Projected long-term productivity in Saskatchewan hybrid poplar plantations: weed competition and fertilizer effects

An ecosystem management model was used to project total aboveground and belowground production in hybrid poplar plantations for two sites in Saskatchewan that were previously in agricultural production and that differed in their soil organic matter and nitrogen content (categorized as poor and rich sites). Stemwood production (the primary measure of treatment response) was always negatively affected by the competition that resulted when weeds were abundant; the ef- fect was more severe on the poor than on the rich site. Stemwood biomass was greater when weed competition was low, but peak production declined over successive rotations on both sites, regardless of whether fertilizer was used. Fertilization always enhanced stemwood production but less so on the rich than on the poor site. A single fertilizer application in the second or seventh year after plantation establishment resulted in consistently higher stemwood production than midrotation fertilization (year 12). Fertilization was more beneficial to stemwood production when weed competition was high than when it was low. Low weed competition in conjunction with early fertilization produced the highest stemwood production. The simulations indicate that the relative benefit of a given management regime cannot be considered independently of the site nutrient status and the particular rotation. Resume´ : Un modele d'amenagement d'ecosysteme a eteutilisepour predire la production totale aerienne et souterraine de plantations de peupliers hybrides dans deux stations en Saskatchewan. Ces deux stations avaient auparavant eteconsa- crees a la production agricole et differaient par le contenu en matiere organique et en azote de leur sol (classees comme station pauvre et station riche). La production de bois de fut, la principale mesure de la reaction aux traitements, a toujours eteaffecteen egativement par la competition lorsque les mauvaises herbes etaient abondantes. L'effet etait plus prononce ´ dans la station pauvre que dans la station riche. La biomasse de bois de futetait plus elevee lorsque la competition des mauvaises herbes etait faible mais la production maximale a diminueavec les revolutions successives dans les deux sta- tions, qu'un fertilisant ait eteutiliseou non. La fertilisation a toujours augmentela production de bois de fut mais pas au- tant dans la station riche que dans la station pauvre. Une seule application de fertilisant la premiere ou la septieme annee apres l'etablissement des plantations a constamment entraoˆneune plus forte production de bois de fut qu'une fertilisation a ` mi-revolution (12 ans). La production de bois de fut a davantage beneficiede la fertilisation lorsque la competition des mauvaises herbes etait forte que lorsqu'elle etait faible. Une faible competition des mauvaises herbes combineeaune fer- tilisation hative a entraoˆnela plus forte production de bois de fut. Les simulations indiquent que les bienfaits relatifs d'un regime d'amenagement ne peuvent etre consideres independamment du statut nutritif de la station et du nombre de revolu- tions qui ont precede ´. (Traduit par la Redaction)