Status and limiting factors of two rare plant speices in montane dry communities of Hawai`i Volcanoes National Park.

Abstract

Two rare plants native to montane dry forests and woodland communities of Hawai`i Volcanoes National Park (HAVO) were studied for more than two years to determine their stand structure, short-term mortality rates, patterns of reproductive phenology, success of fruit production, floral visitor composition, seed germination rates in the greenhouse, and survival of both natural and planted seedlings. Phyllostegia stachyoides, a shrubby Hawaiian mint (Lamiaceae) that is a species of concern, was studied within two small kīpuka at a natural population on the park’s Mauna Loa Strip, and three plantings at sites along the Mauna Loa Road were also monitored. Silene hawaiiensis, a threatened shrub species in the pink family (Caryophyllaceae), was monitored at two natural populations, one on Mauna Loa at the Three Trees Kīpuka and the second on Kīlauea Crater Rim south of Halema`uma`u. Silene hawaiiensis plantings were also made inside and outside ungulate exclosures at the park’s Kahuku Unit.

Phyllostegia stachyoides appeared to have a relatively stable natural population in HAVO with approximately 19% adult plant mortality over three years and recruitment of natural seedlings. Despite high mortality (~98%), some seedlings persisted for more than a year, and recruitment of new plants into the population exceeded the losses of adult plants. Flowering and fruiting phenology was annual and seasonal with peak appearance of buds and flowers in spring and greatest abundance of mature fruit in the summer and fall. Successful production of green fruit from buds and flowers was very high (45%), and green fruit transitioned to mature fruit at a rate of 17.8%. Five insect species were observed visiting flowers, and those with the greatest visitation rates were the alien hover fly Allograpta obliqua (Syrphidae) and the endemic yellow-faced bee Hylaeus difficilis (Colletidae). Both insect species were shown to be carrying pollen of P. stachyoides. Seed germination rates in the greenhouse were variable but ranged as high as 80.4%. Mortality of seedlings planted at three sites along the Mauna Loa Road was very high (~90%) within 2–3 years of planting. There was no significant difference in the mortality or growth of seedlings planted in areas with little grass compared to those in adjacent areas with high grass cover.

Silene hawaiiensis had a stable population structure at the Mauna Loa study area, but its population structure at the Kīlauea study site was flat to declining. Mortality of adult plants was low on Mauna Loa (6.5%), but was greater than 30% at the Kīlauea Crater Rim site. Among regularly monitored plants at the Kīlauea site, losses were observed in all size classes between 2006 and 2008. Natural seedling recruitment was observed in stand structure plots at both sites between 2006 and 2007, but numbers of seedlings were low and did not compensate for losses of adult plants. Reproductive phenology was annual with buds and flowers observed in summer and fall, and fruit formed in the fall and winter. The production of immature fruit capsules from buds and flowers was high (51.2%) and tagged immature fruit became mature fruit at a high rate of 66.7%. Floral visitation rates were very low in timed observations and only three insect species were identified visiting S. hawaiiensis flowers: native yellow-faced bees Hylaeus difficilis and H. volcanicus, and the alien hover fly Allograpta exotica. A seed dispersal experiment at the Kīlauea Crater Rim site demonstrated that wind dispersed seeds could travel at least 40 m from S. hawaiiensis plants with mature open capsules. Seed germination rates varied from 7.0 to 73.0% in greenhouse trials. Mortality of planted seedlings at Kahuku was not significantly greater outside ungulate exclosures than inside, but growth in height and production of reproductive structures was significantly greater in protected areas inside exclosures. In the current study, the seedling stage was the most vulnerable part of the life cycle for both P. stachyoides and S. hawaiiensis, and low seedling recruitment appeared to be the most important limiting factor for these species.