Differences in Fine-Root Biomass of Trees and Understory Vegetation among Stand Types in Subtropical Forests

Variation of total fine-root biomass among types of tree stands has previously been attributed to the characteristics of the stand layers. The effects of the understory vegetation on total fine-root biomass are less well studied. We examined the variation of total fine-root biomass in subtropical tree stands at two sites of Datian and Huitong in China. The two sites have similar humid monsoon climate but different soil organic carbon. One examination compared two categories of basal areas (high vs. low basal area) in stands of single species. A second examination compared single-species and mixed stands with comparable basal areas. Low basal area did not correlate with low total fine-root biomass in the single-species stands. The increase in seedling density but decrease in stem density for the low basal area stands at Datian and the quite similar stand structures for the basal-area contrast at Huitong helped in the lack of association between basal area and total fine-root biomass at the two sites, respectively. The mixed stands also did not yield higher total fine-root biomasses. In addition to the lack of niche complementarity between tree species, the differences in stem and seedling densities and the belowground competition between the tree and non-tree species also contributed to the similarity of the total fine-root biomasses in the mixed and single-species stands. Across stand types, the more fertile site Datian yielded higher tree, non-tree and total fine-root biomasses than Huitong. However, the contribution of non-tree fine-root biomass to the total fine-root biomass was higher at Huitong (29.4%) than that at Datian (16.7%). This study suggests that the variation of total fine-root biomass across stand types not only was associated with the characteristics of trees, but also may be highly dependent on the understory layer.

以往学界多将不同林分类型间的总细根生物量（total fine-root biomass）差异归因于林分层的特征，但针对林下植被（understory vegetation）对总细根生物量的影响，相关研究仍较为匮乏。本研究以中国大田与会同两处的亚热带林分（subtropical tree stands）为研究对象，探究其总细根生物量的变化规律。两处站点均具有相似的湿润季风气候，但土壤有机碳（soil organic carbon）水平存在差异。其一，针对单种林分（single-species stands）设置两组胸高断面积（basal area）梯度（高断面积与低断面积）开展对比分析；其二，选取胸高断面积相当的单种林分与混交林分（mixed stands）进行对比研究。在单种林分中，低胸高断面积并未对应较低的总细根生物量。就大田站点的低断面积林分而言，其幼苗密度上升但株密度（stem density）下降；而会同站点的断面积梯度组间林分结构并无显著差异——这两处的特征分别解释了为何两地均未观测到胸高断面积与总细根生物量间的关联。混交林分的总细根生物量亦未高于单种林分。除树种间缺乏生态位互补（niche complementarity）外，株密度与幼苗密度的差异，以及树木与非树植被间的地下竞争（belowground competition），同样是导致混交林分与单种林分总细根生物量相近的原因。综合所有林分类型来看，土壤更为肥沃的大田站点，其树木细根、非树细根及总细根生物量均高于会同站点。但非树细根生物量对总细根生物量的贡献率在会同站点（29.4%）高于大田站点（16.7%）。本研究表明，不同林分类型间的总细根生物量差异，不仅与树木自身特征相关，还可能高度依赖于林下植被层。