Plant Community Water Use and Invasibility of Semi-arid Shrublands by Woody Species in Southern California

Soil moisture is a key limiting resource in arid and semi-arid environments for woody shrub species. We assessed if three arid communities differed in their level of dry season soil moisture content and if the dominant species in these communities differed in their ability to use soil moisture. Water potential of all of the dominant woody plant species occurring in chaparral, coastal sage, and Mojave Desert communities and soil moisture content of these sites were measured seasonally. Species occurring in the Mojave Desert exhibited the most negative water potentials while the coastal sage community displayed the least negative water potentials. Dry season volumetric soil moisture content of the Mojave Desert site was lowest (7%), the chaparral site was intermediate (10%), and the coastal sage scrub site had the moistest dry season soil (20%). These moisture differences developed even though the coastal sage and chaparral communities both received the same annual rainfall and had similar soil characteristics. Of the three communities, the coastal sage community may be particularly susceptible to invasion by woody shrub species because its soil moisture content would allow for germination and persistence of a wider range of potential invaders. Current differences among sites in numbers of nonnative woody species are consistent with predicted differences in susceptibility to non-native species based on community water use and dry seasonal soil moisture.