Cryptogams build up a living microcosm: Geoecological effects of biocrusts on volcanic tephra (Haleakalā, Maui, Hawai’i)

Abstract

This study analyzes data of moss/lichen biocrusts on volcanic tephra at 2335 m in Haleakalā Crater (Maui, Hawaiʻi). Samples were compared among three adjacent positions: rolling crusts, pinnacled crusts, and bare tephra; two profiles contrasted substrate variation in crust and tephra areas. Field experiments assessed soil shear- and compressive-strength, infiltration rates, and diurnal soil temperatures; a lab experiment measured evaporation from crust specimens for 9 days. Biocrusts significantly affected physical, chemical, and structural soil properties. Fine particles, organic matter, and water storage were appreciably greater in biocrusts. Microsampling indicated upper-pinnacle sections accumulated more fine grains due to capture of volcanic ash, and also contained greater organic matter, than lower-pinnacle portions. Crust areas experienced infiltration rates ~2.8 times faster than tephra; mean diurnal temperatures were 12.1 °C (26.9 vs 39.0 °C) cooler in crusts than in tephra. Saturated crust specimens stored ~2.7 times more moisture than tephra soils; slower evaporation in biocrusts significantly lengthened the period over which poikilohydric cryptogams are able to maintain metabolic activity. Soil shear- and compressive-strength were higher on crusts than on tephra. Biocrust soils showed greater fertility, as available Ca2+, Mg2+, K+, P, NH4+-N, and cation-exchange capacity were higher than in tephra. NH4+-N was more concentrated under rolling crusts than in pinnacled ones; this is mainly ascribed to sampling methodology. Buried profile horizons suggest the site experienced additional tephra deposition during episodic volcanic eruptions. A developmental model assesses interactions among soil and environmental processes, and provides an integrated geoecological view of linkages that influenced biocrust genesis.