CNPS

Vegetation patterns are often dramatic aspects of California’s landscapes. By looking at these patterns in detail, you will notice that they repeat themselves, and that each part of the pattern is composed of a distinctive assemblage of species that present a characteristic appearance based on size, shape, and spacing of the plants. These distinctive parts of the pattern, called plant communities or vegetation types, are the predictable result of plants’ interaction with specific environments. How do you best describe and understand these patterns? This is the work of a vegetation ecologist and biogeographer.

An ecologist refers to the characteristic set of species as the community’s species composition, and the distinguishing physiognomy is referred to as the community’s structure. Each community also reflects distinct set of environmental conditions of climate, soil, water, disturbance, and other variables. For example, a grove of redwood trees is a particular kind of vegetation seen along the central and north coasts of the state that receive summer fog. A patch of chamise and yerba santa shrubs in a tract of chaparral is easily contrasted with a nearby patch of grasses and forbs in the dry hillsides in southern California where a patchy fire recently burned. Ecologists describe and understand these patterns at local, regional, and state-wide scales.
Ecologists have developed methods they use to describe vegetation in a consistent way. First an area is surveyed, recognizing the various kinds of vegetation present; such as forest, willow thicket, wet meadow, and bald areas. This reconnaissance allows the researcher to stratify the land into a set of vegetation assemblages that exist in the landscape. Each assemblage of the vegetation is then sampled to describe its species composition (species presence and cover), structure (plant cover, height, etc.) and environmental conditions (elevation, local topography, soil characters, etc.) for fixed areas (variously referred to as samples, plots, quadrats, relevés, etc.). This sampling is repeated throughout the landscape being assessed for all the kinds of vegetation present.
	The field data are subjected to statistical methods which quantify the repeating patterns in species composition and correlate them with vegetation structure and environmental data. These analyses suggest a circumscribed set of vegetation samples that best describe and encapsulate the landscape’s vegetation in an empirical way. They also allow the researcher to recognize different levels of detail in the data set. Not only can forests be separated from the meadows, but different forests and meadows can be placed in a hierarchy. In other words, the vegetation types are classified. The result of this process is that we now have an objective examination of the landscape’s vegetation. The types can be mapped, interpreted, and compared to those in other classifications for other parts of the state.
	Vegetation ecologists have done this work for “end-users” of the classification over large regions of the state. A Manual of California Vegetation brings together these local and regional studies into one source. As with the Jepson Manual that helps you identify the state’s plants to the family, generic, and species level, the Manual places the state vegetation into a formal classification. For further information, see the overview chapter “CNPS Approach to Classification” and how it relates to the national vegetation classification.
	A majority of people find the “alliance” level to be most useful in characterizing vegetation types of the state because it is floristically-driven and provides a great overview of the common, wide-spread and important plant species and their assemblages of the state. We use the term “vegetation type” for any level of the classification. “Temperate Forest” is a type at the subclass level, “Californian Broadleaf Forest & Woodlands” at the group level. The “group” is composed of various alliances which are composed of “associations”.

An association is often recognized by two or more diagnostic species that are often found in different vegetation layers, which circumscribe the most detailed similarities of species composition and climate, topography, substrate, hydrology, and disturbance. For example, the Quercus kelloggii / Ceanothus integerrimus association is adapted to relatively frequent fires in the lower montane zone of the Sierra Nevada.

An alliance is a group of associations whose diagnostic species define a set of associations with similar composition reflecting regional to sub-regional climate, substrate, hydrology, moisture/nutrient factors, and disturbance regimes, typically broader than at the association level. For example, a vernal pool alliance identified by the presence of Lasthenia glaberrima is composed of six associations, the Arctostaphylos hooveri alliance with one, and Ceanothus cuneatus alliance with nine.

A quick survey of the Manual will give you an indication of the forest, shrub, herbaceous, and sparsely vegetated alliances in the state. The goal of this effort is to have California’s vegetation presented in a consistent and well organized fashion that will serve many purposes.

Introduction