Being reckless in a tough world: Two invading grasses and their relationship with a native desert shrub.
Holzapfel, Claus*,1, Parag, Hadas1, 1 Rutgers University, Newark, NJ
ABSTRACT- Mojave Desert scrub lands are increasingly invaded by non-native, annual grasses. Two of them, the red brome, Bromus madritensis rubens, and the split grass, Schismus arabicus, are now the dominants in the annual plant community associated with desert shrubs. Shrub-associated Bromus populations can have strong direct competitive effects on adults of a dominant shrub (Ambrosia dumosa); dense populations of Schismus are directly competing with shrub seedlings. One of the goals of an eleven year study in the northern Mojave Desert is to document the population dynamics of the two grasses in reaction to the variation of rainfall regime and to relate them to their impact on native annuals and shrubs. Both grass species readily germinate after early season rainfalls and can not build large inter-annual seed reservoirs (seed banks) - in comparison to native annual plants, which are capable of seed bank building, they therefore appear to be reckless. Bromus is subjected to large mortality when prolonged intra-annual dry periods occur, while the desiccations tolerant Schismus can survive such water stress. As a consequence, populations of Bromus are fluctuating strongly while Schismus populations are more stable. The long-term data set demonstrates that short periods of relative inter-annual drought decrease Bromus populations and therefore favor shrub and annual plant performance while good years lead to a recovery of Bromus populations that strongly effect both shrubs and native annuals. Longer periods of drought decrease Bromus densities but also are detrimental for shrub fitness. Schismus populations appear to have rather stable negative effects on shrub seed recruitment. Based on the data we are now able to develop models that allow predictions of the fate of both invading and native species (annuals and shrubs).
Key words: desert, invasion, interaction
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