Studies on the Ecology of the River Rheidol: I. Plant Colonization and Permanent Quadrat Records in the Main Stream of the Lower Rheidol

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Abstract
CallltricKe intermedia and Glyceria fluitans were introduced in soil-filled sacks into the Rheidol, a Cardiganshire stream. Favorable conditions for colonization by these two plants are a summer depth of water of about 15 cm, an open situation especially unshaded by tree overhang and a current speed within a yard of the bank of not more than about 15 cm per second at low water. The absence of the two plants from shaded, stillwater parts of the river is due primarily to the action of low light intensity on growth rather than on initial establishment. The introduction of plants protruding from sacks of soil is suitable for moderately swift streams with stony beds, but the sacks must be placed against the banks and weighted down with stones. Records of permanent quadrats established in the stream bed showed that a marked increase in Glyceria fluitans followed from the introduction of this plant. Subsequent increase in other plants was attributed to the sheltering Glyc effect of the Glyceria. In a quadrat where an obstruction upstream removed the influence of the current, transition in the vegetation from an aquatic to a swamp type was observed. While Scapanla undulata and Fontinalis antipyretica are suited to con-ditions in the open stream, these bryophytes disappear when the current is removed. This probably results mainly from increased competition from angiosperms.