中亚热带森林植物多样性增加导致细根生物量“超产”

Overyielding of fine root biomass as increasing plant species richness in subtropical forests in central southern China

细根在森林生态系统C分配和养分循环过程中发挥着重要作用,但对地下细根与植物多样性之间关系的研究相对较少。该研究选择中亚热带从单一树种的杉木(Cunninghamia lanceolata)人工林到多树种的常绿阔叶林(青冈(Cyclobalanopsis glauca)-石栎(Lithocarpus glaber)林)的不同植物多样性梯度,用根钻法采集细根并测定其生物量,用Win-RHIZO2005C根系分析系统测定细根形态参数,以验证以下3个假设:1)植物种类丰富度高的林分其细根生产存在"地下超产"现象;2)根系空间生态位的分离水平是否随着植物多样性增多而增大?3)细根是否通过形态可塑性对林木竞争做出响应?结果显示:从单一树种的杉木人工林到植物种类较复杂的青冈-石栎常绿阔叶林,0-30cm土层的林分细根总生物量和活细根生物量均呈增加的趋势,即细根总生物量为杉木林(305.20g·m-2)<马尾松(Pinus massoniana)林(374.25g·m-2)<南酸枣(Choerospondias axillaris)林(537.42g·m-2)<青冈林(579.33g·m-2),活细根生物量为杉木林(268.74g·m-2)<马尾松林(299.15g·m-2)<南酸枣林(457.32g·m-2)<青冈林(508.47g·m-2),各森林类型之间的细根总生物量差异显著(p<0.05),但活细根生物量差异不显著。土壤垂直剖面上,除杉木林细根生物量随土层变化不显著外,其他森林类型的活细根生物量和总细根生物量均随土层变化显著,表层细根生物量随树种多样性的升高呈减小趋势,据此推测树种间的生态位分离水平逐渐增大。植物多样性的不同对林分的细根形态及空间分布格局影响不显著,细根形态可塑性对生物量变化响应不明显。

Aims The objectives were to compare the fine root biomass, necromass and their distributions in the soil profile and examine the effects of plant species diversity on the morphological properties of live fine roots at different soil depths among four forests along a gradient of increasing plant species richness in subtropical southern China. Methods The fine root samples were collected in March 2010 in four forests： coniferous monoculture （Cunninghamia lanceolata）, coniferous pioneer species （Pinus massoniana-Lithocarpus glaber）, deciduous （ Choero- spondias axillaris） and evergreen broadleaved （Cyclobalanopsis glauca-Lithocarpus glaber）. In each forest, three soil cores containing fine roots were taken at each of three soil depth （0-10, 10-20 and 20-30 cm） at each of upper, middle and lower slope positions. Living and dead roots were manually separated and then oven dried at 75 ℃ to constant mass to determine weight. Living roots were scanned with a Win-RHIZO 2005C to measure morphological parameters, including total root length, total root volume, number of root tips and forks. Important findings Fine root biomass tended to increase along the gradient from the plantation to evergreen broadleaved forest. Total fine root biomass within the 0-30 cm soil depth differed significantly among the forests, but there were no significant differences in living root biomass. Fine root biomass density decreased exponentially with soil depth at similar rates among the forests. With exception of the plantation, fine root biomass showed clear differences in their distribution in the soil layers, indicating significant belowground spatial niche segregation of the vertical root distribution patterns in the more species-rich stands. Differences in root biomass did not lead to significant differences in fine root morphology on a stand area basis.