Unraveling an uncertain future for the Great Basin: how climate, grass invasion, and wildfire interact to drive ecosystem function

Drylands are the earth's largest terrestrial biome and are rapidly experiencing change. In drylands of the western United States, three of the most influential global change processes are shifting precipitation regimes, annual grass invasion, and wildfire. Understanding how these drivers interact to influence ecosystem function, and how such interactions may change in the future, can provide critical insight for building ecological resilience. This dissertation explores how changing moisture regimes interact with grass invasion and wildfire to modify three key cycles in the Great Basin: carbon (C), nitrogen (N), and water. This work bridges multiple spatial and temporal scales using empirical studies conducted in the field, greenhouse, and laboratory, along with simulation modeling. The work in chapter one used a greenhouse and laboratory experiment to identify how soil aridity influences root exudation by cheatgrass (Bromus tectorum), an invasive annual grass, alters the cycling of carbon (C) and nitrogen (N) in the soil. We observed that cheatgrass was able to uniquely access N in dry soils, and that changes in soil condition that implicate increased C and N cycling were caused when cheatgrass root exudates were experimentally added to soils. This work shows that root exudation by cheatgrass accelerates soil C cycling and suggests that it modifies N cycling and provides cheatgrass with unique access to N under dry conditions. When combined with seasonal dynamics and annual grass phenology, these mechanisms may provide cheatgrass with greater access to soil N compared to native species. In chapter two, I used simulation modeling to mechanistically test the effects of changing precipitation regimes and grass invasion on water and N dynamics at larger, watershed scales. Using the Regional Hydro-Ecologic Simulation System (RHESSys) - a process-based ecohydrologic model that couples the cycling of water, C, and N-I examined how precipitation variability and invasion influence stream discharge and N export. I simulated four climate scenarios with three levels of cheatgrass invasion in a factorial design. Streamflow decreased with both climate change and grass invasion, both of which increased rates of evapotranspiration. Watershed N export increased with climate change and invasion. This work can help us understand how change and annual grass invasion in the Great Basin may interact to influence ecosystem water supply and water quality. Chapter three investigated the role of aridity and burn severity on N retention after wildfire using a field and laboratory approach. I quantified how burn severity and aridity influence some of the key factors driving N uptake by plants and soil microbes after fire. Plant N uptake exhibited a non-monotonic relationship with burn severity, where sites that burned severely had suppressed N uptake for up to two years following fire. Dry conditions muted plant and microbial N uptake for all burn severities. N uptake was lowest after severe fire and when soils were relatively dry. Soil N that exceeded plant and microbial demand when N uptake is suppressed is likely more vulnerable to export. Using a conceptual framework, I hypothesize that postfire N loss is likely to occur at a lower burn severity when conditions are dry after fire. This hypothesis expands existing N saturation theory to better capture the role of fire and its interactions with soil aridity in drylands.

Name thesis:degree_name

Doctor of Philosophy

Level thesis:degree_level

Doctorate Degree

Year dc:date.issued

2024

Author dc:creator

• Strain, Maxwell Kay

Advisor dc:contributor.advisor

• Hanan, Erin

Committee members dc:contributor.committeemember

• Blaszczak, Joanna
• Leger, Elizabeth
• Gustin, Mae
• Verburg, Paul

• Dryland Ecology
• Ecosystem Ecology
• Fire Ecology
• Global Change
• Invasion Ecology
• Plant-Soil Interactions

Language dc:language.iso

en_US, English

Repository record dc:identifier.uri

https://scholarwolf.unr.edu/handle/11714/11166

OAI identifier oai:identifier

oai:scholarwolf.unr.edu:11714/11166