Intraspecific Relationships among Wood Density, Leaf Structural Traits and Environment in Four Co-Occurring Species of Nothofagus in New Zealand

Plant functional traits capture important variation in plant strategy and function. Recent literature has revealed that within-species variation in traits is greater than previously supposed. However, we still have a poor understanding of how intraspecific variation is coordinated among different traits, and how it is driven by environment. We quantified intraspecific variation in wood density and five leaf traits underpinning the leaf economics spectrum (leaf dry matter content, leaf mass per unit area, size, thickness and density) within and among four widespread Nothofagus tree species in southern New Zealand. We tested whether intraspecific relationships between wood density and leaf traits followed widely reported interspecific relationships, and whether variation in these traits was coordinated through shared responses to environmental factors. Sample sites varied widely in environmental variables, including soil fertility (25–900 mg kg–1 total P), precipitation (668–4875 mm yr–1), temperature (5.2–12.4 °C mean annual temperature) and latitude (41–46 °S). Leaf traits were strongly correlated with one another within species, but not with wood density. There was some evidence for a positive relationship between wood density and leaf tissue density and dry matter content, but no evidence that leaf mass or leaf size were correlated with wood density; this highlights that leaf mass per unit area cannot be used as a surrogate for component leaf traits such as tissue density. Trait variation was predicted by environmental factors, but not consistently among different traits; e.g., only leaf thickness and leaf density responded to the same environmental cues as wood density. We conclude that although intraspecific variation in wood density and leaf traits is strongly driven by environmental factors, these responses are not strongly coordinated among functional traits even across co-occurring, closely-related plant species.

植物功能性状（Plant functional traits）能够捕捉植物策略与功能的重要变异。近期相关研究表明，物种内的性状变异程度远超此前的认知。但目前我们对不同性状间的种内变异如何协同调控，以及种内变异受环境驱动的机制仍知之甚少。我们针对新西兰南部4种广泛分布的南山毛榉属（Nothofagus）树木物种，量化了木材密度与支撑叶片经济谱（leaf economics spectrum）的5项叶片性状的种内与种间变异——这5项叶片性状分别为叶片干物质含量、单位面积叶片干质量、叶片大小、叶片厚度与叶片组织密度。我们检验了木材密度与叶片性状间的种内关系是否符合广泛报道的种间关系，同时检验了这些性状的变异是否通过对环境因子的共同响应而得到协调。采样位点的环境因子差异显著，涵盖土壤肥力（全磷含量25–900 mg·kg⁻¹）、降水量（668–4875 mm·yr⁻¹）、年均温（5.2–12.4 °C）以及纬度（南纬41°–46°）。在同一物种内，各叶片性状间呈现显著的相关性，但与木材密度并无显著关联。有部分证据表明木材密度与叶片组织密度、叶片干物质含量间存在正相关关系，但未发现叶片质量或叶片大小与木材密度存在相关性；这一结果表明，单位面积叶片干质量无法作为叶片组分性状（如组织密度）的替代指标。性状变异可由环境因子预测，但不同性状的响应模式并不统一：例如，仅叶片厚度与叶片密度与木材密度响应相同的环境信号。我们的研究结论如下：尽管木材密度与叶片性状的种内变异强烈受环境因子驱动，但即便在共存且亲缘关系相近的植物物种间，这些响应在不同功能性状间也未呈现显著的协同一致性。