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Native coastal flora and plant communities in Hawai`i: their composition, distribution, and status.
|HCSUTR-014WarshauerNativeCoastalFloraFULLREPORT.pdf||18.03 MB||Adobe PDF||View/Open|
|Title:||Native coastal flora and plant communities in Hawai`i: their composition, distribution, and status.|
|Authors:||Warshauer, Fredrick R.|
Jacobi, James D.
Price, Jonathan P.
|Date Issued:||Dec 2009|
|Series:||Technical Report HCSU - 014|
|Abstract:||The Hawaiian coastal flora primarily includes the halophytic (salt-adapted) plants closest to the area of salt spray and wave wash, often concentrated within a distinct strand zone, and usually growing as low mats. However, behind the strand is a zone of vegetation that is quite varied in composition and structure, but somewhat less specialized in life form, which is also adapted to the specific conditions of the coastal
environments and to those at different locales. Coastal plant communities in Hawai‘i are distributed across a very wide range of conditions, and are anything but homogeneous. Primary factors that influence their composition and structure include moisture, substrate, and exposure to wind and salt water. Some of these factors also have roles in dispersal, competition, trauma, and periodic reordering of local community compositions. External to these are anthropogenic impacts which may have a similar scale of influences today.
Between 2000 and 2005 we surveyed a total of 133 coastal sites on the islands of Moloka‘i, Maui, and O‘ahu. On O‘ahu we visited 28 sites, 50 sites were surveyed on Moloka‘i, 36 sites on West Maui, and 19 sites on East Maui. The survey areas were selected primarily to sample a region’s community variety and composition, and were distributed within the diversity of moisture zones found in the coastal regions of each island. A few sites were visited on Kaua‘i and Hawai‘i, but these islands are in need of much more survey work and thus have limited commentary in the current summary. The smaller main islands (Ni‘ihau, Lāna‘i, and Kaho‘olawe), as well as many of the small offshore islets, were not visited at all in this assessment.
During this project we recorded 142 taxa of native plants out of 169 that were expected based on past plant collections and from the literature. A total of 105 coastal plants were recorded on Moloka‘i, 85 on Maui, and 52 on O‘ahu. Thirty-eight species were found on all three of the islands we surveyed, 53 on at least two islands, and 51 plants were found on only one of these islands. We encountered 12 listed endangered taxa, 2 threatened taxa, and 13 species of concern (SOC) but with no official listing status, as well as many other taxa that are now relatively uncommon within the Hawaiian coastal zone.
The number of plants per site varied considerably between the areas surveyed, but the greatest diversity was found on Moloka‘i and Maui, with their richest sites containing 30 and 32 species, respectively. On Maui 22 (40%) of the sites had less than10 native coastal plant species, 30 (55%) had 10 – 20 species, and 3 (5%) with more than 20 species. A different situation was found on Moloka‘i where only 10 (20%) of the
sites had less than 10 native coastal plant species, 28 (56%) had 10 – 20 species, and 12 (24%) with more than 20 species. On O‘ahu seven (25%) sites had less than 10 native plant species, 20 (71%) had 10 – 20 species, and only one site (Ka‘ena Point) had over 20 species, but, in this case just a total of 21 coastal plant species. The current coastal flora of O‘ahu was somewhat reduced compared to the other two islands, likely a result of the much greater human-related impacts on the coastal zone of O‘ahu. Additionally, the remaining coastal vegetation on O‘ahu is nearly all within the dry zone. The most influential site factor for Hawaiian coastal communities is the range of moisture that occurs across any particular area. The greater the moisture zone range,
the more species are likely to be found in a region. The composition of strand communities varies considerably spatially, but in most given locations communitiescontain limited subsets of the species richness potentially available. The coastal vegetation is characterized by low growing and mat forming species in areas closest to the ocean and by taller plants farther inland or where available soil has accumulated
locally. In the arid and dry zones, a few annual species, mostly grasses, are seen; the viii rest of native coastal flora is perennial. Exposure to salt water and onshore flow of salt mist (‘ehukai) comprise the harshest ecological factors within the coastal zone. Exposure to ‘ehukai, strong winds, and brackish basal ground water all influence this generalization and add variegation to an area’s vegetation structure and composition. A range in other site conditions helps to further diversify the structural and species composition of the communities. Understanding of these conditions can assist managers with identification of areas to preserve and manage, and help to guide restoration attempts.
Alien plants represent one of the greatest threats to native coastal vegetation in that any one of several invasive species can completely displace or prevent the
colonization of entire suites of native species. Alien animals are another important threat, one that frequently opens the door for, or tips the balance to, alien plants. The most obvious and widespread animal species are pastured and free-roaming ungulates, particularly cattle, goats, sheep, pigs, and deer. The numbers and distributions of some
ungulate species may have surged and waned variably over time and space, but their impacts are unequivocally negative in the coastal zone in any abundance.
Human development and use of coastal areas continues to exert severe, usually permanent, impacts to remaining native coastal vegetation. Recreational activities, in particular, tend to be a widespread detriment to coastal plant communities. The fragmentation of habitats and compounding affects of an associated array of disturbances resulting from urban and agricultural activities have led to considerable
attrition of species from predominately native dominates areas in the coastal zone. These losses are continuing, perhaps accelerating, with the expanding use of the limited coastal areas throughout the islands for resort, residential, and recreational activities. Given the cumulative stresses that Hawai‘i’s coastal communities have experienced in recent years, the few plant extinctions that have occurred in that habitat
indicates there is still time to employ effective management to prevent more loss of diversity. However, the widespread damage to coastal vegetation and the rarity of so many species indicates the urgency for supplying sufficient targeted management to preserve species and to restore community composition, structure, and function. The coastal vegetation’s adaptation to natural disturbance, as evident from the harsh environment it occupies, coupled with its regenerative capability, may have helped these communities survive thus far. This suggests a good potential for preservation of the biota of these regions if effective and strategic management actions are effected soon. A conservation strategy that incorporates both protected regions and species augmentation may be able to reverse declining trends in Hawaiian coastal communities if applied in time and at sufficient scale. Protection of coastal regions should feature reduction or elimination of the major stress factors that accompany alien plants and ungulates, as well as reducing and compensating for the adverse consequences of land
use. A number of sites on each island stand out with high species diversity and/or populations of rare plant species, as well as still having an established connection with contiguous lowland vegetation. These sites can serve as core areas for a regional approach to managing strips of coastal communities and their associated lowland vegetation.
The following conclusions and potential management strategies have been derived from our survey observations:
A regional approach to coastal resource conservation is likely to be the most effective approach to secure an island’s native coastal vegetation.
It is important to identify and prioritize coastal vegetation areas that still retain connections to native lowland plant communities. Even depleted communities can still contribute to the coastal areas’ biodiversity, and both could be stabilized ix and augmented where warranted. Managing both coastal and lowland areas together can be an efficient strategy for conserving a variety of resources and processes across modestly-sized areas.
Removal or significant reductions of feral ungulates is one of the most pressing management needs along certain stretches of the coastlines of the main Hawaiian Islands. Strategic fencing can be an effective tool for excluding ungulates and potentially predators. However, high installation and maintenance costs limit their use presently, particularly near shorelines. Development of cost effective corrosion-resistant materials and appropriate designs could encourageincreased use of fences in coastal areas.
Given the ongoing spread of numerous alien plants into new regions, immediate removal of the early colonizing individuals of particularly threatening species from native coastal vegetation can proactively prevent an increase in ecosystem disruption.
Expand public education and outreach programs to enlist more support of coastal community conservation from the public.
These surveys have provided more and current information on the ecology, composition, distribution, and status of coastal plant communities and species in selected portions of the main Hawaiian Islands. Although not as rich in endemic species as are upland communities, the Hawaiian coastal flora is relatively diverse, and taken as a whole, is still quite intact with very few historically known species that are now extinct. Although the coastal zone has been heavily impacted over the past 250 years, many high quality examples of diverse plant communities can still be found, particularly in the
wet and mesic habitats on the islands of Maui and Moloka‘i. Management efforts that are regionally focused on reducing the impacts of invasive species (both plants and animals) and maintaining the connection between the coastal strand and lowland vegetation,
coupled with expanding public awareness of the value of coastal communities, can allow for effective restoration and maintenance of this unique set of ecosystems for the future.
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