At local scales, it has been suggested that high levels of resources lead to increased tree growth via trait optimization (highly peaked trait distribution). However, this contrasts with (i) theories that suggest that trait optimization and high growth occur in the most common resource level and (ii) empirical evidence showing that high trait optimization can be also found at low resource levels. This raises the question of how are traits and growth optimized in highly diverse plant communities? Here, we propose a series of hypotheses about how traits and growth are expected to be maximized under different resource levels (low, the most common, and high) in tree seedling communities from a subtropical forest in Puerto Rico. We studied the variation in the distribution of biomass allocation and leaf traits and seedlings growth rate along four resource gradients: light availability (canopy openness) and soil K, Mg, and N contents. Our analyses consisted of comparing trait kurtosis (a measurement of trait optimization), community trait means, and relative growth rates at three resource levels (low, common, and high). Trait optimization varied across the three resource levels depending on the type of resource and trait, with leaf traits being optimized under high N and in the most common K and Mg conditions, but not at any of the light levels. Also, seedling growth increased at high light conditions and high N and K but was not related to trait kurtosis. Our results indicate that local-scale variability of soil fertility and understory light conditions result in shifts in species ecological strategies that increase growth despite a weak trait optimization, suggesting the existence of alternative phenotypes that achieve similar high performance. Uncovering the links between abiotic factors, functional trait diversity and performance is necessary to better predict tree responses to future changes in abiotic conditions.

中文翻译：

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响应局部土壤和光照变化的树苗性状优化和生长

在局部范围内，有人提出高水平的资源通过性状优化（高峰值性状分布）导致树木生长增加。然而，这与 (i) 理论认为性状优化和高增长发生在最常见的资源水平以及 (ii) 经验证据表明高性状优化也可以在低资源水平上找到。这就提出了一个问题，即如何在高度多样化的植物群落中优化性状和生长？在这里，我们提出了一系列关于如何在波多黎各亚热带森林的树苗群落中在不同资源水平（低、最常见和高）下最大化性状和生长的假设。我们研究了沿四个资源梯度的生物量分配和叶片性状和幼苗生长速率分布的变化：光照（冠层开度）和土壤 K、Mg 和 N 含量。我们的分析包括比较特征峰度（特征优化的度量）、社区特征均值和三个资源水平（低、普通和高）的相对增长率。性状优化在三个资源级别上有所不同，具体取决于资源类型和性状，在高 N 和最常见的 K 和 Mg 条件下优化了叶子性状，但在任何光照水平下都没有。此外，在高光条件和高 N 和 K 下幼苗生长增加，但与性状峰度无关。我们的结果表明，尽管性状优化较弱，但土壤肥力和林下光照条件的局部尺度变异导致物种生态策略发生变化，从而促进生长，这表明存在实现类似高性能的替代表型。揭示非生物因素、功能性状多样性和性能之间的联系对于更好地预测树木对未来非生物条件变化的反应是必要的。