Many introductory ecology textbooks illustrate succession, at least in part, by using certain classic studies (e.g. sand dunes, ponds/bogs, glacial till, and old fields) that substituted space for time (chronosequence) in determining the sequences of the succession. Despite past criticisms of this method, there is continued, often uncritical, use of chronosequences in current research on topics besides succession, including temporal changes in biodiversity, productivity, nutrient cycling, etc. To show the problem with chronosequence-based studies in general, we review evidence from studies that used non-chronosequence methods (such as long-term study of permanent plots, palynology, and stand reconstruction) to test the space-for-time substitution in four classic succession studies. In several cases, the tests have used the same locations and, in one case, the same plots as those in the original studies. We show that empirical evidence invalidates the chronosequence-based sequences inferred in these classic studies.

Postfire tree recruitment in the boreal forest is restricted to patches from which the duff (organic layer) has been removed by fire. Duff consumption occurs by smoldering combustion, propagation of which is determined by bulk density, moisture content, and depth. This study investigated interactions among these factors, their spatial distribution, and spatial patterns of duff consumption in two wildfires. A hypothesized positive relationship between moisture content and depth was supported by a laboratory study. Duff characteristic data were collected from two burns and comparable unburned areas of mixedwood forest in western Canada to describe and explain patterns of duff consumption within and between top-slope Pinus banksiana Lamb. -Picea mariana (Mill.) BSP stands and lower slope pure stands of Picea mariana on glaciofluvial hillslopes. In unburned stands, bulk density did not differ significantly between stands, while depths were significantly greater in Picea stands. With moist duff, consumption was significantly greater in Picea than in Pinus-Picea stands as depth limited smoldering in the thin duff of Pinus-Picea stands. With dry duff, smoldering was propagated regardless of depth resulting in no differences between stand types. The spatial correlation between burned patches and fire-killed trees was explained by within-stand spatial variation in duff moisture due to precipitation interception by tree crowns.Résumé : La régénération après feu en forêt boréale est limitée aux parcelles où l'humus (horizon organique) a été dé-truit par le feu. L'humus est consumé par la combustion lente dont la propagation est déterminée par la densité apparente, la teneur en humidité et la profondeur. Cette étude a examiné les interactions entre ces facteurs, leur distribution spatiale et les patrons spatiaux de destruction de l'humus pour deux feux de forêt. L'hypothèse d'une relation positive entre la teneur en humidité et la profondeur a été confirmée par une étude en laboratoire. Les données caractéristiques de l'humus ont été récoltées dans deux brûlis et des superficies non brûlées et comparables de forêt mélangée de l'ouest du Canada pour décrire et expliquer les patrons de destruction de l'humus à l'intérieur et entre des peuplements de Pinus banksiana Lamb. -Picea mariana (Mill.) BSP de haut de pente et des peuplements purs de Picea mariana de bas de pente sur des versants fluvio-glaciaires. Dans les peuplements non brûlés, la densité apparente ne varie pas significativement entre les peuplements alors que la profondeur est significativement plus importante dans les peuplements de Picea. Dans le cas d'un humus humide, la destruction est significativement plus prononcée dans les peuplements de Picea que dans ceux de Pinus-Picea car la profondeur limite la combustion lente dans l'humus mince des peuplements de Pinus-Picea. Avec un humus sec, la combustion lente se propage quelle que soit la profondeur, n'entraînant pas de différence entre les types de peuplement. La corrélation spatiale entre les parcelles brûlées ...

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Abstract. Mimicking of natural disturbance for ecosystem management requires an understanding of the disturbance processes and the resulting landscape patterns. Since fire is the major disturbance in the boreal forest, three widely held beliefs about fire behavior and resulting landscape patterns are examined in light of the empirical evidence available. These beliefs are: (1) that there is a ‘natural’ fire frequency for boreal ecosystems; (2) that the landscape mosaic created by wildfire is generally one of small, younger patches embedded within a matrix of older forest; and (3) that forest flammability is largely controlled by fuel accumulation. Despite the apparently logical basis for such beliefs, they are not well supported by empirical evidence. This discrepancy is explained by problems such as failure to appreciate the relationship between number of fires and area burned and inappropriate extrapolations or generalizations from other regions and vegetation types. The most important implications for management are that the natural disturbance processes producing landscape patterns in the boreal forest generally operate at much larger scales than management units, and that humans may have more indirect (through landuse change) rather than direct (through fire suppression) effects on the frequency of wildfires.