From Glossary of Meteorology
An optical phenomenon in which some species of dew-covered plants become strongly retroreflective.
On a warm summer evening when a beam of light, such as car headlights, illuminates dew- covered trees, the scene is reminiscent of snow-covered trees in the moonlight. Four things must be present for the sylvanshine to be seen: the proper species of tree, the proper season of the year, dew-covered leaves, and a view looking directly down the beam of light illuminating the tree in an otherwise dark scene. The sylvanshine is closely related to the heiligenschein.
Fraser, A. B. 1994. The Sylvanshine: retroreflection in dew-covered trees. Appl. Optics. 33. 4539–4547.