In arid and semiarid environments, water is the main limiting factor for seed germination and recruitment of annual herbaceous plants. In these systems water usually is available as isolated precipitation pulses which are highly variable in time and space. In many arid regions the El Niño Southern Oscillation (ENSO) phenomenon leads to large precipitation pulses every 3-5 years. In Chile, exotic annual herbaceous species are mostly from Mediterranean climate-type environments, and their seeds germinate with smaller rainfall pulses than do native annual herbaceous species. Hence, drier years would be predicted to favor exotic annual herbs over native annual herbs; conversely, the opposite would be predicted for more rainy years. We assessed this hypothesis for annual plant assemblages over 19 years (1989 - 2007) in a semiarid community in Parque Nacional Bosque Fray Jorge, in north-central Chile.

Results/Conclusions  

Species richness of native as well as exotic annual herbs increased with an increased rainfall, but native species did so at higher rates. Threshold responses of native annual herbs to rainfall were more variable than those of exotic annual herbs. Increased species richness was not accompanied by  decreased abundance (i.e., plant cover) under higher rainfall conditions. More diverse annual plant communities are likely to be more productive. Numbers of exotic annual herbaceous species were positively correlated with numbers of native annual herbaceous species.  Despite the significant and positive correlation between annual herbaceous species richness and annual precipitation, a higher correlation was found between cover of annual herbaceous species and annual precipitation.  Decreased future rainfall in this system should have a stronger impact on native annual herbaceous species than on exotic annual herbaceous species.  This research was financed by FONDECYT 1070808, US NSF, and IEB P05-002 ICM.