Integrative Xylem Analysis of Chaparral Shrubs

The chaparral shrub community is dominated by evergreen sclerophyllous shrubs and primarily occurs in southern California (Fig. 7.1a) (Parker et al. in press). The chaparral has probably been more studied in an ecophysiological context than any other natural vegetation type in the world. The species that dominate this vegetation type have proven to be excellent model organisms for a range of physiological studies including studies of photosynthesis (Field and Mooney 1986), water relations (Morrow and Mooney 1974; Poole and Miller 1975), drought tolerance (Harrison et al. 1971; Kolb and Davis 1994), and xylem structure and function (Carlquist and Hoekman 1985; Hargrave et al. 1994; Wagner et al. 1998). This chapter will focus on xylem traits and for a recent broader review see Parker et al. (in press). There are important reasons why chaparral vegetation has drawn the attention of physiologists (reviewed in Field and Davis 1989). One chief reason is that the environment where chaparral occurs is predictably stressful, thus providing a clear ecological context for understanding stress resistance traits. The source of the stress is the Mediterranean-type climate that characterizes the region where chaparral occurs in California. The precipitation falls as rain in the winter months when the temperatures are cool and the summers are hot with a predictable 4–6 months rainless period (Cowling et al. 2005). During the summer and fall, many species develop considerable water deficits and consistently have some of the lowest water potentials ever measured (Oechel 1988; Davis et al. 2002; Jacobsen et al. 2007a, b).