Climate change and natural disturbances are catalysing forest transitions to different vegetation types, but whether these new communities are resilient alternate states that will persist for decades to centuries is not known. Here, we test how changing climate, disturbance and biotic interactions shape the long‐term fate of a deciduous broadleaf forest type that replaces black spruce after severe wildfires in interior Alaska, USA. We simulated postfire deciduous forest that replaced black spruce after severe fires in 2004 for tens to hundreds of years under different climate scenarios (contemporary, mid 21st century, late 21st century), fire return intervals (11–250 years), distances to seed source (50–1,000 m) and browsing intensities (background, moderate, chronic). We identified combinations of conditions where deciduous forest remained the dominant vegetation type and combinations where it returned to black spruce forest, transitioned to mixed forest (where deciduous species and black spruce co‐dominate) or converted to nonforest. Deciduous forest persisted in 86% of simulations and was most resilient if fire return intervals were short (≤50 years). When transitions to another vegetation type occurred, mixed forest was most common, particularly when fire return intervals were long (>50 years) and the nearest seed source was 500 m or farther. Moderate and chronic browsing also reduced deciduous sapling growth and survival, helping black spruce compete if fire return intervals were long and seed source was distant. Dry soils occasionally caused conversion to nonforest following short‐interval fire when simulations were forced with a late 21st‐century climate scenario that projects warming and increased vapor pressure deficit. Return to black spruce forest almost never occurred. Synthesis. Conversion from black spruce to deciduous forest is already underway at regional scales in interior Alaska, and similar transitions have been widely observed throughout the North American boreal biome. We show that this boreal deciduous forest type is likely a resilient alternate state that will persist through the 21st century, which is important, because future vegetation outcomes will shape biophysical feedbacks to regional climate and influence subsequent disturbance regimes.

Citation: Olnes, J., and K. Kielland. 2016. Stage-dependent effects of browsing by snowshoe hares on successional dynamics in a boreal forest ecosystem. Ecosphere 7(10):e01475. 10.1002/ecs2.1475Abstract. Herbivores transform landscapes and affect succession via selective foraging that alters vegetation composition. In the boreal forest, mammalian herbivores, mainly moose, facilitate a shift toward the dominance of heavily defended species over time, such as white spruce. The effects of moose herbivory are intensified by the browsing of snowshoe hares. However, unlike moose, snowshoe hares also browse seedlings of white spruce. We quantified herbivory by snowshoe hares on white spruce along the Tanana River, interior Alaska, and assessed the effects on white spruce demography via two different herbivore exclosure experiments. We hypothesized that both experiments would show reduced plant density and height growth in the presence of hares. We found evidence of extensive browsing of white spruce seedlings by snowshoe hares, which negatively affected white spruce establishment along the Tanana River floodplain. Previous research has indicated that hares facilitate floodplain succession in the forward direction. However, hare herbivory can also retard succession by depressing white spruce establishment and increase the time individual trees spend in the understory. This stage-dependent effect on successional dynamics via a unique temporal variation in snowshoe hare herbivory may contribute to alternative successional trajectories in the boreal forest.

Abstract. Treelines in Alaska are advancing in elevation and latitude because of climate warming, which is expanding the habitat available for boreal wildlife species, including snowshoe hares (Lepus americanus). Snowshoe hares are already present in tall shrub communities beyond treeline and are the main browser of white spruce (Picea glauca), the dominant tree species at treeline in Alaska. We investigated the processes involved in a "snowshoe hare filter" to white spruce establishment near treeline in Denali National Park, Alaska, USA. We modeled the pattern of spruce establishment from 1970 to 2009 and found that fewer spruce established during periods of high hare abundance. Multiple factors interact to influence browsing of spruce, including the hare cycle, snow depth and the characteristics of surrounding vegetation. Hares are abundant at treeline and may exclude spruce from otherwise optimal establishment sites, particularly floodplain locations with closed shrub canopies. The expansion of white spruce treeline in response to warming climate will be strongly modified by the spatial and temporal dynamics of the snowshoe hare filter.

Herbivores have the capacity to modify plant community composition and ecosystem structure and function via browsing. For example, moose and snowshoe hare facilitate succession in Alaska's boreal forest by preferentially browsing early successional species over late successional conifers. Snowshoe hares also eat conifers, including white spruce, and this browsing may affect the pattern of spruce establishment over time. We measured over 800 spruce at 18 locations along the Tanana River floodplain in interior Alaska, USA and demonstrated that the proportion of spruce browsed annually positively correlates with annual hare abundance. Nearly all seedlings sampled had been browsed. Further, we modeled the pattern of spruce establishment over the last 40 years and found that hare abundance, growing season temperature, early season snow depth, and flooding explain the majority of this pattern. This model demonstrated that less spruce established during periods of high hare abundance than during periods of low hare abundance. The extensive browsing of white spruce that occurs during periods of high hare abundance may further compound the negative effects of climate warming on spruce recruitment in these floodplain forests.

We used satellite telemetry to examine bowhead whale movement behavior, residence times, and dive behavior in the Alaskan Beaufort Sea, 2006 – 18. We explored the timing and duration of use of three subregions (western, central, eastern) within the Alaskan Beaufort Sea and applied a two-state switching state-space model to infer bowhead whale behavior state as either transiting or lingering. Transiting whales made direct movements whereas lingering whales changed direction frequently and were presumably feeding. In spring, whales migrated across the Alaskan Beaufort Sea in 7.17 ± 0.41 days, primarily off the continental shelf over deep water. During the autumn migration, whales spent over twice as much time crossing the Alaskan Beaufort Sea than in spring, averaging 18.66 ± 2.30 days, spending 10.05 ± 1.22 days in the western subregion near Point Barrow. Most whales remained on the shelf during the autumn migration and frequently dove to the seafloor, where they spent 45% of their time regardless of behavioral state. Consistent dive behavior in autumn suggests that the whales were looking for food while migrating, and the identification of lingering locations likely reflects feeding. The lack of lingering locations in the eastern and central subregions suggests that prey densities are rarely sufficient to warrant whales pausing their migration for multiple days, unlike in the western subregion near Point Barrow, where bowhead whales regularly lingered for long periods of time.