胶农（林）复合生态系统的种间水分关系及对土壤水分的调控

Hevea brasiliensis (rubber tree), the single viable source of natural rubber, shows strong synchronicity for flushing and shedding, displaying a very different phenology to the native vegetation in Xishuangbanna. As a tree native to the tropical rainforest of the Amazon Basin, its ideal habitat is characterized by small variations in air temperature and precipitation throughout the year. However, in order to meet the insatiable market demand for natural rubber, the cultivation of rubber tree has been extended to higher latitudes and altitudes in Southeast Asia. In Xishuangbanna, the land use has changed dramatically during the past three decades due to the conversion of tropical rainforest to rubber plantations, which has had a profound impact on the eco-environment, including significant water loss and soil erosion, soil degradation, soil hardening and even soil water gradual depletion in watershed scale. For the rubber tree itself, it also suffered serious water stress during the 5-month dry season. In this study, we will conduct field experiments to characterize the interspecific water relationships and their regulating capabilities on soil water in the typical rubber agroforestry systems under different intercropping patterns, by using the stable isotope technique (stable hydrogen, oxygen and carbon isotopes) in some selected small closed watersheds. The experiments consisted of monocultural mature rubber plantation and some typical rubber intercropping systems. These selected rubber polyculture systems were thought to be economically viable in this region, and have been intercropped with coffee, cacao, tea, thyrsoideum, macrophylla and grapefruit etc. Through observing and measuring the isotope ratios of hydrogen and oxygen in the related environmental waters (including rain, soil water, ground and stream water) and plant xylem water, as well as measuring leaf water potential, carbon isotope ratio and proline of leaves, soil water potential and water content, and root distribution patterns in the experimental watersheds, we will examine the water use strategy, water use efficiency and osmotic-adjustment ability of the plants in these different rubber agroforestry systems. Meanwhile, we will compare the competitive relationships for the limited source-water between plants, the differential water resources use among plants, and hydraulic redistribution of soil water under the different rubber agroforestry systems. The objective of this study is to select some suitable intercropping patterns among the established rubber agroforestry systems in this region, which would provide great capability on soil water regulating. These results would be useful for the local rubber planters, smallholders and state farms, not only on the selection of the best management practice and the optimal allocation mode among the established agroforestry systems, but also on the amelioration of soil water status in the watershed scale in xishuangbanna region.

针对西双版纳地区近40年来大面积单一橡胶种植（约占20%土地，近50万公顷）所导致的林地土壤板结缺水、小流域水源不断枯竭和遭受的干旱胁迫等现状，以该地区广泛种植的纯橡胶林和近年来构建的橡胶-咖啡、橡胶-可可、橡胶-龙血树、橡胶-茶叶、橡胶-大叶千斤拔、橡胶-砂仁、橡胶-柚子等不同类型胶农（林）复合生态系统为研究对象，采用氢、氧、碳稳定同位素、植物树干液流等方法研究各复合系统的植物水分获取策略、水分来源及其相互转换规律，揭示各栽培模式的种间水分互惠策略和对有限水分的竞争关系，明确不同复合系统的土壤水分再分配规律及其在调控林地土壤水分、缓解土壤板结缺水、改善土壤蓄水能力等方面的优劣差别，遴选一些种间水分关系配置合理的复合橡胶林模式。为优化该地区大面积的纯橡胶林、改善胶园生态环境、营造和推广可持续利用的生态胶园栽培模式，提供理论依据和切实参考。

项目以纯橡胶林和橡胶-咖啡、橡胶-可可、橡胶-龙血树、橡胶-大叶千斤拔、橡胶-茶叶等不同胶农复合系统为对象，采用同位素等方法研究了各复合系统的植物水分获取策略、水分来源及其相互转换规律，揭示了种间水分互惠策略和对有限水分的竞争关系，明确了土壤水分再分配规律及其在调控林地土壤水分、缓解土壤板结缺水、改善土壤蓄水能力等方面的优劣差别。结果表明：（1）由于橡胶树极其发达的浅层须根，其成年植株表现出了十分灵活的水分获取策略；虽然橡胶-大叶千斤拔复合系统内的植物存在一定的种间水分竞争，但并没有对植物间的水分有效获取产生不利影响，相反还加强了橡胶树的水分利用效率、养分吸收及该复合系统应对极端气候事件（干旱、低温）的能力；（2）在橡胶-可可复合系统内，橡胶、可可的用水策略均为挥霍型水分利用方式，使得该系统的土壤水分低于纯橡胶林；在橡胶-咖啡复合系统内，其种间水分的竞争最强，使得该系统内橡胶树的用水策略调整为保守型，但强烈的水分竞争却不利于作物生产；（3）橡胶树和茶树采用了不同但互补型的土壤水分垂直获取方式从土壤层中分区利用水分，从而减弱了它们之间对干季有限水分的直接竞争、增强了植物的水分利用效率，因而产生了水文生态位分离的效果，利于橡胶树和茶树间的共存；茶树行带间的上下坡土壤中包含较多的水分，然而复合系统潜在的合作式水分再分配功能（即水平、垂直方向的水分传输和运移），可以将茶树下丰富的水分重新分配到系统的各坡位。这种土壤水分垂直方向的互补型分区利用、侧向水平方向的水分再分配和水分运移，确保了该系统植物间的共存，减缓了干旱季节植物受到的水分胁迫；（4）纯橡胶林改造为复合橡胶林之后，土壤有机质、全氮含量、孔隙度、含水量、导水率显著增加，土壤广义土壤结构指数升高、土壤容重降低、土壤团聚体趋于稳定、土壤质量得到了明显的改良，同时也显著地增强了土壤的保水能力，土壤含水量显著增大。