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Summary1 Facilitation between neighbouring plants can promote species survival and regulate community composition. However, the role of facilitation varies along environmental severity gradients. It is important to understand the shape of this relationship to improve our ability to predict the impact of a changing environment on biodiversity. 2 We used Scots pine saplings growing within heather to examine the shape of the relationship between facilitative interactions (protection from browsing) and the severity of the environment (deer browsing intensity). We also investigated whether protection from browsing translated into a biomass response of saplings. 3 In the first winter following planting heather had a facilitative effect on saplings by reducing the probability of browsing. This effect was strongest at intermediate deer browsing intensities, thus producing a hump-backed relationship between facilitative effects and the severity of the environment. 4 Protection from browsing did not lead to longer-term biomass gains for the saplings. The competitive effects of heather on sapling growth therefore outweighed the beneficial effects of protection from browsing. 5 These results provide much-needed information on the shape of the severity-interactions relationship with respect to a key natural disturbance phenomenon (herbivory), and demonstrate that an observable interaction relationship does not necessarily translate into a biomass response. 6 This illustrates the complex and potentially transient nature of plant-plant interactions, and the potential difficulty that would be associated with using shelter effects of heather as a management tool to promote Scots pine regeneration.

Abstract:The impact of forest management operations on soil physical properties is important to understand, since management can significantly change site productivity by altering root growth potential, water infiltration and soil erosion, and water and nutrient availability. We studied soil bulk density and strength changes as indicators of soil compaction before harvesting and 1 and 5 years after harvest and site treatment on 12 of the North American Long-Term Soil Productivity sites. Severe soil compaction treatments approached root-limiting bulk densities for each soil texture, while moderate compaction levels were between severe and preharvest values. Immediately after harvesting, soil bulk density on the severely compacted plots ranged from 1% less than to 58% higher than preharvest levels across all sites. Soil compaction increases were noticeable to a depth of 30 cm. After 5 years, bulk density recovery on coarse-textured soils was evident in the surface (0-10 cm) soil, but recovery was less in the subsoil (10-30 cm depth); fine-textured soils exhibited little recovery. When measured as a percentage, initial bulk density increases were greater on finetextured soils than on coarser-textured soils and were mainly due to higher initial bulk density values in coarse-textured soils. Development of soil monitoring methods applicable to all soil types may not be appropriate, and more sitespecific techniques may be needed for soil monitoring after disturbance. 564Résumé : Il est important de comprendre l'impact des interventions dictées par l'aménagement forestier sur les propriétés physiques du sol étant donné qu'elles peuvent modifier de façon significative la productivité d'une station en altérant le potentiel de croissance des racines, l'infiltration d'eau, l'érosion du sol et la disponibilité de l'eau et des nutriments. Les auteurs ont étudié les changements dans la résistance et la densité apparente du sol en tant qu'indicateurs de la compaction du sol avant la récolte ainsi qu'un et 5 ans après la récolte et la préparation du terrain dans 12 stations du projet nord-américain de productivité des sols à long terme. Les traitements de compaction sévère du sol s'approchaient de la densité apparente inappropriée pour les racines pour chaque texture de sol alors que les degrés modérés de compaction du sol se situaient entre des valeurs allant de sévères à celles obtenues avant la récolte. Immédiatement après la récolte dans les parcelles où la compaction du sol était sévère, la densité apparente du sol variait de 1 % moins élevée à 58 % plus élevée qu'avant la récolte pour l'ensemble des sites. L'augmentation de la compaction du sol était observable jusqu'à une profondeur de 30 cm. Après 5 ans, le rétablissement de la densité apparente dans les sols à texture grossière était évident en surface (0-10 cm) mais pas aussi évident en profondeur (10-30 cm); presque aucun rétablissement n'était apparent dans les sols à texture fine. Mesurée en pourcentage, l'augmentation de la densité apparente initiale était plus for...

We examined fifth-year seedling response to soil disturbance and vegetation control at 42 experimental locations representing 25 replicated studies within the North American Long-Term Soil Productivity (LTSP) program. These studies share a common experimental design while encompassing a wide range of climate, site conditions, and forest types. Whole-tree harvest had limited effects on planted seedling performance compared with the effects of stem-only harvest (the control); slight increases in survival were usually offset by decreases in growth. Forest-floor removal improved seedling survival and increased growth in Mediterranean climates, but reduced growth on productive, nutrient-limited, warm–humid sites. Soil compaction with intact forest floors usually benefited conifer survival and growth, regardless of climate or species. Compaction combined with forest-floor removal generally increased survival, had limited effects on individual tree growth, and increased stand growth in Mediterranean climates. Vegetation control benefited seedling growth in all treatments, particularly on more productive sites, but did not affect survival or alter the relative impact of organic matter removal and compaction on growth. Organic matter removal increased aspen coppice densities and, as with compaction, reduced aspen growth.

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