Abstract

Over the last millennium, since human arrival, Hawaiian dryland ecosystems have been increasingly degraded and fragmented through habitat loss, invasive species, resource exploitation, and fire. Conventional interpretations hold that fire was rare or absent in these systems prior to human settlement. To evaluate this assumption, we reconstructed the disturbance and vegetation history of Kipuka Kalawamauna, a 14,000-year-old substrate on the leeward flank of Mauna Loa (<1,000 mm annual precipitation), where vegetation today is dominated by the native grass Eragrostis atropioidesand the native shrub Dodonaea viscosa. Using LiDAR-derived topographic mapping, we identified a stable soil accumulation zone and excavated a 1.55 m sediment profile. Stratigraphy and radiocarbon dating of microcharcoal and macroscopic charcoal fragments revealed a continuous fire record spanning <200 to 13,480 calibrated years before present (cal yr BP). Charcoal was present in every horizon, demonstrating that fire has occurred in this landscape for more than 13 millennia, albeit at low frequency prior to human arrival. Taxonomic identifications from plant microfossils, including preserved macroscopic charcoal fragments (>250 μm), indicated the past presence of native woody taxa no longer found in Kipuka Kalawamauna, such as Acacia, Cordia, Nothocestrum, Osteomeles, and Leptecophylla. D. viscosa, now abundant, was absent from the identifiable prehistoric assemblage. These identifications, together with the stratigraphic context, demonstrate that the plant community occupying this site in the past was compositionally different from the one present today. Stable carbon isotope (δ¹³C) analyses of soil organic matter indicated varying relative contributions of C ₃ and C ₄ plants throughout the Holocene, with elevated C ₄ inputs in mid-Holocene horizons. Grass cuticle counts confirmed a persistent presence of grasses over the last several thousand years, with higher abundances in more recent horizons. The combined charcoal and plant microfossil evidence documents a long-term history of fire in this subalpine dryland and shows that its vegetation composition has changed considerably over the late Quaternary. While the profile shows an increase in charcoal abundance in the upper horizons consistent with human-era burning, fires were already a persistent—though infrequent—component of the system for thousands of years before human arrival. These results provide a rare, stratified record of fire and vegetation change from a Hawaiian dryland and offer a critical baseline for interpreting contemporary species distributions, understanding the role of disturbance in shaping dryland plant communities, and informing management strategies aimed at conserving and restoring these increasingly rare ecosystems.