Relative importance of different secondary successional pathways in an Alaskan boreal forest

Abstract

Postfire succession in the Alaskan boreal forest follows several different pathways, the most common being self-replacement and species-dominance relay. In self-replacement, canopy-dominant tree species replace themselves as the postfire dominants. It implies a relatively unchanging forest composition through time maintained by trees segregated within their respective, ecophysiological niches on an environmentally complex landscape. In contrast, species-dominance relay involves the simultaneous, postfire establishment of multiple tree species, followed by later shifts in canopy dominance. It implies that stand compositions vary with time since last fire. The relative frequencies of these and other successional pathways are poorly understood, despite their importance in determining the species mosaic of the present forest and their varying, potential responses to climate changes. Here we assess the relative frequencies of different successional pathways by modeling the relationship between stand type, solar insolation, and altitude; by describing how stand age relates to species composition; and by inferring successional trajectories from stand understories. Results suggest that >70% of the study forest is the product of self-replacement, and tree distributions are controlled mainly by the spatial distribution of solar insolation and altitude, not by time since last fire. As climate warms over the coming decades, deciduous trees will invade cold sites formerly dominated by black spruce, and increased fire frequency will make species-dominance relay even rarer.Résumé : La succession après feu dans la forêt boréale de l'Alaska adopte différents modes, les plus communs étant le retour des mêmes espèces et la dominance successive de différentes espèces. Dans le cas du retour des mêmes espèces, les espèces d'arbres qui dominent la canopée sont remplacées par les mêmes espèces qui deviennent dominantes après un feu. Cela implique que la composition de la forêt qui demeure relativement stable dans le temps soit maintenue par des arbres qui sont restreints à leur niche écologique respective dans un paysage complexe du point de vue environnemental. Au contraire, la dominance successive de différentes espèces suppose l'établissement après feu de plusieurs espèces d'arbres suivi par des changements ultérieurs de dominance dans la canopée. Cela signifie que la composition du peuplement change avec le temps après un feu. Les fréquences relatives de ces modes de succession et d'autres sont peu connues malgré leur importance dans la détermination de la mosaïque d'espèces de la forêt actuelle et de leurs différentes réactions potentielles aux changements climatiques. Dans cet article, nous évaluons la fréquence relative de différents modes de succession en modélisant la relation entre le type de peuplement, l'ensoleillement et l'altitude; en décrivant comment la composition en espèces est reliée à l'âge du peuplement et en déduisant le mode de succession à partir du sous-étage d'un peuplement. Les résultats indiquent que plus de ...

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