How to Read the Alliance and Other Descriptions

First Paragraph of the Description

The leading sentence of the first paragraph defines the alliance and indicates which vegetation layer(s) (e.g., tree, shrub, or herbaceous layer) is important in defining it. The plant(s) named first in the leading sentence also denote the diagnostic plants that are dominant, co-dominant, or characteristically present. The plants in the description following the first one(s) are associates: they occur commonly in all or some parts of the alliance. Since species composition can vary regionally, we do not include all possible species in this list.

In forest and woodland descriptions, we describe the understory and herbaceous layers structurally. In cases where forests are of two heights, we describe the tree canopy as “two-tiered.” Other ecologists use the terms “subcanopy” and “understory” for shorter tree layers, and still others use the terms “subcanopy” and “understory” to refer only to tree reproduction. In our descriptions, stands with two-tiered tree canopies may also have a lower layer of tree reproduction. In woodlands, the description of the understory may include plant composition.

In both shrubland and herbaceous descriptions, we commonly list emergent plants that are taller than these layers if they regularly occur. Emergent trees must be at low cover (generally < 10% absolute cover) over the shrub or herbaceous layer; if not, then the stand belongs to a tree alliance. In grassland, meadow, and other non-woody alliances, the emergent shrub and tree layers may include commonly encountered trees or shrub species. In other alliances, too many different tree or shrub species are emergent above the layer to list them.

What Are Dominant, Co-Dominant, and Characteristic Plants?

The plants listed first in the description define and typify the alliance. Dominant plants have high cover compared to other plants. In redwood forests, for example, the cover of redwood trees dominates to the exclusion of other trees. Redwood is a dominant. (It is also a dominant in the sense of height, but we do not use the term in that way in the descriptions.) Many quantitative studies have established rules, such as: if a plant’s relative cover exceeds 50% of a layer’s total cover, the plant is a dominant, and if two plants have similar cover, say between 30% and 60% relative cover in the same layer, the plants are co-dominants. These decision rules used by researchers are listed in the Membership Rules section. Plants called “characteristic” can be of low cover, but their constancy or presence is sufficient to define an alliance. Most often, these plants are conspicuous but of low or variable cover. For example, Joshua trees rarely dominate in parts of the Mojave Desert, but they are quite noticeable and diagnostic of that region. The presence of Joshua trees places a stand in the Yucca brevifolia alliance.

What about the co-dominants not named in the leading part of the first paragraph? A survey of associations listed in many alliance descriptions offers several combinations of co-dominants or dominant trees found in two different canopy tiers. For example, in the Pseudotsuga menziesii alliance, the associations involve the following groups with trees co-dominant across one or more tiers:

A careful reading of the leading part of the Pseudotsuga menziesii alliance description only names P. menziesii as characteristic and allows it to be co-dominant with other trees. In this case, six trees fill that role at the association level. These groups might be seen as suballiances. Should these groups get alliance status? Time will tell. The reason that we gave mixed stands of Lithocarpus densiflorus and P. menziesii alliance status was their commonness and predictability in the forests of northwestern California and southwestern Oregon. These suballiance groups may be better mapping units than the more generally defined alliances. In some cases, recognizing Douglas fir forests as different from Douglas fir-California bay forests may be important; at other times, the alliance or the association level may be best.

We use the following conventions in the alliance names: We usually use the name of the one or two most characteristic species. In rare cases, we use the subspecies level if the ecology is sufficiently different between subspecies. At other times, two species in the same genus have very similar ecology, so we place the specific epithets in parentheses, such as in the Amsinckia (menziesii, tessellata) alliance.

New Names

You will encounter many new scientific names in the alliance descriptions. Although we considered retaining the Jepson Manual (Hickman 1993) as our standard source, we decided to embrace the many monographs and revisions of more recent taxonomy and the names used in the NVCS. These new names are also increasingly common in the ecological literature. While the new edition of the Jepson Manual is available, we have typically used this as a secondary reference (see http://ucjeps.berkeley.edu/IJM.html).

We rely instead on two comprehensive sources of up-to-date plant nomenclature: The Flora of North America North of Mexico (Flora of North America Editorial Committee 1993) and the USDA Plants Database of the United States Department of Agriculture and the National Resources Conservation Service (USDA-NRCS 1999-2013; http://plants.usda.gov). Both references provide us with nationally accepted binomials and a reasonably comprehensive list of synonyms.

Life History Traits of Principal Species Table

This table summarizes the life history of the consistently dominant plant(s) that characterize the alliance. This depiction offers a start to understanding the ecology and adaptations of the alliance: How long do the dominants live? How do they respond to disturbances? Is that response similar throughout the range of the alliance? We do not supply a table for alliances we currently consider provisional or for those that are non-native, semi-natural stands.

Habitats Section

The second paragraph briefly describes the environmental settings associated with the alliance, including elevation. If the plant grows in wetlands, we supply the U.S. Fish and Wildlife Service’s Wetland Inventory rank for California (see USDA-NRCS 1999-2013; http://plants.usda.gov). California is Region O (per the 1996 list), and in Regions AW and WMVC (per the 2013 list).

Alliance Rarity Ranking and Classification Synonymy Section

The third paragraph first lists the California Natural Diversity Database’s rarity ranking for the alliance (CNPS’s data and reports contributed greatly to the ranks). We are using the NatureServe’s Heritage Program methodology defined for Natural Community Conservation Ranks as defined below (and see http://www.natureserve.org). The G indicates the alliance’s rarity and threat globally, and the S indicates the alliance’s rarity and threat in California.

Global and State Ranks

Additional Threat Ranks

If an alliance is marked with a G1 through a G3 code, the alliance and associations are rare and threatened throughout its range. An alliance marked with a G5 and an S1 through an S3 code is secure through its range outside the state but is rare and threatened in California. A G4 S4 alliance may or may not be endemic to the state and is secure statewide.

Classification Synonymy

We then offer a crosswalk to other vegetation and habitat classifications used commonly in California. MCV refers to the first edition of A Manual of California Vegetation. NVCS refers to the National Vegetation Classification System that is supported by the Ecological Society of America, Federal Geographic Data Committee, NatureServe, the Nature Conservancy, and the U.S. Geological Service-National Park Service’s Vegetation Mapping Program in association with U.S. Fish and Wildlife Service. See NatureServe Explorer: an online encyclopedia of life (http://www.natureserve.org/explorer/).

For mapping classifications, “Calveg” refers to the USDA Forest Service’s Classification and Assessment with Landsat (satellite technology) of Visible Ecological Groupings system. This assesses California’s existing vegetation for use in statewide resource planning (Parker and Matayas 1979; http://www.fs.fed.us/r5/rsl/projects/mapping/accuracy.shtml). “Holland” refers to the California Natural Diversity Database (CNDDB) of the California Department of Fish and Game (CDFG). The goal of the CNDDB is to provide up-to-date information on the state’s most imperiled elements of natural diversity (Evens and Keeler-Wolf 2006). The first attempt, Preliminary Descriptions of the Terrestrial Natural Communities of California by Holland (1986), is often cited; it paved the way for this manual.

For other classifications, Munz provides a general classification of the state’s vegetation in the introduction to A California Flora (1959). As the state’s standard for flora description for over 30 years, this flora was one of the first to include vegetation information with the plant descriptions. Even today, well-regarded books such as Introduction to California Plant Life (Ornduff et al. 2003) build upon Munz. The acronym “WHR” refers to another CDFG program, the California Wildlife Habitat Relationships System. This program provides information on the life history, geographic range, habitat relationships, and management of 692 vertebrate species of amphibians, reptiles, birds, and mammals known to exist in the state. The program includes a classification of mainly vegetation-defined habitats (Mayer and Landenslayer 1988; http://www.dfg.ca.gov/biogeodata/cwhr/).

Membership Rules Section

In this section we report the specific membership rules developed by local and regional vegetation studies. The researchers offer specific values to define the terms “dominant” and “co-dominant,” based on analysis and classification of vegetation survey data. These rules may differ among studies for the same alliance, though often they are quite similar. How is that possible? Different researchers have different outlooks and different datasets, and the differences commonly come from the localized understanding of an alliance. Stand species composition, especially in widespread alliances, differs regionally. As people survey more of the state’s vegetation, and as we combine local datasets to analyze them together, we expect the rules will become more similar, at least at the regional level.

Remarks Section

The remarks section further describes the dominant plants and the alliance. The first paragraph provides the specific characteristics of the dominant, adding details to the plant’s life history. We may include taxonomic information involving our choice of scientific names. If another plant consistently co-dominates, we include a life history narrative for it as well. The last paragraph deals with the alliance’s life history and ecology. We often describe the alliance in relation to other ones.

Fire Characteristics Section

We present this section as a table in addition to text. The information summarizes the fire characteristics associated with the alliance or with the semi-natural stands. This information, combined with the ecological attributes of the plants dominating the alliance, provides for a better understanding of the vegetation position in the overall state pattern. For detailed discussions of the terms used in the table, see Fire in California Ecosystems (Sugihara et al. 2006). We do not supply a table for alliances we currently consider provisional. We also do not supply tables for alliances that are not commonly disturbed by fire, but we do briefly discuss other disturbance factors.

Regional Status Section

Some alliances exist only in a small portion of the state, and their species composition and structure are consistent, as are the associated environmental conditions. Other alliances occur extensively throughout the state and vary in species composition and environmental conditions. We use the Forest Service’s National Ecological Region System (McNab and Avers 1994) as it applies to California (Miles and Goudey 1997) in discussing this variation. Ecological regions and subregions are synthetic units derived primarily from considering an area’s soils, climatic zones, landforms, and associated vegetation. The upper three levels— Domain, Division, Province—apply to the ecological region, and the levels Section and Subsection are the higher levels in the ecological subregion. The publication Ecological Subregions of California: Section and Subsection Descriptions has descriptions of California’s sections and subsections (http://web.archive.org/web/20080304224853/http://www.fs.fed.us/r5/projects/ecoregions/).

California includes 2 domains, 4 divisions, 8 provinces, and 19 sections. We list in bold the sections where we know the alliance occurs. This knowledge comes from the local and regional studies that we cite and from any of the authors’ personal observations of the alliance. We also list the possible range of the alliance based on the range(s) of the dominant species. In addition, we represent the range of the alliance at the subsection level in an accompanying map. We do not provide a Regional Status section or range map for provisional alliances, semi-natural stands, or special stands.

The USDA Forest Service has formally established 52 Research Natural Areas (RNAs) in California, which are protected permanently to maintain biological diversity and provide ecological baseline information, education, and research. Each RNA, including its vegetation, is described by Cheng (2004). This report also describes candidate (cRNA), dropped (dRNA), proposed (pRNA), or recommended (rRNA) research natural areas. In alliance descriptions we refer to the RNAs in the Regional Status section by these abbreviations. The oldest is Indiana Summit RNA, established in 1932 on the Inyo National Forest.

Management Considerations Section

In this section, we emphasize management activities that will maintain excellent examples of the alliance. In those cases where the dominant species are nonnative, we consider management needs for their removal. We provide this information for those alliances and non-native stands where we have adequate information, survey data, and/or research. This type of information for provisional alliances and special stands appears in the Remarks section.

Associations and References Sections

Each alliance has at least one association, typically the one defined in the first paragraph. However, many alliances show variety in species composition and structure. The associations capture the variety in the same way that species capture the variety in a genus. In the example of Douglas fir associations, the variation is in the tree layer and/or understory layers. The typical conventions for listing this variation are as follows: When an association has species in the same layer, a dash “-” separates them, such as in Pseudotsuga menziesii-Quercus agrifolia association. When an association has species in two different layers, a slash “/” separates them as in the Pseudotsuga menziesii / Chimaphila umbellata and Pseudotsuga menziesii / Corylus cornuta associations.

We list the associations created by local and regional vegetation studies. Depending on the study, we might change the name order of the association from the original description, placing the characteristic plant name first. From phytosociological studies, we use modified names in this way (see the vernal pool discussion on pages 26-29).

The numbers that follow the association names cite the references from plot-based studies. We cite the studies involved in the Associations section by alphabetically sorting and numbering all of the citations found in each association. We list the citations in the References section with their numbers. We also include other useful literature about the alliance in the References section. The combined information from the first paragraph, the Regional Status section, and the list of associations offers the most complete depiction of the alliance.

Provisional Alliances

We describe provisional alliances when we have sufficient data to propose the vegetation type, but we may not have enough research and regional information to be confident about its status in California’s vegetation. We have arrived at this conclusion in several ways. The NVCS may recognize the alliance in other parts of the nation, but plot data are insufficient to verify it fully in California. In other cases, local or regional studies have proposed the alliance, but the description is based on only a few plots (fewer than 10 samples). Since every sample of vegetation varies in detail from other samples, we need more evidence that this species composition recurs at the alliance level across the landscape.

We provide an abbreviated format for provisional alliance descriptions. This includes the Habitats, Rarity Ranking, and Result sections and another section, called Observations, which details the information that we have in accepting the alliance for the state.

Special Stands

We denote a small set of vegetation types as special stands, which we called “unique stands” in the first edition. Any stand—an actual piece of vegetation in which plant composition and structure are uniform— is unique in some way from all other stands. We define special stands as specific patches of vegetation in the landscape that are unique from other patches; they may appear structurally distinctive as well as be rare.

What allows ecologists to recognize vegetation types is their similarity in species composition and structure. Vegetation types, whether they are alliances or associations, are a collection of stands that have similar species composition, structure, and environmental conditions. One or several species that typically co-occur or even dominate at high cover will define a vegetation type.

Stands of a vegetation type have what ecologists call “redundancy” (Gauch 1982). This redundancy has two aspects: stands are (1) consistently similar among themselves and (2) consistently distinct from stands of another vegetation type. In this way, the ecologist can distinguish and define different alliances and associations. Redundancy is a feature not only of common or extensive vegetation types, but also of rare types. Whether stands are rare or common, redundancy is a principle feature by which ecologists can distinguish between a rare vegetation type and a rare species that does not have to demonstrate redundancy in the landscape.

Following this principle, the ecologist can distinguish between rare vegetation types and stands defined only by the presence of a CNPS-listed plant, or stands that are structurally distinctive. In the case of special stands, the presence of specific rare or threatened CNPS list plants typically define a type, and they usually establish in less redundancy (e.g., fewer than 10 stands) across the landscape. We find that certain rare plants establish with a unique set of species within certain climatic and edaphic conditions, and they typically codominate with a rare assemblage of species. These are special stands.

Might not then all rare plants define a vegetation type? We cannot substantiate this case because many plants are rare in the state with few occurrences, growing with a variety of species in different areas, and they do not dominate the area. In some cases, rare plants may assist in defining alliances, associations, or special stands, but in most cases, the creation of another type adds little to the species’ conservation.

The goals at the species level and plant community level are the same. The special stands that we recognized in the first edition, as well as in this volume, are listed by CNDDB (2003) and the Holland classification (1986), and we see the need to continue to track them and preserve the special features that they produce in the landscape. Some are recognized as mapping units in several regional mapping projects.

These stands do make an interesting group. CNPSlisted plants may be restricted to a rare type, and they may be a part of common ones. For example, Bursera microphylla does occur in localized areas as a dominant defining special stands, but it also mixes as individual trees in the stands of a few other alliances. In the case of Tetracoccus hallii, we have concluded that special stands are evident in certain areas of the Sonoran Desert, but we need further information to understand its relationship within the state’s vegetation.

For now, we provide an abbreviated format for descriptions of special stands. We denote the existing survey data and maps that uphold the special stands as rare units of biodiversity, and they deserve protection. We also offer the special stands as a focus of further study. As CNPS members, agencies, and others collect more data, we can refine our definitions and define other special stands.

Semi-Natural Stands

The flora of California includes plants introduced from other parts of the United States, Europe, and the rest of the world. Many are weeds, a plant “that interferes with management objectives for a given area of land at a given point in time,” according to the Western Society of Weed Science’s fifth edition of Weeds of the West (Whitson et al. 1996). Many are widespread and economically significant in reducing crop yields; others are toxic to livestock. Many ecologists recognize another group of non-native plants variously called “aliens,” “exotics,” or “invasive plants.” They often grow in nonagricultural settings with native plants, and some are valuable as wildlife habitat.

Non-native plants are engulfing California’s wildlands, but each one differs in its impact on the natives with which it grows. We rarely encounter some; others occur in few numbers and mix with natives with little consequence; some reduce native plant populations; and others dominate an environment. Our emphasis in this edition is on the invasive naturalized plants, those that take over in abundance. They form an important part of the state’s vegetation.

Many local and regional vegetation studies have established alliances or created mapping units for invasive plants. However, the authors usually present little or no plot data in their support. This is not surprising. Why spend resources on types that land managers probably hope to eradicate? We want to take account of these vegetation types and mapping units in this edition, but we believe these invasive plant types are not equivalent to alliances. This is why we call them “semi-natural stands.” The stands replace native vegetation, and the non-native plants become dominant.

The creation of the alliance-ranked vegetation types does not mean that we can make a strong distinction between native and non-native vegetation. A quick scan of alliance descriptions will find non-native plants listed as associates in many alliances. Many associations involve a native and non-native plant in the name, so what drives us to single out a subset of invasive plants? We include the cases where the plants are sufficiently dominant to have replaced most of the natives, and, in many situations, the associates are themselves non-native species.

In the first edition, we created a few series (alliances) for non-natives. However, we approach these types differently in this edition. In the case of alliances, the descriptions include information on ways to maintain the natives. In cases where the dominants are non-natives that form semi-natural stands, the emphasis is on the wildlife/plant habitat qualities within them and on ways to restore native vegetation. Some local studies provide descriptions of non-native alliances; however, we often have little information to shape a general description.

At the beginning of the semi-natural vegetation descriptions, we report the main non-native species that define the type, but we do not list associated species because so many of the naturalized types are widespread in the state and grow with a diverse set of native plants. We provide the invasive species ranking rather than the rarity ranking that is given for the alliances and special stands. “Cal-IPC” refers to California Invasive Plant Council’s inventory that classifies invasive non-native plants that threaten wildlands as High, Moderate, or Limited (see http://www.cal-ipc.org/ip/inventory/). “CDFA” refers to the California Department of Food and Agriculture’s noxious weed inventory that classifies non-native plants that adversely impact agricultural products as A, B, or C (see the Encycloweedia website at http://www.cdfa.ca.gov/plant/ipc/index.html). We also list synonymies with the first edition of the Manual (MCV) and with National Vegetation Classification (NVCS) when they exist.

In the Remarks section, we focus on the ecology of the plant(s), and we may include descriptions of ecologically similar plants along with the major species. For example, the Weeds of California and other Western States (DiTomaso and Healy 2007) reports that California has 13 Centaurea (star-thistle and knapweed) taxa as weeds and escaped ornamentals. We discuss C. melitensis, C. solstitialis, and C. sulphurea as a yellow star-thistle type, which appears to contain the most invasive of the genus.

In the Fire Characteristics and Management Considerations sections, we emphasize habitat restoration and plant removal to enhance native plants. We include a fire characteristics section, if the information is known, as part of restoration palate of techniques.

We include the Regional Status and Stands sections only when specific findings exist from local and regional vegetation studies. Most studies recognize mapping units without extensive sampling to classify and establish associations, so we report the studies found by region and list the associated plants found in the studies when possible. In the next edition, we hope that we can report success in slowing the impact by non-native plants on the state’s vegetation.

Unranked Vegetation Types

Some vegetation studies recognize vegetation types that we cannot fully support at this time. Sometimes, the results of local vegetation studies provide little detail and few surveys, and the vegetation being studied may appear related to other existing vegetation types, so it is difficult to place in a statewide classification framework. These kinds of studies lead us to the following questions that need to be answered first: Is this the first evidence of a type that repeats regularly across the landscape? Is this type only local, and is it even a rare type that we should recognize? More information, in the form of survey data and mapping, will tell us whether these unranked vegetation types warrant alliance, association, or special stand status. Appendix 4 provides a list of these unranked types, and Appendix 5 provides an additional list for types that we reassessed and reclassified into existing or new alliances in this edition.