James A. Allen

MANGROVES AS ALIEN SPECIES : THE CASE OF HAWAII

Prior to the early 1900s, there were no mangroves in the Hawaiian Archipelago. In 1902, Rhizophora mangle was introduced on the island of Molokai, primarily for the purpose of stabilizing coastal mud flats. This species is now well established in Hawaii, and is found on nearly all of the major islands. At least five other species of mangroves or associated species were introduced to Hawaii in the early 1900s, and while none has thrived to the degree of R. mangle, at least two have established self-maintaining populations (Bruguiera gymnorrhiza and Conocarpus erectus). Mangroves are highly regarded in most parts of the tropics for the ecosystem services they provide, but in Hawaii they also have important negative ecological and economic impacts. Known negative impacts include reduction in habitat quality for endangered waterbirds such as the Hawaiian stilt (Himantopus mexicanus knudseni), colonization of habitats to the detriment of native species (e.g. in anchialine pools), overgrowing native Hawaiian archaeological sites, and causing drainage and aesthetic problems. Positive impacts appear to be fewer, but include uses of local importance, such as harvesting B. gymnorrhiza flowers for lei-making, as well as some ecological services attributed to mangroves elsewhere, such as sediment retention and organic matter export. From a research perspective, possible benefits of the presence of mangroves in Hawaii include an unusual opportunity to evaluate their functional role in coastal ecosystems and the chance to examine unique or rare species interactions.

Effects of salinity on baldcypress seedlings: Physiological responses and their relation to salinity tolerance

Growth and physiological responses of 15 open-pollinated families of baldcypress (Taxodium distichum var.distichum) subjected to flooding with saline water were evaluated in this study. Ten of the families were from coastal sites in Louisiana and Alabama, USA that have elevated levels of soil-water salinity. The other five families were from inland, freshwater sites in Louisiana. Seedlings from all families tolerated flooding with water of low (2 g 1−1) salinity. Differences in biomass among families became most apparent at the highest salinity levels (6 and 8 g 1−1). Overall, increasing salinity reduced leaf biomass more than root biomass, which in turn was reduced more than stem biomass. A subset of seedlings from the main greenhouse experiment was periodically placed indoors under artificial light, and measurements were made of gas exchange and leaf water potential. Also, tissue concentrations of Cl−, Na+, K+, and Ca2+ were determined at the end of the greenhouse experiment. Significant intraspecific variation was found for nearly all the physiological parameters evaluated, but only leaf concentrations of Na+ and Cl− were correlated with an index of family-level differences in salt tolerance.

Factors influencing the regeneration of the mangrove Bruguiera gymnorrhiza (L.) Lamk. on a tropical Pacific island

Abstract Mangrove swamps occupy approximately two-thirds of the shoreline on Kosrae, Federated States of Micronesia (FSM), and also border the island’s most populated areas. Kosraeans depend on mangrove swamps for a supply of wood to support a growing handicraft industry, for a dependable source of fuelwood, and for habitat to support the harvest of fish and mangrove crabs. One of the more prominent mangrove species on Kosrae is Bruguiera gymnorrhiza , yet it is not the most preferred species for carving or cooking. To evaluate B. gymnorrhiza ’s persistence in the intertidal and to develop a better understanding of factors influencing its regeneration, we investigated predispersal insect colonization of propagules, postdispersal propagule predation by crabs, and the relative effects of natural and artificial shade, salinity, and tidal flooding on early tree seedling survival and growth. Predispersal insect colonization of propagules by boring insects was very high (93%), but the damage did not seem to influence seedling survival. Postdispersal predation of B. gymnorrhiza propagules by crabs was low (17%) and did not change in gap versus understory plots. Predation did vary by intertidal location (lower intertidal>middle intertidal=upper intertidal), with lower predation occurring in an intertidal location with a B. gymnorrhiza -dominated overstory. Shade and tidal inundation reduced seedling growth more than salinity in greenhouse investigations, but sunlight had less positive influence on seedling growth in the field. In general, regeneration and growth occurred successfully under a variety of conditions, indicating that none of the factors investigated serve as strong regulators to B. gymnorrhiza regeneration and early growth on Kosrae.

Patterns of natural and anthropogenic disturbance of the mangroves on the Pacific Island of Kosrae

Mangroves in many parts of the world are subjected to frequent, large-scaledisturbances. A possible exception is Kosrae, Federated States of Micronesia(FSM), a small volcanic island in the west-central Pacific Ocean. Relativesea level has been stable for most of the last 1000 years and the last tropical cyclone to affect the island was in 1905. Many trees on Kosrae,especially individuals of the species Sonneratia alba, thereforeappear to die only after reaching advanced ages and exceptional sizes. Themost widespread anthropogenic disturbance is harvesting of trees for fuelwoodand poles, which is done selectively and generally creates small, dispersedgaps. Other forms of anthropogenic disturbance, such as modifications ofcoastal landforms, alterations of freshwater inflows road construction andconversion to residential or agricultural uses, are still relatively minorbut have led to some irreversible losses. The economy of Kosrae is basedto a large degree on income derived from a Compact of Free Association betweenthe FSM and the United States, an agreement that has an uncertain future.Many of the funding provisions of the Compact expire in 2001 and, if notrenewed, may have dramatic impacts on resource use. This in turn may leadto a much greater level of anthropogenic disturbance of what are now someof the worlds most intact mangrove swamps.

Salinity effects and differential germination of several half-sib families of baldcypress from different seed sources

Saltwater intrusion is responsible for the destruction of large expanses of baldcypress swamps along the Gulf of Mexico in the southern United States. Recent restoration efforts have focused on the identification, and subsequent planting, of moderately salt-tolerant plant material within these “ghost forests.” The long-term persistence of cypress swamps in these areas, however, will ultimately depend on the germination of seeds from the improved parent trees. This study investigates the germination capacity of seeds collected from baldcypress parent trees, located in both freshwater and brackish water environments. Seeds were tested at four different salinity levels under controlled conditions for a period of 65 days. Mean germination under the 0, 2, 4, and 6 g l-1 seawater treatments was 26.3, 22.9, 15.4, and 10.2%, respectively. Although none of the eight families used in this study had a germination greater than 50%, three families demonstrated superiority in their germination capacity under all four salinity treatments. In general, brackish water seed sources had greater germination success than the two families from freshwater sources. Results indicate that families previously identified as moderately salt-tolerant based on physiological and morphological parameters may be equally successful in seed germination capacity under moderately saline conditions.

DOWNED WOOD IN MICRONESIAN MANGROVE FORESTS

Dead, downed wood is an important component of upland forest and aquatic ecosystems, but its role in wetland ecosystems, including mangroves, is poorly understood. We measured downed wood in ten sites on the western Pacific islands of Kosrae, Pohnpei, and Yap, all located within the Federated States of Micronesia. Our goals were to examine patterns of variability in the quantity of downed wood in these mangrove ecosystems, provide a general characterization of downed wood in a region with no previously published accounts, and investigate the relationship between harvesting practices and the amount of downed wood. The overall mean volume of downed wood at our study sites was estimated to be 60.8 m3 ha−1 (20.9 t ha−1), which is greater than most published data for forested wetlands. There were significant differences among islands, with the sites on Kosrae (104.2 m3 ha−1) having a much greater mean volume of downed wood than those on Pohnpei (43.1 m3 ha−1) or Yap (35.1 m3 ha−1). Part of the difference among islands may be attributable to differences in stand age and structure, but the most important factor seems to be the greater amount of wood harvesting on Kosrae, coupled with a low efficiency of use of cut trees. Of a total of 45 cut trees examined on Kosrae, no wood had been removed from 18 (40%); these are believed to be trees cut down because other, more valuable, trees were caught on them as they were felled. Of the other 27 trees, only 24 to 42% of the stem volume (to a 10 cm top) was removed from the forest, the amount varying by species. The impacts of current harvesting practices are unknown but may include important effects on tree regeneration and the abundance and species composition of crab populations.

Plant succession and greentree reservoir management: implications for management and restoration of bottomland hardwood wetlands

Bottomland hardwood forests are distributed along rivers and streams throughout the central and eastern United States, with the greatest concentration in the Southeast. Past and projected losses of bottomland hardwoods and degradation of remaining stands suggest that habitat management and/or restoration strategies that target multiple species and multiple uses will be necessary to maintain, enhance, and restore flora and fauna within bottomland hardwood wetlands. A greentree reservoir is a current management strategy that entails manipulating water regimes to provide habitat for wintering waterfowl. We conducted a literature review and synthesis to determine the potential impacts of greentree reservoir management on plant succession within bottomland hardwood wetlands. Greentree reservoirs can impact vegetation establishment through several processes. Despite shortcomings of greentree reservoirs, designs similar to them could be very beneficial in restoring bottomland hardwood plant and animal communities from degraded forests provided water-level control and maintenance are substantially improved. Emulation of natural hydrologic regimes, including natural variability, could produce diverse bottomland hardwood plant communities and provide habitat for a variety of wildlife species.

Influence of Propagule Flotation Longevity and Light Availability on Establishment of Introduced Mangrove Species in Hawai‘i

ABSTRACT Although no mangrove species are native to the Hawaiian Archipelago, both Rhizophora mangle and Bruguiera sexangula were introduced and have become naturalized. Rhizophora mangle has spread to almost every major Hawaiian island, but B. sexangula has established only on O‘ahu, where it was intentionally introduced. To examine the possibility that differences in propagule characteristics maintain these patterns of distribution, we first reviewed the literature on surface currents around the Hawaiian Islands, which suggest that propagules ought to disperse frequently from one island to another within 60 days. We then tested the ability of propagules of the two species to float for periods of up to 63 days and to establish under two light intensities. On average, R. mangle propagules floated for longer periods than those of B. sexangula, but at least some propagules of both species floated for a full 60 days and then rooted and grew for 4 months under relatively dense shade. A large percentage (~83%) of R. mangle propagules would be expected to float beyond 60 days, and approximately 10% of B. sexangula propagules also would have remained afloat. Therefore, it seems likely that factors other than flotation ability are responsible for the failure of B. sexangula to become established on other Hawaiian islands.

Root and shoot responses of Taxodium distichum seedlings subjected to saline flooding

Abstract Variation among progeny of five half-sib family collections of baldcypress ( Taxodium distichum ) from three freshwater and two brackish-water seed sources subjected to saline flooding was evaluated. Mini-rhizotrons (slant tubes) were used to monitor root elongation for a period of 99 days. Salinity level produced significant effects across all baldcypress half-sib families, with root elongation averaging 1594.0, 956.8, and 382.1 mm, respectively, for the 0, 4, and 6 g l −1 treatments. Combined mean root elongation for families from brackish-water seed sources was greater (1236.7 mm) than for families from freshwater seed sources (794.6 mm). Considerable variation occurred at the highest salinity treatment, however, with one freshwater family maintaining more than 28% more root growth than the average of the two brackish-water collections. Hence, results indicate that short-term evaluation of root elongation at these salinity concentrations may not be a reliable method for salt tolerance screening of baldcypress. Species-level effects for height and diameter, which were measured at day 62, were significant for both parameters. Height increment in the control (7.4 cm), for example, was approximately five times greater than height increment in the 6 g l −1 salinity treatment (1.5 cm). Family-level variation was significant only for diameter, which had an incremental range of 0.2 to 1.5 mm across all salinity levels.

A Tropical Freshwater Wetland: I. Structure, Growth, and Regeneration

Forested wetlands dominated by Terminalia carolinensis are endemic to Micronesia but common only on the island of Kosrae, Federated States of Micronesia. On Kosrae, these forests occur on Nansepsep, Inkosr, and Sonahnpil soil types, which differ in degree of flooding and soil saturation. We compared forest structure, growth, nutrition, and regeneration on two sites each on Nansepsep and Inkosr soils and one site on the much less common Sonahnpil soil type. Terminalia tree sizes were similar on all three soil types, but forests differed in total basal area, species of smaller trees, and total plant species diversity. Terminalia regeneration was found only on the Inkosr soil type, which had the highest water table levels. Other Terminalia species are relatively light demanding, and T. carolinensis exhibited similar characteristics. It is therefore likely that Terminalia requires periodic, but perhaps naturally rare, stand-replacing disturbances (e.g., typhoons) in order to maintain its dominance, except on the wettest sites, where competition from other species is reduced. Terminalia swamps in the Nansepsep soil type appeared to be at the greatest risk of conversion to other uses, but swamps on all three types may face greater pressure as Kosraes population increases and the islands infrastructure becomes more developed.