
shows the management status of lands in the Modoc Plateau Region.
Over sixty-three percent of the region is publicly owned. Public
ownership is distributed among many agenices: U.S. Forest Service
(32.8% of the total region), BLM (22.3%), USFWS (1.9%), DoD (1.5%),
Indian reservations (0.3%), National Park Service (0.8%), state
parks (0.1%), California Fish & Game (1.0%), and other state
land (2.1%). Among all ten regions in California, only the other
three desert regions have a lower proportion of private land.
relatively large proportion of public land in the region, the Modoc
Plateau has the smallest proportion in status 1 and 2 managed areas
among regions except for the Great Central Valley (Table MOD-1).
Twenty-seven status 1 and 2 managed areas (covering 5.2% of the
region) were mapped, seven of which are status 1. Status 1 managed
areas are dominated by Lava Beds National Monument at just under
19,000 ha, and the South Warner Wilderness Area on the Modoc National
Forest at 25,000 ha. Status 1 areas in the Modoc Plateau region
are not protected from grazing. Twenty status 2 areas consist of
eleven Wildlife Areas of California Fish & Game containing nearly
24,000 ha, four USFWS National Wildlife Refuges covering 43,000
ha, and 2 state parks. Waterfowl management areas are managed for
hunting. Fifty-five percent of the region is other public lands
managed at status 3.
Table MOD-1. Area
and percentage of land surface by management status level of the Modoc
Of the 27 status
1 and 2 managed areas, only 5 are larger than 10,000 ha. Three of
these are National Wildlife Refuges managed in large measure for
waterfowl rather than general biodiversity. The largest, the South
Warner Wilderness (including the Raider Basin RNA) is only 25,000
ha and grazing is allowed throughout. Eleven of the 27 managed areas
are less than 1,000 ha in size (although a few are partially located
in adjoining regions). Thus compared to most other regions of California,
the Modoc Plateau region has no large areas managed primarily for
Figure MOD-3. Comparison
of the proportion of managed areas with all lands in the Modoc Plateau
Region by elevation zones.
levels are distributed unevenly across elevation zones (Figure MOD-3)
and are quite dissimilar to other regions in the state. In the 1000-1500
m zone, which is the most prevalent elevation range in the region,
the percentage of status 1 and 2 is relatively higher than all lands
in the zone. In most regions, the lowest elevation zone is typically
under-represented in status 1 and 2. The next most widespread elevation
zone is 1501-2000 m is substantially under-represented. Elevations
above 2000 m occupy only 4% of the region but are almost exclusively
on public lands and contain roughly 17% of the status 1 and 2 managed
relatively uniform climate, substrata, physiography, and up to three
dominant vegetation types were delineated on 1990 Thematic Mapper
satellite imagery. In the relatively sparsely vegetated areas covering
much of the region, satellite data were dominated by reflectance
from rock and soil. Thus, black and white aerial photography (1:62,500,
USDA, 1955-1965) was also examined during the delineation process.
This imagery better revealed changes in tree density and disturbance
patterns, as well as vegetation physiognomy. These were the main
criteria for determining landscape units. Areas with salt affected
soils and relatively young, barren lava flows (e.g., near Lava Beds
National Monument) were also delineated. These were readily observed
on both photos and satellite imagery.
to delineation of landscape units can perhaps best be understood
by illustrating how vegetation/landscape units were defined along
a typical elevational gradient. The longest elevational gradient
in the region, from Surprise Valley westward up the east side of
the Warner Mountains, was delineated into landscape units that may
be described as follows: lakebed, salt-affected soil and vegetation,
irrigated pasture/hayfield, sagebrush with scattered western juniper,
western juniper woodland, lower montane coniferous forest, upper
montane coniferous forest, and alpine landscape. Each of these landscape
types may contain substantial complexity. Final delineation of landscape
units was an iterative process based on evidence from the satellite
imagery, the aerial photography, and the vegetation information
used during the labeling process. Vegetation boundaries shown on
the map sources used for labeling were often used to modify boundaries
of landscape units.
mapped in this region tend to be large, in large part because of
the general nature of Great Basin vegetation. Gradual changes in
species dominance and abundance from basins to alluvial fans to
highlands are typical. Vegetation had to occupy a minimum of 10
percent of the area within a polygon to be included in the description.
Thus, small scale upland vegetation features (e.g. aspen groves
or Washoe pine groves) were generally not put in the description
of dominant vegetation.
was generally not available for herbaceous vegetation. Complexes
of perennial herbaceous species associated with sagebrush, etc.
were therefore defined and recorded in the database in dominant
species fields rather than listing individual herbaceous species.
The relative cover of the entire complex was used to determine where
to place the entity in the species abundance ranking. The appropriate
grass species complex for labeling was determined based on elevation
and/or the occurrence of salt-affected soils.
The best available
source of floristic information was used for labeling. Information
from various sources ranges widely in date and accuracy. It is important
to consider how this might affect the description of landscape units.
Sources of floristic information were the following:
- Soil Conservation
Service 1:24,000 soil vegetation maps, which cover most of the
Type Map (VTM survey maps; Wieslander 1946), which cover the southeast
corner of the region
vegetation maps for Lava Beds National Monument contained in (Erhard
- Modoc National
Forest vegetation map, which was produced in 1978 from aerial
- Bureau of
Land Management range survey maps, covering the arid Surprise
- BLM integrated
management plan database, derived from a August 13, 1989, TM image
centered on the Madeleine Plains. A preliminary version of the
map was used, but then updates were made from the January, 1996
- Field reconnaissance
in areas for which no existing vegetation map was available. Accessible
roads through polygons were driven with numerous stops made to
check vegetation, using binoculars and a 200x stereoscope from
vantage points to survey areas distant from the road, and to identify
conifers on nearby slopes and ridges.
is considered most reliable where obtained from the Soil Conservation
Service 1:24,000 soil vegetation maps, Erhard's map of Lava Beds,
and the VTM survey, all of which were ground based and identified
dominant species rather than communities. There is no evidence of
major vegetation change since the circa 1930s date of the VTM maps;
however, it should be noted that a high percentage of junipers were
observed to be recently dead in the area covered by this map source.
The Forest Service and BLM maps only contained data on community
types defined usually by a dominant overstory species. We generally
assumed that sagebrush and bitterbrush were present in juniper and
other woodlands. Although this assumption is generally valid, the
abundance of sagebrush and bitterbrush should be considered somewhat
overestimated in areas labeled using the Forest Service information,
and these areas underestimate the importance of such species as
serviceberry (Amelanchier utahensis), wild rose (Rosa
gymnocarpa), and bitter cherry (Prunus emarginata). Forest
Service maps indicated some shrublands dominated by silver sagebrush
(Artemisia cana). It would be impossible to distinguish this
sagebrush from big sagebrush in aerial photos, so presumably the
distribution of silver sagebrush represents a prediction based on
the fidelity of this species for the lower portions of basins once
flooded by pluvial lakes. It is unclear how accurately the dominance
of silver sagebrush is mapped. Our own field descriptions are rich
with floristic information, but as they were based on visual inspection
of large landscapes and only estimate the relative abundance of
shrub species, which may appear similar from a distance. Dominance
estimates are therefore more questionable in these areas. In general,
data on upland community types and wildlife habitat types are more
reliable than information on individual species or on wetland or
nearly 18,000 km² (79%) of the Modoc Plateau region as vegetated,
undeveloped and uncultivated land cover. Nearly one-fifth has been
converted to towns or agricultural uses or contains open water or
bare ground. The vegetation layer was delineated into 704 landscape
units, with an average size of 3,247 ha, and provides distributional
information on 83 dominant species, 36 plant community types and
12 land cover/land use types.
Based on our
system for assigning dominant species assemblages into natural community
types, we mapped 36 vegetated community types within the Modoc Plateau
region. Twenty-six types were mapped with an area greater than 25
km². Five community types contribute 74% of the region's total
vegetated area. Great Basin Woodlands, Great Basin Mixed Scrub,
and Eastside Ponderosa Pine Forest are the most extensive types
mapped, covering 3,974 km², 3,551 km², and 2,917 km²,
Six new community
types were described for the region that were not included in the
original CNDDB classification (Holland 1986). These include Salvia
dorri/Chamaebatiaria Scrub, Low Sagebrush Scrub, Silver
Sagebrush Scrub, Cercocarpus ledifolius Woodland, Great Basin
Wet Meadow, and Modoc White Fir Forest.
Table MOD-2. Percent
area of each CNDDB community type at each management status level
in the Modoc Plateau Region. * indicates an addition to the standard
CNDDB classification (Holland 1986).
Community Name (Holland 1986)
Mapped Distribution (km²)
Basin Mixed Scrub
dorri / Chamaebatiaria Scrub *
Sagebrush Scrub *
Sagebrush Scrub *
ledifolius Woodland *
Basalt Flow Vernal Pool
Basin Wet Meadow *
Basin Cottonwood-Willow Riparian Forest
Basin Riparian Scrub
Interior Cypress Forest
Ponderosa Pine Forest
Mixed Coniferous Forest
White Fir Forest *
Pine-Lodgepole Pine Forest
Total (incl nonvegetated)
of the region is publicly owned, primarily in status 3 management
on public lands, most of the community types have similar management
profiles. We call attention, however, to three categories of poor
representation and one in which types appear to be well protected.
Table MOD-2 gives the area of mapped distribution and proportions
in each management level for every community type.
community types occurring mainly on status 4 lands. Four of
the 26 types have areas greater than 25 km² with more than
50% in status 4 lands (primarily privately owned). These types include
Desert Greasewood Scrub, Montane Manzanita Chaparral, Modoc-Great
Basin Cottonwood-Willow Forest, and Sierran Mixed Coniferous Forest.
The majority of Rabbitbrush Scrub (35400) and Non-Native Grassland
(42200) also occur on private land, but are considered transitional,
disturbance-following communities or heavily altered. Many of the
meadow and riparian scrub types were also mapped primarily on private
land but we have lower confidence in the accuracy of our mapping
of these relatively rare communities at the resolution of this gap
analysis. Modoc-Great Basin Cottonwood-Willow Forest is of conservation
concern because it is endemic to the Great Basin and has been ubiquitously
impacted by grazing. More accurate information of the distribution,
extent, and condition of riparian vegetation is still needed for
effective conservation action.
chaparral, and herbaceous types mainly located in unprotected areas.
Nine types were mapped with areas greater than 25 km² and with
less than 10% of their distribution in status 1 or 2 managed areas.
These include Great Basin Mixed Scrub, Big Sagebrush Scrub, Low
Sagebrush Scrub, Silver Sagebrush Scrub, Cercocarpus ledifolius
Woodland, Desert Saltbush Scrub, Desert Greasewood Scrub, Montane
Manzanita Chaparral, and Montane Ceanothus Chaparral. These nine
communities are of concern because of the impacts of long-term cattle
grazing which has selectively favored some plants over others. It
should be noted, however, that grazing is even allowed in many of
the status 1 and 2 areas as well. Northern Basalt Flow Vernal Pools
were only mapped where data existed on the SCS soil-vegetation maps,
although they are relatively common west of Goose Lake and north
of Big Sage Reservoir. This habitat of concern to the Natural Heritage
Division of Fish and Game needs to be mapped in more detail to improve
our understanding of its biological importance and management status
on the Modoc Plateau. The Rabbitbrush type is also not well-represented.
In many cases it occurs on degraded sites, but there may be circumstances
where Chrysothamnus dominates relatively pristine areas,
especially C. viscidiflorus. More information about disturbance
history for this type is needed to adequately assess its management
status. Great Basin Grassland and Great Basin Wet Meadow slightly
exceed the 10% representation threshold. Both types, however, have
been heavily impacted by the effects of grazing, and the grassland
type has been significantly converted to Non-Native Grassland, principally
with the spread of cheatgrass.
and woodland types mainly located in unprotected areas. Eight
types were mapped with areas greater than 25 km² and with less
than 10% of their distribution in status 1 or 2 managed areas. These
include Modoc-Great Basin Cottonwood-Willow Forest, Oregon Oak Woodland,
Great Basin Woodlands, Eastside Ponderosa Pine Forest, Sierran Mixed
Coniferous Forest, Modoc White Fir Forest, Jeffrey Pine Forest,
and Lodgepole Pine Forest. These types are of management concern
because timber harvest and/or grazing prevail where the communities
occur. Oregon Oak, Sierran Mixed Coniferous Forest, Jeffrey Pine
Forest, and Lodgepole Pine Forest are more characteristic of adjoining
regions and just barely cross into the Modoc Plateau. Northern Interior
Cypress Forest is entirely absent from status 1 or 2 areas and is
primarily on status 3 public lands. Although this type was only
mapped over 7.8 km² at Timbered Crater, it represents the largest
population of Baker cypress (Cupressus bakeri), which grows
primarily on volcanic soils in only eight California locations and
one in Oregon (Schoenherr 1992). This species depends on fire for
its reproduction (Vogl and Armstrong 1977), and so fire management
is an important consideration for the maintenance and regeneration
of this rare community type.
types that appear well-protected. Two types were mapped with
areas greater than 25 km² and with more than 25% of their distribution
in status 1 or 2 managed areas. These include Salvia dorri/Chamaebatiaria
Scrub (an endemic type only described in Lava Beds National Monument
by Erhard ) and Transmontane Freshwater Marsh. In addition,
Whitebark Pine-Lodgepole Pine Forest, with only 20 km² in the
region was mapped entirely within status 1 areas. This does not
imply that there is no impact from grazing even in these managed
areas. The marsh type in particular needs to be evaluated in greater
detail with more intensive mapping to establish its actual conservation
needs, as it is a critical habitat for many species in the region.
The Modoc Plateau
Region is the California portion of two ecoregional provinces in
the ECOMAP system, the Intermountain Semi-Desert and a Sierran ecoregion
(Bailey 1995). The management status of community types in the Modoc
region may not be representative of these provinces as a whole.
A vegetation gap analysis of the multi-state Intermountain Semidesert
Province, which includes the Honey Lake basin and the area east
of the Warner Mountains, has recently addressed conservation needs
in this larger context (Stoms et al. 1998). Therefore, our findings
should not be considered comprehensive and apply only to the California
portion of these more ecologically-defined regions. However, most
Great Basin types are not well-represented in biodiversity management
areas in other states within the ecoregion either (Caicco et al.
1995, Edwards et al. 1995). The gap analysis of the Intermountain
Semi-Desert Ecoregion (Stoms et al. 1998) identified highest conservation
priorities for riparian, wetland and meadow types, and the Sarcobatus
vermiculatus alliance (corresponding to the Desert Greasewood
community type in the CNDDB schema).
A formal, objective
process to select priority sites for potential new areas for biodiversity
management has not been implemented yet for the Modoc Plateau region.
We believe that it would be more appropriate to perform such an
analysis in conjunction with the larger Great Basin ecoregional
gap analysis. Nevertheless, certain areas contain concentrations
of vulnerable natural communities or vertebrates mentioned above.
These sites would be of conservation interest at least from the
more local perspective of the Modoc Plateau region within California.
Timbered Crater contains the largest population of Baker cypress,
which depends on fire to open its cones. The Pit River provides
habitat for the scarce cottonwood-willow forest and riparian scrub
communities and consequently areas of high species richness. The
Warner Mountains provide a disjunct montane habitat for many species
that occur nowhere else in the Modoc Plateau or are found some distance
away in the southern Cascades. Small, rare, Washoe pine stands,
not mapped by this project, are also found there.