Seasonal priority effects: Implications for invasion and restoration in California coastal sage scrub

Background/Question/Methods

     Exotic annual grasses (EAGs) threaten native plant communities in California, and pose a challenge for habitat restoration. Several mechanisms have been hypothesized to explain the success of EAGs, including suppression of native seedlings by high litter accumulation, exhaustion of soil moisture by EAGs, and low seed rain of natives relative to exotics. EAGs may also exert priority effects by being active earlier in the growing season; EAGs sometimes germinate with a smaller threshold rain event, whereas native species germinate with cooler temperatures and larger winter rains. We  hypothesized that 1) EAGs (Avena fatua, Bromus hordeaceus) would germinate with an experimentally applied late-summer rain, but native species would not germinate until cool autumn rains, and 2) EAGs which germinated in late summer would not survive until the winter rains begin, due to either desiccation or herbivory. EAGs and native species were seeded into known areas of experimental plots, and a pulse of water was applied in late summer to a disturbed site within a coastal sage scrub community. Four levels of summer watering were included (none=control, addition of 10, 20 or 30 mm). Germination, survival, and community level patterns of abundance were monitored for both native and exotic species.

Results/Conclusions

     EAG germination was substantial in response to the higher level summer watering treatments, particularly for B. hordeaceus. As a consequence, EAG germination following the natural winter rains was markedly lower in the plots that had received a summer rain pulse.  In contrast, no native coastal sage scrub species germinated in response to the late summer watering treatments. Instead, germination of native species commenced, as hypothesized, with the beginning of the cool winter rains. These results support our first hypothesis, consistent with the idea that native species have more complex germination cues than EAGs. In addition, to support our second hypothesis, all of the EAGs that germinated in response to later summer watering perished before the onset of the winter rains.  Additional observations suggest that these early germinating seedlings were apparent to herbivores, and that herbivory rather than desiccation led to their lack of survival. Overall, these results suggest that late summer watering could be an important restoration strategy for invaded native plant communities by depleting the EAG seed bank, and allowing for native species to establish with reduced competition from EAGs.