How do functional traits syndromes covary with growth and reproductive performance in a water-stressed population of Fagus sylvatica?

A central issue in plant evolutionary ecology is to understand how several coordinated suites of traits (i.e. traits syndrome) may be jointly selected within a single species. This study aims to describe patterns of variation and co-variation of functional traits in a water-stressed tree population and test their relationships with performance traits. Within a Mediterranean population of Fagus sylvatica experiencing recurrent summer droughts, we investigated the phenotypic variation of leaf unfolding phenology, Leaf Area (LA), Leaf Mass per Area (LMA), Leaf Water Content (LWC), water use efficiency (WUE) estimated by carbon isotopic discrimination (d13C), twig Huber-value (HV: the stem cross-section divided by the leaf area distal to the stem), wood density (WDens), and leaf nitrogen content (Nmass). First, a Principal Component Analysis revealed that two main axes structured the phenotypic variability: the first axis opposed leaf unfolding earliness and LWC to LMA and WUE; the second axis opposed LA to HV. These two axes can be interpreted as the opposition of two strategies (water economy versus water uptake) at two distinct scales (leaf for the first axis and branches for the second axis). Second, we found that LMA, LA, leaf unfolding and LWC responded differently to competition intensity, while WUE, WDens and HV did not correlate with competition. Third, we found that all studied functional traits were related to growth and/or reproductive performance traits and that these relationships were frequently non-linear, showing strong interactions between traits. By highlighting phenotypic clustering of functional traits involved in response to water stress and by evidencing antagonistic selection favouring intermediate trait values as well as trait combinations, our study brought new insights on how natural selection operates on plant functional traits in a stressful environment.植物进化生态学的核心议题之一，是阐明单一物种种群内多套协同性状（即性状综合征，traits syndrome）如何受到联合选择。本研究旨在描述水分胁迫林木种群功能性状的变异与协变模式，并检验其与表现性状的关联。本研究以经历周期性夏季干旱的地中海欧洲山毛榉（Fagus sylvatica）种群为研究对象，调查了以下功能性状的表型变异：叶片展叶物候、叶面积（Leaf Area, LA）、比叶重（Leaf Mass per Area, LMA）、叶片含水量（Leaf Water Content, LWC）、基于碳同位素判别（carbon isotopic discrimination, d13C）估算的水分利用效率（water use efficiency, WUE）、小枝胡伯值（twig Huber-value, HV：茎横截面积与茎远端叶面积的比值）、木材密度（wood density, WDens）以及叶片氮含量（leaf nitrogen content, Nmass）。首先，主成分分析（Principal Component Analysis）结果显示，两条主轴构成了表型变异的核心结构：第一轴将展叶早性与叶片含水量，与比叶重和水分利用效率呈负相关；第二轴则将叶面积与小枝胡伯值呈负相关。这两条主轴可被解读为两个不同尺度上的两种对立策略——第一轴对应叶片尺度的水分经济策略与水分吸收策略的权衡，第二轴对应枝条尺度的同类权衡。其次，研究发现比叶重、叶面积、展叶物候与叶片含水量对竞争强度的响应模式存在显著差异，而水分利用效率、木材密度与小枝胡伯值则与竞争强度无显著关联。第三，所有被测功能性状均与生长及/或繁殖表现性状存在关联，且此类关联多呈非线性，表明性状间存在强烈的交互作用。本研究通过揭示参与水分胁迫响应的功能性状的表型聚类现象，并证实拮抗选择会青睐中间性状值与特定性状组合，为自然选择如何在胁迫环境下作用于植物功能性状提供了新的认知。