Human activities and rapid global climate change are altering fire regimes with potential threat to the stability of aspen ecosystems in North America. Aspen is an early successional species that plays an important role in post-fire forest reestablishment, but chronic browsing on juvenile aspen by large ungulate herbivores after fire can be detrimental and lead to regeneration failure. Although larger and more severe fires are expected to become more prominent, whether and how this may influence aspen and ungulate communities remains unclear. The objective of this research was to examine how the relationship between aspen and ungulate communities might be influenced by variation in fire severity and size. In 2012, we examined browse patterns, growth responses and defense chemistry (phenolic glycoside and condensed tannins) concentrations of regenerating aspen that experienced variable burn severity in the 2010 Twitchell Canyon Fire, Utah, USA. We found that greater light availability in higher severity burn environments enhanced aspen tolerance and resistance against herbivory by increasing growth potential and defense chemistry concentrations of aspen. These results suggest that burn severity influences plant-herbivore interactions through bottom-up and top-down forces, and that higher fire severity increases post-disturbance vegetation recruitment potential by increasing resilience to herbivory. In 2013, we characterized aspen and ungulate patterns of 25 fires that spread across five National Forests (Uinta-Wasatch-Cache NF, Ashley NF, Fishlake NF, Dixie NF, and Manti-La Sal NF) in the state of Utah. We identified interaction effects between fire size and severity that strongly influenced aspen and ungulate densities. Fire size and severity are important ecological filters that can interact to affect forest reestablishment and community response. This information is useful in developing decision-making tools for wildfire and ungulate management that can more effectively increase the long-term resilience of forests systems.

The relative importance of abiotic versus biotic top-down factors on structuring tundra plant communities is debated. With climate change already strongly affecting the tundra ecosystem, understanding which factors will prevail is vital. Tundra plant communities are presumed to be predominantly structured by their abiotic conditions yet grazing by reindeer (Rangifer tarandus) has a major effect on composition and diversity. It is increasingly recognized, however, that these factors cannot be considered in isolation. Here, I aim to test the relative and interactive effect of abiotic and top- down factors on vegetation structure in the Swedish mountain tundra. Using direct measurements of reindeer grazing via tri-axial accelerometers (from two summers, 2019 and 2020) coupled with remotely-gathered data on landscape features, I examine how species richness and coverage of vascular plants, bryophytes and lichens (sampled in 2022) are driven by grazing duration and abiotic conditions. Abiotic factors, specifically aspect and soil wetness, prevailed as the dominant drivers of local vegetation patterns. Clear interactions between factors were also observed. Reindeer grazing duration had predictable but weak effects on richness, with responses predominantly observed on south-facing slopes. Additionally, soil wetness interacted with grazing duration, with wetter areas grazed far less. These results demonstrate the importance of considering interactions between abiotic and biotic factors, providing a better understanding of how tundra plant communities in northern Sweden might change under future climate change or different grazing regimes. The observed interactions imply divergence in vulnerability between slopes and the potential for effects of herbivory to be altered under future hydrological conditions. I emphasize that future studies should continue to disentangle these relationships.

The Mississippi Alluvial Valley (MAV) has undergone losses of bottomland hardwood forests due to agricultural conversion. Hardwood establishment on marginal croplands has been proposed to mitigate effects of deforestation and related loss of carbon-capture potential. However, a possible concern with reforestation is low seedling survival from mammalian herbivory. I surveyed two afforested fields in the MAV of northwest Mississippi to assess damage and mortality from four herbivores on nine species of hardwood seedlings (n = 868). Percentage survival of seedlings was 35%. Mortality of seedlings caused by herbivores was: hispid cotton rat (Sigmodon hispidus; 6.45%), rabbit ((Sylvilagus spp.; 1.95%), pine vole (Microtus pinetorum; 2.99%), and white-tailed deer (Odocoileus virginiana; 0.69%). Of surviving seedlings (n = 316), 10.82% were damaged by cotton rats, pine vole (2.99%), rabbit (8.06%), and deer (7.02%). Green ash (Fraxinus pennsylvanica), water oak (Quercus nigra), and Nuttall oak (Quercus nuttallii) had greatest survival.

Human activities and expansion have created novel disturbance patterns across Earth’s landscapes. Disturbance is an ecological interruption after which ecosystem recovery or transition into a new state can occur, affecting biodiversity, ecosystem functioning, and theavailability of ecosystem services. Fire and herbivory are two of the most widespread forces of disturbance which shape ecosystems globally. In temperate forest ecosystems, fire affects forest composition, which influences the diversity and resilience of ecosystems (chapters 1 and 2) and forest canopy structure, which is important to snowpack accumulation and the availability of water resources (chapters 3 and 4). In chapter one, the effects of conifer competition, which occurs under fire suppression, and ungulate herbivory on aspen regeneration and recruitmentwere examined. It was found that conifer competition, and ungulate herbivory both drastically reduced successful aspen regeneration and recruitment and had a larger effect than climatic or topographical variables. In chapter two, this understanding was used to investigate mechanicaland fire interventions by the National Forest Service in a mixed aspen conifer forest experiencing fire suppression and heavy ungulate herbivory. Untreated forests failed to recruit aspen suckers successfully due to conifer competition and ungulate browsing. Fire treatments were successful in restoring aspen habitat, but mechanical treatments failed due to heavy ungulate use, despitethe treatment eliciting high sucker densities. In chapter three, fire severity was found to have important implications for snowpack accumulation and snow-water content in alpine forests. High burn severity, which is projected to become more common under future climaticconditions, led to deeper and denser snowpack possibly buffering the effects of water loss in a warmer climate. In chapter four, the interaction between topography and vegetation in burned forest conditions was evaluated. It was found that topographical aspect likely mediates the effect of vegetation on snowpack and may have an opposite effect on snow accumulation and melt on north vs south facing aspects. A synthesis of studies from different regions further supports the idea that this relationship between fire and snow is heavily dependent on latitude, elevation, and slope angle. Together, these findings demonstrate that the resilience and persistence of aspenforest ecosystems in changing disturbance regimes depend on complex interactions among disturbance type, disturbance severity, landscape position, and hydrology. These interactions should be integrated into management strategies to protect ecosystems and ensure ecosystemservices for growing human populations in the western United States.

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