Controls on ecohydrologic properties in desert ecosystems: Differences in soil age and volcanic morphology

• We examine volcanic soil hydraulic properties with differing substrate morphology.
• We describe change in soil hydraulic properties with soil and landscape development.
• Volcanic soil morphology greatly influences soil water retention and transport.
• Soil development differs between volcanic substrate types and is unique to drylands.
• The physical controls on water availability are likely to dictate plant succession.

Research examining the influence of parent material on soil and landscape development is limited in dryland environments. In volcanic soils, most studies examining soil development and succession have been conducted in humid environments, where weathering occurs rapidly. This study was conducted in a semi-arid environment to examine the influence of volcanic parent material morphology on soil development and ecohydrologic properties that may regulate primary succession. Four study sites were selected across young (~ 2.0 ka) and old (~ 12.1 ka) end members of two different parent materials (cinder cones and pahoehoe lava flows). We hypothesized that soils and associated hydraulic properties would evolve more rapidly on the cinder cones compared to the pahoehoe flows owing to greater surface area on the cinder. For each site, we quantified plant cover, soil organic matter, bulk density, texture, soil water retention, and hydraulic conductivity. The old cinder cone soils accumulated approximately 9% soil organic matter and increased in silt plus clay fraction by 23% relative to the young soil. This increased water retention and decreased hydraulic conductivity. Similarly, plant cover increased by 47%. The old pahoehoe site remained dominated by unweathered basalt, where silty eolian material accumulated preferentially in crevices. Soil organic matter and silt plus clay fraction increased by 18% and 49% respectively in old pahoehoe soils, resulting in increased water retention but little change in hydraulic conductivity. Plants were restricted to crevices and cover increased by only 34%. We observed the cinder cone sites had extensive soil and plant community development from the young to old sites. The pahoehoe soils and plant communities established earlier than on cinder cones, but showed only modest functional changes between young and old sites. The combined characterization of soil development, hydraulic properties, and plant community dynamics highlights potential ecohydrologic mechanisms controlling successional processes in young semi-arid landscapes.