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Herbivores can modify the rate of shrub and treeline advance. Both direct and indirect effects of herbivory may simultaneously interact to affect the growth rates of plants at this ecotone. We investigated the effect of snowshoe hare herbivory on the height of white spruce at two treeline locations in Alaska, USA. White spruce is expanding its distribution both upwards in elevation and northward in latitude because of climate warming, and snowshoe hares are already present in areas likely to be colonized by spruce. We hypothesized that herbivory would result in browsed individuals having reduced height, suggesting herbivory is a direct, negative effect on spruce treeline advance. We found an interactive effect between browsing history and spruce age. When young (under 30 years old), individuals that were browsed tended to be taller than unbrowsed individuals. However, older seedlings (over 30 years old) that had been browsed were shorter than unbrowsed individuals of the same age. Hares suppress faster growing individuals that are initially taller by preferentially browsing them as they emerge above the winter snowpack. This reduced height, in combination with increased mortality associated with browsing, is predicted to slow the advance of both latitudinal and altitudinal treeline expansions and alter the structure of treeline forests. [ABSTRACT FROM AUTHOR]

Climate warming is expected to increase the seasonal duration of photosynthetic carbon fixation and tree growth in high-latitude forests. However, photoperiod, a crucial cue for seasonality, will remain constant, which may constrain tree responses to warming. We investigated the effects of temperature and photoperiod on weekly changes in photosynthetic capacity, leaf biochemistry and growth in seedlings of a boreal evergreen conifer, white spruce [ Picea glauca (Moench) Voss]. Warming delayed autumn declines in photosynthetic capacity, extending the period when seedlings had high carbon uptake. While photoperiod was correlated with photosynthetic capacity, short photoperiods did not constrain the maintenance of high photosynthetic capacity under warming. Rubisco concentration dynamics were affected by temperature but not photoperiod, while leaf pigment concentrations were unaffected by treatments. Respiration rates at 25 °C were stimulated by photoperiod, although respiration at the growth temperatures was increased in warming treatments. Seedling growth was stimulated by increased photoperiod and suppressed by warming. We demonstrate that temperature is a stronger control on the seasonal timing of photosynthetic down-regulation than is photoperiod. Thus, while warming can stimulate carbon uptake in boreal conifers, the extra carbon may be directed towards respiration rather than biomass, potentially limiting carbon sequestration under climate change. [ABSTRACT FROM AUTHOR]

We characterized the ectomycorrhizae (ECM) of planted white spruce (Picea glauca (Moench) Voss) seedlings as affected by mechanical site preparation (MSP) of clear-cut conifer-dominated boreal mixedwood forest. Relative abundance, richness, and composition of the ECM community were compared among untreated control, mixed, mounded, and scalped site preparation treatments. On >11 000 root tips, we observed 16 ECM morphotypes. Those common to the nursery in which the seedlings were raised were most abundant (Thelephora americana, Wilcoxina-like (E-strain), Amphinema byssoides, Phialocephala-like (MRA)). Seedlings in the untreated controls had lower abundances of these, but higher abundances of other ECM, which were not present in the nursery of origin but were indigenous to these forest stands. In terms of ECM composition, the “mixed” treatment was most similar to the untreated control, while the “scalped” and “mound” treatments showed significantly different ECM communities than the controls. Our results suggest that MSP may facilitate continued dominance by ECM that establish on seedlings in the nursery while slowing the natural succession towards the natural forest ECM. MSP treatments that leave some surface organic matter relatively intact may impact ECM less than those that remove or bury the organic layer. Nous avons caractérisé les ectomycorhizes (ECM) de plants d’épinette blanche (Picea glauca (Moench) Voss) affectés par la préparation mécanisée de terrain (PMT) à la suite d’une coupe à blanc dans une forêt mixte dominée par des résineux. L’abondance relative, la richesse et la composition de la communauté d’ECM a été comparée entre un témoin non traité et des traitements de préparation de terrain mixtes, d’aménagement de buttes et de scalpage. Nous avons observé 16 morphotypes d’ECM sur plus de 11 000 apex racinaires. Ceux qui étaient communs dans la pépinière où les semis avaient été produits étaient les plus abondants (Thelephora americana, le type Wilcoxina (souche E), Amphinema byssoides, le type Phialocephala (MRA)). L’abondance de ces ECM était plus faible chez les semis des témoins non traités qui comportaient, cependant, une plus forte abondance d’autres ECM qui n’étaient pas présentes dans la pépinière d’origine, mais qui étaient indigènes à ces peuplements forestiers. En termes de composition en ECM, le traitement mixte était le plus semblable au témoin non traité alors que le scalpage et l’aménagement de buttes étaient associés à des communautés d’ECM significativement différentes de celle du témoin. Nos résultats indiquent que la PMT peut faciliter le maintien de la dominance des ECM établies sur les plants en pépinière tout en ralentissant la succession naturelle vers les ECM de la forêt naturelle. Les traitements de PMT qui laissent des superficies relativement intactes de matière organique peuvent avoir moins d’impact sur les ECM que ceux qui enlèvent ou enfouissent la couche de matière organique. [ABSTRACT FROM AUTHOR]

This study examined the impact of increased irrigation efficiency on the hardening and frost tolerance of 2-year-old containerized white spruce seedlings in the context of groundwater protection, irrigation management and the maintenance of seedling quality in northern climates. The seedlings were grown under three different irrigation regimes (IR =30%, 40% and 55% v/v; cm3 H2O/cm3 substrate) and were hardened under conditions of natural photoperiod and temperature. After being subjected to artificial frost tests on four sampling dates during autumn, the seedlings were compared for bud development and frost tolerance. IR had no influence on frost tolerance as determined by measurements of physiological (electrolyte leakage, root water loss) and morphological (shoot damage, root initiation) variables. At the end of the second growing season, there was no significant difference between IRs in seedling height, root collar diameter, shoot dry mass and root dry mass. The results indicate that the amount of water applied to large-dimension 2-year-old white spruce seedlings during the growing season can be significantly decreased without prematurely impeding their growth or hindering their acquisition of frost tolerance. [ABSTRACT FROM AUTHOR]