Can Nitrogen and Phosphorus Explain Invasive Success?: A Test of the Importance of
these Resources in the Coastal Sage Scrub Community
Cynthia Kim Thai
Mentor: Katharine Suding
California’s coastal sage scrub (CSS) communities have been heavily invaded by exotic
plant species. We investigated the role of nitrogen and phosphorus in invasion success.
We hypothesized that high resource conditions would increase invasive success and that
plant traits related to resource use and growth rate could predict competitive outcomes.
Good competitors under high resource conditions should have high tissue N and P,
favoring fast growth, while good competitors at low resource availability are predicted to
have high root:shoot ratios and high levels of mycorrhizal association. We designed a
greenhouse experiment to analyze species growth and competitive interactions in four
fertilizer treatments of variable nutrient levels. We used Artemesia californica and Salvia
mellifera, two native CSS species, and Bromus hordeaceaus and Brassica nigra, two
exotics. We found that exotics had higher shoot biomass in the high nitrogen treatments.
Phosphorous addition had no significant effect on competitive interactions. Exotics,
counter to our prediction, had higher root:shoot ratios than natives, which may have
helped them compete for resources. Natives had more mycorrhizal interactions, which
may have helped them obtain resources in lower resource environments. Also, tissue N
may be more indicative of competitive ability than tissue P. These results suggest that
exotics are likely to be most invasive in high nitrogen environments, and traits like leaf
nitrogen, root:shoot ratio, and mycorrhizal colonization may help predict the relative
success of natives and exotics in the CSS community. Reducing nitrogen availability or
limiting atmospheric N deposition may help to restore and preserve CSS.