Summary1. Whether plant competition grows stronger or weaker across a soil fertility gradient is an area of great debate in plant ecology. We examined the effects of competition and soil fertility and their interaction on growth rates of the four dominant tree species in the sub-boreal spruce forest of British Columbia. 2. We tested separate soil nutrient and moisture indices and found much stronger support for models that included the nutrient index as a measure of soil fertility. 3. Competition, soil fertility and their interaction affected radial growth rates for all species. 4. Each species supported a different alternate hypothesis for how competitive interactions changed with soil fertility and whether competition intensity was stronger or weaker overall as soil fertility increased depended on the context, specifically, species, neighbourhood composition and type of competition (shading vs. crowding). 5. The four species varied slightly in their growth response to soil fertility. 6. Individual species had some large variations in the shapes of their negative relationships between shading, crowding and tree growth, with one species experiencing no net negative effects of crowding at low soil fertility. 7. Goodness-of-fit was not substantially increased by models including competition-soil fertility interactions for any species. Tree size, soil fertility, shading and crowding predicted most of the variation in tree growth rates in the sub-boreal spruce forest. 8. Synthesis. The intensity of competition among trees across a fertility gradient was species-and context-specific and more complicated than that predicted by any one of the dominant existing theories in plant ecology.

. 2012. Growth of aspen and white spruce on naturally saline sites in northern Alberta: Implications for development of boreal forest vegetation on reclaimed saline soils. Can. J. Soil Sci. 92: 213Á227. We examined height and basal area growth over time for trembling aspen and white spruce in plots along a salinity gradient at six naturally saline sites in northern Alberta, as a benchmark for forest productivity on reclaimed saline sites. We measured root distributions and analyzed foliage for ions, nutrients and carbon and nitrogen stable isotope ratios. Both species grew on soil conditions previously considered unsuitable for forest vegetation [pH !8.5; electrical conductivity !10 dS m (1 , sodium adsorption ratio!13 at depth (50Á100 cm)] yet there was little evidence of nutritional toxicities or deficiencies. Aspen basal area growth decreased 50% as salinity increased, but aspen was commercially productive (site index022) on soils with electrical conductivity of 7.8 dS m(1 at 50Á100 cm depth. Growth of white spruce seemed to be unaffected by salinity level differences, but 78% of white spruce site indexes were less than 13 and would be considered non-productive. Both species showed growth declines over time, compared with non-saline reference growth curves, and rooted primarily in the forest floor and top 20 cm of soil. This suggests that rooting limitations may constrain longer-term productivity of forests established on sites with salinity at depth.Key words: Salinity gradient, salt tolerance, forest growth, stem analysis, boreal forest, reclamation Lilles, E. B., Purdy, B. G., Macdonald, S. E. et Chang, S. X. 2012. Croissance du tremble et de l'e´pinette blanche sur des sites salins naturels dans le nord de l'Alberta : implications pour la croissance de la foreˆt bore´ale sur des sols salins restaure´s. Can. J. Soil Sci. 92: 213Á227. Les auteurs ont examine´la hauteur et la croissance de la surface terrie`re de trembles et d'e´pinettes blanches poussant sur des parcelles situe´es le long d'un gradient de salinite´, a`six sites naturellement salinise´s du nord de l'Alberta, en vue d'e´tablir des points de comparaison pour la productivite´forestie`re sur les sites salins restaure´s. Ils ont pour cela mesure´la distribution des racines et analyse´le feuillage afin d'e´tablir la concentration d'ions et d'e´le´ments nutritifs ainsi que le ratio des isotopes stables de carbone et d'azote. Les deux essences croissaient sur des sols qu'on pensait jusque la`impropres a`la ve´ge´tation forestie`re (pH !8,5; conductivite´e´lectrique !10 dS par me`tre, ratio d'adsorption du sodium !13 a`une profondeur de 50 a`100 cm). Pourtant, les arbres pre´sentaient peu d'indices de toxicite´ou de carence nutritives. La croissance de la surface terrie`re du tremble diminue de 50 % avec la hausse de la salinite´, mais l'espe`ce demeure commercialement productive (indice du site 022) sur les sols dont la conductivite´e´lectrique est de 7,8 dS par me`tre, a`une profondeur de 50 a`100 cm. La croissance de l'e´pinette blanche ne semble ...

The combined effects of light, soil fertility, and ontogenetic changes on plant growth rates are poorly understood, yet these three factors play fundamental roles in structuring plant communities. We sought to determine how lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia), interior spruce (Picea glauca × engelmanii (Moench) Voss), and subalpine fir (Abies lasiocarpa (Hook.) Nutt.) sapling growth responds to the combination of light, soil fertility, and ontogeny and how these three dominant conifer species in sub-boreal forests of British Columbia differ in their responses. Using maximum likelihood methods, we found that 0.20-4 m tall sapling growth rates changed during ontogeny and were limited by both light and soil resources. The strongest differences among species' growth rates were due to tree size, with smaller differences due to soil fertility, and there were no differences among species in the shape of their growth responses to light. Rank order in growth rates for small saplings (pine > spruce > fir) inversely corresponded to classic shade-tolerance ratings, thus supporting the carbon balance theory. Interior spruce height growth rates increased relative to lodgepole pine with increasing soil fertility, clearly matching the landscape-scale increase in canopy dominance of interior spruce over lodgepole pine with increasing soil fertility.Résumé : On comprend mal les effets combinés de la lumière, de la fertilité du sol et des changements ontogénétiques sur le taux de croissance des plantes bien que ces trois facteurs jouent un rôle fondamental dans l'établissement de la structure des communautés végétales. Nous avons cherché à déterminer comment la croissance des gaules de pin tordu (Pinus contorta Dougl. ex Loud. var. latifolia), d'épinette de l'intérieur (Picea glauca × engelmanii (Moench) Voss et de sapin subalpin (Abies lasiocarpa (Hook.) Nutt.) réagit à la combinaison de lumière, de fertilité du sol et d'ontogénie et de quelle façon la réaction de ces trois espèces dominantes de conifères diffère dans les forêts subboréales de la Colombie-Britannique. À l'aide des méthodes du maximum de vraisemblance, nous avons trouvé que le taux de croissance des gaules de 0,20 à 4 m de haut a changé durant l'ontogénie et était limité par la disponibilité de la lumière et des ressources du sol. Les plus grandes différences entre le taux de croissance des espèces étaient dues à la dimension des arbres et dans une moindre mesure à la fertilité du sol; il n'y avait pas de différence entre les espèces quant à la forme de leur réaction de croissance à la lumière. L'ordre du classement du taux de croissance des petites gaules (pin > épinette > sapin)

We developed models to describe the responses of four commonly examined leaf traits (mass per area, weight, area and nitrogen (N) concentration) to gradients of light, soil nutrients and tree height in three conifer species of contrasting shade tolerance. Our observational dataset from the sub-boreal spruce forests of British Columbia included subalpine fir (Abies lasioscarpa [Hook.] Nutt; high shade tolerance), interior spruce (Picea glauca × Picea engelmannii [Moench] Voss; intermediate shade tolerance) and lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia; low shade tolerance) saplings from 0.18 to 4.87 m tall, in 8-98% of total incident light, from field sites with <17.6 kg ha(-1) to >46.8 kg ha(-1) total dissolved N. Leaf weights and areas showed strong positive responses to light and height, but little or no response to soil nutrients. Parameter estimates indicated that the shape of leaf weight and area responses to light corresponded with shade tolerance ranking for the three species; pine had the most linear response whereas spruce and fir had asymptotic responses. Leaf N concentration responded positively to soil nutrients, negatively to light and idiosyncratically to height. The negative effect of light was only apparent on sites of high soil nutrient availability, and parameter estimates for the shape of the negative response also corresponded to shade tolerance ranking (apine = -0.79, aspruce = -0.15, afir = -0.07). Of the traits we measured, leaf mass per area showed the least response to light, soil nutrient and height gradients. Although it is a common practice in comparisons across many species, characterizing these conifers by mean values of their leaf traits would miss important intraspecific variation across environmental and size gradients. In these forests, parameter estimates representing the intraspecific variability of leaf trait responses can be used to understand relative shade tolerances.

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