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Appendix MP. The Modoc Plateau Region


Contributing Authors: Dennis Odion, David Stoms, and Frank Davis

Regional Character
Land Stewardship
Plant Community Types

Regional Character

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.

The Columbia 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 precipitation levels.

Low sagebrush (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.

Pinyon pine (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.

Land Stewardship

Modoc Plateau Region Managed Areas

Figure MOD-2. Management status of lands in the Modoc Plateau Region. See text for definitions of management levels.

Figure MOD-2 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.

Despite the 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 Plateau Region.

Status Area (km²) %
1 467 2.1
2 698 3.1
3 12,396 54.8
4 9,038 40.0
Total 22,599 100.0

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 biodiversity.

Elevation Bias in Modoc Region

Figure MOD-3. Comparison of the proportion of managed areas with all lands in the Modoc Plateau Region by elevation zones.

The management 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 areas.

Plant Community Types

Areas having 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.

The approach 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.

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.

Floristic information 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:

Floristic information 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 meadow habitats.

We classified 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, respectively.

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).

CNDDB Code CNDDB Community Name (Holland 1986) CNDDB Rank Status 1 % Status 2 % Status 3 % Status 4 % Total Mapped Distribution (km²) Status 1+2 %
35100 Great Basin Mixed Scrub S4 0.9 1.0 70.2 27.8 3,551.3 1.9
35110 Salvia dorri / Chamaebatiaria Scrub * -- 90.9 0.8 7.5 0.8 30.4 91.7
35210 Big Sagebrush Scrub S4 4.7 0.8 66.5 27.9 1,505.2 5.5
35211 Low Sagebrush Scrub * -- 0.1 1.9 70.1 27.9 1,342.0 2.0
35212 Silver Sagebrush Scrub * -- 0.4 4.2 50.6 44.8 389.8 4.6
35400 Rabbitbrush Scrub S5 0.0 0.0 49.3 50.6 316.4 0.0
35500 Cercocarpus ledifolius Woodland * -- 6.8 0.0 75.6 17.6 113.6 6.8
36110 Desert Saltbrush Scrub S3.2 0.0 0.0 67.4 32.6 158.8 0.0
36130 Desert Greasewood Scrub S3.2 0.0 4.9 16.1 79.1 485.4 4.9
37520 Montane Manzanita Chaparral S4 0.0 0.0 46.0 54.0 209.3 0.0
37530 Montane Ceanothus Chaparral S4/3.3 0.0 0.0 87.9 12.1 58.9 0.0
37810 Buck Brush Chaparral S4 0.0 23.1 69.4 7.4 35.3 23.1
42200 Non-Native Grassland S4 0.5 3.3 42.1 54.1 392.5 3.8
43000 Great Basin Grassland S1.1 0.3 13.1 40.1 46.5 99.7 13.4
44131 Northern Basalt Flow Vernal Pool S2.2 0.0 0.0 96.8 3.2 3.9 0.0
45100 Montane Meadow S3.2 0.0 0.0 10.2 89.8 19.7 0.0
45310 Alkali Meadow S2.1 0.0 0.0 0.0 100.0 10.5 0.0
45500 Great Basin Wet Meadow * -- 0.0 10.4 47.3 42.3 196.8 10.4
52320 Transmontane Alkali Marsh S2.1 0.0 35.5 14.3 50.2 11.7 35.5
52420 Transmontane Freshwater Marsh S2.2 0.0 51.4 9.6 38.9 203.8 51.4
61610 Modoc-Great Basin Cottonwood-Willow Riparian Forest S2.1 0.0 0.6 16.0 83.4 52.0 0.6
63600 Modoc-Great Basin Riparian Scrub S2.1 0.0 5.6 24.8 69.6 24.3 5.6
71110 Oregon Oak Woodland S3.3 0.0 5.3 50.9 43.8 216.4 5.3
71410 Foothill Pine-Oak Woodland S4 0.0 16.4 52.6 31.0 17.5 16.4
72100 Great Basin Woodlands S3.2/4 3.0 0.5 73.8 22.7 3,973.6 3.5
81340 Black Oak Forest S4 0.0 0.0 41.5 58.5 11.9 0.0
81B00 Aspen Forest S3.2 12.0 0.0 77.2 10.8 29.4 12.0
83220 Northern Interior Cypress Forest S2.2 0.0 0.0 82.6 17.4 7.8 0.0
84220 Eastside Ponderosa Pine Forest S2.1 1.0 0.0 69.1 29.9 2,916.6 1.0
84230 Sierran Mixed Coniferous Forest S4 0.0 0.0 49.9 50.1 176.4 0.0
84260 Modoc White Fir Forest * -- 9.1 0.0 54.7 36.3 942.7 9.1
85100 Jeffrey Pine Forest S4 0.0 0.5 82.0 17.5 236.1 0.5
85210 Jeffrey Pine-Fir Forest S4 20.5 0.0 73.9 5.6 107.1 20.5
85310 Red Fir Forest S4 0.0 0.0 100.0 0.0 8.3 0.0
86100 Lodgepole Pine Forest S4 0.0 0.0 90.9 9.1 101.7 0.0
86220 Whitebark Pine-Lodgepole Pine Forest S4 100.0 0.0 0.0 0.0 19.8 100.0

Regional Total-Natural Communities


Regional Total (incl nonvegetated)
2.1 3.1 54.8 40.0 22,858.0 5.2

Because two-thirds 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.

1. Plant 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.

2. Scrub, 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.

3. Forest 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.

4. Community 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 [1979]) 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.

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