高水位条件下池杉根系的生态适应机制和膝根的呼吸特性

  池杉(Taxodium ascendens)属于典型的耐水树种, 掌握其根系对淹水环境的生态适应机制对于研究林木耐水机理十分重要。通过对江苏省里下河低湿地17年生池杉在高水位(6～10月淹水, 全年平均地下水位-5 cm)、中水位(8～9月淹水, 全年平均地下水位-18 cm)和低水位(常年不淹水, 全年平均地下水位-41 cm)条件下的根系进行调查, 结果表明, 池杉在高水位条件下形成细长的气生根, 气生根依附于树干北侧或潜伏于树干外表皮内侧和纵裂的树皮缝隙中; 中水位池杉能形成直径(7.9±2.2) cm、高(7.7±2.7) cm的膝根, 每株立木拥有膝根数(5.8±1.7)个; 低水位池杉也能形成膝根, 但个体小、数量少。林木地下和地上生物量均呈现出明显的高水位<中水位<低水位的趋势, 但是地下/地上生物量的比值却呈相反趋势, 表明池杉耐水性虽然很强, 长期处于较高水位时生长会明显受抑, 尤其是地上生物量生长受抑更显著。高、中和低水位池杉的地径/胸径之比分别是2.66±0.11、2.08±0.10和1.75±0.08, 说明水位较高的环境能促进树干基部的相对粗生长。长期淹水导致地下根的容重降低, 但是气生根和膝根的容重却明显大于地下根。高、中水位池杉细根的Fe和Mn浓度显著高于低水位, 其中Fe的浓度相差10倍以上, 但是叶的Fe、Mn浓度在不同水位之间没有显著差异。膝根的呼吸具有明显的季节差异, 8月和9月平均每个膝根的呼吸速率为2.1～2.5 mgCO₂&#8226;h^–1, 6月和11月为0.7～0.9 mgCO₂&#8226;h^–1, 3月为0.4 mgCO₂&#8226;h^–1; 膝根吸收O₂的摩尔数是释放CO₂摩尔数的4.6倍, 说明膝根吸收的O₂除了供自身呼吸, 大部分是提供给地下根利用。池杉之所以具有较强的耐水性, 与其在缺氧环境中能形成气生根和膝根、树干基部膨大和根系容重降低等有利于改善根系通气条件的生态适应机制密切相关。

ECOLOGICAL ADAPTATION MECHANISMS OF ROOTS TO FLOODED SOIL AND RESPIRATION CHARACTERISTICS OF KNEE ROOTS OF TAXODIUM ASCENDENS

Aims Taxodium ascendens is a flood-tolerant tree species. It is important to understand the mechanisms of flood-tolerance by means of study on the root changes of T. ascendens in flooded conditions. Methods Based on the investigation of the roots of T. ascendens plantations in 17 year-old in Lixiahe wetland, Jiangsu Province, China, the ecological adaptations of the roots were analyzed in the different water table sites divided into three groups: high water table site (HWS, the site was flooded from June to October every year, and the depth of the mean water table in each year was –5 cm), middle water table site (MWS, the site was flooded from August to September every year, and the depth of the mean water table in each year was –18 cm), and low water table site (LWS, the site was not flooded all the time, and the mean water table in each year was –41 cm). Important findings In HWS, T. ascendens formed aerating roots which were very long and thin, and attached on the tree stem or in the outer epidermis or in the crack of tree bark; In the MWS, T. ascendens formed knee roots which were (7.9±2.2) cm in diameter, (7.7±2.7) cm in height; Although T. ascendens also formed the knee roots in LWS, they were smaller than in MWS. The belowground biomass and the aboveground biomass of the trees were in the order of HWS2&#8226;h^–1 in August and September, 0.7～0.9 mgCO₂&#8226;h^–1 in June and November, and 0.4 mgCO₂&#8226;h^–1 in March, respectively. The moles of oxygen absorption was 4.6 times more than that of carbon dioxide flux by knee roots. It appeared that the oxygen absorbed by knee roots was not only supplied to knee roots respiration, but also supplied to underground roots respiration. It was suggested that T. ascendens with high flood-tolerance was due to forming the aerating roots and knee roots, promoting the diameter growth of stem, and decreasing the volume weight of roots in the flooding sites for improving the ventilation condition of roots.