Summary1. Disturbances and resource availability are key factors affecting plant diversity in managed forests. As disturbance regimes vary among silvicultural systems and may simultaneously affect different types of resources, effects on biodiversity can be unpredictable. 2. We compared the effects of two silvicultural systems on understorey plant diversity, including species composition, structural attributes and functional organization. One hundred and thirty-five phytosociological relevés were sampled from 27 forest stands managed under either a traditional coppice-with-standards (CWS, n = 12) or a 'close-tonature' selective cutting system (SC, n = 15), over similar edaphic conditions. Important environmental factors affecting vegetation were deduced using Ellenberg indicator values. Structural diversity was described using traditional indices of α and β diversity. Guilds were defined within the local pool of species using a set of 14 traits and their relationship with silviculture was assessed using correspondence analysis. 3. Post-logged CWS stands share some compositional and structural characteristics with selectively cut stands, including high species richness and a dominance of early successional species. However the species pool for all coppicing areas was higher than for selectively cut areas, suggesting that the high disturbance frequency occurring in the latter may progressively eliminate the most sensitive species. 4. Functional diversity strongly differs between the two systems. Although it is conserved through the silvicultural cycle in the coppice-with-standards system, some guilds were lacking in selectively cut stands. The most negatively impacted guilds were tree and shrub saplings, prostrated ruderals, shade-tolerant perennials and vernal geophytes. The latter two comprise 'true forest species' which may also be considered as 'coppicingmaintained species'. To reach the same values of guild richness (i.e. number of guilds) or redundancy (i.e. proportion of the maximal species richness within each guild), larger areas were required in SC compared with CWS systems. 5. In the SC system, the high proportion of light reaching the forest floor induced a spectacular spread of blackberries Rubus fruticosus agg., which decreased species richness. It also caused shifts in guild composition: graminoids and ferns grew strongly to the detriment of true forest species. 6. Synthesis and applications . Our results suggest long-term negative effects of selective cutting on both structural and functional plant diversity, compared with coppice-withstandards. Cutting intervals are shorter than recovery times, so that early successional species-dominated communities are maintained. Vernal geophytes and shade-tolerant perennials seem to be limited by the frequency of disturbance rather than by the severity of disturbance. We conclude that, from a biodiversity point of view, this 'close-to-nature' system does not cope with the objective of sustainable forest management. The rotations currently in use do not match n...

[1] As a part of the Arctic Ocean Model Intercomparison Project, results from 10 Arctic ocean/ice models are intercompared over the period 1970 through 1999. Models' monthly mean outputs are laterally integrated over two subdomains (Amerasian and Eurasian basins), then examined as functions of depth and time. Differences in such fields as averaged temperature and salinity arise from models' differences in parameterizations and numerical methods and from different domain sizes, with anomalies that develop at lower latitudes carried into the Arctic. A systematic deficiency is seen as AOMIP models tend to produce thermally stratified upper layers rather than the ''cold halocline'', suggesting missing physics perhaps related to vertical mixing or to shelf-basin exchanges. Flow fields pose a challenge for intercomparison. We introduce topostrophy, the vertical component of VÂr r r rD where V is monthly mean velocity and r r r rD is the gradient of total depth, characterizing the tendency to follow topographic slopes. Positive topostrophy expresses a tendency for cyclonic ''rim currents''. Systematic differences of models' circulations are found to depend strongly upon assumed roles of unresolved eddies.

[1] As a part of Arctic Ocean Intercomparison Project, results from five coupled physical and biological ocean models were compared for the Arctic domain, defined here as north of 66.6°N. The global and regional (Arctic Ocean (AO)-only) models included in the intercomparison show similar features in terms of the distribution of present-day water column-integrated primary production and are broadly in agreement with in situ and satellite-derived data. However, the physical factors controlling this distribution differ between the models. The intercomparison between models finds substantial variation in the depth of winter mixing, one of the main mechanisms supplying inorganic nutrients over the majority of the AO. Although all models manifest similar level of light limitation owing to general agreement on the ice distribution, the amount of nutrients available for plankton utilization is different between models. Thus the participating models disagree on a fundamental question: which factor, light or nutrients, controls present-day Arctic productivity. These differences between models may not be detrimental in determining present-day AO primary production since both light and nutrient limitation are tightly coupled to the presence of sea ice. Essentially, as long as at least one of the two limiting factors is reproduced correctly, simulated total primary production will be close to that observed. However, if the retreat of Arctic sea ice continues into the future as expected, a decoupling between sea ice and nutrient limitation will occur, and the predictive capabilities of the models may potentially diminish unless more effort is spent on verifying the mechanisms of nutrient supply. Our study once again emphasizes the importance of a realistic representation of ocean physics, in particular vertical mixing, as a necessary foundation for ecosystem modeling and predictions.

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Current large-scale sea ice models represent very crudely or are unable to simulate the formation, maintenance and decay of coastal landfast ice. We present a simple landfast ice parameterization representing the effect of grounded ice keels. This parameterization is based on bathymetry data and the mean ice thickness in a grid cell. It is easy to implement and can be used for two-thickness and multithickness category models. Two free parameters are used to determine the critical thickness required for large ice keels to reach the bottom and to calculate the basal stress associated with the weight of the ridge above hydrostatic balance. A sensitivity study was conducted and demonstrates that the parameter associated with the critical thickness has the largest influence on the simulated landfast ice area. A 6 year (2001)(2002)(2003)(2004)(2005)(2006)(2007) simulation with a 20 km resolution sea ice model was performed. The simulated landfast ice areas for regions off the coast of Siberia and for the Beaufort Sea were calculated and compared with data from the National Ice Center. With optimal parameters, the basal stress parameterization leads to a slightly shorter landfast ice season but overall provides a realistic seasonal cycle of the landfast ice area in the East Siberian, Laptev and Beaufort Seas. However, in the Kara Sea, where ice arches between islands are key to the stability of the landfast ice, the parameterization consistently leads to an underestimation of the landfast area.

In some coastal regions of the Arctic Ocean, grounded ice ridges contribute to stabilizing and maintaining a landfast ice cover. Recently, a grounding scheme representing this effect on sea ice dynamics was introduced and tested in a viscous‐plastic sea ice model. This grounding scheme, based on a basal stress parameterization, improves the simulation of landfast ice in many regions such as in the East Siberian Sea, the Laptev Sea, and along the coast of Alaska. Nevertheless, in some regions like the Kara Sea, the area of landfast ice is systematically underestimated. This indicates that another mechanism such as ice arching is at play for maintaining the ice cover fast. To address this problem, the combination of the basal stress parameterization and tensile strength is investigated using a 0.25° Pan‐Arctic CICE‐NEMO configuration. Both uniaxial and isotropic tensile strengths notably improve the simulation of landfast ice in the Kara Sea but also in the Laptev Sea. However, the simulated landfast ice season for the Kara Sea is too short compared to observations. This is especially obvious for the onset of the landfast ice season which systematically occurs later in the model and with a slower build up. This suggests that improvements to the sea ice thermodynamics could reduce these discrepancies with the data.

Despite the availability of several atmospheric reanalyses (e.g. ERA-Interim) there exists both considerable uncertainty in surface forcing fields for ice/ocean modelling and sensitivity to the choice of product used. Here we introduce a relatively high-resolution alternative forcing dataset for ice-ocean models derived from the Canadian Meteorological Centre's (CMC) global deterministic prediction system (GDPS). A set of daily 30 h reforecasts is produced using the GDPS 33 km resolution model providing hourly atmospheric forcing fields for the period 2002-2011. The CMC GDPS reforecasts (CGRF) are compared with ERA-Interim and several observational datasets to evaluate their suitability for forcing ocean models. In particular, the surface temperature, humidity and winds of the CGRF show equivalent biases to those found in ERA-interim. Moreover, the higher resolution of the CGRF permit a more detailed representation of atmospheric structures and topographic steering, resulting in finer-scale coastal features and wind-stress curl. Although the CGRF dataset is not a reanalysis and thus is expected to be less well constrained by available observations, its higher resolution and small bias make it an attractive alternative for forcing ice-ocean models.

ABsTrACT. A tidal model of the Canadian Arctic Archipelago was used to map the strength of the tidal currents, tidal mixing (h/U 3 ), and the vertical excursion associated with the tidal currents that drive water upslope and downslope. The hot spots in these quantities correspond to the location of many of the small polynyas in the archipelago, supporting the idea that the tidal currents make an important contribution to the dynamics of many of these recurring polynyas. The potential link with tidal mixing means that these locations may have enhanced plankton production in the summer.Key words: Canadian Arctic Archipelago, polynyas, tidal currents, tidal mixing, tidal mixing fronts résuMé. un modèle des marées de l'archipel Arctique canadien a servi à mapper la force des courants de marée, le mélange de marée (h/U 3 ) et l'excursion verticale associés aux courants de marée qui poussent l'eau en ascendant et en descendant. les points chauds de ces quantités correspondent à l'emplacement d'un grand nombre des petites polynies de l'archipel, ce qui vient appuyer l'idée selon laquelle les courants de marée jouent un rôle important dans la dynamique d'un grand nombre de ces polynies récurrentes. le lien susceptible d'exister avec le mélange de marée implique que la production de plancton à ces emplacements pourrait être rehaussée à l'été.Mots clés : archipel Arctique canadien, polynies, courants de marée, mélange de marée, fronts de mélange de marée Traduit pour la revue Arctic par nicole Giguère.