Relationship between herbivory and leaf traits in mangroves on Iriomote Island, southern Japan

Leaf herbivory potentially affects carbon (C) and nutrient cycling in forest ecosystems. It is especially critical to study the leaf herbivory rates in mangroves to understand the pattern of resource changes from the detrital pathway to the grazing pathway. However, determinants of leaf herbivory rates in mangroves are poorly understood. We examined the differences in leaf herbivory rates among leaf phenotypes (sun or shade leaves), leaf ages (young or old leaves), and season (summer or winter) and investigated the relationships between the leaf herbivory rates and 12 leaf traits (including chemical, physiological, and physical/mechanical traits) in each of six communities from the seaward fringe to the inland of subtropical mangroves on Iriomote Island, southern Japan. The herbivory rates decreased from seaward to landward mangrove species, excluding one species in the landward mangroves. The herbivory rates also decreased as the concentration of condensed tannins or the tannins:nitrogen ratio increased but did not correlate with the other leaf traits. Overall, season, leaf phenotype, and leaf age had no effect on the herbivory rate. However, in the two seaward and one rhizophoraceous species, the herbivory rates were higher in summer than in winter and were affected by the leaf phenotypes. The concentration of leaf condensed tannins moderately influenced leaf herbivory, fluctuating from seaward to landward; therefore, it was one of the factors driving the grazing pathway in mangroves. Understanding these mechanisms is critical for a comprehensive appreciation of the high productivity of and nutrient cycling in mangrove forest ecosystems.

叶片植食作用（leaf herbivory）可能会影响森林生态系统的碳（C）与养分循环。研究红树林中的叶片植食率，对于理解资源从碎屑食物链途径（detrital pathway）向牧食食物链途径（grazing pathway）的转变模式尤为关键。然而，目前学界对红树林叶片植食率的调控因子仍知之甚少。本研究以日本南部西表岛亚热带红树林从向海边缘向内陆分布的6个群落为研究对象，分析了不同叶片表型（leaf phenotype，包括阳生叶与阴生叶）、叶片年龄（leaf age，包括幼叶与老叶）以及季节（夏季与冬季）下的叶片植食率差异，并探究了叶片植食率与12项叶片性状（leaf traits，涵盖化学、生理以及物理/机械性状）之间的关联。研究结果显示，红树林物种的植食率整体随向海至内陆的分布梯度呈下降趋势，仅内陆红树林中的1个物种为例外。植食率随缩合单宁（condensed tannins）浓度或单宁氮比（tannins:nitrogen ratio）的升高而降低，但与其余叶片性状无显著关联。整体来看，季节、叶片表型与叶片年龄对植食率并无显著影响。但在2个向海分布物种与1个红树科物种中，夏季植食率显著高于冬季，且植食率受叶片表型的调控。叶片缩合单宁浓度对叶片植食作用存在中等程度的调控效应，且随向海至内陆的分布梯度发生波动，因此其是驱动红树林牧食食物链途径的关键因子之一。阐明这些调控机制，对于全面理解红树林生态系统的高生产力与养分循环过程至关重要。