title: Linking Extreme High Air Temperature Events to Wildfire and Environmental Variation to Water Balance Partitioning in the Western US
Creators: name: Savage, Neil 
Publisher: name: University of Nevada, Las Vegas
Year: 2021

description: Chapter 1

Wildfire activity has increased across the western United States in recent decades, causing significant damage to ecosystem services and human communities. The Sierra Nevada region, in particular, has experienced substantial increases in the ignition frequency, severity, and extent of large wildfires. To partly disentangle the complex processes underlying wildfire risk, I developed a simple approach to link natural wildfire ignition patterns to changing seasonal temperatures and extreme high and low air temperature events across the Sierra Nevada region from 1992—2015. Extreme event analyses focused on the association between the magnitude and frequency of occurrence of extreme temperature events and wildfire ignition in spring, summer, and fall wildfire seasons, with a particular focus on contrasting these patterns between fire and non-fire locations. Temperatures increased over the study period, including those of extreme high and low temperature events. Generally, fire locations were more likely to experience extreme high temperature events and less likely to experience extreme low temperature events. During summer and fall, fire locations also experienced relatively hotter extreme high temperature events. Fire ignition probability was most strongly associated with the frequency of extreme high air temperature event occurrence in seasons preceding and including the spring, summer, and fall fire seasons. As the number of extreme high temperature events increased over these seasons, fire ignition probabilities increased substantially (maximum ignition increases of +1346% in spring, +265% in summer, and +148% in fall) and were consistent across the Sierra Nevada region. Thus, the sustained occurrence of extreme high air temperature events indicates increased wildfire risk, and ignition in summer and fall fire seasons may be further enhanced by hotter extreme events. Relationships between high temperatures and natural wildfire ignition can help to identify locations of increased wildfire risk, and to some degree minimize uncertainty associated with the multiple factors that shape wildfire characteristics. This insight can guide management actions to reduce wildfire risk across the Sierra Nevada region and in other similar ecoregions.

Chapter 2

Piñon pine-juniper woodlands are strongly influenced by variation in the ecosystem water balance and the linkage between soil moisture and tree transpiration. Water balance differs across fine-scale climate, landscape, and tree stand heterogeneity, and these differences are challenging to resolve mechanistically. I used a low-dimensional water balance simulation model to quantify variation in the water balance across heterogeneous piñon pine-juniper sites in southern Nevada. My simulations focused on 2 contrasting higher and lower precipitation mountain ranges in southern Nevada, and resolving the interactive effects of 3 conditions across these areas: elevation (2000, 2200, and 2400 m), unsheltered microsites with low and higher canopy cover, and south, neutral, and north facing aspects. Across all sites and simulations, variation in ecosystem water balance was imparted by increasing precipitation with site elevation, as well as lower partitioning of water into the soil profile at sites with higher tree stand density and greater interception. This supports the potential for strong regional similarity in the water-driven mechanisms influencing piñon pine-juniper woodlands. Notably, tree transpiration magnitude as well as transpiration as a function of incoming rainfall declined above tree densities of ∼16—20 m2 ha-1, illustrating the role of canopy and litter interception on ecosystem water balance. This may help to explain why past drought impacts to piñon pine-juniper ecosystems have not been limited to lower elevational ranges. My study elucidates how variation in climate, landscape, and tree stand characteristics interact to shape ecologically meaningful variation in piñon pine-juniper water balance and tree functioning.