Award Date


Degree Type


Degree Name

Doctor of Philosophy in Geoscience



First Committee Member

Brenda Buck, Chair

Second Committee Member

Margaret Rees

Third Committee Member

Stephen Rowland

Fourth Committee Member

Deborah Soukup

Graduate Faculty Representative

Lloyd Stark

Number of Pages



Biological soil crusts (BSCs) are complex matrices of soil particles, mosses, lichens, and cyanobacteria that prevent erosion and influence water and energy balances, soil fertility, and vascular plant germination. The processes that form BSCs, the factors that control their distribution, and the ecosystem feedbacks that they sustain are poorly understood. This dissertation employed a novel interdisciplinary approach to address those research unknowns through investigations of the micromorphological structure, soil-geomorphic relationships, and biogeochemical feedbacks of BSCs in the Mojave Desert.

A micromorphological study of BSCs resulted in a succession model that illustrates how crust formative processes and structures change through time. Tall moss-lichen pinnacled crusts actively capture dust, precipitate authigenic minerals, experience alternating expansion-contraction from wet-dry cycles, and form a surface seal that together with dust capture produces Av horizons. The resulting unique bio-sedimentary structures control ecological function and further promote BSC growth. This is the first study to demonstrate a biological process leading to the formation of Av horizons and suggests that BSCs are previously under-recognized critical agents of arid pedogenesis and landscape development.

From a soil-geomorphic study of BSCs a model was developed wherein the ratio of fine sand to rocks controls the relative distribution of three surface cover types - cyanobacteria crusts, moss-lichen crusts, and desert pavements with low to moderate moss-lichen cover. The biological and geological feedbacks that sustain these cover types vary predictably across intermontane basins, yielding new insights for land management. Moreover, the physical processes that control BSCs are common to most deserts, making these results applicable worldwide.

An ecological study of BSCs resulted in a conceptual model wherein the sand-to rock-ratio, which constrains interspace cover by BSCs and desert pavements, ultimately determines the magnitude of the fertile island effect. Inferred biological-geological feedbacks produce three unique biogeochemical patterns that vary predictably across the landscape. These surface cover patterns are consistent within many deserts, potentially reflecting overarching controls of resource allocation that operate despite differences in total site productivity.


Biogeochemistry; Biological soil crust; Cryptobiotic soil; Desert pavement; Desert soils; Fertile island; Geomorphology; Soil crusting; Soil formation; Soil microbial ecology; United States – Mojave Desert; Vesicular horizon


Desert Ecology | Geomorphology | Soil Science | Terrestrial and Aquatic Ecology

File Format


Degree Grantor

University of Nevada, Las Vegas




2011 Outstanding Dissertation Award from the Graduate Council Student Awards Committee

AWilliams_SupplimentaryData_2011.pdf (499 kB)
Supplimentary Data Spreadsheet

AWilliams_GuideToData_2011.pdf (23 kB)
Guide to Supplementary Data

AWilliams_Maps_2011.pdf (134674 kB)
Supplementary Maps


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