A native nitrogen-fixing shrub facilitates weed invasion

Abstract

Invasions by exotic weedy plants frequently occur in highly disturbed or otherwise anthropogenically altered habitats. Here we present evidence that, within California coastal prairie, invasion also can be facilitated by a native nitrogen-fixing shrub, bush lupine (Lupinus arboreus). Bush lupines fix nitrogen and grow rapidly, fertilizing the sandy soil with nitrogen-rich litter. The dense lupine canopy blocks light, restricting vegetative growth under bushes. Heavy insect herbivory kills lupines, opening exposed nitrogen-rich sites within the plant community. Eventual re-establishment of lupine occurs because of an abundant and long-lived seed bank. Lupine germination, rapid growth, shading and fertilization of sites, and then death after only a few years, results in a mosaic of nutrient-rich sites that are available to invading species. To determine the role of bush lupine death and nitrogen enrichment in community composition, we examined nutrient dynamics and plant community characteristics within a site only recently colonized by lupine, comparing patches where lupines had recently died or were experimentally killed with adjacent areas lacking lupine. In experimentally killed patches, instantaneous pool sizes of exchangeable ammonium and nitrate nitrogen were higher than in adjacent sites free of lupine. Seedlings of the introduced grass Bromus diandrus accumulated 48% greater root biomass and 93% more shoot biomass when grown in a greenhouse in soil collected under experimentally killed lupines compared to B. diandrus seedlings grown in soil collected at least 1 m away from lupines. At the end of the spring growing season, total above-ground live plant biomass was more than twice as great in dead lupine patches as in the adjacent lupine-free grassland, but dead lupine patches contained 47% fewer plant species and 57% fewer native species. Sites where lupines have repeatedly died and reestablished during recent decades support an interstitial grassland community high in productivity but low in diversity, composed of mostly weedy introduced annual plants. In contrast, at a site only recently colonized by bush lupines, the interstitial grassland consists of a less productive but more diverse set of native and introduced species. We suggest that repeated bouts of lupine germination, establishment, and death can convert a rich native plant community into a less diverse collection of introduced weeds.

Arrival order among native plant functional groups does not affect invasibility of constructed dune communities

“…It appears that native functional groups have a nurse effect on bitou seedlings and moderate environmental conditions. Nurse effects of native species on invader germinants have been documented by previous studies which have reported facilitative effects through reducing light and temperature extremes (Lenz & Facelli 2003) and increasing water and nutrient availability (Maron & Connors 1996;Cavieres et al 2005) under canopies of existing native species compared with uncolonised areas. In our study, bare ground cover was significantly higher in the unplanted than planted treatments.…”

Mechanisms underlying the impacts of exotic plant invasions

“…Only after the death of nitrogen fixers do other invaders tend to increase (e.g. Maron & Connors 1996;Adler et al 1998). Thus, while it is clear that invaders can alter nitrogen cycling, it is less clear that this impact per se changes community structure.…”

Role of invasive Melilotus officinalis in two native plant communities

“…Native species often are better adapted to lower N levels present in the unaltered ecosystem (Arendt 1997;Hobbie 1992;McLendon and Redente 1992). In contrast, many invasive exotic species are well adapted to exploiting high N sites and have high dispersal rates that allow rapid colonization (Cousens and Mortimer 1995;Maron and Connors 1996;Rejmanek and Richardson 1996). Once at a N-rich site, exotic species can exploit N more quickly than later successional natives by increasing growth rates in response to higher nutrient availability (Carson and Barrett 1988;Fogarty and Facelli 1999).…”