Impacts of Wildfire on Hawaii Island’s Pre-Contact Landscape

Abstract:

The arid western slopes of Hawaii’s Mauna Kea volcano record pre-historic landscape changes that accompanied occupation by Native Hawaiians. Stratigraphy in the Keamuku area shows that a long (c. 38-42 ka) period of airfall and eolian deposition and pedogenesis was terminated by at least 8 layers of charcoal-rich sediment, interbedded with sand and shells of land snails that were transported by running water. Streams across Keamuku then cut several meters down through these deposits to bedrock of the Hamakua Volcanics. Radiocarbon ages indicate that charcoal-rich layers were deposited from the 13^th-14^th Century A.D. (e.g., 1207 +/- 48; 1287 +/- 128 A.D.) through the 17th - 18th century (e.g., 1648 +/- 157; 1651 +/- 160 A.D.; mean probability +/- 2 sigma. Stream incision commenced sometime thereafter. We measured saturated hydraulic conductivities (K_sat) with a mean of 56 mm/hour in nearby soils with tree and shrub cover. This value exceeds common hourly rainfall intensities, so runoff from these landscapes is unlikely without disturbance. Work at a wildfire boundary in Molokai, Hawaii, shows that just after fire disturbance, saturated hydraulic conductivities of similar Hawaiian soils are one half to one fifth of unburned equivalents.

One interpretation is that during the 13^th-19th centuries and later, humans burned shrub- or tree-covered landscapes reducing soil infiltration capacities. Over the course of several hundred years of burning, one or more large storms with sustained hourly rainfall intensities exceeding the infiltration capacity of the altered land surface occurred soon enough after fires to generate runoff in a place that had not previously experienced it. This runoff carved the existing gully network across over 100 km² of Keamuku area, as wildfires pushed the shrub-/tree-line upslope. This interpretation joins a growing body of thought that pre-historic human’s use of fire fundamentally altered landscapes.