Key message

Though competition operates distinctly above and below ground, competition with near neighbours explained more variation in crown and root system morphology of young tropical trees than did abiotic conditions.

Abstract

Comparisons of above- and below-ground tree architecture and variation remain scarce in the literature of competition dynamics in forests. Trees compete above and below ground with neighbours, but the extent of such a neighbourhood and the relative importance of above- or below-ground neighbours in determining tree growth rates remain poorly understood. We combined terrestrial laser scans and a root excavation campaign in the tropical planted Sardinilla forest experiment with annual diameter-at-breast height measurements to test how the crowns and root systems of five tropical tree species (N = 128) varied with biotic neighbourhood and abiotic environmental characteristics. Crown and root system extent were compared, and the Hegyi competition index calculated at three spatial scales was regressed on focal tree biomass. We identified which neighbourhood and environmental variables explained most of the variation in mean crown and root system extent and compactness (a measure of morphological homogeneity) at the plot level and for species separately. Root systems were 2.6 ± 1.1 times wider than crowns on average and focal tree biomass was the best predicted by competition with near neighbours. At the plot level, more variation in crown and root system traits was explained by biotic than abiotic characteristics (65.8 and 26.0%, respectively). More intense competition in the near neighbourhood reduced crown extent of one study species (Anacardium excelsum) and root system extent of 4 out of 5 study species, and it also led to more compact crowns and root systems of Tabebuia rosea. After accounting for spatial autocorrelation in focal tree location, soil nitrogen concentration was not found to significantly affect the studied morphological traits. These results emphasize the role of competition in determining tree morphology generally, while also supporting the claim that competition operates distinctly above and below ground.

中文翻译：

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邻里对热带树冠和根系的范围和紧密度的影响

关键信息

尽管地上和地下的竞争明显不同，但与近邻的竞争比非生物条件解释了年轻热带树冠和根系形态的更多变化。

抽象的

在森林竞争动态的文献中，对地上和地下树木结构和变化的比较仍然很少。树木在地上和地下与邻居竞争，但这种邻居的范围以及地上或地下邻居在确定树木生长速度方面的相对重要性仍然知之甚少。我们将陆地激光扫描和热带种植沙丁鱼森林中的根部挖掘活动与年度胸径测量相结合，以测试五种热带树种（N = 128) 随生物邻域和非生物环境特征而变化。比较冠和根系范围，并在三个空间尺度计算的合宜竞争指数对焦点树生物量进行回归。我们确定了哪些邻域和环境变量分别解释了地块水平和物种的平均冠和根系范围和紧密度（形态均匀性的度量）的大部分变化。根系平均比树冠宽 2.6 ± 1.1 倍，焦点树生物量是与近邻竞争的最佳预测。在地块水平上，生物特征解释了冠部和根系特征的更多变化，而非生物特征（分别为 65.8% 和 26.0%）。邻近地区更激烈的竞争减少了一个研究物种的冠部范围（腰果猪血）和4超过了5种研究的根系程度，这也导致更紧凑的牙冠和牙的根系统红花风铃木。在考虑了焦点树位置的空间自相关后，未发现土壤氮浓度对研究的形态性状有显着影响。这些结果强调了竞争在总体上决定树木形态方面的作用，同时也支持了竞争在地上和地下明显运作的说法。