Exotic annual grasses and forbs are invading California’s deserts. These exotic invaders have the ability to out-compete native species, especially annuals, for space and resources. Anthropogenic nitrogen deposition artificially fertilizes the soil, which favors exotic invaders by increasing resource availability in a resource-limited ecosystem. Invasive species have a negative impact on native species abundance in natural systems. In California’s deserts, many of the native annual species are adapted to relying on a soil seed bank. The seed bank is a bet-hedging strategy to ensure species survival over the long term when the environment is unpredictable. In this study, we examined how exotic invasions and nitrogen deposition affect the soil seed bank. We used four sites along a natural west-to-east nitrogen gradient in Joshua Tree National Park that have been part of a long-term nitrogen fertilization study. At each site, soil cores 9cm x 5cm were collected from both high fertilized (30 kgN/ha) and control (0 kgN/ha) plots. These soils were mixed with sand and spread out in a greenhouse in 20cm x 20cm trays, watered liberally, and subsequent germination was recorded. As seedlings were identified, they were removed from the trays. Three rounds of watering occurred over the course of 14 months.
Results/Conclusions
We compared the seed bank composition between fertilization treatments as well as between sites. There was no significant difference due to nitrogen fertilization in the seed bank, although grass cover was significantly high with nitrogen in field vegetation surveys. Exotic percent cover was inversely related to native percent cover and seed bank density. Seed bank variation was found among sites, which may be related to site-specific factors such as historic level of invasion and nitrogen deposition, as well as soil characteristics. Examinations of soil seed banks can provide valuable information about the status of plant populations, invasions, and potential for restoration. Seeds stored in seed banks also preserve genetic diversity. The soil seed bank can be a valuable restoration tool for managers if it is intact and can also be a target for invasive weed control if exotic seed has overwhelmed the seed bank. Knowledge of soil seed banks enhances ecosystem understanding, population dynamics, and the potential for conservation and restoration.
