Invasion of Prosopis juliflora Figure 6. Root length (proportion of control, %) of Brassica campestris (upper panel), B. juncea (middle panel) and Sorghum bicolor (lower panel) seedlings grown in soil treated with different amounts (60, 150 and 300 mL/g soil) of Prosopis cineraria (gray bars) or Prosopis juliflora (black bars) leaf leachate. Soil treated with distilled water served as untreated control (white bars). Error bars represent +1SE of mean. F and P values are shown for two way ANOVAs. doi:10.1371/journal.pone.0044966.g006 native ranges in China and India. These differences were correlated with differences in the allelopathic effects of volatile organic compounds on species native to the different ranges. Callaway et al. [2] found that the biomass of native species in Acroptilon repens stands was 25–30 times lower in the non-native range than in the native range. It is important to note that lower plant species richness in plots invaded by P. juliflora compared to plots not yet invaded by it, could be due to the negative impact of the invader or colonization of sites that inherently have low species richness; mechanisms that cannot be separated through measure- ments of correlative patterns. However, consistently stronger negative canopy effects of P. juliflora in two non-native ranges compared to its native range, differences in canopy effects between congeners in the non-native range, and direct comparisons of litter from the congeners support the hypothesis that P. juliflora is at least in part a ‘‘driver’’ of decreased diversity rather than a ‘‘passenger’’ responding to other factors that also decrease diversity [39]. Strong negative impacts of P. juliflora on the richness, evenness and densities of other plant species have also been reported in the United Arab Emirates where it is also invasive [20]. The observed effects of P. juliflora canopies on species richness are less likely to be due to higher stand densities because similar negative canopy effects of P. juliflora on species richness have also been observed at lower stand densities (unpublished work). Congeneric comparisons demonstrated stronger impacts of the invasive P. juliflora than the native P. cineraria on plant diversity. Prosopis cineraria, a native Indian species, has been reported to facilitate growth of crop species and is an important species in agroforestry [40]; however, we measured its neutral effects on native diversity. Although biogeographic comparisons have been performed for other invasive plants such as Ageratina adenophora [38], Alliaria petiolata [41], Centaurea stoebe [42], Acroptilon repens [2] and C. diffusa [43], to our knowledge, this congeneric comparison is unique. Since P. cineraria is not an invasive species, we did not study, reciprocally, its positive or negative effects on species exotic to its native range India. Our results show that like other species of Prosopis, both P. juliflora and P. cineraria form resource islands by accumulating total organic N and organic carbon in their rhizosphere soil. Unlike P. cineraria, P. juliflora also accumulates soluble salts, exchangeable-P and total phenolics in its soil. However, it appears that the presence of high concentrations of total phenolics in P. juliflora soil may override its potential positive effects on soil fertility. Our study indicates that litter may play at least some role in the impact of P. juliflora on soil nutrients, soil phenolics, and the reduction of native species diversity in India. Leaf litter of P. juliflora killed far more seedlings of native Indian species than litter of P. cineraria. These effects of leaf litter appeared to be through their biochemical effects, results consistent with the Novel Weapons Hypothesis [44]. Bacteria and fungi isolated from P. juliflora and P. cineraria leaf litter and applied to seedlings killed almost all native Indian species, but there was no hint of a difference in effects between the congeners. Also, much higher amounts of total phenolics in soil amended with P. juliflora leaf leachate compared to that amended with P. cineraria leachate suggest that the release of more total phenolics into soil may play an important role in the stronger effects of P. juliflora litter. In a similar comparison of the phytotoxicity of P. juliflora and P. cineraria rhizosphere soils, Inderjit et al. [25] reported that Bambusa arundinacea seedlings grown in P. juliflora soil were smaller than those grown in P. cineraria soils, a result that could be due to a higher total phenolic content of P. juliflora soils than P. cineraria soils, as we measured here. Also, there were much higher amounts of total phenolics leached from P. juliflora litter than P. cineraria litter over a 14 d period (Figure 7B), suggesting a higher and more continuous supply of phenolics to soil from the litter of P. juliflora. Figure 7. Total phenolic content (mean + SE) of soil amended with Prosopis cineraria (gray bar) and P. juliflora (black bar) leaf leachates and unamended soil (control, white bar). (A) Different letters above bars indicate significant differences (ANOVA, post-ANOVA Tukey’s test; p,0.05). (B) Total phenolic content (mean 6 SE) of soil treated with no litter (white circles), P. cineraria (gray circles) or P. juliflora leaf litter (black circles) at rate of 12 mg/g soil, and incubated at 30–34uC under 12 h/12 h light/dark cycle for 0, 1, 2, 3, 4, 6, 8, 10 and14 days. doi:10.1371/journal.pone.0044966.g007 PLOS ONE | www.plosone.org 11 September 2012 | Volume 7 | Issue 9 | e44966