Relationships between Scotch broom (Cytisus scoparius), soil nutrients, and plant diversity in the Garry oak savannah ecosystem

Abstract

Scotch broom (Cytisus scoparius), is a leguminous shrub, native to the Mediterranean, which has invaded most of the remaining Garry oak savannah ecosystems in Oregon, Washington, and British Columbia. Here, it is considered to be a threat to the native plant community. We tested the hypothesis that broom would increase available soil nitrogen by comparing soil nutrients in contiguous broom-invaded and non-invaded sites. We then looked for changes in patterns of diversity in the herbaceous community that might indicate a role of Scotch broom in changing conditions following its invasion. Finally we carried out greenhouse assays to test whether broom had a greater impact on the growth of a native and an introduced grass compared to that of a native shrub. Broom was associated with only a weak trend in increased soil nitrogen, but a significant decrease in soil phosphorus was observed. Patterns of plant diversity differed between two sites. At one site, 60% of the plants whose abundances increased in the broom-invaded plots were introduced species while native species abundances decreased in the broominvaded plots compared to broom-free plots. At the other site, 60% of the plants that caused the differences between broom-invaded and un-invaded plots were native species that were less abundant in the broom-invaded plots. Finally, in greenhouse assays grass growth was not affected as a result of being grown with broom; however, grasses appeared to produce more flowers when grown with broom. We conclude that broom does not necessarily modify soil nitrogen availability but may deplete soil phosphorus availability and that broom invasion can be associated with increase of exotic species and/or the decline of native species.

Growth response of broom (Cytisus scoparius) growing with and without radiata pine (Pinus radiata) seedlings to different P levels in soils

“…The significant effect of P fertiliser on the increase of shoot P concentration is most probably due to the very low available P concentration in soil at the addition of 0 mg⋅kg -1 . Shaben & Myers (2010) found that broom and Garry oak savannah ecosystem had a significant depletion effect in soil available P. This result confirmed Williams's (1981) findings that broom reaches optimum growth rates in soils with a high level of inorganic P to support its ability to fix N, grows rapidly and sets large amount of seed. These abilities make it to be a very successful pioneer in new habitats.…”

Introduced populations of Genista monspessulana (French broom) are more dense and produce a greater seed rain in California, USA, than native populations in the Mediterranean Basin of Europe

“…For example, as for C. scoparius Fogarty and Facelli 1999), G. monspessulana may have other traits that allow it to be a better competitor than native plants occurring in introduced habitats. C. scoparius has been shown to alter soil nutrient dynamics through an accumulation of nitrogen fixed by rhizobial bacteria and subsequent leaf deposition (Haubensak and Parker 2004;Prévosto et al 2006) and depletion of soil phosphorus (Shaben and Myers 2010). An increase in soil nitrogen availability has been shown to facilitate the establishment of other fast-growing invasive species (Corbin and D'Antonio 2004), and for both G. monspessulana (Haubensak 2001) and C. scoparius (Haubensak and Parker 2004;Prévosto et al 2006) soil nutrient enrichment can create a positive feedback facilitating the establishment of new broom seedlings thereby outcompeting resident native plant species (Fogarty and Facelli 1999).…”

Shrubs affect soil nutrients availability with contrasting consequences for pasture understory and tree overstory production and nutrient status in Mediterranean grazed open woodlands

“…In the meantime, Retama diminished P availability in soil and P concentration in both pastures and trees. Shaben and Myers (2010) showed that a similar species than Retama (Cytisus scoparius) could deplete soil P availability in an oak savanna. P provides energy for, among other physiological functions, N 2 -fixation (Vance et al 2003), thus Retama may impose a certain competition for this nutrient.…”