VEGETATION PATTERNS IN THE SEMI-DESERT PLAINS OF BRITISH SOMALILAND
VEGETATION PATTERNS IN THE SEMI-DESERT PLAINS OF BRITISH SOMALILAND
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DOI：
10.2307/1789384
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发表时间：
1950-10-01
影响因子：
3
通讯作者：
Macfadyen, W. A.
中科院分区：
文献类型：
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作者：
Macfadyen, W. A.
在豪德和Sawl Haud的两个大型半沙漠平原的航空照片显示，在许多情况下，新月形植被弧和水道在罕见的情况下，有节奏的模式。野外调查结果表明，这些弧形是由小乔木、灌木、草本和草本植物的密集度有节奏地排列而成的。每年降雨量低于3-4英寸。导致普遍缺乏明确的排水通道。植被弧精确地指示了非常低的斜坡的方向，并在这些毫无特色的平原上定义了流域。弧形层的方向总是这样的，即弧形的弦与排水方向成直角，弧形是向上凸起的。提出了一个假设来解释弧的起源。罕见的暴雨过后，水流流过这个几乎平坦的表面，沿着地面上松散的有机物质，并以线状物的形式将其存款下来，这些线状物位于等高线上，因此向上凸起。该存款含有高比例的动物排泄物和种子，种子将发芽产生一个弧形的植被。随后的暴雨，每隔几年发生一次或两次，可能会以同样的节奏重复这一过程，因为一个弧形一旦建立，之后就会像筛子一样，把更多的碎片筛进去。植被弧是建立良好的，包含约50岁的相思树，并表明半沙漠表面的稳定性和目前没有明显的土壤侵蚀。在航空照片上，水道看起来像是没有灌木丛的笔直的痕迹，通常是多条或平行的，位于坡度很低的地形上，那里没有正常的切割排水线。水道通常出现在植被弧之间，与其弦成直角，因此沿着最大坡度向下延伸。它们可能起源于未形成图案的灌木丛，也可能是一条单独的车道延续了一条切割的排水线;它们似乎在植被弧区和具有切割的排水线的区域之间形成了一个中间联系。水道的特点一般是多条水道平行，每条水道的宽度很大，水流没有明显的侵蚀作用。
Air photographs of 2 large semi-desert plains in the Haud and the Sawl Haud show rhythmic patterns of crescentic vegetation arcs in many cases and water lanes in rarer cases. Field investigation showed that the arcs were made by rhythmical arrangement of concentrations of small trees, shrubs, herbs and grasses. Rainfall of less than 3-4 inches per yr. results in the general absence of defined drainage channels. The vegetation arcs indicate precisely the direction of the very low slopes, and define the watersheds in these featureless plains. The tiers of arcs are invariably oriented so that the chords of the arcs are at right angles to the direction of drainage and the arcs are convex upslope. An hypothesis to account for origin of the arcs is presented. Sheets of water flowing over this nearly flat surface after the very rare rainstorms sweep along the loose organic material lying on the ground and deposit it in the form of strand lines, which are on the contour and hence convex upslope. The deposit contains both animal excreta in high proportion and seeds which will germinate to produce an arc of vegetation. Subsequent rainstorms, occurring at the rate of not more than 1 or 2 every few years, would probably repeat the process in the same rhythm, since an arc once established would afterward act as a sieve to enmesh more debris. Vegetation arcs are well-established, containing acacia trees about 50 yrs. old, and indicate the stability of the semi-desert surface and the absence of appreciable soil erosion at present. Water lanes appear on air photographs as straight tracks fairly clear of brush, often multiple or parallel, on terrain of very low slope where normal incised drainage lines are absent. Water lanes generally occur among vegetation arcs, cutting them at right angles to their chords, and thus run down the maximum slope. They may arise from and end in unpatterned brush, or a single lane may continue an incised drainage line; they seem to form an intermediate link between vegetation arc areas and regions with incised drainage lines. Water lanes generally are characterized by exact parallelism when multiple, the great width of each separate channel, and the lack of noticeable erosive effect from water flow.