Exponential nutrient additions have been shown to induce steady‐state nutrition in plants and enhance seedling growth and nutrient status, which suggests improved nutrient efficiencies during the growing season. The objective of this study was to compare periodic growth, plant nutrient status, and N uptake of desert mesquite (Prosopis chilensis Mol.) seedlings that received 200 mg N seedling−1 as complete fertilizer delivered either conventionally (as single dose or constant top dressing) or exponentially (as pure or modified exponentially increasing additions) during a 12‐wk rotation. Steady‐state nutrition (characterized by a stable tissue nutrient concentration) was induced for the cultural period in the modified‐exponential fertility regime. The nutrients were supplied exponentially except for higher initial amounts that compensated for the incomplete accessibility of growing medium to the small root systems. These seedlings showed consistently higher N accumulation that matched dry‐matter production, reflecting higher fertilizer‐N uptake efficiency throughout the growing season. During the same growing season, pure exponential nutrient‐addition rates appeared to achieve steady‐state conditions 4 wk later because of incomplete interception of applied nutrients by small root systems during establishment. Conventional fertility regimes caused nutrient dilution in plants with time. Nutrient uptake did not match growth in these seedlings, resulting in nutrient stress at varying stages of seedling development.
Enhanced nutrient mobilization from old to current growth is a key mechanism that confers competitive ability to nutrient-loaded black spruce (Picea mariana (Mill.) BSP) seedlings planted on high-competition forest sites in northern Ontario. This study examines effects of nutrient loading and differing vegetation management (herbicide, fertilization, shading) on growth, partitioning, and net retranslocation of N between current and old shoots of newly planted black spruce seedlings on six contrasting boreal sites. Nutrient loading during nursery culture improved seedling growth and N uptake and retranslocation after transplanting. Herbicide application accelerated growth and N uptake only on the high-competition Alnus, hardwood, and mixedwood sites but had no effect on the less competitive feathermoss and Vaccinium sites. Weed removal on competitive sites reduced N retranslocation significantly. Field fertilization reduced N retranslocation in seedlings and promoting tree growth and N uptake on the less competitive feathermoss and Vaccinium sites. However, fertilization stimulated weeds on high-competition sites suppressing seedling biomass and N uptake. Factors that seem to affect net N retranslocation include the type and level of internal N reserves, biomass and N accumulation and partitioning, external N supply, and other stresses that limit plant growth such as competition.Résumé : Une meilleure mobilisation des nutriments des vieilles pousses vers les nouvelles constitue un mécanisme clé qui confère un avantage compétitif aux semis d'épinette noire (Picea mariana (Mill.) BSP) riches en nutriments qui sont plantés dans des sites forestiers où la compétition est forte dans le nord de l'Ontario. Cette étude examine les effets de l'enrichissement en nutriments et du mode de gestion de la végétation (herbicide, fertilisation, ombrage) sur la croissance, la répartition et la translocation nette de N entre les vieilles et les nouvelles pousses chez des semis d'épinette noire récemment plantés dans six sites différents en forêt boréale. L'enrichissement en nutriments pendant la culture en pépinière a amélioré la croissance, le prélèvement de N et sa translocation après la plantation. L'application d'herbicide a accéléré la croissance et le prélèvement de N seulement dans les sites de forêt mixte, de forêt feuillue et en présence d'aulne, où il y avait une forte compétition. Par contre, elle n'a eu aucun effet dans les sites à mousse hypnacée ou à Vaccinium où la compétition était faible. L'élimination des herbacées a significativement réduit la translocation de N dans les sites où la compétition était forte. La fertilisation a réduit la translocation de N dans les semis et favorisé la croissance des arbres et le prélèvement de N dans les sites à mousse hypnacée ou à Vaccinium où il y avait moins de compétition. Cependant, la fertilisation a favorisé les herbacées dans les sites où la compétition était forte et a entraîné une réduction de la biomasse des semis et du prélèvement de N. Les facteurs qui semblent affect...
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