We have developed extensions of traditional distance-dependent, spatial competition analyses that estimate the magnitude of the competitive effects of neighboring trees on target tree growth as a function of the species, size, and distance to neighboring trees. Our analyses also estimate inter- and intra-specific competition coefficients and explicitly partition the competitive effects of neighbors into the effects of shading versus crowding. We tested the method using data from forests of northern, interior British Columbia dominated by western hemlock (Tsuga heterophylla (Raf.) Sarg.) and western redcedar (Thuja plicata Donn ex D. Don). For both species, the most parsimonious regression models included terms for the effects of tree size, crowding, and shading and separate competitive effects of four different groups of competing species. The models explained 33%–59% of the variation in radial growth of the two species. For both species, growth declined more steeply as a function of crowding than shading. There was striking asymmetry in the strength of interspecific competition between hemlock and redcedar, with crowding by hemlock having a strong per capita effect on redcedar, while crowding by redcedar had relatively little effect on the radial growth of hemlock.

Summary 1.The neutral theory debate has highlighted the scarcity of robust empirical estimates of the magnitude of competitive effects and responses within guilds of co-occurring tree species. Our analysis quantifies the relative magnitude of all possible pairwise competitive interactions within a guild of nine co-occurring tree species in temperate forests of northern, interior British Columbia, and explicitly partitions the competitive effects of neighbours into the effects of shading versus the residual effects of 'crowding', assumed to reflect below-ground competition. 2. Models that treated neighbours as equivalent in their competitive effects were the most parsimonious for the five species with the smallest sample sizes. For the remaining species (samples sizes of > 150 individuals), the best models estimated separate competition coefficients for all nine species of neighbours. We take this as evidence that species do indeed differ in their competitive effects, but that there can be a minimum sample size required to discriminate between them. 3. There was a strong size-dependency in potential growth. Six species showed an optimal growth at a size between 5 and 20 cm diameter. Potential growth declined moderately to strongly as diameter increased. Sensitivity to crowding varied as a function of tree size for five of the nine species; however, this response was not consistent by tree species. 4. The magnitude of reduction in growth due to crowding was greater on average than the reduction in growth due to shading, except for the two least shade tolerant conifers. Sensitivity to shading among the conifer species was correlated with their shade tolerance. 5. The per capita effects of crowding by different species of neighbours varied widely. A large number of the estimated pairwise per capita competition coefficients were very low. The relative magnitude of the strength of intra-versus interspecific competition also varied widely among the tree species. 6. Synthesis . Model selection techniques effectively separated above-and below-ground competition in complex forests, and allowed us to assess differences among species in competitive effects and responses. While below-ground effects were strong, they were due to proximity of neighbours from a very specific (and small) subset of strong competitors within the guild. Response to crowding varied with tree size but the nature of the relationship varied widely among the species.

Summary 1We compared three commonly used empirical seed/seedling dispersal functions for trees (lognormal, 2Dt, by analysis of published studies where the location of the source is known, as well as by inverse modelling within an old growth hardwood forest in southern Quebec. Almost all the species were wind-dispersed. 2 For the discrete source studies, the lognormal was clearly superior, while for the inverse modelling the performance of the three dispersal functions was somewhat more even. We speculate that collisions with boles spuriously enhanced the likelihood of the 2Dt and the Weibull with inverse modelling, as both these functions assume that the greatest seed/seedling density will occur at the base of the maternal parent bole. 3 We conclude that the lognormal function is to be preferred because, as well as providing a framework for mechanistic interpretation, it tends to provide a closer approximation to observed dispersal curves. 4 We also argue that mean distances travelled by seed crops are far more extensive than indicated by previous studies that used the Weibull function.

We examine the influence of (i) the spatial distribution and abundance of parent trees (as seed sources) and (ii) the abundance and favourability of seedbed substrates, on seedling recruitment for the major tree species in northwestern interior cedar-hemlock forests of British Columbia, under four levels of canopy openness (full canopy, partial canopy, large gap, and clearcut). Substrate distribution varied with canopy openness, and substrate favourability was a function of both canopy openness and seedling species. Lack of suitable substrates was the predominant factor limiting seedling density under full canopies. Partial canopy and gap sites provided a broad range of favourable substrates in close proximity to parent trees, resulting in the highest observed seedling densities. There was much higher effective dispersion of seedlings away from parent trees in gaps than in the partially cut stands. Seedling dispersion to clearcut sites was poor with seedlings being tightly restricted to a narrow band along the forest edge. Thus, seedling recruitment in these forests was a reflection of the interaction between the abundance of seed and substrate favourability, and the relative importance of these factors varied significantly with canopy structure. Résumé : Les auteurs ont examiné l'influence (i) de la répartition spatiale et de l'abondance d'arbres parentaux consi-dérés comme sources de semences et (ii) de l'abondance et de la convenance du substrat des lits de germination, sur le recrutement des semis des principales espèces d'arbres dans les forêts de thuya et de pruche de la partie nord-ouest de l'intérieur de la Colombie-Britannique, en fonction de quatre niveaux d'ouverture du couvert (couvert complet, couvert partiel, grande ouverture et coupe à blanc). La répartition du substrat variait avec le degré d'ouverture du couvert et la convenance du substrat était fonction à la fois de l'ouverture du couvert et de l'espèce du semis. Le manque de subs-trats appropriés était le principal facteur qui limitait la densité des semis sous un couvert complet. Les plus hautes den-sités de semis ont été observées sous couvert partiel et dans les ouvertures qui procuraient un large éventail de substrats appropriés dans l'environnement immédiat des arbres parentaux. La dispersion des semis loin des arbres pa-rentaux était beaucoup plus efficace dans les ouvertures que dans les peuplements partiellement coupés. La dispersion des semis dans les sites coupés à blanc était faible, les semis étant limités à une bande étroite située le long de la li-sière de la forêt. Ainsi, le recrutement des semis dans ces forêts était le résultat de l'interaction entre l'abondance des semences et la convenance du substrat, et l'importance relative de ces facteurs variait de façon significative selon la structure de la canopée. [Traduit par la Rédaction] LePage et al. 427

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