Phenology, reproductive potential, seed dispersal and predation, and seedling establishment of three invasive plant species in a Hawaiian rain forest
(Medeiros, 2004: dissertation)
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Citation: Medeiros, A.C. 2004. Phenology, reproductive potential, seed dispersal and predation, and seedling establishment of three invasive plant species in a Hawaiian rain forest. Ph.D. Dissertation, Department of Zoology, University of Hawaii at Manoa, Honolulu.
Overview: After rain forest of Haleakala National Park was fenced in the late 1980s, native vegetation responded vigorously yet three problematic plant invaders (Clidemia hirta, Hedychium gardnerianum, and Psidium cattleianum) continued to spread unabated and became of great concern to Park managers. This contribution provides a quantitative assessment of crucial life history junctures (quantitative phenology, reproductive potential, seed dispersal, seed predation, seedling establishment) to assist Haleakala NP and other managers of Hawaiian rain forests. It also provides detailed information for potentially identifying key characteristics in prevention, rapid response, and prioritization of incoming invasive species.
Abstract: The objective of this study was to investigate phenology, reproductive potential, seed dispersal and predation, and seedling establishment of three important weed species (Clidemia hirta, Hedychium gardnerianum, Psidium cattleianum) of Hawaiian rain forests. The phenology results revealed that Clidemia ripe fruit production was highest from October through January, Hedychium from October through December, and Psidium in November. Compared to Psidium of similar sizes at lower elevations, the production of ripe fruit by Psidium at the study site appeared limited. Clidemia was estimated to produce more than four orders of magnitude more seeds than Hedychium and Psidium of equal cover. Individual Clidemia plants, which averaged 0.37 m2 cover, produced an average of 3,815,552 seeds/yr. Birds, especially two common non-native species (Zosterops japonicus and Leiothrix lutea), are dispersing all three weed species as well as at least six native plant species, one non-native species, and four unidentified species. Under experimental conditions, the passage time for Clidemia seeds was as long as 210 minutes, for Hedychium seeds as long as 270 minutes, and with limited results, for Psidium seeds less than 60 minutes. Non-native rats (Rattus spp.) were discovered to be dispersing viable Clidemia seeds that did not differ significantly in germinability or germination rate from control seeds. Non-native rats were also discovered to have caused a 48.3% reduction in Hedychium seed production through flower bud, flower, and pre-dispersal seed predation. The final objective of this investigation was to determine which sites are suitable for germination and initial seedling growth of the study species. Results from Clidemia sites (but not Hedychium and Psidium) indicated a large, viable seed bank and/or substantial seed rain. Clidemia produced the greatest number and heights of seedlings in scalped and grassy sites. Hedychium produced the greatest number of seedlings in epiphytic sites and the least in grassy sites. Psidium were found in three site types, with the tallest seedlings being found in fernland sites. Data from the last part of this investigation suggest that Clidemia is disturbance-adapted and less adapted to protected, shaded sites, whereas Hedychium and Psidium are greater threats to recovering and intact rain forests.
Synopsis: Selected ecological traits (reproductive phenology, avian seed dispersal, mammalian seed predation and dispersal, and seedling establishment sites) and implications for management of three problematic fleshy-fruited weed species Clidemia hirta (L.) D. Don (clidemia), Melastomataceae, Hedychium gardnerianum Ker.-Gawl., Zingiberaceae (kahili ginger), and Psidium cattleianum Sabine, Myrtaceae (strawberry guava) (hereafter Clidemia, Hedychium, and Psidium) are discussed. The classic weed model suggests that a characteristic of invasive plant species is the ability to produce large numbers of small seeds annually with minimum duration between episodes of fruit production. Of the three study species Clidemia most clearly fits this description. Regarding seed size, Clidemia has very small seeds (0.7 mm x 0.5 mm) compared to Psidium and Hedychium that produce seeds approximately 3 mm x 4mm. Regarding numbers of seeds, Clidemia was estimated to produce more than four orders of magnitude more seeds than Hedychium and Psidium of equal cover. Based on data from this study, it is estimated that the mean annual reproductive potential of a square meter area of Clidemia is 10,312,292 seeds; for Hedychium is 2,024 seeds; and for Psidium is 568 seeds. Regarding the production of fruit annually with minimum duration Clidemia once again is the species that most clearly fits the existing weed model. Clidemia fruit production was highest from October through January when each plant ripened an average of 5.6 fruits per day, that is, 4883 seeds per plant per day. However, Clidemia flowered and fruited year round while Hedychium fruited only in a relatively short but intense period from late fall through mid-winter. Though Psidium produced flower buds and flowers year-round, many of these apparently aborted without maturation as ripe fruit production (which peaked in fall) occurred only at very low levels. This study determined that seeds of all three study species were primarily dispersed by non-native opportunistic passerines (Zosterops japonicus and Leiothrix lutea), as well as 11 other plant species including at least six important native plant genera (Cheirodendron, Melicope, Broussaisia, Pipturus, Coprosma, Vaccinium). Zosterops and Leiothrix captures within fruiting Clidemia patches averaged 314 seeds/capture and 102 seeds/capture respectively. One captured Zosterops excreted over 1800 Clidemia seeds, or approximately 2.2 fruits. In comparison, the other two study species had much lower seed dispersal rates. For Hedychium, Zosterops and Leiothrix captures averaged 0.29 seeds/capture and 0.72 seeds/capture respectively and for Psidium, Leiothrix captures averaged 1.8 seeds/capture with no Zosterops captures. Passage time of seeds through the gastrointestinal (GI) tract of birds was tested experimentally and determined to be for Hedychium, Clidemia, and Psidium seeds was as long as 270 minutes, 210 minutes, and less than 60 minutes respectively. Evidence was that non-native rodents (Rattus) with well-developed reputations as seed predators dispersed viable Clidemia seeds, though not increasing the seeds germinability and germination rate, and through bud, flower, and fruit predation, caused a 48.3% reduction in seed production for Hedychium. Experimental trials demonstrated that 1) Clidemia had a extremely large soil seed bank and that this species produced the greatest number and heights of seedlings in scalped and grassy sites 2) Hedychium produced the greatest number of seedlings in epiphytic sites and the least in grassy sites, and 3) Psidium were found in all three site types, with the tallest seedlings being found in fernland sites. Experimental data suggests that Clidemia is disturbance-adapted and less suited to protected, shaded sites, whereas Hedychium and Psidium are greater threats to feral pig-free forests.
The project has demonstrated that biological control appears to be the only effective long-term solution for relatively intractable weed species in Hawaii’s rain forest, such as clidemia, kahili ginger, and strawberry guava. Mechanical and chemical control are going to ultimately prove inadequate for addressing these species. Of the three study species, Psidium appears to pose the greatest threat to the long-term status of rain forests of the Hawaiian Islands and the Pacific. This study also indicated that the creation of a predictive weed model to be used in screening introductions is far from simple as with these three highly invasive study species, commonalities appear no more frequent than differences amongst them.
Thanks to Art Medeiros for sending us this document and allowing us to post it online!
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This page was created on 27 July 2005 by PT, and was last updated on
27 July 2005