Tracing Intraspecific Variation in Milkweed Functional Traits Back to Germination Timing

Variability in plant functional traits is key to population resilience in the face of changing environments. This intraspecific trait variation is especially important at the vulnerable seedling stage, but the sources of trait variation among seedlings are not fully understood. In this study, we tested the hypothesis that population-level trait variation can be partly attributed to variation in developmental timing. We recorded the date of germination of individual showy milkweed (Asclepias speciosa) seedlings, then tracked their functional traits during the first growing season. Showy milkweed is native to the western United States and is a major host plant for monarch butterflies (Danaus plexippus) in the Pacific Northwest. We found that germination timing occurred over a 26-day range even in a common environment. This variation in developmental timing was a significant predictor of later intraspecific trait variation among milkweed seedlings. Variation in height and aboveground leaf area ratio were particularly influenced by developmental timing. These findings suggest that developmental timing variability may be an important yet under-studied source of intraspecific trait variation, with implications for ecological interactions and resilience. Key Points To better understand functional variation in showy milkweed seedlings, we recorded germination dates and functional traits over the first growing season. Germination timing was variable among individuals and this developmental variability contributed significantly to later variation in functional traits. Later-germinating milkweed seedlings were less vigorous (shorter, with less structural support for leaves) at the same date than earlier-germinating neighbors.