Imperata cylindrica, an invasive grass, shifts nitrogen pools in a young pine forest

Imperata cylindrica, a C₄ rhizomatous perennial grass that invades a range of sites, is one of the most troublesome weed species in the world.  Several studies have examined control of this species, but few have explored the impacts of this species on ecosystem processes.  Using ¹⁵N-labeled fertilizer, we examined how invasion by I. cylindrica into a one-year-old Pinus taeda forest would alter the cycling of N using a completely randomized design consisting of five replications of two treatments, native understory species and I. cylindrica, in Santa Rosa County, Florida. I. cylindrica was significantly more competitive in acquiring applied N than native species as evidenced by a large proportion of ¹⁵N in its biomass (p=0.002).  I. cylindrica and the native vegetation maintained roughly the same amount of aboveground biomass.  However, the belowground biomass of I. cylindrica on average was seven times higher than that of the native species.  As a result, it retained significantly more N per hectare belowground compared to the native species (p<0.001)(32.9 kg N/ha vs. 14.7 kg N/ha).  In the native species treatment, more N was stored aboveground compared to belowground.  Thus, invasion by I. cylindrica could lead to a shift in N pools from above to belowground in ecosystems where it is forming monospecific stands in the understory.