Soil fertility and disturbance interact to drive contrasting responses of co-occurring native and nonnative species.

Some plant functional groups such as nonnative invasive and nitrogen (N)-fixing plants are widely thought to have consistent, coordinated differences in their functional traits relative to other groups such as native and non -N-fixing plants. Recent evidence suggests that these trait differences between groups can be context dependent, varying with environmental factors such as resource availability and disturbance. However, many previous comparisons among plant groups differing in invasion status have not standardized growth form between groups or have compared species that do not co-occur, which could result in invasion status per se being confounded with other factors. We determined growth and leaf functional trait responses of 20 co-occurring woody species, that is, five species within each of four functional groups (native N-fixers, native non -N-fixers, nonnative [invasive] N-fixers and nonnative [invasive] non-N-fixers), to factorial combinations of soil fertility and defoliation treatments in a mesocosm experiment to test each of two hypotheses. First, we hypothesized that nonnative invasive and N-fixing species will have functional traits associated with rapid resource acquisition whereas natives and non -N-fixing species will have traits linked to resource conservation. Second, we hypothesized that plant growth and leaf traits of nonnative and N-fixing species will be more strongly influenced by environmental factors (i.e., soil fertility and disturbance) than will natives and non-N-fixers. Plant growth, foliar nutrients, and leaf structural traits varied among plant functional groups in a manner consistent with our first hypothesis. Support for our second hypothesis was mixed; origin (native vs. nonnative) and soil fertility rarely interacted to determine plant growth or variation in leaf traits whereas interactions involving N-fixing ability and soil fertility were common. Further, there were no consistent interactive effects between plant groupings and disturbance. Our results demonstrate that variation in growth and functional traits among plant species were driven primarily by the relatively large responses of nonnative N-fixers to soil fertility, rather than by consistent differences between other plant functional groups. These findings highlight the importance of resource availability in determining trait or performance differences among plant functional groups, and provide insights into the assembly of plant functional traits in novel communities of co-occurring native and nonnative species.