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The biology and ecology of Passiflora mollissima in Hawaii ?


La Rosa AM


PCSU Technical Report



Passiflora mollissima (HBK). Bailey a perrenial climber from the Andean highlands is currently proliferating in koa Acacia-koa) and \\\ohi\\\a (Metrosideros collina) forests on the islands of Hawai\\\i and Kaua\\\i. Since its introduction to the islands in the early part of this century it has spread rapidly and now occupies nearly 500 km2. Infestations range from scattered individuals with low cover to complete dominance of an area inhibiting the growth and reproduction of forest trees.This vine commonly known in Hawai\\\i as \\\"banana poka\\\" has been variously referred to as Passiflora mollissima or P. mixta or as the product of introgressive hybridization between P. mollissima and an unknown species. quantitative and qualitative characters were evaluated in an attempt to characterize the Hawaiian taxon. Comparison of these data with the most recent taxonomic treatment of Passiflora showed that most characters are within the range of those of P. mollissima. It is concluded that the Hawaiian taxon should be considered Passiflora mollissima sensu lato. Brief mention is made of the similarity between the Hawaiian population and several populations in other Pacific areas. The ecology and life history of P. mollissima were studied in three areas on the island of Hawai\\\i. In this species seeds require a short period of after ripening. Germination is staggered and most seedlings emerge within 4 to 12 weeks. Gemination success is not affected by light intensity but the rate increases with increasing light and associated higher temperatures. At very low light intensities (2.0% RLI) the germination rate is significantly inhibited. Ingestion of seeds by feral pigs has little effect on germination; they function principally as effective short-distance dispersal agents. Densities of germinants ranged from 540 to 554000 indiv./ha. and were considerably higher in areas of heavy pig activity. Growth of seedlings is slow compared with other stages in the life cycle. Heaviest mortality occurs between the germinant and seedling stage. The juvenile phase is characterized by rapid growth rates (up to 3 m/yr) and a distinctive morphology which is also found in rapidly-growing (flushing) adult shoots. Survival from the juvenile to the adult phase of the life cycle is very high. Under favorable conditions individuals may go from seed to reproductive maturity in a single year. All populations sampled were stable and reproducing. The reproductive pattern of P. mollissima promotes both allogamy and autogamy although selfing is favored. A high degree of self-compatibility is indicated although autogamy occurs in low frequency in the absence of pollinators which are principally exotic insects. Pollinator efficiency is quite low however (30%). Passiflora mollissima exhibits continuous growth and reproduction in Hawai\\\i both individuals and populations are asynchronous. Seasonal differences in behavior are reflected in the relative magnitude of activity throughout the year. Peak activity of reproductive phases tends to be concentrated into a single annual period while growth is less seasonal. All phases of flowering are concentrated in the drier summer months (May-Aug.) fruiting in the wet winter months (Dec.-Mar.). No strong correlations were found between climatic variables and phenological activity however. The relative shade tolerance of P. mollissima and its mode of establishment in closed-canopy forests of Hawai\\\i was studied using a series of shade levels under both natural and artificial conditions. Net assimilation rates and relative growth rates increase with increasing light intensity. Optimal growth occurs in full sun (100% RLI). Intermediate levels of shade (2-14% RLI)induce etiolation and result in rapid height growth. Greatest shade tolerance is exhibited a t these intermediate intensities. At very low light levels (0.35% RLI) growth is severely restricted and some individuals are regularly below the compensation point. As growth is proportional to light intensity the magnitude of a disturbance or gap size has a direct effect on the rate and pattern of establishment. Following a disturbance increases in cover and biomass of P. mollissima are proportional to levels of canopy disturbance and associated higher light intensities. Density is not proportional to the amount of biomass and cover present. The rapid and synchronous colonization of plots under conditions of 130% canopy removal also suggests that disturbance stimulates the germination of seeds present in the soil. From these data it is concluded that although closed-canopy forests in Hawai\\\i are not optimal habitats they are nonetheless susceptible to invasion and infestation by this exotic vine following regular canopy disturbance.


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