Invasion of Prosopis juliflora Figure 1. Prosopis juliflora in its native range of Venezuela (A); the invaded range of Haryana, India (B), along the National Highway to Rajasthan (C), at the boundaries of an agricultural field in India (D), and in Hawaii, USA (E); Prosopis cineraria in its native range, Rajasthan, India (F). Photo credits: Pascual J. Soriano (A); Inderjit (B, C, D and F) and Timothy J. Gallaher (E). doi:10.1371/journal.pone.0044966.g001 17]. Accordingly, Prosopis cineraria, indigenous to North-Western India, can facilitate native species [18]. Farmers keep P. cineraria in their fields because their crops grow better under the trees than in the open fields [19] (Figure 1F). Aggarwal et al. [19] also found that soil nitrogen, phosphorus, and potassium were higher under P. cineraria canopies than in open fields, and that the biomass of Pennesetum typhoides was three times higher when grown in P. cineraria soil than in open soil. Adding nutrients to non-P. cineraria soil reduced these differences, but P. typhoides grown in P. cineraria soils was always at least two times larger than when grown in soil from outside of P. cineraria canopies regardless of nutrient additions. These studies suggest that in their native ranges, Prosopis species often have neutral to positive effects on the species beneath them. Prosopis juliflora was first introduced to India in 1877 where it has become invasive. However it is also a source of fuel wood, fodder, charcoal and timber [3]. In India, Aggarwal et al. [18] found that the canopies of the invasive P. juliflora had far fewer understory species than any of four other species measured, whereas the native congener, P. cineraria, was associated with higher subcanopy diversity than any other species. In the Arabian Peninsula where P. juliflora is invasive has strong negative impacts on native species despite increases in the concentrations of some nutrients in subcanopy soil [20]. Exotic plants can also have different impacts on nutrient cycling than native species through nutrient release and biochemical effects [21–24]; suggesting the potential importance of compari- sons of the effects of P. juliflora to its native congener, P. cineraria, on the chemical characteristics of soil. Inderjit et al. [25] compared the effects of soils from the rhizospheres of P. juliflora and P. cineraria and found that soil under P. juliflora had higher concentrations of total phenolics and inhibited total biomass of Bambusa arundinacea more than soil from under the native congener. Goel et al. [26] reported allelopathic potential of P. juliflora leaf leachate prepared in hot water and decomposing litter residues. The amino acid L- tryptophan has been isolated from foliage leachate of P. juliflora and has been shown to have allelopathic potential on Echinochloa crus-galli in filter paper bioassays [27]. The apparent contrasts in the ecology of P. juliflora in its native and non-native ranges, and between the effects of P. juliflora and P. cineraria in the latter’s native range, is curious. This biogeograph- ical contrast parallels those exhibited by many other invasive species and raises questions about interactions among species from different parts of the world might be affected by evolutionary mismatches [1,28]. We hypothesized that P. juliflora in its invaded PLOS ONE | www.plosone.org 2 September 2012 | Volume 7 | Issue 9 | e44966