A Guide to the Revegetation and Environmental Restoration of Closed Landfills

Chapters 4-5

Chapter 4: Types of Vegetative Communities

A "community" is defined as "an aggregation of living organisms having mutual relationships among themselves and to their environment."² A plant community includes each element of the vegetation characterized by a dominant species. For restoration of a plant community to be successfully achieved for any project, an understanding of the basics of plant communities must be explored. For a project proponent to install a vegetation community that will have the highest chance of succeeding, the planner must be aware of the types of plant communities that exist throughout California and the one at his project site. The operator must consider climate conditions, soil types, and compositions in the project area and demonstrate an awareness of the surface topography of the area surrounding the project site where restoration will occur.

Even in using nursery stock instead of California native plant stocks in a revegetation project, soil types, climate, and equivalency in plant types are important to successful survival of the final planting.

Throughout the State of California, plant communities have developed and evolved into distinct assemblages of plants and distribution patterns. Coastal plant communities differ greatly from desert plant communities. Alpine conifer forests will differ from valley chaparral. Species of plants will differ from one northern oak woodland community in northern California versus a southern oak woodland community in southern California, although they may look superficially alike. Even the western coastal conifer makeup is different from the conifer forests in the western Sierras. An awareness of these subtle differences may help make the difference in a restoration or revegetation project being a success or a potential failure.

California’s vegetative communities fall within four major vegetative zones.³ (Micro-environments are found within each major zone, containing their own distinctive plant communities.) Following are the four major zones.

Coastal Zone

This vegetation zone embraces the majority of northern California from Modoc County to the northeast, across the northern counties to include the mountainous areas of Siskiyou, Shasta and Trinity counties. This zone includes the coastal counties from Del Norte south to San Diego County, bounded by the coastal ranges on its eastern margin.

Plants in this zone are varieties that are highly moisture dependent, preferring a more temperate average climate ranging from the 50s (^OF) to the 90s rarely. Rainfall is abundant to moderately available while frequent occurrences of fog from the marine air layer of the Pacific Ocean increase atmospheric moisture levels. The conifers dominate the northern forests, including the Redwoods (Sequoia sempervirons) and Douglas Fir (Pseudotsuga menziesii). These species dominate as the overstory species. Hardwoods (Western Hemlock, oak, and others) can be commingled in some areas, occupying the intermediate layer of vegetation.

Smaller shrubs fall in the understory at the closest to ground level. More southerly forests will be populated with coastal species of hardwood (deciduous) forests. A pocket of alpine desert plant communities can be found in northern and eastern Siskiyou County and Modoc County.

The Interior Zone includes the entire Central Valley and a narrow band that follows along the eastern slopes of the Coast Ranges, including the west halves of Los Angeles, San Bernardino, Riverside and San Diego counties. Climatic conditions in this zone are drier than the Coastal Zone, being partially influenced by the initial rain shadow effect of the Coast Ranges. Temperatures can vary from the low 30s (^OF) into the 100-plus degree range. Atmospheric moisture is generally dry during the summer and fall months. A short period of heavy rains occurs between September and March.

This region is dominated by grasslands and Oak Chaparral communities, often with higher concentrations of vegetation along river and creek channels (riparian environments). Many of these riparian environments are dominated by cottonwoods (Populus trichocarpa or tremuloides) as well as willow (Salix subspecies) in the overstory layer. Digger Pine (Pinus sabiniana) can be found in the drier hilly areas of this zone. Embracing the San Gabriel and San Bernardino Mountains, the Interior Zone holds distinctive mountain plant communities in these ranges.

This zone includes the region running along the western slopes of the Sierra Nevada, from southern Modoc County, across all western Sierra counties southerly to Tehama County and including north Kern County. Dry temperate to warm summers and cold, snowbound winters at the higher elevations generally dominate climatic conditions in the Mountain Zone.

The western Sierra receives high volumes of rain, and thunderstorms are frequent. Much of the eastward migration of storm moisture conveyed to this point is precipitated out before crossing to the east desert regions. Vegetation in this area is dominated again by conifers such as Ponderosa Pine (Pinus ponderosa) and Fir (Abies grandis or A. concolor) in the overstory. These species of conifers are more tolerant of dryer, hotter climates than the conifers of the coastal varieties. Oak savannah or chaparral may dominate the southern portion of this zone. Arid conditions may dominate in the extreme southern zone. Hardwoods such as Valley Oaks (Quercus lobata) are found more at the lower elevations and as intermediate species at middle montane elevations.

The Desert Zone takes in northern Modoc County, the areas north of Lake Tahoe, and all of the easternmost counties to the southern California border. This zone includes the area bordered by the east slopes of the Sierra Nevada. The climate in this zone ranges from cool winter days to extremely hot summer daytime temperatures. The climate is generally arid with relatively less snowfall than the Mountain Region. Soil conditions are dry, sandy or stony, often forming a "desert pavement," creating harsh conditions for natural plant growth. Precipitation is limited because of the rain shadow of the Sierra. What rain there is may come primarily as cloudbursts creating brief flash flood events. Vegetation in this region consists of xerophytes—plants highly tolerant of harsh desert conditions. Junipers (Juniperis) including J. californica and J. communis in the north, and J. ostosperma in the Mojave region, succulents, creosotes and other shrubs, and assorted species of yuccas or other desert vegetation will dominate this plant community.

The desert environment is particularly sensitive to impacts from man. Desert regions, both low-altitude and alpine, possess very subtle features difficult to replicate. These regions take a long time to "heal" after excavations have been performed, and the slow rates of growth and relative sparseness of native plant species in the desert region will reveal scarring longer than other impacted areas. Barren rocky regions may make restoration nearly impossible to achieve as desert pavement and the phenomenon of "desert varnish," a dark glaze over the rocky surfaces, are difficult to reconstruct, requiring natural weathering to complete the process. Alpine desert areas as in central Siskiyou county display subtle signs of frost polygon-like forms in the soil, presenting cell-like arrangements of surface stones, surrounding low hummocks over large tracts of open grassland.

"Desert" refers to a natural environmental community created in evolutionary response to hot arid climates. Desertification is an environmental condition, usually resulting from the adverse activities of man. These activities, such as mismanagement of irrigation water, salt leaching, and concentration of other minerals in the soil and wind erosion of soils from tilling operations result from agricultural activities in fertile or marginally fertile lands of the desert regions. In the arid soils, chemicals accumulate and the soil surface forms a thin crust, relatively impermeable to the sparse available rains of those regions. The result is conditions that are hostile to plants and animals and loss of natural soil nutrients that inhibits revegetation efforts.

Figure 1 Map of California Showing Vegetative Zones Relative to Counties (21 KB)

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Chapter 5: Precipitation and Moisture

The types of plants to be selected and the irrigation plan intended for a specific landfill site will depend upon the average natural precipitation in a particular area. The project planner must take this variation into consideration. The precipitation pattern of California is atypical of most precipitation and climate distributions worldwide.

Most climate patterns follow defined responses to geographic features, resulting in gradated changes across a climate regime. This usually results in wetter coastal regions gradating to drier or arid environments inland. In California, the association between weather and precipitation, the widely varied terrain and regional temperatures creates a far more intricate melange of environmental zones. Precipitation may vary widely in two different areas even though they may both be within the same vegetation zone and geographic regime.

California’s terrain is divided lengthwise by two major mountain chains running the length of the state. Two regional mountain systems are located in the north central state and a long, transverse range in southern California. The Central Valley occupies the mid-portion of the state, while low desert and high desert plateaus and mountain complexes occupy the northeast and easternmost margins of California. The state’s length results in a broad temperature range from the north latitudes to the south. These wide-ranging environmental influences result in wide variation in temperatures and precipitation, all within small distances.

As an example, in the coastal vegetation zone, precipitation varies from 10 inches average annually, southeast of Monterey, to 100 inches or more north of Eureka; a 90-inch difference. Temperatures can be very cold on the north coast, yet warm in Monterey. Precipitation in the Central Zone varies between 10 and 70 inches. The eastern flank of the Sierra and the south desert region (Desert Zone) range from 2 to 20 inches average annual precipitation.

A balance must be developed between the natural precipitation and average temperatures, and the planned irrigation volumes for landfill revegetation at a specific project site. (See Figure 2).

Figure 2 Map of California Showing Average Annual Precipitation (63 KB)

Figure 3a Map of California Showing Locations of Active Landfills (14 KB)

Distribution of California’s Landfills
Californians live throughout the state, in the most remote areas of the mountains, to the farthest reaches of the desert. This distribution places these sources of waste within virtually every climatic, temperature and precipitation zone in the state. Figure 3a shows the locations of the State’s 186 active landfills. This wide dispersal of landfills demonstrates the diversity of waste management requirements and postclosure maintenance and land use demands placed upon operators and postclosure re-vegetation programs, both active and proposed.

Figure 3b Map of California Showing Locations of Active and Inactive Landfills (15 KB)

Figure 3b shows 262 landfills, active and inactive, within California. The majority of these landfills have not employed an environmental restoration program. Most employ programs compliant with regulations, employing the basic techniques of vegetative cover and standard engineering practice. Many employ aesthetic programs, incorporating golf courses or other recreational facilities in the postclosure use plan.

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Last updated: April 18, 2008