Fire and substrate interact to control the northern range limit of black spruce (Picea mariana) in Alaska

Black spruce (Picea mariana (Mill.) BSP) is a common treeline species in eastern Canada but rare at treeline in Alaska. We investigated fire and substrate effects on black spruce populations at six sites along a 74 km transect in the Brooks Range, Alaska. Our southern sites, on a surface deglaciated >50000 years ago, had significantly more acidic soils, more black spruce, and higher seed viability than our northern sites, which were deglaciated approximately 13 000 years ago. Despite similar fire history at five of our six sites, postfire recruitment dynamics varied with surface age. Sexual reproduction was vigorous in both postfire and nonfire years in populations on the older surface. On the younger surface, vigorous sexual reproduction was restricted to postfire decades and clonal reproduction by branch layering predominated in nonfire years. At the northernmost site, which was unburned, black spruce reproduced almost exclusively by layering. The species' northern range limit thus reflects an interaction between fire and substrate: on recently deglaciated surfaces, sexual reproduction is restricted to postfire years. This substrate-induced dependence on fire may restrict the range of black spruce to sites that burn sufficiently often to allow occasional sexual reproduction. L'epinette noire (Picea mariana (Mill.) BSP) est une espece commune a la limite des arbres dans l'est du Canada mais rare a la limite des arbres en Alaska. Nous avons etudie les effets du feu et du substrat sur les populations drepinette noire dans six stations le long d'un transect de 74 km dans les monts Brooks, en Alaska. Dans les stations situees au sud, oh la glaciation a pris fin il y a plus de 50 000 ans, les sols sont significativement plus acides; il y a significativement plus d'epinettes noires et la viabilite des graines est significativement plus elevee que dans les stations situees au nord oU la glaciation a pris fin il y a environ 13 000 ans. Bien que l'historique des feux soit semblable dans cinq des six stations, la dynamique du recrutement apres feu varie avec l'age de la surface. La reproduction sexuee est vigoureuse dans les populations qui occupent les surfaces plus vieilles, autant lors des annees qui suivent un feu que lors des annees ou l'action du feu est inexistante. Sur la surface plus jeune, une reproduction sexuee vigoureuse est limitee aux decades qui suivent un feu et la reproduction clonale par marcottage predomine lors des annees oil l'action du feu est inexistante. Dans la station situee le plus au nord, qui n'a pas et. [Traduit par la Redaction]
Introduction Temperatures in Alaska have been rising at the rate of approximately 0.4[degrees]C per decade since the 1960s (Chapin et al. 2005); much larger increases in temperature are expected in [...]