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Appendix GV. The Great Central Valley Region


Contributing Author: David Stoms

Regional Character
Land Stewardship
Plant Community Types

Regional Character

The Great Central Valley (GCV) Region (Figure GV-1) encompasses 58,627 km², consisting of the drainages of the Sacramento River in the north (15,787 km²) and the San Joaquin River in the south (42,840 km²). To account for the size and heterogeneity of the region, we also conducted a gap analysis on the two subregions. The boundary between the subregions bisects the Delta, and in the east follows a low divide between the Cosumnes and Mokelumne river drainages. The upper elevational limit of the region occurs at the transition between grassland of the Valley and the foothill woodlands in the Sierra Nevada, Central Western, Northwestern, Southwestern, and Cascade Ranges regions. The Carrizo Plain is included in the region because its vegetation is similar to that of the pre-settlement San Joaquin Valley, even though it is located at 700 m elevation. The woodlands of the Temblor Range are excluded from the region, forming a disjunct part of the Central Western Region.


Figure GV-1. Landsat image of the Great Central Valley (GCV) Region. Solid lines mark the subregion boundaries.

The GCV is an enormous plain with very little topographic relief. Sediments lie up to 10 km deep over the Sierra Nevada basement rock. The only major topographic feature is Sutter Buttes, an igneous outcrop rising over 600 m above the Sacramento River Valley. Soils tend to be fertile, lacking only adequate water in the south to be naturally productive. Hardpan formed beneath the surface in other areas. In spring, low lying "hogwallows" in the valley over this hardpan fills with water, forming vernal pools which often have unique flora. Soils around the flood basins and vernal pools are naturally alkaline as the water evaporates and leaves behind dissolved salts. Other vernal pools occur in the north end of the valley on volcanic soils, while terrace pools are found on ancient flood terraces on higher ground on the eastern side of the region. Rolling mounds, called "mima mounds" up to 2 m high often surround these terrace pools (Schoenherr 1992). Ancient sand dunes occur next to the Kettleman Hills along the shoreline of extinct Lake Corcoran (Norris and Webb 1990).

The arid climate favored the formation of grassland over all well-drained areas of the valley. Perennial bunchgrasses dominated, especially Nassella pulchra (Purple needlegrass, formerly Stipa pulchra). Huge freshwater marshes formed in the depressions in the Tulare Basin at the southern end of the region. Vernal pools were scattered throughout the region in small depressions over hardpans of various materials such as clay or basalt. The persistence of water later in the spring and the high salinity of the soils created a unique flora associated with these vernal pools. In the saline soils surrounding the shallow lakes and marshes, alkali scrub communities were found. Extensive riparian forests and woodlands bordered streams and rivers on the well-watered natural levees and terraces. Every one of these types has been extensively reduced and degraded throughout the valley as a result of the complex interaction of grazing, agricultural and urban development, draining of the vast wetlands, monumental modifications of the surface hydrology for flood control and irrigation, ground water pumping, controlling of fire, pesticide applications, firewood cutting, and the introduction of exotic species (Barry 1972, Katibah 1984, U. S. Fish and Wildlife Service 1978, Holland 1978).

In the mountainous and desert regions of the state, the public domain land remained basically in federal hands. In the flat, fertile Great Central Valley, however, nearly all land not already in Spanish or Mexican land grants was homesteaded in the 19th century. A mere 2% of the region that was retained in the public domain under the management of the Bureau of Land Management (BLM). Virtually all managed areas in this region were acquired by government agencies or private conservation organizations in this century. Many small to medium-sized military bases are distributed throughout the region. Greater than 90 percent of the land area is in private ownership, primarily dedicated to urban or agricultural uses. Only a small fraction of private land is managed for biodiversity protection, such as The Nature Conservancy preserves and National Audubon Society sanctuaries. Private duck clubs own the majority of remaining non-public wetlands in the Central Valley, which they manage for waterfowl habitat and sport hunting (Frayer et al. 1989). Many of these clubs struggle to remain open in the face of stiff competition for water needed to maintain the ponds in drought years.

The geometry of the Public Land Survey System and human settlement has been superimposed on the geography of natural topography and hydrology of the Great Central Valley, giving a superficial impression that the region is homogeneous, if not monotonous. However, the underlying variation in climate and soils still exert their influences to maintain significantly different habitats and species assemblages. The Delta still retains extensive wetlands. Rice fields in the north can provide wintering habitat for migratory waterfowl while the southern valley has evergreen citrus orchards. Forests still cling to the banks of rivers in the north, in stark contrast to the alkali sinks in the San Joaquin Valley where water is less plentiful. And when extreme winter precipitation does come, satellite images reveal the normally hidden shapes of Tulare Lake and other historic wetlands. Nature reserves must be distributed throughout the valley to fully protect the diversity of species and habitats.

Many of the best examples of relic pristine habitats are now being preserved by the joint efforts of numerous agencies and organizations. Although biodiversity has been permanently reduced, there are still opportunities to protect moderately high quality sites and to restore degraded habitats to better conditions than currently exist. Previous conservation efforts have focused on individual elements of biodiversity such as threatened and endangered species, specific habitats such as California prairie and riparian woodlands, or specific geographic locations.

Land Stewardship

Figure GV-2 shows the management status in the Great Central Valley Region. Ninety-four percent of the region is privately owned. The remainder in public ownership is widely distributed among BLM (2.3%), DoD military bases (1.0%), USFWS (0.8%), US Forest Service (0.1%), state parks (0.2%), California Fish & Game (0.8%), other state lands (0.1%), local governments (0.1%), and private conservation groups (0.2%). The proportions are remarkably similar proportions in both subregions. The main difference between subregions is that most BLM land is in the San Joaquin Valley, while National Wildlife Refuges make up a larger proportion of the Sacramento Valley subregion.

Great Central Valley Region Managed Areas

Figure GV-2. Management status of lands in the Great Central Valley Region. See text for definitions of management levels.

Eighty-two status 1 and 2 managed areas were mapped for the Great Central Valley Region, covering a combined 3.1% of the entire region (Table GV-1). The 28 status 1 managed areas are dominated by 13 Nature Conservancy preserves and 13 ecological reserves of the California Department of Fish & Game. The status 2 areas include 15 USFWS national wildlife refuge units, 29 state wildlife units, 3 state park units, 2 water district tracts, 2 BLM ACEC's, and the Carrizo Plain Natural Area macropreserve managed jointly by the BLM, The Nature Conservancy, and California Department of Fish and Game. The Carrizo managed areas collectively encompass 62,852 ha of lands managed for biodiversity. Another 1.6% is other public lands managed at status level 3. As with ownership profiles, the distribution of management levels by subregion closely follow the region statistics.

Of the status 1 and 2 managed areas, only two of the BLM ACEC's are greater than 10,000 ha in size. Both are on the western edge of the San Joaquin Valley subregion. The average size is only 2,500 ha. Half of the managed areas are less than 1,000 ha, including most of the non-governmental organization preserves, state parks, and Fish & Game ecological reserves and wildlife areas. This pattern of small managed areas reflects the highly fragmented nature of the Great Central Valley, where large, contiguous blocks of habitat seldom exist.

Table GV-1. Area and percentage of land surface by management status level of the Great Central Valley Region, the Sacramento Valley (SV) and San Joaquin Valley (SJV) Subregions.

Great Central Valley Region Sacramento Valley Subregion San Joaquin Valley Subregion
Status Area (km²) % Area (km²) % Area (km²) %
1 204 0.3 76 0.4 128 0.3
2 1,643 2.8 454 2.9 1,189 2.8
3 945 1.6 138 0.9 808 1.9
4 55,670 95.2 14,994 95.7 40,676 95.0
Total 58,463 100.0 15,662 100.0 42,801 100.0

Plant Community Types

Initial landscape boundaries were derived from the USGS Digital Land Use and Land Cover maps (DLULC, USGS 1986) mapping. This set of maps identify cover and use classes according to Level 2 of the Anderson classification system (Anderson et al. 1976), which only distinguishes broad types. Examples of Anderson Level 2 classes include Herbaceous Rangeland and Evergreen Forest/Woodland. Crops are also distinguished from orchards; residential, commercial, industrial, and other urban uses are defined. The DLULC mapping was nominally at 1:250,000 scale from National High-Altitude Photography (NHAP) taken during the mid-1970's. Because the photography was at the 1:60,000 scale, USGS obtained a minimum mapping unit size of 16 ha for most classes and 4 ha for selected land uses and open water.

We first generalized these maps by combining urban classes and deleting very small map units to achieve the spatial resolution objective of GAP (minimum mapping unit of 100 ha, or 1 km²). This base map was then edited subjectively by photointerpretation of patterns in the satellite imagery to improve registration of distinct edges and to account for recent and use changes. Final delineation of landscape units was an iterative process based on evidence from the satellite imagery, 1990 air photos, existing vegetation maps and field reconnaissance. Wetlands were added from the 1:24,000 scale digital National Wetlands Inventory (NWI) maps from the U. S. Fish and Wildlife Service, compiled from aerial photography in the mid 1980's using a classification based on hydrologic system and vegetation structure (Cowardin et al. 1979). The detailed map units of NWI, which were available in digital form only from Fresno northward at the time, were generalized into larger landscapes for GAP purposes. Other source maps for delineating landscapes include: relic patches of native perennial bunchgrass prairie (Barry 1972 and redrawn by Dremann 1988); Carrizo Plain from the Bureau of Land Management, Bakersfield District; Southern San Joaquin Valley from the California Energy Commission (Anderson et al. 1991); and riparian forests (Nelson and Nelson 1984).

Floristic information was derived mainly unpublished maps produced by the VTM survey and from our own field survey. We also relied on the maps by Barry (1972) for species composition of relic prairie grasslands. We did not attempt to survey the region to identify new native grassland sites beyond those inventoried by Barry or from the NDDB. Some species information was also obtained from the Central Valley Riparian Mapping Project (CVRMP) maps (Nelson and Nelson 1984). Site visits were also used to resolve discrepancies between sources or to verify older information. Of the 2,216 landscape units mapped in the region, 322 of them, covering almost 11,000 km² or 20%, were visited in the field during 1995. The database for the GCV Region provides distributional information on 57 dominant species, 34 plant community types and 20 human-dominated or unvegetated land use/cover classes, and 33 wildlife habitat types.

Because source information ranged widely in date and reliability, the current database is uneven in both level of detail and accuracy. We did not have the resources to assess the statistical accuracy of the vegetation map and associated database. However, we have appraised the product using less formal methods that have guided our use of the product. The vegetation map probably overestimates the extent of agricultural types and underestimates the extent of shrubland, riparian hardwood and wetland types where these occur as tiny habitat remnants within a cultivated matrix. Floristic information is more reliable in the San Joaquin Valley subregion than in the Sacramento Valley subregion, which was only partially covered by our field survey. Floristic information is also more reliable on public lands than private lands. Data on community types for small managed areas is generally based on published information about the vegetation of the site, and so the contents, if not the landscape boundaries, of such sites should be reliable. The data on upland community types and wildlife habitat types are more reliable than information on individual species or on wetland or vernal pool habitats.

We classified 18,520 km² (31.6%) of the GCV region as vegetated other than agricultural or urban land uses, water, or bare ground. Sixty-three percent has been converted to agricultural uses with an additional 4.1 percent urban or industrial uses, including oil field development. Average size for the 974 vegetated landscapes is 1,905 ha. Excluding non-native grassland map units which tend to be large, the average size is 805 ha.

Based on our system for converting dominant species assemblages into natural community types, we mapped 34 community types within the GCV region out of 40 types described by CNDDB (Holland 1986). Nineteen other land use/cover classes were also mapped. Seventeen of the 34 natural community types were mapped with an area greater than 25 km². Non-native Grassland and Valley Saltbush Scrub are by far the most extensive types, covering 14,000 km² (76%) and 1,900 km² (10%), respectively. Four other community types contribute an additional 8% of the region's total area of native vegetation­Diablan Sage Scrub, Coastal and Valley Freshwater Marsh, Great Valley Cottonwood Riparian Forest, and Blue Oak Woodland. The following six natural communities described by the CNDDB (Holland 1986) for the GCV region were not mapped because they were not detectable by our methods: Stabilized Interior Dunes, Sierra-Tehachapi Saltbush Scrub, Alkali Seep, Alkali Playa Community, Buttonbush Scrub, and Elderberry Savanna.

Table GV-2. Percent area of each CNDDB community type at each management status level in the Great Central Valley Region.

CNDDB Code CNDDB Community Name (Holland 1986) CNDDB Rank Status 1 % Status 2 % Status 3 % Status 4 % Total Mapped Distribution (km²) Status 1+2 %
23300 Monvero Residual Dunes S1.2 0.0 31.0 0.1 68.9 3.0 31.1
32600 Diablan Sage Scrub S3.2 0.0 5.3 19.1 75.6 326.6 5.3
35400 Rabbitbrush Scrub S5 0.0 0.0 32.7 67.3 2.1 0.0
36210 Valley Sink Scrub S1.1 1.8 9.4 0.0 88.7 250.4 11.2
36220 Valley Saltbush Scrub S2.1 0.9 11.9 6.5 80.7 1,900.7 12.8
36320 Interior Coast Range Saltbush Scrub S2.1 1.5 16.1 8.9 73.5 27.7 17.6
37200 Chamise Chaparral S4 0.0 0.0 0.0 100.0 1.5 0.0
37400 Semi-Desert Chaparral S3.2 2.2 17.5 47.3 33.0 99.8 19.7
37900 Scrub Oak Chaparral S3.3 0.0 0.0 6.8 93.2 6.3 0.0
37A00 Interior Live Oak Chaparral S3.3 0.0 0.0 0.0 100.0 26.4 0.0
39000 Upper Sonoran Subshrub Scrub S3.2 1.4 50.5 3.8 44.3 175.2 51.9
42110 Valley Needlegrass Grassland S3.1 34.7 0.0 0.0 65.3 4.1 34.7
42120 Valley Sacaton Grassland S1.1 0.0 64.7 0.0 35.3 9.1 64.7
42200 Non-native Grassland S4 0.9 4.5 3.6 91.1 13,996.9 5.4
44110 Northern Hardpan Vernal Pool S3.1 0.0 0.0 0.0 100.0 0.1 0.0
44120 Northern Claypan Vernal Pool S1.1 21.3 0.0 0.0 78.7 1.6 21.3
45310 Alkali Meadow S2.1 0.0 0.1 2.1 97.8 35.2 0.1
52200 Coastal Brackish Marsh S2.1 0.8 17.0 1.5 79.2 266.1 17.8
52310 Cismontane Alkali Marsh S1.1 0.0 10.7 0.0 89.3 9.3 10.7
52410 Coastal & Valley Freshwater Marsh S2.1 0.2 40.8 0.0 59.0 449.0 41.0
61410 Great Valley Cottonwood Riparian Forest S2.1 1.5 17.8 1.1 79.5 308.8 19.3
61420 Great Valley Mixed Riparian Forest S2.2 5.0 13.3 1.1 80.7 41.5 18.3
61430 Great Valley Valley Oak Riparian Forest S1.1 1.2 4.6 1.3 92.9 71.2 5.8
62100 Sycamore Alluvial Woodland S1.1 0.0 0.0 0.0 100.0 2.3 0.0
63410 Great Valley Willow Scrub S3.2 1.0 0.0 1.4 97.6 18.9 1.0
63420 Great Valley Mesquite Scrub S1.1 0.0 0.3 0.0 99.7 22.6 0.3
63810 Tamarisk Scrub S4 0.0 12.7 1.5 85.8 53.0 12.7
63820 Arrowweed Scrub S3.3 0.0 0.0 7.4 92.6 13.4 0.0
71130 Valley Oak Woodland S2.1 0.0 0.1 0.6 99.3 29.7 0.1
71140 Blue Oak Woodland S3.2 0.8 2.0 3.6 93.6 317.3 2.8
71150 Interior Live Oak Woodland S3.2 0.1 0.0 1.0 98.9 17.0 0.1
71160 Coast Live Oak Woodland S4 0.0 0.0 12.7 87.3 0.6 0.0
71430 Juniper-Oak Cismontane Woodland S3.2 2.1 15.5 1.2 81.2 8.0 17.6
72200 Mojavean Pinyon-Juniper Woodland S3.2/4 0.0 0.0 46.9 53.1 24.6 0.0

Region Total - Natural Communities


Region Total - All Lands
0.3 2.8 1.6 95.2 58,627 3.1

We call attention to three groups of distribution by management level, including one in which types appear to be well protected. The area of mapped distribution and the proportions in each management level are given in Table GV-2.

1. Plant community types mainly on status 4 lands. Six of the 34 types have areas greater than 25 km² with more than 90% in status 4. These types include Interior Live Oak Chaparral, Non-Native Grassland, Alkali Meadow, Great Valley Valley Oak Riparian Forest, Valley Oak Woodland, and Blue Oak Woodland. The Interior Live Oak Chaparral and Blue Oak Woodland types are more common in neighboring regions and are marginal to the Great Central Valley. Non-Native Grassland is widespread and heavily altered by grazing and invasions of exotic plants. The remaining three types are all ranked as 1.1 or 2.1 by the state (California Department of Fish and Game, unpublished list, dated 4/12/91). That is, they are considered rare and very threatened by losses to agricultural and urban conversion or to degradation from grazing, water diversions, or chemical pollution. Remaining patches are often tiny remnants surrounded by disturbed lands, so that they are hardly pristine representatives. Small amounts of Sycamore Alluvial Woodland and Great Valley Willow Scrub and Mesquite Scrub occur almost exclusively on private land. Valley Sink Scrub, Valley Saltbush Scrub, and Great Valley Mixed Riparian Forest have greater than 80% in status 4. Tamarisk Scrub, also with greater than 80% in status 4, is considered an exotic community type.

2. Plant community types mainly located in unprotected areas. Six of the 34 types were mapped with areas greater than 25 km² and with less than 10% of their distribution in Status 1 or 2 areas, which are designated for conservation of native biodiversity. These include Diablan Sage Scrub, Interior Live Oak Chaparral, Alkali Meadow, Great Valley Valley Oak Riparian Forest, Valley Oak Woodland, and Blue Oak Woodland. The Interior Live Oak Chaparral and Blue Oak Woodland types are more typical of the surrounding foothill regions. Mojavean Pinyon-Juniper Woodland falls just below the size threshold, but is contiguous with similar habitat in the Central Western and Southwestern regions. This type is at the margin of its distribution in the southwestern corner of the Great Central Valley region and may have unusual floristic combinations of species.

3. Community types that appear well-protected. Two of the 34 types were mapped with areas greater than 25 km² and with more than 25% of their distribution in Level 1 areas. These include Upper Sonoran Subshrub Scrub and Coastal and Valley Freshwater Marsh. Upper Sonoran Subshrub Scrub is ranked as 3.2, or threatened, whereas Coastal and Valley Freshwater Marsh is ranked as 2.1, or very threatened.

Several other types are of concern to the state but not identified above because their management profiles fall in between those of groups 1-2 and group 3 above. Included in this group are Valley Saltbush Scrub, Interior Coast Range Saltbush Scrub, Great Valley Cottonwood Riparian Forest, and Great Valley Mixed Riparian Forest. All four types are ranked by the State, however, as either threatened or very threatened, and have suffered extreme reductions in their distributions. Similarly, Valley Needlegrass and Valley Sacaton Grassland are largely protected in BMA's although their extent is less than 25 km². Both are also ranked as very threatened. The explanation for the high proportion of status 1 and 2 land for these types is that they have been actively protected in recent years in recognition of their rarity, the biological significance, and the substantial historical losses. As the last remnants of these types, additional management action may still be needed to preserve them.

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