Community Classification and Nomenclature
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T HREE Community Classification and Nomenclature TODD KEELER-WOLF, JULIE M. EVENS, AYZIK I. SOLOMESHCH, V. L. HOLLAND, AND MICHAEL G. BARBOUR Grass-dominated vegetation covers approximately one-fourth Mueller-Dombois and Ellenberg 1974). In addition, each type of California’s area, and it is well known that virtually all of exists in a precontact and a postcontact condition. Four of it has been significantly modified by the invasion of natural- them are regional in distribution, but serpentine grassland is ized annual and perennial grasses and forbs. Less commonly azonal and not limited to a single geographic region. understood is the fact that this type conversion resulted in Before we tour these major types, we offer several defini- very few extinctions. Bartolome et al. (in press) concluded tions. These definitions are largely our own creation, because that—although local extirpation, reduced abundance, and the literature is so inconsistent in their treatment. range retraction characterize the status of the once-dominant native taxa—only a few species have retreated to the point of Vegetation and Community Type Definitions complete extinction. Only four of the 29 taxa presumed to be extinct throughout all of California could have once been Grassland is vegetation that belongs to the Herbaceous For- components of the valley grassland (CNPS 2001). Native mation Class, defined by Grossman et al. (1998) as “Herbs species remain rich in number, even if individually their cover (graminoids, forbs, and ferns) dominant (generally forming at is low. In some areas, their cumulative cover is greater than least 25% cover; trees, shrubs, and dwarf shrubs generally less that of the exotics. than 25% cover).” For any given California grassland com- This is an exciting time for grassland classification: The munity, three to four grass species typically constitute the number of defined and named community types is rapidly majority of the aboveground biomass, but forb species rich- climbing, and the scale at which community types can be ness can be four times that of the grasses and grass-allies (Sims recognized is becoming finer and more localized. On the and Risser 2000). More commonly, grassland is described as other hand, grassland classification is still unfinished: lacking woody plants. Many grassland taxa extend as an Defined and mapped types are scattered polygons within a understory beneath foothill savannas and woodlands and much larger blank matrix that remains undescribed. It is as beneath riparian woodland. However, these formations dif- though we are looking at a jigsaw puzzle of California with fer from grassland by having a distinctive physiognomy, and at least half the pieces missing. Furthermore, the classifica- they will not be discussed in this chapter. Open savannas, tion level for existing pieces is not uniform. Some associa- however, with Ͻ 10% cover by tree canopies, appear to have tions have not yet been aggregated into alliances, and some no significant effect on grassland communities (Sawyer and alliances have not yet been subdivided into associations. Keeler-Wolf 1995; Sawyer et al. 2007), so readers should be Following a number of authors, we recognize five major aware that information in this chapter also applies to sparsely grassland types within the state’s boundaries (Figure 3.1). treed grasslands. Their common names are valley/south coastal grassland, Climatically, North American grassland communities are north coastal grassland, serpentine grassland, cold desert very diverse and have few factors in common (Barbour and grassland, and warm-desert grassland. Many synonyms exist Christensen 1993). In California, they occupy habitats where for these types. They occupy such large areas, and are so floris- the annual precipitation ranges from a low of 120 mm, in the tically and environmentally different, that each may be the southwestern part of the San Joaquin Valley, to 2000 mm, in equivalent of classes or orders, in the sense of traditional Braun- the Coast Ranges of northwestern California (Bartolome et al. Blanquet phytosociological classification (see, for example, in press). The annual precipitation-to-evaporation ratio (P:E) is 21 FIGURE 3.1. Modern distribution of major grasslands, including areas type-converted from grassland to agricultural and urban uses. Solid lines separate the state into four grassland regions: north coastal (NC), Valley and south coastal (CV), cold desert (CD), and warm desert (WD). The boundaries of north coastal grassland are based on Ford and Hayes (in press); boundaries for the other three major grasslands are based on Kuchler’s (1977) map of potential natural vegetation and on geographic subdivisions recognized by the Jepson Manual (Hickman 1993). The boundaries are not intended to indicate complete dominance by grassland within them, only to indicate the limits of each grassland type. Patches of grassland do occur in other parts of the state, but they are not mapped here. from 0.7 to 0.3, soils are deep and fine-textured, and the fire- 500–1000 mm precipitation annually (see, for example, return interval is shorter than a decade (Barbour et al. 1999). Bailey 1995). However, a climatic distinction between prairie Where the P:E ratio Ͼ1.0, as in cismontane elevations above and grassland is just as often ignored in the literature, with the 1200 m, herb-dominated vegetation can be present, but only term prairie applied almost randomly. Thus, we choose to where local hydrology prevents the growth of woody vegeta- treat prairie and grassland as complete synonyms, and for that tion, such as in meadows. We do not include meadows in this reason the vegetation sometimes called “north coastal prairie” chapter, because they occur in a climate more appropriate for is here referred to as “north coastal grassland.” forest vegetation or they occupy seasonally saturated soils Steppe is as problematic a term as prairie. In this chapter we (see, for example, Brown 1982). We also omit wetlands: tidal define steppe as a complete synonym for grassland, even marshes, freshwater marshes, and vernal pools. though others have attempted to define it as different from Prairie is defined in this chapter as a synonym for grassland. grassland. Bailey (1995), for example, differentiates steppes The term has been used by some ecologists to denote subhu- from grasslands as being more open, occupying landscapes mid grasslands that have a P:E ratio close to 1.0 and are dom- colder and drier (more temperate and more continental), and inated by tall perennial grasses. An example is the tall grass dominated by short-stature grasses associated with shrubs prairie in the central portion of North America, which receives and trees as scattered individuals or in clumps. The ratio of 22 OVERVIEW herb-to-woody cover in typical steppe has not been widely An alliance is a group of floristically related associations defined. West and Young (2000) and Daubenmire (1970, that collectively occupy a larger range than does any single 1975) describe relictual sagebrush steppe in the intermoun- association. Its makeup is broader than any one association tain region as having 20–80% cover by shrubs and 50–80% (that is, more floristically, structurally, and ecological variable; cover by grasses and forbs, attaining a combined, total plant Jennings et al. in press). Nationally, there are 1,900 alliances, cover of 60–200%. Prominent growth forms in that steppe meaning that each alliance contains (on average) about three (in terms of relative floristic richness) are hemicryptophytic associations. In California, Sawyer et al. (2007) have so far herbs and a soil crust composed of mosses, lichens, and algae. recognized some 400 alliances, and we predict that more The term steppe has been variously applied to bunchgrass than 50 grassland alliance types will ultimately be recog- vegetation lacking any woody component (for example, to nized when vegetation classification is closer to completion the valley grassland of California), to bunchgrass vegetation than it is at present. with shrubs regularly present (equally often called a “shrub- steppe” or “scrub grassland”), and to bunchgrass vegetation Valley/South Coastal Grassland with trees regularly present (also called a “tree-steppe”). The basic classification unit, and often the unit most local Several names have been given to upland herbaceous vegeta- in its range, is the association. As initially defined at the Inter- tion in California’s Central Valley: Valley needlegrass grassland national Botanical Congress of 1910, an association (group of (with Nassella pulchra and N. cernua), California prairie, and stands) is a plant community of definite floristic composition California annual grassland, among others (Heady et al. 1988 that presents a uniform physiognomy and that grows in uni- and 1992; Holland 1986; Kuchler 1977; Figure 3.2). “Valley” form habitat conditions (Flahault and Schroeter 1910). Asso- ciations are abstractions, and they are represented in reality by concrete examples, called stands. The Ecological Society of America’s Vegetation Classification Panel (Jennings et al. in press) adopted a more American-biased definition of associa- tion as being “a vegetation classification unit defined on the basis of a characteristic range of species composition, diag- nostic species occurrence, habitat conditions, and physiog- nomy,” a definition that clearly states that stands within a particular association are expected to be somewhat different from each other. Box 3.1 provides an example of the kind of vegetation