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Vegetation-management practices are applied in temperate-zone forests on the assumption that changing the competitive environment between conifers and unwanted vegetation will improve conifer productivity. We review this assumption using research examining interactions between paper birch (Betula papyrifera Marsh.) and conifers in the highly productive Interior Cedar Hemlock zone of British Columbia. We have found that both competition and facilitation are important in young plantations, where paper birch competes for light, reducing growth of shade-intolerant conifers, but having a facilitative effect on shade-tolerant conifers. This facilitative effect may result from greater ectomycorrhizal diversity, population sizes of Armillaria ostoyae (Romagn.) Herink antagonistic bacteria, and associative nitrogen fixation in plantations where interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) is mixed with paper birch. Where paper birch is manually cut or girdled, conifers grow faster in diameter, but more die as a result of A. ostoyae root disease, and these responses increase with increasing weeding intensity. The weeding treatments do not affect plant community species richness but reduce paper birch dominants and increase understory structural diversity. British Columbia forest policy has been slow to respond to these findings, and we suggest that as a result, the forested landscape incurs substantial risk. We propose additional pathways for managing Interior Cedar Hemlock mixtures to ensure that the natural mix of forest types in the landscape is maintained.

We studied the regeneration niche of Picea engelmannii and Abies lasiocarpa in wet sub-zones of the Engelmann spruce-subalpine fir (ESSF) zone of southern British Columbia. We characterized the niche on transects from clearcuts into adjacent old-growth stands, in terms of aspect, elevation, micro-relief, light, seedbed, canopy cover, and competing vegetation. Fourteen transects were sampled at four watersheds. The sampling design ensured similar representation of small (1-30 cm), medium (31-100 cm) and tall (101-300 cm) saplings, and of both species. In total, 465 Picea and 623 Abies saplings were sampled. For every third sapling, an adjacent microsite with no sapling was selected using a random procedure and correspondingly characterized, enabling a pairwise comparison of microsites with and without saplings. Recruitment of both species was related to rotten wood seedbeds, i.e., mainly logs, but also stumps and root plates. This effect was stronger for Picea saplings, and it decreased with increasing sapling size, suggesting lower survival on rotten wood seedbeds. A higher occurrence of tall Abies saplings, in comparison to small ones, under the cover of canopy trees suggests higher survival under canopy than in gaps. Levels of direct light did not generally differ either between microsites with and without regeneration, or between Picea and Abies saplings. The microsite effects on sapling occurrence were quite consistent across gentle south-and north-facing slopes, upper and lower ESSF forests, sites, and along clearcut -old-growth gradients. The results suggest a strong dependence of Picea and Abies establishment on logs in ESSF forests.Key words: seedling establishment, microsite, seedbed, nurse logs, direct light, advance regeneration, ecological niche Nous avons étudié la niche de régénération de Picea engelmannii et d'Abies lasiocarpa dans les sous-zones humides de la zone de l'épinette d'Engelmann -sapin sub-alpin (ESSF) dans le sud de la Colombie-Britannique. Nous avons caractérisé la niche selon des tracés transversaux allant des coupes à blanc vers les vieux peuplements adjacents, au niveau de l'aspect, de l'altitude, du micro-relief, de la luminosité, du milieu de germination, du couvert des cimes et de la végétation concurrente. Quatorze tracés ont été échantillonnés dans quatre bassins versants. La conception de l'échantillonnage assurait une représentation similaire des semis de petite taille (1 -30 cm), de taille moyenne (31 -100 cm) et de grande taille (101 -300 cm) et des deux espèces. Au total, 465 semis de Picea et 623 semis d'Abies ont été échantillonnés. À chaque troisième semis, un microsite adjacent sans semis a été sélectionné au moyen d'une procé-dure aléatoire et caractérisée de la même façon, permettant une comparaison par éléments pairés des microsites ayant et n'ayant pas de semis. Les recrues des deux espèces étaient reliées à un milieu de germination composé de bois en décomposition, c'est-à-dire, principalement des billots, mais également des souches et des mottes. Cet effet étai...

We used transect surveys at a large-scale experimental site at Sicamous Creek, B.C., to measure the effects of five treatments on windthrow: 10-ha clearcuts, arrays of 1-ha patch cuts, arrays of 0.1-ha patch cuts, individual-tree selection cuts, and uncut controls. We also examined edge effects and conditions predisposing trees to windthrow. Windthrow of subalpine fir (Abies lasiocarpa (Hook.) Nutt.) in the 2.7 years following harvesting increased from 0.6% of basal area per year in uncut forest to 0.8-1.8% per year in harvested treatments, with highest rates in individual tree selection units and lowest rates in 0.1-ha patch-cut arrays. Engelmann spruce (Picea engelmannii Parry ex Engelm.) showed similar patterns of windthrow but lower rates (0.2-0.7% of basal area per year in harvested treatments). Windthrow was concentrated near north and east edges of 1-ha and 10-ha openings but was dispersed throughout the more uniform treatments. Windthrown trees did not differ from random trees in diameter but had lower height/diameter ratios, probably reflecting the greater windthrow observed in subxeric sites on complex, elevated topography. The rates and distribution of windthrow in different harvest treatments have implications for ecological processes, salvage, long-term windthrow potential, and mitigation possibilities.

The effects of partial cutting on seedling growth of three conifer species were studied at a very dry, hot interior Douglasfir site near Kamloops, British Columbia. Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco), lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.), and ponderosa pine (Pinus ponderosa Dougl. ex. P. & C. Laws.) seedlings were planted in mechanically prepared 50 cm ϫ 50 cm patches under different canopy conditions created by harvesting 60% of the original stand volume. The prepared areas were selected to represent canopy closures from open to closed, slopes from 0 to 60%, and all aspects. After six years, survival of Douglas-fir, lodgepole pine and ponderosa pine was 78%, 76% and 70%, respectively. Light level had a strong influence on survival and condition. Growth of all species increased linearly with light, and was greatest for lodgepole pine, followed by ponderosa pine and Douglas-fir. Multiple regression analysis showed that six-year seedling size was most significantly affected by total light, and only occasionally by aspect, slope, or crown closure. The best models explained 53%, 47% and 42% of the variation in diameter of lodgepole pine, ponderosa pine, and Douglas-fir, respectively. Natural abundance 13 C was positively correlated with light and soil moisture availability, reflecting higher photosynthetic capacity of all species in the wetter, open canopy conditions. Patterns in isotopic discrimination also indicated greater water use efficiency of Douglas-fir and ponderosa pine than lodgepole pine under low light conditions. Underplanting stands thinned to a basal area of less than 15m 2 per ha offers a solution to regeneration difficulties on hot, dry Interior Douglas-fir sites.Key words: partial cutting, Douglas-fir, lodgepole pine, ponderosa pine, light, soil moisture, 13 C, growth, survival, Opax Mountain Silvicultural Systems Project RÉSUMÉLes effets d'une coupe partielle sur la croissance des semis de trois espèces de conifères ont été étudiés sur une station de sapin Douglas très sèche et chaude de l'Intérieur près de Kamloops en Colombie-Britannique. Des semis de sapin Douglas (Pseudotsuga menziesii var. glauca (Beissn.) Franco), de pin lodgepole (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.)et de pin ponderosa (Pinus ponderosa Dougl. ex. P. & C. Laws.) ont été plantés dans des parcelles préparées mécaniquement de 50 cm ϫ 50 cm sous différentes conditions de couvert forestier créées par la récolte de 60 % du volume initial du peuplement. Les parcelles préparées ont été choisies dans le but de représenter le niveau de fermeture du couvert forestier allant d'ouvert à fermé, sur des pentes de 0 à 60 % selon tous ces aspects. Après six ans, la survie des semis de sapin Douglas, de pin lodgepole et de pin ponderosa était respectivement de 78 %, 76 % et de 70 %. Le niveau de luminosité a eu une influence déterminante sur la survie et l'état des semis. La croissance de toutes les espèces a augmenté linéairement en fonction de la lumi...