1 INTRODUCTION

The seedling stage represents one of the most vulnerable and elusive periods of the plant life cycle (Leck et al., 2008). Seedling recruitment can be one of the greatest bottlenecks to population growth (e.g. Eriksson & Ehrlén, 2008) and determinants of conservation and restoration success (e.g. Shackelford et al., 2021). However, despite their outsized importance, small sizes and short developmental timescales of seedlings can make it challenging to measure function and performance. Changes in temperature, water and herbivory pressure can rapidly impact seedlings, making it difficult to pinpoint when and why mortality occurs in natura. Further, seedling growth strategies can vary ontogenetically (i.e. at discrete stages of seedling development; Boege & Marquis, 2005; Moles & Leishman, 2008; Garbowski et al., 2021) and respond to abiotic and biotic factors in the surrounding environment (Kitajima & Fenner, 2000). Despite this understanding, our ability to successfully predict seedling dynamics in non-model systems, including most ecological systems and systems in disequilibrium (e.g. disturbed sites), remains limited.

Studying seedling functional traits (i.e. measurable attributes that influence seedling growth, survival and fitness) may improve prediction in multiple ecological and evolutionary subdisciplines. For example, seedling leaf and root traits have been shown to interact with selection pressures and explain variation in recruitment success (e.g. Daws et al., 2007; Harrison & LaForgia, 2019; Khurana & Singh, 2001; Krannitz et al., 1991; Moles & Westoby, 2004). Emerging evidence suggests that a functional perspective on seedling ecology has the potential to shed light on climate change mitigation (e.g. Lewandrowski et al., 2021; Walck et al., 2011), restoration and conservation of species and ecosystem services (e.g. Havrilla et al., 2021; Larson et al., 2015) and the eco-evolutionary dynamics of plant life (e.g. Razzaque & Juenger, 2022; Simons, 2009). However, relative to the wide availability of trait data measured from adult plants in global trait databases (e.g. Kattge et al., 2020), there remains a dearth of data and information on seedling traits for most species (Saatkamp et al., 2019).

Practical and methodological challenges associated with studying seedling dynamics may contribute to there being limited functional trait ecology research on seedlings, particularly for anything other than tree species. The ease of studying and measuring seedlings is likely determined by multiple, interacting research challenges, including access to appropriate experimental resources (e.g. Casad et al., 2021; Cooper & Berry, 2020), existing knowledge about target species (e.g. Buhk et al., 2007) and perhaps most obviously, the small size and relative delicateness of the organisms being studied (e.g. Tang et al., 1992). Because of these challenges, there has been limited research on the functional traits of seedlings outside of agricultural systems (e.g. Cooper & MacDonald, 1970) and model plant species (e.g. Eastmond et al., 2015). While the number of studies on seedling function has been increasing steadily over the past 20 years (Figure 1), the vast majority has been directed towards trees, with only a small portion of seedling trait research centered on herbaceous and woody shrub plant species (26.87%, Appendix 1a).

Our objective is to advance seedling functional ecology research by communicating the value of these data, streamlining decision-making in research design and providing insight on the numerous considerations and challenges to be addressed as this field takes off. First, we define and describe the term seedling and the botanical and ecological characters used to describe seedlings. We then describe why seedling studies are essential to functional ecology and the critical importance of comparable and standardised methods in seedling research. At last, we provide an overview of major steps in the research process to consider when designing a seedling trait study. In doing so, we provide numerous experimental considerations that can influence the outcome of seedling studies and likely mediate establishment trajectories. For example, we explain important ecological and evolutionary considerations when sourcing seeds and describe the environmental and experimental conditions typically required to answer research questions focused on seedling traits. We also define important ontogenetic considerations and approaches to measuring seedlings through time and describe the importance of ontogeny and trait selection. We describe different experimental approaches, their applications and limitations with examples from the existing seedling trait literature. We focus primarily on ecological studies but leverage important insights from evolutionary biology and agricultural science. While experimental design decisions for each study will be determined based on goals of the experiment, we aim to guide future work along established and comparable methodological pathways. With a common vision, the growing literature on seedling functional ecology will be primed to provide more robust insight and solutions to global challenges during the Anthropocene.

中文翻译：

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促进幼苗功能生态学的可比研究

1 简介

幼苗阶段代表植物生命周期中最脆弱和最难以捉摸的时期之一（L​​eck 等，  2008）。幼苗补充可能是种群增长的最大瓶颈之一（例如Eriksson＆Ehrlén，  2008）以及保护和恢复成功的决定因素（例如Shackelford等人，  2021）。然而，尽管幼苗非常重要，但其尺寸小和发育时间短，使得测量功能和性能变得具有挑战性。温度、水和食草压力的变化会迅速影响幼苗，因此很难确定自然死亡的时间和原因。此外，幼苗生长策略可以在个体发育上有所不同（即在幼苗发育的不同阶段；Boege & Marquis,  2005；Moles & Leishman,  2008；Garbowski et al.,  2021）并对周围环境中的非生物和生物因素做出反应（Kitajima &芬纳，  2000）。尽管有这样的认识，我们成功预测非模型系统（包括大多数生态系统和不平衡系统（例如受干扰地点）的系统）中幼苗动态的能力仍然有限。

研究幼苗功能性状（即影响幼苗生长、存活和适应性的可测量属性）可以改善多个生态和进化子学科的预测。例如，幼苗的叶和根性状已被证明与选择压力相互作用并解释了招募成功的变化（例如Daws等人，  2007年；Harrison＆LaForgia，  2019年；Khurana＆Singh，  2001年；Krannitz等人，  1991年）。莫尔斯和韦斯托比，  2004）。新出现的证据表明，幼苗生态学的功能视角有可能为减缓气候变化（例如Lewandrowski等人，  2021年；Walck等人，  2011年）、物种和生态系统服务的恢复和保护（例如Havrilla等人）提供启示。 .,  2021 ; Larson et al.,  2015）和植物生命的生态进化动力学（例如 Razzaque & Juenger,  2022 ; Simons,  2009）。然而，相对于全球性状数据库中成年植物测量的性状数据的广泛可用性（例如 Kattge 等人，  2020），大多数物种的幼苗性状数据和信息仍然缺乏（Saatkamp 等人，  2019） 。

与研究幼苗动态相关的实践和方法学挑战可能导致对幼苗的功能性状生态学研究有限，特别是对于树种以外的任何物种。研究和测量幼苗的难易程度可能取决于多个相互作用的研究挑战，包括获得适当的实验资源（例如 Casad 等人，  2021 年；Cooper & Berry，  2020 年）、有关目标物种的现有知识（例如 Buhk 等人，2020 年）。 ，  2007），也许最明显的是，所研究的生物体体积小且相对脆弱（例如 Tang 等人，  1992）。由于这些挑战，对农业系统之外的幼苗的功能性状（例如Cooper＆MacDonald， 1970）和模式植物物种（例如Eastmond等人，  2015 ）的研究有限 。虽然过去 20 年来有关幼苗功能的研究数量稳步增加（图 1），但绝大多数都是针对树木，只有一小部分幼苗性状研究集中在草本和木本灌木植物物种上（26.87 %，附录 1a)。

我们的目标是通过传达这些数据的价值、简化研究设计中的决策并提供对该领域起飞时需要解决的众多考虑因素和挑战的见解，来推进幼苗功能生态学研究。首先，我们定义和描述术语“幼苗”以及用于描述幼苗的植物学和生态特征。然后，我们描述了为什么幼苗研究对于功能生态学至关重要，以及幼苗研究中可比和标准化方法的至关重要性。最后，我们概述了设计幼苗性状研究时要考虑的研究过程中的主要步骤。在此过程中，我们提供了许多实验考虑因素，这些因素可以影响幼苗研究的结果并可能介导建立轨迹。例如，我们解释了采购种子时重要的生态和进化考虑因素，并描述了回答针对幼苗性状的研究问题通常所需的环境和实验条件。我们还定义了重要的个体发育考虑因素和随时间测量幼苗的方法，并描述了个体发育和性状选择的重要性。我们通过现有幼苗性状文献中的例子描述了不同的实验方法、它们的应用和局限性。我们主要关注生态研究，但利用进化生物学和农业科学的重要见解。虽然每项研究的实验设计决策将根据实验目标确定，但我们的目标是沿着已建立的和可比较的方法学途径指导未来的工作。有了共同的愿景，越来越多的关于幼苗功能生态学的文献将为人类世期间的全球挑战提供更强有力的见解和解决方案。