By Dr. Kevin D. Barrett, Botany Department ‘19
The tires on our Jeep barely grip the twin strips of overgrown asphalt. I catch myself asking the battered skid plates underneath to forgive my fondness for potholes. But there is hardly any room for evasion. On our left is an impenetrable green wall of trees, ferns, tree-ferns, and moss. To the right, a truncated grass verge reminds us of the shear 2,000 ft. plunge to the valley below. Rising fog obscures the ledge—and my anxiety. Several yards ahead, the beaten and soggy service road disappears into a cloud. We return to the mist-laden forest crowning Kohala Mountain on the island of Hawaiʻi.
For over two decades, UW-Madison Botany Professor Sara Hotchkiss and her graduate students have been studying the ecosystems that lie within the cloud-covered heights of the Hawaiian Islands. These “cloud forests” contain some of Hawaiʻi’s least disturbed biological communities and provide habitat for countless endemic species. They are also essential to the production and storage of freshwater on islands surrounded by thousands of miles of ocean.
The clouds result from the predominant feature of Hawaiian climate. On Kohala Mountain, as on other Hawaiian peaks, moist ocean air drawn inland from northeasterly trade winds ascends the mountain’s windward flank. The air cools as it rises, causing water vapor in the air to condense and form clouds. Cooled clouds release moisture as rain and fog along the ascent before depleting on the drier leeward side. Kohala’s hydrology is striking because the mountain’s dense foliage intercepts the passing fog which then enters the water cycle as “fog drip” substantially increasing local precipitation sums. And in turn, the moisture subsidy from fog drip supplies the growth of Kohala’s lush plant communities.
On this visit to Kohala forest, Professor Hotchkiss and I are in search of an alluring ecosystem type that mottles the forest’s wettest zones: the montane bog. The bogs support unique assemblages of plants adapted to flooded soil and low oxygen, such as ericaceous shrubs, sedges, and large swathes of Sphagnum moss. Anoxic conditions slow the decay rate of plant matter resulting in thick deposits of peat that have accumulated beneath surface vegetation. These deposits preserve archives of historical plant and animal life and, therefore, a record of environmental variability that has left an imprint on Kohala’s biosphere.
Our interest is the history of Kohala’s hydrological variability. Water is, and has been, the principal resource motivating management of Kohala forest for at least a century. Between 1910-1950, extensive agricultural ditches and aqueducts were built to transport water from the wet summit to neighboring sugar cane plantations, who also kept meticulous rainfall records. To uncover earlier hydrological patterns, we can look for clues preserved in Kohala’s bogs.
Two biological clues that have been useful in the study of Kohala’s hydrology are microscopic members of the testate amoebae and Cladocera. Testate amoebae are moisture-sensitive protists that produce their own shell. Cladocera are freshwater zooplankton sensitive to water depth and nutrients—and the central figures of Edward Asahel Birge’s doctoral thesis on Lake Mendota. Testate amoebae and Cladocera can be utilized as indicator species: organisms whose presence, absence, or abundance reflect an environmental condition, in this instance the surface wetness of Kohala’s bogs. We made detailed surveys of testate amoeba and cladoceran community composition in the bogs and analyzed empirical relationships between community composition and bog wetness. And because the shells of testate amoebae and the chitinous exoskeletons of Cladocera preserve well in these acidic and anaerobic environments, we can use those relationships to infer past changes in relative bog wetness from the composition of subfossil communities in sediment cores.
We are beginning to find that during the early 19th century in Kohala, when irrigation canals were being constructed, bogs had some of their wettest conditions of the past few hundred years. The implication is that the bogs have been drying for the better part of the last century. And other water resources in the region continue to thin. Stream flows and rainfall in Kohala are both reduced in recent decades. And contemporary threats, such as development, municipal water demand, and biological disturbance compound the challenges for a sustainable watershed.
Fortunately, Kohala’s native bogs and forested watershed enjoy elevated protection from Hawaiʻi’s Natural Area Reserve System (NAR). The NAR has identified that the primary environmental threat to Kohala’s watershed is damage from feral pig populations. Non-native pigs devour and trample fragile forest components causing disturbance to ground cover, creating openings for non-native species, and degrading the quality of the forest and the local hydrological cycle.
My colleague, PhD candidate Michael Peyton, along with NAR partners, is quantifying the densities of pig populations in Kohala forest and their influence on plant communities. Michael is using an array of trail cameras distributed broadly around Kohala forest to understand the movement and behavior of pigs in different habitats. Quantifying native and non-native plant functional traits shows Michael how certain plants, such as non-native grasses and herbs, excel in the presence of pig activity, while others, such as slow-growing native understory trees and shrubs, wane. Michael is also isolating fragments of ancient pig DNA preserved in Kohala’s many bog sediments to piece together the history of pig activity in the forest.
The idiosyncrasies of Kohala forest and its denizens dance around my mind on the drive back down the mountain road, our crew wrapped in soaked clothes we pretended were “waterproof”—veterans wear a trash bag. The descent escapes the fog and opens our view of the coastline and ocean. Dizzying ecological questions become muted. It is impossible not to appreciate the watershed-thinking that permeates Hawaiian cultural memory. Ancient land divisions honor the interdependence of subsistence living in lowlands with the care of forests in uplands through wedge-shaped contours that follow natural watershed boundaries. I think of our Jeep tracing the flow of water from the forested summit, wao akua, realm of the spirits concealed in clouds, down through streams and springs to the fields of human industry below. I should really pay better attention to the road. More potholes.