流域水文变化对胡杨荒漠河岸林林窗及形成木特征的影响

林窗作为森林生态系统得以长期维持的主要驱动力之一，能够促进森林的自然更新与演替、优化森林结构和养分循环，从而提高生物多样性及生态系统服务功能。基于塔里木河中游历年径流量、耗水量与地下水埋深等水文数据，利用回归模型、拟合优度t检验与种群结构动态定量相结合的方法，分析了研究区水文条件对胡杨荒漠河岸林林窗数量及其形成木种群结构与动态的影响。结果表明：1）1970年前塔里木河中游水量较为充足，在一定程度上可满足胡杨生存发育的需水量。自1980年来，塔里木河中游年径流量（R²=0.785，P < 0.01）与耗水量（R²=0.524，P < 0.01）呈明显减少趋势、地下水日益匮乏，其埋深呈直线下滑趋势（R²=0.8618，P < 0.01）。这导致河道两岸的胡杨林生态用水日益紧缺，从而引起胡杨林林窗现象频繁出现，尤其是在1997-2006年期间。2）随着中游年径流量与耗水量逐渐减少、地下水埋深逐渐增加，林窗内胡杨形成木数量则变为增长趋势。3）林窗形成木除1957-1966、1967-1976年属衰退型外，1977-1986、1987-1996、1997-2006和2007-2016年均呈现增长型特征。特别是林窗内胡杨形成木Ⅱ级幼龄树大幅度增加，其表现出胡杨林年龄结构的老龄化特征。4）由模型显示，林窗及其形成木数量与径流量之间存在显著性负相关，径流量降至17.81×10⁸ m³时，林窗内的胡杨将面临高峰死亡。由此可知，在干旱区水文条件是决定胡杨生死的主导因子，也是塔里木河中游荒漠河岸林林窗形成的主要原因。

Influence of river hydrological change on the characteristics of canopy gaps and gap makers of Populus euphratica desert riparian forest

Populus euphratica is a member of the Salicaceae family, and is a key tree species of the desert riparian forest. Forest gaps can promote the natural renewal and succession of forests and optimize forest structures and nutrient cycling, thereby, which is regarded as a main driver for increasing biodiversity and ecosystem services. In this study, field data were obtained from six plots with a typical sampling method within study area. The effects of hydrological conditions on the number of canopy gaps, formation and dynamics of gap maker populations were analyzed by using a combined method of the regression model, the goodness-of-fit t test and the dynamic quantitative of population structure, based on the hydrological data such as runoff, water consumption and groundwater depth in the middle reaches of the Tarim River. The result showed that:1) Before the 1970s, the amount of water in the middle reaches of the Tarim River was sufficient, to a certain extent, which could meet the water requirement for the survival and development of P. euphratica forests to some extent. Since the beginning of 1984, the annual runoff (R²=0.785, P < 0.01) and water consumption (R²=0.5238, P < 0.01) in the middle reaches of the Tarim River have shown a significant decreasing trend, the groundwater is increasingly scarce and its depth is in a declining trend (R²=0.8618, P < 0.01), especially during the period of 1997-2006. 2) As the annual runoff and water consumption gradually decrease, the groundwater depth also increases; the number of gap maker population becomes a growing trend. 3) In addition to the recession type in 1957-1966 and 1967-1976, the forest gap maker population showed a growing trend in 1977-1986, 1987-1996, 1997-2006 and 2007-2016. Particularly, the juvenile P. euphratica tree gap makers in the forest gap increased significantly, which showed the aging characteristics of P. euphratica forests. 4) According to the model output, there is a significant negative correlation between runoff and the number of canopy gaps, and gap makers. Large number of P. euphratica in forest gaps will face with mortality, when runoff is decreased to the 17.81×10⁸ m³. Thus, hydrological conditions in the arid area are the dominant factors determining the life and death of P. euphratica, and also the main driver for the formation of desert riparian forest gaps in the middle reaches of the Tarim River.