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

Abstract

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.

Common milkweed seeds exhibit latitudinal clines more consistent with adaptation to growing season length than temperature

“…Characterizing fitness in the field will be necessary to fully test this idea (Donohue et al 2010). In a recent study of common milkweed seed establishment in low‐ and mid‐latitude field sites (Missouri and Illinois, U.S.A.), seeds sourced from high‐latitude populations germinated earlier, but there was no evidence of an effect of latitude of origin on survivorship or above‐ground biomass accumulation by July (Finch et al 2023). We note that common milkweed invests heavily in below‐ground growth and suggest testing whether population differences in germination time correspond with differences in root:shoot ratio, overwintering success or flowering time, especially at high‐latitude field sites.…”

“…Geographic variation in A. syriaca traits, including growth, phenology, defense, seed mass, and germination rates have been documented in parts of the range (Malcolm et al 1989; Woods et al 2012; Finch et al 2018); however, there is not strong evidence for adaptation to local conditions (Finch et al 2023). Asclepias syriaca plants from higher latitudes produce larger seeds (Woods et al 2012) with higher germination success (Finch et al 2018) and faster germination times (Finch et al 2023). These latitudinal clines are consistent with adaptation to temperature, growing season length (GSL), or some other aspect of the environment that varies with latitude (De Frenne et al 2013).…”

Native bacteria and cyanobacteria can influence seedling emergence and growth of native plants used in dryland restoration

“…We also estimated the final biomass produced from 100 seeds sown as the total dry aboveground biomass that would be produced if we sowed 100 seeds for each treatment. This measure differs from the mass per surviving seedling as it includes non‐emerged and non‐survived seedlings (individuals with zero biomass following Finch et al, 2022).…”