南方“竹—牛—草”复合生态系统的系统动力学分析

为了探索我国南方亚热带地区竹林下放牧的可行性，我们设计了一种“竹—牛—草”复合生态系统，进行竹林下养牛试验。通过试验、调查和测定，构成一个比较完整的数学模型。利用系统动力学对这一系统进行定量分析。模型考虑了毛竹的产量及其经济收益、养牛所需的成本和收益等经济因素，还考虑到牛的践踏对于土壤物理性质的影响及对草被生长的影响，草被的载畜能力及其它反馈机制。这一综合性的系统动力学模型，主要用于评估在何种条件下这一系统具有高经济效益。模型还设定了七种选择方案，每种方案都以竹子的砍伐年龄为标准，试图对竹林下放牧的长期生态后果进行评估。得出的结论是：通过林下放牧或舍饲养牛并不能有效地改善竹林区的经济现状，但如果技术条件足够并有广大的销售市场，采用舍饲方式饲养生长迅速的肉牛或以售牛奶为其经营方针，则经济效益有可能大幅度提高。

A STUDY ON A BAMBOO-CATTLE-GRASS AGROFORESTRY SYSTEM USING SYSTEMS DYNAMICS

Bamboo forests are widely distributed in the subtropics of China. Agroforestry systems have been recommended, e.g. raising cattle in bamboo stands, to enhance the economic return. A Bamboo-Cattle-Grass agroforestry system has been designed to explore the feasibility of that kind of systems.Data were collected from the experiments conducted in Dagangshan, Jiangxi province, and a simulation model was developed. Productivity, cost,economic return, impacts of crown density on undergrowth, impacts of cattle on volume weight of soils etc.and feedback loops among them were taken into consideration in the model. Seven scenarios are set up based on cut-age of bamboo stands. The results show that as the crown density becomes lower the economic return grows up, but a disastrous ecological impact follows.The soil is compacted by cattle and low carrying capacity and serious land degradation will happen.Introduction of quality grass species and cage-feeding method can much improve the productivity of grass and soil conditions,but the economic return is not high. Neither grazing under canopy norcagefeeding of cattle favoures the improvement of economics of bamboo regions.If appropriate techneques and markets are available meat-targeted ormilktargeted cattle in cage-feeding may be profitable. However, these conditions are generally not available in the present bamboo regions. The later policy may have hopeful perspectives. A successful evaluation needs a futhersocial-economic-production model.