Dryland rock detention structures increase herbaceous vegetation cover and stabilize shrub cover over 10 years but do not directly affect soil fertility

Abstract

Rock detention structures (RDS) such as check dams, gabions, and one rock dams are commonly used to mitigate erosion impacts in dryland ephemeral stream channels. RDS increase local water infiltration and floodplain connectivity, reduce sediment transport, and enhance vegetation growth and establishment. In addition to increasing overall vegetation cover, RDS may also buffer against a cycle of vegetation growth and collapse during years of extremely variable precipitation, helping to maintain stable cover. Although widely employed by land managers, success as reported in scientific literature varies, especially with regard to RDS effects on vegetation and soil fertility. We present the results of a 10-year field experiment in southeastern Arizona, USA, designed in collaboration with local land practitioners to measure local in-channel effects of RDS. Over 10 years, cover of herbaceous vegetation (forbs and grasses) doubled from 11 % to 22 % in channels treated with RDS, but did not significantly increase in untreated control channels. Shrub cover in treated channels was significantly less variable than in control channels over time. We analyzed the complex relationships between RDS, vegetation cover, and soil fertility using structural equation modeling (SEM), which represented conditions of the tenth year alone. SEM revealed that RDS did not directly affect soil fertility, as measured by total soil nitrogen, total soil carbon, soil organic matter, microbial richness, and potential nutrient cycling capacity. Notably, SEM did not yield the same trends as temporal monitoring, possibly because our structural equation models could not capture change over time. This discrepancy highlights the need for long-term, frequent monitoring of aboveground and belowground conditions to evaluate treatment success on a management scale. Overall, installing rock detention structures in ephemeral channels in arid and semiarid regions is a low-cost, feasible way to increase channel sediment aggradation, forb, and grass cover; stabilize shrub cover; and combat dryland degradation.

Keywords: Check dam; Ephemeral stream; Erosion control; Gabion; Rangeland management.