Global climate change models for the Midwestern US predict changes in both the amount and interval/frequency of precipitation. Precipitation variability has long been known to affect the composition of plant communities in arid systems, but few studies have addressed this in more mesic systems where water is not as limiting a resource as nitrogen in terrestrial plant communities. We conducted a greenhouse experiment to explore the affects of variation in the frequency and amount of water available on germination of seeds from the seedbank of an agricultural weed community dominated by annuals. Total water equal to the 10-year average rainfall and a drought treatment of half that amount was crossed with three intervals lengths between watering events; 4, 8, or 12 days between water additions. Total germination was recorded was recorded over 39 days. Due to high mortality in some treatments, species level data could not be analyzed, but we were able to compare the responses of monocots and dicots.

Results/Conclusions

We found that total germination responded positively to lengthening intervals in the drought treatment, but was unaffected by interval length at average amounts of water. Both monocot and dicot germination increased with longer intervals under drought conditions, but dicots responded more strongly than monocots to changes in interval in the drought treatment. At average amounts of total water, monocots germination decreased slightly with longer intervals while dicot germination was unaffected by interval length. These results demonstrate that variation in both the total amount and the interval between precipitation events can have significant and interacting effects on weed seed germination. For annual plants germination responses may play a prominent role in determining community composition. Therefore, changes in precipitation amount and the interval between precipitation events should be considered when examining the impact of climate change on annual communities such as weeds.