Vascular Plant Response to Microbiotic Soil Surface Crusts

Microbiotic soil surface crusts are a common and widespread feature of the world's arid and semiarid lands (e.g. West 1990; Eldridge and Greene 1994).

The structure of the microbiotic crusts of Nizzana, their role in the different ecotopes and their physiological activity have been addressed in other chapters in this volume (Chaps. 10, 17, 20, 21). This chapter will focus on the effects of those crusts on vascular plants.

Several reviews on microbiotic crusts (e.g. Harper and Marble 1988; Johansen 1986; Isichei 1990; West 1990; Metting 1991; Johansen 1993; Eldridge and Greene 1994; Belnap et al. 2001) have concluded that crusts have either no effect or positive effects on vascular plants. Negative effects are usually stressed much less (but see Eldrigde and Greene 1994). Yet, the abovementioned authors had to base their conclusions on previous studies which were mainly observational (Kleiner and Harper 1972; Nebecker and St. Clair 1980; Anderson et al. 1982a, b; Graetz and Tongway 1986; Johansen and St. Clair 1986; Marble and Harper 1989; Tongway and Smith 1989; Beymer and Klopatek 1992; Eldridge 1993). These studies usually compared one particular measure of vascular plant abundance (e.g. cover) at grazed sites with disturbed crusts and at undisturbed sites with well-developed crusts. A shortcoming of such an approach is the lack of distinction between the direct impact of the source of the disturbance (i.e. trampling and grazing) on vascular plants, and the effect of the crust itself.