California native and exotic perennial grasses differ in their response to soil nitrogen, exotic annual grass density, and order of emergence Citation

49 Early emergence of plant seedlings can offer strong competitive advantages over 50 later-germinating neighbors through the preemption of limiting resources. This 51 phenomenon may have contributed to the persistent dominance of European annual 52 grasses over native perennial grasses in California grasslands, since the former 53 species typically germinate earlier in the growing season than the latter and grow 54 rapidly after establishing. Recently, European perennial grasses have been spreading 55 into both non-native annual and native perennial coastal grass stands in California. 56 These exotic perennials appear to be less affected by the priority effects arising from 57 earlier germination by European annual grasses. In addition, these species 58 interactions in California grasslands may be mediated by increasing anthropogenic or 59 natural soil nitrogen inputs. We conducted a greenhouse experiment to test the effects 60 of order of emergence and annual grass seedling density on native and exotic 61 perennial grass seedling performance across different levels of nitrogen availability. 62 We manipulated the order of emergence and density of an exotic annual grass 63 (Bromus diandrus) grown with either Nassella pulchra (a native perennial grass), 64 Festuca rubra (native perennial) or Holcus lanatus (exotic perennial), with and 65 without added nitrogen. Earlier B. diandrus emergence and higher B. diandrus 66 density resulted in greater reduction in the aboveground productivity of the perennial 67 grasses. However, B. diandrus suppressed both native perennials to a greater extent 68 than it did H. lanatus. Nitrogen addition had no effect on the productivity of native 69 perennials, but greatly increased growth of the exotic perennial H. lanatus, grown 70 with B. diandrus. These results suggest that the order of emergence of exotic annual 71 versus native perennial grass seedlings could play an important role in the continued 72 dominance of exotic annual grasses in California. The expansion of the exotic 73 perennial grass H. lanatus in coastal California may be linked to its higher tolerance 74 of earlier-emerging annual grasses and its ability to access soil resources amidst high 75 densities of annual grasses. 76 77 78