-Conventional nursery culture of containerized black spruce (Picea mariana Mill. B.S.P.) seedlings usually involves a lateseason interval, commonly called the "hardening period", when fertilization and water are withheld to promote frost-hardiness. Considerable growth may occur during this time which may lead to internal nutrient dilution, a condition often detrimental to subsequent field performance. Continued late season fertilization (at 6 or 12 mg N seedling -1 ) of seedlings during the hardening period was tested as a technique to prevent late season nutrient dilution and possibly to increase nutrient reserves. Root growth was increased much more than shoot growth during this period. Late-season fertilization raised N, P and K uptake as much as 164, 70 and 32% respectively, compared to conventionally fertilized seedlings with no late-season fertilization. Depending on dose rate and pre-hardening nutrient loading, this technique demonstrates the potential to build internal nutrient reserves in seedlings. Nutrient dilution was temporarily averted by lateseason fertilization suggesting that intensive and prolonged nutrient supplementation during the hardening period may further delay or eliminate nutrient dilution in seedlings.black spruce / hardening period / nitrogen / nutrient dilution / nutrient loading Résumé -Fertilisation en fin de saison des plants de Picea mariana cultivés en serre : dynamique de la biomasse et des éléments nutritifs. Dans les pépinières, l'élevage en container de plants de Picea mariana (Mill. B.S.P.) comporte normalement en fin de saison une phase appelée « période d'endurcissement » pendant laquelle fertilisation et arrosage sont supprimés pour améliorer la résistance au froid. La croissance, au cours de cette période, peut être importante d'où une dilution interne des éléments nutritifs affectant souvent les performances ultérieures sur le terrain. On a testé une technique consistant à prolonger la fertilisation pendant la période d'endurcissement (6 à 12 mg N par plant) pour éviter, en fin d'élevage, une dilution des éléments nutritifs, voire en augmenter la teneur. Pendant cette période, le gain de croissance du système racinaire a été plus élevé que celui des parties aériennes. Cette fertilisation en fin de saison se traduit par un prélèvement en N, P et K accru de respectivement 164, 70 et 32 % par rapport à celui observé avec le régime de fertilisation classique. Dépendant du régime de fertilité antérieur avant endurcissement et de la dose d'éléments nutritifs adoptée, cette technique dé-montre qu'il est possible d'agir sur la quantité de réserves en éléments des plants. Une fertilisation en fin de saison interrompt temporairement le processus de dilution des éléments. Ceci permet de penser que l'apport intensif et prolongé d'éléments pendant la période d'endurcissement peut retarder ou éviter la dilution en éléments des plants.Picea mariana / période d'endurcissement / azote / dilution des éléments nutritifs / changements nutritifs Ann. For. Sci. 59 (2002) [255][2...

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Application. Exponential fertilization of red pine seedlings grown under limited irrigation decreased shoot/root ratio, increased root nutrient reserves and enhanced drought avoidance properties when compared to conventional fertilization. Enhanced preconditioning from exponential fertilization may promote outplanting performance.Abstract. Red pine seedlings were grown for 16 weeks under contrasting fertilization (conventional, exponential) and moisture (wet, moist, dry) regimes to assess preconditioning effects of treatments on biomass production, nutrient uptake and allocation, and water relations. Growth, nutrient status, and water relations were affected more by moisture availability than by fertilization regime. Exponential fertilization under limited irrigation lowered shoot/root mass ratio, increased root nutrient reserves, and enhanced drought avoidance compared to conventional fertilization regimes. Drought treatments decreased nutrient uptake in the shoots of both fertilization regimes by 24%, but increased nutrient accumulation in the roots by 39% in the exponential regime compared to 17% in the conventional. These results may explain improved outplanting performance noted for exponentially fertilized container stock.

Conventional fertilization of black spruce (Piceamariana (Mill.) B.S.P.) container stock usually does not conform to steady-state nutrient conditions and may limit subsequent outplanting performance. Steady-state nutrient preconditioning of seedlings, characterized by maintaining stable tissue nutrient (N, P, and K) concentrations during the exponential growth phase, was induced by an exponentially based fertilization regime that compensated for low nutrient reserves in germinating seeds. By the end of the greenhouse rotation, this regime reduced the shoot/root ratio of the trees, but fell short of increasing seedling growth and nutrient status when compared with seedlings conventionally fertilized with equivalent amounts of nutrients. However, first-season height growth and shoot biomass of trees outplanted on different surface soils in pot bioassays were significantly improved by steady-state nutrient preconditioning and were accompanied by increased plant nutrient uptake, particularly N at the half-dose level and P at the full-dose level. Out-planting performance was higher on an upland site type, although relative response was greater on nutrient-poorer, lowland substrates. The preconditioning response from exponentially based fertilization was attributed to lower shoot/root mass ratio and more effective nutritional adaptation of the seedlings to the field environment. Under steady-state nutrient culture, seedling nutrient uptake conforms more closely with stable nutrient supply in nature, since expanding root systems exploit new soil volumes exponentially. Field performance of exponentially fertilized seedlings may be further enhanced when combined with balanced nutrient loading in the greenhouse phase.

Four full-sib families of interior spruce (Picea glauca (Moench) Voss) x Picea engelmanii Parry ex Engelm.) with contrasting growth rates (two fast-growing and two slow-growing families) were grown aeroponically with either a 2% relative nitrogen addition rate or free access to nitrogen. Fast-growing families showed greater plasticity in allocating biomass to shoots at high nitrogen supply and to roots at low nitrogen supply than slow-growing families. Compared with the slow-growing families, short-term net ammonium uptake rate measured with an ion selective electrode was significantly greater in fast-growing families at high ammonium supply, but not at low supply. Net nitrate uptake showed the same trend, but differences among families were not significant. Results indicate that differences in seedling growth rate are partly a result of physiological differences in net nitrogen uptake efficiency and nitrogen productivity.