Life's more than one niche: demographic constraints on northern black spruce range expansion in Yukon, Canada

Abstract

Species’ distributions are often characterized by the species’ niche represented in geographic space. Species’ niches can be divided demographically, resulting in multiple demographic niches with unique dimensions. This approach determines which life stages have either the narrowest niche breadth or the least available niche space at the range edge, and thus act as key demographic hurdles on range expansion. We quantified microsites inhabited by different life stages of black spruce (Picea mariana) at subarctic treeline in Yukon, Canada to characterize demographic niche breadth and assess how available niche space changed towards the range edge. We found that restricted suitable emergent niche space and viable seed availability towards the range limit were the overwhelmingly main limiting demographic bottlenecks on northern range expansion. Our findings suggest that demographic niches can identify life stage specific hurdles to range expansion, contributing to our understanding of how species’ distributions will respond to climate change.