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Abstract: Distylous species typically experience self-incompatibility with one morph often having partial selfcompatibility Small populations may therefore experience greater rates of selfing/intramorph crosses leading to skewed morph ratios and reduced [...]

We explored interactions among plant growth forms in the understory of mature boreal mixedwood forests in western Canada by investigating the competitive influence of erect shrubs on herbs (forbs and grasses). We established 10 pairs of plots; all erect shrubs were removed in one plot of each pair (removals) and left intact in the other plot (controls). Two years later, we harvested all aboveground biomass of the herbaceous layer (herb biomass: this included graminoids, forbs, trailing shrubs, and species with a woody base but not woody stems) from the 20 plots. We tested for significant differences in understory species biomass and composition between control and removal plots and examined the influence of 25 environmental factors on species composition of the herbaceous layer. Competition intensity was measured by the natural logarithm of response ratio (In RR) index based on herb biomass. After erect shrub removal, there was a significant increase in herb biomass, mostly due to an increase of the most common species (e.g.. Cornus canadensis Linnaeus, Linnaea borealis Linnaeus). The values of competition intensity (In RR) varied among herb species but were, overall, positive, indicating a release from competition following shrub removal. Composition of the herbaceous layer was significantly different between removal and control plots and was also significantly related to seven environmental factors, which explained 40% of the variation in composition. Our study suggests that there is asymmetric competition for light between erect shrub and herb species in boreal ecosystems. Key words: removal experiment, functional diversity, understory community, species interactions, dominant species, boreal forest. Les auteurs ont etudie les interactions entre les formes de croissance des plantes en sous etage de forets boreales mixtes matures dans l'ouest du Canada, en examinant l'influence competitive des arbustes dresses et des herbacees. Ils ont etabli 10 paires de parcelles, supprimant tons les arbustes dresses dans une parcelle de chaque paire (supprimes) et en laissant intacte l'autre parcelle (temoins). Deux ans plus tard, ils ont recolte toute la biomasse epigee de la strate herbacee (biomasse herbacee; incluant les plantes gramineennes et non-gramineennes, les arbustes rampants et les especes munies d'une base ligneuse mais sans tige ligneuse) des 20 parcelles. Ils ont effectue des lests de differences significatives pour la biomasse des especes de sous etage et la composition entre les parcelles temoins et de suppression, pour examiner l'influence de 25 facteurs environnementaux sur la composition en especes de la strate herbacee. Ils ont mesure l'intensite de la competition par le log du Rapport de Reaction (In RR), un index base sur la biomasse herbacee. Suite a la suppression des arbustes dresses, on observe une augmentation significative de la biomasse herbacee, due en grande partie a une augmentation des especes les plus communes (e.g. Cornus canadensis Linnaeus, Linnaea borealis Linnaeus). La valeur de l'intensite de la competition (In RR) varie entre les especes herbacees, mais demeure dans l'ensemble positive, ce qui indique une liberation de la competition suite a l'enlevement des arbustes. La composition de la strate herbacee s'avere significativement differente, entre les parcelles avec suppression et temoins, et montre egalement une relation significative avec sept facteurs environnementaux, lesquels expliquent 40 % de la variation de la composition. Les resultats suggerent l'existence d'une competition asymetrique pour la lumiere entre les especes arbustives dressees et herbacees, dans les ecosystemes boreaux. Mois-cles : experience d'elimination, diversite fonctionnelle, communaute de sous etage, interactions interspecifiques, especes dominantes, foret boreale. [Traduit par la Redaction], Introduction Interactions among plant species play an important role in regulating composition of local communities and ecosystems (Brooker 2006) and in mediating ecosystem functioning when a given species or functional [...]

Epiphytic mosses and liverworts contribute substantially to the bryophyte diversity of circumpolar boreal forests but are susceptible lo altered growing conditions after forest harvesting. Management practices that retain some trees after harvest may enhance epiphyte survival; however, the effectiveness or this emerging method needs lo be assessed. We examined the survival, composition, and nearest neighbour relationships of epiphytic bryophytes on trembling aspen (Populus tremuloides Michx.) across a range (10%-100%) of dispersed green-tree retention 5 years after harvest in boreal mixed- wood forest. Growth of the forest floor moss Hylocomium splendens (Hedw.) Schimp. in B.S.G. was used as an indicator of changes in moisture availability for epiphytes following harvesting. Epiphyte richness and abundance increased with canopy retention and were positively correlated with local abundance of coniferous trees. Positive associations among neighbouring species in intact forest demonstrated that interspecies relationships form naturally. However, there was a shift in species composition after harvesting and fewer interspecific associations with declining retention. These trends were accompanied by reduced Hylocomium splendens growth, which implies that moisture may be an important driver of epiphyte response. Although different levels of canopy retention were similarly capable of maintaining some epiphytes, the loss of species associated with intact forest will require consideration of alternative management practices for their conservation. Key words: corticolous, Hylocomium splendens, liverwort, moss, poikilohydric. variable retention. Les mousses et les hepatiques epiphytes contribuent subsiantiellement a la diversite des bryophytes des forets boreales circumpolaires, mais sont sensibles aux modifications des conditions de croissance, suite aux operations forestieres. Les methodes d'amenagement qui conservent certains arbres apres la recolte peuvent ameliorer la survie des epiphytes; cependant, on doit evaluer l'efficacite de ces methodes en emergence. Les auteurs ont examine la survie, la composition et les relations avec les voisins les plus rapproches des bryophytes epiphytes sur le peuplier faux-tremble (Populas tremuloides Michx.) pour l'ensemble d'une aire (10% a 100%) comportant des arbres vivants conserves et disperses, 5 ans apres la recolte dans une foret boreale mixte. Ils ont utilise la croissance d'une mousse du parterre forestier, l'Hylocomium splendens (Hedw.) Schimp, in B.S.G., colome indicatrice des modifications de la disponibilite de l'humidite pour les epiphytes, suite a la recolte des arbres. La richesse et l'abondance des epiphytes augmentent avec la retention de la canopee et montrent des correlations positives avec l'abondance localisee de coniferes. Les associations positives avec les especes avoisinantes dans la foret intacie demontrent que des relations interspecifiques se forment nalurellement. Cependant, on observe un deplacement de la composition en especes apres la recolte et moins d'associations interspecifiques avec une diminution de la retention. Ces tendances s'accompagnent d'une reduction de la croissance de l'Hylocomium splendens, ce qui implique que l'humidite pourrait etre un agent causal important de la reaction des epiphytes. Bien que differents degres de conservation de la canopee peuvent etre egalement aptes a maintenir certaines epiphytes, la perte d'especes associees aux forets intactes necessitera la definition de methodes alternatives d'amenagement pour leur conservation. Mots-cles: corticole, Hylocomium splendens, hepatiques, mousse, poikilohydrique, retention variable. [Traduit par la Redaction], Introduction Forest management practices that retain a greater volume of standing trees after harvest are gradually becoming more widely accepted in place of traditional clearcut methods in North American and [...]