History
Land-based sources of pollution are a growing threat to coastal ecosystem integrity across the Hawaiian Islands. Stresses on upstream watershed ecosystems from land-use change, management actions, and natural disasters contribute to the degradation of Hawaiian coral reefs. Addressing these diffuse sources of disturbance in the landscape requires a watershed-based perspective of the land.
Ninety-two percent of the lands comprising the watershed of Pelekane Bay were once covered with diverse mesic (moist) forest containing trees that are now rare or threatened: sandalwood (Santalum ellipticum), koai‘a (Acacia koaia), naio (Myoporum sandwicense), Hawaiian olive (Nestigis sandwicensis) and wiliwili
(Erythrina sandwicensis).
The remaining mature native vegetation in the gulches, where cattle cannot reach, are the last intact ecosystems from which we can learn the structure and function of the forest that formerly covered the leeward slopes of Kohala Mountain. These forest buffers slow the overland flow of runoff and store it for extended release. Riparian vegetation also traps sediment and utilizes excess nutrients in runoff. Restoration of riparian forest and scrubland in the Pelekane Bay Watershed will significantly reduce sediment and nutrient loads from flowing into the bay.
Hawaiian tropical dry forests, now one of the most critically endangered ecosystems in the world, were once present on the lower sections of the watershed. Clearing and burning of lowland dry forests began with the arrival of Polynesians, and the last remnants are being destroyed today through development, expansion of agriculture and pasture, and wildfires. There are now large areas with no vegetation or, in some places, only sparse grass and scattered exotic kiawe (mesquite) trees (Prosopis pallida). These trees are deep-rooted and will desiccate an area by drawing down groundwater.
A few lone native trees dot the landscape, and a few native shrubs survive in small patches across the slopes. Two exotic grasses cover the ground here: buffel grass (Cenchrus ciliari), an excellent forage for cattle, and fountain grass (Pennisetum clandestinum), a fire-adapted, unpalatable bunch grass. In their native state, these areas rarely burned, but in recent decades a growing human population has created a greater ignition potential, and the invasion of fountain grass has increased the intensity and extent of wildfires. Fire favors non-native species that are adapted to a frequent fire cycle, and post-fire, these species regenerate to dominate the landscape. Cattle do not graze these grasses, so their biomass increases at an uncontrolled rate, which in turn increases the amount of fuel available to feed a fire. Fires threaten homes, farms, and businesses, and they dramatically alter the landscape.
Once the landscape has been burned, rendering it devoid of trees, shrubs, and ground cover, there is nothing to hold the soil in place during storm events. The heat of a fire also changes the physical characteristics of soil particles, making them hydrophobic, or unable to absorb water. As a result, sudden storms create flash flood conditions with huge amounts of water flowing over the land surface. Rather than being absorbed into the soils, enormous amounts of sediment are instead carried downstream and deposited in Pelekane Bay. With the topsoil washed away, what remains is bare hardpan, depleted of organic nutrients and without a seed bank to allow regeneration of existing species.
Sudden, severe storm events also create head-cutting gullies where large blocks of soil crumble and are washed downstream. These gullies erode rapidly by gouging and widening dramatically with every new storm.
There are no native land mammals in Hawai‘i, so wildlife in the Pelekane Bay Watershed consists of feral animals that mostly roam uncontrolled across historic sites, residential areas, and ranches. In the dry lower sections of the watershed, thousands of goats browse freely on vegetation in pastures, yards, and gardens, trampling the soil and creating dusty conditions that are inhospitable for plant germination and growth.
Continued erosion and sediment deposition in Pelekane Bay has led to chronically impaired nearshore marine waters. Hawai‘i’s Unified Watershed Assessment identified Pelekane Bay Watershed as a Category 1: in most urgent need of restoration. Pelekane Bay is also on the Department of Health’s 2004 Section 303(d) list of impaired waters.
With the support of NOAA’s Coastal Restoration grant, KWP is translating the initiatives laid out in its management plan into positive actions on the ground to restore the Pelekane Bay Watershed.
