This past year, the Dutch postal authorities have issued four sheetlets with tree stamps, based on the four seasons. Although the overall design of the sheetlets may be a bit busy, there's plenty of information to be picked up for thematic collectors, especially on the tabs. The other day I received the sheetlet of winter trees from my brother-in-law.
The stamps depict the Alder (alnus glutinosa) and the White Willow (salix alba). Previous stamps in the series tended to concentrate on details like bark or leaf, but what I like about these is that they show the tree as a whole and leave the detail for the tabs. The willow, especially, is a very atmospheric stamp. Could almost have been a Christmas stamp!