Physiological variation among native and exotic winter annual plants associated with microbiotic crusts in the Mojave Desert

Microbiotic crusts are important components of many aridland soils. Researc h on crusts typically focuses on the increase in soil fertility due to N-fi xing micro-organisms, the stabilization of soils against water and wind ero sion and the impact of disturbance on N-cycling. The effect of microbiotic crusts on the associated plant community has received little attention. We quantified the influence of crusts on the production, species diversity, nu trient content and water relations of winter annual plant species associate d with microbiotic soil crusts in the northeast Mojave Desert. Shoot biomas s of winter annuals was 37% greater and plant density was 77% greater on cr usts than were biomass and density on soils lacking crust cover (=bare soil s). This greater production of annuals on crusts was likely due to enhanced soil conditions including an almost two-fold increase in soil organic matt er and inorganic N compared to bare soils. Crusted soils also had 53% great er volumetric water content than bare soils during November and December, t he time when winter annuals become established. As plant development progre ssed into spring, however, soil water availability decreased: More negative plant xylem water potentials were associated with greater plant biomass on crusted soils. Plants associated with microbiotic soil crusts had lower co ncentrations of N in shoots (mg N g(-1) dry mass). However, total shoot N ( mg N m(-2)) was the same in plants growing on the different soil types when biomass production peaked in April. Shoots had similar patterns in their c oncentration and content of P. Species diversity of annuals was not statist ically different between the two soil types. Yet, while native annuals comp rised the greatest proportion of shoot biomass on bare soils, exotic forbs and grasses produced more biomass on crusts. Total shoot nutrient content ( biomassxconcentration) of the two exotic annual species examined was dramat ically greater on crusts than bare soils; only one native species had great er shoot content of N and P when growing on crusts than bare soils. Microbi otic crusts appear to increase site fertility in the northeast Mojave Deser t, but nutrients and water distributed within a greater biomass of annual p lants growing on microbiotic crusts likely resulted in lower concentrations of nutrients in plant tissue and lower xylem pressure potentials than plan ts growing on bare soils. Exotic annuals growing on crusts appear to respon d to the higher N availability by growing faster, potentially outcompeting native annual species.