Carl A. Roland

Landscape-scale patterns in tree occupancy and abundance in subarctic Alaska

Recent studies suggest that climate warming in interior Alaska may result in major shifts from spruce-dominated forests to broadleaf-dominated forests or even grasslands. To quantify patterns in tree distribution and abundance and to investigate the potential for changes in forest dynamics through time, we initiated a spatially extensive vegetation monitoring program covering 1.28 million ha in Denali National Park and Preserve (DNPP). Using a probabilistic sampling design, we collected field measurements throughout the study area to develop spatially explicit Bayesian hierarchical models of tree occupancy and abundance. These models demonstrated a strong partitioning of the landscape among the six tree species in DNPP, and allowed us to account for and examine residual spatial autocorrelation in our data. Tree distributions were governed by two primary ecological gradients: (1) the gradient from low elevation, poorly drained, permafrost-influenced sites with shallow active layers and low soil pH (dominat...

Patterns in the occupancy and abundance of the globally rare lichen Erioderma pedicellatum in Denali National Park and Preserve, Alaska

Abstract We investigated habitat attributes related to the occupancy of the globally rare and endangered epiphytic lichen, Erioderma pedicellatum, in a newly discovered (2009) population center in Denali National Park and Preserve (DNPP), Alaska. We measured forest, tree and epiphytic lichen community characteristics on eighty-five systematically selected plots in four study areas. We aggregated these data at three spatial scales (tree, plot and study area) at which to compare E. pedicellatum occupancy (probability of occurrence) and abundance to environmental covariates. We observed 2,035 E. pedicellatum thalli on 278 individual Picea glauca stems. The species occurred in 61% of the plots measured. Occupancy of E. pedicellatum at the individual tree-scale was influenced by stem diameter, study area, live crown length, plot P. glauca basal area, plot canopy cover and distance to open water. Our models for E. pedicellatum occupancy at the plot-scale identified study area, P. glauca density, deciduous basal area (Betula neoalaskana and Populus trichocarpa), and tall shrub cover (≥ 200 cm in height) as significant covariates. Our estimates of the DNPP population size increase the world population by at least tenfold to approximately 100,000 thalli. We suggest important topics for further research on E. pedicellatum.

Erioderma pedicellatum (Hue) P.M. Jørg, New to the United States and Western North America, Discovered in Denali National Park and Preserve and Denali State Park, Alaska.

Abstract. Erioderma pedicellatum (Hue) P.M. Jørg is a globally rare, foliose cyanolichen known from Scandinavia and southeastern Canada. In August 2007, several thalli of E. pedicellatum were collected in Denali National Park and Preserve on the south side of the Alaska Range, Alaska, USA. Subsequently, several additional thalli were observed in nearby Denali State Park. Together they represent the first report of E. pedicellatum within the United States and western North America and mark a major range extension for one of the most rare and endangered lichens in the world. The existence of a third global population center raises intriguing questions about the nature of the genetic relationships among eastern North American, western North American and Scandinavian populations.

A Bryophyte Species List for Denali National Park and Preserve, Alaska, with Comments on Several New and Noteworthy Records

Abstract. We present a substantially revised and updated bryophyte flora for the 2.4 million ha Denali National Park and Preserve (DNPP) based on wide-ranging field investigations conducted over the period 2001–2011. We visited 39 inventory sites and 45 vegetation monitoring study areas within DNPP, increasing the number of bryophyte taxa previously known by 147. 499 taxa are reported in this updated flora (380 moss taxa in 134 genera and 119 liverwort taxa in 43 genera). We provide geographic and ecological summaries through annotations and distribution maps for 24 of the most noteworthy finds, as well as comments on patterns of bryophyte community composition and dominance in this large and previously poorly-studied region.

Weather-driven change in primary productivity explains variation in the amplitude of two herbivore population cycles in a boreal system

Vertebrate populations throughout the circumpolar north often exhibit cyclic dynamics, and predation is generally considered to be a primary driver of these cycles in a variety of herbivore species. However, weather and climate play a role in entraining cycles over broad landscapes and may alter cyclic dynamics, although the mechanism by which these processes operate is uncertain. Experimental and observational work has suggested that weather influences primary productivity over multi-year time periods, suggesting a pathway through which weather and climate may influence cyclic herbivore dynamics. Using long-term monitoring data, we investigated the relationships among multi-year weather conditions, measures of primary productivity, and the abundance of two cyclic herbivore species: snowshoe hare and northern red-backed vole. We found that precipitation (rain and snow) and growing season temperatures were strongly associated with variation in primary productivity over multi-year time horizons. In turn, fourfold variation in the amplitude of both the hare and vole cycles observed in our study area corresponded to long-term changes in primary productivity. The congruence of our results for these two species suggests a general mechanism by which weather and climate might influence cyclic herbivore population dynamics. Our findings also suggested that the association between climate warming and the disappearance of cycles might be initiated by changes in primary productivity. This work provides an explanation for observed influences of weather and climate on primary productivity and population cycles and will help our collective understanding of how future climate warming may influence these ecological phenomena in the future.

Bottom-up processes drive reproductive success in an apex predator

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.

Evaluating relocation extent versus covariate resolution in habitat selection models across spatiotemporal scales

Abstract Many habitat selection studies have focused on the importance of spatiotemporal scales and sample size, yet often hidden within is a trade-off between using more animal locations versus more predictive covariates. Few have evaluated the outcome of choosing between these two different paths even though the trade-off can have significant impacts on the conclusions drawn from habitat modeling. We evaluated the covariate resolution versus relocation trade-off across multiple spatiotemporal scales by building habitat selection models using a data-mining approach with 22 years of VHF collar data from the Denali Caribou Herd. We asked whether caribou selected winter habitat based on forage resources after accounting for snow depth. Habitat selection models at three temporal scales (decadal, inter-annual and intra-annual) provide correlative evidence of active selection for winter forage. Based on model performance and predicted occupancy, we found that acquiring appropriate covariate layers is critical and likely even more important than using more animal locations. Based on analyses that utilized more covariates, caribou predominantly selected high graminoid cover, their second most important winter food, over areas with high lichen cover, their primary winter food, at all spatial scales implying a need to balance food quality and accessibility. However, habitat selection differed between temporal scales. Two years in a row, caribou switched to flatter areas with higher lichen cover and low shrub cover while intra-annual variation in habitat selection showed caribou congregated in areas of higher lichen cover and lower snow depths as winter progressed. We conclude that large patches of tussock tundra are high-use areas for wintering caribou in Denali at decadal scales while lichen-rich areas were used at inter-annual and intra-annual temporal scales. Lower snow levels in the future may allow heavier use of lichen woodlands but increased wildfire activity and shrub encroachment may counteract increased lichen availability in woodlands.