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Inferences about the mechanisms of distributional change are often made from simple assessments of variation in the geographical positions of populations. However, direct assessments of species' responses to local habitat change may be necessary for proper understanding of the drivers of distributional dynamics.Amplified climate warming is inducing cascading impacts in boreal‐tundra regions including the expansion of conifers and deciduous shrubs (shrubs). In Denali National Park (Denali), Alaska, passerine birds are exhibiting rapid upslope shifts in distribution but the relative roles of conifer and shrub (woody vegetation) expansion in driving these shifts are unknown. Without directly assessing passerine‐vegetation dynamics, the assumption has been that the observed upslope shifts are indicative of shrub‐adapted passerines tracking the upslope expansion of shrubs. Here, we jointly investigate the processes of conifer and shrub expansion and their relationship to changes in passerine abundance in Denali.We used a remotely sensed vegetation cover timeseries (1985–2020) to assess the topographic and edaphic correlates of conifer and shrub expansion. We then assessed the impacts of changes in shrub and conifer cover on the relative abundance of 12 passerine species (1995–2020).Shrub and conifer colonization rates were highest at intermediate elevations near treeline. However, forest‐ and shrub‐adapted passerines differed in terms of the location in which their response was concentrated relative to treeline. The population growth rates of forest‐adapted passerines exhibited stronger effects of woody vegetation expansion at sites that were initially above treeline (IAT). In contrast, the population growth rates of shrub‐adapted passerines exhibited the negative effects of conifer expansion together with the positive effects of shrub expansion at initially below treeline sites. However, they showed a weak response to woody vegetation expansion at sites that were IAT.Below treeline conifer infilling appears to be pushing the elevational distributions of shrub‐adapted passerines upslope rather than these species following the pull of modest shrub expansion above treeline, as previously assumed.Overall, our findings illustrate the need for explicitly accommodating heterogeneity in habitat change at small spatial scales to properly view the distributional response, particularly when habitat change is concentrated at ecotones. [ABSTRACT FROM AUTHOR]

Boreal forest and tundra ecosystems are undergoing rapid climatic and environmental changes with consequences for ecosystem structure, function, and services. Although riparian zones occupy a small footprint within subarctic landscapes, they have disproportionately high value as foci of hydrological processes, biogeochemical cycling, ecological disturbance, biodiversity, and wildlife activity. Recent observations of increased winter discharge, reduced peak flows, and increased connectivity between catchments, streams, and groundwater in subarctic riparian zones have prompted predictions of altered riverine disturbance regimes, increased channelization, and a decline in the extent of active floodplains. However, few observational data exist concerning the spatiotemporal dynamics of subarctic floodplain vegetation, which can serve as a bioindicator to corroborate such predictions. We analyzed the distribution and extent of riparian ecotypes across a network of streams in 12 Interior Alaska watersheds using high‐resolution image pairs from circa 1981–2010. All stream reaches encompassed pronounced elevational gradients and included elevational forest–tundra ecotones. We classified riparian ecotypes using an image‐based point‐intercept sampling approach, calculated the probability of ecotype transitions, and evaluated relationships between ecotype transitions and environmental covariates. Our results reveal widespread increases in the stature, density, and extent of riparian vegetation spanning gradients of elevation, floodplain morphology, and climate. Ecotype transitions occurred at >20% of sample points, and there was a strong imbalance toward forward successional transitions (16.5%) versus backward transitions (4.0%). That is, we observed a strong tendency toward increasing cover, stature, and density of vegetation communities across our extensive sampling domain across our approximately 30‐year sample period. This relatively rapid riparian "greening" signal tended to be most pronounced in our glaciated watersheds. Although the streams we studied displayed high local variability in ecotype transitions, our results support hypotheses of increasing channelization and reduced extent of unvegetated surfaces on subarctic floodplains. They also likely reflect a trend toward more rapid and extensive plant recruitment and growth due to processes associated with conspicuous warming in northern ecosystems, consistent with greening documented in other subarctic landscape segments along gradients of elevation and latitude. [ABSTRACT FROM AUTHOR]

Ongoing environmental change across the Arctic is affecting many freshwater ecosystems, including small thaw ponds that support macroinvertebrates, thus potentially affecting important forage for fish and bird species. To accurately predict how fish and wildlife that depend on these macroinvertebrates will be affected by ecosystem change at high latitudes, understanding proximate factors that influence macroinvertebrate abundance is critical.To better understand factors that affect spatial and seasonal (i.e. phenology) patterns in abundance, we collected macroinvertebrates throughout the growing season of a single year from 33 thaw ponds on the Arctic Coastal Plain in northern Alaska. We used hierarchical N‐mixture models to provide detection‐corrected estimates of abundance (of the population exposed to sampling) in relation to pond type and seasonal patterns in environmental variables (i.e., cumulative water temperature, nutrient levels) for five taxonomic groups representing key food items for birds and fish—Anostraca (Arthropoda: Branchiopoda), Chironomidae (Insecta: Diptera), Cladocera (Arthropoda: Branchiopoda), Limnephilidae (Insecta: Trichoptera), and Physidae (Mollusca: Gastropoda).For three of five taxa (Anostraca, Cladocera, Limnephilidae), abundance varied across pond types and was lower in pond types where water temperatures increased more rapidly. Further, seasonal temperature profiles in ponds affected phenology, suggesting that seasonal patterns in abundance were influenced by changes in water temperature.These findings suggest that increases in water temperature in northern areas could alter macroinvertebrate phenology, possibly with consequences for upper level predators if availability of macroinvertebrate prey is reduced or shifted seasonally. Our results will facilitate improved predictions of how changing abiotic conditions could affect inland waters in northern areas, a critical need for conservation of Arctic wildlife and ecosystems. [ABSTRACT FROM AUTHOR]

Moose management throughout much of Alaska and Canada relies on aerial count data, which are commonly collected using the geospatial population estimator (GSPE) protocol. The GSPE uses a model‐based analytical approach and finite‐population block kriging to estimate abundance from a collection of sampled survey units. Widespread implementation and well‐documented analytical software have resulted in reliable estimates of moose abundance, density, and composition across a large proportion of their range. Analysis is conducted almost exclusively using the GSPE software, which fits a fixed model structure to data collected within a single year. The downside of this approach to analysis is that the fixed model structure is inefficient for estimation, leading to more field effort than would otherwise be necessary to achieve a desired level of estimator precision. We developed a more easily modified and flexible Bayesian spatial general additive model approach (BSG) that accommodates spatial and temporal covariates (e.g., habitat characteristics, trend), multiple survey events, prior information, and incomplete detection. Using a series of 6 GSPE surveys conducted in Yukon‐Charley Rivers National Preserve, Alaska, USA, from 2003–2019, we established the equivalence of the 2 approaches under similar model structures. We then extended the BSG to demonstrate how a more comprehensive approach to analysis can affect estimator precision and be used to assess ecological relationships. The precision of annual abundance estimators from the BSG were improved by an average of 43% over those based on the standard GSPE analysis, highlighting the very real costs of assuming a fixed (i.e., suboptimal) model structure. The population increased at a rate of 2.3%/year (95% CrI = 0.8–3.8%), and the increase was largely explained by a parallel increase in wildfire extent (i.e., high quality moose habitat). These results suggest that our approach could be used to increase estimator efficiency or decrease future survey costs without any modifications to the basic protocol. While modification of the GSPE software is possible, practitioners may find the BSG approach more convenient for quickly developing model structures for a particular application, thereby allowing them to extract more information from existing and future datasets. [ABSTRACT FROM AUTHOR]

Wildlife population estimators often require formal adjustment for imperfect detection of individuals during surveys. Conventional distance sampling (CDS) and its extensions (mark‐recapture distance sampling [MRDS], temporary emigration distance sampling [TEDS]) are popular approaches for producing unbiased estimators of wildlife abundance. However, despite extensive discussion and development of distance sampling theory in the literature, deciding which of these alternatives is most appropriate for a particular scenario can be confusing. Some of this confusion may stem from an incomplete understanding of how each approach addresses the components of the detection process. Here we describe the proper application of CDS, MRDS, and TEDS approaches and use applied examples to help clarify their differing assumptions with respect to the components of the detection process. To further aid the practitioner, we summarize the differences in a decision tree that can be used to identify cases where a more complex alternative (e.g., MRDS or TEDS) may be appropriate for a given survey application. Although the more complex approaches can account for additional sources of bias, in practical applications one also must consider estimator precision. Therefore, we also review the concept of total estimator error in the context of comparing competing methods for a given application to aid in the selection of the most appropriate distance sampling approach. Finally, we detail how the use of more advanced techniques (i.e., informed priors, open‐population distance sampling models, and integrated modeling approaches) can further reduce total estimator error by leveraging information from existing and ongoing data collection. By synthesizing the existing literature on CDS, MRDS, TEDS and their extensions, in conjunction with the concepts of total estimator error and the components of the detection process, we provide a comprehensive guide that can be used by the practitioner to more efficiently, effectively, and appropriately apply distance sampling in a variety of settings. [ABSTRACT FROM AUTHOR]

Managers rely on accurate estimators of wildlife abundance and trends for management decisions. Despite the focus of contemporary wildlife science on developing methods to improve inference from wildlife surveys, legacy datasets often rely on index counts that lack information about the detection process. Data integration can be a useful tool for combining index counts with data collected under more rigorous designs (i.e., designs that account for the detection process), but care is required when datasets represent different population processes or are mismatched in space and time. This can be particularly problematic in cases where animals aggregate in response to a spatially or temporally limited resource because individuals may temporarily immigrate from outside the study area and be included in the abundance index. Abundance indices based on brown bear (Ursus arctos) feeding aggregations within coastal meadows in early summer in Lake Clark National Park and Preserve, Alaska, USA, are one such example. These indices reflect the target population (brown bears residing within the park) and temporary immigrants (i.e., bears drawn from outside the park boundary). To properly account for the effects of temporary immigration, we integrated the index data with abundance data collected via park‐wide distance sampling surveys, the latter of which properly addressed the detection process. By assuming that the distance data provide inference on abundance and the index counts represent some combination of abundance and temporary immigration processes, we were able to decompose the relative contribution of each to overall trend. We estimated that the density of brown bears within our study area was 38–54 adults/1,000 km2 during 2003–2019 and that abundance increased at a rate of approximately 1.4%/year. The contribution of temporary immigrants to overall trend in the index was low, so we created 3 hypothetical scenarios to more fully demonstrate how the integrated approach could be useful in situations where the composite trend in meadow counts may obscure trends in abundance (e.g., opposing trends in abundance and temporary immigration). Our work represents a conceptual advance supporting the integration of legacy index data with more rigorous data streams and is broadly applicable in cases where trends in index values may represent a mixture of population processes. [ABSTRACT FROM AUTHOR]

Quantitative studies of regional variation in plant diversity across eastern Beringia (northern Alaska and adjacent areas) are lacking due to an absence of datasets of sufficient scale and scope. We interrogated a landscape‐scale plant diversity dataset collected across two regions of interior Alaska with different disturbance, topographic, and climate attributes to investigate hypotheses regarding drivers of plant species richness. Our approach integrated a multi‐scale sampling design with an analytical framework focused on quantifying how components of plant diversity (growth forms, biogeographic groups, and dominant species) respond to site factors that vary along landscape gradients. Our results revealed essential similarities in both the composition of the overall floras and the influences on local and meso‐scale species richness across both regions. However, these continuities at smaller scales contrasted with differences in landscape‐level distribution of plant diversity patterns along elevation gradients. Our findings suggest that local drivers of richness and occupancy interacted with differing macro‐scale attributes (e.g., relative continentality) to produce distinctive landscape‐level diversity patterns. Our results confirm that high levels of local and meso‐scale plant richness in interior Alaska depend on conditions that foster richness of herbaceous and northerly distributed species groups. However, we found that important differences in landscape‐level richness patterns were driven by regional differences in climate, topoedaphic variables, and disturbance. In the warmer region, woody species and boreal plant communities extended to higher elevations and common species occupancy showed marginally greater influence of fire. Overall richness was relatively low in alpine areas of the warmer region but heterogeneous edaphic and topographic circumstances stimulated higher species turnover in lower elevations there, increasing landscape‐level richness. In contrast, in the cooler region, woody species showed restricted distribution across the elevation gradient while site attributes associated with increased species richness aligned with the elevation gradient and thus peak richness occurred in the alpine zone. Our results show how total and growth‐form richness, as well as community composition, vary regionally in relation to important drivers (including growing season warmth) across interior Alaska. Consequently, our study provides new insights into the potential trajectories of future change in biodiversity patterns in this rapidly warming region. [ABSTRACT FROM AUTHOR]

Body size reflects realized physiological niche width at the population level and provides insights into the potential resiliency of a species to natural and anthropogenic perturbations to ecosystems. We analyzed patterns of American black and brown bear body size through a meta‐analysis of data from 18 studies conducted across North America, to evaluate the effects of species, sex, latitude, sympatry, and harvest. We used a bootstrap model selection procedure, which accounted for differences in sample size and population variation between studies, to investigate patterns in body mass. As expected, we found that brown bears were generally larger than black bears and both species were sexually dimorphic (i.e., males were generally larger than females). Black bear body size was not related to latitude, whereas brown bear body size was greatest at intermediate latitudes, possibly due to the presence of salmon. Neither sympatry nor harvest was associated with body size for either species at the geographic scale of our meta‐analyses, but both may warrant consideration at the local scale. Body size, as an index of population health, reflects phenotypic plasticity within species and populations and may serve as a useful indicator of niche utilization. Indeed, understanding large‐scale physiological patterns such as these can assist in understanding past, present, and future changes to realized niches and subsequent resiliency of species and populations. [ABSTRACT FROM AUTHOR]

The timing of life cycle events has strong fitness consequences, suggesting that monitoring of arrival and departure timing may help understand spatial and population dynamics. Several existing models with inference to arrival and departure in unmarked populations are applicable to detection/non‐detection data which is a reduced information summary of the underlying population and phenological dynamics. These models also do not directly address the dependence of seasonal variation in availability (e.g. song rate) on arrival timing, often treating the seasonal distribution of availability as fixed across years despite allowing variation in arrival. Model development in an abundance framework has largely occurred in the context of stopover populations, rather than populations that exhibit some period of closure between arrival and departure phases.We developed an N‐mixture model that accommodates the dependence of seasonal availability on arrival timing, providing inference about abundance and both arrival and departure timing based on repeated count data. The model is applicable to populations in which there exists some period of closure between the arrival and departure phases. We developed two general formulations of the model, both of which include a model for the arrival process, but differ in the model for seasonal availability. The first formulation is applicable to cases in which seasonal availability is a function of cue production. The other is applicable to situations where seasonal availability is a function of departure of individuals or their transition to a state in which they remain unavailable for detection.We demonstrated through simulation that both versions provide unbiased and precise estimates of phenology and abundance and illustrated the cue production formulation using data collected in Denali National Park, Alaska for three passerine species.We expect that our inference framework will be broadly applicable in studies of unmarked populations where joint assessment of population, spatial and phenological dynamics is of interest. [ABSTRACT FROM AUTHOR]

Changes in climate are driving widespread landscape changes in northern ecosystems, altering the size and distribution of wildlife populations over multi-decadal time scales. Extreme weather events are also expected to become more common over time, affecting a variety of species, and mountain ungulates may be particularly susceptible because they occupy habitats with particularly harsh winter weather conditions. To explore the impacts of weather conditions and adverse weather events as population drivers, we surveyed Dall's sheep throughout their latitudinal range in Alaska and assessed lamb production and population trend in relation to end of the continuous snow season (CSS) as a measure of spring onset. In 2013, spring onset was extraordinarily late, providing an opportunity to directly assess the impacts of variability in weather on sheep population dynamics. We hypothesized that the timing of the end of the CSS would have greater impacts in arctic areas where conditions are presumably most limiting. We found that lamb production was negatively related to the annual timing of the end of the CSS and was near 0 in arctic areas in 2013. The 2013 event was also associated with ~40-70% declines in overall sheep numbers in arctic areas, indicating adult survival was also impacted. Overall, our results suggest that expected increases in adverse weather events may have direct, lasting impacts on mountain ungulate populations and that these impacts can be expected to be most extreme for populations occurring at northern range limits where growing season conditions are most restricted. [ABSTRACT FROM AUTHOR]

Abstract: Management or conservation targets based on demographic rates should be evaluated within the context of expected population dynamics of the species of interest. Wild populations can experience stable, cyclical, or complex dynamics, therefore undisturbed populations can provide background needed to evaluate programmatic success. Many raptor species have recovered from large declines caused by environmental contaminants, making them strong candidates for ongoing efforts to understand population dynamics and ecosystem processes in response to human‐caused stressors. Dynamic multistate occupancy models are a useful tool for analyzing species dynamics because they leverage the autocorrelation inherent in long‐term monitoring datasets to obtain useful information about the dynamic properties of population or reproductive states. We analyzed a 23‐year bald eagle monitoring dataset in a dynamic multistate occupancy modeling framework to assess long‐term nest occupancy and reproduction in Lake Clark National Park and Preserve, Alaska. We also used a hierarchical generalized linear model to understand changes in nest productivity in relation to environmental factors. Nests were most likely to remain in the same nesting state between years. Most notably, successful nests were likely to remain in use (either occupied or successful) and had a very low probability of transitioning to an unoccupied state in the following year. There was no apparent trend in the proportion of nests used by eagles through time, and the probability that nests transitioned into or out of the successful state was not influenced by temperature or salmon availability. Productivity was constant over the course of the study, although warm April minimum temperatures were associated with increased chick production. Overall our results demonstrate the expected nesting dynamics of a healthy bald eagle population that is largely free of human disturbance and can be used as a baseline for the expected dynamics for recovering bald eagle populations in the contiguous 48 states. [ABSTRACT FROM AUTHOR]

Abstract: Effective wildlife management and conservation depends on a detailed understanding of population dynamics at large spatial scales. Distance sampling is an efficient tool for landscape‐scale monitoring, and the recent development of the open‐population distance sampling framework for analysis allows efficient estimation of population parameters for unmarked populations. Applications of open‐population distance approaches are relatively rare, and further development is required to generalize this powerful class of models for use in complex sampling scenarios. Here, we extend this class of models to situations where individuals occur in groups and address: the incorporation of group composition information, fluctuations in aggregation among individuals into groups, and non‐repeated survey designs. We also describe a new parameter, κ, representing the change in the rate of aggregation of individuals into groups over time. Using simulations, we assess the performance of this extended framework, and demonstrate its utility through an applied example using a Dall's sheep (Ovis dalli dalli) dataset collected in Alaska from 2009 to 2016. Our simulation study demonstrated that our extended model reliably recovered the data‐generating parameter values without bias. In the applied example, we found that an abrupt decline in abundance observed in the Dall's sheep population was driven primarily by reductions in lambs and ewe‐like sheep that coincided with the late onset of spring conditions in 2013. Rams were much less affected, suggesting declines in the ewe‐like class drove overall population trajectory in this population. This work demonstrates how ecological questions related to the dynamics of group‐dwelling species may be assessed in complex sampling situations using an open‐population distance sampling framework. The resulting framework is broadly applicable to a variety of species and will be especially useful for evaluating population dynamics in unmarked populations of group‐dwelling species at large spatial scales. [ABSTRACT FROM AUTHOR]

Abstract: One of the central goals of the field of population ecology is to identify the drivers of population dynamics, particularly in the context of predator–prey relationships. Understanding the relative role of top‐down versus bottom‐up drivers is of particular interest in understanding ecosystem dynamics. Our goal was to explore predator–prey relationships in a boreal ecosystem in interior Alaska through the use of multispecies long‐term monitoring data. We used 29 years of field data and a dynamic multistate site occupancy modeling approach to explore the trophic relationships between an apex predator, the golden eagle, and cyclic populations of the two primary prey species available to eagles early in the breeding season, snowshoe hare and willow ptarmigan. We found that golden eagle reproductive success was reliant on prey numbers, but also responded prior to changes in the phase of the snowshoe hare population cycle and failed to respond to variation in hare cycle amplitude. There was no lagged response to ptarmigan populations, and ptarmigan populations recovered quickly from the low phase. Together, these results suggested that eagle reproduction is largely driven by bottom‐up processes, with little evidence of top‐down control of either ptarmigan or hare populations. Although the relationship between golden eagle reproductive success and prey abundance had been previously established, here we established prey populations are likely driving eagle dynamics through bottom‐up processes. The key to this insight was our focus on golden eagle reproductive parameters rather than overall abundance. Although our inference is limited to the golden eagle–hare–ptarmigan relationships we studied, our results suggest caution in interpreting predator–prey abundance patterns among other species as strong evidence for top‐down control. [ABSTRACT FROM AUTHOR]

Obtaining useful estimates of wildlife abundance or density requires thoughtful attention to potential sources of bias and precision, and it is widely understood that addressing incomplete detection is critical to appropriate inference. When the underlying assumptions of sampling approaches are violated, both increased bias and reduced precision of the population estimator may result. Bear ( Ursus spp.) populations can be difficult to sample and are often monitored using mark-recapture distance sampling ( MRDS) methods, although obtaining adequate sample sizes can be cost prohibitive. With the goal of improving inference, we examined the underlying methodological assumptions and estimator efficiency of three datasets collected under an MRDS protocol designed specifically for bears. We analyzed these data using MRDS, conventional distance sampling ( CDS), and open-distance sampling approaches to evaluate the apparent bias-precision tradeoff relative to the assumptions inherent under each approach. We also evaluated the incorporation of informative priors on detection parameters within a Bayesian context. We found that the CDS estimator had low apparent bias and was more efficient than the more complex MRDS estimator. When combined with informative priors on the detection process, precision was increased by >50% compared to the MRDS approach with little apparent bias. In addition, open-distance sampling models revealed a serious violation of the assumption that all bears were available to be sampled. Inference is directly related to the underlying assumptions of the survey design and the analytical tools employed. We show that for aerial surveys of bears, avoidance of unnecessary model complexity, use of prior information, and the application of open population models can be used to greatly improve estimator performance and simplify field protocols. Although we focused on distance sampling-based aerial surveys for bears, the general concepts we addressed apply to a variety of wildlife survey contexts. [ABSTRACT FROM AUTHOR]

Reliable assessments of low-density carnivore populations such as brown bears Ursus arctos are often limited by a lack of sufficient information for strong inference at appropriate scales. Standard approaches often rely on physical marking of individuals or the use of inherently field-intensive hair-snag or distance sampling techniques. Although these tools are very useful, logistical and monetary costs often limit their successful application, particularly in large, remote areas. We developed a novel photographic mark-resight approach using physical characteristics and spatial locations of individual brown bears to temporarily mark individuals over a short revisit interval. We applied this approach along with site-occupancy techniques to evaluate a low-density brown bear population in northwestern Alaska. Based on the mark-resight approach, we estimated there were 420 [95% CrI:274–650] independent and 713 [95% CrI:474–1070] total brown bears in our 19,998km 2 study area. When expressed as densities, these estimates were consistent with those of other low-density populations from the surrounding area. Estimated den and bear site-occupancy rates were similar, 0.48 [95% CrI:0.37–0.63] and 0.40 [95% CrI;0.28–0.55], respectively. Close congruence among occupancy and abundance estimates supported the robustness of our new mark-resight approach and provided additional metrics for population monitoring. Together, these parallel metrics provide a general framework for monitoring low density populations of brown bears and other rare carnivores when physical marking or intensive survey techniques are impractical. [ABSTRACT FROM AUTHOR]

There has been relatively little study of the capacity of subarctic passerines to adjust their phenologies to rapid changes on their breeding grounds. Here, we assess variation in passerine arrival timing in Denali National Park, Alaska, from 1995 to 2015, a period that included both the warmest and coldest recorded mean spring temperatures for the park. Using an open-population occupancy modeling approach in which arrival events are random variables, we investigated interannual variation in the arrival distribution for 10 Nearctic-Nearctic migrants, three Nearctic-Neotropical migrants, and one Palearctic migrant. Neotropical-Nearctic migrants varied in terms of the flexibility of their arrival timing, but generally showed plastic phenologies, suggesting resilience under extreme spring conditions. In comparison, Nearctic-Nearctic migrants showed similar or greater plasticity in arrival timing. A majority of species showed synchronous-asynchronous fluctuation in arrival (i.e., synchronous arrival in some years, asynchronous in others) in combination with various levels of the mean response (i.e., early, average, and late arrival), suggesting the presence of interactions between environmental conditions at multiple scales and inter-individual variation. The presence of synchronous-asynchronous fluctuation in arrival suggests that weakening of the north-south temperature gradient under continued Arctic amplification may strongly affect arrival variances. Our results also suggest that complex interactions between distributional and phenological changes may be possible. For example, the arrival distribution of Fox Sparrow (Passerella iliaca) became more synchronized over time, a pattern that coincided with a dramatic increase in occupancy probability through expansion of its elevational distribution. Overall, our findings suggest that monitoring of the mean-variance relationship may lead to a deeper understanding of the factors shaping phenological responses. [ABSTRACT FROM AUTHOR]

The growth response of boreal forest trees to projected changes in climate will have wide ranging and cascading impacts on disturbance regimes, climate feedbacks, carbon storage, and habitat ranges for flora and fauna. Recent findings in Alaska suggest the boreal biome is shifting in response to changes in climate. It is unlikely such a shift will occur in a uniform manner over a landscape variable in topography and site conditions, although there is little consensus of how local site characteristics will interact with climate to alter tree growth patterns. Few studies of tree growth responses to climate in the boreal forest have used randomized study designs encompassing the full range of site conditions, leaving open the question of whether observed patterns in tree growth at local scales are wholly representative of the boreal forest. We addressed these issues using a systematic sampling design to quantify the landscapescale patterns in annual growth for Picea mariana and Picea glauca, the two most abundant tree species in interior Alaska. We used measurements of annual tree growth based on increment cores taken from trees distributed across a 1.28 million-ha study area in Denali National Park and Preserve. We found site-specific variables, namely near-surface permafrost, slope angle, and elevation, strongly modified the magnitude, shape, and, in some cases, the direction of growth response to climatic conditions for both species. With predicted warming, our results suggest both P. mariana and P. glauca will decrease growth in areas underlain by shallow permafrost. P. mariana growth may increase on flat terrain, but decrease on steep slopes, while P. glauca growth will show the greatest increases at high elevations during very warm summers. Our results suggest future shifts in these species distributions in response to a changing climate will be conditional on variation in site factors operating at local scales. [ABSTRACT FROM AUTHOR]

Despite predictions of poleward and upslope shifts in the distribution of breeding passerines under climate change, studies often report variable responses with some species shifting opposite of the expected direction and others showing range stability. While changes in climate could affect distribution directly, passerines show strong preferences for particular structural vegetation characteristics, suggesting that long‐term changes in vegetation may mediate a species' distributional response to climate, and consequently, may be responsible for the observed heterogeneity. We assessed changes in the elevational distribution and occupancy probability of 17 passerine species in Denali National Park, Alaska, from 1995 to 2013 across an elevational gradient containing multiple topographically defined ecotones (treeline and shrubline). An upward distributional shift was pervasive among shrub‐tundra species, corresponding with observed expansion of shrub cover at higher elevations. Forest‐associated passerines showed a weaker response as a group with some species showing range stability or retraction, while others exhibited modest expansions that were consistent with an advancing treeline ecotone. Denali's mean summer temperature increased significantly over the past century, but remained relatively stable over our study period, implying that longer term changes in climate indirectly influenced bird distribution through changes in woody vegetation. Further, heterogeneity in the response of forest‐associated species was consistent with a slower rate of forest development and expansion as compared with shrub colonization. Together, our results indicate that the elevational range dynamics of passerines may be related to species‐specific associations with different vegetation communities and variation across these vegetation communities in the timescale over which distributional change is occurring. [ABSTRACT FROM AUTHOR]

Selective harvest regimes are often focused on males resulting in skewed sex-ratios, and for many ungulate species this strategy is sustainable. However, muskoxen (Ovibos moschatus) are very social and mature bulls (≥4 years old), particularly prime-age bulls (6–10 years old), play important roles in predator defense and recruitment. A year-round social structure incorporating large males into mixed-sex groups could make this species more susceptible to the effects of selective harvest if population composition and sex-ratios influence overall survival and reproductive success. Using detailed data collected on the muskox population occupying the Seward Peninsula, Alaska during 2002–2012, we formulated the hypothesis that the selective harvest of mature bulls may be related to documented changes in population composition and growth rates in this species. In addition, we reviewed existing published information from two other populations in Alaska, the Cape Thompson and Northeastern populations, to compare population growth rates among the three areas under differential harvest rates relative to our hypothesis. We found that on the Seward Peninsula, mature bull:adult cow ratios declined 4–12%/year and short-yearling:adult cow ratios (i.e., recruitment) declined 8–9%/year in the most heavily harvested areas. Growth rates in all 3 populations decreased disproportionately after increases in the number of bulls harvested, and calf:cow ratios declined in the Northeastern population as harvest increased. While lack of appropriate data prevented us from excluding other potential causes such as density dependent effects and changes in predator densities, our results did align with our hypothesis, suggesting that in the interest of conservation, harvest of mature males should be restricted until causal factors can be more definitively identified. If confirmed by additional research, our findings would have important implications for harvest management and conservation of muskoxen and other ungulate species with similar life-histories. [ABSTRACT FROM AUTHOR]