Arbuscular Mycorrhizae Effects on Growth of Two Hawaiian Species: Indigenous Osteomeles anthyllidifolia (Rosaceae) and Invasive Psidium cattleianum (Myrtaceae)

Two important plant species of Hawai‘i, the indigenous Osteomeles anthyllidifolia (Sm.) Lindl., a component of Hawai‘i’s most endangered habitat, and the highly invasive Psidium cattleianum Sabine were grown with or without arbuscular mycorrhizal fungi in a soilless mix at different soil-solution phosphorus (P) levels. At P levels similar to those in the field (0.007 mg P/liter), shoot biomass of inoculated plants of O. anthyllidifolia was 189% greater than that of controls, and that of P. cattleianum was 93% greater. Root weight of O. anthyllidifolia and leaf-tissue P of both species also were significantly higher in inoculated plants. At a higher concentration of soil-solution P (0.020 mg P/liter), inoculated plants of O. anthyllidifolia had 176% more biomass than controls, and those of P. cattleianum had 49% more. In a growth medium with soilsolution P equivalent to that of good agricultural soil (0.200 mg P/liter), inoculated plants of O. anthyllidifolia were 101% larger than controls. Results suggest that presence of arbuscular mycorrhizal fungi is of vital importance to establishment of O. anthyllidifolia in Hawaiian soils and that their absence may limit P. cattleianum invasion of sites that are highly deficient in available P. The successful establishment and growth of seedlings of many plant species in the field is greatly influenced by the presence and abundance of arbuscular mycorrhizal fungi in the soil (e.g., Reeves et al. 1979, Janos 1980b, Grime et al. 1987, Read and Birch 1988, St. John 1999). Arbuscular mycorrhizal fungi form mutualisic associations with the roots of the majority of plant species (Smith and Read 1997) and substantially enhance the growth of plants by greatly improving access to immobile nutrients (especially phosphorus but also copper and zinc) (Miyasaka and Habte 2001), an important benefit in soils low in available phosphorus (Smith and Read 1997). In addition, arbuscular mycorrhizal fungi increase the uptake of inorganic nitrogen (Govindarajulu et al. 2005), improve drought tolerance of host plants (Auge 2001), and can provide protection against some pathogens (e.g., Pozo et al. 2002) and heavy metals (Meharg 2003). Because plant species and cultivars vary greatly in their capacity to grow vigorously in the absence of arbuscular mycorrhizal fungi, the presence or absence of a critical population of arbuscular mycorrhizal fungi (along with other biotic and abiotic factors) can play an important role in regulating the composition of the plant communities that form at particular sites (e.g., Reeves et al. 1979, Janos 1980b, Grime et al. 1987, Read and Birch 1988, St. John 1999, Richter and Stutz 2002, Stampe and Daehler 2003). Although arbuscular mycorrhizal fungi have been noted in the roots of many Hawaiian species (e.g., Gemma et al. 1992, Koske et al. 1992), studies to assess their effect on the growth of important species in Hawai‘i are relatively few (e.g., Habte and Manjunath 1991, Miyasaka et al. 1993, Gemma et al. 2002, Stampe and Daehler 2003). In the study reported here we examined the mycorrhizal dependency of a native and an invasive species. Pacific Science (2006), vol. 60, no. 4:471–482 : 2006 by University of Hawai‘i Press All rights reserved 1 Manuscript accepted 2 January 2006. 2 Department of Biological Sciences, University of Rhode Island, Kingston, Rhode Island 02881. 3 Corresponding author: phone: 401-874-2629; fax: 401-874-5974; e-mail: [email protected]. Osteomeles anthyllidifolia (Sm.) Lindl. (‘ūlei) is an indigenous perennial shrub occurring at 2–2,320 m elevation on the islands of Hawai‘i, Maui, Kaua‘i, Moloka‘i, and O‘ahu in a variety of habitats (e.g., coastal cliffs, lava fields, and mesic forests), including the dry forest, Hawai‘i’s most endangered habitat (Wagner et al. 1990, Mehrhoff 1996). Psidium cattleianum Sabine grows as a shrub or small tree and is a troublesome invasive in tropical and subtropical areas throughout the world, including Hawai‘i, Tahiti, Mauritius, Florida, and Norfolk Island (Cronk and Fuller 1995, Langeland and Burks 1998). The species was described by Smith (1985:200) as ‘‘the worst pest in Hawai‘i’s rain forests.’’ Introduced to Hawai‘i from Brazil as a fruit tree in about 1825 (Wagner et al. 1990), P. cattleianum (‘‘strawberry guava’’) now occurs on Hawai‘i, Kaua‘i, Lana‘i, Maui, Moloka‘i, and O‘ahu at elevations between 150 and 1,300 m (Smith 1985), where it forms dense, monotypic stands that exclude native species. In Hawai‘i, the roots of O. anthyllidifolia and P. cattleianum are highly colonized by arbuscular mycorrhizal fungi, and both species were putatively classified as being highly dependent on the basis of field surveys (Koske et al. 1992), suggesting that they would be excluded from sites lacking a minimal population of arbuscular mycorrhizal fungi (Koske and Gemma 1997). If so, this would be of critical importance in the restoration of native sites by O. anthyllidifolia and could help in assessing the likelihood of sites being successfully invaded by P. cattleianum. The goal of this study was to evaluate the effect of arbuscular mycorrhizal fungi on growth of these two species at different levels of soilsolution phosphorus. materials and methods