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1. Boreal forest soils contain large stocks of soil carbon (C) that may be sensitive to changes in climate and disturbance. Destablization of boreal forest soil C through changes in C inputs, belowground C pools and/or wildfire could feedback to accelerate rising atmospheric CO2 concentration. Additionally, increasing frequency of severe fires may be changing the dominant forest types and reshaping aboveground C stocks. 2. Although controls on ecosystem C pools have received considerable attention, many studies have been limited to locations near the road system, leading to uncertainty in current and future C stocks across boreal Alaska. Here, we leveraged 545 randomly selected and spatially balanced Forest Inventory and Analysis (FIA) plots across ~13.5 million hectares in interior Alaska to examine the factors governing soil and live tree C pools. 3. Forest type mediated the effects of mean summer air temperature and the probability of near-surface permafrost on soil C. Meanwhile, forest type, stand age and aspect were the primary drivers of live tree C. Overall, plots with a known history of wildfire during the past 70 years did not have significantly different soil C stocks than plots without a known history of fire, likely due to the historical predominance of low severity fires. 4. Where wildfire likely initiated a transition to deciduous trees (19% of plots), live tree and soil C pools were reduced by 16% and 20%, respectively. Ecosystem C likely recovered over time, as maturing deciduous stands rapidly gained C in live trees. Deciduous stands without a known fire had comparatively very large live tree C stocks, suggesting a significant change in the distribution of ecosystem C following severe fire. 5. Synthesis. Our results highlight the nuanced interactions among wildfire, landscape position and forest type that will play important roles in shaping future boreal ecosystem C stocks. [ABSTRACT FROM AUTHOR]

Aim A critical concern for boreal ecosystems centres on broad-scale responses to warming, i.e. declining growth and mortality, or enhanced growth and greater productivity. However, few studies have synthesized tree growth along biogeographic gradients to address this issue. This study develops a broader understanding of how growth of a dominant conifer has responded to recent warming near the western forest margin of Alaska. Location Alaska, United States. Methods Thirty Picea glauca sites in south-west Alaska (1216 trees ≥ 4 cm dbh) were evaluated for growth differences from the southern boreal forest to the western forest-tundra margin across low-elevation forests and woodlands, and altitudinal tree line. Regional climate records were used to evaluate (1) whether tree growth near western tree line showed greater sensitivity to temperature and/or precipitation than southern boreal sites, (2) if the climate-growth response varied through time, across ecoregions and ecosystems, with spruce beetle disturbance, and by tree age, and (3) if there was a temperature threshold that limited growth. Results Positive growth trends since the 1980s in many open stands were consistent with the predicted expansion of western and altitudinal tree line. However, growth levelled off with temperatures ≥ 13°C at all but altitudinal tree line. An increasingly positive effect of precipitation on growth occurred after 1985, particularly at boreal tree line and closed forest sites. Closed-canopy forests showed lower rates of growth, greater spruce beetle activity and less potential for resiliency to warming than other ecosystem types. Main Conclusions Warming has led to markedly different growth responses according to ecosystem type and ecoregion near the forest-tundra margin. Altitudinal tree line sites showed consistently positive growth with warmer temperatures in recent decades, whereas low-elevation forests had reduced growth and greater beetle mortality. Strong positive correlations between growth and summer-fall precipitation since the mid-1980s suggest that precipitation will become an increasingly important factor with further warming. [ABSTRACT FROM AUTHOR]

Long-term experiments provide a way to test presumed causes of successional or environmentally driven vegetation changes. Early-successional nitrogen (N)-fixing plants are widely thought to facilitate productivity and vegetation development on N-poor sites, thus accounting for observed vegetation patterns later in succession. We tested this facilitative impact on vegetation development in a 23-yr field experiment on an Interior Alaska ( USA) floodplain. On three replicate early-successional silt bars, we planted late-successional white spruce ( Picea glauca) seedlings in the presence and absence of planted seedlings of an early-successional N-fixing shrub, thinleaf alder ( Alnus incana). Alder initially facilitated survivorship and growth of white spruce. Within six years, however, after canopy closure, alder negatively affected spruce survivorship and growth. Our three replicate sites followed different successional trajectories. One site was eliminated by erosion and supported no vegetation development during our study. The other two sites, which differed in site moisture, diverged in vegetation composition. Structural equation modeling ( SEM) suggested that, in the drier of these two sites, alder inhibited spruce growth directly (presumably by competition) and indirectly through effects mediated by competition with other woody species. However, at the wetter site, alder had both positive and negative effects on spruce growth, with negative effects predominating. Snowshoe hares ( Lepus americanus) in alder thickets further reduced height growth of spruce in the wetter site. We conclude that net effects of alder on white spruce, the late-successional dominant, were primarily inhibitory and indirect, with the mechanisms depending on initial site moisture. Our results highlight the importance of long-term research showing that small differences among initial replicate sites can cause divergence in successional trajectories, consistent with individualistic distributions of species and communities along environmental gradients. This divergence was detectable only decades later. [ABSTRACT FROM AUTHOR]

Increasing temperatures have resulted in reduced growth and increased tree mortality across large areas of western North American forests. We use tree-ring isotope chronologies (δ13C and δ18O) from live and dead trees from four locations in south-central Alaska, USA, to test whether white spruce trees killed by recent spruce beetle ( Dendroctonus rufipennis Kirby) outbreaks showed evidence of drought stress prior to death. Trees that were killed were more sensitive to spring/summer temperature and/or precipitation than trees that survived. At two of our sites, we found greater correlations between the δ13C and δ18O chronologies and spring/summer temperatures in dead trees than in live trees, suggesting that trees that are more sensitive to temperature-induced drought stress are more likely to be killed. At one site, the difference between δ13C in live and dead trees was related to winter/spring precipitation, with dead trees showing stronger correlations between δ13C and precipitation, again suggesting increased water stress in dead trees. At all sites where δ18O was measured, δ18O chronologies showed the greatest difference in climate response between live and dead groups, with δ18O in live trees correlating more strongly with late winter precipitation than dead trees. Our results indicate that sites where trees are already sensitive to warm or dry early growing-season conditions experienced the most beetle-kill, which has important implications for forecasting future mortality events in Alaska. [ABSTRACT FROM AUTHOR]

Interactions between vegetation and fire have the potential to overshadow direct effects of climate change on fire regimes in boreal forests of North America. We develop methods to compare sediment-charcoal records with fire regimes simulated by an ecological model, ALFRESCO (Alaskan Frame-based Ecosystem Code) and apply these methods to evaluate potential causes of a mid-Holocene fire-regime shift in boreal forests of the south-central Brooks Range, Alaska, USA. Fire-return intervals (FRIs, number of years between fires) are estimated over the past 7000 calibrated 14C years (7-0 kyr BP [before present]) from short-term variations in charcoal accumulation rates (CHARs) at three lakes, and an index of area burned is inferred from long-term CHARs at these sites. ALFRESCO simulations of FRIs and annual area burned are based on prescribed vegetation and climate for 7-5 kyr BP and 5-0 kyr BP, inferred from pollen and stomata records and qualitative paleoclimate proxies. Two sets of experiments examine potential causes of increased burning between 7-5 and 5-0 kyr BP. (I) Static-vegetation scenarios: white spruce dominates with static mean temperature and total precipitation of the growing season for 7-0 kyr BP or with decreased temperature and/or increased precipitation for 5-0 kyr BP. (2) Changed-vegetation scenarios: black spruce dominates 5-0 kyr BP, with static temperature and precipitation or decreased temperature and/or increased precipitation. Median FRIs decreased between 7-5 and 5-0 kyr BP in empirical data and changed-vegetation scenarios but remained relatively constant in static-vegetation scenarios. Median empirical and simulated FRIs are not statistically different for 7-5 kyr BP and for two changed-vegetation scenarios (temperature decrease, precipitation increase) for 5-0 kyr BP. In these scenarios, cooler temperatures or increased precipitation dampened the effect of increased landscape flammability resulting from the increase in black spruce. CHAR records and all changed-vegetation scenarios indicate long-term increases in area burned between 7-5 and 5-0 kyr BP. The similarity of CHAR and ALFRESCO results demonstrates the compatibility of these independent data sets for investigating ecological mechanisms causing past fire-regime changes. The finding that vegetation flammability was a major driver of Holocene fire regimes is consistent with other investigations that suggest that landscape fuel characteristics will mediate the direct effects of future climate change on boreal fire regimes. [ABSTRACT FROM AUTHOR]

Northern and high-latitude alpine treelines are generally thought to be limited by available warmth. Most studies of tree-growth–climate interaction at treeline as well as climate reconstructions using dendrochronology report positive growth response of treeline trees to warmer temperatures. However, population-wide responses of treeline trees to climate remain largely unexamined. We systematically sampled 1558 white spruce at 13 treeline sites in the Brooks Range and Alaska Range. Our findings of both positive and negative growth responses to climate warming at treeline challenge the widespread assumption that arctic treeline trees grow better with warming climate. High mean temperatures in July decreased the growth of 40% of white spruce at treeline areas in Alaska, whereas warm springs enhance growth of additional 36% of trees and 24% show no significant correlation with climate. Even though these opposing growth responses are present in all sampled sites, their relative proportion varies between sites and there is no overall clear relationship between growth response and landscape position within a site. Growth increases and decreases appear in our sample above specific temperature index values (temperature thresholds), which occurred more frequently in the late 20th century. Contrary to previous findings, temperature explained more variability in radial growth after 1950. Without accounting for these opposite responses and temperature thresholds, climate reconstructions based on ring width will miscalibrate past climate, and biogeochemical and dynamic vegetation models will overestimate carbon uptake and treeline advance under future warming scenarios. [ABSTRACT FROM AUTHOR]

Needle hardiness of introduced yellow pine, Pinus banksiana Lamb., lodgepole pine, P. contorta Dougl. and native white spruce, Picea glauca (Moench) Voss, were assessed by the effective prefreezing temperature method. Yellow pine needles were less hardy than lodgepole pine or white spruce needles in Alaska on each date measured. Although hardiness decreased in springtime in all species, decreases in hardiness in yellow pine began before temperatures were above -20°C, apparently in response to day length, while decreases in hardiness in lodgepole pine and white spruce began only when mean temperatures were above 0°C. Hardiness was increased by decreasing the water content of yellow pine and spruce needles. However, only the latter increased its field hardiness by decreased water contents, and only to a small degree. Large decreases in phospholipid occurred during the dehardening period, indicating the presence of major membrane-associated changes. However, changes in hardiness did not closely parallel those in phospholipid; hardiness decreased before phospholipid did in spruce and after phospholipid did in lodgepole pine. In yellow pine, changes in hardiness were more closely related to changes in phospholipid content. Decreases in phospholipid appeared to be correlated with the day length in all species. [ABSTRACT FROM AUTHOR]