On the odonate fauna of a Finnish forest pond occasionally drying up

The author made some observations on the drag-onfly life of a forest pond (Finland, Ta, Kangasala, Grid 6821: 339) since 1979. Although the variations of the water level have been considerable there always was at least some water in the pond round the year. The summer 1983 was exceptionally dry in Finland and the pond dried up towards the beginning of August staying waterless until the first snow. This made it possible to observe the drought resistance of some dragonflies. This question is treated only very rarely in Finnish odonatological literature (K.A. VALLE, 1952. Sudenkorennol. Suomen eläimet 7, WSOY, Turku, p. 133). In the following only the most essential data and observations are given. Spring melting waters are the main water source of the pond. These waters flow both from the southern knoll and from the gently sloping spruce forest in all other directions. The maximal measurements of the pond are some 60 m x 40 m corresponding to a water level of 120 cm. Because the pond lies in the shadow the evaporation is so slight that summer rains are normally enough to prevent the drying up of the pond. Before the spring 1983 the southern knoll was totally harvested causing changed evaporation conditions. This and the exceptionally dry summer led to the total drying up of the pond. When the place was visited on 11.VII.1983, the pond was waterless and the bottom largely dry. Beneath a thin layer of mire the bottom consists of a waterproof clay. The dragonflies favour especially the West and North edges of the pond with dense Carex rostrata vegetation. Other vascular plants abunding in the pond are Alisma plantago-aquatica, Potamogeton natans, Sparganium minimum, S. emersum, Juncus filiformis, Alopecurus aequalis and Potentilia palustris. The spring values of the water level are usually rather stable (110-120 cm), but the variations in the water level during summers may be quite different, depending on the rains and on the overcast. In 1983 the pond was continuously waterless for some 3.5 months until it froze on November 10. In summer 1984 the following extreme values of the water level were measured: after melting (27.IV.), 115 cm, the minimum (I4.IX.)45 cm, in frozen state (17.XI) 120 cm.