WHITE SPRUCE MEETS BLACK SPRUCE: DISPERSAL, POSTFIRE ESTABLISHMENT, AND GROWTH IN A WARMING CLIMATE

Local distributions of black spruce (Picea mariana) and white spruce (Picea glauca) are largely determined by edaphic and topographic factors in the interior of Alaska, with black spruce dominant on moist permafrost sites and white spruce dominant on drier upland sites. Given the recent evidence for climate warming and permafrost degradation, the distribution of white spruce is expected to expand, but the transition from black to white spruce may be dispersal limited: unlike the semi-serotinous black spruce, postfire regeneration of white spruce relies on seed dispersal from unburned areas. To determine the relative roles of dispersal, establishment, and growth in recruitment of white and black spruce, we studied postfire spruce regeneration in a 21-year-old burn across a white spruce–black spruce transition in the interior of Alaska. Although prefire spatial distributions of adults of the two species were well separated along the topographic sequence from upland to floodplain sites, the spatial distributions of recruits overlapped considerably. Even >700 m away from its seed source, white spruce sapling density on typical black spruce sites was high enough to form fully stocked stands. In contrast, black spruce regeneration was sparse on typical white spruce upland sites. Establishment rates of both species, estimated from a statistical model, were highest in mossy, wet depressions, which tended to have a thick residual postfire organic layer (∼10 cm). On all site types, height growth rates inferred from age–height relationships were comparable for recruits of both species. On typical black spruce sites ≥300 m into the burn, white spruce was younger (and, therefore, shorter) than black spruce due to the timing of masting events following the fire. There was no indication that dispersal, establishment, or edaphic constraints on juvenile growth limit white spruce's capacity to invade typical black spruce stands during the recruitment stage in our study area. It is unlikely that white spruce recruits would persist to the adult stage if the permafrost returned to the original prefire levels during future postfire succession. However, if permafrost continues to degrade under climate warming, transition to a white spruce-dominated landscape could be rapid.