Characteristics of the Psidium cattleianum (Myrtaceae) Seed Bank in Hawaiian Lowland Wet Forests

Psidium cattleianum Sabine (strawberry guava) is one of Hawai‘i’s most disruptive alien plants. Dense stands can suppress growth and establishment of native species, support high populations of crop-damaging fruit flies, and preclude restoration or management of native forests. Our research investigated factors affecting persistence of P. cattleianum seeds in lowland wet forest soils. We collected soil cores from four forested sites immediately after fruit fall and 6.5 months later. We found abundant germination of P. cattleianum seeds immediately after fruit drop. Soil collected under mature P. cattleianum clumps yielded 761 viable seeds/m. We found no viable seeds 6.5 months after fruit drop. We evaluated seed longevity using seed bags buried below the litter layer that we retrieved after 28, 56, 196, and 365 days. Seeds either germinated or deteriorated rapidly after fruit drop; after 28 days, 22.3% of the buried seeds were viable and there were no viable seeds at 196 days. Predator effects were assessed using trays with a known number of seeds with and without predator exclosures. After 28 days, 37% of the seeds in the open trays were damaged by predators. The lack of a persistent seed bank likely is due to a combination of rapid, high germination rates, postdispersal seed predation, and seed mortality. We suggest that chemical or mechanical control efforts would be most efficient and effective if conducted at least 3 months after the fruiting season, when the vast majority of seeds have either germinated or died. Psidium cattleianum Sabine (Myrtaceae), also known as strawberry guava, is considered one of Hawai‘i’s most severe invasive weeds (Smith 1985, Wagner et al. 1999, Motooka et al. 2003). Psidium cattleianum is common on all the major Hawaiian islands and throughout the Pacific where it continues to increase in density and impact. Rapid growth rates ( J.S.D. and A.L.U., unpubl. data), prolific fruit production, and high rates of suckering produce dense stands quickly from small populations (Huenneke and Vitousek 1990). Psidium cattleianum is a serious threat to native forest ecosystems in Hawai‘i and elsewhere in the Pacific due to its ability to invade even relatively undisturbed wet forests and form dense thickets (Smith 1985, Huenneke and Vitousek 1990). The fruits of P. cattleianum support high populations of fruit flies (e.g., Dacus dorsalis Hendel) that are considered one of the most important agricultural pests in Hawai‘i (Vargas et al. 1990). The efficacy of any control method for P. cattleianum is influenced by the characteristics of its seed bank, because a persistent seed bank contributes to high population growth rates and reduces the likelihood of local eradication. We investigated longevity of seeds of Psidium cattleianum in Hawaiian soils, the characteristics of the seed bank, and factors affecting the duration of seeds in lowland wet forest soils in Hawai‘i. materials and methods