Although Webster conceived many of her signature works in response to the nuclear threats of the 1980s, they now read as remarks on climate change and as templates for making art in a world overrun with stuff that never biodegrades. Some are subtle inducements to commune with fragile forms of life. Take “Moss Bed, King” (1986), a mound of emerald green moss the size and shape of a king-size mattress, on view at Dia Beacon. It requires regular watering and other forms of care to last.