Thermal Influence of Volcanic Tephra on Maximum Soil Temperatures: Geoecological Implications for Germination and Distribution of Argyroxiphium sandwicense (Haleakalā, Maui, Hawai’i)

Mid-day maximum soil temperatures were measured at 10 study plots during different hot summer days in Haleakalā Crater, Maui, with thermocouple thermometers on five adjacent microsite types: bare surface soils, soils under black tephra, soils under reddish tephra, soils shaded by silverswords, and soils under plant litter. The main tephra morphologies and geomorphic environments, as well as their geoecological association with silversword rosettes (Argyroxiphium sandwicense), were also assessed; silversword density was substantially greater on reddish tephra-covered areas than under black tephra fragments. Silversword seeds are extremely sensitive to high temperatures and fail to germinate after a short exposure to soil temperatures ≥35 °C. Thermal data sets were statistically compared with parallel box plots; the ability of various microsites to provide safe sites for silversword growth was also assessed. Bare soils and black tephra reached the highest median temperatures, up to 48.7 °C and 40.3 °C, respectively; reddish tephra remained much cooler, with all median temperatures ≤30.8 °C. Rosette-shaded soils and soils under silversword litter were the coolest, with temperatures below 18.7 °C and 18.5 °C, respectively. Temperatures in all microsites, except those under black tephra, were significantly lower (p < 0.0001) than on contiguous bare ground. It was concluded that reddish tephra provides the ideal conditions for silversword regeneration.