RSS 1.0Biological Sciences
Recent Submissions
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Seeing the forest through the trees: how site conditions mediate white and black spruce responses to climate in Interior AlaskaThe boreal forest provides essential ecosystem services and helps regulate global climate. With climate change occurring at a faster rate at high latitudes, including in the boreal forest biome, it is critical to understand how boreal forests are responding to these unprecedented changes. Despite much effort, uncertainty remains as to how boreal forest productivity has and will change with ongoing climate changes. Some of the uncertainty reflects the complex mosaic of regional climatic patterns, direct and indirect species-specific responses to regional climate, and heterogenous local site conditions that affect boreal forest productivity. I focused on the latter uncertainty: the potential role of topographic, edaphic, and biotic conditions in mediating the climate-growth responses of boreal tree species. My overarching goal was to quantify the radial growth response of black spruce (Picea mariana) and white spruce (Picea glauca), the two most common tree species in interior Alaska, to climate variability across a suite of site conditions to better understand the observed and predicted variation in climate driven productivity across a variable landscape. I employed a systematic sampling design to quantify the landscape-scale patterns in both environmental conditions and incremental annual growth of trees distributed across a 1.28 million-ha study area in Denali National Park and Preserve (and beyond in Chapter 4). I also used targeted sampling of carbon isotopes in tree rings to investigate potential drought stress. I found that near-surface permafrost, slope angle, and elevation strongly modified the magnitude, shape, and, in some cases, the direction of radial growth response of both species. For white spruce, the negative growth response to warm and dry summer conditions intensified in high competition stands and in areas receiving high potential solar radiation. During years with high cone and seed production, white spruce shifted its current year's carbon resources from radial growth to reproduction and showed signs of drought stress. I also observed differences between black and white spruce climate-growth responses, with near-surface permafrost driving their contrasting responses to June-July temperatures and with black spruce growth showing an overall more positive response to summer precipitation. These results demonstrate that local site and stand variables can force contrasting growth responses to similar climate conditions and help predict how future black and white spruce growth may play out with climate changes across a heterogeneous landscape. My results underscore the pivotal role of near surface permafrost in both the climate-growth responses and competitive dynamics of black and white spruce. Consequently, my results emphasize the importance of ongoing and predicted changes in the distribution and prevalence of permafrost for the future of the boreal forest.
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Snow as structural habitat for wolverines in a changing ArcticArctic snowpack provides critical wintertime habitat for animals to facilitate thermoregulation and avoid predators. Wolverines (Gulo gulo) are iconic among such animals, relying on snow burrows for resting sites and reproductive dens. Most of the knowledge regarding this mesocarnivore's association with snow, however, has so far originated in more southerly latitudes. In this dissertation, I investigated Arctic wolverines' behaviors associated with snow, focusing on how specific snow properties influence resting, habitat selection, and avoiding predators. Motivated by the paucity of published descriptions of wolverine resting burrows and reproductive dens on tundra, I first described terrain features and architecture of such sites. I found that resting burrows typically consist of a single tunnel leading to a resting chamber, sometimes associated with non-snow structure such as stream cutbanks and river shelf ice. By contrast, reproductive dens typically consist of longer tunnels associated with snowdrift-forming terrain. Second, I used GPS collar data from 21 adult wolverines, coupled with snowpack information at 10 meter pixel resolution, to evaluate wolverine habitat selection and movement response to snow depth, density, and melt status. I found that wolverines select deeper, denser snow, except when snow is melting, likely reflecting resting site use. Third, I developed a machine learning model to classify wolverine behaviors using tri-axial accelerometers based on direct observations of three captive wolverines, and applied this model to free-living wolverines in Arctic Alaska. I found that the model performs better when allowed to predict behaviors as "unknown," and that it accurately predicts resting, food handling, running, and scanning surroundings. Finally, based in part on this classification model, I evaluated the extent to which wolverines' use of snow burrows and surface beds for resting sites is influenced by thermoregulatory needs versus predation avoidance. I found evidence in support of both demands driving resting behavior; wolverines trade thermoregulation off against predation avoidance by resting on the snow surface on warm, sunny days, but use snow burrows on cold, dark days to meet both demands simultaneously. Collectively, this dissertation demonstrates the importance of Arctic snowpack to wolverines, a topic of increasing importance as the snow season shortens with climate change, and serves as a model for investigating behavioral processes associated with snow among other species.
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Benthic carbon demand and community structure across the Pacific Arctic continental shelvesHigh latitude continental shelves are experiencing rapid environmental change. The Pacific Arctic, which includes the northern Bering and southern Chukchi Sea continental shelves, is undergoing warming temperatures, reductions in sea ice, and changes to the marine ecosystem. Fieldwork was conducted across the northern Bering and southern Chukchi Sea continental shelves in June 2017 and June 2018 on the R/V Sikuliaq. The overall objective of this dissertation was to characterize benthic community structure, function, and carbon demand in the Pacific Arctic to serve as baselines for assessing impacts of environmental change. Spatial patterns of macrofauna and meiofauna were characterized, including abundances, biomass, composition, and vertical distribution within the sediment. Polychaete structure and function were assessed in detail by identifying polychaetes to family level and assigning each a functional guild based on feeding mode, motility, and feeding structures. Nematodes were identified to genus level and characterized by feeding type and life-history strategy. Clusters of polychaete functional guilds and nematode genera assemblages were similar and occupied different general regions within the Pacific Arctic: northern Bering Sea, Bering Strait, offshore Chukchi Sea, and coastal Chukchi Sea. These polychaete and nematode assemblages were associated with different depositional and food environments, characterized by grain size and the amount and quality of sediment organic matter. In addition, metabolic and carbon demand of dominant macrofaunal were estimated based on oxygen consumption rates. Species-specific rates suggest that shifts in macrofaunal community composition in the region will impact benthic carbon demand. Overall, the research presented here provides critical baseline data for benthic community structure, function, and carbon demand in the Pacific Arctic and can be used to evaluate change and constrain region-specific ecosystem models, especially in the context of a rapidly changing environment.
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Drivers and mechanisms of migration in an Arctic caribou herdMigration is one of the world's great natural wonders and the scale of some migratory journeys is astounding. Yet migration is globally imperiled and effective conservation of the remaining migrations will require a thorough understanding of the drivers and mechanisms underlying how migrants complete such journeys. In this dissertation, I present three chapters that sought to better understand spring and autumn migration for the Western Arctic Herd, a population of barren-ground caribou (Rangifer tarandus) that complete some of the longest terrestrial migrations on the planet. In the first chapter, I applied and validated an analytical method to infer parturition events from GPS data with robust statistical confidence. In the second chapter, I examined the parturition events detected with these methods to better understand the drivers and mechanisms of spring migration because the calving grounds are the destination for pregnant females in spring. I quantified annual spatial patterns of calving and assessed what environmental factors influenced calving site selection by caribou through time. I found evidence of both memory and perception influencing spring migration, such that caribou use memory to return to an area of generally high-quality forage at the time of calving, and consequently adjust calving sites each year based on experienced conditions. In the third chapter, I sought to understand the environmental cues caribou respond to in deciding when to migrate in autumn. I found that decreasing temperatures and the timing of first snowfall events of the season had the greatest influence on migratory movements, but notably, caribou re-assessed decisions throughout the migration period as the conditions they experience changed. I also found that the cues caribou used are similar across individuals despite the herd being broadly dispersed in late summer, and the variability in migration timing observed each year is likely due to variability in environmental conditions experienced across the range. These findings pertaining to the drivers and mechanisms of migratory behavior, and broader aspects of movements by caribou, are highly relevant for conservation and management of the species across the circumpolar North. Moreover, the observation that caribou movement exhibits strong responses to particular climate phenomena, such as temperature and precipitation, have important implications for how caribou might respond as the climate of the Arctic continues to change.
