Advance, steadfast, retreat? Growth and establishment of white spruce seedlings at treelines in Alaska

Abstract

Range dynamics of tree species are largely driven by seedling establishment and survival, mainly at and beyond current treelines. Although numerous studies document the growth of mature white spruce (Picea glauca) trees in Alaska, research on seedling establishment and growth, particularly dendroecological studies addressing climate sensitivity, remains limited. This study examines seedling densities and age distributions, growth performance, and climate–growth correlations of white spruce seedlings at five Alaskan sites: a “dry treeline” with potential drought limitations and the adjacent “dry forest,” a reference forest in Interior Alaska, and a “cold treeline” with expected cold temperature limitations and the adjacent “cold forest” in the Brooks Range. Seedling densities were highest at the cold treeline and especially the cold forest, moderate at the reference forest and dry forest, and very low at the dry treeline. Growth rates and height:diameter ratio were greatest at the dry treeline, with slightly lower growth at the cold treeline and cold forest. We found some evidence for drought sensitivity in spring at the dry treeline and in summer at the dry forest, while warm temperatures in fall positively influenced seedling growth at both sites. Notably, growth was more often significantly correlated with climate during the transitional seasons compared to only once in summer and not at all in winter. This pattern likely reflects that seedling growth, especially at the cold sites, is shaped less by direct macroclimatic influences and more by indirect effects on growing season dynamics, such as snow accumulation and melt. Despite growth peaking at the dry treeline, low seedling densities suggest a potential retreat of Pi. glauca at dry, low-elevation treelines in Alaska. In contrast, high seedling densities and growth rates at the cold treeline and cold forest indicate favorable conditions for seedling establishment and densification. The complexity of seedling growth and climate–growth correlations demands careful consideration, as growth is only partially influenced by macroclimate and is likely more strongly shaped by factors like competition, snowpack, soil moisture and temperature, and nutrient availability. Overall, our local data lend credibility to the hypothesized biome shift of white spruce, albeit unfolding at a slow pace.