Abstract. The structure and function of Alaska's forests have changed significantly in response to a changing climate, including alterations in species composition and climate feedbacks (e.g., carbon, radiation budgets) that have important regional societal consequences and human feedbacks to forest ecosystems. In this paper we present the first comprehensive synthesis of climate-change impacts on all forested ecosystems of Alaska, highlighting changes in the most critical biophysical factors of each region. We developed a conceptual framework describing climate drivers, biophysical factors and types of change to illustrate how the biophysical and social subsystems of Alaskan forests interact and respond directly and indirectly to a changing climate. We then identify the regional and global implications to the climate system and associated socio-economic impacts, as presented in the current literature. Projections of temperature and precipitation suggest wildfire will continue to be the dominant biophysical factor in the Interior-boreal forest, leading to shifts from conifer-to deciduous-dominated forests. Based on existing research, projected increases in temperature in the Southcentral-and Kenai-boreal forests will likely increase the frequency and severity of insect outbreaks and associated wildfires, and increase the probability of establishment by invasive plant species. In the Coastal-temperate forest region snow and ice is regarded as the dominant biophysical factor. With continued warming, hydrologic changes related to more rapidly melting glaciers and rising elevation of the winter snowline will alter discharge in many rivers, which will have important v www.esajournals.org 1 November 2011 v Volume 2(11) v Article 124 consequences for terrestrial and marine ecosystem productivity. These climate-related changes will affect plant species distribution and wildlife habitat, which have regional societal consequences, and trace-gas emissions and radiation budgets, which are globally important. Our conceptual framework facilitates assessment of current and future consequences of a changing climate, emphasizes regional differences in biophysical factors, and points to linkages that may exist but that currently lack supporting research. The framework also serves as a visual tool for resource managers and policy makers to develop regional and global management strategies and to inform policies related to climate mitigation and adaptation.

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To test the hypothesis that seedling growth and water use increase with soil temperature and improved soil aeration and vary with species, we evaluated the above- and below-ground growth and water use of seedlings of four northern boreal conifer species: black spruce ( Picea mariana (Mill.) B.S.P.), white spruce ( Picea glauca (Moench) Voss), tamarack ( Larix laricina (Du Roi) K. Koch), and lodgepole pine (Pinus contorta Dougl. ex Loud.) grown under different temperature and near-flooded soil conditions. Seedlings were grown in specialized pots that maintained the water table level at either 15 cm (high water table treatment: very wet) or 30 cm (low water table treatment: moderately wet) below the soil surface, and whole-seedling transpiration was assessed. Soil temperature (5, 10, or 20 °C) was controlled with a water bath surrounding the pots. Although some species were sensitive to the high water table treatment, soil temperature was the driver of seedling growth and water use. We ranked the ability of the seedlings of the species to tolerate the cold soil conditions examined as black spruce > lodgepole pine > tamarack > white spruce. The ranking of the ability to tolerate near-flooded conditions was tamarack and lodgepole pine > black spruce > white spruce.

Three studies examined the establishment and early growth of trembling aspen ( Populus tremuloides Michx.) and balsam poplar ( Populus balsamifera L.) from seed. To better understand the differences in initial developmental patterns between both species, we monitored germination and early growth in a washed sand medium with a balanced fertilizer added. Two additional studies used the Ae, Bm, and Bt horizons of a Brunisolic Gray Luvisol soil to test the impact of different soil horizons and conditions (compaction and moisture) on the establishment and early growth of trembling aspen and balsam poplar seedlings. Balsam poplar had faster radicle and leaf area development than trembling aspen and grew similarly in all soil horizons, while trembling aspen only grew well in the Ae horizon, where it outgrew balsam poplar. The superior growth of trembling aspen in the Ae horizon was associated with higher P and organic C relative to the lower horizons. Although germination was lowest on the low-compaction – low-moisture treatment for both species, balsam poplar establishment and early growth were higher than for trembling aspen in all combinations of compaction and moisture. Compared with trembling aspen, the wider establishment niche of balsam poplar is attributed to its faster root development and ability to grow in a range of soil substrates and conditions.

How robust is our assessment of impacts to ecosystems and species from a rapidly changing climate during the 21st century? We examine the challenges of uncertainty, complexity and constraints associated with applying climate projections to understanding future biological responses. This includes an evaluation of how to incorporate the uncertainty associated with different greenhouse gas emissions scenarios and climate models, and constraints of spatiotemporal scales and resolution of climate data into impact assessments. We describe the challenges of identifying relevant climate metrics for biological impact assessments and evaluate the usefulness and limitations of different methodologies of applying climate change to both quantitative and qualitative assessments. We discuss the importance of incorporating extreme climate events and their stochastic tendencies in assessing ecological impacts and transformation, and provide recommendations for better integration of complex climate–ecological interactions at relevant spatiotemporal scales. We further recognize the compounding nature of uncertainty when accounting for our limited understanding of the interactions between climate and biological processes. Given the inherent complexity in ecological processes and their interactions with climate, we recommend integrating quantitative modeling with expert elicitation from diverse disciplines and experiential understanding of recent climate-driven ecological processes to develop a more robust understanding of ecological responses under different scenarios of future climate change. Inherently complex interactions between climate and biological systems also provide an opportunity to develop wide-ranging strategies that resource managers can employ to prepare for the future.

Abstract• Aspen (Populus tremuloides Michx.) regenerates at high densities following manual cleaning.• Ten-year-old stands located near Lac La Biche and Peace River, Alberta were manually cleaned to three densities (0, 500 or 1 500 stems ha −1 ) at three times (bud set, dormancy or bud flush) to test the hypothesis that maintaining residual aspen reduces regeneration.• At Lac La Biche up to 98% of the aspen regeneration died in the partially-cleaned plots compared to 67% at Peace River five years post-treatment. A spring frost in the second growing season at Lac La Biche is hypothesized to be the inciting factor predisposing the stump sprouts to infection by decay fungi such as Armillaria root rot, resulting in reduced density and height of the aspen regeneration at Lac La Biche relative to Peace River. Drought and ungulate herbivory provided additional stresses. The high mortality of aspen regeneration at Lac La Biche shifted the understory regeneration from aspen to balsam poplar (Populus balsamifera L.).• These results indicate that maintaining 1 500 stems ha −1 of residual aspen will not effectively control the re-sprouting of aspen; however, the vulnerability of aspen regeneration to spring frost and other stressors can nearly eradicate the re-growth of aspen. Mots-clés :tremble / Populus tremuloides / nettoiement / gel / Armillaria (pourriture des racines) Résumé -Le gel de printemps et la pourriture fongique sont impliqués dans la suppression de la repousse des trembles rejetant après un nettoiement partiel dans des peuplements juvéniles.• Le tremble (Populus tremuloides Michx.) se régénère à haute densité à la suite d'un nettoiement manuel.• Des peuplements âgés de dix ans situés près de Lac La Biche et de Peace River, en Alberta, ont été nettoyés manuellement à trois densités (0, 500 ou 1 500 tiges ha −1 ) à trois moments (bourgeon formé, dormance des bourgeons ou débourrement) pour tester l'hypothèse que le maintien de trembles résiduels réduit la régénération.• Au Lac La Biche, on constate jusqu'à 98 % de mortalité de la régénération du tremble dans les parcelles partiellement nettoyées par rapport au 67 % de Peace River cinq ans après le traitement. Un gel printanier dans la deuxième saison de croissance, à Lac La Biche est supposé être le facteur prédisposant les pousses de la souche à être infectées par les pourritures fongiques comme Armillaria (pourriture des racines), ce qui réduit la densité et la hauteur de la régénération du tremble à Lac La Biche par rapport à Peace River. La sécheresse et les ongulés herbivores fournissent un stress supplémentaire. L'importante mortalité de la régénération du tremble à Lac La Biche modifie la régénération du sous-étage du tremble en peuplier baumier (Populus balsamifera L.).• Ces résultats indiquent que le maintien de 1 500 tiges ha −1 de trembles résiduels ne contrôlera pas efficacement les rejets du tremble, cependant la vulnérabilité de la régénération des trembles au gel de printemps et aux autres facteurs de stress peut presque éliminer le recru des tr...

The probability of lodgepole pine (Pinus contorta Dougl. ex Loud.) having main-stem galls caused by western gall rust, Endocronartium harknessii (J.P. Moore) Y. Hiratsuka, surviving to rotation is unknown. To evaluate survival, 400 galled trees with at least one stem gall and 400 trees without stem galls were measured in 1992 in two precommercially thinned stands approximately 20 years old. The survival of trees was assessed in 2003. Nonlinear regression using iteratively reweighted least squares was employed to estimate the survival of galled trees as a function of the proportion of the main stem encircled by galls. Galls encircling >79% and >91% of the stem in the two stands increased the risk of mortality relative to non-galled trees, with the risk increasing steeply with percent gall encirclement; smaller stem galls did not cause tree mortality. The 11-year pattern of survival of galled trees was similar for infections that occurred on the main stem and those that had reached the stem from a nearby branch infection. Based on an earlier model of gall expansion, 38%-43% of stem-galled trees would be expected to survive until age 80. Scribing of stem galls to prevent their expansion does not appear to be a feasible management strategy. Résumé :La probabilité qu'un pin tordu (Pinus contorta Dougl. ex Loud.) avec une galle sur la tige principale causée par Endocronartium harknessii (J.P. Moore) Y. Hiratsuka) survive jusqu'à la fin de la période de révolution est inconnue. Pour évaluer la survie, 400 arbres avec au moins une galle sur le tronc et 400 arbres sans galles sur le tronc ont été mesurés en 1992 dans deux peuplements âgés approximativement de 20 ans qui avaient subi une éclaircie pré-commerciale. La survie des arbres a été évaluée en 2003. La régression non linéaire utilizant les moindres carrés pondérés par itération a été utilizée pour estimer la survie des arbres avec des galles en fonction de la proportion de la tige principale encerclée par les galles. Les galles qui encerclaient plus de 79 ou 91 % du tronc selon le peuplement augmentaient le risque de mortalité comparativement aux arbres sains et le risque augmentait rapidement en fonction du pourcentage d'encerclement. Les galles plus petites sur le tronc ne causaient pas de mortalité. Le patron de survie d'une durée de 11 ans des arbres avec des galles était semblable pour les infections qui s'étaient produites sur le tronc ou à partir d'une branche près du tronc. En se basant sur un modèle précédent d'expansion des galles, 38-43 % des arbres survivants avec des galles sur le tronc devraient vivre jusqu'à l'âge de 80 ans. La stratégie d'aménagement qui consiste à faire une rainure autour des galles dans le but de prévenir leur expansion ne semble pas viable.[Traduit par la Rédaction] Wolken et al. 885

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