Partial windthrow as a driving process of forest dynamics in old-growth boreal forests

As climate changes, boreal forest ecosystems may become subject to disturbances that were previously uncommon in some regions. In recent decades, large tracts of northeastern boreal forest of Canada have been affected by different types of climatic events causing a lot of partial and some total stand mortality. Since these disturbances may become more important drivers of forest dynamics, there is a need to document their impact on forest structure. The objectives of this study were to describe temporal dynamics of partial windthrows and determine the effect of partial windthrow on stand composition and understory vegetation. The study was conducted in the North-Shore region of Quebec (Canada). Eighteen plots in closed forests were paired with 18 adjacent windthrow areas, in which trees experienced similar edaphic and climatic conditions. Dendroecological analyses, combined with vegetation sampling, were conducted on each site to determine stand structure and vegetation development through time. Significant increases in balsam fir and shade-tolerant species were observed in windthrow gaps. Tree mortality in windthrown stands was a slow process until the mid-1990s, a period during which spruce budworm defoliation may have played a role in weakening trees and making them more vulnerable to partial windthrow. Greater mortality observed following the mid-1990s was most certainly related to a regional storm. The initial composition of stands plays an important role in driving postwindthrow succession, as balsam fir is more susceptible to treefall. As opposed to stand-replacing windthrow and spruce budworm outbreaks that generate various postdisturbance responses, partial windthrow appears to only create opportunities for pre-established balsam fir to undergo release in gaps. Key words: windthrow, plant communities, growth release, Picea mariana, Abies balsamea, vegetation shift, spruce budworm outbreak. Compte tenu du changement climatique, les ecosystemes forestiers boreaux pourraient devenir assujettis a des perturbations naturelles qui etaient peu communes auparavant. Au cours des dernieres decennies, de grandes superficies de la foret boreale ont ete affectees par des perturbations causant la mortalite partielle ou totale des peuplements forestiers. Ces perturbations peuvent devenir plus importantes dans la dynamique naturelle de la foret boreale pressant la documentation de leur effet sur la structure de la foret. Les objectifs de cette etude etaient de decrire la dynamique temporelle des chablis partiels et determiner leur effets sur la composition forestiere et du sous-bois. Cette etude a ete realisee dans la region de la Cote-Nord, Quebec (Canada). Dix-huit placettes experimentales en foret fermee ont ete appariees avec dix-huit parcelles en foret affectee par les chablis ou les arbres poussaient dans des conditions climatiques et edaphiques similaires. Des analyses dendrochronologiques combinees avec des releves de vegetation ont ete realisees dans chacune des parcelles pour determiner l'evolution du couvert forestier et de la vegetation dans le temps. La mortalite des arbres dans les chablis a ete progressive jusqu'au milieu des annees 1990, periode au cours de laquelle la defoliation causee par la tordeuse des bourgeons de l'epinette a pu jouer un role en affaiblissant les arbres, les rendant plus vulnerables au chablis. Une tempete regionale a egalement accelere la mortalite des arbres a la fin des annees 1990. La composition initiale des peuplements joue un role important dans la succession apres chablis ou le sapin baumier est plus susceptible au chablis. Contrairement au chablis total et aux epidemies de tordeuse des bourgeons de l'epinette qui peuvent generer une variete de reponses post perturbation, le chablis partiel semble seulement creer des opportunites permettant a la regeneration preetablie de sapin baumier d'occuper les trouees. Mots-cles: chablis, communautes vegetales, detente de croissance, Picea mariana, Abies balsamea, epidemies de tordeuse des bourgeons de l'epinette.
Introduction Wind damage has a strong ecological impact on a range of forest ecosystems, from tropical (Boose et al. 1994) to temperate (Peterson and Pickett 1991) and boreal forests (Kuuluvainen [...]