Recruitment varies among milkweed seed sources for habitat specialist but not generalist

If sufficient seedling establishment can be achieved, seed-based restoration provides an affordable, active restoration approach that can be implemented quickly at scale. However, establishment has served as a major restoration bottleneck, highlighting the need for improved understanding of seed germination niche and interactions with site conditions. Germination niche breadth (NB) is expected to increase with gene flow, resulting in broader environmental tolerance range, reduced sensitivity to site conditions, and less variation among seed sources. To investigate how germination NB relates to inter- and intraspecific variation in establishment from seed, we compared field recruitment for two milkweeds (Asclepias), the larval host plant of the monarch butterfly and thus a high priority group for habitat creation. Consistent with species-level NB derived from laboratory trials, there was strong evidence that early life stages of the habitat specialist (Asclepias incarnata) varied among seed collection regions (separated by 423–572 km) but no evidence that the generalist (A. syriaca) varied among seed sources collected across an approximately 750-km transect. Regeneration trends demonstrate that A. incarnata is significantly more sensitive to seed source and therefore requires more restricted seed zones. However, climate change may necessitate that we separate seed collection zones from seed application zones, upending the traditional framework of seed transfer zones. Until taxon-specific studies have identified the scale of adaptive, phenotypic variance, restoration practitioners should continue to adjust the scale of seed collection zones for milkweeds and other taxa based on species traits known to influence gene flow, such as abundance and habitat specificity.