不同土壤水盐条件下多枝柽柳(Tamarix ramosissima)对胡杨(Populus euphratica)幼苗的影响

目前胡杨更新困难，种群处于退化阶段，而与其生态位高度重叠的多枝柽柳却在扩张。为探讨多枝柽柳对胡杨生长的影响，本研究设置了3个水平的水分、盐分梯度，对胡杨进行了单种和混种的盆栽控制实验，通过测定胡杨幼苗的生长和存活状况，分析不同水、盐梯度下多枝柽柳如何影响胡杨生长。结果表明：（1）多枝柽柳伴生降低当年生胡杨幼苗的存活率，随着水分条件改善，胡杨存活率提高，而盐分对存活率没有显著的影响。（2）水分、盐分和伴生模式几个因子对胡杨的生长特征的影响存在显著的交互作用（P<0.05）。各水盐条件下，多枝柽柳会不同程度降低胡杨幼苗的株高、主根长和地上地下生物量，而根冠比增大。混种条件下，胡杨幼苗对水盐的响应更为敏感。（3）相对竞争强度对水盐环境有明显的响应（P<0.05），随水分条件改善，地上、地下相对竞争强度降低。多枝柽柳对胡杨的生长产生不良影响的机制是通过快速消耗土壤的水分，从而导致胡杨幼苗水分亏缺。土壤水分条件是胡杨和多枝柽柳幼苗共存的关键因素，在水分供给不足的情况下，对水分偏好的差异导致胡杨无法通过实生苗进行有效的更新从而加剧胡杨种群的衰退，而多枝柽柳更为耐旱的特性使其逐步占据河岸的生境。我们的研究结果强调了两个树种对环境因子的适应性差异决定了河岸带植被发育过程中植物-土壤的相互作用导致胡杨在演替过程中表现出的衰退现象。

Effects of Tamarix ramosissima seedlings on Populus euphratica seedlings under various water and salinity conditions

Populus euphratica is now facing with hard regeneration and degeneration due to restriction of water and soil salinity in hyperarid area in Northwest China, while the co-dominated shrub Tamarix ramosissima in riparian forests keeps expanding. As the two species are both primarily constructive in riparian forests, the relationship between them plays an important role in maintaining desert riparian ecosystem. At present, whether Tamarix ramosissima has become a threat to regeneration of Populus euphratica in the oases with changing hydrological environment remains unknown since previous studies focused primarily on impacts of abiotic stresses and the relationship between the two species is still unclear. Thus, a controlled greenhouse experiment was carried out to investigate how annual Populus euphratica seedlings response to neighboring Tamarix ramosissima seedlings under various water and salinity conditions. Populus euphratica seedlings were neighbored by their conspecific individuals or Tamarix ramosissima seedlings. Three levels of water conditions (low-water, medium-water and high-water treatment) and salinity conditions (low-salinity, medium-salinity and high-salinity treatment) intersected with each other, resulting in 18 treatments in total. The number of surviving Populus euphratica seedlings in each treatment were calculated and growth indexes were measured in this experiment. The results showed that neighboring Tamarix ramosissima seedlings inhibited growth and survival of Populus euphratica seedlings remarkably. Survival of Populus euphratica seedlings neighbored by Tamarix ramosissima seedlings significantly decreased, but it increased as water condition improved. Salinity had no effects on survival of seedlings. The significantly interactive effects were observed among water, soil salinity and neighbor conditions. Height, main root length, aboveground and belowground biomass of Populus euphratica seedlings decreased while being neighbored by Tamarix ramosissima under each treatment (P<0.001), among which main root length had the slightest reduction. However, root/shoot ratio increased, indicating a competition induced modification of biomass allocation of Populus euphratica seedlings. Moreover, Populus euphratica seedlings were more vulnerable to drought and salinity stress while competing with Tamarix ramosissima. Relative competitive intensity (RCI) for both aboveground and belowground decreased as water availability improved. Therefore, sufficient water supply is a key factor to the two species to co-exist, otherwise Populus euphratica seedlings would die of drought because of its poorer water absorbing capacity than Tamarix ramosissima, which might be a reason for the failed regeneration of Populus euphratica and gradual expansion of Tamarix ramosissima to broader range of riparian zone. Our research highlighted difference in adaptation between the two species determined plant-soil interaction in the development of riparian vegetation, resulting in the degeneration of Populus euphratica during succession.