Summer lingers in the form of the wine and olive oil made from the grapes and olives of the summer. Olives are pressed to yield "sun-gold oil" and grapes are harvested "to brim our vats where summer lingers in the red froth."
To some extent, summer is still present in the vines and branches and stems being burned in the fireplace to fend off the winter chill. In the same way, the needles on the pine trees remain, narrowing the beams of sunlight to needle-like slivers of light. The broad leaves of deciduous trees, the ones that create shade in the summer, have already fallen from the trees - they do not remain.
The primary souvenior of summer, however, is the wine and the enjoyable memories that arise while drinking it. In that pleasure, "the grape will redden on your fingers through the lit crystal of the cup." In that product, summer will linger for as long as the wine lasts.