Comparisons between wildfire and forest harvesting and their implications in forest management
Dates
Year
2001
Citation
McRae, D. J., Duchesne, L. C., Freedman, B., Lynham, T. J., and Woodley, S., 2001, Comparisons between wildfire and forest harvesting and their implications in forest management: Environmental Reviews, v. 9, no. 4, p. 223-260.
Summary
Emulation silviculture is the use of silvicultural techniques that try to imitate natural disturbances such as wildfire. Emulation silviculture is becoming increasingly popular in Canada because it may help circumvent the political and environmental difficulties associated with intensive forest harvesting practices. In this review we summarize empirical evidence that illustrates disparities between forest harvesting and wildfire. As a rule, harvesting and wildfire affect biodiversity in different ways, which vary a great deal among ecosystem types, harvesting practices, and scale of disturbance. The scales of disturbance are different in that patch sizes created by logging are a small subset of the range of those of wildfire. In particular, [...]
Summary
Emulation silviculture is the use of silvicultural techniques that try to imitate natural disturbances such as wildfire. Emulation silviculture is becoming increasingly popular in Canada because it may help circumvent the political and environmental difficulties associated with intensive forest harvesting practices. In this review we summarize empirical evidence that illustrates disparities between forest harvesting and wildfire. As a rule, harvesting and wildfire affect biodiversity in different ways, which vary a great deal among ecosystem types, harvesting practices, and scale of disturbance. The scales of disturbance are different in that patch sizes created by logging are a small subset of the range of those of wildfire. In particular, typical forestry does not result in the large numbers of small disturbances and the small number of extremely large disturbances created by wildfires. Moreover, the frequency of timber harvesting is generally different from typical fire return intervals. The latter varies widely, with stand-replacing fires occurring in the range of 20 to 500 years in Canada. In contrast, harvest frequencies are dictated primarily by the rotational age at merchantable size, which typically ranges from 40 to 100 years. Forest harvesting does not maintain the natural stand-age distributions associated with wildfire in many regions, especially in the oldest age classes. The occurrence of fire on the landscape is largely a function of stand age and flammability, slope, aspect, valley orientation, and the location of a timely ignition event. These factors result in a complex mosaic of stand types and ages on the landscape. Timber harvesting does not generally emulate these ecological influences. The shape of cut blocks does not follow the general ellipse pattern of wind driven fires, nor do harvested stands have the ragged edges and unburned patches typically found in stand-replacing fires. Wildfire also leaves large numbers of snags and abundant coarse woody debris, while some types of harvesting typically leave few standing trees and not much large debris. Successional pathways following logging and fire often differ. Harvesting tends to favor angiosperm trees and results in less dominance by conifers. Also, understory species richness and cover do not always recover to the pre-harvest condition during the rotation periods used in typical logging, especially in eastern Canada and in old-growth forests. As well, animal species that depend on conifers or old-growth forests are affected negatively by forest harvesting in ways that may not occur after wildfire. The road networks developed for timber extraction cause erosion, reduce the areas available for reforestation, fragment the landscape for some species and ecological functions, and allow easier access by humans, whereas there is no such equivalency in a fire-disturbed forest.Original Abstract: La sylviculture simulee est l'utilisation de techniques sylvicoles cherchant a imiter des perturbations naturelles telles que l'incendie de foret. La popularite de la sylviculture simulee ne cesse d'augmenter au Canada, parce qu'elle peut aider a circonscrire les difficultes politiques et environnementales associees avec les pratiques de recolte forestiere intensives. Les auteurs revoient et resument les preuves empiriques qui illustrent les differences entre la recolte forestiere et l'incendie de foret. En general, la recolte et l'incendie affectent la biodiversite de differentes facons qui varient beaucoup selon les types d'ecosysteme, les techniques de recolte et l'echelle de perturbation. Les echelles de perturbation sont differentes, en ce que la dimension des trouees creees par la recolte constitue un petit sous-ensemble de l'echelle de celles occasionnees par l'incendie. Particulierement, la pratique forestiere typique ne conduit pas aux grands nombres de petites perturbations et aux petits nombres de grandes perturbations qui font suite a l'incendie. De plus, la frequence des recoltes est generalement differente des intervalles de retour de l'incend e. Ces dernieres varient largement, avec des intervalles de remplacement du peuplement allant de 20 a 500 ans. Au contraire, la frequence des recoltes est definie principalement par le temps necessaire pour atteindre des dimensions marchandes, ce qui se situe entre 40 et 100 ans. Dans plusieurs regions, la recolte forestiere ne maintient pas la distribution naturelle d'ages des peuplements, notamment les classes d'age les plus vieilles. L'incidence de l'incendie dans le paysage est largement fonction de l'age du peuplement et de son inflammabilite, de la pente, de l'aspect, de l'orientation de la vallee, et de la localisation ainsi que du moment de la mise a feu. Ces facteurs conduisent a l'apparition d'une mosaieque complexe de types de peuplements et d'ages dans le paysage. La recolte forestiere n'imite generalement pas ces influences ecologiques. La forme de decoupage des parcelles ne suit generalement pas le patron elliptique des incendies de foret, et les parcelles ne montrent pas le decoupage irregulier et les ilots non-perturbes qu'on retrouve dans les peuplements venus apres feu. Les incendies laissent de nombreux chicots et d'abondants macrorestes ligneux, alors que certains types de recolte laissent typiquement peu d'arbres sur pieds et peu de macrorestes ligneux. Les etapes de la succession, suite a la recolte et au feu, different frequemment. La recolte favorise les arbres angiospermes et conduit a une moindre dominance des coniferes. De plus, la richesse en especes des sous-etages et leurs couvertures ne retournent pas toujours aux conditions precedent la recolte, au cours des intervalles de rotation typiques entre les recoltes, surtout dans l'est canadien et dans les forets surannees. Aussi, les especes animales qui dependent des coniferes ou des vieilles forets sont negativement affectees par la recolte forestiere de facons qui peuvent ne pas survenir apres l'incendie. Les reseaux routiers developpes pour la recolte causent de l'erosion, reduisent les surfaces disponibles pour le reboisement, fragmentent le paysage pour certaines especes et fonctions ecologiques, et facilitent l'acces par les humains, situations qui n'existent pas dans les forets perturbees par l'incendie.
