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Status and limiting factors of three rare plant species in the coastal lowlands and mid-elevation woodlands of Hawai`i Volcanoes National Park.

Item Summary

Title: Status and limiting factors of three rare plant species in the coastal lowlands and mid-elevation woodlands of Hawai`i Volcanoes National Park.
Authors: Pratt, Linda
VanDeMark, Joshua
Euaparadorn, Melody
Keywords: germination rates
declining population
endangered species
Portulaca sclerocarpa
Sesbania tomentosa
show 4 moreBobea timonioides
fruit production
seed germination
soil seed bank

show less
Issue Date: 26 Jan 2016
Series/Report no.: TR-024
Abstract: Two endangered plant species (Portulaca sclerocarpa, `ihi mākole, and Sesbania
tomentosa, `ōhai) and a species of concern (Bobea timonioides, `ahakea) native to the coastal lowlands and dry mid-elevation woodlands of Hawai`i Volcanoes National Park were studied for more than two years to determine their stand structure, short-term mortality rates, patterns of reproductive phenology, success of fruit production, seed germination rates in the greenhouse, presence of soil seed bank, and survival of both natural and planted seedlings. The role of rodents as fruit and seed predators was evaluated using exclosures and seed offerings in open and closed stations or cages. Rodents were excluded from randomly selected plants of P. sclerocarpa and from branches of S. tomentosa, and flower and fruit production were compared to that of adjacent unprotected plants. Tagged S. tomentosa fruit were also monitored monthly to detect rodent predation.

Natural populations of all three rare plant species showed declines over the period of monitoring. Bobea timonioides had the stand structure of a senescent population, and about a third of these long-lived trees have died since a previous survey 13 years ago. The size of monitored natural P. sclerocarpa plants decreased during the study, and mortality rate was 23% over two years. A comparison of current and long-term population data showed the same declining trend. Stand structure and mortality of natural S. tomentosa could not be determined at Kīpuka Nēnē, but half of monitored plants died over a year at coastal `Āpua Point. Portulaca sclerocarpa and S. tomentosa showed pronounced seasonal patterns in their reproductive phenology, and B. timonioides appeared to have a continuous pattern of flower and fruit production. Buds and flowers of P. sclerocarpa at its natural population peaked during spring and fall months, and fruit capsules were borne most of the year. At the upland S. tomentosa site, peak bud and flower production occurred in spring and summer, and greatest fruit abundance was in summer and winter months. The coastal site for this species showed greatest flowering in the fall and mature fruit were persistent year-long. Fruit set was high for P. sclerocarpa, very low for S. tomentosa, and undetermined for B. timonioides. Pollination was studied for only S. tomentosa at Kīpuka Nēnē, where six insect species were floral visitors, and native Hylaeus, or yellow-faced bees, and Apis mellifera honeybees were found to be transporting pollen of the rare plant.

Seed germination rates determined by greenhouse studies were moderately high for B. timonioides and S. tomentosa and low but variable for P. sclerocarpa. Field-sowed seeds of B. timonioides and P. sclerocarpa did not germinate, but up to 31% germination was observed for S. tomentosa seed-sowing trials. No difference in germination or seedling survival was observed for S. tomentosa in plots with and without grass. A small seed bank was detected for S. tomentosa during one season at both Kīpuka Nēnē and `Āpua Point. Mortality of planted seedlings or cuttings was high for all three rare species, but recent plantings of P. sclerocarpa seedlings showed a low mortality rate at one site. Natural seedlings were observed at two P. sclerocarpa planting sites, but all succumbed during dry periods. Rodents were found to be seed predators of both P. sclerocarpa and S. tomentosa, but had no detected impact on B. timonioides. A third of tagged seed pods of S. tomentosa displayed signs of rat predation, and another third disappeared.

The most important limiting factors identified for P. sclerocarpa were loss of seeds to rodent predation and low seedling recruitment. Sesbania tomentosa shared these two limiting factors and also lost flowers to alien insect predation and displayed very low fruit set caused by either a lack of effective pollination or self-compatibility problems. Lack of natural seedling recruitment, perhaps caused by current harsh site conditions, appeared to be the most significant limiting factor for B. timonioides.
Pages/Duration: 88
URI/DOI: http://hdl.handle.net/10790/2638
Appears in Collections:Hawaii Cooperative Studies Unit (HCSU)



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