Are dormant plants hedging their bets? Demographic consequences of prolonged dormancy in variable environments

During the growing season, some individuals in perennial plant populations may remain alive below ground while others emerge. This phenomenon, known as prolonged dormancy, seems maladaptive, because prolonged dormancy delays growth and reproduction. However, prolonged dormancy may offer the benefit of safety while below ground, leading to the hypothesis that prolonged dormancy is a bet hedging strategy. We evaluated this hypothesis using a 25-year demographic study of Astragalus scaphoides, an iteroparous perennial plant. First, we determined the relationship between prolonged dormancy and fitness using data from individuals in our population. This analysis showed that prolonged dormancy decreased arithmetic mean fitness and reduced variance in fitness. Geometric mean fitness was maximized at intermediate levels of prolonged dormancy. Empirical patterns of lifetime reproductive success confirm this relationship. We also compared fitness of plants in our population to hypothetical plants without prolonged dormancy, which generally revealed benefits of prolonged dormancy, even if plants could forego prolonged dormancy without costs to other vital rates. Therefore, prolonged dormancy may indeed function as a bet hedging strategy, but the benefits of remaining below ground only outweigh the costs for a subset of individuals. Bet hedging has been demonstrated in plants with simple life histories, such as annuals and monocarpic perennials; we present evidence that bet hedging may be important for plants with more complex life histories.

在生长季中，多年生植物种群内的部分个体可能保持地下存活状态，其余个体则会萌发出土。这一被称为长期休眠（prolonged dormancy）的现象看似是非适应性的，因其会延缓植株的生长与繁殖过程。然而，长期休眠可为地下存活的个体提供生存庇护，由此提出假说：长期休眠是一种避险策略（bet hedging）。我们以囊果黄芪（Astragalus scaphoides）——一种多次结实的多年生植物——为研究对象，通过长达25年的种群统计学研究对该假说展开验证。首先，我们借助研究种群内个体的观测数据，明确了长期休眠与适合度（fitness）之间的关联。分析结果显示，长期休眠会降低算术平均适合度，同时缩小适合度的变异幅度；几何平均适合度则在长期休眠处于中等水平时达到峰值。植株终身繁殖成功的实证模式也印证了这一关联。我们还将研究种群内植株的适合度与无长期休眠的假想植株进行了对比，结果普遍表明长期休眠具备生存优势——即便植株可以在不牺牲其他关键生命率的前提下放弃长期休眠。由此可见，长期休眠或许确实属于避险策略，但仅在部分个体中，地下存活带来的收益才会超过其所付出的代价。此前，避险策略已在生活史较为简单的植物类群中得到证实，例如一年生植物与一次性结实多年生植物；本研究提供的证据则表明，避险策略对于生活史更为复杂的植物类群同样可能具有关键意义。