【作者】 李有志；

【导师】 谢永宏；

【作者基本信息】 湖南农业大学 ， 生态学， 2014， 博士

【摘要】 大规模的人工林,尤其是速生型人工林的单一种植,引发了有关生态环境的广泛担忧,如地力衰退、土壤旱化、地下水位降低等。关于生物多样性的研究主要集中在动物多样性上,对植物多样性的研究相对较少,且结论相互矛盾。本文以洞庭湖湿地杨树人工林(Populus deltoides)为研究对象,通过大面积的野外调查、固定样地连续监测与室内控制实验,揭示了速生型人工林对林下植物多样性的影响及机理。主要结论如下：(1)杨树人工林虽然能提高林下植物多样性指数,但改变了物种组成。以原生植被南荻(Miscanthus sacchariflorus)为对照,通过对洞庭湖湿地杨树人工林大面积的野外调查与对11龄与5龄杨树林、南荻三种植物群落固定样地(以下简称固定样地)的持续监测表明,杨树人工林有利于林下物种数目与Shannon-Wiener指数的提高。与南荻相比,杨树人工林具有较低的湿生植物、阴性植物与本地植物比例,较高的中生植物、阳性植物、中性植物与外来植物比例。然而,杨树人工林林下物种生活史组成(一两年生、多年生植物)与生长型组成(木本植物、藤本植物与草本植物)与南荻无显著差异。(2)杨树人工林通过改变影响林下植物多样性的关键环境因子而引起物种更替和多样性变化。通过对大范围野外调查的杨树人工林林下植物多样性与生态环境因子的冗余分析(RDA)与回归模型的优化分析表明：杨树种植密度、林下光照率与土壤水分含量是影响林下植物多样性及组成的关键环境因子。其次,通过对固定样地三种植物群落环境因子的连续监测表明：与南荻相比,杨树人工林具有较高的林下光照率与较低的土壤水分含量；杨树人工林改变了土壤粒径组成,致使粘粒与粉粒的降低、砂粒的增加,导致土壤中层(20-60cm)电导率的降低,以及下层(60-100cm)pH值的升高,对土壤全磷含量无明显影响,总体上降低了土壤全钾含量,引起土壤40-60cm全氮含量的降低,降低趋势随林龄的增加而增大。(3)叶片高气孔导度、高蒸腾速率以及低水分利用率是杨树人工林降低土壤水分含量的主要生理学机制。通过对固定样地三种植物群落在生长期间(5-9月)的光合与蒸腾特征逐月监测表明：三种群落植物叶片光合速率、气孔导度、蒸腾速率与水分利用率具有明显的季节动态；与南荻相比,11龄与5龄杨树人工林均具有较低的光合速率与水分利用率,较高的气孔导度与蒸腾速率。(4)低叶片数与低叶面积指数是杨树人工林提高林下光照利用率的主要形态学机制。通过对固定样地三种植物群落生长期间叶片数目、叶面积指数的逐月监测表明：三种植物群落叶片数目、叶长、叶宽、叶面积、叶面积指数均具有明显的季节动态,且叶面积指数与林下光照率呈显著负相关关系；与南荻相比,11龄与5龄杨树人工林群落具有较低的叶片数目、叶长、叶面积指数。(5)杨树林下不同水分需求生态型植物所表现出对水位的不同适应性,是导致人工林林下植物物种组成变化的主要原因。通过对杨树人工林林下中生植物白茅与湿生植物薹草在五种水位梯度(-20cm、-10cm、0cm、10cm、20cm)下的幼苗生长控制实验表明：-20cm与-10cm是白茅生长的最适水位,而-10cm是薹草生长的最适水位；两种植物在适宜的水位梯度下,通过高株高、高叶片数目、高叶面积等形态学特征以及高光合速率等光合生理特征,促进生物量的积累。更多还原

【Abstract】 Large areas of tree plantations, especially fast-growing tree plantations, have provked increasing concern about the negative effects on ecological environments, such as declinded soil fertility, dried soil, and decreased water table. Previous studies regarding the biodiversity mainly focused in animal diversity and less concerned plant diversity. More importantly, the conclusions regarding the effects of tree plantation on understory plant diversity were contradictory. In this paper, taking poplar plantations (Populus deltoids) in the Dongting Lake wetlands as objects, the mechanisms and effects of tree plantations on undestoty plant diversity were studied through large-area field investigations, fixed-plots monitorings, and indoor controlled experiments.(1) Poplar plantations increased diversity index of understory plant, but changed species composition. Effects of plopar plantations on understory plant diversity indicators and species composition were evalued by conduting a large-area field investigation on areas of poplar plantations and reeds(Miscanthus sacchariflorus, a native herbaceous plant) in the Dongting Lake wetlands, and by performing fixed-plots monitoring for two years among areas of11-year old and5-year old poplar plantations and reeds. Generally, poplar plantations resulted in a higher species number and Shannon’s diversity. Species compositions were different between areas with poplar and reed populations:a lower ratio of hygrophytes but a higher ratio of mesophytes, a higher ratio of heliophytes but a lower ratio of neutrophilous or shade plants, and a higher ratio of extotic plants and a lower ratio of native plants in areas having poplar plantations compared to those having reeds. However, there was no difference in the ratio of these plants claffied according to their life form and growth types.(2) Understory plant diversity was affected by the changed key environmental factors in poplar plantations. Stand density, understory light availability, and soil water content were the key environmental factors affecting understory plant diversity and species composition by conducting a RDA (Redundancy analysis) and optimized regression model analysis between understory plant diversity and species composition, and environmental factors in large-area field investigation among poplar plantations. In the growing seasons, understory light availability was higher and soil water content was lower in areas having poplar plantations than those having reeds by conducting one years monitoring among11-year old and5-year old poplar plantations and Reeds. Additionally, poplar plantation changed soil particle-size distribution and decreased the percentage of clay and silt but increased that of sand. Poplar plantation decreased electrical conductivity in middle soil (20-60cm), total potassium in all layers of soil, but increased pH in subsoil (60-100cm). There was no difference in phosphorus among three vegetations. Additionally, poplar plantations also decreased total nitrogen in soil layer of40-60cm, and the effect was increased with increased stand age.(3) A higher stomatal conductance and transpiration rate, but a lower water-use efficiency were the main physiological mechanism for the lower soil water content in poplar plantations. Plant photosynthetic and transpiration characteristics between poplar plantations and Reeds were compared by performing a one year monitoring among11-year old and5-year old poplar plantations and Reeds in the growing seasons from April to September. Results showed that photosynthetic rate, stomatal conductance, transpiration rate, and water-use efficiency of individual leaves of three vegetations were varied during the growing season. Compared to Reeds, both2poplar plantations have a lower photosynthetic rate and water-use efficiency, but a higher stomatal conductance and transpiration rate.(4) A lower leaf number and leaf area index were the main morphological mechanism for the higher understory light availability in poplar plantations. The leaf characteristics, including leaf number, leaf area, and leaf area index, were investigated by conducting a one years monitoring among11-year old and5-year old poplar plantations and Reeds. Results showed that leaf number, leaf length, leaf width, leaf area, and leaf area index in three vegetations were varied during the growing season. Leaf area index in three vegetations were negatively correlated with understory light availability. Compared to Reeds, both two poplar plantations have a lower leaf number, leaf length, and leaf area index.(5) Different adaptability to water tale among indentical ecological types according to water requirement was the main reason for the changes of species composition in poplar plantations. Adaptive abilities between two dominant plant species under poplar plantations (Imperata cylindrical, a mesophytes, and Carex tristachya, a hygrophytes) planted in plastic tanks with five water table gradient (-20cm,-10cm,0cm,10cm,20cm) were studies. Results showed that the most favorable water tables for plant growth were different between two plants:-20cm and-10cm for I. cylindrical and-10cm for C. tristachya. Biomass of both two plants were increased in favorable water table by higher plant height, higher leaf number, and higher leaf area to acquiring more light resources and by a higher photosynthetic rate compared to in unfavorable water table.更多还原