黄土高原子午岭天然林与刺槐人工林地土壤干化状况对比

Comparison of Soil Desiccations in Natural and Acacia Forests in the Ziwuling Mountain of the Loess Plateau

详细调查和对比分析了子午岭(富县)天然林地和人工林地土壤水分状况和植被状况,发现天然林地水分亏缺不严重,只在浅层形成了轻度亏缺,并在雨季后能得到及时的恢复;同时这种亏缺并没有影响到天然植被的发育与演替。人工林地土壤水分则亏缺明显,深层亏缺尤为严重,土壤含水量最低平均达到5.90%,已接近凋萎湿度,且这种水分亏缺严重影响了人工林的生存和发展,部分林地已出现了明显的衰退迹象。在此基础上,说明了天然林地土壤水分状况明显优于人工林地的原因是天然林在林分结构上形成了典型的“乔-灌-草”复层稳定的空间层片结构,具有很强的自我调节能力,使天然林在物种的组成成分上逐渐向喜干物种方向发展,以适应气候旱化的总趋势;而人工林则因其结构简单,物种单一,自我调节能力差,在连续干旱的条件下因植被对土壤水分的过度消耗,形成了严重程度不一的土壤干化层。同时,从天然植被的自然水分状况入手,提出了“林地稳定土壤持水量”的概念,来界定土壤干化现象:依据土壤水分的亏缺现状初步将干化程度分为轻度干层(8%～10%)、中度干层(6%～8%)和严重干层(<6%)3个等级;依据土壤水分的补偿深度又将其划分为临时性干层和持续性干层。研究认为,当前人工林地的土壤水分出现严重亏缺而形成土壤干化层是人为不合理经营造成人工林群落林分结构不完善的结果,如果人为干预适当,能创造出类似于天然植被的生存条件,人工植被完全有可能得到正常的生存和发展。

Detailed investigation and comparisons of soil water and vegetations in natural forests were conducted,revealing that natural forests had less severe deficiency of soil water which only appeared in shallow soil layer and rehabilitated timely after rain season;meantime, the deficiency still had not affected the development and evolution of forest vegetations.Artificial forests had severe deficiency of soil water,especially so in deeper soil layers where the content of soil water amounted to 5.90%,a value nearly approaching the wilting soil humidity,and this deficiency severely affected the existence and development of artificial forests so that some of them had showed obvious degeneration symptoms.Accordingly, the reason why the soil moisture in natural forests was remarkably better than that in artificial forests was that natural forests formed the stand structure typical of “arbor-shrub-grass and thus had very strong self-regulating capacity so that the species of natural forests gradually developed into the ones adapted to dry conditions so as to follow the general climatic drying tendency; moreover,artificial forests had poor self-regulating capacity because of their simple structure and uniformed species composition and thus consumed too much soil water and formed dry a desiccated layers under long-lasting dry conditions to varying degree.Meantime,the concept of “stable water-holding capacity of forestlands” was put forward in terms of the soil moisture in natural forests to define the phenomena of the desiccated soil layer;according to the deficiency of soil water,the severity of soil desiccation was divided into light desiccation (water content:8%~10%), moderate desiccation ((water content:6%~8%) and severe desiccation (water content:less than 6%);according the compensation depth of soil water,the desiccated layer was divided into temporarily desiccated layer and permanently desiccated layer.The study held that the occurrence of the desiccated layer due to severe deficiency of soil water was because of the improper stand structure of the communities in artificial forests resulting from the mismanagement of artificial forests; with proper intervention,it was fully possible to create similar growing conditions to those of natural forests under which artificial vegetations could survive and develop.