Many semi-arid systems are experiencing increasing rates and severity of environmental change. In this context, seeds have become a focal point for supporting recovery and stability in plant communities, especially through the practice of seed-based restoration. Seeds that vary in their environmental requirements to germinate could help spread risks to plant recovery through time and contribute to vegetative resilience. However, different mechanisms of germination delay related to dormancy-breaking, light, or temperature requirements are rarely characterized as key aspects of plant functioning or integrated into community assembly models – which also hinders their strategic use in seed-based restoration. We identified 50 grass, forb, and shrub species relevant to restoration in the semi-arid grass- and shrublands of the western US and screened these for a suite of seed germination and early seedling attributes. We then asked: 1) Are there independent mechanisms of germination delay related to light, temperature, and dormancy-breaking? 2) Are these related to subsequent strategies in germinated seedlings? We hypothesized that species would exhibit a diverse set of tradeoffs related to stress avoidance and tolerance, and specifically, that seed traits related to avoidance (e.g., dormancy) would be associated with lower seedling stress tolerance (e.g., lower specific leaf area). We found wide variation in the degree of germination sensitivity to light, temperature, and dormancy-breaking, and these were not strongly correlated across species – suggesting that seed germination timing may reflect unique combinations of delay mechanisms within each species. For example, total germination (%) responded significantly to light condition for about a third of species – including many with minor sensitivity (≤15% difference) and fewer with more extreme sensitivity (≥40% difference). In contrast, dormancy-breaking increased total germination for most species with low initial levels (≤70%), but response magnitude varied widely. We also found some preliminary ties to seedling function: species requiring warmer germination temperatures tended to have thinner leaves and more root biomass, which may impact how seedlings of cold-avoiding species tolerate freezing or drought, respectively (currently being tested).
Conclusions: Germination delay is a complex reflection of variation in seed dormancy and sensitivity to multiple environmental factors. This variation likely contributes to differences among species and populations in risk avoidance (e.g., freezing, light limitation) that may be especially important when seedlings are less tolerant of such risks. Quantifying germination delay strategies can provide a foundation to model risk avoidance in plant communities, and subsequently, enhance it in seed-based restoration.
