Tarka and other otters barely survive a particularly harsh winter, and this idea of the summer being stored away in the sun until the winter melts away is warming.  Elsewhere Williamson speaks of a ‘sun-hot boulder’ which a lamb uses as a radiator to sleep against.

‘Tarka was gone in the mist and rain of the day, to hide among the reeds of the marsh pond – the sere and icicled reeds, which now could sink to their ancestral ooze and sleep, perchance to dream of sun-stored summers raising the green stems.’

Source: Henry Williamson, Tarka the Otter: His joyful water-life and death in the two rivers, illus. C.F. Tunnicliffe (Harmondsworth: Puffin Books, 1976 (1927)), p. 122