Dissertation/Thesis Abstract

Tree Regeneration Following Large Wildfires in Southwestern Ponderosa Pine Forests
by Owen, Suzanne Marie, Ph.D., Northern Arizona University, 2019, 187; 13879631
Abstract (Summary)

Wildfires in southwestern US ponderosa pine (Pinus ponderosa Lawson & C. Lawson) forests have recently increased in size and severity, leaving large, contiguous patches of tree mortality, and raising concerns about post-fire recovery. Ponderosa pines are a dominant species in the Southwest and they evolved with low- to moderate-severity fire regimes. They are poorly adapted to regenerate after large, high-severity fires because they do not have serotinous cones, re-sprouting capabilities, or long-lived seed banks. Additionally, high-severity fires can favor competing understory plants or induce long-term changes to soil nutrient dynamics and surface fuel loads, potentially altering ponderosa pine regeneration niches. Furthermore, high-severity wildfires and the loss of ponderosa pines may alter fungal community composition, including pine-symbiotic ectomycorrhizal (EM) fungi and saprotrophic fungi, which are important for forest recovery and productivity. My research objectives were to understand the effects of fire severity > 10 years post-fire on: (1) the spatial patterns, and interactions of regenerating ponderosa pine and sprouting tree species, (2) ponderosa pine regeneration niches and seedling growth, and (3) fungal sporocarp and root tip EM community composition and colonization. My study sites for the first objective included large, 4-ha plots located in two types of high-severity (100% tree mortality) burn, either adjacent to residual live forest edges (edge plots) or > 200 m from any residual live trees (interior plots) in two Arizona wildfires, the 2000 Pumpkin and 2002 Rodeo-Chediski Fires. Study sites for the last objectives included both high-severity edge and interior plots, as well as moderate-severity, and unburned plots on the Pumpkin fire. I found that ponderosa pine regeneration densities were lower in the interior compared to the edge of large high-severity burn patches, and regeneration was spatially heterogeneous on both wildfires. However, I did not find evidence that regenerating ponderosa pines were negatively impacted by the presence of re-sprouting trees. More time could eventually lead to higher ponderosa pine densities, but given the predictions for warming climates and increased fire severity, this heterogeneous stage of forest development, that includes native sprouting species could be more resilient to drought and high-severity fires than dense pine stands. I also found that high-severity burns altered understory vegetation and surface fuel loads, but soil properties were similar compared to moderate-severity and unburned areas. I discovered certain niche characteristics that favored ponderosa pine growth, and found that growth was consistently greater in the high-severity edge plots compared to other treatments. Niche characteristics such as overstory tree canopy (-), course woody debris (CWD; +), forb cover (+), Muhlenbergia montana (+) cover, and soil phosphate (+) were important explanatory variables favoring ponderosa pine growth. Surrounding exotic plant cover did not negatively affect ponderosa pine growth. These results suggest that if ponderosa pines can survive the first few critical years after germination, proximity to some forbs and CWD may help increase their growth rates. However, the fact that greater fine fuel biomass and CWD were adjacent to pine seedlings might put them at increased risk from surface fires until they are taller and more fire resistant. My data also revealed that high-severity burn patches had long-term consequences for EM sporocarp communities, and EM inoculum species pools. High-severity burn plots had a unique sporocarp community composition, and a shift in dominant sporocarp functional groups, with 5-13 times lower EM sporocarp densities, and 4-7 times lower EM sporocarp species richness compared to unburned and moderate-severity plots. In contrast, saprotrophic sporocarp densities and richness were similar among treatments, even with the large amount of woody debris in the high-severity burn patches. These results suggest that large patches of high-severity fire have long-term consequences for EM sporocarp communities, which may reduce reproduction of some species or influence recovery. Managers may want to prioritize the protection of surviving trees within or surrounding large burn patches, as well as naturally regenerating ponderosa pines, to conserve local adaptations, future ponderosa pine seed and EM sources. Also, planted pines may have improved growth near some CWD or forb cover, but they may be at risk for future fires until they are tall enough to survive higher flame lengths. By understanding how above- and belowground communities recover after disturbances, we can improve management plans aiming for forest health and resiliency.

Indexing (document details)
Advisor: Fulé, Peter Z., Sieg, Carolyn H.
Commitee: Baggett, L. Scott, Gehring, Catherine A.
School: Northern Arizona University
Department: Forestry
School Location: United States -- Arizona
Source: DAI-B 80/11(E), Dissertation Abstracts International
Subjects: Ecology, Forestry
Keywords: Fire severity, Forest resilience, Mycorrhizal fungi, Pinus ponderosa, Spatial heterogeneity, Wildfires
Publication Number: 13879631
ISBN: 978-1-392-27542-9
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