Serpentine Vegetation of Western North America

As discussed in chapter 11, the general patterning of vegetation on serpentine up and down the western North American continent is relatively straightforward. However, many of the distinctive nuances relating to the structure and composition of the vegetation, particularly in comparison to adjacent nonserpentine vegetation have yet to be described. In this chapter we use vegetation as a tool to describe the variation of biotic diversity on serpentine throughout western North America. Vegetation is valuable in this regard because, by describing it, one assembles the information on all plants growing in different patterns in a landscape. This chapter expands on some of the concepts mentioned in chapter 11 and addresses some of the specific questions of interest to ecologists and biologists regarding the influence of serpentine on groups of plant species, using examples from western North America. Western North America provides an excellent template for understanding general questions about serpentine effects on species and vegetation. The broad latitudinal distribution and the local topographic and geologic diversity of serpentine exposures throughout this area produce an array of gradients of temperature, moisture, soil development, disturbance patterns, and day length to produce multiple ecological gradients operating at multiple scales. Also, within western North America a wide number of species from many different genera and families are influenced by serpentine. Vegetation classification is a tool used for several purposes, including efficient communication, data reduction and synthesis, interpretation, and land management and planning. Classifications provide one way of summarizing our knowledge of vegetation patterns. Although there are many different classification concepts, all classifications require the identification of a set of discrete vegetation classes. The fundamental unit of these discrete classes that is identifiable in the field is the stand. A stand is defined by two main unifying characteristics (CNPS 2003): 1. It has compositional integrity. Throughout the site, the combination of plant species is similar. The stand is differentiated from adjacent stands by a shift in plant species composition that may be abrupt or indistinct. That shift relates to a concomitant shift in certain ecological features such as temperature, moisture, or soil fertility that maintain control over the plant species composition.