Modelling recruitment of Populus tremuloides, Pinus banksiana, and Picea mariana following fire in the mixedwood boreal forest D.F. Greene and E.A. Johnson Abstract: We examined the relationship between the post-fire regeneration density of Populus tremuloides Michx., Pinus banksiana Lamb., and Picea mariana (Mill.) BSP and their pre-fire basal area density at the spatial scale of 70 m (the width of the stands studied) in four fires in central Saskatchewan and one in Quebec. For these three species with mechanisms for in situ reproduction, there were highly significant relationships between regeneration density and pre-fire basal area density (basal area per area). Given equal source basal area densities, Populus tremuloides has an advantage, relative to the other two species, in initial regeneration densities, but the advantage is not great because the asexual stems thin rapidly. The overriding conclusion is that, for these three species, there is little change in species composition following fire. Simple predictive equations performed reasonably well for Pinus banksiana and Picea mariana: F B D D = 806 0 95 . and F B D D = 593 0 86 . , where F D is regenerative stem density (no./m 2 ), and B D is basal area density (m 2 /m 2 ). For asexual reproduction by Populus tremuloides, the thinning begins immediately following fire, and the regeneration model was F B t D D = + - 11 600 1 0 79 1 64 . . ( ) , where t is years since fire. Résumé : Nous avons étudié la relation entre la densité de la régénération après feu de Populus tremuloides Michx., Pinus banksiana Lamb. et Picea mariana (Mill.) BSP et leur densité en surface terrière avant feu à l’échelle spatiale de 70 m (la largeur des peuplements étudiés), suite à quatre feux survenus dans le centre de la Saskatchewan et un autre au Québec. Dans le cas de ces trois espèces qui ont des mécanismes de reproduction in situ, il y avait des relations très significatives entre la densité de la régénération et la densité en surface terrière (surface terrière par unité de surface) avant feu. Étant donné des densités en surface terrière égales au départ, le Populus tremuloides a un avantage, relativement aux deux autres espèces, dans la densité initiale de la régénération, mais cet avantage est mince parce que les tiges asexuées s’éclaircissent rapidement. La conclusion la plus importante, dans le cas de ces trois espèces, est qu’il y a peu de changement dans la composition des espèces après feu. De simples équations de prédiction performaient raisonnablement bien pour le Pinus banksiana et le Picea mariana : F B D D = 806 0 95 , et F B D D = 593 0 86 , où F D est la densité des tiges régénératives (nombre/m 2 ) et B D est la densité en surface terrière (m 2 /m 2 ). Dans le cas de la reproduction asexuée chez le Populus tremuloides, le processus d’éclaircie débute immédiatement après le feu et l’équation qui permet de modéliser la régénération est F B t D D = + - 11 600 1 0 79 1 64 , , ( ) où t est le nombre d’années écoulées depuis le feu. [Traduit par la rédaction] Greene and Johnson 473 In the Western Canada mixedwood boreal forest the pre- dominant type of disturbance, stand-replacing fire (Johnson 1993), ensures the removal of advanced regeneration and any tree seeds in the soil seedbank (Archibold 1979, 1980; Johnson and Fryer 1996). Thus, there are only three methods of post-disturbance colonization available to tree species: asexual reproduction within the burn (Populus tremuloides Michx., Populus balsamifera L., Betula papyrifera Marsh.), aerial seed banks within the burn (Pinus banksiana Lamb., Picea mariana (Mill.) BSP), and dispersal from an unburned edge into the burn (the first five species plus Picea glauca (Moench) Voss, Larix laricina (DuRoi) K. Koch, and Abies balsamea (L.) Mill.). Here we will focus on in situ regeneration by the three most common species (Populus tremuloides, Pinus bank- siana, and Picea mariana), deferring the results for dispersal from living source trees to a subsequent paper. These three species colonize the burn primarily within the first few years following the fire (Cayford 1960; Zasada et al. 1991; St. Pier- re et al. 1992; Greene et al. 1999) and thus understanding the initial recruitment densities is of considerable interest. In this paper we will model the initial recruitment episode following fire. The exercise is of applied interest because the model may serve as a standard for comparison with regener- ation (natural or artificial) following harvesting or as an as- sessment of the recruitment that might have been obtained in the absence of post-fire salvaging. Our primary objective is to model the initial recruitment density as a function of the dimensionless pre-fire basal area density (area/area), assuming that this latter quantity is Can. J. For. Res. 29: 462–473 (1999) © 1999 NRC Canada 462 Received March 26, 1998. Accepted December 17, 1998. D.F. Greene. 1 Department of Geography and Department of Biology, Concordia University, 1455 de Maisonneuve Blvd., Montréal, QC H3G 1M8, Canada. e-mail: greene@alcor.concordia.ca E.A. Johnson. Department of Biological Sciences and Kananaskis Field Stations, University of Calgary, Calgary, AB T2N 1N4, Canada. 1 Author to whom all correspondence should be addressed.