Mechanisms And Effects Of Poplar Plantations On Understory Plant Diversity In The Dongting Lake Wetlands

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.