Stem density induces differential effects of litter on native and invasive plants

Litter plays an important role in plant invasion. Our previous studies have found that litter had two opposite effects as Spartina alterniflora invading native plant Phragmites australis community. Litter stimulated the growth of S. alterniflora on the edge of the community, while it inhibited the growth in the old community. Accordingly, we hypothesized that litter may play different roles in different stages of plant invasions. We selected three types of communities: P. australis monoculture (PAM), S. alterniflora monoculture (SAM), S. alterniflora–P. australis mixture (SPM) and conducted a comparative study by keeping and removing litter at Chongming Dongtan in the Yangtze River estuary, China. Our results revealed that litter removal significantly increased light intensity in SAM, and enhanced it in SPM, but had no significant influence on it in PAM. The effect of litter on light intensity resulted in a higher chlorophyll content and higher plant height of S. alterniflora monoculture after litter removal, which eventually led to a higher single weight and aboveground biomass for PAM and SAM after litter removal, and to a higher aboveground biomass for S. alterniflora in SPM. Litter removal significantly enhanced the dominance of S. alterniflora relative to P. australis in SPM. We concluded that the effects of invasive plants' litter on the invasion processes depend on the stem density of invasive plants relative to native plants. If an invasive species had a higher plant density, its litter may not have a shelter effect on native species, but could restrict its own growth.