Abstract: Nutrient availability is important in grassland competition dynamics, as it is commonly enhanced through nitrogen-based fertilizers, yet also thought of as a limiting factor during competition between native and invasive species. Most of the current literature on grassland species focuses on community effects of nitrogen, while the detailed morphological effects of both nutrient addition and restriction, especially in stages of early growth, remain unclear.
I compared the effect of a fertilizer containing nitrogen, phosphorus, and potassium (24% N, 8% P, 16%K) on the above and below ground traits of California native, Hemizonia congesta (hayfield tarweed), and its non-native counterpart, Centaurea solstitialis (yellow starthistle) at different stages of early growth. The experiment involves a germination experiment followed by a 10 week greenhouse experiment, observing trait differences of both species under a high and low nutrient treatment. The measured traits, including biomass, leaf area, length, and various root traits, act as proxies for productivity, resource acquisition, and overall fitness.
As predicted under high nutrient conditions, C. solstitialis exhibited greater above ground biomass, height, and leaf area than H. congesta, along with greater root volume and root biomass. In the low nutrient treatment, H. congesta exhibited greater height overall, along with greater root length, root volume, and root biomass for the first 3 weeks of growth. C. solstitialis had greater leaf area overall than H. congesta in low nutrient conditions, as well as greater above ground biomass, root volume, and root biomass after 6 weeks of growth.
This study will shed light on how native and invasive species respond to varying nitrogen availability, informing the role of nutrients in early plant establishment within the California grassland. For grassland managers and restoration practitioners, the response of native and invasive species to different nitrogen availability could inform timing decisions of when to apply or immobilize nitrogen, utilizing it as a tool to limit the spread of C. solstitialis or promote the growth of natives such as H. congesta.
