This paper reviews the effects that resource manipulations, especially nutrient additions, have on productivity, diversity, species interactions and succession in natural and semi-natural low-growth-form terrestrial restrial vegetation. In most studies, fertilization has resulted in an increase in productivity with a concomitant decrease in species diversity. Plant community productivity in nutrient-poor habitats (e.g. arctic tundra) however, has been relatively little affected by nutrient additions. Results from several fertilization experiments in nutrient-poor habitats indicate that competition may be an important component of community dynamics. Resource additions generally do not affect co-occurring species equally. In many cases, this results from an alteration of the relative competitive abilities of species. Several short-term fertilization studies have suggested that the rate of succession is increased by the addition of nutrients. Opposite trends however, have been reported in long-term studies. Contradictory results among some resource manipulation studies appear to stem largely from design limitations (e.g. only one resource was manipulated). Variability in habitat fertility, the type of resource added and the duration of enrichment also contribute to the difficulties involved in interpreting and comparing field experiments. A pluralistic approach combining glasshouse studies with both neighbour and resource manipulations in the field can be expected to lead to a greater depth of understanding of current controversies over the effects of habitat fertility on the intensity of competition and the relationship between competitive ability and tolerance of resource impoverishment.

Macronutrient inputs to annual cropping systems can benefit weeds as well as crops, sometimes decreasing or eliminating the benefits of fertilization. This interaction between fertility management and integrated weed management is becoming increasingly important as these fields increase their focus on efficiency and prevention, respectively. The risk of increased weed competition reflects the fact that weed biomass and height may be highly responsive to nitrogen, phosphorus, and/or potassium. This generalization is supported by monoculture studies of species such as redroot pigweed (Amaranthus retroflexusL.), common lambsquarters (Chenopodium albumL.), and barnyardgrass [Echinochloa crus-galli(L.) P. Beauv.] and by ecological theory. However, field studies indicate variation in the effects of macronutrients on weed–crop competition and crop yield, even within species groups. To address challenges in interpreting, comparing, and extrapolating from these diverse reports, we propose a conceptual framework that summarizes the mechanisms underlying observed variation within and between studies. This framework highlights functional traits and trends that help predict yield outcomes in binary weed–crop interactions. Important factors include timing of emergence, maximum heights of the weed and crop, and relative responsiveness to the added nutrient. We also survey recent work on the effects of nutrient source (e.g., the composition of organic amendments) on weed–crop competition. Because different sources vary in their nutrient release dynamics and supplied nutrient ratios, they may have dramatically different effects on weed–crop competition and crop yield. Finally, we offer a guide to best practices for studies of fertility effects on weed–crop competition. Although this review highlights several topics requiring further research, including fertility effects on multispecies interactions and interactions with other environmental factors, emerging methods offer considerable promise. Ultimately, an improved understanding of nutrient effects on weed–crop competition will contribute to the efficient and effective management of diverse cropping systems.

Cynanchum rossicum (dog-strangling vine) and C. louiseae (black dog-strangling vine) are introduced, perennial herbs or small twining vines in the Milkweed family (Asclepiadaceae). Generic placement of these two species has been problematic, but are here treated as members of Cynanchum subgenus Vincetoxicum. The species occur primarily in natural upland areas, including understories of woodlands, pastures, old fields, shores, flood plains and ruderal areas of southern Ontario and Quebec and the northeastern United States. Ecosystems on well-drained, stony soils are often densely colonized, but both species can tolerate a wide moisture regime. Plants establish in full sun or under forest canopies and may form monospecific stands in all light conditions. They often form dense colonies which smother other vegetation and reduce invertebrate and vertebrate biodiversity. Reproduction is by polyembryonic, wind-dispersed seeds. Effective control is primarily by herbicides, since mechanical control is difficult and no biological control agents have been developed. A third European species, C. vincetoxicum, has been reported as an occasional garden escape in southern Ontario and the northeastern United States, but has not yet become naturalized. Information on this species is included because of its close relationship with C. rossicum and its better known biology. Key words: Dog-strangling vine, swallow-wort, Cynanchum, Vincetoxicum, CYKNI, invasive plant, weed biology