Unraveling when elevated nitrogen conditions will result in plant invasions

Background/Question/Methods

Understanding the mechanisms that contribute to invasions has been central to plant ecology for decades; however ecologists are now burdened with trying to predict when an exotic species may capitalize on the ecosystem impacts from other environmental drivers to become established or spread. One such driver is nitrogen (N) deposition, which is already resulting in diversity loss and a conversion from perennials to annuals in herbaceous communities.  The goal of our study was to understand how N enrichment can affect the colonization success within currently invaded and uninvaded California grasslands. We set up a split plot experiment with monocultures of either an exotic annual grass (Avena fatua) or native perennial grass (Stipa pulchra) along an N gradient (low, ambient, high) and manipulated light availability (low and ambient). We conducted reciprocal invasions within each plot, where each species was added to its own community type as well as the other. We measured survivorship for the perennial and spikelet production for the annual to assess the fitness of an individual when rare (invading heterospecific) or dominant (invading conspecific). This invasibility criterion – ability to increase when rare – can indicate whether the invasion would result in coexistence, resistance, or complete transformation of the community.

Results/Conclusions

The exotic annual had greater fitness when rare (invading the perennial community), and its fitness more than tripled under elevated N conditions within the native community. The native perennial’s survivorship was higher when establishing in its own community type under ambient and high N conditions. Resource manipulations approximating species effects did not replicate the strong reductions in fitness that the perennial experienced within the annual community, suggesting the contribution of other factors affecting the perennial’s colonization ability within the exotic annual community. Reducing N availability equalized the survivorship for an invading perennial between the annual and perennial communities, but did not alter the fitness of the exotic annual, which was still greater within the native community. These results suggest that lowering resource availability within an exotic annual community may help the reestablishment of native perennials and may prevent the invasion of the exotic annuals in native communities under future high N conditions, promoting coexistence. Furthermore, these results also indicate that management efforts should take into consideration potential species effects that may thwart the reintroduction of natives within currently invaded areas.