High relative growth rate (RGR) of an invasive plant species is an important characteristic that allows plant invasion in disturbed areas, especially environments rich with resources. This trait allows invaders to take advantage of small increases in soil nitrogen (N). As N becomes available, high RGR characteristics of invasives facilitate establishment of extensive root and shoot systems as compared to native counterparts. Competition remains a key factor in the dynamics of ecosystems that are low in nitrogen. The objective of the present study is to examine (1) varying level of N (0 and 400 mg N/kg soil) on growth rate of different species, (2) growth differences of plants under different competitive scenarios and (3) underlying mechanisms contributing to RGR differences among native and exotic plants. We grew two perennial natives (Sandberg bluegrass and Bluebunch wheatgrass) and two annual invasives (Cheatgrass and Medusahead) in a diallel greenhouse experiment either alone or in 1:1 binary combinations with low and high nitrogen. Shoot weight, root weight and leaf area were measured for each species at 7, 15, 30, 60 and 90 days after seeding. RGR and its components were quantified over five harvests.

Results/Conclusions

Growth analysis revealed significant variation in RGR among annual and perennial grasses. However, low soil N did not result in proportionally greater reduction in RGR of exotics than natives. In interspecific competition, natives were more suppressed than in intraspecific competition under both N conditions. Conversely, intraspecific competition was more intense than interspecific competition for invasives. Differences in RGR were mainly due to variation in net assimilation rate (NAR). There were no differences in leaf mass ratio (LMR) and specific leaf area (SLA) of invasives and natives. Maintaining low N levels is necessary to limit the spread of invasives and increased soil N enhances the competitive ability of invasive plants.