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Decadal scale lake drying in interior Alaska results in lake margin colonization by willow shrub and graminoid vegetation, but the effects of these changes on plant production, biodiversity, soil properties, and soil microbial communities are not well known. We studied changes in soil organic carbon (SOC) and nitrogen (N) storage, plant and microbial community composition, and soil microbial activities in drying and non‐drying lakes in the Yukon Flats National Wildlife Refuge. Historic changes in lake area were determined using Landsat imagery. Results showed that SOC storage in drying lake margins declined by 0.13 kg C m−2yr−1over 30 years of exposure of lake sediments, with no significant change in soil N. Lake drying resulted in an increase in graminoid and shrub aboveground net primary production (ANPP, +3% yr−1) with little change in plant functional composition. Increases in ANPP were similar in magnitude (but opposite in sign) to losses in SOC over a 30‐year drying trend. Potential decomposition rates and soil enzyme activities were lower in drying lake margins compared to stable lake margins, possibly due to high salinities in drying lake margin soils. Microbial communities shifted in response to changing plant communities, although they still retained a legacy of the previous plant community. Understanding how changing lake hydrology impacts the ecology and biogeochemistry of lake margin terrestrial ecosystems is an underexamined phenomenon with large impacts to landscape processes. Lake dominated ecosystems in Alaska have experienced periods of widespread lake drying in the past. We understand little of what occurs on these new nearshore habitats following lake drying, especially changes in soil properties and soil microbial communities. Using a combination of remote sensing and field sampling on hundreds of lakes in the Yukon Flats National Wildlife Refuge, we learned that lake margin soils lose significant quantities of soil C in the decades following lake drying, and at the same time soils become increasingly saline and soil microbial activites are reduced. Overall, lake drying has large impacts on the surrounding ecosystems and it is important to understand these changes for management. Lake drying in the Yukon Flats National Wildlife Refuge triggers a succession of plant and microbial communities and losses of lake margin soil organic carbon (SOC)Lake drying also results in increases in the salinity of lake margin soils, slowing rates of microbial activitiesAlthough lake drying resulted in losses of SOC, increases in Aboveground Net Primary Production (ANPP) compensated for these losses Lake drying in the Yukon Flats National Wildlife Refuge triggers a succession of plant and microbial communities and losses of lake margin soil organic carbon (SOC) Lake drying also results in increases in the salinity of lake margin soils, slowing rates of microbial activities Although lake drying resulted in losses of SOC, increases in Aboveground Net Primary Production (ANPP) compensated for these losses

ABSTRACTOver the last century in the circumpolar north, notable terrestrial ecosystem changes include shrub expansion and an intensifying wildfire regime. Shrub invasion into tundra may be further accelerated by wildfire disturbance, which creates opportunities for establishment where recruitment is otherwise rare. The Seward Peninsula currently experiences more frequent and larger fires than other tundra regions in Alaska. There are areas of overlapping burn scars dating back to the 1950s. Using a chronosequence approach, we examined vegetation and ecosystem dynamics in tussock tundra. Increasing burn severity and fire frequency corresponded with an increase in grass cover and a decrease in shrub basal area. We used multivariate ordination analysis to create a single integrator variable of fire effect that accounted for time after fire, burn severity, and number of times burned. This fire effect was significantly associated with decreases in soil organic layer thickness and overall plant biomass. Unlike previous studies in Arctic Alaska tundra, we found that increases in fire frequency and severity did not increase shrub cover and biomass. Instead, intensifying fire disturbance, and particularly repeat fires, led to grass dominance. Our findings support the hypothesis that intensifying tundra fire regimes initiate alternative post-fire trajectories that are not shrub dominated and that are structurally and functionally quite different from sedge or shrub-dominated tundra.

ABSTRACTDespite its wide geographic distribution and important role in boreal forest fire regimes, little is known about the climate-growth relationships of black spruce (Picea mariana[Mill.] B.S.P.). We used site-and tree-levelanalyses to evaluate the radial growth responses to climate of black spruce growing on a north-facing toposequence in interior Alaska for the period A.D. 1949–2010. At the site level, correlations between growth and climate were negative for temperature and positive for precipitation. The signs and strengths of these correlations varied seasonally and over time. These site-leveldifferences probably arise from tree interactions with non-climatic factors that vary with topography and include active layer thickness, soil temperature, solar radiation, microsite, and tree architecture. We infer that black spruce suffers from drought stress during warm, dry summers and that the causes of this moisture stress relate to topography and the seasonality of drought. Tree-levelanalyses reveal that divergent inter-tree growth responsesamong individual trees at the same site also occur, with the lower slope positions having the greatest frequency of mixed responses. The overall complexity of black spruce’s climate-growth relationships reflects the plastic growth strategy that enables this species to tolerate harsh, high-latitude conditions across a transcontinental range.

AbstractClimate-induced changes in the tundra fire regime are expected to alter shrub abundance and distribution across the Arctic. However, little is known about how fire may indirectly impact shrub performance by altering mycorrhizal symbionts. We used molecular tools, including ARISA and fungal ITS sequencing, to characterize the mycorrhizal communities on resprouting Betula nanashrubs across a fire-severity gradient after the largest tundra fire recorded in the Alaskan Arctic (July-October 2007). Fire effects on the components of fungal composition were dependant on the scale of taxonomic resolution. Variation in fungal community composition was correlated with fire severity. Fungal richness and relative abundance of dominant taxa declined with increased fire severity. Yet, in contrast to temperate and boreal regions with frequent wildfires, mycorrhizal fungi on resprouting shrubs in tundra were not strongly differentiated into fire-specialists and fire-sensitive fungi. Instead, dominant fungi, including taxa characteristic of late successional stages, were present regardless of fire severity. It is likely that the resprouting life history strategy of tundra shrubs confers resilience of dominant mycorrhizal fungi to fire disturbance by maintaining an inoculum source on the landscape after fire. Based on these results, we suggest that resprouting shrubs may facilitate post-fire vegetation regeneration and potentially the expansion of trees and shrubs under predicted scenarios of increased warming and fire disturbance in Arctic tundra.

AbstractThis study examined the climate sensitivity of the growth of riparian Alnus incanassp. tenuifolia(thinleaf alder), a keystone nitrogen-fixer, on the Tanana River floodplain of interior Alaska. We investigated correlations between alder radial growth and inter-annual variation in monthly meteorology and hydrology, spatial patterns of alder climate sensitivity in relationship to depth of subsurface water, and long-term climatic trends. Annual radial growth of alder was positively correlated with June precipitation, river discharge, and Palmer Drought Severity Index values and was negatively correlated with June temperature, suggesting a susceptibility of growth to early-season moisture limitation, due to the co-occurrence of dry meteorological conditions and low levels of hyporheic flow. Alder radial growth was positively correlated with August discharge and July temperature, suggesting that moisture may also be limiting in August, but not in July. The sensitivity of alder growth to variation in temperature and precipitation was greater on higher terraces where depth to water table was greater, while the sensitivity to variation in river level was greater on lower terraces, suggesting that subsurface water more strongly influences moisture availability at these sites. Long-term climatic trends in this region suggest increasing drought conditions; however, the overall influence on alder growth and associated rates of nitrogen fixation are uncertain due to the contrasting relationship of growth with climate in June/August versusJuly.