Mechanical site preparation and nurse plant facilitation for the restoration of subarctic forest ecosystems

Sustainable forest management implies successful regeneration following disturbances. Tree regeneration in subarctic ecosystems can, however, be constrained by limitations to seedling establishment related to cold soils, slow decomposition rates, and competition by ericaceous species. We established a field trial at the northern limit of commercial forests in Quebec, Canada, to evaluate to what extent mechanical site preparation (MSP) and planting of a nurse [N.sub.2]-fixing species could promote conifer establishment on a site burned in 2007. The experiment comprised four treatments applied in 2010: standard MSP (disc trenching), standard MSP plus planting of Alnus crispa, intensive MSP, with larger furrows than standard MSP, and a control. Main plots were divided and planted in 2011 with Picea mariana (Mill.) Britton, Stearns & Poggenb. or Pinus banksiana Lamb. We monitored seedling survival, growth, nutrition, and microsite over a 3-year period. Results revealed interactions between treatments and planted species. Mechanical site preparation resulted in higher conifer growth relative to the control conditions, and planting Alnus resulted in growth gains similar to those obtained from intensive MSP. We measured competitive interactions between Alnus and the conifers that might eventually cancel out the initial benefits derived from facilitation by planting the nurse species. Longer term monitoring of interspecific interactions is needed to unravel the mechanisms responsible for the facilitative effect and identify the best management practices. Key words: reforestation, restoration, boreal forest, conifer, nutrition, competition, regeneration. L'amenagement durable des forets implique que la regeneration forestiere s'etablisse avec succes apres les perturbations. La regeneration forestiere dans les ecosystemes subarctiques peut toutefois etre problematique en raison des limitations que les sols froids, les faibles taux de decomposition, et la vegetation concurrente dominee par les ericacees imposent a l'etablissement des semis et a la productivite vegetale. Nous avons effectue une experience sur le terrain a la limite nordique des forets commerciales du Quebec, au Canada, pour evaluer dans quelle mesure la preparation mecanique du sol (PM) et la plantation d'une espece de soutien fixatrice d'azote pouvait ameliorer l'etablissement des coniferes sur un site recemment brule. L'experience comprenait quatre traitements appliques en 2010 : PM standard (scarifiage a disques), PM standard plus plantation d'Alnus crispa, PM intensive avec des sillons plus larges que la PM standard et un temoin. Les parcelles principales ont ete divisees et plantees en 2011 avec Picea mariana (Mill.) Britton, Stearns & Poggenb. ou Pinus banksiana Lamb. Nous avons mesure la survie, la croissance et la nutrition des plants, ainsi que les caracteristiques des microsites oU ils avaient ete plantes, sur une periode de 3 ans. Les resultats ont revele des interactions entre la PM et l'espece plantee. La PM a stimule la croissance des coniferes par rapport au temoin et la plantation d'Alnus a engendre des gains de croissance similaires a ceux associes a la PM intensive. Nous avons mesure des interactions competitives entre Alnus et les coniferes, lesquelles pourraient eventuellement annuler les effets benefiques initiaux de facilitation s'il advenait qu'Alnus domine la station. Un suivi a plus long terme de meme que l'etude des interactions interspecifiques sont necessaires afin d'elucider les mecanismes responsables de l'effet de facilitation et d'identifier les meilleures pratiques d'amenagement. Mots-cles : reboisement, restauration, foret boreale, conifere, nutrition, competition, regeneration.
Introduction Tree density and stocking are critical factors that determine forest productivity (Drew and Flewelling 1979). They both depend directly upon successful tree regeneration following natural and anthropogenic disturbances (Burton [...]