陕北水蚀风蚀交错区生物结皮的形成过程与发育特征

生物结皮的形成发育具有明显的时空分异性，荒漠地区的诸多研究结果不能直接推演至黄土高原地区。研究黄土高原生物结皮的形成过程与发育特征，是对全球生物结皮认知的重要补充，也是揭示其生态功能的基础。论文选择陕北黄土高原水蚀风蚀交错区的典型小流域，通过定位动态观察和全面调查测算，探讨生物结皮的发育过程。结果表明：①流域内生物结皮经历了由退耕裸地—物理结皮—生物结皮的发育阶段。退耕撂荒的第1 年，物理结皮便迅速形成；第2 年，土表颜色明显变深，藓结皮开始发育。流域内藓结皮占绝对优势，发育稳定后（15 a）覆盖度可达90%。②不同发育年限生物结皮的物种组成及种数差异很大。发育初期（<6 a）发现苔藓植物2 科4 属5 种，其中芦荟藓（Aloina rigida (Hedw.) Limpr.）和短喙芦荟藓（A. brevirostris (Hook. & Grev.) Kindb.）为先锋优势种。发育稳定的生物结皮（15 a）鉴定出苔藓植物2 科8 属13 种。其中，丛藓科占6 属8 种，以对齿藓属（Didymodon）的尖叶对齿藓（D. constrictus (Mitt.) Saito）、黑对齿藓（D. nigrescens (Mitt.) Saito）、硬叶对齿藓尖叶变种（D. rigidulus Headw. var. ditrichoides (Hedw.) Zand.）所占种数最多。不同阶段的藓结皮均有藻类伴生，发育初期（<6 a）的藓结皮中鉴定出藻类植物26 科35 属50 种。③水热同期的雨季是生物结皮形成的关键时期。发育2 a 的生物结皮厚度达到6.3 mm，超过稳定结皮厚度（9.8~11.8 mm）的50%，抗剪强度约30 kPa。发育初期（<6 a），容重无明显变化（1.40~1.43 g·cm^-3），发育稳定（15 a）后容重下降到1.0~1.3 g·cm^-3。该区生物结皮的形成过程同荒漠区类似，但表现出发育速度快、稳定周期短的特点；藻类、苔藓组成及其优势种[坑形细鞘丝藻（Leptolyngbya foveolara）、尖叶对齿藓]与荒漠区明显不同。

The Development Characteristics and Formation Process of Biological Soil Crusts in Wind-Water Erosion Crisscross Region,Northern Shaanxi Province,China

The formation, development and ecological functions of biological soil crusts (BSCs) are characterized by significant temporal and spatial differences, results of many researches from the desert region can not be directly deduced in the Loess Plateau Area. To study the formation process and developmental characteristics of BSCs in the Loess Plateau is not only an important complement to the global cognition of BSCs, but also the precondition to reveal its ecological functions. Selecting typical small watershed (Liudaogou) in the Loess Plateau as a case, this study explores the developmental characteristics of BSCs by positioned dynamic observation, comprehensive investigation and estimation. The results showed as follows: 1) BSCs in Liudaogou watershed experienced a development process of bare farmland-physical crust-biological soil crusts. Physical crust rapidly formed in the first year of farmland abandonment; in the second year, moss-dominated crust developed obviously, and the soil color became apparently darker than before. Moss crust dominated absolutely in the watershed, and its coverage was up to 90% when it developed into a stable stage (>15 years). 2) Years of development had a significant impact on the composition and amounts of species of BSCs. In the initial development stage (<6 years) of BSCs, bryophytes had 5 species under 4 genera belonging to 2 families, of which Aloina rigida (Hedw.) Limpr. and Aloina brevirostris (Hook. & Grev.) Kindb mosses both were the pioneer species. In the stable stage (>15 years) of BSCs, bryophytes had 13 species under 8 genera belonging to 2 families, of which the family Pottiaceae had 6 genera with 8 species, and the dominant species were Didymodon constrictus (Mitt.) Saito, Didymodon nigrescens (Mitt.) Saito and D. rigidulus Headw var ditrichoides (Hedw.) Zand. In addition, in the initial stage (<6 years), 26 families of algae including 50 species under 35 genera were identified in BSCs. 3) Rainy season with warm weather was a critical period of BSCs formation. The thickness of BSCs developed 2 years reached 6.3 mm, which exceeded the stable depth of crust (9.8-11.8 mm) by 50% and the shear strength of BSCs was about 30 kPa. In the initial stage (<6 years), bulk density showed no significant change (1.40-1.43 g ·cm^-3), but it reduced to 1.0 to 1.3 g ·cm^-3 when development of BSCs went into the stable stage. The formation process of BSCs in this area was similar to that in desert region, but the rate of development is faster and the period to stabilization is shorter. The composition of algae, moss and their dominated species (Leptolyngbya foveolara, Didymodon constrictus (Mitt.) Saito) was apparently different from those in desert region.