An Overview of the Status and Protection of Maui’s Watershed Forests
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2024-09
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Abstract
Today and likely into the future, pure freshwater resources are one of Maui’s greatest gifts and greatest needs, an essential element to existing ways of life, the island’s economy, and a requirement if we are ever to achieve food independence. Though often unrecognized in this role, native watersheds provide one of Maui’s most important natural resources, essential for life as we now know it.
The watershed forests of windward exposures of Maui that receive the bulk of the island’s precipitation are still functionally intact over great expanses, retaining their multi-layered, largely native tree and fern dominated forest systems. Because of this, despite the fact that Maui’s native watershed forests have been greatly modified and reduced, windward forests above 3,000 feet to tree line remain fundamentally intact and functionally vital for water extraction purposes.
Despite this relatively apparent state of richness currently found in these windward forests, there are factors in motion which appear to threaten the status of this critical natural capitol resource for future generations. Given the current status of Maui’s watersheds and their management, the best-case scenario for windward forests is to maintain a type of ecological stability as time progresses.
The unusual and near complete dominance of windward Hawaiian watershed forests by a single tree species, ‘ōhi‘a lehua (Metrosideros polymorpha), creates ecological vulnerability for rapid and dramatic changes to forest structure if the health of that dominant tree species is threatened.
Other factors that threaten this forest type are climate change and invasive species. Climate change predictions for Hawaiʻi forecast decreasing precipitation, increasing heat, and greater number and frequency of drought and storm events. Invasive species are non-native species of plant, animal, and pathogen deliberately or incidentally introduced to a new region. A recent dramatic example of pathogen impact on Hawaiian watershed forests was the recent emergence of two Metrosideros-specific pathogenic fungi, collectively known as Rapid ‘Ōhi‘a Death.
Decline of the dominant tree species of Hawaiian watersheds, ‘ōhi‘a lehua, would likely trigger successional changes involving the explosive spread of invasive plant species responding to an increase in light and resource availability. Potentially these changes could initiate a cascade of ecological processes that would challenge the long-term stability of watershed forests and water extraction and cause fundamental changes in the ways future generations conduct our lives and manage limited water supplies
The degradation and potential loss of native Hawaiian watershed forests is an excellent example of the ‘tragedy of the commons.’ Originally an economic theory, ‘tragedy of the commons’ describes how highly valued community resources can be degraded or even lost because, without direct ownership, no one entity appears to have singular responsibility for the resources’ fate. Studying the interaction of people and ecosystems, Elinor Ostrom, Stanford University, demonstrated that if appropriate systems are instituted, valuable community resources can, in contrast, be responsibly managed and utilized by stakeholders, as well as play a key role in protecting and perpetuating these resources. The community- and partner-based protection and restoration of Maui County’s watershed forests has the potential to be hailed as a classic example of ‘restoration of the commons’, where, with community awareness and support, public and private groups work together to cooperatively provide stewardship for an invaluable community asset.
On Maui, watershed forest degradation accelerated dramatically after European contact, approximately 250 years ago, with the introduction of non-native animals and plants. On leeward slopes, wildfires and feral cattle herds were the primary drivers for the near-complete loss of watershed forests on the islands of Maui, Molokaʻi, Lānaʻi, and Kahoʻolawe. From 1790 to 1850, native forests which formerly occurred from near sea level to tree line on leeward exposures were decimated and replaced largely by suites of African, Australian, and Central and South American plant species.
Upper elevation native windward ‘ōhi‘a lehua (Metrosideros polymorpha) forests, where most potable water gathering systems are based, receive the bulk of tradewind-associated precipitation in Maui County. Worldwide, native forests, with their diverse, multi-layered canopies and understories, are the most efficient natural systems in processing, filtering, and storing precipitation.
Methods to protect Hawaiian windward watershed forests were developed in the 1980s and have remained the primary standard practices for today’s watershed managers. Sadly, despite this, even priority watershed areas are progressively being degraded (especially up to moderately high elevations, ca. 4,500 feet (1,370 m) by landscape-level plant invasions. Conversion of native forests to non-native forests decreases overall aquifer recharge and has reduced ability to manage sediment and sheet flow during torrential rain events.
Rapid ‘Ōhi‘a Death (ROD) likely constitutes the worst invasive pathogen threat ever encountered by Hawaiian watershed forests. This assessment is partially regarding the importance of ‘ōhi‘a lehua making up 80% of the trees in our watershed forests, and partially on the impact of ROD, killing over an estimated one million ‘ōhi‘a lehua trees over a four-year period (2017-2021), largely on Hawaiʻi Island.
In addition to Rapid ‘Ōhi‘a Death, two other drivers, invasive species and climate change, also threaten to upset the status quo of native windward forests of Maui County and potentially reduce water quantity and quality.
Prior to the arrival of humans, the Hawaiian Islands received a new plant species about once every 30,000 years. Since humans first arrived, 20,000-30,000 non-native plant species have been introduced to the islands. Currently, a new non-native plant species arrives about every five days. Without rigorous quarantine measures, threats from new species continue to mount.
Despite the fact that they are among the most intact of remaining native Hawaiian ecosystems, windward ‘ōhi‘a lehua forests are vulnerable to large-scale displacement by certain notoriously invasive tropical plant species. Plant invasions at this scale cause drastic changes in species composition and reduced resistance to natural and climatic disturbances, as well as alterations to nutrient, carbon, and water cycles. With yet uncertain implications, climate change is an additional stressor of Hawaiian watershed forests, both independently as well as synergistically interacting with other stressors like diseases and invasive plants.
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Medeiros A., von Allmen, E. 2024. An Overview of the Status and Protection of Maui’s Watershed Forests. Technical Report #206. Pacific Cooperative Studies Unit, University of Hawaiʻi, Honolulu, Hawaiʻi. 93 pp.
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Maui County, Hawaiʻi
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