Moisture availability and ecological restoration limit fine fuels and modelled wildfire intensity following non-native ungulate removal in Hawaii.
Abstract
The removal and exclusion of non-native ungulates for conservation of biodiversity is common globally, including on tropical Pacific Islands. However, the poorly quantified effects of ungulate exclusion on fuels and wildfire may represent an important conservation trade-off. We measured fuels (live and dead fuel loads, type, height and continuity) and modelled potential wildfire behaviour (flame height and rate of spread) inside and outside of 13 ungulate exclosures, three of which received active ecological restoration (e.g. planting of native shrubs and trees), across a 2,740 mm mean annual rainfall (MAR) gradient on the Island of Hawaii. Differences in fuel characteristics and modelled wildfire behaviour inside versus outside of ungulate exclosures were assessed using linear mixed effects analyses. Non-native ungulate removal, in general, increased fine fuel loading (average change ranged from -0.7 to 11.3 Mg/ha), shrub fuel loading (-0.1 to 5.6 Mg/ha) and modelled flame lengths (-0.2 to 1.9 m). Post-removal fine fuel loading and modelled flame lengths increased linearly and positively with moisture availability, by as much as 56% in the wettest sites. Dead fine fuels, a key driver of fire intensity and combustion rates, also increased with ungulate removal. Sites undergoing active ecological restoration exhibited reductions in fuel loading, with greater reductions over time since ungulate removal; by the 10th year following ungulate removal, fine fuel loading was reduced at restoration sites by 41%. Synthesis and applications. Dry and mesic environments where wildfire occurrence is more frequent are of particular concern following non-native ungulate removal, but with drought even typically wet environments can be at high risk of wildfire following ungulate removal. Given limited resources, managers in mesic and wet forests might focus first non-native ungulates in non-native grass-dominated areas that contribute disproportionately to wildfire spread and are adjacent to fenced higher value areas, while managers in dry to mesic areas might focus on removing non-native ungulates. Where possible, land managers in all situations should invest in long-term fuel management strategies such as active ecological restoration to reduce fuel loads and the occurrence and intensity of wildfires.