Data from: Can intercropping with the world’s three major beverage plants help improve the water use of rubber trees?

The dramatic expansion of rubber plantations in mainland South-East Asia and south-west China has caused many eco-environmental problems, especially negative hydrological consequences. These problems have gradually worsened and pose formidable threats to rubber agriculture, especially in the light of increasingly frequent extreme weather events. Although rubber-based agroforestry systems are regarded as the best solution for improving the sustainability of rubber agriculture and environmental conservation, plant water use and related interactions have rarely been examined in such systems.
We primarily used stable isotope (δD, δ18O and δ13C) methods to test whether intercropping could improve the water use and extreme weather tolerance (extreme cold and drought in our study) of rubber trees in three types of promising agroforestry systems (i.e. rubber with tea, coffee and cocoa) in Xishuangbanna, China.
We found that the rubber tree is a drought-avoidance plant with strong plasticity with respect to water uptake. This characteristic is reflected by its ability to cope with serious seasonal drought, allowing it to avoid interspecific competition for water. The rubber trees showed wasteful water behaviour unless they were intercropped with tea or coffee. However, these intercropped species exhibited drought-tolerance strategies and maintained lower water use efficiencies to strengthen their competitive capacity for surface soil water. The stable δ13C values of the intercrop leaves indicated that all the agroforestry systems have stable internal microclimatic environments or higher resistance.
Synthesis and applications. This study suggests that interspecific competition for water can enhance the water use efficiency of drought-avoidance plants (i.e. rubber trees) and lead to complementarity between the root distributions of plants in rubber agroforestry systems (i.e. rubber with tea, coffee and cocoa). All agroforestry systems have higher resistance, but tea was the most suitable intercrop in terms of water use because the interspecific competition for water was moderate and the agroforestry system retained much more soil water and improved the water use efficiency of the rubber tree. Considering the root characteristics of the tea trees, we suggest that the crops selected for intercropping with rubber trees should have a relatively fixed water use pattern, short lateral roots and a moderate amount of fine roots that overlap with the roots of the rubber trees in the shallow soil layer.

东南亚大陆及中国西南地区的橡胶种植园（rubber plantations）规模急剧扩张，引发了诸多生态环境问题，尤以负面水文后果（hydrological consequences）最为突出。上述问题日益恶化，加之极端天气事件（extreme weather events）愈发频发，对橡胶种植业构成了严峻威胁。尽管以橡胶为主的农林复合系统（agroforestry systems）被视为提升橡胶种植业可持续性与环境保护成效的最优方案，但此类系统中植物水分利用（plant water use）及其相关相互作用却鲜有研究。
本研究以中国西双版纳地区三种极具应用前景的橡胶农林复合系统（即橡胶-茶、橡胶-咖啡、橡胶-可可）为研究对象，主要采用稳定同位素（stable isotope，δD、δ18O及δ13C）技术，探究间作（intercropping）能否改善橡胶树的水分利用状况及极端天气耐受能力（本研究中特指极端低温与干旱胁迫）。
研究结果表明，橡胶树属于具有较强水分吸收（water uptake）可塑性的避旱型植物（drought-avoidance plant），该特性使其能够应对严重的季节性干旱，进而规避种间竞争（interspecific competition）。未进行间作的橡胶树存在水分利用低效的问题，而与茶树或咖啡树间作的橡胶树则无此弊端。此外，间作物种均采取耐旱策略，维持较低的水分利用效率（water use efficiencies）以增强其对表层土壤水分的竞争能力。间作作物叶片的稳定δ13C值表明，所有农林复合系统均具备稳定的内部微气候环境（microclimatic environments）或更强的环境抗性。
综合与应用：本研究证实，橡胶农林复合系统（即橡胶-茶、橡胶-咖啡、橡胶-可可）中，种间水分竞争可提升避旱型植物（即橡胶树）的水分利用效率，并促成植物根系分布（root distributions）的生态位互补。所有受试农林复合系统均具备较高的环境抗性，其中以橡胶-茶复合系统在水分利用方面表现最优——其种间水分竞争强度适中，能够留存更多土壤水分并提升橡胶树的水分利用效率。结合茶树的根系特性，我们建议，与橡胶树间作的作物应具备相对固定的水分利用模式、较短的侧根，以及与橡胶树浅层土壤根系重叠度适中的细根数量。