Alvarado-Celestino, ErnestoCronan, James Breswter2026-04-202026-04-202026-04-202025Cronan_washington_0250E_28983.pdfhttps://hdl.handle.net/1773/55508Thesis (Ph.D.)--University of Washington, 2025Conservation lands within the range of longleaf pine (Pinus palustris) are valuable because they protect remnants of an ecosystem with globally high measures of biological diversity. The longleaf pine ecosystem is adapted to frequent, low-intensity fire, however, naturally occurring fire no longer burns with sufficient frequency to maintain the ecological structure and function necessary to sustain high levels of biological diversity. To compensate, aggressive prescribed fire programs are necessary. The fragmented nature of this ecosystem combined with the difficulty, complexity, and expense of applying regular prescribed fire in a densely populated landscape raises numerous management challenges. The research presented in this dissertation addresses many of the knowledge gaps confronting land managers using prescribed fire in longleaf pine ecosystems. The focus here is to provide information that describes how changes in the fire regime affect understory vegetation from the perspective of both fire hazard and community ecology: the two main management objectives for prescribed burning in longleaf pine forests. Research addressing these objectives is presented in three chapters. First, in Chapter 3 I examined the effects of season-of-burn on shrub and herbaceous fuel layers and predicted fire behavior on frequently burned mesic pine flatwoods in northern Florida. Compared to single dormant season burns, single growing season burns caused no changes to live understory fuels and had no detectable effect on fire behavior. Shrub coverage and predicted fire behavior were, however, significantly different between two geographic regions sampled. Additionally, shrub height was significantly affected by fire temperature. Predicted fire behavior was strongly correlated with measures of the litter and herb strata. Results from this study suggest that land managers should not initially expect large changes in understory fuel properties or potential fire behavior related to a shift from dormant to growing season prescribed burning, and that geographic location and fire intensity can have significant effects on live fuels and potential fire behavior. Second, in Chapter 4 I evaluated the effects of frequent prescribed fire and forest structure on understory composition in mesic longleaf pine flatwoods. To do this, understory biomass and forest structure were measured in 22 stands of longleaf pine that were burned regularly for at least 20 years. Constrained multivariate ordination and boundary line regression were used to test whether explanatory variables (i.e., fire history and forest structure) affected understory biomass, grouped by genera and plant functional groups. Fire frequency, as measured by the mean fire interval (MFI) and coarse woody debris biomass had marginally significant effects on understory composition. These variables, though uncorrelated, weakly aligned with the first Principal Components axis where sites were arranged along a compositional gradient from typical understory shrubs and wiregrass (Aristida beyrichiana) to sites characterized by vines and understory hardwoods. Boundary line regression showed a significant negative relationship between forb abundance and MFI; however, this relationship did not extend to graminoids. Boundary line regression did not show any significant effects of MFI on measures of woody plant biomass. Season-of-burn had no significant effects on understory plants in the constrained ordination or boundary line regression. These results suggest that other environmental factors and fire regime characteristics that are more favorable to fire sensitive species would be necessary to significantly influence understory vegetation dynamics. Third, in Chapter 5 I used the Fuelbed Dynamics Model (FDM), a landscape-succession model I developed that incorporates fire and other disturbances, to simulate changes in fuels at Eglin Air Force Base (Eglin) in northwestern Florida for 50-year time periods under four scenarios of prescribed burning. The simulation outputs quantified the effect of different levels of prescribed burning on fire hazard across the longleaf pine-dominated landscape at Eglin where prescribed fire is the primary method used to manage vegetation and mitigate fire hazard. If the rate of prescribed burning is insufficient to maintain a low fire hazard landscape, Eglin’s forests could pose substantial wildfire risk to the base’s infrastructure, surrounding wildland urban interface communities, and longleaf pine communities. Simulations indicated that fire hazard for surface and canopy fuels would rise under reduced prescribed fire scenarios (20k and 30k ha yr-1), would remain relatively unchanged at the baseline level of burning (40k ha yr-1), and decline if prescribed burning was increased to 50k ha yr-1. The MFI and overstory cover data suggest that fire hazard would ultimately decrease towards the end of the simulation for reduced burn scenarios over a substantial portion of Eglin relative to baseline and increased burn scenarios as forest mesophication favored fire impeding species in management units were fire was excluded and longleaf pine was replaced by broadleaf species and sand pine (Pinus clausa).application/pdfen-USCC BYfire behaviorfire ecologyfuelsPinus palustrisprescribed firewildfire riskNatural resource managementEnvironmental managementForestryForestryThesis