塔里木荒漠河岸林物种多样性沿地下水埋深梯度的分布格局

探讨荒漠河岸林土壤水分、物种多样性的空间变异性及其相互关系，可为干旱区天然林保护、可持续经营和生态恢复提供科学依据。以塔里木荒漠河岸林为研究对象，基于野外样带调查和采样测定，系统分析了地下水埋深（GWD）梯度下林地土壤水分与物种多样性的空间变异及其权衡关系。结果表明：随GWD增加和土壤水分减少，荒漠河岸林群落物种数减少、结构简化、群落发生退化，退化顺序为浅根系的中生草本植物和灌木，最后留存的是抗旱性较强的乔灌木或灌木；同时土壤水分和物种丰富度、物种多样性指数均呈显著的线性递减趋势，而物种均匀度指数降幅较小。GWD与土壤水分、物种多样性之间均呈极显著的相关（P<0.01），土壤水分与物种多样性的相对收益随GWD增加而逐渐降低，表明GWD是控制荒漠河岸林土壤水分和物种多样性空间变异的关键因素。荒漠河岸林土壤水分与物种多样性权衡关系的转折点为GWD 4.5m左右，转折点以下（GWD<4.5m）二者沿GWD以相同速率变化，呈协同关系；转折点以上（GWD>4.5m）土壤水分与物种多样性的权衡明显增大，土壤水分相对收益剧降，即维持当前相应的物种多样性以消耗土壤水分为代价，系统通过反馈调节使物种多样性降低。综上表明，维持塔里木荒漠河岸林物种多样性和生态系统功能的合理GWD在4.5m左右，这为塔里木河流域荒漠河岸林保育与生态输水工程实施提供科学依据。

Species diversity distribution pattern of desert riparian forest along a groundwater depth gradient in the Tarim Basin

Groundwater is the most important water source for plant growth and vegetation survival in arid desert area, which directly affects the composition, distribution and development of desert vegetation. Desert riparian forest is a valuable natural forest resource in extreme arid region, and also a distinctive ecosystem with the most active life phenomena, high biodiversity and primary productivity. The largest area of desert riparian forest is distributed in the Tarim Basin, northwest China. It protects biodiversity and desert ecosystem stability, and serves as a natural barrier to safeguard the ecological security of oasis. Since the 1950s, a large deal of land has been reclaimed and the water resources have been excessively utilized in the area. It has resulted in a reduction in the groundwater level, weak vegetation growth, and high desert plant mortality, which has affected the sustainable development of agriculture and the economy of oasis. So, it is urgent to restore the degraded desert riparian forest ecosystem. To establish a scientific basis for protection, sustainable management and ecological restoration of natural forest in arid areas, understanding the spatial variations of soil water availability and biodiversity in desert riparian forests, as well as their interrelationships, is critical for protecting, managing and/or restoring the natural forests on drylands. We surveyed 22 riparian forest sites in the Tarim Basin to examine how soil moisture and species diversity covary along a groundwater depth gradient (GWD) from 1.2m to 5.6m. Results showed that the number of species significantly decreased, community structure became simplified and vegetation degraded as soil moisture reduced and GWD increased. The vegetation degraded in the following sequence:herb and shrub species with shallow roots were early losers in face of water shortage at greater GWD sites, and only tree and shrub species with high drought-tolerance persisted at the extreme of the GWD gradient. There was a significant linear correlation between soil moisture and GWD, and the strength of correlation between GWD and soil moisture was significantly higher than that between GWD and species diversity. Soil moisture and species richness, species diversity index showed a significant linear declining tendency with increasing GWD, while species evenness index decreased at a relatively low rate (i.e., gentle slope). In the meantime, the net negative effect of GWD increases on soil moisture was higher than on species diversity, suggesting a key of GWD in determining the spatial variability of species diversity and soil moisture. Linear piece-wise quantile regression was employed to determine the inflection points of trade-off between species diversity and soil moisture along the GWD gradient. It revealed an obvious trend of trade-off along the GWD gradient. The inflection point of trade-off was detected at a GWD of about 4.5m. Where GWD was less than 4.5m, the soil moisture and species diversity changed at the same rate and exhibited a synergistic relationship. Alternatively, with GWD>4.5m, the trade-off was strengthened obviously and the relative returns of soil moisture decreased sharply. So, species diversity was maintained at the cost of soil water consumption, and the species diversity reduced through a feedback regulation. To sum up, the critical GWD for maintaining species diversity and ecosystem functions of desert riparian forests is about 4.5m, which provides a scientific guideline for the conservation of desert riparian forests and the implementation of ecological water conveyance project in the Tarim Basin.