Abstract

Fires are becoming larger and more frequent in the deserts of North America, driven largely by the expansion of invasive annual grasses. Rodent consumers can curb invasive grass expansion through plant herbivory, but the top-down effects of this pressure on soil resources are understudied. In shrub dominated deserts, islands of fertility under shrub canopies are surrounded by nutrient poor interspace soils that dictate patterns of vegetative growth. Little is known of how top-down controls from rodents and fire may interact with fertile island topography. These studies explore the effects of fire, rodent activity, and fertile island topography on plant available soil resource distribution in the sagebrush steppe and the Mojave Desert of the North American West. The study design consisted of two long-term experimental sites installed in 2011, one in the Mojave Desert and one in a sagebrush steppe site in the Great Basin (Utah, USA) with plots randomly assigned to be burned or unburned and rodents allowed or excluded. In the Fall of 2021 and 2023 (sagebrush steppe site) and Spring of 2019 (Mojave Desert site) soil samples were collected to a depth of 5 cm in each plot. Studies at the sagebrush steppe site explored the effects of fire and rodents (Chapter 1) and fire and fertile islands (Chapter 2) on soil nitrate, ammonium, phosphorus, potassium, organic matter, pH, and salinity. Fire had a strong positive effect on soil nitrate and phosphorus in both years, in addition to salinity in 2021 and potassium in 2023. Rodents had a positive effect on soil nitrate and potassium and lowered pH. Rodents also strongly raised nitrate and phosphorus and lowered pH under burned conditions, resulting in significant interaction terms. Fertile islands contained elevated nitrate, potassium, organic matter, pH, and salinity over interspace soils. Fire homogenized the distribution of soil phosphorus, potassium, and salinity between fertile islands and interspaces by increasing interspace soils to fertile island concentrations. Mojave Desert soils were analyzed for soil moisture, total mineral nitrogen, nitrate, and ammonium and the effects of fertile islands, fire, and rodents explored (Chapter 3). Shrub fertile islands increased soil nitrate 96% (P < 0.001) and total mineral nitrogen 28% (P = 0.05) while lowering volumetric water content 52% (P < 0.001) over interspace soils. Fire decreased fertile island nitrate 25% eight years post-fire (P = 0.09). Rodents did not have a significant effect on measured soil resources (P >0.1). While trends differed between sites, in the sagebrush steppe soil resources are concentrated beneath shrub canopies, but fire increases interspace concentrations to these levels over a decade post-burn, with implication for vegetative expansion. Rodents may also increase resources, particularly after fire. In the Mojave Desert, shrubs strongly concentrate nitrate and reduce moisture beneath their canopies, but nitrate is reduced after fire with implication for long-term nutrient dynamics in this system. These findings suggest that fertile islands, fire, and rodents play key roles in the distribution of nutrients and properties in these systems, but that there may be spatiotemporal differences in how and when they are expressed.

Degree

MS

College and Department

Life Sciences; Plant and Wildlife Sciences

Rights

https://lib.byu.edu/about/copyright/

Date Submitted

2024-04-23

Document Type

Thesis

Handle

http://hdl.lib.byu.edu/1877/etd13619

Keywords

fertile island, trophic cascade, biological invasion, Great Basin, Mojave Desert

Language

english

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