毛竹种群向针阔林扩张的根系形态可塑性

为了弄清毛竹(Phyllostachys edulis)向针阔林扩张过程中根系的形态可塑性反应,在浙江天目山自然保护区毛竹向针阔林扩张的典型过渡地带,连续区域上设置毛竹纯林、针阔-毛竹混交林(以下简称过渡林)、针阔林3种样地。用根钻法采集样地毛竹根系、针阔树根系并比对其生物量密度、细根比根长、相邻同级侧根节点距等形态特征参数变化。结果表明:随着毛竹的扩张程度增加,林内根系生物量密度增加;且与针阔树竞争过程中毛竹将更多的根系放置于表层;同时在水平方向上随离样株距离的增加未出现明显变化,而针阔树根系则随离样木距离的增加而逐渐减少;毛竹根系比根长明显增加,平均增幅15%;一、二级侧根节点距则均有所下降,毛竹侧根数量增多。这些结果表明毛竹种群可通过根系生物量密度、细根比根长、相邻同级侧根节点距等形态可塑性方式实现向周边森林扩张。

The response of root morphological plasticity to the expansion of a population of Phyllostachys edulis into a mixed needle-and broad-leaved forest

Moso bamboo(Phyllostachys edulis), an economically important plant, is often cultivated not only for its delicious shoots and versatile culms, but also as an important biomass resource in southern China. However, with its robust growth and strong rhizomes, Moso bamboo populations have been expanding rapidly into adjacent forests. Root plasticity may enable the bamboo to efficiently forage for nutrients in heterogeneous soils. The ability of a Moso bamboo plant to respond to competition through root plasticity may be an important trait for the species, but little information is available on how root systems respond to belowground competition during colony expansion. To provide baseline information on the morphological plasticity of the response of Moso bamboo roots during population expansion, we continuously sampled three forest types. These were 1) Moso bamboo forest; 2) mixed bamboo, needle-and broad-leaved forest(or transition forest); and 3) mixed needle-and broad-leaved forest. This study was conducted in a typical transition zone in the TianMu Mountain Nature Reserve, Zhejiang Province, China, where there is ongoing bamboo expansion. Rhizomes and roots from Moso bamboo, and roots from mixed needle-and broad-leaved trees were collected by the soil core method to analyze root morphological parameters, including root biomass density, special root length, and internodes length of lateral roots of the same order. The results showed that(1) the root/rhizome system biomass of the three forest types could be ranked as Moso bamboo forest(Moso bamboo root/rhizome system biomass=1630.46 g/m²) > Moso bamboo in the transition forest(902.76 g/m²) > the mixed needle-and broad-leaved forest(the needle-and broad-leaved trees root system biomass=412.76 g/m²) > the needle-and broad-leaved trees root system biomass in the transition forest(272.36 g/m²). As Moso bamboo expanded into a mixed needle-and broad-leaved forest, Moso bamboo culm density increased gradually, with a corresponding increase in root biomass.(2) In a vertical direction, the root system biomass of Moso bamboo in the 0-10 cm soil layer increased from 44% of all root biomass in the Moso bamboo forest to 56% in the transition forest. Simultaneously, the root biomass of needle-and broad-leaved trees shifted from the upper to lower soil layers. The root system of Moso bamboo managed to place more biomass in upper soil layers when it encountered other trees, allowing it to gain a competitive advantage.(3) In the horizontal direction, the root biomass of Moso bamboo did not significantly decline with an increase in distance from the sample tree, while the root biomasses of needle-and broad-leaved trees decreased with increasing distance from the sample tree.(4) The special root length of Moso bamboo in the transition forest increased by an average of 15% compared to the Moso bamboo forest, while the internodes length of first and second lateral roots decreased. The number of bamboo lateral roots also increased. Moso bamboo, with its ability to increase the special root length and decrease the length of the lateral root internodes, would outcompete trees in the belowground environment. These findings indicate that the morphological plasticity of Moso bamboo, shown by its root biomass, special root length, and internode length of the same order lateral roots, easily results in the expansion of Moso bamboo populations into surrounding forest.