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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Insect herbivory on forest canopies strongly affects ecosystem nutrient cycling and availability in a variety of ways, e.g. by changing the quantity, quality and timing of nutrient input to forest soils. A qualitative method for measurements of ingestion, egestion and exuviation rates of the insect Chrysomela tremulae on leaves of the hybrid Populus tremula x tremuloides were tested in this study with the aim to detect differences in relative nutrient cycling efficiencies. The assimilation efficiency (AD), efficiency of conversion of digested food (ECD) and efficiency of conversion of ingested food (ECI) increased from 1st, through 2nd and 3rd instar larvae with generally higher efficiencies for nitrogen than carbon. Effects on nutrient limitations for the insect were also tested by increasing the C:N ratio of insect diet. A carbohydrate solution was painted onto leaves which resulted in a significant increase in C:N (p<0.0001). This lead to a trend of lengthened developmental time for each ontogenetic stage, as well as higher ingestion rate and lower egestion- and exuviation rates. However, a different method of increasing the C:N ratio is recommended in future experiments since the leaves never truly absorbed the solution.